A new indicator for knock detection in gas SI engines

Abstract

Determination of knock onset for any engine tuning remains a difficult work for many engine manufacturers. This study investigates different combinations of existing knock indices in order to produce an upgraded indicator, which is easier to calibrate. Experiments are conducted on a single-cylinder gas engine bounded to combined heat and power (CHP). Effects of spark advance, volumetric efficiency and equivalent ratio are studied under constant speed operation. The ratio IMPO/( MAPO× W) (with IMPO defined as the Integral of Modulus of Pressure Oscillations, MAPO as the Maximum Amplitude of Pressure Oscillations and W as the width of the computational window) is proposed as suitable indice. In any engine setting, it remains constant under no knocking conditions. When knock occurs, a model deduced from dimensionless analysis allows determination of the oversteps of Knock Limited Spark Advance from a single IMPO/( MAPO× W) measurement with an accuracy better than 1 CA. Knock is then studied for different gas qualities by adding propane or carbon dioxide to the fuel. The results show that there is no significant effect of the fuel composition on the proposed indicator, making the model able to calculate KLSA overstep in all the situations.