A Review on the Effect of Fuel Additives and EGR on Knocking Behavior of Spark Ignition Engine

Engine design and selection of fuels for automotive applications are required to minimize noise and exhaust emissions without compromising fuel economy. The knocking combustion investigation is essential as it directly affects the performance and durability as well as the thermal efficiency of the engine. Several fuel additives were suggested in the previous studies to mitigate the knocking combustion in spark ignition (SI) engines. The present study reviews the effect of antiknock fuel additives such as ethanol, methanol, prenol, n-butanol, furan mixtures, etc., on knocking behavior in SI engines. Additionally, this paper aims to present a systematic review of the studies conducted to investigate the effect of EGR on the knocking in SI engines. The EGR is often considered an effective means to suppress knocking in SI engines. The thermal effect of EGR in controlling the knocking is well known as EGR affects the temperature and pressure history of the combustion chamber. The main constituting components of EGR are CO₂, H₂O, NO, and N₂. It is found from the published studies that CO₂ has a strong chemical effect on knocking through a chemical reaction between CO₂ and hydrogen. CO₂ also plays a significant role in the suppression of the temperature rise. The CO₂ has a more substantial effect than H₂O on laminar burning velocity. Nitric oxide (NO) present in the EGR also affects autoignition and knocking. It is demonstrated that knocking can be suppressed with NO addition, but the fuel must have a strong negative temperature coefficient in some specific conditions. Based on the literature review, future research directions are also proposed for further studies.