The term '2-stroke' is used to describe this type of engine because the piston passes through two strokes (from the top of the cylinder to the bottom, then back to the top) to complete all the phases of operation. The 4-stroke engine's piston has to pass through four strokes or two complete revolutions of the crankshaft, before it begins to repeat itself.

The 2-stroke engine doesn’t need valves (although some use them), but instead use ports, the inlet, the exhaust and the transfer ports. The rest of the engine should be familiar to us all. There is a cylinder, in which the piston reciprocates. The piston is connected to the crankshaft. The top of the cylinder is sealed by the cylinder head which also houses the combustion chamber and the glow plug (except for diesels). The bottom of the cylinder is sealed by the piston. The bottom half of the engine, the crankcase, is sealed and connected to the cylinder via a transfer port.

The Inlet Port, to which the carburettor is connected feeds into the crankcase. There are two common methods by which this is done. The inlet port either begins in the front housing and travels down the centre of the crankshaft, or is mounted on the back plate (rear induction). The rear induction types use a disc or drum valve assembly to seal this port closed at the appropriate times.. The front induction use the rotation of the specially designed (timed) crankshaft to serve as a valve.

The Exhaust Port connects the cylinder to the type of muffler/pipe fitted (if any). It is open and closed by the movement of the piston.

The Transfer Ports are passages through which the crankcase gasses flow to the cylinder. They feed into the cylinder opposite to the exhaust port and are positioned so that their top edge is approximately the same height as the centre of the exhaust port. These ports are opened and closed by the movement of the piston.

All model 2-stroke engines are lubricated by oil mixed with the fuel. Some common variations in designs are, Ball bearings, shaped piston crowns (top), rings on the piston, etc. In most cases the cylinder consists of a sleeve mounted within the engine block, and are air cooled.

Now to its operation. The engine has two chambers in which different events occur at the same time ...

The piston moves up the cylinder, it closes the transfer port and increases the volume of the crankcase. This produces a low pressure in the crankcase which allows atmospheric pressure outside of the engine to force air through the carburettor (picking up fuel and oil) into the crankcase. When the piston moves down, the crankcase volume decreases, so the air/fuel/oil mix is pressurised, being trapped within by which-ever inlet valve system is used. ... See diagram below

The above diagram is a ‘normal’ set of timings figures for a high performance / racing engine.

A ‘sports’ engine is likely to have timings such as Exhaust = 150^o, Transfers 115^o and intake 40 - 55^o.

As the piston approaches the bottom of its stroke it passes and exposes the transfer port allowing the pressurised air/fuel/oil charge to flow into the cylinder. When the piston next moves up, it closes the transfer port, followed by the exhaust port. Further movement of the piston compresses the charge in the combustion chamber. Due to the compression and the glow plug, the temperature of the charge rapidly rises to ignition temperature, especially in the vicinity of the glow plug where combustion begins.

As combustion occurs, sufficient heat is released to cause rapid expansion of the gases which provide the force required to push the piston down. Most of this energy is spent by the time the piston has moved down far enough to expose the exhaust port. When it does so the still very hot gases rush out through this port leaving a low pressure area within its wake. This low pressure area in the cylinder then aids the flow of the fresh charge from the transfer port(s) when opened.

Ignoring the kinetic energy in the crank shaft etc, all the force experienced by the piston on each down stroke originates from the expanding gases, created by the combustion process. The amount of expansion force exerted is dependent on the following factors;

The amount of air entering the cylinder during induction.

The type of fuel.

The amount of fuel.

The ratio of fuel and air.

The amount of swirl in the combustion chamber.

The temperature of the air entering the cylinder.

The degree of vaporisation of the fuel.

The amount of compression of the charge before ignition.

The rate of heat loss to cylinder and combustion chamber walls.

The temperature of the combustion chamber during compression.

The rate of combustion.

The time of ignition relative to piston position and movement.

Please note; the effectiveness of transferring this energy to power is then determined by the amount of drag imposed on the system, i.e. good bearings, fits and clearances etc.

We have all heard that the greater the compression ratio an engine has the greater the power produced, and this is true. There are limiting factors, and the most important of these is the fuel. The compression ratio of an engine is calculated by adding the volume of the cylinder (that volume through which the top of the piston passes when travelling from one end of its stroke to the other) (swept volume), to the volume of the combustion chamber (above the piston when at the top of its stroke), then dividing the answer by the combustion chamber volume. It could also be described as... all the volume above a piston at the bottom of the stroke, divided by all the volume above a piston at the top of it’s travel / stroke..

Compression Ratio = Swept Vol. + Clearance Vol. / Divided by Clearance Vol.

The greater the compression ratio, the more the air/fuel mixture is compressed before the desired ignition time. The more the air/fuel is compressed the greater the amount of heat generated by compression. The real danger here is that the fuel will reach self ignition temperature and explode (known as detonation) rather than burn. On automobile engines it is common practise to lower compression ratios when fitting turbo charges and the like, to prevent the resultant pressure increase (and therefore temperature increase) from raising fuel temperature to self ignition temperature.

Compression ratios can be increased by fitting a head with a smaller combustion chamber, or shaving some material from the bottom of the head. They can be decreased by fitting two head gaskets (the easiest) , fitting another head with a larger combustion chamber, physically increasing the size of the existing combustion chamber or using a shorter reach glow plug.

Detonation may be eliminated by reducing compression ratio, decreasing the temperature of the cylinder head and walls, using a cooler glow plug, using a fuel with higher octane rating, etc. Detonation can have serious effects on an engine. These are;

Overheating

Pitting to burning of the head and/or aluminium piston

Rapid wear to big end, gudgeon and crankshaft bearings

Excessive strain on gudgeon pin, big end journal and con rod.

Blown glow plugs.

Many of the larger engines now finding favour with large scale models etc use a timed spark ignition process rather than the conventional glo plug system. Most of these engines are 2 strokes and operate on a petrol based fuel, although they can (often with modification to the carburation) operate on a methanol based fuel .... for more power but are far more expensive to run.

Diesels are similar in all respects except for the fuel ignition and fuel. The ignition is based on using a low flashpoint temperature fuel and high compression to ignite the fuel. Generally the engine features a ‘contra-piston for the adjustment of the compression ratio. No glo plug etc is used.