Four Strokes versus Two Strokes
2-Strokes vs. 4-Strokes
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If you spend any time in the dirt bike or motocross world at all, you’ll probably catch a debate about 2-strokes compared to 4-strokes. Or if you’re thinking about buying a new or used dirt bike for the first time, you’ll wonder if you should go with a two-stroke or four-stroke. Here are a few basics on the two types of engines.

2-Stroke Operation

1) The fuel/air mixture is pulled into the crankcase (not the cylinder) by the vacuum created during the upward stroke of the piston.

2) During the downward stroke, the valve into the crankcase closes. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

3) Toward the end of the stroke, the piston exposes the intake port that allows the compressed fuel/air mixture in the crankcase to get past the piston into the main cylinder. On the way in, the fresh fuel/air mixture pushes the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. The only way to make sure the cylinder is full of the fuel/air mixture, is to push “too much” through the cylinder, so some unburned fuel/air gets into the exhaust system.

4) The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. It's a little confusing, but at the same time, another intake stroke is taking place in the crankcase -- underneath the piston -- as in step #1.

5) 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. And again, a little confusing, but underneath the piston, the fuel/air mixture ie being compressed.

You'll find an EXCELLENT animation of a working 2-stroke engine, as well as diagrams, here.

4-Stroke Operation

1) During the intake stroke, the piston moves down, pulling fresh fuel/air mixture through the open intake valve into the cylinder. An exhaust valve in the cylinder is held closed at this stage.

2) With the intake and exhaust valve both closed, the piston moves up in the cylinder, propelled by the momentum of the flywheel. As the piston moves up, the fuel/air mixture is compressed.

3) At the top of the compression stroke the spark plug fires, igniting the compressed fuel/air mixture. The expanding gasses resulting from this controlled explosion push the piston downward.

4) At the bottom of this power stroke, the exhaust valve is opened (the intake valve remains closed) and, as the piston rises again, exhaust is pushed out of the cylinder.

You'll find an EXCELLENT animation of a working 4-stroke engine, as well as diagrams, here.

2-Stroke Engine Advantages

The spark plug fires once every revolution of the crankshaft.

Basically, they produce twice the power of a four stroke engine of the same size.

The two-stroke engine is much simpler than four strokes, with lighter construction and fewer parts.

2-Stroke Engine Disadvantages

You have to buy two-stroke engine oil, measure it, and mix it with your fuel.

Technically, they wear out faster because lubrication is not as efficient as in a four stroke engine with heavier oil.

Because unused fuel is exhausted with each cycle of the engine, they use more fuel.

Because unused fuel is exhausted with each cycle of the engine , they pollute more.

4-Stroke Engine Advantages

They generally last longer than two stroke engines because of the more efficient lubrication of moving parts.

Four-strokes use less fuel.

Four-stroke engines pollute less than two-strokes.

4-Stroke Engine Disadvantages

Construction/assembly/operation of the engine is more complicated. More moving parts means more things that can go wrong (Although I personally have had very little trouble in this respect. How about any of you?).

Four-strokes are half as powerful as two stroke engines of the same size.