Insights into the Role of Autoignition during Octane Rating

The paper describes the emulation of the knock measurement equipment on the CFR engine used during octane rating. It was found during engine measurements that the low-pass filtered rate-of-change of the pressure signal from Primary Reference Fuels (PRF) established the definitive metric of standard knock intensity and that non-PRF fuels exhibited similar pressure rise rates at standard knock intensity. The effect of fuel octane number, compression ratio and air-fuel ratio was also clearly distinguishable.

Further modelling interpretation revealed that a non-instantaneous, cascading autoignition was a likely origin for the measured pressure development. Thermal inhomogeneities, which were the result of a heat loss gradient in the trapped mass, served to explain the observed behaviour of both RON and MON test conditions and a range of fuels of different octane numbers. A predictive model was ultimately applied to demonstrate that the pressure development and pressure rise rate can be predicted from the physical conditions in the engine, coupled to a fuel ignition delay descriptor.