Today, the interest towards 2-stroke, opposed-piston compression-ignition engines is higher than ever, after the announcement of imminent production of a 2.7L 3-cylinder light truck engine by Achates Powers. In comparison to other 2-stroke designs, the advantages in terms of scavenge and thermal efficiency are indisputable: a perfect "uniflow" scavenge mode can be achieved with inexpensive and efficient piston controlled ports, while heat losses are strongly reduced by the relatively small transfer area. Unfortunately, the design of the combustion system is completely different from a 4-stroke DI Diesel engine, since the injectors must be installed on the cylinder liners: however, this challenge can be converted into a further opportunity to improve fuel efficiency, adopting advanced combustion concepts. This paper is based on a previous study, where the main geometric parameters of an opposed piston engine rated at 270 kW (3200 rpm) were defined with the support of CFD 1D-3D simulations. The current work will focus on the influence of an innovative combustion system, developed by the authors by means of further CFD-3D analyses, holding constant the boundary conditions of the scavenging process. The numerical study eventually demonstrates that an optimized 2-S OP Diesel engine can achieve a 10% improvement on brake efficiency at full load, in comparison to an equivalent conventional 4-stroke engine, while reducing in-cylinder peak pressures and turbine inlet temperatures.

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