Impact of Fuel Octane Rating and Aromatic Content on Stochastic Pre-Ignition

The effects of aromatic content and octane rating of gasoline fuels on stochastic pre-ignition (SPI) behaviors were investigated at typical operating conditions using a modern 2.0 L turbocharged engine. In-cylinder pressure time history measurements made during a speed-load test sequence designed to stimulate SPI were used to determine both the frequency of SPI occurrence and the in-cylinder peak pressure during such events. Six fuels were tested with varying levels of aromatic content (15 - 35% by vol.) and two octane rating levels (∼88 & 94 anti-knock index). The engine was operated using a production-intent calibration with equivalence ratio near one. Pressure and temperature in the intake manifold were held constant near two bar and 35°C respectively. Significant SPI activity was observed, with abnormal event frequencies up to ∼1 SPI event per 1,000 engine cycles and in-cylinder peak pressures up to ∼200 bar. Aromatic content correlated directly with frequency of SPI occurrence and had minimal effect on in-cylinder peak pressures; whereas, octane rating correlated inversely with in-cylinder peak pressures during SPI and had minimal effect on frequency. Analysis revealed that for the higher octane fuels only deflagration was evident and for the lower octane fuels deflagration transitioned to a violent auto-ignition of the unburned charge. The strong effect of aromatic content and the discovery of a minimum concentration below which no SPI was observed under the operating conditions evaluated suggests that the chemical and/or physical pathways of this fuel component were critical to SPI behavior at these conditions.