Fuel Octane and Composition Effects on Efficiency and Emissions in a High Compression Ratio SIDI Engine

The effects of fuel octane have been assessed on the efficiency and emissions of a high compression ratio (ε=13) spark ignition direct injection (SIDI) engine. Under low load stratified operation (1200 rpm, ∼20% load), a low octane fuel (RON=84, comprised of toluene, iso-octane, and n-heptane) yielded higher brake thermal efficiency and significantly lower hydrocarbon emissions than a base gasoline (RON=91). The indicator diagram for the low octane fuel showed evidence for two stage heat release, suggesting the presence of spark induced compression ignition (SICI). These results suggest that higher efficiency under low load stratified conditions can be obtained with lower octane fuels that undergo SICI combustion. The effect of fuel octane under high load was assessed at WOT with a high RON model fuel (RON=103, comprised of toluene, iso-octane, and n-heptane). This high octane fuel exhibited a torque benefit compared to pure iso-octane (RON=100) and premium gasoline (RON=99) that is significantly greater than expected based on RON alone. The results suggest that a high RON fuel, in particular one that is high in aromatics, yields significant torque benefits under high load.