L4 - Efficiency & Power

objectives

Work

• Engines and motors do work by making things move.
• Petrol and diesel engines spend the energy stored in their fuel.
• Electric motors spend energy supplied by a battery or generator.
• The human body is also a form of engine. It spends the energy stored in food.

Efficiency

• The chart above shows the efficiencies of some typical engines and motors.
• The low efficiency of fuel-burning engines is not due to poor design.
• When a fuel burns, it is impossible to transform its thermal energy into kinetic (motion) energy without wasting much of it.

Power

As energy and power are related, there is another way of calculating the efficiency of an engine:

Power Problems

Example 1: The crane on the right lifts a 100 kg block of concrete through a vertical height of 16 m in 20 s. If the power input to the motor is 1000 W. what is the efficiency of the motor?

Example 2* The car on the right has a steady speed of 30 m/s. If the total frictional force on the car is 700 N, what useful power output does the engine deliver to the driving wheels?