The Interaction of Fuel Anti-Knock Index and Cooled EGR on Engine Performance and Efficiency

Experiments were performed on a 2.4L boosted, MPI gasoline engine, equipped with a low-pressure loop (LPL) cooled EGR system and an advanced ignition system, using fuels with varying anti-knock indices. The fuels were blends of 87, 93 and 105 Anti-Knock Index (AKI) gasoline. Ignition timing and EGR sweeps were performed at various loads to determine the tradeoff between EGR level and fuel octane rating. The resulting engine data was analyzed to establish the relationship between the octane requirement and the level of cooled EGR used in a given application. In addition, the combustion difference between fuels was examined to determine the effect that fuel reactivity, in the form of anti-knock index (AKI), has on EGR tolerance and burn rate. The results indicate that the improvement in effective AKI of the fuel from using EGR is constant across commercial grade gasolines at about 0.5 ON per % EGR. In addition, there was a detectable difference in burn rates between fuels, the magnitude of which varied with operating condition. At the only condition where all three fuels could be compared on an equal basis, no detectable difference was found in EGR tolerance between fuels. Emissions results confirm the effect of higher fuel reactivity on burn rate, with lower AKI fuels leading to higher NOx emissions at a constant EGR level and load.