SOLUTION

Thermal efficiency is the ratio of how effectively an engine converts a fuel’s heat energy into usable work. Each fuel has a certain amount of heat or thermal energy. The energy of the fuel has the potential to produce a certain amount of work when burned in the engine. This thermal efficiency is a theoretical value. 

Diesel fuel has more heat energy than coal and gasoline. Diesel engines also have higher compression ratios. Both of these are the reason why diesel gets better fuel economy than gasoline engines and steam engines. The efficiency of the diesel-electric locomotive (26 to 30%) is more than that of the steam locomotive (6 to 8%) and less than that of an electric locomotive (34 to 38%).