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Technical Paper

Study on Knocking Characteristics for High-Efficiency Operation of a Super-Lean Spark Ignition Engine

This study investigated the influence of EGR and spark advance on knocking under high compression ratio, ultra-lean mixture and supercharged condition using premium gasoline as a test fuel. A high-compression ratio, supercharged single cylinder engine was used in this experiment. As a result, the period from ignition to autoignition was prolonged. In addition, knock intensity was drastically reduced. In other words, it is inferred that by combining an appropriate amount of EGR and spark advance, high efficiency operation avoiding knocking can be realized.
Technical Paper

Influence of Autoignition and Pressure Wave Behavior on Knock Intensity Based on Multipoint Pressure Measurement and In-Cylinder Visualization of the End Gas

In this study, the effect of autoignition behavior in the unburned end-gas region on pressure wave formation and knock intensity was investigated. A single-cylinder gasoline engine capable of high-speed observation of the end gas was used in the experiments. Visualization in the combustion chamber and spectroscopic measurement of light absorption by the end gas were carried out to analyze autoignition behavior in the unburned end-gas portion and the reaction history before autoignition. The process of autoignition and pressure wave growth was investigated by analyzing multipoint pressure histories. As a result, it was found that knocking intensity increases through interaction between autoignition and pressure waves.
Journal Article

Analysis of Interaction between Autoignition and Strong Pressure Wave Formation during Knock in a Supercharged SI Engine Based on High Speed Photography of the End Gas

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.
Technical Paper

A Study of Combustion in an HCCI Engine Using Non-Equilibrium Plasma Discharge Assist

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.
Technical Paper

A Study of Knocking in a Lean Mixture Using an Optically Accessible Engine

Improving the thermal efficiency of internal combustion engines requires operation under a lean combustion regime and a higher compression ratio, which means that the causes of autoignition and pressure oscillations in this operating region must be made clear. However, there is limited knowledge of autoignition behavior under lean combustion conditions. Therefore, in this study, experiments were conducted in which the ignition timing and intake air temperature (scavenging temperature) of a 2-stroke optically accessible test engine were varied to induce autoignition under a variety of conditions. The test fuel used was a primary reference fuel with an octane rating of 90. The results revealed that advancing the ignition timing under lean combustion conditions also advanced the autoignition timing, though strong pressure oscillations on the other hand tended not to occur.
Technical Paper

A Study of Streamer-Discharge-Assisted Autoignition Combustion using an Optically Accessible Engine

It is difficult to control the ignition timing of Homogeneous Charge Compression Ignition (HCCI) engines because they lack a physical means of igniting the mixture. Another issue of HCCI engines is their narrow operating range owing to the occurrence of misfiring at low loads and abnormal combustion at high loads. As a possible solution to these issues, this study focused on the generation of a streamer discharge using nonequilibrium plasma as a means of assisting HCCI combustion. A two-stroke engine that allowed visualization of the entire bore area was used in this study. A primary reference fuel blend (50 RON) was used as the test fuel. The streamer discharge was continuously generated in the end-gas region during a 360 deg. interval from the scavenging stroke to the exhaust stroke using a spark plug from which the ground electrode had been removed. Experiments were conducted in which the applied voltage of the streamer discharge was varied to investigate its effect on combustion.
Technical Paper

Influence of Initial Turbulence in RCM on Spontaneous Ignition of End Gas under SI Combustion

The influence of the eddy scale of initial turbulence in RCM on the pressure rise rate after spontaneous ignition of end gas was investigated. The combustion time of the end gas after spontaneous ignition was observed by using high-speed direct photography. As a result, the large scale eddy reduced the pressure rise rate after spontaneous ignition. The temperature inhomogeneity of end gas was higher with the large scale eddy. The combustion time of end gas after spontaneous ignition was prolonged by variation in local ignition delay due to inhomogeneity. The large scale eddy may prevented the knocking occurrence.
Technical Paper

A Study of HCCI Combustion Assisted by a Streamer Discharge Based on Visualization of the Entire Bore Area

