Crevice volume effect on spark ignition engine efficiency

Abstract

A set of experiments and a simulation study are completed to quantify the effect of the piston crevice on engine efficiency. The simulation study breaks down the loss mechanisms on brake efficiency at different displacement volumes (300 - 500 cc) and compression ratios (8-20). Experiments focus on indicated efficiencies for a narrow range of compression ratios (9.24- 12.57) with different piston crevice volumes. Piston crevice volume is increased in two steps by machining a groove into the piston top land, and is decreased by raising the top ring. Indicated efficiency is measured at various loads (0.4 - 1.0 bar MAP), speeds (1500, 2000, 2500 rpm), and coolant temperatures (50°C and 80°C). All data points compared in this study are recorded at MBT timing with a relative air-fuel ratio ([gamma]) of 1. For the baseline case (CR = 9.24, speed = 2000 rpm, coolant = 80°C), increased crevice volume results in an indicated efficiency degradation of 0.3-0.5%-points per 1000 mm3. This absolute decrease corresponds to a 1.2-1.5% relative decrease for a 100% increase in crevice volume; referenced to the control piston crevice modification. Decreasing crevice volume leads to a gain in indicated efficiency of 2.3-3.5%-points per 1000 mm3 , which corresponds to a 6.9- 11.8% relative increase for a 100% decrease in crevice volume; referenced to the control piston crevice modification. Results of the experimental investigation, when compared across compression ratio, engine speed, and coolant temperature, show that the crevice effect on efficiency is largely independent of these three parameters. Large gains from decreased piston crevice volume prompt renewed discussions on piston top land, top ring, and crown design.