Modern diesel engines have about 50% thermal efficiency.
Just sayin'.
Closer to 40 than 50 and only when running at optimum RPMs and doesn’t account for transmission losses and other loads not related to moving the vehicle such as pollution control.
Diesel locomotives are closest to ideal efficiency as the engine is not mechanically locked to the wheels letting them run at optimum RPMs and reduce transmission loss.
There are some power stations that can get about 50% efficiency but it requires space age materials to handle the heat as the efficiency is directly related to gas temp and ambient temp as described by Carnot.