This study investigated the effect of streamer discharge on autoignition and combustion in a Homogeneous Charge Compression Ignition (HCCI) engine. A continuous streamer discharge was generated in the center of the combustion chamber of a 2-stroke optically accessible engine that allowed visualization of the entire bore area. The experimental results showed that the flame was initiated and grew from the vicinity of the electrode under the application of a streamer discharge. Subsequently, rapid autoignition (HCCI combustion) occurred in the unburned mixture in the end zone, thus indicating that HCCI combustion was accomplished assisted by the streamer discharge. In other word, ignition timing of HCCI combustion was advanced after the streamer discharging process, and the initiation behavior of the combustion flame was made clear under that condition.
Journal Article

Visualization and Spectroscopic Measurement of Knocking Combustion Accompanied by Cylinder Pressure Oscillations in an HCCI Engine

Combustion experiments were conducted with an optically accessible engine that allowed the entire bore area to be visualized for the purpose of making clear the characteristics that induce extremely rapid HCCI combustion and knocking accompanied by cylinder pressure oscillations. The HCCI combustion regime was investigated in detail by high-speed in-cylinder visualization of autoignition and combustion and emission spectroscopic measurements. The results revealed that increasing the equivalence ratio and advancing the ignition timing caused the maximum pressure rise rate and knocking intensity to increase. In moderate HCCI combustion, the autoignited flame was initially dispersed temporally and spatially in the cylinder and then gradually spread throughout the entire cylinder.
Technical Paper

Behavior of Shock Wave and Pressure Wave of SI Knocking with Super Rapid Compression Machine

Behaviors of shock waves in knocking phenomena were observed in detail and influences of low temperature reaction on the flame and spontaneous ignition of end gas were investigated through experiments using high-speed direct and schlieren photography. As a result, it was found that light emission of shock waves, that is an indicator of pressure, rose when the shock waves collided with the cylinder wall and that pressure waves arose by low temperature reaction before knocking occurrence. Flame oscillation was caused by pressure waves. It is presumed that pressure waves influence spontaneous ignition.
Technical Paper

Characteristics of HCCI Combustion in Homogenized Temperature Fields using a Super Rapid Compression Machine

Homogeneous Charge Compression Ignition combustion in homogenized temperature fields was investigated experimentally using a super rapid compression machine. Temperature fields before a blue flame occurs are thought to control the burning process. The time of blue flame occurrence, burn rate and ignition delay time were measured. Temperature homogeneity before a blue flame occurred was controlled by two means. One was by the piston shape which controlled the roll-up vortex and the other was by the heat release of low temperature reactions that has a homogenizing effect. It was found that homogenized temperature fields contribute to the occurrence of a homogeneous blue flame while inhomogeneous fields produce an uneven occurrence.
Technical Paper

Evaluation of Homogeneous Charge Compression Ignition at High Engine Speeds using a Super Rapid Compression Machine

We had improved RCM and developed a Super Rapid Compression Machine (SRCM) that realizes an extremely rapid compression compared with the conventional RCM. In this study, the performance of the developed SRCM was evaluated. The SRCM was used to investigate on the effects of equivalence ratio on HCCI of n-heptane and iso-octane fuel/air mixture. Experimental results for ignition delay time, τ, and combustion time, t, were obtained from the cylinder pressure histories. The HCCI at high engine speeds was clarified by Optical observation using a high speed camera. As a result, the ignition delay time and combustion time are found to saturate above equivalence ratio of 0.6 at constant compression ratio. In the HCCI combustion in high compression ratio case, shock wave occurs from the core region of the roll-up vortex cause by piston motion. The HCCI combustion has many peaks over a wide range of frequency.
Technical Paper

Air Fuel Ratio Estimation of a Glow Engine through Sound

We examined if air fuel ratio could be estimated by engine sound for a two-stroke glow engine. A time resolved FFT (Fast Fourier Transform) was adopted to analyze the engine sound. As a result, strong acoustic radiation were shown twice at TDC and BDC in the frequency range below 2.5 kHz in each cycle. For the case of high air fuel ratio, a peak of sound pressure in the range of 2.5 to 5 kHz exists in between TDC and BDC. This sound is supposed to be a noise due to anomalous combustion. Since the noise appears just after the combustion noise, the anomalous combustion noise is supposed to be a knocking noise. When the periodic noise in the range up to 2.5 kHz appears weakly twice in one cycle, the air fuel ratio is approaching low limit for a stable run of the engine at 12000 rpm. When the periodic noise in the range of 2.5 to 5 kHz appears once in one cycle between the periodic noise in the range up to 2.5 kHz, the air fuel ratio is approaching high limit.