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The 2-Stroke Engine

operation of 2-stroke engine, as used in motorbikes, chain saws, strimmers and stuff!

In "Suck; Squash; Bang; Blow!", I introduced and explained the Otto cycle, the principle on which the internal combustion engine works, and in "The 4-Stroke Engine", I showed it in operation, in a 'real' engine, with this neat little animation.

The 4-stroke reciprocating piston engine, illustrates the Otto-cycle very clearly, each of phase of the cycle completed in a separate stroke of the piston, in it's cylinder, as it 'reciprocates' up and down, attached to a crank-shaft.

The piston makes four 'strokes' to complete one cycle, and everything happens above the piston, and is relatively easy to understand.

The 2-Stroke engine, is  a bit different. Same things are going on; it still works on the principle of the Otto cycle, and there are still four stages to the combustion cycle, only it does it all in just two strokes of the piston, rather than four.

And it does this by utilising the other end of the cylinder, closed by the crank-case, underneath the piston.

As the piston rises, so that chamber is getting bigger, while the combustion chamber is getting smaller.

Make sense?

Right, well, its not quite as simple as all that. In some ways the 2-Stroke engine is less complicated than the 4-stroke, because there are fewer moving parts, but the way that it works means that the actual phases of the Otto cycle don't co-inside quite as conveniently with the piston strokes, so it can take a bit to wrap your head round.




1 2 3 4
Suck Squash Bang Blow
Induction Compression Power Exhaust
Charge (fuel & air) drawn into the engine Charge put under pressure Charge ignited, and allowed to burn, releasing energy Burned charge expelled from the engine, ready for a fresh cycle to begin
In the first stage of induction, charge is sucked into the crank-case by the piston rising in the cylinder. In the second stage of induction, the charge, under pressure in the crank-case is allowed into the cylinder via the transfer port, opened by the piston passing beneath a window in the cylinder wall The piston rises, compressing the charge above the piston At the top, the charge is ignited, and the burning gases push the piston back down again At the bottom, the piston passes beneath the window in the cylinder wall, opening the exhaust port, letting the exhaust gasses out under their own pressure.

And colour coded and labelled so you can identify the bits, we have:-

A) Spark Plug
See article "the Spark Ignition system", The spark plug sets fire to the charge in the cylinder at the beginning of the power stroke.
B) Cylinder Head
The cylinder head is the 'lid' on the engine, and holds the charge and everything in the cylinder. In the 2-stroke engine it does not have any valves in it, the charge comes in, and the exhuast gets out through windows or 'ports' in the wall of the cylinder, towards the bottom, where they are effectively opened and closed by the piston rising and falling over them.
C) Transfer Port
The 2-stroke has a passage between the crank case and combustion chamber, opened and closed by the passing of the piston as it rises and falls, allowing the charge into the combustion chamber from where its sucked into the crank-case.
D) Piston
The piston is a plug to seal the bottom of the cylinder; but its a clever plug that can move up and down, allowing the volume in the cylinder to change. Connected to the crankshaft via the con-rod, it converts the 'pressure' made by combustion and turns it into force to push the crank-shaft around. It usually has sealing or 'piston rings' around it to let it seal against the cylinder without being too tight a fit.
E) Crank-Case / engine 'block'
The 'block' is the casing of the engine, and supports all the other bits that go round, do work or move about. The main chamber, where the crank-shaft sits is called the crank-case, the chamber that the piston goes up and down in is called the barrel. Car engines generally have the crank-case and barrel cast in one piece of metal, which is then referred to as the 'block'. Motorbike engines often have the barrels cast separately from the crank cases. But designs vary, and some engines have the barrel cast integrally with the cylinder head. But, the important thing is that there is some structure, holding everything else together, and providing the enclosed space inside which combustion takes place, hence the 'internal' combustion engine.
G) Connecting Rod / Con-Rod
The con rod, is a rod, with a hinge at either end. It connects the piston to the crankshaft, and transmits the forces from the crankshaft to the piston or the piston to the crank, and in conjunction with that, allows the reciprocating, up and down motion of the piston to be converted to rotary motion on a shaft.
H) Inlet Port
A hole in the cylinder head, controlled by the inlet valve to allow the 'charge' into the engine.
J) Exhaust Port
A hole in the cylinder head controlled by the exhaust valve, to allow the exhaust gasses out of the engine
K) Combustion Chamber / Cylinder
The combustion chamber is usually the space in the cylinder head, above the cylinder when the piston is at the top of its stroke. The cylinder is the space beneath that, which the piston goes up and down in. They are both different regions of the big hole in the engine block, that's sealed by the cylinder head and the valves, where the 'action' happens!

Confusing isn't it. But, that's how the operations of the engine align with the phases of the otto-cycle. And its cloudy because we some of its happening under the piston, and some of its happening above it, and in actual fact, we have two combustion cycles being done at the same time.

