The effect of piston design on hydrocarbon emissions in a spark ignited engine.

This thesis describes an investigation into the effect of piston design on hydrocarbon emissions from an spark ignited engine. The experimental investigation tested a series of three piston configurations against a standard design based on production dimensions. These tests examined the effect of top and 2nd land crevice volumes and absorption and desorption from lubricant on the cylinder liner as sources of hydrocarbon emissions. The operational conditions were steady state for all engine parameters. The work was performed on a modern four cylinder 16 valve engine with multi point fuel injection. Two fuels were used, a reference unleaded petrol and trimethyle pentane.The results have shown that significant reductions in hydrocarbon emissions can be achieved by certain design changes. Reducing the top land height from 6mm to 2.8mm reduced emissions by up to 25% and creating a smoother surface on the cylinder liner wall reduced emissions by up to 28%. A method of assessing residual lubricant on the cylinder walls was developed from surface finish measurement and showed that the smoother surface finish would retain less oil and reduce the amount of fuel absorbed by the oil. The 2nd land volume was a secondary source having an effect at low speed low load conditions.The hydrocarbon species were investigated with gas chromatography, the concentrations of these species were observed to change with each design, but not necessarily proportional to the total hydrocarbon emissions.A model was developed to allow the prediction of changes to hydrocarbon emissions by altering various piston design parameters. In addition to modelling mixture flow into piston crevice volumes and absorption of fuel by lubricant on cylinder walls a basic combustion analysis allowed the prediction of combustion gas temperatures and the end point of combustion. In-cylinder oxidation could then be approximated. Results from this model gave good agreement with experimental results and was then used to assess the optimum piston design to reduce hydrocarbon emissions.This research has demonstrated that component design and specification can be used to reduce hydrocarbon emissions from a spark ignited engine. The most significant parameters have been identified and methods of measurement developed. After considering current material and design constraints the dimensions for a low hydrocarbon emission piston was modelled and predicted 30% reduction in emissions.