Reduction of Hydrocarbon Emissions from SI-Engines by Use of Carbon Pistons

The use of pistons made of fine grain carbon was investigated in a spark-ignition engine within a European Community funded research project (TPRO-CT92-0008). Pistons were designed and manufactured and then tested in a single cylinder engine. Due to the carbon material's lower coefficient of thermal expansion the top land clearance between piston and cylinder can be reduced by a factor of three in comparison to standard aluminium designs. Under steady-state part-load operating conditions the emission of unburned hydrocarbons can be reduced by more than 15% compared to aluminium pistons, without significant penalties in NOx-emissions. Simultaneously, a small improvement in fuel economy of about 2% is observed. At full-load blow-by leakage flow is reduced by more than 50%. The piston crown temperature is about 30 °C higher with the carbon piston than with the standard aluminium piston, due to the lower thermal conductivity of the carbon material. Under cold-start and warm-up conditions the reduction in hydrocarbon emissions can be as high as 28%, depending on the piston design. For maximum benefit the carbon piston crown has to be coated, in order to close the pistons porosity.