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Efficiency in heat engines is limited by the Carnot principle. (no pun intended) The greater the difference in temperature between the extremes in the system, the higher the percentage of the energy that can be extracted from the heat. So, even though most of the energy in gasoline winds up as waste heat, it is so degraded that further recovery is inefficient. A low-pressure steam engine, in turn, is bulky, and either quite intricate or high-maintenance. Steam-cleaned cylinders are hard to keep oiled and rust-free.
The best turbo-compound engine got up to 45% efficiency, trying to cool the exhaust but turbocharged ship diesels are getting over 50%, mostly by raising the peak temperature.
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