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A Stirling engine uses the Stirling cycle, which is unlike the cycles used in internal-combustion engines.

The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet.
The Stirling cycle uses an external heat source, which could be anything from gasoline to solar energy to the heat produced by decaying plants. No combustion takes place inside the cylinders of the engine.


The Stirling Cycle
The key principle of a Stirling engine is that a fixed amount of a gas is sealed inside the engine. The Stirling cycle involves a series of events that change the pressure of the gas inside the engine, causing it to do work.
There are several properties of gasses that are critical to the operation of Stirling engines:

If you have a fixed amount of gas in a fixed volume of space and you raise the temperature of that gas, the pressure will increase.
If you have a fixed amount of gas and you compress it (decrease the volume of its space), the temperature of that gas will increase.

There are four parts to the Stirling cycle. The two pistons in the animation above accomplish all of the parts of the cycle:

Heat is added to the gas inside the heated cylinder (left), causing pressure to build. This forces the piston to move down. This is the part of the Stirling cycle that does the work.
The left piston moves up while the right piston moves down. This pushes the hot gas into the cooled cylinder, which quickly cools the gas to the temperature of the cooling source, lowering its pressure. This makes it easier to compress the gas in the next part of the cycle.
The piston in the cooled cylinder (right) starts to compress the gas. Heat generated by this compression is removed by the cooling source.
The right piston moves up while the left piston moves down. This forces the gas into the heated cylinder, where it quickly heats up, building pressure, at which point the cycle repeats.

The Stirling engine only makes power during the first part of the cycle. There are two main ways to increase the power output of a Stirling cycle:

Increase power output in stage one - In part one of the cycle, the pressure of the heated gas pushing against the piston performs work. Increasing the pressure during this part of the cycle will increase the power output of the engine. One way of increasing the pressure is by increasing the temperature of the gas. When we take a look at a two-piston Stirling engine later in this article, we'll see how a device called a regenerator can improve the power output of the engine by temporarily storing heat.

Decrease power usage in stage three - In part three of the cycle, the pistons perform work on the gas, using some of the power produced in part one. Lowering the pressure during this part of the cycle can decrease the power used during this stage of the cycle (effectively increasing the power output of the engine). One way to decrease the pressure is to cool the gas to a lower temperature.

Displacer-type Stirling Engine
Instead of having two pistons, a displacer-type engine has one piston and a displacer. The displacer serves to control when the gas chamber is heated and when it is cooled. This type of Stirling engine is sometimes used in classroom demonstrations. You can even buy a kit to build one yourself!

In order to run, the engine (shown in the picture) requires a temperature difference between the top and the bottom of the large cylinder. In this case, the difference between the temperature of your hand and the air around it is enough to run the engine.


The power piston - This is the smaller piston at the top of the engine. It is a tightly-sealed piston that moves up as the gas inside the engine expands.
The displacer - This is the large piston in the drawing. This piston is very loose in its cylinder, so air can move easily between the heated and cooled sections of the engine as the piston moves up and down.
The displacer moves up and down to control whether the gas in the engine is being heated or cooled. There are two positions:
When the displacer is near the top of the large cylinder, most of the gas inside the engine is heated by the heat source and it expands. Pressure builds inside the engine, forcing the power piston up.
When the displacer is near the bottom of the large cylinder, most of the gas inside the engine cools and contracts. This causes the pressure to drop, making it easier for the power piston to move down and compress the gas.


Why Aren't Stirling Engines More Common?
There are a couple of key characteristics that make Stirling engines impractical for use in many applications, including in most cars and trucks.
Because the heat source is external, it takes a little while for the engine to respond to changes in the amount of heat being applied to the cylinder -- This means that it takes time for the heat to be conducted through the cylinder walls and into the gas inside the engine

The engine requires some time to warm up before it can produce useful power.
The engine can not change its power output quickly.
These shortcomings all but guarantee that it won't replace the internal-combustion engine in cars. However, a Stirling-engine-powered hybrid car might be feasible.

In thermodynamics as a point of view, this type of engines depends on the fact of ENTROPY GENERATION which says that the heat exhusted to the sink which is necessary to complete the cycle can be used to increase the internal energy of the working fluid at earlier steps, but there is always exists a rejected portion of the exhusted heat otherwise the cycle won't be completed accordind to the 2nd law of thermodynamics.1

It will be also better for you to check the internal combustion engine cycle from any thermodynamics book to have a better idea about what i am talking about.1

There are many other engines which works on the same principleof ENTROPY GENERATION but the application among them is the only difference. e.g. Ericsson Cycle or Engine

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