Heat Engines & Heat Pumps

A Heat Engine is a device that uses heat to perform work.  Some heat engines you may be familiar with are steam engines and car engines (internal combustion engines).  See the Otto Cycle here.

• Heat leaves a high temperature reservoir and enters the heat engine.
• Some of this heat is converted into work.
• The remaining heat energy is leaves the heat engine and enters a low temperature reservoir.

The efficiency of a heat engine is a measure of how much Input Heat leaves the engine as output Work.

Efficiency = Work Leaving Engine / Heat Entering Engine

e  = W / Q_H

If a heat engine was 100% efficient that would mean that all the Heat entering the engine would leave the engine as work.  If you can build such an engine you are sure to be a rich and famous individual!  (Bill Gates will be a pauper compared to you.) 

According to the law conservation of energy the amount of heat entering the engine, Q_H must equal the amount of work,W and heat, Q_L leaving the engine.

Q_H  = W + Q_L

e  = ( Q_H  - Q_L ) / Q_H

A Heat Pump is a device that is the reverse of a heat engine.  Refrigerators and air conditioners are two common types of heat pumps.