Experimental study on knock sources in spark ignition engine with exhaust gas recirculation

The presented study aims to experimentally investigate the sources of influence of exhaust gas recirculation on the tendency toward knock in the spark ignition engine. The three main sources of influence of exhaust gas recirculation on the engine tendency towards knock are known. The influence on flame propagation changes the profile of combustion and therefore the end-gas pressure and temperature profile. The thermal influence changes the thermal properties of the end-gas mixture and consequently its temperature profile, while the chemical influence changes the kinetic behaviour of the end-gas mixture. The study is based on the results from experimental setup with spark ignition engine that uses cooled exhaust gas recirculation system and air heater installed into the intake manifold. Experimental tests that employ a new approach were performed, where intake temperature is varied by air heater when engine is operated with different levels of exhaust gas recirculation. In this way the end-gas temperature and exhaust gas recirculation percentage were varied while the influence on flame propagation was partially compensated by the change of spark timing. The obtained results show that there is no clear chemical influence of the exhaust gases on the tendency towards knock as the cases with low and high levels of exhaust gas recirculation are all mixed when the temperature of the end-gas is set to the same values. This leads to the overall conclusion that the predominant factor in a tendency towards knock is the end-gas temperature profile.