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

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

Efficiency = (Useful output energy / Total input energy) x 100%

The useful output energy is the energy that is actually used to perform work, such as the energy used to move a car or power a generator. The total input energy is the energy that is supplied to the engine, such as the chemical energy in fuel or the electrical energy supplied to an electric motor.

To calculate the efficiency of an engine, you need to measure the useful output energy and the total input energy. For example, to calculate the efficiency of a car engine, you could measure the distance travelled and the amount of fuel consumed. The useful output energy would be the energy used to move the car, which is equal to the force applied to the wheels multiplied by the distance travelled. The total input energy would be the chemical energy in the fuel, which can be calculated using the energy density of the fuel and the amount of fuel consumed.

Once you have measured the useful output energy and the total input energy, you can plug these values into the efficiency formula and calculate the efficiency of the engine. For example, if a car travelled 100 km using 10 litres of fuel, and the energy density of the fuel is 34 MJ/L, then the total input energy is:

Total input energy = 10 L x 34 MJ/L = 340 MJ

If the force applied to the wheels was 5000 N and the car travelled at a constant speed of 60 km/h, then the useful output energy is:

Useful output energy = 5000 N x 60 km/h x 1000 m/km = 1.8 MJ

Using these values, the efficiency of the car engine can be calculated as:

Efficiency = (1.8 MJ / 340 MJ) x 100% = 0.53%

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