Papers - KAWAHARA Nobuyuki

Effect of swirl flow on the main chamber combustion dynamics of methane in a passive pre-chamber spark ignition engine

MD. Tanvir Khan, Nobuyuki Kawahara, Yoshimitsu Kobashi, Isao Yamane, Towa Hirayama, Ayumi Shimizu, Sekai Miyamoto

Fuel   391   134735 - 134735   2025.7

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.fuel.2025.134735

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Development of Ignitability Evaluation Index by Optical Combustion Analyzer

Nayuha Moriguchi, Yoshimitsu Kobashi, Nobuyuki Kawahara, Masahide Takagi, Satoshi Kawauchi

The 28th Small Powertrains and Energy Systems Technology Conference (SETC 2024), Poster   2024.11

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Publishing type：Research paper (international conference proceedings)  

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Improvement of Lean Burn Characteristics with Ozone Addition in a Diesel Micro-Pilot Natural Gas Engine Reviewed

Yoshimitsu Kobashi, Shoki Miyata, Nobuyuki Kawahara, Ryuya Inagaki

The 28th Small Powertrains and Energy Systems Technology Conference (SETC 2024), JSAE 20249104/ SAE 2024-32-0104   2024.11

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Publishing type：Research paper (international conference proceedings)  

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Hydrogen concentration measurements using spark induced breakdown spectroscopy in a real engine Reviewed

Dongchan Kim, Quancen Wan, Qing Nian Chan, Yoshimitsu Kobashi, Nobuyuki Kawahara, Sanghoon Kook

Proceedings of the Combustion Institute   40 ( 1-4 )   105209 - 105209   2024.6

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.proci.2024.105209

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Droplet diameter and axial velocity measurements near a nozzle exit of an automotive fuel injector using PDA Invited

Nobuyuki Kawahara

2023.10

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Authorship：Lead author  

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Visualization of Combustion and Flow Phenomena in a Methane-Fueled Passive Pre-Chamber Ignited Gas Engine Reviewed

MD Tanvir KHAN, Nobuyuki Kawahara, Yoshimitsu Kobashi, Towa Hirayama, Ayumi Shimizu, Sekai Miyamoto

2023 JSAE/SAE Powertrains, Energy and Lubricants Int. Meeting, JSAE 20239119, SAE 2023-32-0057   2023.8

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Authorship：Corresponding author  

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PREMIER Combustion of Natural Gas Ignited with Diesel Fuel in a Dual Fuel Engine - Effects of EGR and Supercharging on End-gas Auto Ignition and Thermal Efficiency- Reviewed

Yoshimitsu Kobashi, Kengo Kishimoto, Nobuyuki Kawahara

2023 JSAE/SAE Powertrains, Energy and Lubricants Int. Meeting, JSAE 20239121, SAE 2023-32-0016   2023.8

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Spatially and temporally resolved plasma formation on alumina target in microwave-enhanced laser-induced breakdown spectroscopy Reviewed

Yuji Ikeda, Joey Kim Soriano, Nobuyuki Kawahara, Ikuo Wakaida

Spectrochimica Acta Part B: Atomic Spectroscopy   197   106533 - 106533   2022.11

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.sab.2022.106533

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Evaluation of ignition characteristics of diesel spray using large eddy simulation Reviewed

Nobuyuki Kawahara

22nd Annual Conference on Liquid Atomization and Spray Systems-ILASS-Asia 2022, Paper No. 40   2022.10

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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Plasma formation and its sustainment in time and space in microwave enhanced laser induced breakdown spectroscopy Reviewed

Yuji Ikeda, Joey Kim Soriano, Nobuyuki Kawahara

20th Int. Symp. on the Application of Laser and Imaging Techniques on Fluid Mechanics (LISBON)   2022.7

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Authorship：Last author   Publishing type：Research paper (international conference proceedings)  

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Spark-induced breakdown spectroscopy in a direct-injected natural gas spark-ignition engine Reviewed

Nobuyuki Kawahara, Kohei Miyao

20th Int. Symp. on the Application of Laser and Imaging Techniques on Fluid Mechanics (LISBON)   2022.7

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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Visualization of End-gas Auto-ignition during PREMIER Combustion in a Dual-fuel Gas Engine Reviewed

Nobuyuki Kawahara, Eiji Tomita

The 10th International Conference on Modeling and Diagnostics for Advanced Engine Systems (COMODIA 2022), C9-3   2022.7

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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Measurement of Cyclic Variation of the Air-to-Fuel Ratio of Exhaust Gas in an SI Engine by Laser-Induced Breakdown Spectroscopy Reviewed

Yuji Ikeda, Nobuyuki Kawahara

Energies   15 ( 9 )   3053 - 3053   2022.4

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Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Temporally and spatially resolved laser-induced breakdown spectroscopy (LIBS) was applied to a four-stroke, single-cylinder test engine’s cyclic exhaust gas to demonstrate engine performance. The LIBS technique provided quantitative air-to-fuel ratio (A/F) measurements by generating localized breakdown plasma during the compression and exhaust strokes. The results showed that the timing and duration settings of the emission energy ionization and molecular spectra affect the intensity peaks. Optimum measurements performed between 200 ns and 10 ms after breakdown resulted in observed atomic spectra of CI (248 nm), Hβ (485 nm), Hα (656 nm), NI (745, 824 nm), and OI (777, 844 nm). The intensities of CI (248 nm) and Hα (656 nm) decreased with increasing A/F, whereas the intensity ratios of NI and OI remained constant. A decrease in the intensity ratio of C/O and Hα/O was observed as the A/F increased. This study is a major step toward defining a means of using LIBS to control the A/F ratio in gasoline engines by focusing on the exhaust gas rather than the flame.

DOI： 10.3390/en15093053

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PREMIER combustion characteristics of a pilot fuel-ignited dual-fuel biogas engine with consideration of cycle-to-cycle variations Reviewed

Taku Imamoto, Nobuyuki Kawahara, Eiji Tomita

Fuel   314   123049 - 123049   2022.4

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.fuel.2021.123049

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Fuel spray impingement and liquid film formation in a gasoline direct-injection spark-ignition engine Reviewed

Y. Tomomatsu, N. Kawahara, E. Tomita

International Journal of Environmental Science and Technology   20 ( 1 )   477 - 488   2022.3

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s13762-022-04010-4

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Other Link： https://link.springer.com/article/10.1007/s13762-022-04010-4/fulltext.html

Droplet diameter measurement near a nozzle exit of a common-rail Diesel injector using PDA Reviewed International journal

Nobuyuki Kawahara, Rikuya Yamada, Eiji Tomita

15th International Conference on Liquid Atomization and Spray Systems, ICLASS2021, Paper No. 247   2021.8

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

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Differences between PREMIER combustion in a dual-fuel gas engine and knocking with pressure oscillations Invited

Nobuyuki Kawahara

7th International Seminar on Motor Vehicle Environmental Protection   2021.7

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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Performance, emissions and end-gas autoignition characteristics of PREMIER combustion in a pilot fuel-ignited dual-fuel biogas engine with various CO2 ratios Reviewed International journal

Alireza Valipour Berenjestanaki, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

Fuel   286   119330 - 119330   2021.2

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Biogas, a renewable and alternative energy, has recently gained attention as an environmentally friendly fuel for power generation in internal combustion (IC) engines. This study aimed to investigate the effect of the carbon dioxide (CO2) to methane (CH4) ratio in a dual-fuel gas engine. It is known that gaseous fuel combustion at high loads is accompanied by knocking if an appropriate combustion controlling strategy is not applied. Therefore, premixed mixture ignition in the end-gas region (PREMIER) combustion was proposed under high load conditions. Experiments were carried out using a dual-fuel gas engine under supercharged conditions, with an intake pressure of 200 kPa. Simulated biogas consisting of CH4 and CO2 was used as a primary fuel and diesel was used as a pilot fuel. The pilot fuel injection timing was varied during the experiments. The indicated mean effective pressure (IMEP) and thermal efficiency increased as injection timing was advanced. With the addition of CO2, the thermal efficiency of PREMIER combustion was slightly increased, although the IMEP was slightly decreased. The unburned gas temperature for PREMIER combustion increases as a function of the CO2 content of the mixture. Due to compression by flame propagation, high unburned gas temperatures in dual-fuel gas engines are important for achieving autoignition inside the end-gas region under PREMIER combustion conditions.

DOI： 10.1016/j.fuel.2020.119330

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Multidimensional CFD Simulation of PAH Formation Process in Diesel Spray Combustion Reviewed

Nobuyuki Kawahara, Hirotomo Daido, Eiji Tomita

21st Annual Conference on Liquid Atomization and Spray Systems-ILASS-Asia 2020, ILASS-Asia2020-3-12   2020.10

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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Quantitative evaluation of liquid film formation process from DISI injector using LIF method Reviewed

Yasutaka Tomomatsu, Nobuyuki Kawahara, Eiji Tomita

21st Annual Conference on Liquid Atomization and Spray Systems-ILASS-Asia 2020, ILASS-Asia2020-3-11   2020.10

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Publishing type：Research paper (international conference proceedings)  

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Binary collisions and coalescence of droplets in low-pressure fuel injector Reviewed International coauthorship International journal

Fatma Ahmed, Nobuyuki Kawahara, Eiji Tomita

Thermal Science   ( 00 )   185 - 185   2020

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Publishing type：Research paper (scientific journal)   Publisher：National Library of Serbia  

The phenomena of binary collisions and coalescence of droplets was
investigated from experimental studies but still are missing from real
applications such as from fuel injector. The main purpose of the current
study is to investigate the phenomena of binary collisions and coalescence
of droplets from a practical port fuel injector (PFI). To accomplish this,
direct microscopic images are taken from high-speed video camera coupled
with a long-distance microscope and Barlow lens using the backlighting
method. Experimental optimization of the spatial resolution and the depth
-of -field of the long-distance microscope and Barlow lens are achieved.
Experimental results from the direct microscopic images are compared with
predictions from empirical equations for different collision regimes.
Droplet sizes and velocities of experimental coalescence droplets from
collisions are compared with the values predicted by the equations. The main
results of this study are: The probability of collision and coalescence is
very low in a PFI. The tangential velocity components of small droplets play
an essential role in shape deformation during collisions and coalescence of
the droplets. The previous published empirical equations to calculate
dimensionless parameters, the Weber number (We), the droplet diameter ratio
(?), and impact parameter (B) are applicable to the coalescence of droplets
in a PFI.

