Abstract
This study is to clarify the effect of the optimum compression and expansion ratio control on the thermal efficiency by variable valve timings with a single-cylinder heavy duty diesel engine equipped with external supercharging system. Test was done in various combinations of effective compression and expansion ratio with three different pistons having different geometric compression ratios from 18.0 to 26.0. The results showed that improvement in the gross indicated thermal efficiency by increasing effective expansion ratio was still significant with the highest compression ratio, while brake thermal efficiency was deteriorated by the steeply increased mechanical and pumping losses. On the other hand, it was confirmed that the excessive exhaust heat loss caused by the reduction in heat loss from the wall was able to transform into work by adjusting effective expansion ratio larger than fixed effective compression ratio.
