Autoignition of hydrocarbons in relation to engine knock
A single piston Rapid Compression Machine (RCM) has been used to investigate the autoignition of hydrocarbons under conditions of temperature and pressure similar to those which occur in the end-gas of a spark-ignition engine under knocking conditions. Extents of reactant consumption have been measured during the course of autoignition following rapid compression of hydrocarbon-air mixtures. Evidence for the occurrence of low temperature oxidation during the compression stroke has been found and its effect on the overall ignition delay has been determined by numerical methods. The influence of diethylamine on this reactivity during compression and on the overall ignition delay has been investigated experimentally. The amine was shown to exert an inhibiting influence on the low temperature oxidation of n-heptane and n-pentane. Measurements of autoignition delays have been made over a range of compressed gas temperatures for different hydrocarbons (C4-C8). The results illustrate the relationship between autoignition delay and octane rating and the effect of molecular structure on the reactivity of hydrocarbons in an RCM. In general, as the Research Octane Number (RON) increases the duration of the ignition delay following compression to 900 K increases, however, a quantitative correlation of the two could not be made. Spatial imaging techniques (schlieren imaging, image intensified Charge Coupled Device (CCD)), used in the investigation of the spatial development of autoignition in the RCM, confirm the existence of spatial temperature inhomogeneities within the combustion chamber during the post-compression period. These imaging techniques have also been used in the study of spark-ignition and “knock” in the RCM. The ignition of methane in the RCM has been studied. The experimental results suggest that the presence of higher alkanes (ethane, propane and n-butane) or carbonaceous particles enhances the initiation of ignition of methane-oxygen mixtures. The effects of pressure and temperature on this behaviour have been explored experimentally.