Abstract
A free-piston engine is a novel internal combustion engine which has the advantages of a variable compression ratio and multi-fuel adaptability. This paper focuses on numerical simulation for combustion process and spray angle optimization of an opposed-piston free-piston engine. The working principle and spray-guided central combustor structure of the engine are discussed. A three-dimensional computational fluid dynamic model with moving mesh is presented based on the tested piston motion of the prototype. Calculation conditions, spray models, and combustion models were set-up according to the same prototype. The effects of spray angle on fuel evaporation rate, mixture distribution, heat release rate, in-cylinder pressure, in-cylinder temperature, and emissions were simulated and analyzed in detail. The research results indicate that the performance of the engine was very sensitive to the spray angle. The combustion efficiency and the indicated thermal efficiencies of 97.5% and 39.7% were obtained as the spray angle reached 40°.
Highlights
The internal combustion engine (ICE) has been widely used in the automotive and aviation fields since its birth in 19th century and has been widely used in people’s daily lives
Guo et al [11] indicated that the trends of the peak in-cylinder pressure versus the injection timing were identical with the convex function curve, and the trapezium fuel injection rate-profile is more appropriate and can keep high efficiency of a free-piston engine than other injection rate-profiles; they did not provide the parameters of the combustion process in detail
The spray process is one of the indispensable stages in the cylinder. It has a significant effect where m means the mass of droplets; c is the drag coefficient; T is the temperature of droplets; on the fuel atomization, evaporation and mixing process, so it directly determines the quality of the α represents the convective heat transfer coefficient through the film around the droplets; A is the combustion process
Summary
The internal combustion engine (ICE) has been widely used in the automotive and aviation fields since its birth in 19th century and has been widely used in people’s daily lives. The results indicated that higher injection pressure is favorable for better fuel atomization and better combustion This finding lacks tested results which should be compared with simulation results. Guo et al [11] indicated that the trends of the peak in-cylinder pressure versus the injection timing were identical with the convex function curve, and the trapezium fuel injection rate-profile is more appropriate and can keep high efficiency of a free-piston engine than other injection rate-profiles; they did not provide the parameters of the combustion process in detail. The spray angle is one of the key parameters which affect mixture formation and combustion efficiency of the engine, and it has influence on fuel economy and emission properties indirectly [17]. The results obtained in this paper provide a useful guidance for the improvement of the opposed-piston free-piston engine
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