DOI： 10.2298/tsci191120185a

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CO2 Concentration Measurements inside Expansion-Compression Engine Under High EGR Conditions using an Infrared Absorption Method Reviewed International coauthorship International journal

Fatma B, M. Ahmed, Mohamed F. C. Esmail, Nobuyuki Kawahara, Eiji Tomita

Ain Shams Engineering Journal   11 ( 3 )   787 - 793   2020

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Ain Shams Engineering Journal

DOI： 10.1016/j.asej.2019.12.003

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Attainment of High Thermal Efficiency and Near-zero Emissions by Optimizing Injected Spray Configuration in Direct Injection Hydrogen Engines Reviewed

Masakuni Oikawa, Yasuo Takagi, Yuji Mihara, Nobuyuki Kawahara, Eiji Tomita, Ken Naitoh

SAE Technical Papers   ( December )   2019.12

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Publishing type：Research paper (international conference proceedings)  

© 2019 SAE Japan and SAE International. The authors have previously proposed a plume ignition and combustion concept (i.e., PCC combustion), in which a hydrogen fuel is directly injected to the combustion chamber in the latter half of compression stroke and forms a richer mixture plume. By combusting the plume, both cooling losses and NOx formation are reduced. In this study, thermal efficiency was substantially improved and NOx formation was reduced with PCC combustion by optimizing such characteristics as direction and diameter of the jets in combination with combustion of lean mixture. Output power declined due to the lean mixture, however, was recovered by supercharging while keeping NOx emissions at the same level. Thermal efficiency was further improved by slightly re-optimizing the jet conditions. The results showed that the hydrogen engine can attain near-zero emissions of NOx reduced to the single-digit ppm level with high thermal efficiency close to 50 percent and can thus be truly called a near-zero emission engine.

DOI： 10.4271/2019-01-2306

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Plasma Temperature of Spark Discharge in a Lean-burn Spark-ignition Engine Using a Time Series of Spectra Measurements Reviewed

Nobuyuki Kawahara, Masanobu Watanabe, Eiji Tomita, Yukihide Nagano, Toshiaki Kitagawa

SAE Technical Papers   ( December )   2019.12

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Publishing type：Research paper (international conference proceedings)  

© 2019 SAE Japan and SAE International. In this research, a spark plug with an optical fiber has been developed to obtain the emission spectra from the spark discharge and flame kernel. This developed spark plug with an optical fiber can obtain the time series of emission spectra from the spark discharge and Initial flame kernel in the real spark-ignition engine using EMCCD spectrometer. The plasma vibrational temperature of the spark discharge can be measured using the emission spectra from the electrically excited CN violet band system. The plasma of the spark discharge and gas rotational temperature of the initial flame kernel can be also measured using emission spectra from OH∗ radicals (P and R branches). The plasma temperature of the spark discharge was almost 8,000 K and the gas temperature of the Initial flame kernel approached that of the adiabatic flame temperature.

DOI： 10.4271/2019-01-2158

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Droplet diameter distribution measurement of a gasoline DISI injector using PDA Reviewed

Nobuyuki Kawahara, Bunta Mori, Eiji Tomita

20th Annual Conference on Liquid Atomization and Spray Systems-ILASS-Asia 2019, ILASS-Asia2019-064   2019.12

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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End-gas autoignition characteristics of PREMIER combustion in a pilot fuel-ignited dual-fuel biogas engine Reviewed International journal

Alireza Valipour Berenjestanaki, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

Fuel   254   115634 - 115634   2019.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

To improve the thermal efficiency of internal combustion engines at high loads, premixed mixture ignition in the end-gas region (PREMIER) combustion is proposed as a precursor to knocking. In the current work, a pilot fuelignited dual-fuel gas engine was operated at constant speed under different intake pressures (101, 150, and 200 kPa). A simulated biogas was served as the primary fuel and diesel as the pilot fuel. The maximum mean effective pressure and thermal efficiency were evident during PREMIER operation because of autoignition in the end-gas region. Similar to knocking combustion, PREMIER combustion features two stages but neither pressure oscillation nor a rapid pressure rise is observed. We here define a new parameter, the PREMIER intensity (PI), which reflects the strength of PREMIER combustion. As the injection timing was advanced and the pressure boosted, more cycles underwent end-gas autoignition; the associated heat release increased and, consequently, the PI value rose. When the pressure and temperature of a premixed fuel mixture rose as injection timing was advanced, end-gas autoignition commenced earlier. The end-gas autoignition delay became shorter as intake pressure was increased and injection timing advanced.

DOI： 10.1016/j.fuel.2019.115634

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3D CFD of Auto-ignition at the End Gas Region in Dual-fuel Gas Engine Reviewed

Nobuyuki Kawahara, Eiji Tomita

12th Asia-Pacific Conference on Combustion-ASPACC2019, 2019-1211   2019.7

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Evaluation of Combustion Characteristics of Ultra Low Sulphur Fuel Oil by Using OCA (Optical Combustion Analyzer) and Performance and Exhaust Emissions in a Two Stroke Diesel Engine Reviewed

Eiji Tomita, Yuya Kubota, Nobuyuki Kawahara, Kazuya Tsuboi, Ken Miyauchi, Morio Kondo

29th CIMAC World Congress, 2016-181   2019.6

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Differences between PREMIER combustion in a natural gas spark-ignition engine and knocking with pressure oscillations Reviewed International journal

Kawahara Nobuyuki, Kim Yungjin, Wadahama Hisashi, Tsuboi Kazuya, Tomita Eiji

PROCEEDINGS OF THE COMBUSTION INSTITUTE   37 ( 4 )   4983 - 4991   2019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

PREMIER (PREmixed Mixture Ignition in the End-gas Region) combustion occurs with auto-ignition in the end-gas region when the main combustion flame propagation is nearly finished. Auto-ignition is triggered by the increases in pressure and temperature induced by the main combustion flame. Similarly to engine knocking, heat is released in two stages when engines undergo this type of combustion. This pattern of heat release does not occur during normal combustion. However, engine knocking induces pressure oscillations that cause fatal damage to engines, whereas PREMIER combustion does not. The purpose of this study was to elucidate PREMIER combustion in natural gas spark-ignition engines, and differentiate the causes of knocking and PREMIER combustion. We applied combustion visualization and in-cylinder pressure analysis using a compression-expansion machine (CEM) to investigate the auto-ignition characteristics in the end-gas region of a natural gas spark-ignition engine. We occasionally observed knocking accompanied by pressure oscillations under the spark timings and initial gas conditions used to generate PREMIER combustion. No pressure oscillations were observed during normal and PREMIER combustion. Auto-ignition in the end-gas region was found to induce a secondary increase in pressure before the combustion flame reached the cylinder wall, during both knocking and PREMIER combustion. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. This caused a sudden pressure difference and imbalance between the flame propagation region and the end-gas region, followed by a pressure oscillation. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2018.08.055

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Experimental and Numerical Analysis of Laser-ignition of Wet Ethanol with Elevated Water Content Reviewed International coauthorship

Kazi Mostafijur Rahman, Nobuyuki Kawahara, Eiji Tomita

International Journal of Automotive Engineering and Technologies   8 ( 2 )   61 - 69   2019

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Publishing type：Research paper (scientific journal)  

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Advanced Combustion in Natural Gas-Fueled Engines

Ulugbek Azimov, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

Energy, Environment, and Sustainability   215 - 250   2019

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Publishing type：Part of collection (book)  

Current energy and emission regulations set the requirements to increase the use of natural gas in engines for transportation and power generation. The characteristics of natural gas are high octane number, less amount of carbon in the molecule, suitable to lean combustion, less ignitibility, etc. There are some advantages of using natural gas for engine combustion. First, less carbon dioxide is emitted due to its molecular characteristics. Second, higher thermal efficiency is achieved owing to the high compression ratio compared to that of gasoline engines. Natural gas has higher octane number so that knock is hard to occur even at high compression ratios. However, this becomes a disadvantage in homogeneous charge compression ignition (HCCI) engines or compression ignition engines because the initial auto-ignition is difficult to be achieved. When natural gas is used in a diesel engine, primary natural gas–air mixture is ignited with small amount of diesel fuel. It was found that under high pressure, lean conditions, and with the control of certain parameters, the end gas is auto-ignited without knock and improves the engine combustion efficiency. Recently, some new fuel ignition technologies have been developed to be applied to natural gas engines. These are the laser-assisted and plasma-assisted ignition systems with high energy and compact size.

