The work presented here seeks to compare different means of providing scavenging systems for an automotive 2-stroke engine. It follows on from previous work solely investigating uniflow scavenging systems, and aims to provide context for the results discovered there as well as to assess the benefits of a new scavenging system: the reverse-uniflow sleeve-valve.
For the study the general performance of the engine was taken to be suitable to power a medium-duty truck, and all of the concepts discussed here were compared in terms of indicated fuel consumption for the same cylinder swept volume using a one-dimensional engine simulation package. In order to investigate the sleeve-valve designs layout drawings and analysis of the Rolls-Royce Crecy-type sleeve had to be undertaken.
A new methodology for optimization was developed and the analysis process also took into account work done by the charging system, this being assumed to be a combination of supercharger and turbocharger to permit some exhaust waste heat recovery.
As a result of this work it was found that the opposed-piston configuration provides the best attributes since it allows maximum expansion and minimum heat transfer. It gave net specific fuel consumption results which were 9.6% lower than the loop-scavenged engine (which was marginally the worst of the configurations investigated). The other uniflow systems were next, with the reverse sleeve valve being the most promising (3.4% better than the loop-scavenged engine).
Furthermore, although the general performance of the loop-scavenged configuration was closer to the other designs than was initially expected, it was found to be compromised by its requirement to have intake and exhaust ports at the same height in the cylinder, thus lengthening the gas exchange events for any given angle-area and consequently reducing the effective (or trapped) compression and expansion ratios. This was despite the use of a charge trapping valve to provide asymmetric port timing and minimize charge short-circuiting, the adoption of which was felt to be a factor in its better-than-expected performance. Finally, the reverse-loop-scavenged poppet-valve type was found to be so compromised by breathing and valve train kinematics that it was not taken to a full optimization.
For the opposed-piston engine, once the port timing obtained by the optimizer had been established, a supplementary study was conducted looking at the effect of relative phasing of the crankshafts on performance and economy. This was found to have a small effect on fuel consumption for a significant change in compression ratio, suggesting that, if available, variable crankshaft phasing could be a very important control actuator for gasoline compression ignition in such an engine.
Importantly, it was found that existing experiential guidelines for port angle-area specification for loop-scavenged, piston-ported engines using crankcase compression could also be applied to all of the other scavenging types, this having been done here in order to provide a starting point for the work. This important result has not been demonstrated before for such a wide range of architectures. The optimizer employed then allowed further improvements to be made over the starting point. The paper therefore presents a fundamental comparison of scavenging systems using a new approach, providing insights and information which have not been shown before.