The effects of exhaust gas recirculation on diesel combustion and emissions

An investigation was conducted with the aim of identifying and quantifying the effects of exhaust gas recirculation (EGR) on diesel engine combustion and exhaust emissions. Five effects of EGR were identified and investigated experimentally: the reduction in oxygen supply to the engine, participation in the combustion process of carbon dioxide and water vapour present in the EGR, increase in the specific heat capacity of the engine inlet charge, increased inlet charge temperature and reduction in the inlet charge mass flowrate arising from the use of hot EGR. The experimental methodology developed allowed each one of these effects to be investigated and quantified separately. The investigation was carried out on a high-speed, direct injection diesel engine, running at an intermediate speed and load. A limited number of tests were also conducted in an optically accessible diesel engine, which established the effects of EGR on local flame temperature. Finally, tests were conducted with simulated EGR being used additionally to the engine air supply. This contrasts with the conventional use of EGR, whereby EGR replaces some of the air supplied to the engine. It was found that the first effect of EGR (reduction in the oxygen flowrate to the engine) was substantial and resulted in very large reductions in exhaust NOx at the expense of higher particulate emissions. The second and third effects (participation of carbon dioxide and water vapour in the combustion process and increase in the charge specific heat capacity) were almost insignificant. The fourth effect (higher inlet charge temperature) increased both exhaust NOx and particulate emissions. The fifth effect (reduction in the inlet charge due to thermal throttling) reduced NOx but raised particulate emission. Finally, when EGR was used additionally to the inlet air charge (rather than displacing air), substantial reductions in NOx were recorded with little increase in particulate emission.