Influence of Coolant Temperature and Flow Rate, and Air Flow on Knock Performance of a Downsized, Highly Boosted, Direct-Injection Spark Ignition Engine

The causes of engine knock are well understood but it is important to be able to relate these causes to the effects of controllable engine parameters. This study attempts to quantify the effects of a portion of the available engine parameters on the knock behavior of a 60% downsized, DISI engine running at approximately 23 bar BMEP. The engines response to three levels of coolant flow rate, coolant temperature and exhaust back pressure were investigated independently. Within the tested ranges, very little change in the knock limited spark advance (KLSA) was observed. The effects of valve timing on scavenge flow and blow through (the flow of fresh air straight into the exhaust system during the valve overlap period) were investigated at two conditions; at fixed inlet/exhaust manifold pressures, and at fixed engine torque. For both conditions, a matrix of 8 intake/exhaust cam combinations was tested, resulting in a wide range of valve overlap conditions (from 37 to -53°CA). The results indicate that at the fixed manifold pressure test condition, the effects of cam timing on air flow masked the effects of in-cylinder conditions on KLSA. At fixed engine torque, however there was a 1.2°CA variation in KLSA with no distinguishing trend between valve overlap and KLSA. A trade-off between valve timing and the impact that this has on the boost system requirements was found to be of far greater significance than any benefits that valve timing had on spark advance at this load.