Faculty Profiles - KAWAHARA Nobuyuki

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

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

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

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

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

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

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

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

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

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

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

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

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

Advanced Combustion in Natural Gas-Fueled Engines

Ulugbek Azimov, Nobuyuki Kawahara, Kazuya Tsuboi, Eiji Tomita

Energy, Environment, and Sustainability 215 - 250 2019

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

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

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

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

SAE Technical Papers 2018-April 2018.1

© 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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