Numerical study of the influence of diesel post injection and exhaust gas expansion on the thermal cycle of an automobile engine

This study deals with the development of a numerical tool developed to analyze the thermomanagement of the heat rejection from the fuel combustion in the case of a four cylinder 2 L – 110 HP direct injection Diesel engine. It is composed of two main elements: the first one simulates all the phenomena linked to the combustion, the second one is about thermal exchanges in the heart of the engine. We only deal with the first one here. The combustion study is based on two aspects: the consideration of multi injection (the pilot injection, the main injection and the post injection) and the simulation of compressible flows through the exhaust valves. This model allows us to predict the combustion chamber energy balance (heat fluxes, mechanical work) and the fuel consumption with an experimental tolerance within 3%. Engine tests have been made at the laboratory to validate this result. Fuel consumption calculation is linked to the accurate estimation of the exhaust gas flow and temperature. The main interest of this method is that it takes into account the sonic blockage at the exhaust valve opening. This work has also allowed to precise the post injection importance during the engine temperature rise. The increase in thermal rejects is used to accelerate the warm up of the different engine fluids.