Knock is studied in a single cylinder direct injection spark ignition engine with variable intake temperatures at wide open throttle and stoichiometric premixed ethanol-air mixtures. At different speeds and intake temperatures spark angle sweeps have been performed at non-knocking conditions and varying knock intensities. Heat release rates and two zone temperatures are computed for both the mean and single cycle data.
The in-cylinder pressure traces are analyzed during knocking combustion and have led to a definition of knocking conditions both for every single cycle as well as the mean engine cycle of a single operating point. The timing for the onset of knock as a function of degree crank angle and the mass fraction burned is determined using the “knocking” heat release and the pressure oscillations typical for knocking combustion.
A detailed chemical kinetic model for ethanol combustion is used to compute ignition delay times (IDT) for stoichiometric ethanol-air mixtures at pressures and temperatures relevant to the conditions measured on the engine test bench. A multi-step Arrhenius type correlation has been fit to the data including the observed flattening of the IDT for ethanol at relatively low temperatures (<850K) and compared to other data available in literature.
Empirical knock prediction models available in literature are tested against the available measurement data and improvements to the models are formulated. The importance of accurate IDT values as well as a model for the reducing probability of knock towards the end of combustion for the precision of a knock model is illustrated.
