Instructables

Tuning Two-Stroke Engines

Step 2: Two Stroke Theory

Picture of Two Stroke Theory
here is an overview of how two stroke motors work. Two stroke motors are more efficient that four strokes for a few reasons. 1 is the displacement, they generally have smaller combustion chambers, which means less fuel burned. 2 is the fact that they have fewer moving parts, which decreases the constant load on the engine, allowing it to burn the air-fuel mixture more efficiently, and reach much higher revolutions per minute.

Here is the process of the engine.

Intake, The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston. The illustrated engine features a poppet intake valve, however many engines use a rotary value incorporated into the crankshaft. During the downward stroke the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

Transfer/Exhaust. Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well. I'm going to interrupt here for a second to explain how that fresh mixture is prevented from being expelled.

so, you think an exhaust pipe is just meant to get the exhaust away from the engine, right? well in the 4 stroke application that's completely true, but in the two stroke, you must use a specially formed pipe, that uses something called backpressure, to force the expelled mixture back into the combustion chamber, this can be seen in the animation quite clearly.

Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture, insuring that the mixture explodes properly. (At the same time, another intake stroke is happening beneath the piston).

Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.
 
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some of this information is potentially misleading. what i was told was a 2 stroke engine has the same horsepower as a 4 stroke engine about twice its size in displacement. however, i thought that 2 strokes consumed more gallons per hour than a 4 stroke of equal horsepower and displacement. now granted, 2 strokes have tons of nuts, i just thought they were always less fuel friendly than 4 strokes
Nope, you're right. 2 strokes are much more weight/size efficient, but in no way are more fuel efficient.

When you see that green fuel-air mix rebound back in the animated pic - not all of it makes it back into the cylinder... Especially when not running at the resonant frequency of the pipe.
With a CVT (modern moped belt transmission) the engine is maintained at the optimum RPM through a wide range of road speeds - with a geared transmission, much gas is wasted outside of the powerband as well as the bike acting like a pig at low RPM.

On the other hand the latest direct fuel injection 2-strokes have completely insane power and efficiency - they avoid the fuel loss problem (and burning all that oil, too) by injecting fuel after the ports close, like a diesel. I personally test rode a 50cc bike with DFI up to 90kph, TOTALLY STOCK.

Source: I own a 2-stroke moped that has tons of nuts for its 50cc displacement. It also eats gas compared to similar, gutless 4 strokes on the road (almost twice as much, in fact - 3L/100km)