DOI： 10.1007/978-981-13-3307-1_8

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後方散乱X線を用いた層流予混合火炎の温度分布と化学状態の計測 Reviewed

櫻井浩, 河原伸幸, 冨田栄二, 鈴木宏輔, 伊藤真義, 櫻井吉晴

日本エネルギー学会機関紙えねるみくす   97 ( 4 )   2018.4

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Numerical Investigation of Natural Gas-Diesel Dual Fuel Engine with End Gas Ignition Reviewed

Apoorv Talekar, Ming Chia Lai, Eiji Tomita, Nobuyuki Kawahara, Ke Zeng, Bo Yang

SAE Technical Papers   2018-April   2018.1

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© 2018 SAE International. All Rights Reserved. The present study helps to understand the local combustion characteristics of PREmixed Mixture Ignition in the End-gas Region (PREMIER) combustion mode while using increasing amount of natural gas as a diesel substitute in conventional CI engine. In order to reduce NOx emission and diesel fuel consumption micro-pilot diesel injection in premixed natural gas-air mixture is a promising technique. New strategy has been employed to simulate dual fuel combustion which uses well established combustion models. Main focus of the simulation is at detection of an end gas ignition, and creating an unified modeling approach for dual fuel combustion. In this study G-equation flame propagation model is used with detailed chemistry in order to detect end-gas ignition in overall low temperature combustion. This combustion simulation model is validated using comparison with experimental data for dual fuel engine.

DOI： 10.4271/2018-01-0199

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Characterisation of DME-HCCI combustion cycles for formaldehyde and hydroxyl UV-vis absorption Reviewed International coauthorship International journal

Ulugbek Azimov, Nearchos Stylianidis, Nobuyuki Kawahara, Eiji Tomita

FUEL   210   578 - 591   2017.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

We investigated time-resolved ultraviolet-visible (UV-vis) light absorbance to identify the formation behaviour of formaldehyde (HCHO) and hydroxyl (OH) within the wavelength range of 280-400 nm in a homogeneous charge compression ignition (HCCI) engine fuelled with dimethyl ether (DME). The time-resolved HCHO and OH profiles at different initial pressures showed that HCHO absorbance increased in the low-temperature reaction (LTR) and thermal-ignition preparation (TIP) regions and decreased gradually as the combustion approached the high-temperature reaction (HTR) region. At higher intake pressures, HCHO absorbance decreased and OH absorbance increased. The time-resolved absorbance spectra of HCHO, with peaks at 316, 328, 340, and 354 nm for all combustion cycles, were evaluated and it was found that average absorption at 328 nm was slightly higher than at 316, 340, and 354 nm. For knocking combustion cycles, the absorbance of HCHO in the LTR region was high for cycles with low knock intensity and low for cycles with high knock intensity, showing a high level of OH absorbance. Chemical kinetics analyses showed that for different fuel/oxidiser ratios, initial O-2 concentration and intake temperature had no effect on in-cylinder temperatures in the LTR or TIP regions. However, they did have significant effects on HTR combustion. In-cylinder temperature in the LTR region had less effect on HCHO and H2O2 formation than pressure.

DOI： 10.1016/j.fuel.2017.09.003

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Improvement of thermal efficiency and reduction of NOx emissions by burning a controlled jet plume in high-pressure direct-injection hydrogen engines Reviewed International journal

Yasuo Takagi, Hiroki Mori, Yuji Mihara, Nobuyuki Kawahara, Eiji Tomita

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   42 ( 41 )   26114 - 26122   2017.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

A new combustion process called the Plume Ignition Combustion Concept (PCC), in which the plume tail of the hydrogen jet is spark-ignited immediately after the completion of fuel injection to accomplish combustion of a rich mixture has been proposed by the authors. This PCC combustion process markedly reduces nitrogen oxides (NOx) emissions in the high-output region while maintaining high levels of thermal efficiency and power. On the other hand, as burning lean mixture of fuel and air is the conventional way to improve thermal efficiency and reduce NOx, a high lambda premixed mixture of hydrogen and air formed by injecting hydrogen in the early stage of the compression stroke has been used in direct injection hydrogen engines. It was recently reported, however, that this mixture condition does not always offer expected improved thermal efficiency under even lean mixture conditions by increasing unburned hydrogen emissions caused by incomplete flame propagation in the non-uniform and extremely lean portion of the mixture. In this study, the effect of retarding the injection timing to late in the compression stroke but slightly advanced from original PCC was examined as a way of reducing unburned hydrogen emissions and improving thermal efficiency. These effects result from a centroidal axially stratified mixture that positions a fairly rich. charge near the spark plug. This stratified mixture is presumably effective in reducing incomplete flame propagation thought to be the cause of unburned hydrogen emissions and also promoting increasing burning velocity of the mixture that improve thermal efficiency. Finally, this research is characterized by measuring the hydrogen fuel concentration at the point and the time of spark ignition quantitatively by spark-induced breakdown spectroscopy in order to identify the changes in mixture ratio mentioned above caused by the parameters involved. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2017.08.015

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Chemical kinetics and CFD analysis of supercharged micro-pilot ignited dual-fuel engine combustion of syngas Reviewed International coauthorship International journal

Nearchos Stylianidis, Ulugbek Azimov, Alireza Maheri, Eiji Tomita, Nobuyuki Kawahara

FUEL   203   591 - 606   2017.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

A comprehensive chemical kinetics and computational fluid-dynamics (CFD) analysis were performed to evaluate the combustion of syngas derived from biomass and coke-oven solid feedstock in a micro-pilot ignited supercharged dual-fuel engine under lean conditions. The developed syngas chemical kinetics mechanism was validated by comparing ignition delay, in-cylinder pressure, temperature and laminar flame speed predictions against corresponding experimental and simulated data obtained by using the most commonly used chemical kinetics mechanisms developed by other authors. Sensitivity analysis showed that reactivity of syngas mixtures was found to be governed by H-2 and CO chemistry for hydrogen concentrations lower than 50% and mostly by H-2 chemistry for hydrogen concentrations higher than 50%. In the mechanism validation, particular emphasis is placed on predicting the combustion under high pressure conditions. For high hydrogen concentration in syngas under high pressure, the reactions HO2 + HO2 = H2O2 + O-2 and H2O2 + H = H-2 + HO2 were found to play important role in in-cylinder combustion and heat production. The rate constants for H2O2 + H = H-2 + HO2 reaction showed strong sensitivity to high-pressure ignition times and has considerable uncertainty. Developed mechanism was used in CFD analysis to predict in-cylinder combustion of syngas and results were compared with experimental data. Crank angle-resolved spatial distribution of in-cylinder spray and combustion temperature was obtained. The constructed mechanism showed the closest prediction of combustion for both biomass and cokeoven syngas in a micro-pilot ignited supercharged dual-fuel engine. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.fuel.2017.04.125

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High-speed visualization of fuel spray impingement in the near-wall region using a DISI injector Reviewed

N. Kawahara, K. Kintaka, E. Tomita

SELECTED PAPERS FROM THE 31ST INTERNATIONAL CONGRESS ON HIGH-SPEED IMAGING AND PHOTONICS   10328   2017

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We used a multi-hole injector to spray isooctane under atmospheric conditions and observed droplet impingement behaviors. It is generally known that droplet impact regimes such as splashing, deposition, or bouncing are governed by the Weber number. However, owing to its complexity, little has been reported on microscopic visualization of poly-dispersed spray. During the spray impingement process, a large number of droplets approach, hit, then interact with the wall. It is therefore difficult to focus on a single droplet and observe the impingement process. We solved this difficulty using highspeed microscopic visualization. The spray/wall interaction processes were recorded by a high-speed camera (Shimadzu HPV-X2) with a long-distance microscope. We captured several impinging microscopic droplets. After optimizing the magnification and frame rate, the atomization behaviors, splashing and deposition, were recorded. Then, we processed the images obtained to determine droplet parameters such as the diameter, velocity, and impingement angle. Based on this information, the critical threshold between splashing and deposition was investigated in terms of the normal and parallel components of the Weber number with respect to the wall. The results suggested that, on a dry wall, we should set the normal critical Weber number to 300.

DOI： 10.1117/12.2270691

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Spark discharge ignition process in a spark-ignition engine using a time series of spectra measurements Reviewed International coauthorship International journal

Nobuyuki Kawahara, Shota Hashimoto, Eiji Tomita

PROCEEDINGS OF THE COMBUSTION INSTITUTE   36 ( 3 )   3451 - 3458   2017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

The spark discharge ignition process was investigated using simultaneous temperature measurements of the spark discharges and the initial flame kernel. We were able for the first time to measure a time series of emission spectra from the spark discharge and initial flame kernel inside a spark-ignition engine using a spectrometer coupled with a spark plug and optical fiber. The plasma vibrational temperature of the spark discharge can be measured using time series emission spectra from the electrically excited CN* violet band system. The gas rotational temperature of the initial flame kernel can also be measured using emission spectra from OH* radicals (P and R branches). Simultaneously, visualization of the spark discharge and a time series of emission spectra inside a spark-ignition engine were performed under homogeneous mixture conditions, to eliminate the effects of stratification of temperature and mixture concentrations around the spark plug. We discuss thermal energy transfer from the spark discharge to the combustible mixture. The main conclusions that can be drawn from this study are as follows. CN* emission can be detected from the spark discharge, visualized using a high-speed camera during the arc discharge phase. Our results confirmed that the plasma temperature of the spark discharge was nearly 6800 K and that thermal energy was transferred from the spark plasma channel to the combustible mixture. The gas temperature of the initial flame kernel approached that of the adiabatic flame temperature. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.

