AbstractA theoretical study was conducted to assess the impact of intake-air nitrogen enrichment on the performance characteristics and soot and nitrogen oxide (NO) emissions of a high-speed direct injection (HSDI) single-cylinder diesel engine. Closed-cycle engine simulation code based on a multizone combustion model was used to perform simulations considering three fuels—diesel fuel, Jet Propellant-8 (JP-8), and Rapeseed Methyl Ester (RME)—and four intake-air nitrogen contents—79%, 81%, 83%, and 85% v/v, at an engine speed of 1,200 rpm under various loads. Two modes of diesel engine combustion are considered: constant fueling rate and, hence, rich in-cylinder average fuel/oxygen equivalence ratio (RAFOER) and reduced fueling rate and, hence, constant average fuel/oxygen equivalence ratio (CAFOER). Theoretical results for the cylinder pressure, bulk gas temperature, and in-cylinder soot and NO concentrations are derived at a speed of 1,200 rpm, and at full engine load considering the three examined fuels and the four examined intake-air nitrogen contents. Predictions for the percentage change of indicated power, exhaust gas temperature, exhaust soot, and exhaust NO are also generated for all examined fuels and at all examined nitrogen contents. The most important findings of the present study are as follows. First, intake-air nitrogen enrichment results in the achievement of NO emissions–free diesel operation, while on the other hand, for increased air nitrogen contents, the extremely limited soot oxidation rate keeps the exhaust soot at high levels. Second, under CAFOER conditions, the combustion of a well-known biodiesel such as RME can lead to reductions in both soot and NO emissions, thereby improving the well-known diesel engine soot/NO trade-off limitation.