The application of ceramic and catalytic coatings to reduce the unburned hydrocarbon emissions from a homogeneous charge compression ignition engine

An experimental and theoretical study of the effect of thermal barriers and catalytic coatings in a Homogeneous Charge Compression Ignition (HCCI) engine has been conducted. The main intent of the study was to investigate if a thermal barrier or catalytic coating of the wall would support the oxidation of the near-wall unburned hydrocarbons. In addition, the effect of these coatings on thermal efficiency due to changed heat transfer characteristics was investigated. The experimental setup was based on a partially coated combustion chamber. The upper part of the cylinder liner, the piston top including the top land, the valves and the cylinder head were all coated. As a thermal barrier, a coating based on plasma-sprayed Al₂O₃ was used. The catalytic coating was based on plasma-sprayed ZrO₂ doped with Platinum. The two coatings tested were of varying thickness' of 0.15, 0.25 and 0.6 mm. The compression ratio was set to 16.75:1. All regulated emissions were measured while operating the engine over a range of equivalence ratios, inlet temperatures and fuels. It was found that the unburned hydrocarbon (HC) emissions decreased and carbon monoxide (CO) emissions increased with increasing air/fuel ratio. Lower emissions of HC and CO were found in the case where a thin thermal barrier was used. All other setups resulted in higher emission levels and lower thermal efficiency. Since the thermal boundary layer is colder than the bulk of the charge, the local heat release is delayed close to the walls. The effect of the coatings has been examined by engine testing and a simple heat-transfer analysis. It can be concluded that the thermal boundary layer is effected by the coatings and that the delay is decreased with increasing coating thickness. Catalytic coatings proved to have a negative effect on unburned hydrocarbon emissions through catalytic flame quenching.