Stirling engine is a heat engine that is vastly different from
engine in your car. Invented by Robert Stirling in 1816,
the Stirling engine has the potential to be much more
efficient than a gasoline or diesel
engine. But today, Stirling engines are used only in some
very specialized applications, like in submarines
or auxiliary power generators for yachts, where quiet
operation is important. Although there hasn't been a
successful mass-market application for the Stirling engine,
some very high-power inventors are working on it.
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.
There are hundreds of ways to put
together a Stirling engine. In this edition of HowStuffWorks,
we'll learn about the Stirling cycle and see how two different
configurations of this engine work.
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.
Let's go through each part of the
Stirling cycle while looking at a simplified Stirling engine.
Our simplified engine uses two cylinders. One cylinder is
heated by an external heat source (such as fire), and the
other is cooled by an external cooling source (such as ice).
The gas chambers of the two cylinders are connected, and the
pistons are connected to each other mechanically by a linkage
that determines how they will move in relation to one another.
There are four parts to the Stirling cycle. The two pistons
in the animation above accomplish all of the parts of the
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 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
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
This section described the ideal Stirling cycle. Actual
working engines vary the cycle slightly because of the
physical limitations of their design. In the next two
sections, we'll take a look at a couple of different kinds of
Stirling engines. The displacer-type engine is probably the
easiest to understand, so we'll start there.
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 above 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
the figure above, you can see two pistons:
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.
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
The engine repeatedly heats and cools the gas,
extracting energy from
the gas's expansion and contraction.
Next, we'll take a look at a two-piston Stirling engine.
Two-piston Stirling Engine In this engine,
the heated cylinder is heated by an external flame. The cooled
cylinder is air-cooled, and has fins on it to aid in the
cooling process. A rod stemming from each piston is connected
to a small disc, which is in turn connected to a larger
flywheel. This keeps the pistons moving when no power is being
generated by the engine.
The flame continually heats the bottom cylinder.
In the first part of the cycle, pressure builds, forcing
the piston to move to the left, doing work. The cooled
piston stays approximately stationary because it is at the
point in its revolution where it changes direction.
In the next stage, both pistons move. The heated piston
moves to the right and the cooled piston moves up. This
moves most of the gas through the regenerator and
into the cooled piston. The regenerator is a device that can
temporarily store heat -- it might be a mesh of wire that
the heated gasses pass through. The large surface area of
the wire mesh quickly absorbs most of the heat. This leaves
less heat to be removed by the cooling fins.
Next, the piston in the cooled cylinder starts to
compress the gas. Heat generated by this compression is
removed by the cooling fins.
In the last phase of the cycle, both pistons move -- the
cooled piston moves down while the heated piston moves to
the left. This forces the gas across the regenerator (where
it picks up the heat that was stored there during the
previous cycle) and into the heated cylinder. At this point,
the cycle begins again.
You might be wondering why there are no mass-market
applications of Stirling engines yet. In the next section,
we'll take a look at some of the reasons for this.
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 -- it takes time
for the heat to be conducted through the cylinder walls and
into the gas inside the engine. This means that:
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
car might be feasible.
For more information on Stirling engines and related
topics, check out the links on the next page!