Two strokes use the same priciple of expanding hot gas to drive them, see the first part of our description of four stroke engines for more details.

Two-stroke engines have two important advantages over four-stroke engines:

• Two-stroke engines do not have valves, which simplifies their construction.
• Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution) -- this gives two-stroke engines a significant power boost.

These two advantages make two-stroke engines lighter, simpler and less expensive to manufacture. They also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination gives two-stroke engines a great power-to-weight ratio.

The Two-stroke Cycle

The biggest difference to notice when comparing to a Four Stroke engine is that it fires once every revolution in a two-stroke engine.

The below illustrations show the various stages of the two stroke cycle. The figure shows a typical cross flow design. You can see that two-stroke engines are ingenious little devices that overlap operations in order to reduce the part count.

You can understand a two-stroke engine by watching each part of the cycle. It isn't clear to see in any of the below diagrams but most model two stroke engines have a hollow crankshaft that connects the carburettor to the inside of the crankcase. As the crankshaft revolves it acts as a valve only allowing air/fuel from the carburettor into the crankcase at the correct times.

Start with the point where the spark/glow plug fires. Fuel and air in the cylinder has been compressed, and when the spark/glow plug fires the mixture ignites. The resulting explosion drives the piston downward. The crankshaft currently has the just closed the carburettor off from the crankcase.

Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder.

As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel. The crankshaft will open the connection between the carburettor and the crankcase, as soon as the intake port is covered again by the piston.

Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum is combined with the rotary valve in the crank shaft opening and thus allows the air/fuel/oil in from the carburettor.

It's called a two-stoke engine because there is a compression stroke and then a power stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust strokes.

You can see that the piston is really doing three different things in a two-stroke engine:

• On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel mixture and capturing the energy released by the ignition of the fuel.
• On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air/fuel from the carburettor through the crankshaft and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
• Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

The more asstute among you will of realised that it is possible for some of the fresh charge to get sucked out along with the spent charge, this means that some power is getting wasted. What we need is a method to keep all the fresh charge in the cylinder, so we can have as big an explosion as possible. To do this you use a tuned pipe. I tuned pipe fits on the exhaust of the engine much like a silencer. What a tuned pipe does is restrict the exhaust gases so they create a back pressure standing wave, that stops then fresh charge from getting out. Tuned pipes are great for giving you engine a power boost.

Finally here is an exploded picture of an actual model two stroke engine. Click on the image for a larger picture.

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