Experimental Investigation of Anodic Oxidation Coatings to Reduce Wall Heat Losses on a Diesel Engine

Theoretical investigations of combustion chamber coatings in order to reduce wall heat losses predicted in the past as well as currently a reduction of fuel consumption and an increase of exhaust gas temperature. However, these effects could only be confirmed by measurement in exceptional cases. During this investigation, two sealed anodic oxidation coatings “plasma electrolytic oxidation” and “sealed hard anodizing” were evaluated according to the current state of the art.
At the beginning of the investigation, the potential of these coatings was assessed by using various simulation tools. According to the material properties, the coating “plasma electrolytic oxidation” provides the highest potential. A simulation of a piston crown coating with a thickness of 60 µm with today's established 0D methods results in a reduction of fuel consumption of 4.3 g/kWh and an increase of exhaust gas temperature of 9.2 K at an operating point with high engine load. The experimental investigation of coated pistons on the single-cylinder research engine, showed no measurable effects that would indicate an actual insulating effect of the coatings. Despite the use of high-precision measuring technology the simulation results could not be confirmed. The measurement data was statistically analysed in order to avoid neglecting neither the measurement accuracy nor the reproducibility of the results. The only verifiable difference between the test series that had been recognized was that with the coating “sealed hard anodizing” a slightly lower air delivery ratio was measured. But this effect is not beneficial for the engine operation. A final analysis suggests that the heat transfer between working gas and combustion chamber wall seems to change significantly when a coating is applied. An in-depth research of the basic mechanisms of heat transfer in the boundary layer is necessary to clarify these circumstances