One of the differences can be found in the amount of heat loss. Experiments in Wisconsin might have been performed at higher coolant temperatures, which can result in lower cylinder wall heat loss. Furthermore, the slight difference in engine speed may have a small influence, but mainly the hardware of the base engine is expected to contribute to differences in thermal efficiency. A second significant contribution to a difference in thermal efficiency can be the amount of blow by of the base engine. This blowby has to be further investigated if high absolute efficiencies are desired to be achieved in future experiments.