Heat engine

From Exampleproblems

In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred to the sink from the source, and in this process some of the heat is converted into work.

1 Everyday examples
2 Engineering and physical concepts
3 Efficiency
4 Heat engine processes
5 See also
6 References
7 External links

Everyday examples

Examples of everyday heat engines include: the steam engine, the diesel engine, and the gasoline (petrol) engine in an automobile. All of these familiar heat engines are powered by the expansion of heated gases. The general surroundings are the heat sink, providing relatively cool gases which when heated, expand rapidly to drive the mechanical motion of the engine.

Engineering and physical concepts

Examples of heat engines:

Vapor power cycles. In these cycles and engines the working fluids are gases and liquids:
- Rankine cycle (steam engine)
- Regenerative cycle Are more efficient than Rankine cycle.
Gas power cycles. In these cycles and engines the working fluid are always like gas:
- Carnot cycle (Carnot heat engine)
- Brayton cycle or Joule cycle (gas turbine)
- Ericsson Cycle
- Stirling cycle (Stirling engine)
- Internal combustion engine (ICE):
  - Otto cycle (eg. Gasoline/Petrol engine, high-speed diesel engine)
  - Diesel cycle (eg. low-speed diesel engine)
  - Atkinson Cycle
  - Lenoir cycle (eg pulse jet engine)
Direct Conversion
- thermoelectric (Peltier-Seebeck effect)
- thermionic emission
Refrigeration (a refrigerator is a heat pump: a heat engine in reverse. Work is used to create a heat differential.)

Efficiency

The efficiency of a heat engine relates how much useful power is output for a given amount of heat energy input.

From the laws of thermodynamics:

$E_W \ = \ E_H \ - \ E_C$

where

E W

is the useful energy from the engine.

E H

is the heat energy taken from the high temperature system

E C

is the heat energy delivered to the cold temperature system

In other words a heat engine absorbs heat energy from the high temperature heat source, converting part of it to useful work and delivering the rest to the cold temperature heat sink.

The efficiency of a given heat engine is defined by:

$e = \frac{E_W}{E_H} = 1 - \frac{E_C}{E_H}$

The theoretical maximum efficiency of any heat engine depends only on the temperatures it operates between. This efficiency is usually derived using an ideal imaginary heat engine such as the carnot heat engine, although other engines using different cycles can also attain maximum efficiency. This efficiency is:

$e = \frac{T_h - T_c}{T_h} \equiv 1 - \frac{T_c}{T_h}$

where $T h$ is the absolute temperature of the hot source and $T c$ that of the cold sink, usually measured in kelvins.

The reasoning behind the proof of this theorem relates to the laws of thermodynamics. It is first assumed that if a more efficient heat engine than a Carnot engine is possible, then it could be driven in reverse as a heat pump. Mathematical analysis can be used to show that this assumed combination would result in a net decrease in entropy. Since no exceptions have ever been found to the Laws of Thermodynamics (which require that entropy for a closed system never decreases) it is concluded that it is not possible to build a heat engine more efficient than a Carnot Cycle engine.

Empirically, no engine has ever been shown to run at a greater efficiency than a Carnot Cycle heat engine.

Heat engine processes


Cycle/Process	Compression	Heat Addition	Expansion	Heat Rejection
Carnot	adiabatic	isothermal	adiabatic	isothermal
Otto (Petrol)	adiabatic	isometric	adiabatic	isometric
Diesel	adiabatic	isobaric	adiabatic	isometric
Brayton (Jet)	adiabatic	isobaric	adiabatic	isobaric
Stirling	isothermal	isometric	isothermal	isometric
Ericsson	isothermal	isobaric	isothermal	isobaric

Each process is one of the following:

isothermal (at constant temperature, maintained with heat added or removed from a heat source or sink)
isobaric (at constant pressure)
isometric/isochoric (at constant volume)
adiabatic (no heat is added or removed from the working fluid)