DOI： 10.1016/j.proci.2016.08.029

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Extension of PREMIER combustion operation range using split micro pilot fuel injection in a dual fuel natural gas compression ignition engine: A performance-based and visual investigation Reviewed

Cagdas Aksu, Nobuyuki Kawahara, Kazuya Tsuboi, Morio Kondo, Eiji Tomita

FUEL   185   243 - 253   2016.12

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The effects of split pilot fuel injection on engine performance and PREmixed Mixture Ignition in the End Gas Region (PREMIER) combustion characteristics were investigated in a single-cylinder dual-fuel natural gas engine ignited with diesel fuel. In particular, the effect of second spray timing on combustion mode was examined. PREMIER combustion was observed in a wider range of operating conditions with split injection strategy compared to single injection strategy. We determined that it was possible to both decelerate heat release and suppress knocking to PREMIER combustion, and accelerate heat release and promote normal combustion to PREMIER combustion, with suitable second injection timing. The maximum of thermal efficiency of PREMIER combustion operation with split injection was close to the results obtained with knocking operation. In-cylinder images showed that split injection strategy advances or retards the progress of combustion by controlling the size and rate of growth of flame kernels, depending on the timing of the second injection. The combustion progress is earlier when the pilot fuel delivered during the first injection autoignites during the second injection. Kernel growth and the final size were adversely affected when the second injection was initiated after pilot fuel autoignition. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.fuel.2016.07.120

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Combustion characteristics and NOx emissions of biogas fuels with various CO2 contents in a micro co-generation spark-ignition engine Reviewed

Yungjin Kim, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

APPLIED ENERGY   182   539 - 547   2016.11

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Biogas is a renewable alternative fuel for internal combustion engines that has several advantages over conventional fuels, including lower costs and reduced levels of harmful emissions. In particular, it exhibits a neutral recirculation loop for carbon dioxide (CO2), which is one of the main causes of global warming. In this study, we investigated biogas fuels with various compositions using a micro co-generation engine system. The ratio of methane to CO2 and engine load were varied, and the intake air and fuel flow rates were controlled to change the equivalence ratio. The results show that for a given engine load, the ignition delay and combustion period increased with CO2 content, and the combustion speed decreased. The fuel consumption increased slightly with CO2 content; however, the thermal efficiency improved using a lean burn strategy, resulting in lower nitrogen oxide (NOx) emission, and moreover, the use of biogas with the stoichiometric air-fuel ratio appears effective in reducing NOx emissions and can improve the fuel economy at higher loads. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.apenergy.2016.08.152

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Quantum cascade laser assisted time-resolved measurements of carbon dioxide absorption during combustion in DME-HCCI engine Reviewed

Ulugbek Azimov, Nobuyuki Kawahara, Eiji Tomita

FUEL   182   807 - 815   2016.10

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We conducted experiments to investigate in-cylinder light absorption by carbon dioxide (CO2) during homogeneous charge compression ignition (HCCI) engine combustion. The combustion was fuelled with dimethyl ether. An in situ laser infrared absorption method was developed. We used an optical fibre spark plug sensor and the light source was a 4.301 mu m quantum cascade laser (QCL). We applied Lambert Beer's law in the case of a single absorption line of CO2. We were able to measure the transient CO2 formation during the HCCI combustion inside the engine cylinder. Our experiments showed that the laser light transmissivity level decreased with the intensity of the infrared (IR) signal. We compared the change in the transmissivity to the spatially integrated HCCI flame luminosity level and observed significant correlations between the flame luminosity level, heat release rate and transmissivity. Time-resolved experiments showed that the CO2 absorbance increases when the second peak of the rate of heat release (ROHR) is maximised. After combustion, the CO2 concentration was approximately 4 vol%, which agrees with the amount of CO2 formed during complete combustion. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.fuel.2016.06.040

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Local fuel concentration measurement through spark-induced breakdown spectroscopy in a direct-injection hydrogen spark-ignition engine Reviewed

Kazi Mostafijur Rahman, Nobuyuki Kawahara, Daichi Matsunaga, Eiji Tomita, Yasuo Takagi, Yuji Mihara

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   41 ( 32 )   14283 - 14292   2016.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Quantitative measurements of local fuel concentrations were conducted in a direct injection hydrogen spark-ignition research engine using the spark-induced breakdown spectroscopy (SIBS) technique. For SIBS measurements, a new sensor was developed from a commercially available M12-type spark plug with no major modifications to the electrodes. The new plug sensor showed better durability and required less maintenance when used in a hydrogen research engine. Emission spectra from the plasma generated by the spark plug were collected through an optical fibre housed in the centre electrode of the plug and resolved spectrally for atomic emissions of Ha, 0(I), and N(I). The main focus of the present work was to characterise the effects of ambient pressure at ignition timing on spectral line emissions and to improve the accuracy of SIBS measurements by taking into account the pressure dependency of atomic emissions. A significant effect of the corresponding pressure at ignition timing was observed on spark-induced breakdown spectroscopic measurements and emission line characteristics. Retarded spark timing (i.e. higher ambient pressure at the ignition site) resulted in lower spectral line intensities as well as weaker background emissions. It is well established that with relatively higher pressure and density of atoms or molecules, the cooling of expanding plasma accelerates, and the collision probability increases, leading to both a weaker broadband continuum and atomic emissions. A "calibration MAP" representing the correlation of air excess ratio (relative air/fuel ratio) with both intensity ratio and pressure at ignition timing was created and subsequently used for quantitative measurements of local fuel concentrations for both port injection and direct injection strategies to demonstrate and explore the effects of pressure dependency of atomic emission on the accuracy of the SIBS measurements. Local stratification of the fuel mixture in the vicinity of the spark gap location associated with direct injection strategies was confirmed; the coefficient of variation of the local air excess ratio was relatively small for measurements made using the calibration map. This demonstrated that the measurement accuracy of local fuel concentrations through a spark plug sensor can be improved significantly when the pressure dependency of atomic emissions is taken into account. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2016.05.280

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Densitometry and temperature measurement of combustion gas by X-ray Compton scattering Reviewed

Hiroshi Sakurai, Nobuyuki Kawahara, Masayoshi Itou, Eiji Tomita, Kosuke Suzuki, Yoshiharu Sakurai

JOURNAL OF SYNCHROTRON RADIATION   23 ( 2 )   617 - 621   2016.3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INT UNION CRYSTALLOGRAPHY  

Measurement of combustion gas by high-energy X-ray Compton scattering is reported. The intensity of Compton-scattered X-rays has shown a position dependence across the flame of the combustion gas, allowing us to estimate the temperature distribution of the combustion flame. The energy spectra of Compton-scattered X-rays have revealed a significant difference across the combustion reaction zone, which enables us to detect the combustion reaction. These results demonstrate that high-energy X-ray Compton scattering can be employed as an in situ technique to probe inside a combustion reaction.

DOI： 10.1107/S1600577516001740

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Combustion characteristics of wet ethanol ignited using a focused Q-switched Nd:YAG nanosecond laser Reviewed

Kazi Mostafijur Rahman, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

FUEL   165   331 - 340   2016.2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

Ethanol is a particularly promising bio-fuel, which is already widely used in the transportation sector. Much research has focused on laser ignition in internal combustion engines, because it offers a number of significant advantages. The motivation of this study is to reduce the higher cost of anhydrous ethanol by enabling the direct use of wet ethanol for combustion applications. This study reports the effects of water content on the laser ignition characteristics of air-ethanol flames with 0-40% water by volume added to the ethanol. Laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. High-speed visualisation of the combustion was achieved, together with analyses of the in-chamber pressure history, mass fraction burned, and ignition delay over a range of equivalence ratios. Removing the final 20% of water from ethanol is particularly energy intensive. The results of this investigation revealed that the flame growth rate and flame propagation velocity are both increased by adding up to 20% water (by volume) to ethanol, indicating a positive effect of the water at relatively low concentrations. It was also found that the ignition delay was shorter as the water content was increased up to a volume fraction of 20%. Therefore, it can be hypothesised that the presence of water enhances the ionisation process and increases the rate of radical generation during laser-induced breakdown of wet ethanol. Numerical analysis using the commercial software package CHEMKIN-PRO revealed a reduction in adiabatic flame temperature and laminar burning velocity due to the addition of water to ethanol. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.fuel.2015.10.067

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Effect of ambient pressure on local concentration measurement of transient hydrogen jet in a constant-volume vessel using spark-induced breakdown spectroscopy Reviewed

Md Tasyrif Abdul Rahman, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   40 ( 13 )   4717 - 4725   2015.4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We report an investigation of the effects of the ambient pressure on fuel concentration measurements of an injected jet of hydrogen using spark-induced breakdown spectroscopy (SIBS) in a constant-volume vessel. Measurements were carried out using hydrogen injected into a nitrogen environment with different ambient pressures, and the local concentrations were measured at various spark locations. The optical emission from the spark discharge at 501 nm (corresponding to nitrogen) and 656 nm (corresponding to hydrogen) was observed using SIBS. Spectrally resolved emission from the plasma was detected simultaneously using a spectrometer. The potential to determine the hydrogen/nitrogen ratio in the spark gap using was demonstrated. Spectral calibration was carried out using a hydrogen/nitrogen mixture, and hydrogen was injected at a pressure of 5.0 MPa into nitrogen with ambient pressures in the range 0.5-1.5 MPa. The results show an increase in the background radiation, as well as of the peaks corresponding to hydrogen and nitrogen atomic emission lines, as the ambient pressure increased. An increase in the density of nitrogen inside the chamber influenced the structure of the hydrogen jet, slowing the spray and reducing penetration, which altered the equivalence ratio at the location of the spark. When the spark occurred during injection, the behavior of the hydrogen jet was quasi-steady state; when the spark timing followed the injection, however, an unsteady state was observed. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2015.01.121

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Visualization and concentration measurement of a direct-injection hydrogen jet in a constant-volume vessel using spark-induced breakdown spectroscopy Reviewed

Md Tasyrif Abdul Rahman, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   39 ( 31 )   17896 - 17905   2014.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

