Mitigating Intensities of Super Knocks Encountered in Highly Boosted Gasoline Direct Injection Engines 2015-01-0084

Turbocharged gasoline direct injection (TGDI) engines can achieve a very high level of brake mean effective pressure and thus the engines can be downsized. The biggest challenge in developing highly-boosted TGDI engines may be how to mitigate the pre-ignition (PI) triggered severe engine knocks at high loads and low engine speeds. Since magnitudes of cylinder pressure fluctuations during aforementioned engine knocks reach those for peak firing pressures in normal combustion, they are characterized as super knocks. It is widely believed that the root cause for super knocks is the oil particles entering the engine cylinder, which pre-ignite the cylinder mixture in late of the compression stroke. It is neither possible nor practical to completely eliminate the oil particles from the engine cylinder; a reasonable approach to mitigate super knocks is to weaken the conditions favoring super knocks. This paper reports the results of an experimental investigation on the conditions that potentially lead to PI and methods to suppress super knocks. Various parameters that could affect engine combustion were studied. It was found that intensities of super knocks varied considerably with the air-fuel ratio for the mixture, the engine cooling temperature, and volatility of the crankcase oil. It was demonstrated that through an appropriate control of the engine operation parameters, the TGDI engine can be operated super-knock free.