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The loss mechanisms associated with engine downsizing, boosting and compression ratio change are assessed. Of interest are the extents of friction loss, pumping loss, and crevice loss. The latter does not scale proportionally with engine size. These losses are deconstructed via a cycle simulation model which encompasses a friction model and a crevice loss model for engine displacement of 300 to 500 cc per cylinder. Boost pressure is adjusted to yield constant torque. The compression ratio is varied from 8 to 20. Under part load, moderate speed condition (1600 rpm; 13.4 Nm/cylinder brake torque), the pumping work reduces significantly with downsizing while the work loss associated with the crevice volume increases. At full load (1600 rpm; 43.6 Nm/cylinder brake torque), the pumping work is less significant. The crevice loss (normalized to the fuel energy) is essentially the same as in the part load case. The sensitivities of the respective loss terms to downsizing are reported.

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.

The effects of piston top-land crevice size on the indicated net fuel conversion efficiency are assessed in a single cylinder SI engine with 465 cc displacement and 11.2 compression ratio. The operating conditions are at 3.6 and 5.6 bar net indicated mean effective pressure (NIMEP), and at 1500 and 2000 rpm speeds. The cold crevice volume is varied from 524 mm3 to 1331 mm3 by changing the top land height from 3 to 7 mm, and by changing the top-land clearance from 0.247 to 0.586 mm. For a 100 mm3 increase in the top land crevice volume (estimated hot value), the indicated net fuel conversion efficiency decreases by 0.1 percentage point at 1500 rpm, and by 0.13 percentage points at 2000 rpm. The results are not sensitive to the two NIMEP values tested. These values are consistent with a simple crevice filling and discharge/oxidation model.