In this work, spark-induced breakdown spectroscopy (SIBS) was employed to investigate the mixing process of a hydrogen jet in a constant-volume vessel. The local fuel concentration of the hydrogen jet was measured at several locations, using a SIBS sensor. A high-speed camera was used to visualize spark discharge fluctuations, and hydrogen jet concentration measurements were conducted simultaneously. Spectrally resolved atomic emissions from the plasma generated by the spark plug were examined to determine the local equivalence ratio. Direct visualization of the spark discharge provided useful information about the influence of spark discharge characteristics related to the spark timing. Using the developed SIBS sensor, atomic emission spectra were obtained from hydrogen H-alpha at 656 nm and nitrogen N (I) at 501 nm. Comparison of the intensity peaks of atomic emissions from hydrogen and nitrogen allows the local hydrogen concentration in a measured volume to be determined, and hence also the local equivalence ratio. The measurement results demonstrate the local variation in the equivalence ratio throughout the jet and along its axis. From the results, the spatial structure of the hydrogen jet affects the hydrogen/nitrogen mixing and could be clarified with SIBS technique when the spark is discharged. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2014.08.146

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Laser-induced plasma generation and evolution in a transient spray Reviewed

Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

OPTICS EXPRESS   22 ( 1 )   A44 - A52   2014.1

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Language：English   Publisher：OPTICAL SOC AMER  

The behaviors of laser-induced plasma and fuel spray were investigated by visualizing images with an ultra-high-speed camera. Timeseries images of laser-induced plasma in a transient spray were visualized using a high-speed color camera. The effects of a shockwave generated from the laser-induced plasma on the evaporated spray behavior were investigated. The interaction between a single droplet and the laser-induced plasma was investigated using a single droplet levitated by an ultrasonic levitator. Two main conclusions were drawn from these experiments: (1) the fuel droplets in the spray were dispersed by the shockwave generated from the laser-induced plasma; and (2) the plasma position may have shifted due to breakdown of the droplet surface and the lens effect of droplets. (C) 2013 Optical Society of America

DOI： 10.1364/OE.22.000A44

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Combustion and Emission Characteristics of a Premixed Charge Compression Ignition Engine Reviewed

Robert Kiplimo, Eiji Tomata, Nobuyuki Kawahara, Sumito Yokobe

Journal of Sustainable Research in Engineering   1 ( 1 )   8 - 16   2014

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Jet-guided combustion characteristics and local fuel concentration measurements in a hydrogen direct-injection spark-ignition engine Reviewed

Mithun Kanti Roy, Nobuyuki Kawahara, Eiji Tomita, Takashi Fujitani

PROCEEDINGS OF THE COMBUSTION INSTITUTE   34   2977 - 2984   2013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Spark-ignition (SI) hydrogen engines based on direct injection (DI) promise significant advantages in terms of thermal efficiency and power output, and present a means of overcoming problems related to knocking, backfiring, and preignition. A better understanding of the effects of hydrogen jets on the fuel concentration distribution and mixing process in a DISI engine should provide new and useful insights into combustion optimization. The objective of the present work was to gain a deeper comprehension of the characteristics of late-injection hydrogen combustion. An experimental combustion setup was applied to a fired, jet-guided DISI engine operated at 600 rpm in stratified mode. GDI injector with the jet directed toward the spark plug was used to develop the stratified combustion concept. A high-speed camera synchronized with the spark was focused on a 52 mm-diameter field of view through a window at the bottom of the piston crown. A series of single-shot images captured at different intervals was used to study the time evolution of the flame distribution. Variations in the fuel injection timing relative ignition timing were found to impact the development of the early flame, as well as the flame propagation. This research also employed spark-induced breakdown spectroscopy (SIBS) to measure the local fuel-air concentration in the spark gap at the time of ignition under stratified-charge conditions. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2012.06.103

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UV-visible light absorption by hydroxyl and formaldehyde and knocking combustion in a DME-HCCI engine Reviewed

Ulugbek Azimov, Nobuyuki Kawahara, Eiji Tomita

FUEL   98   164 - 175   2012.8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

Experiments were conducted in a compression-expansion test engine to investigate the combustion characteristics in a homogeneous charge compression ignition (HCCI) engine fuelled with dimethyl ether. Two types of analyses were performed. In the first, ultraviolet-visible (UV-Vis) light absorbance was investigated to identify the formation behaviour of HCHO and OH during HCCI combustion. In the second, knocking combustion was investigated by analysing the spatially integrated flame luminosity and in-cylinder pressure oscillations. The time-resolved HCHO and OH profiles at different equivalence ratios showed that HCHO absorbance increased in the low-temperature reaction (LTR) and thermal-preparation regions and gradually decreased as the combustion approached the high-temperature reaction (HTR) region. The in-cylinder temperature in the LTR region had little effect on the rate of the maximum pressure rise, and this did not change much at different equivalence ratios. The results demonstrated that there was a marked difference between the intensity of the flame emissions of non-knocking and knocking events. The time-resolved integrated absorbance spectra of HCHO with peaks at 328, 340, and 354 nm that occurred before the OH peaks appeared suggested that when a certain threshold ratio of (dP/d theta R-LT)/(dP/d theta(HTR)) was reached, the amount of HCHO decreased due to reactions in the thermal-preparation region while the tendency to knock increased. (C) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.fuel.2012.03.033

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Effects of spray impingement, injection parameters, and EGR on the combustion and emission characteristics of a PCCI diesel engine Reviewed

Robert Kiplimo, Eiji Tomita, Nobuyuki Kawahara, Sumito Yokobe

APPLIED THERMAL ENGINEERING   37   165 - 175   2012.5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The effects of spray impingement, injection parameters, and exhaust gas recirculation (EGR) on the combustion characteristics and exhaust emissions of a premixed charge compression ignition (PCCI) diesel engine were investigated using a single-cylinder test engine and an optically accessible engine. Tests were carried out under constant speed with variable injection pressures and EGR rates. Exhaust emissions and in-cylinder pressures were measured under all experimental conditions. Analyses were conducted based on diesel spray evolution and combustion process visualisation coupled with performance and exhaust emissions. Higher injection pressures led to lower smoke, hydrocarbons (HC), and nitrogen oxide (NOx) emissions but had roughly the same CO emissions compared with lower injection pressures. Higher EGR rates led to the simultaneous reduction in NOx and soot emissions due to lower combustion temperatures compared to conventional diesel combustion. However, HC and CO emissions increased due to fuel impingement, bulk quenching, and over-mixing, leading to an air fuel mixture that was too lean to burn. An optimum spray targeting spot was identified, leading to lower emissions of soot, CO, and HC but higher NOx emissions without EGR. The simultaneous reduction in NOx and soot was achieved using the optimum spray targeting spot by introducing EGR, which was accompanied by homogenous combustion and a low luminosity flame attributed to fuel impingement on the piston bowl wall. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.applthermaleng.2011.11.011

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Multidimensional CFD simulation of syngas combustion in a micro-pilot-ignited dual-fuel engine using a constructed chemical kinetics mechanism Reviewed

Ulugbek Azimov, Masahiro Okuno, Kazuya Tsuboi, Nobuyuki Kawahara, Eiji Tomita

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   36 ( 21 )   13793 - 13807   2011.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

A multidimensional computational fluid dynamics (CFD) simulation of a constructed syngas chemical kinetic mechanism was performed to evaluate the combustion of syngas in a supercharged dual-fuel engine for various syngas initial compositions under lean conditions. The modelled results were validated by comparing predictions against corresponding experimental data for a supercharged dual-fuel engine. The predicted and measured in-cylinder pressure, temperature, and rate of heat release (ROHR) data were in good agreement. The effect of the hydrogen peroxide chain-propagation reaction on the progress of combustion under supercharged conditions was examined for different types of syngas using various initial H-2 concentrations. The effect of the main syngas kinetic mechanism reactions on the combustion progress was analysed in terms of their contribution to the total heat of the reaction. The best results compared with experimental data were obtained in the range of equivalence ratios below about 0.8 for all types of syngas considered in this paper. As the equivalence ratio increased above 0.8, the results deviated from the experiment data. The spatial distribution of the in-cylinder temperature and OH center dot within this equivalence-ratio range showed the completeness of the combustion. The present CFD model captured the overall combustion process well and could be further developed into a useful tool for syngas-engine combustion simulations. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2011.07.140

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Premixed mixture ignition in the end-gas region (PREMIER) combustion in a natural gas dual-fuel engine: operating range and exhaust emissions Reviewed

U. Azimov, E. Tomita, N. Kawahara, Y. Harada

INTERNATIONAL JOURNAL OF ENGINE RESEARCH   12 ( 5 )   484 - 497   2011.10

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SAGE PUBLICATIONS LTD  

This paper is concerned with engine experiments and spectroscopic analysis of premixed mixture ignition in the end-gas region (PREMIER) combustion in a pilot fuel ignited, natural gas dual-fuel engine. The results reveal the characteristics and operating parameters that induce and affect this combustion mode. The PREMIER combustion is followed by natural gas flame propagation. Pilot-injected diesel fuel ignites the natural gas/air mixture, and the flame propagates before the natural gas/air mixture is autoignited in the end-gas region. This combustion cycle differs from a knocking cycle in terms of combustion and emission characteristics. The PREMIER combustion can be controlled by pilot fuel injection timing, the equivalence ratio, and the exhaust gas recirculation (EGR) rate, and can be used as an effective method for high load extension on a dual-fuel engine. An analysis of the relationship between the maximum in-cylinder pressure and its crank angle (CA) is used to compare combustion dynamics during conventional, PREMIER, and knocking combustion. In PREMIER combustion, the heat release gradually transforms from the slower first-stage flame rate to the faster second-stage rate. During PREMIER combustion, the maximum indicated mean effective pressure (IMEP) and thermal efficiency increase by about 25 per cent compared with those of conventional combustion, and low carbon monoxide (CO) and total hydrocarbon (HC) emissions can be achieved. However, nitrogen oxide (NO(x)) emissions increase. Spectroscopic analysis shows that the intensity of the OH* emissions in the end-gas region increases as the combustion mode transforms from conventional to PREMIER to knocking. In all three modes, emission fluctuations above 650 nm can be observed in the end-gas region. These emissions are attributed to the luminosity from soot particles formed during the concurrent diesel fuel combustion.

