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.
