The efficiency of an engine is determined by dividing the useful output energy by the total input energy.

Efficiency serves as a measure of how effectively an engine transforms input energy into useful output energy. It is expressed as a percentage and can be calculated using the following formula:

Useful Output Energy refers to the energy that is actually utilized to perform work, such as the energy required to move a vehicle or power a generator. In contrast, Total Input Energy represents the energy supplied to the engine, which could be the chemical energy contained in fuel or the electrical energy delivered to an electric motor.

To compute the efficiency of an engine, you must measure both the useful output energy and the total input energy. For instance, to assess the efficiency of a car engine, one might record the distance traveled and the amount of fuel consumed. The useful output energy can be calculated as the product of the force applied to the wheels and the distance traveled. The total input energy, on the other hand, can be derived from the chemical energy present in the fuel, which is calculated using the energy density of the fuel and the total volume of fuel consumed.

Once you have obtained the values for both useful output energy and total input energy, you can substitute these values into the efficiency formula to determine the engine’s efficiency.

For example, consider a scenario where a car travels $100 \, \text{km}$ using $10 \, \text{litres}$ of fuel, with the energy density of the fuel being $34 \, \text{MJ/L}$. The total input energy can be calculated as follows:

If the force applied to the wheels is $5000 \, \text{N}$ and the car maintains a constant speed of $60 \, \text{km/h}$, the useful output energy is calculated as:

With these values, the efficiency of the car engine can be computed as follows:

Thus, the efficiency of the car engine in this example is approximately $0.53\%$.