Influence of thermal piston insulation on the indicated efficiency of SI-engines

Abstract

High efficiency gasoline combustion engines with up to 50 % indicated efficiency are the target of current research work. To reach this goal, it’s essential to reduce all the combustion engine losses as much as possible. One major loss, with about 30 % of the total fuel energy, is the wall heat transfer loss. To reduce this loss, one solution is a temperature swing insulation, which is characterized by a low thermal conductivity and at the same time a low volumetric heat capacity. In order to analyze the thermal efficiency potential of different insulation materials, a thermal insulation model for one-dimensional engine process simulations has been developed and is presented in this paper. The insulation is discretized by a thermal node model and is coupled to an engine process simulation using the simulation tool GT-Power. The heat transfer and temperature swing behavior of the thermal insulation model is validated against 3D-CFD engine cycle simulations. A predictive detailed chemistry combustion model is used in 3D-CFD simulations to evaluate the influence of thermal insulations on the knocking tendency. Two different thermal insulation materials are investigated using the developed simulation model. One is yttria stabilized zirconia (YSZ) as a common thermal sprayed coating. The second material is produced by an electrolytic oxidation of the piston surface (anodizing). With thermal piston insulation, there is only a small increase in thermal efficiency in the range of 0.2 % to 0.6 % possible with both materials, whereby the potential increases at low speeds and medium loads. The reason can be found in the relative heat losses, which increases with a constant center of combustion and increasing load. An efficiency loss analysis shows that the exhaust losses increase by about two third of the reduced heat losses. Due to its lower volumetric heat capacity and similar heat conductivity, the anodized piston surface shows a thermal efficiency advantage compared to YSZ. The simulation results are finally validated by measurements conducted with a single-cylinder research engine for an YSZ coated piston surface as well as a hard anodized piston surface.