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The effect of an active thermal coating on efficiency and emissions from a high speed direct injection diesel engine
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
This study looked into the application of active thermal coatings on the surfaces of the combustion chamber as a method of improving the thermal efficiency of internal combustion engines. The combination of low thermal conductivity and low heat capacity that these coatings are offering, reduces the temperature gradient between the hot combustion gases and the cooler surroundings during the engine cycle thus leading to lower heat transfer losses and increased piston work. The use of such coatings also results in increased exhaust temperatures which has the potential to improve the cold start performance of after-treatment systems. The active thermal coating was applied to a production aluminium piston and its performance was compared against a reference aluminium piston on a single-cylinder diesel engine. The two pistons were tested over a wide range of speed/load conditions and the effects of EGR and combustion phasing on engine performance and tailpipe emissions were also investigated. A detailed energy balance approach was also employed to study the thermal behaviour of the active thermal coating. In general, improvements in indicated specific fuel consumption were not statistically significant for the thermally coated piston over the whole test matrix. Further to that, a reduction in brake efficiency of up to 1.5 percentage points was observed, depending on test conditions, which contradicts published research. Changing the combustion phasing did not show any improvement in brake efficiency. Exhaust temperature showed a marginal increase with the thermally coated piston of up to 6 °C. However, the normalised exhaust enthalpy showed a reduction apart from the higher speed/load conditions when no EGR was applied. The heat transferred to the coolant was reduced by as much as 1.5 percentage points, in agreement with the expected reduction in piston heat transfer, across all operating conditions. Finally, soot emissions were increased with the thermally coated piston, with the biggest discrepancies observed at the lower speed/load conditions.