Optimization of combustion noise and thermal efficiency in diesel engines over a wide speed and load operational range

Pre-mixed diesel combustion has the potential of offering high thermal efficiency with low emissions; however, this may result in loud combustion noise because of the high maximum rate of pressure rise. Combustion noise and thermal efficiency work in a trade-off relation, and it has not been possible to achieve high thermal efficiency with low combustion noise, so far. Our laboratory has worked on combustion noise simulations calculated from the heat release history, and it is now possible to calculate a heat release shape for high thermal efficiency with low combustion noise. In this article, the objective of the research is the reduction of combustion noise by multiple fuel injections with high indicated thermal efficiency for a wide range of engine speeds and loads. The engine employed in the simulations and experiments is a supercharged, single-cylinder direct-injection diesel engine, with a high-pressure common rail fuel injection system. The heat release is approximated by Wiebe functions, and the combustion noise and indicated thermal efficiency are calculated in simulations. The engine operational range was divided into 12 conditions, four engine speed conditions each at three engine load conditions, and the optimum heat release shape for low combustion noise with high indicated thermal efficiency was calculated by a genetic-based algorithm method. The parameters for the genetic-based algorithm simulation were the number of injections, each injection timing, the heating value in each heat release, and the combustion period of each injection. The optimum heat release shape is a delta triangle (Δ)-shaped heat release (the heat release increase in the expansion stroke) with a high degree of constant volume for all conditions; however, the optimum number of heat releases and the injection timing are different depending on the engine speed and load conditions. The simulated results were confirmed by engine tests.