Impact of thermal swing piston top land coatings on gasoline engine performance and raw emissions

Upcoming legislations aim to significantly reduce carbon dioxide and hydrocarbon emissions compared to current regulations. Accordingly, options have to be evaluated to not only convert the raw emissions in a catalytic converter, but also to reduce the raw emissions from the combustion process in the first place. Therefore, thermal piston top land coatings, which lead to an oscillating piston surface temperature were investigated on a single cylinder research engine using a gasoline RON95E10 fuel. Measurements were conducted at a compression ratio of 12.2 and a long Miller intake event. In this manuscript the results achieved with yttria stabilized zirconia in comparison to an uncoated piston are displayed. Significant effects of the coatings on the indicated efficiency could be observed mainly at low engine speeds and loads due to the high share of fuel energy in the wall heat losses and incomplete combustion. At these operating points, the reduction of wall heat losses can almost be fully transferred into indicated efficiency, leading to an increase in indicated efficiency of 6.3% by the yttria stabilized zirconia coating. At higher engine speeds and loads, the advantage in indicated efficiency vanishes and a decrease of 0.5% can be observed. Near full load operation the indicated efficiency slightly lower. However, the effects of a thermal swing coating on combustion efficiency and wall heat losses strongly depend on the operation conditions.