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Homogeneous Charge Compression Ignition (HCCI) engines have attracted much attention and are being widely researched as engines characterized by low emissions and high efficiency. However, one issue of HCCI engines is their limited operating range because of the occurrence of rapid combustion at high loads and misfiring at low loads. It is known that knocking accompanied by in-cylinder pressure oscillations also occurs in HCCI engines at high loads, similar to knocking seen in spark-ignition engines. In this study, HCCI combustion accompanied by in-cylinder pressure oscillations was visualized by taking high-speed photographs of the entire bore area. In addition, the influence of internal exhaust gas circulation (EGR) on HCCI knocking was also investigated. The visualized combustion images revealed that rapid autoignition occurred in the end-gas region during the latter half of the HCCI combustion process when accompanied by in-cylinder pressure oscillations.

One issue of downsized and supercharged engines is low-speed pre-ignition (LSPI) that occurs in the low-speed and high-load operating region. One proposed cause of LSPI is the influence of the engine oil and its additives. However, the effect of engine oil additives on pre-ignition and the mechanism involved are still not fully understood. This study investigated the influence of engine oil additives on abnormal combustion in a spark-ignition engine. A four-stroke air-cooled single-cylinder engine with a side valve arrangement was used in conducting combustion experiments. The research methods used were in-cylinder pressure analysis, in-cylinder visualization and absorption spectroscopic analysis. Engine oil additives were mixed individually at a fixed concentration into a primary reference fuel with an octane number of 50 and their effect on knocking was investigated.

Unresolved issues of Homogeneous Charge Compression Ignition (HCCI) combustion include an extremely rapid pressure rise on the high load side and resultant knocking. Studies conducted to date have examined ways of expanding the region of stable HCCI combustion on the high load side such as by applying supercharging or recirculating exhaust gas (EGR). However, the effect of applying EGR gas to supercharged HCCI combustion and the mechanisms involved are not fully understood. In this study, the effect of EGR gas components on HCCI combustion was investigated by conducting experiments in which external EGR gas was applied to supercharged HCCI combustion and also experiments in which nitrogen (N2) and carbon dioxide (CO2) were individually injected into the intake air pipe to simulate EGR gas components. In addition, HCCI combustion reactions were analyzed by conducting chemical kinetic simulations under the same conditions as those of the experiments.

Homogenous Charge Compression Ignition (HCCI) combustion experiments were conducted in this study using a single-cylinder test engine fitted with a sapphire observation window to facilitate visualization of the entire cylinder bore area. In addition to in-cylinder visualization of combustion, spectroscopic measurements were made of light emission and absorption in the combustion chamber to investigate autoignition behavior in detail. Engine firing experiments were conducted to visualize HCCI combustion over a wide range of compression ratios from 12:1 to 22:1. The results showed that increasing the compression ratio advanced the ignition timing and increased the maximum pressure rise rate, making it necessary to moderate combustion. It was also found that autoignition can be induced even in a mixture lean enough to cause misfiring by raising the intake air temperature so as to advance the overall combustion process.

This study examined Homogeneous Charge Compression Ignition (HCCI) combustion characteristics in detail on the basis of in-cylinder combustion visualization, spectroscopic measurements of light emission and absorption and chemical kinetic simulations. Special attention was focused on investigating and comparing the effects of the fuel octane number and residual gas on combustion characteristics. The results made clear the relationship between the production/consumption of formaldehyde (HCHO) in the HCCI autoignition process and flame development behavior in the cylinder. Additionally, it was found that both the fuel octane number and residual gas have the effect of moderating low-temperature oxidation reactions. Furthermore, it was observed that residual gas has the effect of shifting the temperature for the occurrence of the hot flame to a higher temperature range.

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.

Knocking combustion experiments were conducted in this study using a test engine that allowed the entire bore area to be visualized. The purpose was to make clear the detailed characteristics of knocking combustion that occurs accompanied by cylinder pressure oscillations when a Homogeneous Charge Compression Ignition (HCCI) engine is operated at high loads. Knocking combustion was intentionally induced by varying the main combustion period and engine speed. Under such conditions, knocking in HCCI combustion was investigated in detail on the basis of cylinder pressure analysis, high-speed photography of the combustion flame and spectroscopic measurement of flame light emissions. The results revealed that locally occurring autoignition took place rapidly at multiple locations in the cylinder when knocking combustion occurred. In that process, the unburned end gas subsequently underwent even more rapid autoignition, giving rise to cylinder pressure oscillations.

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.