Reduction of heat transfer and heat load in internal combustion engines

Licentiate thesis, 2019

Thermal insulation of the combustion chamber in an engine has great potential to reduce fuel consumption and CO2 emissions. Research on thermal barrier coatings (TBC) has been performed since the early eighties to address this potential. However, reported results for engine efficiency improvements show a large spread and there is no consensus on the actual benefits of the application of TBCs. Recent work indicates that a high surface roughness, typical for many TBCs, increases fuel consumption.

The purpose of the work described in this thesis was to make an accurate assessment of the indicated efficiency and to quantify the effect of surface roughness for two representative thermal barrier coatings applied on the piston crown in a single cylinder light duty diesel engine. Cylinder pressure data and measured heat losses to the piston cooling oil formed the basis for the evaluation.

A robust and automated measurement method was developed and combined with statistical modeling of the data. This approach increased the precision of the results and made it possible to separate the effect of different factors like surface roughness, compression ratio and coating application.

The compression ratio is an important variable in the analysis of the cylinder pressure data. However, it was not possible to measure the volume of the coated pistons accurately due to the porosity of the coatings. A method to determine the compression ratio from motored pressure traces was found in literature and further developed. This unique method does not require an estimation of the heat losses and is therefore especially suited for pistons with thermal insulation.

The indicated efficiency was measured with a 95% confidence interval of ±0.1 percentage point. Analysis of the results showed that the typically high surface roughness of plasma sprayed zirconia and anodized alumina increases fuel consumption by up to 1%. The high surface roughness enhanced heat losses and delayed combustion. The coatings themselves reduced heat losses to the combustion chamber walls and increased exhaust enthalpy, but they did not improve indicated efficiency.