Hideo SHOJI’s research while affiliated with Nihon University and other places

In this study, knocking over a wide range of engine speeds was visualized using an optically acssessible engine. In addition, knock under a high compression ratio and supercharged, lean combustion was investigated. The results revealed that under high-speed knock, the flame propagation velocity declined when low-temperature oxidation reactions occurred. Subsequently, autoignition began locally and expanded gradually. Eventually, it was observed that a highly brilliant autoignited flame appeared and propagated through the unburned end gas at a high speed of approximately 1700-1800 m/s. This suggests that high-speed knock causes "developing detonation" in which combustion proceeds at a supersonic speed while pressure waves and the reaction front mutually interact. It was also found that strong knock occurred under supercharged, ultra-lean conditions (Compression Ratio: CR=14, Equivalence Ratio: ϕ =0.5, Intake Pressure: Pin = 140 kPa,. In addition, the application of exhaust gas recirculation markedly reduced strong pressure oscillations.

Homogeneous Charge Compression Ignition (HCCI) has attracted much attention as a combustion system for internal combustion engines because it is capable of achieving high efficiency and clean exhaust emissions. However, it is difficult to control the ignition timing of HCCI engines and their range of stable operation is limited to low loads. Approaches to resolving these issues include supercharging and the application of exhaust gas recirculation (EGR), which are effective techniques for moderating combustion during high load operation. However, one drawback of EGR is that it tends to cause unstable combustion. Therefore, the idea of applying Low-Frequency plasma to assist combustion was investigated as a method of stabilizing combustion while still maintaining a high EGR rate. A self-excited pulse generator configured with an inverter circuit was used in this study to apply high-voltage alternating current for forming a Low-Frequency plasma discharge between electrodes inserted into the combustion chamber. The effect of this plasma discharge on HCCI combustion was examined. The results showed that the Low-Frequency plasma assist had the effect of stabilizing combustion under conditions where combustion normally becomes unstable. It was found that this method made it possible to achieve stable combustion at higher engine loads.

div class="section abstract"> Engine knock is the one of the main issues to be addressed in developing high-efficiency spark-ignition (SI) engines. In order to improve the thermal efficiency of SI engines, it is necessary to develop effective means of suppressing knock. For that purpose, it is necessary to clarify the mechanism generating pressure waves in the end-gas region. This study examined the mechanism producing pressure waves in the end-gas autoignition process during SI engine knock by using an optically accessible engine. Occurrence of local autoignition and its development process to the generation of pressures waves were analyzed under several levels of knock intensity. The results made the following points clear. It was observed that end-gas autoignition seemingly progressed in a manner resembling propagation due to the temperature distribution that naturally formed in the combustion chamber. Stronger knock tended to occur as the apparent propagation speed of autoignition increased. It is particularly notable that a condition was observed in which the apparent propagation speed of autoignition through the end gas clearly exceeded the speed of sound. Exceptionally strong knock occurred at that time. The measured results for the actual development of autoignition made it clear that extremely strong knock, such as what occurs under high-speed, high-load operation and also that induced by low-speed pre-ignition (LSPI) in a supercharged downsized engine under low-speed, high-load operation (Super-Knock), is presumably caused by such phenomena. In other word, extremely strong knock is caused by detonation phenomena produced in autoignition process. </div

div class="section abstract"> Homogeneous Charge Compression Ignition (HCCI) combustion has attracted widespread interest because it achieves high efficiency and can reduce particulate matter (PM) and nitrogen oxide (NOx) emissions simultaneously. However, because HCCI engines lack a physical means of initiating ignition, it is difficult to control the ignition timing. Another issue of HCCI engines is that the combustion process causes the cylinder pressure to rise rapidly. The time scale is also important in HCCI combustion because ignition depends on the chemical reactions of the mixture. Therefore, we investigated the influence of the engine speed on autoignition and combustion characteristics in an HCCI engine. A four-stroke single-cylinder engine equipped with a mechanically driven supercharger was used in this study to examine HCCI combustion characteristics under different engine speeds and boost pressures. The results revealed that the engine speed range can be expanded by a mechanically driven supercharger. It was also found that there was no change in the ignition delay time under conditions of a constant equivalence ratio even at different engine speeds. The findings of this study provide important knowledge for controlling the ignition timing. </div

div class="section abstract"> This study focused on a non-equilibrium plasma discharge as a means of assisting HCCI combustion.Experiments were conducted with a four-stroke single-cylinder engine fitted with a spark electrode in the top of the combustion chamber for continuously generating non-equilibrium plasma from the intake stroke to the exhaust stroke. The results showed that applying non-equilibrium plasma to the HCCI test engine advanced the main combustion period that otherwise tended to be delayed as the engine speed was increased. In addition, it was found that the combined use of exhaust gas recirculation and non-equilibrium plasma prevented a transition to partial combustion while suppressing cylinder pressure oscillations at high loads. </div

div class="section abstract"> This study investigated the effects of recirculated exhaust gas (EGR) and its principal components of N₂, CO₂ and H₂O on moderating Homogeneous Charge Compression Ignition (HCCI) combustion. Experiments were conducted using two types of gaseous fuel blends of DME/propane and DME/methane as the test fuels. The addition rates of EGR, N₂, CO₂ and H₂O were varied and the effects of each condition on HCCI combustion of propane and methane were investigated. The results revealed that the addition of CO₂ and H₂O had the effect of substantially delaying and moderating rapid combustion. The addition of N₂ showed only a slight delaying and moderating effect. The addition of EGR had the effect of optimally delaying the combustion timing, while either maintaining or increasing the indicated mean effective pressure and indicated thermal efficiency ηi. </div

div class="section abstract"> Lean-burn technology is regarded as one effective way to increase the efficiency of internal combustion engines. However, stable ignition is difficult to ensure with a lean mixture. It is expected that this issue can be resolved by improving ignition performance as a result of increasing the amount of energy discharged into the gaseous mixture at the time of ignition. There are limits, however, to how high ignition energy can be increased from the standpoints of spark plug durability, energy consumption and other considerations. Therefore, the authors have focused on a multistage pulse discharge (MSPD) ignition system that performs low-energy ignition multiple times. In this study, a comparison was made of ignition performance between MSPD ignition and conventional spark ignition (SI). A high-speed camera was used to obtain visualized images of ignition in the cylinder and a pressure sensor was used to measure pressure histories in the combustion chamber. The results revealed that MSPD ignition formed a stronger flame kernel than conventional SI under conditions inconducive to flame kernel formation. This was found to be effective in promoting stable combustion. </div

div class="section abstract"> Internal combustion engines have been required to achieve even higher efficiency in recent years in order to address environmental concerns. However, knock induced by abnormal combustion in spark-ignition engines has impeded efforts to attain higher efficiency. Knock characteristics during abnormal combustion were investigated in this study by in-cylinder visualization and spectroscopic measurements using a four-stroke air-cooled single-cylinder engine. The results revealed that knock intensity and the manner in which the autoignited flame propagated in the end gas differed depending on the engine speed. </div

This study investigated the effect of ignition timing on knocking in an SI engine. Autoignition and abnormal combustion behavior were examined by using light absorption spectroscopy and in cylinder flame visualization techniques. The result showed that the knocking intensity under the same autoignition timing was declined by advancing ignition timing.