DOI： 10.1177/1468087411409664

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Effect of syngas composition on combustion and exhaust emission characteristics in a pilot-ignited dual-fuel engine operated in PREMIER combustion mode Reviewed

Ulugbek Azimov, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   36 ( 18 )   11985 - 11996   2011.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The objective of this study was to investigate the performance and emissions of a pilot-ignited, supercharged, dual-fuel engine powered by different types of syngas at various equivalence ratios. It was found that if certain operating conditions were maintained, conventional engine combustion could be transformed into combustion with two-stage heat release. This mode of combustion has been investigated in previous studies with natural gas, and has been given the name PREmixed Mixture Ignition in the End-gas Region (PREMIER) combustion. PREMIER combustion begins as premixed flame propagation, and then, because of mixture autoignition in the end-gas region, ahead of the propagating flame front, a transition occurs, with a rapid increase in the heat release rate. It was determined that the mass of fuel burned during the second stage affected the rate of maximum pressure rise. As the fuel mass fraction burned during the second stage increased, the rate of maximum pressure rise also increased, with a gradual decrease in the delay between the first increase in the heat release rate following pilot fuel injection and the point when the transition to the second stage occurred. The H(2) and CO(2) content of syngas affected the engine performance and emissions. Increased H2 content led to higher combustion temperatures and efficiency, lower CO and HC emissions, but higher NOx emissions. Increased CO2 content influenced performance and emissions only when it reached a certain level. In the most recent studies, the mean combustion temperature, indicated thermal efficiency, and NOx emissions decreased only when the CO2 content of the syngas increased to 34%. PREMIER combustion did not have a major effect on engine cycle-to-cycle variation. The coefficient of variation of the indicated mean effective pressure (COV(IMEP)) was less than 4% for all types of fuel at various equivalence ratios, indicating that the combustion was within the stability range for engine operation. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2011.04.192

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Comparison of performance and emissions of a supercharged dual-fuel engine fueled by hydrogen and hydrogen-containing gaseous fuels Reviewed

Murari Mohon Roy, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada, Atsushi Sakane

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   36 ( 12 )   7339 - 7352   2011.6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study investigated the engine performance and emissions of a supercharged engine fueled by hydrogen (H(2)), and three other hydrogen-containing gaseous fuels such as primary fuels, and diesel as pilot fuel in dual-fuel mode. The energy share of primary fuels was about 90% or more, and the rest of the energy was supplied by diesel fuel. The hydrogen-containing fuels tested in this study were 13.7% H(2)-content producer gas, 20% H(2)-content producer gas and 56.8% H(2)-content coke oven gas (COG). Experiments were carried out at a constant pilot injection pressure and pilot quantity for different fuel air equivalence ratios and at various injection timings. The experimental strategy was to optimize the pilot injection timing to maximize engine power at different fuel air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. Better thermal efficiency was obtained with the increase in H(2) content in the fuels, and neat H(2) as a primary fuel produced the highest thermal efficiency. The fuel air equivalence ratio was decreased with the increase in H(2) content in the fuels to avoid knocking. Thus, neat H(2)-operation produced less maximum power than other fuels, because of much leaner operations. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. The emissions of CO and HC with neat H(2)-operation were 98-99.9% and NOx about 85-90% less than other fuels. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2011.03.070

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Cycle-resolved residual gas concentration measurement inside a heavy-duty diesel engine using infrared laser absorption Reviewed

Nobuyuki Kawahara, Eiji Tomita, Atsushi Ohtsuki, Yuzo Aoyagi

PROCEEDINGS OF THE COMBUSTION INSTITUTE   33   2903 - 2910   2011

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Cycle-resolved residual gas fraction measurements were made inside a heavy-duty diesel engine using an infrared absorption method. An in situ laser infrared absorption method was developed using an optical fiber sensor and a 4.301-mu m quantum cascade laser (QCL) as the light source. We discuss the feasibility of obtaining in situ CO2 concentration measurements inside the engine combustion chamber using the newly developed optical fiber sensor system. Lambert-Beer's law can be applied for the case of a single absorption line of CO2, and the dependence of the CO2 molar absorption coefficient on the ambient pressure and temperature of was determined using a constant volume vessel. This coefficient decreased with increasing pressure, indicating almost constant at pressures over 1.0 MPa. CO2 concentration measurements were made in a compression-expansion engine in order to calibrate the measurement system. The feasibility of the optical fiber sensor system was then investigated in a heavy-duty diesel engine. We were able to measure the CO2 concentration inside the combustion chamber under various engine load conditions and were able to determine the internal exhaust gas recirculation (EGR) ratio. This measurement technique proved to be valuable in obtaining the cycle-to-cycle CO2 concentration of the residual gas in a heavy-duty diesel engine. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2010.07.017

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Effects of Compression Ratio and Simulated EGR on Combustion Characteristics and Exhaust Emissions of a Diesel PCCI Engine Reviewed

Robert Kiplimo, Eiji Tomita, Nobuyuki Kawahara, Sumito Yokobe

JOURNAL OF THERMAL SCIENCE AND TECHNOLOGY   6 ( 3 )   463 - 474   2011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC MECHANICAL ENGINEERS  

The effects of compression ratio and simulated exhaust-gas recirculation (EGR) on combustion characteristics and exhaust emissions of a diesel PCCI engine were investigated using a single-cylinder test engine. Tests were carried out under constant speed with various compression ratios and EGR rates. Exhaust emissions and in-cylinder pressure were measured for all experimental conditions. Analyses based on engine performance and exhaust emissions were conducted. An optimum compression ratio that provided better indicated thermal efficiency and IMEP while yielding lower emissions of smoke, HC, and CO, and reasonable NOx without EGR was identified. High rates of EGR led to the simultaneous reduction of NOx and soot emissions due to a lower combustion temperature compared with conventional diesel combustion, with a slight penalty in HC and CO.

DOI： 10.1299/jtst.6.463

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In situ fuel concentration measurement near a spark plug in a spray-guided direct-injection spark-ignition engine using infrared absorption method Reviewed

Nobuyuki Kawahara, Eiji Tomita, Takuya Kadowaki, Tetsuya Honda, Hideaki Katashiba

EXPERIMENTS IN FLUIDS   49 ( 4 )   925 - 936   2010.10

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Vaporized fuel concentration in a spray-guided direct-injection spark-ignition (SG-DISI) engine was measured using an optical sensor installed in a spark plug. A laser infrared absorption method was applied to quantify the instantaneous gasoline concentration near the spark plug. This paper discusses the feasibility of obtaining in situ air-fuel ratio measurements with this sensor installed inside an SG-DISI engine cylinder. First, the effects of the spray plume from a multi-hole injector on the vaporized fuel concentration measurements near the spark-plug sensor were examined using a visible laser. We determined the best position for the sensor in the engine, which was critical due to the spray and vapor plume formation. Then, a 3.392-mu m He-Ne laser that coincided with the absorption line of the hydrocarbons was used as a light source to examine the stratified mixture found during ultra-lean engine operation. A combustible mixture existed around the spark plug during the injection period when a preset air-fuel ratio of 45.0 was used with different fuel injection timings and net mean effect pressure conditions. The effects of the orientation of the spark plug on the measured results and ignitability of the SG-DISI engine were examined. Orienting the spark plug vertically to one of the spray plumes provided more accurate results and better engine reliability. The study demonstrated that it was possible to qualify the air-fuel ratio near the spark plug during the injection period using the developed spark-plug sensor in an SG-DISI engine.

DOI： 10.1007/s00348-010-0884-2

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Direct microscopic image and measurement of the atomization process of a port fuel injector Reviewed

Mohamed Esmail, Nobuyuki Kawahara, Eiji Tomita, Mamoru Sumida

MEASUREMENT SCIENCE AND TECHNOLOGY   21 ( 7 )   2010.7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The main objective of this study is to observe and investigate the phenomena of atomization, i.e. the fuel break-up process very close to the nozzle exit of a practical port fuel injector (PFI). In order to achieve this objective, direct microscopic images of the atomization process were obtained using an ultra-high-speed video camera that could record 102 frames at rates of up to 1 Mfps, coupled with a long-distance microscope and Barlow lens. The experiments were carried out using a PFI in a closed chamber at atmospheric pressure. Time-series images of the spray behaviour were obtained with a high temporal resolution using backlighting. The direct microscopic images of a liquid column break-up were compared with experimental results from laser-induced exciplex fluorescence (LIEF), and the wavelength obtained from the experimental results compared with that predicated from the Kelvin-Helmholtz break-up model. The droplet size diameters from a ligament break-up were compared with results predicated from Weber's analysis. Furthermore, experimental results of the mean droplet diameter from a direct microscopic image were compared with the results obtained from phase Doppler anemometry (PDA) experimental results. Three conclusions were obtained from this study. The atomization processes and detailed characterizations of the break-up of a liquid column were identified; the direct microscopic image results were in good agreement with the results obtained from LIEF, experimental results of the wavelength were in good agreement with those from the Kelvin-Helmholtz break-up model. The break-up process of liquid ligaments into droplets was investigated, and Weber's analysis of the predicated droplet diameter from ligament break-up was found to be applicable only at larger wavelengths. Finally, the direct microscopic image method and PDA method give qualitatively similar trends for droplet size distribution and quantitatively similar values of Sauter mean diameter.

