Investigation of fuel effects on the knock under lean burn conditions in a spark ignition engine

Abstract

In this study, the fuel effects on the knock under super lean burn conditions were investigated using a single-cylinder spark ignition (SI) engine with a compression ratio of 15 at an excess air ratio of 1.8. Four types of test fuels with different ignition characteristics were used. The knock occurred before the minimum spark advance for the best torque in all conditions. The octane number and octane index did not show good correlation with the crank angle position at which 50% of the heat was released (CA50) at the knock limit obtained in the experiments. The crank angle when the calculated Livengood-Wu integral (LWI) value = 1 did not precisely predict the knock onset. The F value proposed in this study was in reasonable agreement with CA50 at the knock limit. It was composed of the following three factors: the crank angle (its unit is degree after top dead center) when LWI = 1, h value representing the differences in the low-temperature oxidation behavior, and gradient of ignition delay times against temperatures at the crank angle when LWI = 1. This indicates that the three factors composing the F value affected knock occurrence under super lean burn conditions using an SI engine with high compression ratio.