Dilution of partially-premixed combustion (PPC) using different combinations of excess air (λ>1) and exhaust gas recirculation (EGR) was investigated in a single-cylinder, heavy-duty diesel engine equipped with common-rail fuel injection. The experiments were limited to a single fuel injection event using ultra-low sulphur diesel fuel at a low engine load (∼3 bar BMEP) and engine speeds of 900 and 1350 rpm. The start of injection was varied to optimize the combustion performance and emissions.
The experimental results show that increasing air dilution at constant EGR reduced BSFC slightly. CO and HC emissions decreased significantly due to the increased oxygen concentration, but NOx and soot emissions increased. For a given level of charge dilution, there was an optimal EGR rate to minimize BSFC. NOx emissions decreased significantly as the proportion of dilution by EGR was increased, but CO and HC emissions increased due to the reduced in-cylinder temperature and oxygen concentration, which increased the combustion duration.