Effects of Fuel Composition on Auto-Ignition and Detonation Development in Boosted Spark-Ignited Engines

The development of highly boosted and high compression spark-ignition engines with enhanced thermal efficiencies is primarily limited by knock and super-knock. Super-knock is an excessively high intensity knock which has been related to a developing detonation process. This study investigates the knocking tendency of different gasoline surrogate fuels with varying research octane numbers (RON), octane sensitivity (S) and composition. The ξ/ɛ diagram with an enclosed detonation peninsula is used to assess the knocking tendency of different fuels. The diagram plots ξ, the ratio of acoustic to auto-ignitive velocity, against ɛ, the ratio of the transit time of an acoustic wave through a hot spot, to the heat release time (τ_e). Constant volume simulations of auto-ignition delay times (τ_i) and excitation times (τ_e) obtained from chemical kinetic calculations, enable calculations of ξ and ɛ. Their location for different fuels and operating conditions on the ξ/ɛ diagram, relative to the detonation peninsula, defines their mode of reaction propagation and the severity of a detonation. It was shown that excitation times are not affected by RON and S of the fuel. However, they are strongly dependent on the mixture composition. Fuels exhibiting a strong negative temperature chemistry (NTC) region are found to enter detonation development and explosion region, and are more likely to result in super-knock events in boosted spark-ignition engines.