Effects of Compression Ratio on Spark-Ignited Engine Efficiency

As CO₂ emissions standards continue to tighten, engine efficiency has jumped to the forefront of automotive engine focus. A proven way to realize efficiency gains is through the increase of engine compression ratio, yet the data available that quantifies this trend are more limited than one would expect. In this paper results from various experimental and simulation studies are compiled to quantify the effect of compression ratio on modern spark- ignited engine efficiency. Four studies are taken from research conducted at the Sloan Automotive Laboratory at MIT and three are from the recent literature. Compression ratios range between 8 and 13.4 in these studies, and gross indicated efficiency, net indicated efficiency, and brake efficiency were compiled. Curves of efficiency versus compression ratio are fit to the data points for each of the studies and normalized about a compression ratio of 10. Average curves for each of the three efficiency types across all data available show that increasing from a compression of 10 to 13 results in relative increases of 5.1% for brake efficiency, 4.6% for gross indicated efficiency and 4.5% for net indicated efficiency (at constant displaced volume). About two-thirds of this increase is realized by the compression ratio increase from 10 to 11.5. The standard deviations of these data sets are 1.8%.