Tech Talk
Two-Stroke Engine Theory

The Two-Stroke Engine

by Brian Pollard

Three moving parts, no valves, high revving and lots of speed...
The two stroke engine is designed using a very simple concept - "less is more!"

The engine has been seen in many variants including air cooled, the simplest form of engine, liquid cooled with the more complex problems of heat transfer, and with multi cylinders, with, again, the heat transfer problems.

The two stroke engine consists of... Throughout the engine joints of suitable gasket material are made to contain the engines gases and fluids. The engine is lubricated by the use of oil, added to the fuel before use. This oil contaminates the fuel and causes excessive unburnt fuel to be apparent on start up and whenever the load, or demand, is placed upon the engine.

The process of induction has been covered with the use of various rotary valves alongside the conventional piston port engines.

Piston port engines have carburettors, which meter the fuel/air, and suck a mixture of petrol and air into their crankcase, compress it primarily and transfer the partially compressed mixture, via transfer ports, to the combustion chamber, where secondary compression takes place as the piston moves up the bore.

The compressed mixture is ignited by the spark plug flashing at just the right moment and the consequence is an internal explosion, within the combustion chamber, that drives the piston down with a great force and this force is translated into rotary motion by the crankshaft.

Any waste gases hanging around after combustion are used to 'plug' the exhaust port for the next inlet stroke.

Reed valves, so called because they resemble a row of reeds bending in the breeze, are sometimes used to slightly control the flow of air/fuel, at carburettor/cylinder barrel joint, during the two stroke cycle. The principle of the reed valve is a spring steel flap opening when inlet pressure demands and closing when inlet pressure is low. The reed valve works very hard due to the high revs per minute that a two stroke engine works at. They work well but are prone to failure if they are asked to constantly open and shut at too high a frequency during use.

Rotary valves are more efficient but introduce a further moving part and its associated problems. The rotary valve is simply a disc with a hole in it, at the precise point necessary to introduce fuel/air mixture directly into the crankcase for primary compression. For this type of induction the carburettor and disc valve are mounted on the side of the crankcase.

The two stroke engine rotates internally through 360 degrees, naturally. One complete stroke consists of the piston starting at the top of its cycle, moving down the cylinder on the induction/primary compression stroke. After reaching the bottom of its stroke it proceeds to move up the cylinder throughout the secondary compression/exhaust stroke. This is an oversimplification of the two stroke cycle due to the fact that during each rotation of the engine many things are happening within the engine in terms of gas flow. For instance, during the downward path of the piston the gases in the crankcase are being compressed and are moving through the transfer ports up into the combustion chamber. During the upward movement of the piston the gases are being drawn into the crankcase via the inlet port and the exhaust gases are expelled into the exhaust pipe. The frequency of the gases introduced and expelled during each cycle is a major factor in two stroke design. The inlet and exhaust gases are moving at speeds designed to cause pressure waves. These waves assist the filling of the crankcase and the extraction of the waste gases. The exhaust system is 'tuned' to assist in this filling and extraction process. It is sometimes referred to as an expansion box or chamber. A rule of thumb is to create an expansion box approximately twelve times greater capacity than the cylinder volume. The shape of the expansion box should encourage gases to move quickly. This is only a starting point and much work is done in tuning an exhaust system for maximum performance. A tale pipe changed by cutting one inch off its length, or by welding a washer with a smaller hole in it to the tail pipe exit, can dramatically alter the performance of the engine. When tuning an engine always think of the engine as a total measurement from the main jet in the carburettor to the tip of the exhaust pipe. Adjusting this length will alter performance.

All two stroke engines have power bands, the point at which they work best. We must establish which area, or rev band, we need the engine to perform at. When this has been established we can tune the exhaust system to achieve this requirement. In general two stroke are high revving engines with narrow power bands. As the revs per minute increase so does the heat within the engine. This heat steals power and is to be controlled at every opportunity.

Two stroke engines get very hot and generate heat at every touching part. Notable touching parts are the piston skirt and the adjacent cylinder barrel. When a two stroke overheats seizure of the two touching parts takes place. They weld together momentarily and stop the engine rotating. This has the effect of locking the back wheels of the vehicle and causing the person being carried a heart stopping 'moment'.

Further problems associated with excessive heat generation are in the areas of the combustion chamber. Two strokes like to run on lots of air. This weakening of the standard fuel/air mixture can cause intense heat to be present in the areas of the piston crown. Melted piston crowns are to be avoided if racing is to be enjoyed. Monitoring piston crown temperature is almost impossible. The nearest attempt to monitor this area is with a cylinder head temperature probe.

When setting carburettor jet sizes, for mixture control, we must make regular piston crown checks for colouring. By lifting the cylinder head we can accurately see the colour of the piston crown and assess the mixture strength.

As a rule of thumb light brown is good, black is too rich and white is too weak. Too weak will result in meltdown/seizure.

Plug checks help, as they are in the same environment as the piston crown, but checking the piston crown is the most accurate method. Plug checks, or piston crown checks, must be done at the end of a long hard run. The engine should be cut on ‘full song’ to accurately read the colour of the engines fuel/air mixture.

Water cooling, or liquid cooling to be exact, is a solution to heat build up. It comes with added weight and further temperature monitoring problems.

The two stroke engine is great fun for sport where torque is of low importance. It brings engine tuning to the masses due to the simplicity of the design while having sufficient depth to still allow top tuners to have fun and produce amazing engines. Tuning two strokes is a big subject with content sufficient for another article ...

This article was produced by Brian Pollard, author of "Preparing the Gx160 for 'open' racing" which is available on CDROM, in multimedia format, as an e-book, and in paper form. All enquires should be sent to the above e-mail address.

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