Knock prevention of CHP engines by addition of N2 and CO2 to the natural gas fuel

This work focuses on the prevention of knock in the case of spark ignition (SI) engines supplied by natural gas network. The effects of the addition of two inert gases (N2 and CO2) are experimentally studied. The added volumetric quantities are between 0% and 25% for N2 and between 0% and 15% for CO2. The thermal efficiency and the emissions of the engine are very slightly affected by the addition, whereas a significant increase of the knock limited spark timing (KLST) is always measured. A twice-higher augmentation of KLST is noted when CO2 is added compared to N2 for an equivalent volumetric concentration. The overall augmentation varies between +1 and +6 °CA depending on engine operation. Finally, a law for predicting the KLST augmentation implied by the addition of inert gases is deduced from all the measurements.