DOI： 10.1088/0957-0233/21/7/075403

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An experimental investigation on engine performance and emissions of a supercharged H-2-diesel dual-fuel engine Reviewed

Murari Mohon Roy, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada, Atsushi Sakane

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   35 ( 2 )   844 - 853   2010.1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study investigated the engine performance and emissions of a supercharged engine fueled by hydrogen and ignited by a pilot amount of diesel fuel in dual-fuel mode. The engine was tested for use as a cogeneration engine, so power output while maintaining a reasonable thermal efficiency was important. Experiments were carried out at a constant pilot injection pressure and pilot quantity for different fuel-air equivalence ratios and at various injection timings without and with charge dilution. The experimental strategy was to optimize the injection timing to maximize engine power at different fuel-air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. The engine was tested first with hydrogen-operation condition up to the maximum possible fuel-air equivalence ratio of 0.3. A maximum IMEP of 908 kPa and a thermal efficiency of about 42% were obtained. Equivalence ratio could not be further increased due to knocking of the engine. The emission of CO was only about 5 ppm, and that of HC was about 15 ppm. However, the NOx emissions were high, 100-200 ppm or more. The charge dilution by N-2 was then performed to obtain lower NOx emissions. The 100% reduction of NOx was achieved. Due to the dilution by N-2 gas, higher amount of energy could be supplied from hydrogen without knocking, and about 13% higher IMEP was produced than without charge dilution. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2009.11.009

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Characterization of the Spray of the DISI Multi-hole Injector by Means of Phase Doppler Anemometer(共著） Reviewed

Fatma AHMED, Nobuyuki KAWAHARA, Eiji TOMITA, Mamoru SUMIDA

Journal of Thermal Science and Technology   35 ( 2 )   844 - 853   2010

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Characterization of the Spray of the DISI Multi-hole Injector by Means of Phase Doppler Anemometer Reviewed

Fatma Ahmed, Nobuyuki Kawahara, Eiji Tomita, Mamoru Sumida

JOURNAL OF THERMAL SCIENCE AND TECHNOLOGY   5 ( 1 )   36 - 50   2010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC MECHANICAL ENGINEERS  

The spray from a multi-hole injector applied to direct injection spark ignition (DISI) engine was investigated. In order to understand the detailed structure of the transient spray near the nozzle a combined LDA/PDA system was used. PDA system was optimized in order to detect relative smaller droplets. Size-classified technique was used to get deep information about the spray characterizations near nozzle. The experiments were performed at 7 MPa of injection pressure. At early stage of spray the droplet velocity distribution in the centre of the spray showed high value. Smaller droplets under 15 mu m showing followability to air flow, while larger droplets over 20 mu m will have drag due to momentum decay. Droplets of 15<D <= 20 mu m in diameter is criteria for follow/penetration near the nozzle. Near the nozzle, the atomization predominantly occurs at the centre region of the spray.

DOI： 10.1299/jtst.5.36

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Evaluation of the Flame Lift-off Length in Diesel Spray Combustion Based on Flame Extinction Reviewed

Ulugbek Azimov, Nobuyuki Kawahara, Eiji Tomita, Kazuya Tsuboi

JOURNAL OF THERMAL SCIENCE AND TECHNOLOGY   5 ( 2 )   238 - 251   2010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC MECHANICAL ENGINEERS  

The interaction of spray and combustion processes forms a complex system of physical phenomena undergoing in IC engines. Studying this interaction is important to determine strategies for simultaneously reducing soot and NOx emissions from diesel engines. Spray combustion interactions are evaluated by the flame lift-off length - the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length. Numerical studies have been performed with the ECFM3Z model and n-Heptane fuel to determine the flame lift-off length under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O-2 concentrations and ambient gas densities.

DOI： 10.1299/jtst.5.238

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Performance and emissions of a supercharged dual-fuel engine fueled by hydrogen-rich coke oven gas Reviewed

Murari Mohon Roy, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada, Atsushi Sakane

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   34 ( 23 )   9628 - 9638   2009.12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study investigated the engine performance and emissions of a supercharged dual-fuel engine fueled by hydrogen-rich coke oven gas and ignited by a pilot amount of diesel fuel. The engine was tested for use as a cogeneration engine, so power output while maintaining a reasonable thermal efficiency was important Experiments were carried out at a constant pilot injection pressure and pilot quantity for different fuel-air equivalence ratios and at various injection timings without and with exhaust gas recirculation (EGR). The experimental strategy was to optimize the injection timing to maximize engine power at different fuel-air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. The engine was tested first without EGR condition up to the maximum possible fuel-air equivalence ratio of 0 65. A maximum indicated mean effective pressure (IMEP) of 1425 kPa and a thermal efficiency of 39% were obtained. However, the nitrogen oxides (NOx) emissions were high. A simulated EGR up to 50% was then performed to obtain lower NOx emissions. The maximum reduction of NOx was 60% or more maintaining the similar levels of IMEP and thermal efficiency. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. (C) 2009 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved.

DOI： 10.1016/j.ijhydene.2009.09.016

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Fuel concentration measurement of premixed mixture using spark-induced breakdown spectroscopy Reviewed

N. Kawahara, E. Tomita, S. Takemoto, Y. Ikeda

SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY   64 ( 10 )   1085 - 1092   2009.10

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study determined the local equivalence ratio of a CH(4)/air mixture in a laminar premixed flame using spark-induced breakdown spectroscopy (SIBS) with a fiber-coupled intensified charge coupled device (ICCD) spectrometer. Spectrally resolved emission spectra of plasma generated by a spark plug were investigated for their potential to measure local fuel concentrations in a premixed mixture. The influence of key parameters, such as the camera gate timing and spark energy, on the intensity of radical emission was illustrated. The intensity ratio of CN/NH had a greater sensitivity to the equivalence ratio than did that of CN/CH, and the local equivalence ratio could be obtained with high resolution by measuring the local intensity ratios of CN/NH. Moreover, a spark-plug sensor with an optical fiber was developed for application in spark-ignition engines. The atomic emission intensity during the breakdown and arc phases of spark discharge could be obtained using the fiber-optic spark-plug sensor. The H(alpha)/O intensity showed better linearity than the CN/NH intensity ratio in lean mixtures. The results presented here confirm the use of SIBS as a diagnostic tool for spark-ignition engines. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.sab.2009.07.016

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Performance and emission comparison of a supercharged dual-fuel engine fueled by producer gases with varying hydrogen content Reviewed

Murari Mohon Roy, Eiji Tomita, Nobuyuki Kawahara, Yuji Harada, Atsushi Sakane

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   34 ( 18 )   7811 - 7822   2009.9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study investigated the effect of hydrogen content in producer gas on the performance and exhaust emissions of a supercharged producer gas-diesel dual-fuel engine. Two types of producer gases were used in this study, one with low hydrogen content (H(2) = 13.7%) and the other with high hydrogen content (H(2) = 20%). The engine was tested for use as a co-generation engine, so power output while maintaining a reasonable thermal efficiency was important. Experiments were carried out at a constant injection pressure and injection quantity for different fuel-air equivalence ratios and at various injection timings. The experimental strategy was to optimize the injection timing to maximize engine power at different fuel-air equivalence ratios without knocking and within the limit of the maximum cylinder pressure. Two-stage combustion was obtained; this is an indicator of maximum power output conditions and a precursor of knocking combustion. Better combustion, engine performance, and exhaust emissions (except NO.) were obtained with the high H(2)-content producer gas than with the low H(2)-content producer gas, especially under leaner conditions. Moreover, a broader window of fuel-air equivalence ratio was found with highest thermal efficiencies for the high H(2)-content producer gas. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2009.07.056

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Visualization of auto-ignition and pressure wave during knocking in a hydrogen spark-ignition engine Reviewed

Nobuyuki Kawahara, Eiji Tomita

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY   34 ( 7 )   3156 - 3163   2009.4

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Visualizations of auto-ignition inside the end-gas region due to flame propagation and pressure waves that occur during knocking were carried out in a hydrogen spark-ignition engine using a high-speed camera. Our results demonstrated that auto-ignition in an endgas region that was compressed by the propagating flame front could be visualized using the high-speed color camera. A large amount of unburned mixture caused by the auto-ignited kernel explosion generated the strong pressure waves. Strong pressure wave oscillations induced by the initial auto-ignition could be visualized using a video camera with a speed of 250 kframes/s. The auto-ignition and pressure waves caused the thermal boundary layer to breakdown near the cylinder wall and piston head, therefore combustion of the lubricant oil grease was visible inside burned gas region. This auto-ignition and pressure waves may result in damage to the cylinder wall and piston head during engine knocking. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved

DOI： 10.1016/j.ijhydene.2009.01.091

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Laser-induced radical generation and evolution to a self-sustaining flame Reviewed

