Effect of Fuel-Air Mixture Dilution on Knock Intensity in an SI Engine 2018-01-0211

Occurrence of knock in spark ignition (SI) engines is usually suppressed by inhibiting auto-ignition of the fuel-air mixture. A steep increase in pressure by auto-ignition of the local mixture is thought to initiate the pressure oscillation, which results in knock. Therefore, in order to prevent knock, the strength of the pressure oscillation would be decreased by reducing the local heat release of the end gas. In this study, the oxidation reaction rate of the auto-ignition was attempted to be reduced by dilution of the mixture. The effect of mixture dilution on the strength of pressure oscillation, that is knock intensity, was examined using a rapid compression machine (RCM) and a single cylinder SI engine. The test result of compression ignition of homogeneous mixture using RCM showed that increase in dilution ratio could decrease the knock intensity even if the input heat increased and the auto-ignition timing advanced. The maximum load without knock as well as without end gas auto-ignition at MBT spark timing was explored for various dilution ratios with inert gas or air in the SI engine. It was cleared that the knock-limited load was higher than the auto-ignition-limited one under high dilution conditions, while both were equal under low dilution conditions. That is to say, knock intensity with auto-ignition of the end gas was also reduced by mixture dilution regardless of dilution gas composition. The knock intensity was well arranged in accordance with the maximum temperature and pressure of the cycle, which are the representative values in the auto-ignition process.