Question

An engine is 40 % efficient. What is the meaning of this statement?

Answer 1

Hint: The efficiency of the engine is the ratio of output and input energy . This can also be interpreted as the what percentage of the input energy is utilised to do output work.

Complete step by step solution:
The meaning of the statement “ An efficiency of the engine is 40 percent “ is that only 40 percent of input energy is utilised to do output work. The ratio of output and input energy is 0.4. Or, in terms of percentage, it is 40 percent. Here 40 percent of input energy is utilised to do output work, remaining 60 percent is dissipated to the environment in the form of heat. Heat may arise due to the friction between the movable parts of the machine.

Note: If all the input energy is utilised to do output work, then the efficiency of the engine is 100 percent. But no engine or machine can be 100 percent efficient. Some waste heat is always produced in a heat engine. No engine is free of the effects of gravity. Also, friction is always present between the two movable parts of the machine even with the lubrication. So the output work is always less than the input energy. This is also consistent with the second law of thermodynamics which says that it is impossible for heat engines to achieve 100 percent thermal efficiency. The maximum efficiency of a heat engine is around 70 percent.