In the ATV world, there are two main types of engines used to supply power for the units. For the most part, two and four stroke engines are similar in that they both burn gasoline, and they both supply power to the wheels of the ATV. But it is how they produce this power where they differ from each other.
The main difference between a two-stroke engine and a four-stroke engine is in how each produces power. In a two stroke engine, the piston takes two strokes to complete one power cycle; whereas a four-stroke engine requires four strokes of the piston to complete one power cycle. A stroke is defined as moving from top dead center (TDC) to bottom dead center (BDC) or vice versa. The goal of a power cycle is to compress and ignite the fuel mixture, and then expel the exhaust gases from the combustion chamber. A four-stroke engine accomplishes the power cycle through the intake stroke, compression stroke, power stroke and finally exhaust stroke. When the piston is at TDC, the intake valve opens and the fuel/air mixture is drawn in as the piston drops. At BDC, the intake valve closes and the piston returns to TDC, compressing the fuel/air mixture, which is ignited by the spark plug near TDC. The ensuing explosion forces the piston down. This is the power stroke which produces the engine's power and returns the piston back to BDC. Once there, the exhaust valve opens and the piston returns to TDC, forcing the spent exhaust gases out of the cylinder. Here, the exhaust valve closes and the cycle starts over.
In a two-stroke engine, the power cycle is completed in a much more abbreviated fashion, allowing smaller displacement engines to match the power output of a larger four-stroke. The power cycle of a two stroke engine begins at BDC, where the rotating crankshaft forces the fuel/air mixture into the cylinder. The piston is then forced up, compressing the mixture where near TDC the spark plug ignites the mixture. This forces the piston down and is the power stroke of the engine. Once at BDC, a fresh dose of the fuel-air mixture is forced into the cylinder and the exhaust gasses are forced out, and the cycle begins again. Because the fuel mixture's path to the combustion chamber flows through the crankcase, the mixture needs oil to be mixed with it to lubricate the inner workings of the engine. In a four-stroke, the fuel mixture never flows through the crankcase, so the oil and fuel are kept separate, and the oil does not burn like it does in a two stroke engine.
There are benefits to both engine designs. The main benefits of two-strokers are that they have a higher power to displacement ratio. This, along with there being no oil reservoir, means that two-stroke engines are much lighter than comparable four-strokes; although the maximum power of a two-stroke is available at much higher RPMs (revolutions per minute) than for the comparable four-stroke. Also, two stroke engines have fewer moving parts, so they are easier for beginners to fix if something goes wrong.
The main benefits to the four-stroke engine is they are more fuel efficient than the two-strokes and there is no mixing of fuel and oil. Also, four-stroke engines are more reliable than comparable two-strokes due largely in part to the power being available at lower RPMs. Four stroke engines are a much more user friendly engine as there is no calculating oil mixtures, just scheduled oil changes, the power delivery is more user friendly, and the engine is easier on your wallet as they are more fuel efficient.