Effects of Combustion Phasing, Relative Air-fuel Ratio, Compression Ratio, and Load on SI Engine Efficiency 2006-01-0229

In an effort to both increase engine efficiency and generate new, consistent, and reliable data useful for the development of engine concepts, a modern single-cylinder 4-valve spark-ignition research engine was used to determine the response of indicated engine efficiency to combustion phasing, relative air-fuel ratio, compression ratio, and load. Combustion modeling was then used to help explain the observed trends, and the limitations on achieving higher efficiency. This paper analyzes the logic behind such gains in efficiency and presents correlations of the experimental data. The results are helpful for examining the potential for more efficient engine designs, where high compression ratios can be used under lean or dilute regimes, at a variety of loads.

Extensive data from this study, across a wide range of engine operating conditions, show that the well-known loss of Net Indicated Mean Effective Pressure (NIMEP; the ratio of net work per cycle to cylinder volume displaced per cycle), with spark retard varies with operating conditions, mostly from variations in burn durations. However, a combustion phasing parameter, here termed “combustion retard”, which represents the shift of the crank angle for 50% mass fraction burned from the optimal angle, was found to correlate with high accuracy all the changes in indicated torque output.

At the baseline compression ratio of 9.8:1, as the engine was operated under mid-load and increasing relative air-fuel ratio, the efficiency curve versus dilution showed two distinct regimes. Through the first regime, efficiency increased with dilution until it peaked at a certain relative air-fuel ratio (range 1.5 to 1.6). Beyond this peak efficiency ratio began a second regime characterized by a falling efficiency due to increasing combustion duration and variability. Modeling and data analysis were used to investigate the contributions of pumping losses, mixture composition (ratio of specific heats), heat loss, burn durations, and combustion variability to the overall efficiency trend. It was determined that the leveling off in efficiency at high air-fuel ratios is due to a lengthening of burn duration beyond a critical value (10-90% burn angle of 30 degrees). Increasing compression ratio increases flame speed, extending the air-fuel ratio for peak efficiency an additional 0.1 lambda. Increasing combustion variability only affects the downward slope in efficiency at high air/fuel ratios. Increasing load extends the peak efficiency to leaner conditions.

Above a compression ratio of 9.8:1, relative mid-load net efficiency improvement is about 2.5% per unit compression ratio. Efficiency peaks at a compression ratio of about 15:1 with a maximum benefit of 6-7%. Efficiency improves more with compression ratio at high speeds and loads due to the reduced importance of heat loss. Wide-open throttle indicated torque at MBT spark timing behaves similarly to mid-load efficiency, with a maximum benefit of 8-9% at a 14:1 compression ratio. These data are particularly useful considering the limited available publications containing consistent compression ratio effect data for a wide range of operating conditions.

Relative net efficiency improvement from increasing load is about 6% per bar net indicated mean effective pressure at mid-load. About 80% of the improvement is from reduced pumping losses and 20% is from heat loss becoming a smaller portion of the overall charge energy. Correlations of efficiency with load are also presented.