Impact of Detailed Fuel Chemistry on Knocking Behaviour in Engines

Demand for more efficient gasoline vehicles has driven the develop‐ ment of downsized, engines, which benefit from higher octane. Features on modern SI engines such, direct injection, inter-cooling in boosted engines, cooled EGR and Millerisation lead to a much lower temperature for a given pressure in a real engine as compared to the test conditions in the CFR engine used to define the Research Octane Number (RON) and Motor Octane Number (MON) octane rating scales. Because the end-gas in modern engines experiences a different pressure/ temperature history during knocking cycles, as compared to the CFR engine, there is a growing body of evidence to suggest that for a given RON, it may actually be beneficial to have a high octane sensitivity (RON-MON) or in other words a lower MON. To explore this further, tests have been conducted in a single cylinder DISI engine over the whole speed load map using three different compression ratios, and fuels with two different levels of RON but with three octane sensitivity levels ranging from 5 to 15. These results have been further interpreted by reference to chemical kinetic models for gasoline autoignition, which can be used to ration‐ alise how the influence of sensitivity varies over the speed/load map.