Two�Stroke Engine Fundamentals�

Engine Types
The engines used to power snowmobiles and most other vehicles today are basically the same. Most of those are known as "Internal Combustion Reciprocating Engines." There are five events that need to occur for an engine of this type to run.

The five events are:
1. Intake of an air/fuel mixture.
2. Compression of the mixture.
3. Ignition of the mixture.
4. Expansion of the ignited mixture.
5. Exhaust of the burnt mixture.

The source of power comes from the expansion of gasses resulting from the ignition of�the air/fuel mixture. When this mixture is ignited in�a closed cylinder that contains a piston, the expansion of the gasses force the piston away from the explosion.�The piston is attached to�a crankshaft by a connecting rod so when�the piston moves, the shaft turns.

A stroke occurs when the piston travels from top dead center (TDC) to bottom dead center (BDC) or vice-versa.

The main difference between a two stroke engine, usually found in a snowmobile, weed whipper, chainsaw, outboard, etc. and a four stroke engine which is like the one in your car, is basically in how the intake of air/fuel and the exhaust of the burnt charge is accomplished.

Four Stroke Engine
In a four stroke engine it takes two complete revolutions of the crankshaft to complete a power producing cycle. The first stroke is the intake stroke which draws a fresh charge of air/fuel in through an open valve at the top of the cylinder as the piston approaches BDC. At the end of this intake stroke the intake valve which is being controlled by a cam timed to the crank, has closed. The piston heads back to TDC on the second stroke which is the compression stroke. During this stroke the air/fuel mixture is compressed as the piston comes back to TDC. When the piston reaches TDC, or even a few degrees before, a spark plug fires igniting the air fuel mixture. The resulting explosion forces the piston away from TDC toward BDC. This is the third stroke�or the power stroke. As the piston reaches BDC and starts back toward TDC again, the cam opens an exhaust valve. This fourth stroke back to TDC is the exhaust stroke in which the burnt gasses are expelled out the exhaust valve by the rising piston. The whole process repeats over and over until there is no combustion or something breaks.

Two Stroke Engine
In a two stroke engine it takes one complete revolution of the crankshaft to complete a power producing cycle. The piston works like a sliding valve for the intake and exhaust ports which�are both open when the piston is at BDC.

In a two stroke engine, the�exhaust port is open to atmospheric pressure at the same time that the intake is open. Because of this, the air/fuel charge must be forced into the cylinder by other means than the vacuum caused by the piston moving in it's stroke toward BDC. I'll get to that in a minute, but for now let's assume that there is a charge of air/fuel in the cylinder.

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The five events that occur in a four stroke engine are still present it a two stroke, the big difference is that the intake and exhaust happen almost simultaneously. The first stroke happens as the crankshaft is turned from BDC, and the piston moves up toward TDC, both ports are closed, and the imaginary air/fuel mixture is compressed. When the piston get to, or right before TDC,� the plug fires and ignites the mixture causing it to, of course, expand. Just like in the four stroke engine, the piston is forced down toward BDC on the second stroke. Now the exhaust port is positioned in the cylinder slightly above the intake port so the burnt gasses can escape out of the cylinder through the exhaust pipe before the intake port is opened by the downward moving piston. As the piston completes it's stroke to BDC, the intake port is opened and a fresh charge of air/fuel is forced into the cylinder by�magic. (No, just�wait, I'm getting to the pressurization thing.) Since the exhaust port is still open, the incoming pressurized charge of air/fuel helps to push the remaining burnt gasses out of the cylinder. This is referred to as "scavenging." As the piston starts back to TDC, the new charge is compressed and the process starts again.

Ok, now for the magic. Well, not really. The two stroke engine works both sides of the piston to accomplish in two strokes what the four stroke does in four strokes. Huh?�

The fresh air/fuel mixture is drawn into the engine and forced into the cylinder by crankcase pressure created by the strokes of the piston in the cylinder. By sealing the engine's crankcase airtight and connecting the carb to the intake port, the upward, or compression stroke of the piston draws air/fuel into the crankcase. When the piston reaches TDC a one way valve is closed so the crankcase can be pressurized. When on the stroke toward BDC, the transfer port is opened, the pressurized chare equalizes into the cylinder. This equalization helps force, or scavenge the burnt exhaust gasses out of the cylinder.