Understanding the Octane Appetite of Modern Vehicles

Octane appetite of modern engines has changed as engine designs have evolved to meet performance, emissions, fuel economy and other demands. The octane appetite of seven modern vehicles was studied in accordance with the octane index equation OI=RON-KS, where K is an operating condition specific constant and S is the fuel sensitivity (RON-MON). Engines with a displacement of 2.0L and below and different combinations of boosting, fuel injection, and compression ratios were tested using a decorrelated RON-MON matrix of eight fuels. Power and acceleration performance were used to determine the K values for corresponding operating points. Previous studies have shown that vehicles manufactured up to 20 years ago mostly exhibited negative K values and the fuels with higher RON and higher sensitivity tended to perform better. On investigating this trend for 2012 MY vehicles, it was found that K values were negative and higher in magnitude in several cases in comparison with results from prior research in this area. The octane response correlated well with the spark advance strategy employed by their respective engine management systems. However, in some instances the K values computed were highly negative (-4 and beyond) which would correspond to unrealistic pressure and temperature conditions in the engine from the perspective of chemical kinetics of auto-ignition. It is speculated that this anomalous result has arisen from the spray-mixing properties of some of the fuels in the matrix. Notwithstanding these points, it was consistently found that fuels with higher RON and higher sensitivity performed better showing that the octane response of modern vehicles is moving further away from MON.