Hot air engine
Encyclopedia
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...
which uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
into mechanical work
Mechanical work
In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...
. These engines may be based on a number of thermodynamic cycles encompassing both open cycle devices such as those of Sir George Cayley
George Cayley
Sir George Cayley, 6th Baronet was a prolific English engineer and one of the most important people in the history of aeronautics. Many consider him the first true scientific aerial investigator and the first person to understand the underlying principles and forces of flight...
and John Ericsson
John Ericsson
John Ericsson was a Swedish-American inventor and mechanical engineer, as was his brother Nils Ericson. He was born at Långbanshyttan in Värmland, Sweden, but primarily came to be active in England and the United States...
and the closed cycle engine of Robert Stirling
Robert Stirling
The Reverend Dr Robert Stirling was a Scottish clergyman, and inventor of the stirling engine.- Biography :Stirling was born at Cloag Farm near Methven, Perthshire, the third of eight children...
.
In a typical implementation, air is repeatedly heated and cooled in a cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
and the resulting expansion and contraction is used to move a piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
and produce useful mechanical work
Mechanical work
In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...
.
Definition
The term "hot air engine" specifically excludes any engine performing a thermodynamic cycleThermodynamic cycle
A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...
such as the Rankine cycle
Rankine cycle
The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...
, in which the working fluid undergoes a phase change. Also excluded are conventional internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
s in which heat is added to the working fluid by combustion of fuel within the working cylinder. Continuous combustion types such as George Brayton
George Brayton
George Brayton was born in Rhode Island, son of William H. and Minerva Brayton. He was an American mechanical engineer who lived with his family in Boston, and who is noted for introducing the continuous combustion process that is the basis for the gas turbine, and which is now referred to as...
's Ready Motor and the related gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
could be seen as borderline cases.
History
The expansive property of heated air was known to the ancients and Hero of AlexandriaHero of Alexandria
Hero of Alexandria was an ancient Greek mathematician and engineerEnc. Britannica 2007, "Heron of Alexandria" who was active in his native city of Alexandria, Roman Egypt...
's Pneumatica contains descriptions of devices which might be used to automatically open temple doors when a fire was lit on a sacrificial altar. Devices called hot air engines, or simply 'air engines', have been recorded from as early as 1699, around the time when the laws of gasses
Gas laws
The early gas laws were developed at the end of the 18th century, when scientists began to realize that relationships between the pressure, volume and temperature of a sample of gas could be obtained which would hold for all gases...
were first set out, and early patents include those of Henry Wood, Vicar of High Ercall near Coalbrookdale
Coalbrookdale
Coalbrookdale is a village in the Ironbridge Gorge in Shropshire, England, containing a settlement of great significance in the history of iron ore smelting. This is where iron ore was first smelted by Abraham Darby using easily mined "coking coal". The coal was drawn from drift mines in the sides...
Shropshire (English patent 739 of 1759) and Thomas Mead, an engineer from Sculcoats Yorkshire (English patent 979 of 1791), the latter in particular containing the essential elements of a displacer type engine (Mead termed it the transferrer). It is unlikely that either of these patents resulted in an actual engine and the earliest workable example was probably the open cycle furnace gas engine of the English inventor Sir George Cayley
George Cayley
Sir George Cayley, 6th Baronet was a prolific English engineer and one of the most important people in the history of aeronautics. Many consider him the first true scientific aerial investigator and the first person to understand the underlying principles and forces of flight...
c.1807
It is likely that Robert Stirling
Robert Stirling
The Reverend Dr Robert Stirling was a Scottish clergyman, and inventor of the stirling engine.- Biography :Stirling was born at Cloag Farm near Methven, Perthshire, the third of eight children...
's air engine of 1818 which incorporated his innovative Economiser patented in 1816 was the first to be put to practical work. The economiser, now known as the regenerator, stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency of Stirling's engine and should be present in any air engine that is properly called a Stirling engine
Stirling engine
A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
.
Thermodynamic cycles
A hot air engine thermodynamic cycleThermodynamic cycle
A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...
can (ideally) be made out of 3 or more processes (typical 4). The processes can be any of these:
- isothermal processIsothermal processAn isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir , and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir...
(at constant temperature, maintained with heat added or removed from a heat source or sink) - isobaric processIsobaric processAn isobaric process is a thermodynamic process in which the pressure stays constant. The term derives from the Greek isos, , and barus,...
(at constant pressure) - isometric / isochoric processIsochoric processAn isochoric process, also called a constant-volume process, an isovolumetric process, or an isometric process, is a thermodynamic process during which the volume of the closed system undergoing such a process remains constant...
(at constant volume) - adiabatic processAdiabatic processIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
(no heat is added or removed from the working fluid)- isentropic processIsentropic processIn thermodynamics, an isentropic process or isoentropic process is one in which for purposes of engineering analysis and calculation, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the...
, reversible adiabatic process (no heat is added or removed from the working fluid - and the entropyEntropyEntropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
is constant)
- isentropic process
- isenthalpic processIsenthalpic processAn isenthalpic process or isoenthalpic process is a process that proceeds without any change in enthalpy, H; or specific enthalpy, h....
(the enthalpyEnthalpyEnthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
is constant)
Some examples (not all hot air cycles, as defined above) are as follows:
Yet another example is the Vuilleumier cycle
Vuilleumier cycle
The Vuilleumier cycle is a thermodynamic cycle with applications in low-temperature cooling. In some respects it resembles a Stirling cycle or engine, although it has two "displacers" with a mechanical linkage connecting them as compared to one in the Stirling cycle. The hot displacer is larger...
.
See also
- Thermoacoustic hot air engine
- Carnot heat engineCarnot heat engineA Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824...
- Timeline of heat engine technologyTimeline of heat engine technologyThis Timeline of heat engine technology describes how heat engines have been known since antiquity but have been made into increasingly useful devices since the seventeenth century as a better understanding of the processes involved was gained...
External links
- Introduction to Stirling-Cycle Machines
- Pioneers in Air Engine Designs (Select the desired biography)
- Stirling Engine's Function Principle
- Project for making one at home
- Apparatus for the Method of Heat Differentiation Vuilleumier patent