The Interaction between Fuel Anti-Knock Index and Reformation Ratio in an Engine Equipped with Dedicated EGR

Experiments were performed on a small displacement (< 2 L), high compression ratio, 4 cylinder, port injected gasoline engine equipped with Dedicated EGR^® (D-EGR^®) technology using fuels with varying anti-knock properties. Gasolines with anti-knock indices of 84, 89 and 93 anti-knock index (AKI) were tested. The engine was operated at a constant nominal EGR rate of ∼25% while varying the reformation ratio in the dedicated cylinder from a ϕ_D-EGR = 1.0 - 1.4. Testing was conducted at selected engine speeds and constant torque while operating at knock limited spark advance on the three fuels. The change in combustion phasing as a function of the level of overfuelling in the dedicated cylinder was documented for all three fuels to determine the tradeoff between the reformation ratio required to achieve a certain knock resistance and the fuel octane rating. These results indicated that the effective increase in knock resistance of the engine was linear with the reformation ratio and that the slope of this improvement was roughly constant across fuel types. In general, a 10% increase in overfuelling in the dedicated cylinder was equal to a 1.8 AKI increase in fuel octane. In addition to the constant torque testing, a series of tests across a range of engine speeds was performed to determine the knock-limited peak torque at varied enrichment rates with the three test fuels. As expected, the higher octane fuels with higher enrichment levels showed the highest knock limited torques but with some overlap between the fuels, indicating that overfuelling in the dedicated cylinder could be used to overcome knock challenges with low octane fuels. In addition, to understand the impact of ethanol, two fuels were procured with a 93 AKI, one as an E0 and the other as an E10 fuel.