An Investigation into the Impact of Cylinder Count on the Efficiency of Opposed Piston Two-Stroke Compression Ignition Engines

<div>The gas exchange process of opposed piston two-stroke (OP2S) diesel engines is primarily driven by the pressure differential between the intake and exhaust, making them susceptible to cylinder-to-cylinder crosstalk, and therefore to cylinder count. This study examined how cylinder count influences brake efficiency in OP2S engines. Using an experimentally validated 1D engine model, three architectures, ranging from two to four cylinders, were created and simulated across their full operating ranges. To isolate the impact of cylinder count, all configurations employed identical cylinder and port geometries, and identical but scaled electrically assisted turbocharger based airpaths. The engines were also controlled to consistent trapped conditions at a given operating condition, resulting in comparable closed-cycle efficiencies. Comparisons were then made using both scaled electrified airpaths and by assuming isentropic airpath work, to assess the impact of airpath efficiency on the results. With electrified airpaths, the two- and four-cylinder architectures had approximately 4.1%_rel and 2.2%_rel lower brake efficiencies, respectively, than the three-cylinder configuration on average. Additionally, the three-cylinder engine was found to be less sensitive than the other architectures to airpath efficiency, as on a per-cylinder basis it had up to a 17% lower power requirement for the turbocharger compressor, and recovered up to 3% less energy from the turbocharger turbine. These trends were also present when assuming isentropic airpath work, with the magnitude of the efficiency penalty of the two- and four-cylinder architectures reducing to 0.8%_rel and 0.6%_rel, respectively due to the lower overall magnitude of airpath power requirements. In all cases, the dominant contributor to the above results was the differing scavenging characteristics of the engines due to cylinder-to-cylinder interactions, demonstrating that cylinder count has a measurable impact on OP2S efficiency, and should be a key factor in designing an efficient OP2S engine.</div>