Cylinder-to-cylinder variation of knock and effects of mixture formation on knock tendency for a heavy-duty spark ignition methanol engine

The knock characteristics of a high-pressure port fuel injection heavy-duty SI engine fueled with pure methanol were investigated experimentally and numerically. This paper aims to provide the optimization direction of the injection strategy from the perspective of knock suppression and thermal efficiency improvement. Both experiment and simulation prove that injection during the intake stroke with high injection pressure can mitigate knock because the strong interaction between spray and intake flow helps to form the more homogeneous and lower temperature mixture which prolongs the ignition delay of end-gas. As the injection timing (αinj) varies from 320°CA to 480°CA, the combustion speed and thermal efficiency increase first and then decrease, while the knock tendency is the opposite. Take the combustion at αinj of 320°CA as a baseline, the faster combustion speed at αinj of 380°CA caused by the homogeneous mixture contributes to the higher thermal efficiency, while that of 480°CA caused by the higher temperature further enhances the knock tendency. There is a significant cylinder-to-cylinder variation of the knock tendency. The knock tendency of the cylinder with the less fresh air charge is lower because of the lower in-cylinder temperature and pressure.