Diesel engines already have very high compression ratios. Compression ratio directly correlates with the ratio of temperature at combustion to the temperature o

Diesel engines already have very high compression ratios. Compression ratio directly correlates with the ratio of temperature at combustion to the temperature outside the engine, which is the limiting factor in the Carnot efficiency. A gasoline engine will be able to get up to maybe 12:1 before knock is an issue, high-efficiency diesels can hit 25:1. Gas turbines can hit even higher ratios, but are far more complex & expensive (e.g. the GE GEnx-1B78/P2 turbofan has an overall compression ratio of 58 and thermal efficiency of 58%, but it has a list price somewhere around $20 million and is bigger than the entire truck cab).