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1 Journal of KONES

1 2010

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Visualization of auto-ignition of end gas region without knock in a spark-ignition natural gas engine

Tomita E., Kawahara N., Zheng J.

Journal of KONES

2010

Vol. 17, No. 4

521-527

It is important to increase thermal efficiency in a spark-ignition engine for reducing carbon dioxide gas in exhaust emissions. One of the solutions is to increase the compression ratio of the engine. Then, knock is a barrier in increasing in compression ratio because of higher pressure and unburned gas temperature in the end gas region. Therefore, there have been many studies on knock. However, there was an interesting phenomenon under some conditions in a gas engine. There is no pressure oscillation in the end stage of combustion and rate of heat release increased. It is considered that the auto-ignition in the end gas region occurs without knock. In this study, the combustion behavior in the end gas region was visualized with an ultra-high-speed camera. In normal combustion case, the flame propagates in the end gas region. In the knock case, the auto-ignited part burns suddenly and leads pressure oscillation. Under the critical conditions, some shows knock and some shows normal flame propagation. However, in some cases, even if auto-ignition occurs, pressure wave is not produced. Then there is no pressure oscillation, that is, mild auto-ignition without knock can be confirmed. And the rate of heat release shows two peaks. The first peak is due to flame propagation and the second peak is due to auto-ignition in the end gas region. The combustion near the end stage become short and thermal efficiency is expected to be increased because of increase in degree of constant-volume.