Engine power is measured as input, indicated, brake and friction power

Input power

• The calorific valueThe amount of energy a fuel stores per unit volume (J m$^{-3}$) or per unit mass (J kg$^{-1}$). of a fuel is the amount of energy it stores per unit volume (or per unit mass).
• The flow rateThe volume (m$^3$ s$^{-1}$) or mass (kg s$^{-1}$) of fuel that flows per second into the engine. of fuel is the volume or mass that flows per second.
• The input power is:

$$\text{Input power} = \text{calorific value} \times \text{fuel flow rate}$$

• Crucially, the calorific value and flow rate must be in matching units: if one is in terms of mass, the other must also be in terms of mass.

Indicated power

• The indicated power is the power developed inside the cylinders of the engine. It depends on the number of cycles (strokes) per second.
• In a four-stroke engine, one complete cycle requires two revolutions of the crankshaft. So the number of cycles per second is:

$$\text{Number of cycles per second} = \frac{\text{revolutions per second}}{2}$$

• The indicated power is:

$$\text{Indicated power} = (\text{area of } p\text{-}V \text{ loop}) \times (\text{cycles per second}) \times (\text{number of cylinders})$$

• The area of the p-V loop represents the work done per cycle per cylinder.

Output (brake) power

• The brake power is the actual power output by the engine. It is the same as the rotational power:

$$P = T\omega$$

• Where $T$ is the torque (N m) and $\omega$ is the angular velocity (rad s$^{-1}$).

Friction power

• Part of the indicated power is used to overcome frictional forces within the engine. This means the brake power is always lower than the indicated power.

$$\text{Friction power} = \text{indicated power} - \text{brake power}$$