Thermal Efficiency: Thermal efficiency expresses the mechanical energy that is delivered to the vehicle relative to the energy content of the fuel. In the typical engine, 35% of the energy that is available in the fuel is lost as heat to the coolant and lubricating oil, 40% is lost as heat and unburned fuel in exhaust gases, and another 5% is lost in engine and drivetrain friction. This means that
only about 20% is available to drive the vehicle and accessories. These percentages vary somewhat with operating conditions but are valid on the average.