Let me put it differently:

In the top of the engine; piston rises, compressing a charge that has been pumped into the cylinder when it was at the bottom Still in the top of the engine, when the piston reaches the top of its stroke, the compressed charge is ignited, pushing the piston back down again Still in the top of the engine; when the piston gets down the bottom, the exhaust gasses rush out, and a fresh charge is squashed in
Mean while in the bottom of the engine, while the charge in the cylinder is being compressed; a new charge is being sucked into the crank case ready for the next cycle Then, when the pistons on its way down under the power of the combustion in the cylinder above, the charge in the crank case is being compressed ready to be transfered to the cylinder for the next combustion cycle When the piston gets to the bottom of the stroke, and the exhaust gasses are rushing out of one side, on the cylinder above, the transfer port has been opened and the compressed charge in the crank case is squeezed from underneath to over the top.

so, the 'business' is mainly going on over the top of the piston, and is basically three pictures; the compression and power strokes as the 4-stroke engine, but the induction and exhaust omitted by using the pumping of the piston in the crank case to pump the charge in, and simply letting the exhaust gasses escape of their own accord, between the two strokes.

Make sense? Well, have a look at an animation of it. Might make it easier.

As with the 4-stroke, there are variations and permutations; in fact they are a lot more common on two stroke engines.

As with the 4-stroke, though, it's name is derived from the fact that only two strokes of the piston are required to complete one combustion cycle.

2 strokes of the piston

1 'cycle' of combustion

(1 revolution of the crank-shaft)

But, as with the 4-stroke, it is possible to connect the piston in other ways, and the crank shaft might turn a different number of revolutions, depending on how its connected.

It is also possible, to dispense with the crank case induction, and use separate supercharger to squash the charge into the cylinder when the piston is at the bottom of its stroke.

And some variations on the 2-stroke, use valves.

In the picture sequence, the flow of charge into the crank case is controlled by the piston 'skirt' as it passes windows in the cylinder. This is the simplest way to 'valve' a 2-stroke, and is simply known as 'piston ported'. In the animation, the flow of charge into the crank case is shown controlled by a valve.

Two common types of valve used are the reed valve; a flap opened and closed by the pressure differential between the crank case and the atmosphere, opening when there's suck, and closing when there's squash.

The other is a variation on piston porting, where a disc on the end of the crank shaft has windows in it, which pass windows in the crank case, and when they windows align, charge can flow, when they don't, it wont.

The permutations are quite infinite, and there have been 2-strokes that used mechanically operated 4-stroke type over-head valves, rather than relying on piston porting; and there is a curious arrangement that had some popularity in the 1930's & 50's called the 'split single', which was a twin cylinder 2-stroke, with the inlet ports in one cylinder, the exhaust ports in another and a tunnel in the cylinder head between the combustion chambers of each.......

I don't want to over complicate things or go into too much unnecessary detail. Most 2-stroke engines, tend to use crank case induction, and they tend to use piston porting, though high performance versions may use disc or reed valving for the induction to the crank case.

One feature of the 2-stroke worth mentioning is that because the charge is swept through the crank case, its difficult to lubricate the bearings of the crank shaft, and con-rod.

Consequently, 2-strokes normally employ a 'total loss' lubrication system, where a little oil is added to the fuel, so that it will lubricate things as it passes through, before being burned with the fuel.

This is not good for efficiency, or emissions, but then neither is the manner of getting the charge in, and exhaust out of the engine. But, because they can burn a charge every revolution, rather then every other revolution, and they don't have so many moving parts, they do have the potential to make a lot of power from their capacity and weight.

I want to mention one last thing about 2-strokes, and that is the expansion chamber exhaust, because many 2-stroke engines wont work without one.

This is a cleverly designed pipe with a varying section, so that as the gasses pass through it, it creates a pressure wave that is reflected back at the exhaust port.

As the animation shows, this pressure wave has the effect of pushing gas back into the cylinder; and the idea is to time that pressure wave so that it pushes back any fresh charge that's escaped through the exhaust port whilst the transfer and exhaust ports are both open, as they must be, governed as they are by the height of the piston in relation to the windows in the wall of the cylinder.

The 'Power Valve' or similar systems used on many Japanese motorbikes with various acronyms, are all refinements of this principle, using valves to mask the port or chambers in the exhaust, to change the resonance effects governing this pressure wave, and so make it more useful.

I have not mentioned the exhaust in relation to the 4-stroke, as it doesn't play a crucial role in the engine's working. On a 2-stroke, though, where the business of getting the charge in and the exhaust out of the engine is not positively 'pumped'  as it is in a 4-stroke, the engine often relies on pressure differences, timings and harmonics just to actually function, and the exhaust crucially effects the engines operation and performance, and it is often absolutely essential to the engine's operation.

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