Control of oxides of nitrogen from diesel engines using diluents while minimising the impact on particulate pollutants

Abstract

The investigation was conducted on a high-speed direct-injection diesel engine and was concerned with the effects of exhaust gas recirculation (EGR) on diesel engine combustion and emissions. In particular, the effects on combustion and emissions of carbon dioxide (CO 2) and water vapour (H 2O), principal constituents of EGR, were analysed and quantified experimentally. The effects of increased inlet temperature and thermal throttling of the inlet charge, both arising from the use of hot EGR, were also investigated. Finally, tests were carried out during which the CO 2 added to the engine air flow increased the charge mass flow rate to the engine, rather than displacing some of the oxygen (O 2) in the inlet air. It was found that when CO 2 or H 2O displaced O 2 in the inlet charge, both the chemical and thermal effects on exhaust emissions were small. However, the dilution effect was substantial, and resulted in very large reductions in exhaust NO x at the expense of higher particulate and unburnt hydrocarbon emissions. Higher inlet charge temperature increased exhaust NO x and particulate emissions, but reduced unburnt hydrocarbon emissions. Reduction in the inlet charge due to thermal throttling reduces NO x emissions but raises all the other pollutants. Finally, when CO 2 was additional to the inlet air charge (rather than displacing O 2), large reductions in NO x were recorded with little increase in particulate emissions.