Partially Premixed Combustion in a Diesel Engine Induced by a Pilot Injection at the Low-pressure Top Dead Center

Abstract

An experimental investigation performed on a single-cylinder heavy-duty diesel research engine operating at low-speed, medium load is presented in this paper. This study is focused on combining a homogeneous charge compression ignition and a conventional diesel combustion processes to produce a partially premixed combustion by means of an original injection strategy. This strategy is based on introducing an early pilot injection at the low-pressure top dead center, between the exhaust and the intake strokes, followed by a conventional injection close to the compression top dead center. The main advantage of this concept compared to other early injection strategies consists of injecting always inside the piston bowl, far of the cylinder liner walls, thereby avoiding the oil dilution problems. The effect of the distribution of the fuel injected between low-pressure and compression top dead centers, and also the effect of the start of injection timing for the conventional injection are investigated in terms of combustion process characteristics, engine out emissions, and fuel consumption. The results obtained show how NOx−soot emissions can be reduced with a slight increment in fuel consumption by increasing the fraction of fuel mass injected at low-pressure top dead center and delaying the conventional injection at the same time. Unburned hydrocarbon emissions increase together with the fuel mass injected at the low-pressure top dead center, whereas CO emissions initially increase before being reduced.