The influence of simulated residual and NO concentrations on knock onset for PRFs and gasolines

Modern engine developments result in very different gas pressure-temperature histories to those in RON/MON determination tests and strain the usefulness of those knock scales and their applicability in SI engine knock and HCCI autoignition onset models. In practice, autoignition times are complex functions of fuel chemistry and burning velocity (which affects pressure-temperature history), residual gas concentration and content of species such as NO. As a result, autoignition expressions prove inadequate for engine conditions straying far from those under which they were derived. The currently reported study was designed to separate some of these effects. Experimental pressure crank-angle histories were derived for an engine operated in skip-fire mode to eliminate residuals. The unburned temperature history was derived for each cycle and was used with a number of autoignition/knock models. A simple empirical expression proved no less effective than more complex formulations in predicting knock onset for iso-octane and PRFs over a wide range of residual free operating conditions. Prediction of knock onset for two commercial gasoline fuels proved less reliable, but was improved using an octane index correction method. Computations of knock onset times proved sensitive to simulated residual gas/EGR and NO concentrations. The influence of NO proved variable and contrary for iso-octane, gasolines, primary and toluene reference fuel mixtures.