The Effects of Thick Thermal Barrier Coatings on Low-Temperature Combustion
An experimental study was conducted on a Ricardo Hydra single-cylinder light-duty diesel research engine. Start of Injection (SOI) timing sweeps from -350 deg aTDC to -210 deg aTDC were performed on a total number of five pistons including two baseline metal pistons and three coated pistons to investigate the effects of thick thermal barrier coatings (TBCs) on the efficiency and emissions of low-temperature combustion (LTC). A fuel with a high latent heat of vaporization, wet ethanol, was chosen to eliminate the undesired effects of thick TBCs on volumetric efficiency. Additionally, the higher surface temperatures of the TBCs can be used to help vaporize the high heat of vaporization fuel and avoid excessive wall wetting. A specialized injector with a 60° included angle was used to target the fuel spray at the surface of the coated piston. Throughout the experiments, the equivalence ratio, ϕ, was maintained constant at 0.4; the combustion phasing was consistently matched at 6.8 ± 0.4 deg aTDC. It can be concluded that the thick TBC cases achieved 1 to 2 percentage points improvement in combustion efficiency, and generally, a ~2 percentage points increase in indicated engine efficiency. It is also noticed that applying a dense top sealing layer to the TBC further improves the UHC emissions compared to the TBC coated piston with an unsealed surface. From the heat release analysis, it can be concluded that the TBCs have no significant impact on the heat release process and knock intensity while matching the combustion phasing; however, it reduces the intake temperature requirement by up to 20 K. The exhaust gas temperatures were expected to increase for the TBC cases, but the expected increase in exhaust temperature was not conclusive from the results observed in this study.