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

The proportion of output to input energy determines an engine's efficiency. This may also be seen as the proportion of incoming energy that is used to produce work.

According to the definition of "the efficiency of the engine is 40%," only 40% of the energy input is used to produce work. The energy intake to output ratio is 0.4. Or it is 40%, expressed as a percentage. Here, 40 percent of the energy intake is used to produce work, while the remaining 60 percent is lost as heat to the environment. Friction between the machine's moveable parts may cause heat to develop.

The engine is said to be 100% efficient if all input energy is used to produce work. However, no machine or engine can operate at 100% efficiency. A heat engine will always create some waste heat. The effects of gravity are present in every engine. Additionally, even with lubrication, there is always friction between the two moving elements of the machine. As a result, the output work is never equal to the input.  A heat engine's maximum efficiency is around 70%.