J. L. Beduneau, N. Kawahara, T. Nakayama, E. Tomita, Y. Ikeda

COMBUSTION AND FLAME   156 ( 3 )   642 - 656   2009.3

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Images and emission spectra of sparks produced by laser-induced breakdown of methane and propane air mixtures were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified high-speed camera and a single/multi-fiber Cassegrain optics system coupled to an ICCD spectrometer. Emission spectra of OH*, CN*, CH*, and C-2* radicals were also collected using spectra boxes. The results provided information about the different stages of the laser-induced breakdown, with a specific focus on the transition from a flame kernel to a self-sustaining flame. The plasma shape and emission spectrum were very reproducible. The differences in the size of the flame kernel and the evolution of the radical emissions were analyzed for mixtures that fired or misfired. The impact of the level of radicals in the flame kernel was a critical parameter for the firing process, starting around 1 mu s after the laser-induced breakdown. The transition from plasma cooling to the classical chemical reactions in the combustion zone was analyzed.
Even though the flame kernel size was directly linked to the spark energy, this was not a key parameter toward evolution to a self-sustaining flame. The Taylor blast wave theory was used to plot the location of the shock based on the evolution of the flame kernel size. The location was calculated using a laser-supported detonation model. A very good correlation was observed with the hot gas ignition process. Our results allowed us to obtain information about the process leading to firing or misfiring for similar environments, resulting in a better understanding of the laser breakdown phenomena and the means of utilizing this technique in an industrial context. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.combustflame.2008.09.013

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High temporally resolved optical measurement for laser ignition process of laminar premixed mixtures Reviewed

Takashi Nakayama, Nobuyuki Kawahara, Eiji Tomita, Yuji Ikeda

Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B   74 ( 743 )   1633 - 1640   2008.7

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

This paper describes laser ignition process in a laminar premixed mixture of propane and air. The plasma was generated by focusing a second harmonic and several-nanosecond pulse emitted from a Q-switched Nd : YAG laser. Minimum ignition energy (MIE) was measured in different equivalence ratio. The growth process in the plasma and the hot kernel (flame kernel) around the MIE were observed using high-speed schlieren photography and a multi-fiber coupled to an ICCD spectrometer. The kernel size, the kernel expansion velocity and the emission spectrum history form the plasma and the kernel were determined under the ignition case and misfire case. Further, temperature of the plasma and the kernel were determined from the CN molecular spectra by comparing experimental and simulated (LIFBASE) spectra. The main results of this study are as follows : (1) In laser ignition method, combustion reaction begins through the cooling process in which the ion recombines to the atom, furthermore the atom recombines to the molecular after breakdown. (2) By analyzing the CH emission intensity history, it was considered that combustion reaction occurs confirmed about 100 μs after laser pulse. (3) The temperature of the plasma and the kernel decrease with time and there were almost no difference between fire and misfire. (4) The minimum ignition energy is the smallest near the stoichiometric mixture.

DOI： 10.1299/kikaib.74.1633

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Spatially, temporally, and spectrally resolved measurement of laser-induced plasma in air Reviewed

N. Kawahara, J. L. Beduneau, T. Nakayama, E. Tomita, Y. Ikeda

APPLIED PHYSICS B-LASERS AND OPTICS   86 ( 4 )   605 - 614   2007.3

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using a framing intensified charged coupled device (CCD) camera and a multi-fiber Cassegrain optics system coupled to an intensified CCD spectrometer. The results provided information about the different stages of laser-induced breakdown. The plasma shape and emission spectrum were very reproducible. Different ionization levels in the plasma kernels, which were observed using the high spatial resolution of the multi-fiber Cassegrain optics system, occurred during the plasma formation and cooling and at different locations within the plasma. This was due mainly to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout. These observations were correlated with the plasma visualizations obtained with the framing ICCD camera. The plasma emission analysis permitted us to study the temperature evolution along the plasma during the laser-induced breakdown process. The analysis demonstrated the validity of a laser-supported wave model during the first stages of laser-induced breakdown and illustrated the weak dependence of the plasma temperature on the input energy.

DOI： 10.1007/s00340-006-2531-4

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Auto-ignited kernels during knocking combustion in a spark-ignition engine Reviewed

Nobuyuki Kawahara, Eiji Tomita, Yoshitomo Sakata

PROCEEDINGS OF THE COMBUSTION INSTITUTE   31 ( 2 )   2999 - 3006   2007

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

In this study, auto-ignition of end-gas due to flame propagation and intensity oscillations caused by shockwaves that occur during knocking combustion were visualized in a compression-expansion engine using a high-speed video camera. Chemical luminescence emissions of the end-gas were detected to analyze the chemical reactions caused by the auto-ignition. Four main conclusions were drawn from this study. When the end-gas region was compressed due to the propagating flame front, auto-ignited kernels appeared near a negative curvature of the flame front. This negative curvature was related to low-temperature chemistry. The large amount of unburned mixture generated a strong pressure wave caused by the auto-ignited kernels explosion. Visualized images of a regular propagating flame front and auto-ignited kernels confirmed that the knocking intensity had a strong relationship with the mass fraction of the unburned mixture. Oscillations of OH* radicals were synchronized with the in-cylinder pressure oscillations, which were produced due to the resulting shockwave. Before auto-ignition of the end-gas occurred, weak OH* radicals and very weak HCHO* radicals appeared in the end-gas region due to low-temperature chemistry. The OH* radicals played an important role in the low-temperature kinetic reactions. This confirms low-temperature chemical reaction of auto-ignited kernel in the end gas region. OH* radicals are a good indicator of the transition from low-temperature chemistry to high-temperature auto-ignition. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2006.07.210

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Cycle-resolved measurements of the fuel concentration near a spark plug in a rotary engine using an in situ laser absorption method Reviewed

Nobuyuki Kawahara, Eiji Tomita, Kenta Hayashi, Michihiko Tabata, Kouhei Iwai, Ryoji Kagawa

PROCEEDINGS OF THE COMBUSTION INSTITUTE   31 ( 2 )   3033 - 3040   2007

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

Cycle-resolved measurements of the fuel concentration near a spark plug in a commercial rotary engine were performed. An in situ laser infrared (IR) absorption method was developed using a spark plug sensor and a 3.392-mu m He-Ne laser as the light source. This wavelength coincided with the absorption line of hydrocarbons. The newly developed IR spark plug sensor had a higher signal-to-noise ratio than its previous version due to the optimization of its quartz lens and two optical fibers. The new sensor provided quantitative cycle-resolved fuel concentration measurements around the spark plug with a high temporal resolution. At lean preset air/fuel (A/F) ratios, fuel was mixed with the surrounding air gradually by the rotor motion in the plug hole of the rotary engine. Strong mixture inhomogeneities were measured during the compression stroke; the magnitude of these inhomogeneities decreased throughout the compression stroke. Cycle-resolved measurements were made to investigate the effects of the fuel concentration near the spark plug on the combustion characteristics of the commercial rotary engine. There was a strong correlation between the fuel concentration measured with the spark plug sensor and the combustion characteristics during the initial combustion period, which occurred faster when conditions were slightly richer than stoichiometric near the spark plug. The indicated mean effective pressure (IMEP) was slightly related to the A/F ratio near the spark plug. It was possible to measure the cycle-resolved A/F ratio near the spark plug and investigate its cycle-to-cycle fluctuations to achieve stable operation using the newly developed spark plug sensor. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

DOI： 10.1016/j.proci.2006.08.088

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軽油着火過給天然ガスエンジンにおける燃焼および排気特性 Reviewed

深谷信彦, 冨田栄二, 河原伸幸, 丸山慶士, 薦田哲男

自動車技術会論文集   38 ( 2 )   125 - 130   2007

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Combustion and Exhaust Emissions Characteristics in the Supercharged Engine Ignited with Gas Oil Fueled by Pyrolysis Gas Generated from Biomass Reviewed

FUKATANI Nobuhiko, TOMITA Eiji, KAWAHARA Nobuyuki, MARUYAMA Keiji, KOMODA Tetsuo

Transactions of the Japan Society of Mechanical Engineers B   73 ( 730 )   1337 - 1344   2007

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Pyrolysis gas produced from woody and waste biomass is considered as one of very important fuels to establish a sustainable society. In this study, asupercharged single cylinder gas engine with micro pilot ignition by gas oil was used. This ignition system has an advantage of large energy source, long period for ignition and stable ignition compared to the spark ignition system. Four types of artificial pyrolysis gases that were compressed into high pressure vessels were used. Compositions of these gases were H_2 , CO, CO_2 , CH_4 and N_2. Combustion was visualized by using a high-speed color video camera from the bottom of the quartz piston. The pressure history was analyzed to obtain the rate of heat release in order to investigate combustion characteristics. Exhaust emissions of NO_x, HC, CO and smoke were measured. Many ignition kernels produced by the iginition of gas oil initiate flame propagation of gas/air mixture and then flames move toward the wall of cylinder. Effects of injection timing, gas composition and equivalence ratio on engine performance were studied.

DOI： 10.1299/kikaib.73.1337

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Method for Predicting Scuffing on Poston Ring and Cylinder Liner in Marine Diesel Engines: Damage Prevention Using Observation of Combustion Flame:Damage Prevention Using Observation of Combustion Flame Reviewed

Takasu Isao, Kan Tadayuki, Takasaki Koji, Tomita Eiji, Kawahara Nobuyuki

JOURNAL OF THE MARINE ENGINEERING SOCIETY IN JAPAN   39 ( 4 )   250 - 258   2004

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Language：Japanese   Publisher：The Japan Institute of Marine Engineering  

Residual low-grade fuels are often used for marine diesel engines for reasons of economy. Some marine engines have been damaged due to the difficulty of combustion control of such fuels. Many factors affect scuffing of the piston ring and cylinder liner. Because of the complexity and difficulties, however, no one can predict scuffing. Problems have to be solved one by one. This study focuses on scuffing that is caused by the fuel itself. A spray flame was observed with a high-speed video camera in a constant volume vessel, in which ambient pressure and temperature were controlled. A previous study found that after-burning and long ignition delay were problems, so this analysis focused on combustion phenomena after the end of injection. A function of the period from the end of injec-tion till quenching of the flame and of the flame length at quench showed good correlation with the degree of scuffing damage. These two parameters are more valuable for predicting scuffing than are other methods before bunkering fuel in the ship.

DOI： 10.5988/jime.39.250

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