Stirling Engines Explained
As you look at a simple Stirling engine, you may wonder how on earth it works.
How can it provide any sort of power from such a small amount of heat?
Could it be perhaps something like your hand or a hot coffee cup that will drive it?
What you need to understand is that the Stirling Engine works by using the differential of air pressure (or some other sort of gas) to drive the power piston along its stroke, usually by some sort of displacement system.
This simple Low-Temperature Stirling Engine is basically a large, lightweight block of something like polystyrene being lifted and lowered by the crankshaft, moving it away from the heated bottom plate towards the cooler top plate before returning to its original position
As this movement takes place, air (or perhaps some other lightweight gas) trapped in the large bottom cylinder, is displaced towards the other plate with a temperature difference.
As the volume of air in the bottom cylinder remains the same, the pressure must increase or decrease as it nears the opposite plate. This drives the power piston either up or down as the pressure changes, which in turn lifts or lowers the polystyrene displacement piston, causing the crankshaft to rotate. For this style of engine to work there needs to be a relatively heavy flywheel used compared to the remainder of the engine.
Other types of Stirling Engine use naked flames heating a metal or glass cylinder in much the same way, but because of the extra heat involved, the heated part and the displacer piston can be much smaller.
And as usual with this type of engine, your eyes and mind are automatically attracted to the heat source and the heated end of the engine, as it is a heat engine after all and the power comes from the heat, doesn’t it?
To explain this better, with most engine cylinders of this type sitting horizontally, the term for the top of the stroke versus the bottom is basically this power piston sits in a cylinder with one end sealed (the top) and the other end open to the atmosphere (the bottom) nearer the crankshaft.
It’s best if you visualize this as a cylinder sitting above a crankshaft, much like the old, uncomplicated, single-cylinder motorbikes used to be.
The power source is provided partially by atmospheric air pressure driving a piston along its stroke towards the top by having reduced air (or gas) pressure above the piston, akin to how a vacuum cleaner, if attached to the top of the cylinder would suck the piston upwards along its stroke.
To accomplish this, the piston in the power cylinder must be a very close fit to its cylinder wall but with virtually no friction between the two. No oil can be added between the piston and cylinder wall as that would simply put a brake on piston travel.
At the same time, the pressure above the piston needs to be reduced as much as possible to help draw that piston along its stroke as there is no way atmospheric pressure can be adjusted without effort too easily.
So we now have the power piston moving along to the top of its stroke from reduced air (or gas) pressure above the piston and greater atmospheric pressure below the piston!
Variable Pressures Required For Stirling Engines
To produce a variable pressure above the piston so that the power piston can move down the cylinder again, this cylinder must have a connection to another variable cylinder capacity, again sealed with a close-fitting piston set at a different position on the crankshaft so that the fixed amount of air (or gas) between the two cylinders can be transferred each way between the cylinders as the crankshaft rotates.
Understandably, you would expect everything to remain in a neutral position if they were set at 180 degrees apart if it was left like that, but here’s where the HOT end comes into action.
There needs to be an interaction between the two cylinders to move the power piston down its stroke.
The Hot end, or cylinder, is basically a steel, iron, or glass tube with a relatively loose-fitting piston displacing the air trapped in the hot cylinder, not really compressing it, but transferring the volume to the power cylinder.
This Hot end is called a displacer cylinder, generally, and to accomplish this transfer of air from one cylinder to the other, the piston needs to be a close fit to its own cylinder at the crank end of the cylinder so that virtually no air escapes to the atmosphere with the heated air having its pressure raised above atmospheric pressure.
At the same time, the displacer piston needs to allow heated air to pass along its outside before being directed to the power cylinder.
As you may think, you pass the heated air to the power cylinder, which is cooler, which shrinks the volume of compressed air, which in turn draws the power piston towards the top of its stroke.
Confusing, to say the least???
Setting the timing right between the two pistons is required, but this is not critical just to get the engine running, provided it is near enough 90 degrees apart with the flywheel (on this engine) pin driving the con rod leading in the rotation of the crank, meaning the displacer cylinder needs to act first to transfer heated air into the power cylinder with the power piston’s con rod and pin following on by 90 degrees.
This pushes the power piston down its stroke and once the displacer piston begins to return to the bottom end of its stroke it reduces the air pressure minimally in the heated cylinder while normal air pressure is forcing the power piston towards the top of its stroke and returning the cooled air to be reheated again.
Now, if you’ve got your head round that, you will find heat has to be added to the heated end of the displacer cylinder for around two minutes from cold, sometimes less, but if you spin the flywheel over, you will find it gradually starts to provide some compression (resistance) as the engine warms up, until, you will find it gradually starts to try running on its own.
A mere ten seconds or so later, your engine will burst into life and continue running while the heat source is there.
Take it away and the residual heat within the engine will keep it running for some time – this is dependant on any friction involved, and simply adding the heat once again before it stops turning over will gradually boost its speed once again.
You may have guessed, this type of engine is dependant on the heat supplied to keep it running, but if you are using something like a small candle, you will never get this engine going as it would not provide enough heating.