How It Works

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Advantages And Disadvantages

Common Applications

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How It Works, Part I

Two-stroke engines have one valve, the reed valve. This valve allows the air/fuel/oil mixture into the crankcase. The actual piston itself allows the air/fuel/oil mixture and burned gases into and out of the combustion cylinder.

As the piston moves downward, it compresses the air/fuel/oil mixture in the crankcase. At the same time, the exhaust port is open and the compression from the piston causes most all of the burned gases to exit through the exhaust port.

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How It Works, Part II

Once the piston bottoms out, the intake port is open and the pressurized air/fuel/oil mixture is forced into the cylinder. At the same time, the fresh intake charge forces out the last of the exhaust.

The momentum of the piston drives it 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 opens the reed valve and sucks air/fuel/oil in from the carburetor.

Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle.

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How It Works, Part III

Basically, the piston is 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 carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.The sides of the piston act like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

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Two-Stroke Engine Diagram

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Advantages And Disadvantages

Advantages Two-stroke engines do not

have valves, which simplifies their construction and lowers their weight.

Spark plugs fire once every rotation which gives them a bigger power boost.

Two-stroke engines can work in any orientation.

Two-stroke engines are lighter, simpler, and less expensive to manufacture.

Disadvantages Lack of a dedicated lubrication

system means parts wear faster. Two-stroke oil is expensive and

a large amount is required for two-stroke motors.

Two-stroke engines do not use fuel efficiently.

Two-stroke motors produce a lot of pollution because they burn oil and there are no valves; unburned air/fuel can escape through the exhaust port.

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Common Applications

Although two-stroke engines are not very economical for cars, they do have many applications:Chain SawsDirt BikesJet SkisLeaf BlowersMopedsRadio-controlled Model AirplanesSmall Outboard Boats

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