Heat Engines

Heat engine is a device by which a system is made to undergo a cyclic process that results in conversion of heat to work.

(1) It consists of a working substance–the system. For example, a mixture of fuel vapour and air in a gasoline or diesel engine or steam in a steam engine are the working substances.

(2) The working substance goes through a cycle consisting of several processes. In some of these processes, it absorbs a total amount of heat Q1 from an external reservoir at some high temperature T1.

(3) In some other processes of the cycle, the working substance releases a total amount of heat Q2 to an external reservoir at some lower temperature T2.

(4) The work done (W ) by the system in a cycle is transferred to the environment via some arrangement (e.g. the working substance may be in a cylinder with a moving piston that transfers mechanical energy to the wheels of a vehicle via a shaft).

The basic features of a heat engine are schematically represented in figure below.The cycle is repeated again and again to get useful work for some purpose.


If Q1 is the heat input i.e., the heat absorbed by the system in one complete cycle and W is the work done on the environment in a cycle. In a cycle, a certain amount of heat (Q2) may also be rejected to the environment. Then, according to the First Law of Thermodynamics, over one complete cycle

W = Q1 – Q2 and efficiency can be represented as

For Q2 = 0, η = 1, i.e., the engine will have 100% efficiency in converting heat into work. The First Law of Thermodynamics i.e., the energy conservation law does not rule out such an engine. But experience shows that such an ideal engine with η = 1 is never possible, even if we can eliminate various kinds of losses associated with actual heat engines.

There is a fundamental limit on the efficiency of a heat engine set by an independent principle of nature, called the Second Law of Thermodynamics.

Related Posts

Internal Energy
Zeroth law of thermodynamics
Thermodynamics Introduction
Heat transfer by radiation
Heat transfer by convection
Heat transfer and conduction
Heat and Temperature