A Stirling engine is a heat engine that operates by cyclic compression and
expansion of air or other gas (the working fluid) at different temperatures, such
that there is a net conversion of heat energy to mechanical work.More
specifically, the Stirling engine is a closed-cycle regenerative heat engine with a
permanently gaseous working fluid. Closed-cycle, in this context, means a
thermodynamic system in which the working fluid is permanently contained
within the system, and regenerative describes the use of a specific type of
internal heat exchanger and thermal store, known as the regenerator. The
inclusion of a regenerator differentiates the Stirling engine from other closed
cycle hot air engines.
Applications of the Stirling engine range from heating and cooling to underwater
power systems. A Stirling engine can function in reverse as a heat pump for
heating or cooling. Other uses include combined heat and power, solar power
generation, Stirling cryocoolers, heat pump, marine engines, low power aviation
engines, and low temperature difference engines.
• By definition a regenerator is a cyclic device. On the
first part of the cycle the hot gas flows through the
regenerator from the heater to the cooler, and in so
doing transfers heat to the regenerator matrix. This is
referred to as a "single blow". Subsequently during
the second part of the cycle the cold gas flows in the
reverse direction, absorbing the heat that was
previously stored in the matrix. Thus at steady state
the net heat transfer per cycle between the working
gas and the regenerator matrix is zero.
• The primary effect of regeneration in a Stirling engine
is to increase the thermal efficiency by 'recycling'
internal heat that would otherwise pass through the
engine irreversibly. As a secondary effect, increased
thermal efficiency yields a higher power output from a
given set of hot and cold end heat exchangers. These
usually limit the engine's heat throughput. In practice
this additional power may not be fully realized as the
additional "dead space" (unswept volume) and
pumping loss inherent in practical regenerators
reduces the potential efficiency gains from regenerator
DIFFERENT STAGES OF ALPHA AND BETA
red- hot region
blue- cold region
During process 1-2, the piston in the hot cylinder
moves from BDC to TDC due to heating of the gas.
Here piston in cold cylinder is fixed.
During process 2-3, the gas in hot cylinder moves
to cold cylinder through regenerator giving heat to
During process 3-4, the gas in cold cylinder gets
compressed and the piston here is powered by
During 4-1, the gas again flows from cold to hot
cylinder through regenerator absorbing the
previously given heat from it.
During process 1-2, the gas at cold end expands
Now as the gas is heated it pushes the power piston
to its maximum point and now the displacer starts
coming down and gas moves to cold end. This is 2-3 .
Now process 3-4 is compressed by power piston at
cold end due to the momentum of flywheel.After
compression the remaining gas flows to the hot end
thereby raising the displacer.
Here regenerator is placed in the gas path around
the displacer. Specifically it is in the position of the
displacer and moving, often as a volume of wire
Advantages of stirling cycle
Stirling engines can run directly on any available heat source, not just one produced by
Some types of Stirling engines have the bearings and seals on the cool side of the
engine, where they require less lubricant and last longer than equivalents on other
reciprocating engine types.
The engine mechanisms are in some ways simpler than other reciprocating engine
types. No valves are needed, and the burner system can be relatively simple. Crude
Stirling engines can be made using common household materials.
A Stirling engine uses a single-phase working fluid that maintains an internal pressure
close to the design pressure, and thus for a properly designed system the risk of
explosion is low.
In some cases, low operating pressure allows the use of lightweight cylinders.
They can be built to run quietly and without an air supply, for air-independent
propulsion use in submarines.
They start easily (though slowly) and run more efficiently in cold weather, in contrast
to the internal combustion, which starts quickly in warm weather, but not in cold
A Stirling engine used for pumping water can be configured so that the water cools the
compression space. This increases efficiency when pumping cold water.
They are extremely flexible. They can be used as CHP (combined heat and power) in
the winter and as coolers in summer.
Efficiency of stirling engine
ΔW12= -nRT2 lnV2/V1,
ΔW34= -nRT1 lnV1/v2
On the isothermal curves the change in inner
energy ΔU=ΔW+ΔQ is zero.
ΔQ = Q12+Q41 = nRT2
ln(V2/V1) + CV(T2-T1)
ΔW = W12+ΔW34= -nR(T2T1) ln V2/V1
( T2+CV(T2-T1)nR ln
1. Power and torque issues