Mabee Inused the wrong term. My definition of thermal efficiency is the number of BTUs of heat turned into useable HP as a ratio to the amount of BTUs in a pound, or gallon, of fuel.
One hp is 42.375 btu/minute, or 2542.47 btu/hour. Diesel fuel has something like .13 or .14 million BTU per US gallon. That is 130000-140000 PTU per US Gallon. At 100% efficiency, an engine running on this fuel would produce (140,000/2542.5)= roughly 55 HP for one hour on one american gallon.
If the engine only produces 27.5 HP for an hour, or 55 hp for 30 minutes on a gallon, it is 50% efficient - and since the only place the rest of the BTUs go is producing heat and noise, the thermal efficiency is just SLIGHTLY under 50%
I do not have the BTU content of gasoline right at my fingertips - but it is lower per gallon - closer on a per lb basis - Gasoline has an SG of roughly .74, and Diesel about .84, so gasoline is LIKELY about
123,000 BTU per gallonA gasoline engine converts about 15% of the fuel it burns into power to move the car, and another small percentage to run power steering, friction losses in the drivetrain, running AC and charging system, etc. Overall - something around 23-25% "efficiency" The base 15% is closer to 24% in a diesel, with the actual "efficiency pushing 30%
You are talking more of cooling efficiency, I think - and cooling efficiency increases when operating temperatures increase, because the "Delta T" between the coolant and the outside air is higher. This is NOT what I was talking about.