Overall, thermal and mechanical efficiency each measure a different aspect of engine performance

Three types of efficiency

• The efficiency of an engine can be measured in three different ways: overall, thermal and mechanical.
• Each one compares a different pair of power quantities from the previous topic (input, indicated and brake power).

Overall efficiency

• The overall efficiency compares the useful power output (brake power) with the total power input from the fuel:

$$\text{Overall efficiency} = \frac{\text{brake power}}{\text{input power}}$$

• This is also equal to the product of the thermal and mechanical efficiencies:

$$\text{Overall efficiency} = \text{thermal efficiency} \times \text{mechanical efficiency}$$

Thermal efficiency

• The thermal efficiency tells us how well the engine converts the chemical energy in the fuel into useful power inside the cylinders:

$$\text{Thermal efficiency} = \frac{\text{indicated power}}{\text{input power}}$$

• This depends on the thermodynamic cycle and the properties of the fuel. Not all of the fuel's energy can be converted into work because of heat losses and incomplete combustion.

Mechanical efficiency

• The mechanical efficiency tells us how much of the indicated power is actually delivered as useful output:

$$\text{Mechanical efficiency} = \frac{\text{brake power}}{\text{indicated power}}$$

• This depends on the amount of energy lost to friction within the moving parts of the engine. The key part is that the difference between the indicated and brake power is the friction power.