Thermal barrier coatings in compression ignition engines: Analysis of combustion strategies and insights into convection vive

Thermal barrier coatings (TBCs) show promise to improve engine efficiency by reducing convection heat transfer losses through elevated surface temperatures. However, in mixing controlled combustion systems, experiments with TBCs often fail to produce efficiency benefits. It was previously hypothesized that this is due to high local heat fluxes from impinging jets causing local surface temperatures to become excessively high, enabling convection vive: exothermic reactions in the thermal boundary layer that increase the convection heat transfer coefficient. In this work, experiments were conducted with a low thermal effusivity TBC operating in either kinetically controlled combustion or mixing controlled combustion with ethanol. The results showed that at loads of 3, 6, and 10 bar IMEPg, the TBC provided an efficiency benefit of up to ∼1 percentage points (pp) in both combustion strategies. At 15 bar IMEPg, only the kinetically controlled combustion strategy showed an efficiency benefit of 0.3pp. The mixing controlled combustion strategy showed an efficiency penalty of 0.2pp with the TBC despite having roughly equivalent closed cycle heat transfer as an uncoated piston. Combined with an increased exhaust temperature, this was attributed to the following phenomenon: convection vive occurs durng the heat release process, increasing heat transfer. Following combustion, elevated surface temperatures reduce heat transfer losses. The total heat transfer is the same, but the change in heat transfer phasing reduces thermodynamic efficiency and results in higher exhaust losses. These results highlight that convection vive is a limiting factor for mixing controlled combustion systems with TBCs; their design should take place with convection vive in mind.