Question 7: What is meant by heat engine? What is its main purpose? How is its efficiency defined?

ANSWER

Heat Engine

Heat engine is a device that converts heat energy into mechanical energy.

Construction/Parts of heat engine

A heat engine consists of the following parts.

1.       Heat source or heat reservoir

Heat reservoir is a large source of heat energy at a higher temperature T₁. It is also called High Temperature Reservoir (HTR). Its temperature remains practically constant during any in or out transfer of heat.

2.       Heat sink or Cold Reservoir

It is a large reservoir at a lower temperature T₂. It is also called Low Temperature Reservoir (LTR). Its temperature also practically remains constant during any in/out flow of heat.

3.       Working Substance

The working substance, usually a gas, undergoes a cyclic process (ΔU = 0). During this process it absorbs some heat Q₁ from HTR, uses it to do some work on the surrounding, and heat Q₂ is rejected to the LTR.

Working Principle

The working substance extracts (absorbs) heat Q₁from the HTR, do some work ΔW on the surrounding by utilizing part of it and rejects heat Q₂ to the LTR. The process is carried out cyclically, i-e, ΔU = 0

Purpose of heat engine

Main purpose of heat engine is to utilize heat energy and convert it into useful work.

Efficiency of Heat engine

The efficiency η of a heat engine is defined as the ratio of its output to the input.
Where output is the work done (ΔW) and input is the heat absorbed, Q₁.
Therefore,

As Q₂/Q₁always has some positive value, therefore, η is always less than 1. Therefore, in terms of % efficiency, the η is always less than 100%.
This shows it is impossible to construct a 100% efficient heat engine.
Alternatively, we can say it is impossible to construct a heat engine which will produce no effect other than extracting heat from a reservoir and the performance of an equal amount of work. This is in fact the Second Law of Thermodynamics.