A study of the influence of exhaust gas recirculation and stoichiometry on the heat release in the end-gas prior to knock using rotational coherent anti-Stokes-Raman spectroscopy thermometry

Abstract

Heat release in the end-gas prior to autoignition was investigated using different experimental methods including transducers for heat flux and pressure as well as rotational coherent anti-Stokes-Raman spectroscopy, which is a laser-based method for non-intrusive instantaneous thermometry of the gas. The time history was examined in the cases of mixtures of various stoichio-metries, where some were diluted with exhaust gas recir-culation (EGR). The measured temperature history was compared with the isentropic temperature calculated from the cylinder pressure trace. This comparison revealed a difference in heat release from low-temperature reactions in the end-gas for the various mixtures tested at a constant indicated mean effective pressure and a fixed position of 50 per cent burnt charge. It is shown that lean mixtures tend to exhibit the highest knock intensity, mainly due to a decrease in specific heat, as compared to the richer mixtures, which result in an earlier knock onset and as a consequence higher knock intensity. Furthermore, the comparison of temperatures indicates that the rich mixtures have a high heat release from low-temperature chemistry, which to some extent negates the higher specific heat of the charge. As a consequence, a slight enrichment of the charge can lead to higher knock intensity in comparison with a stoichiometric mixture. In spite of the lower specific heat of the charge when a stoichiometric charge was diluted with cooled EGR, these mixtures showed a very low tendency to knock.