Two-stroke and four-stroke dirt bikes are defined by the engine, which is either a two-stroke or a four-stroke design. A two-stroke engine is a design that has been used for decades in dirt bike applications and provides power and weight advantages over a four-stroke engine. Modern technology advances have greatly improved the viability of four-stroke dirt bikes to the point where there are valid reasons to choose either type of engine for a dirt bike.
A two-stroke engine is the traditional powerplant of a dirt bike. The engine spark plug fires once every revolution of the crankshaft. It contains only three moving parts: a crankshaft, a piston and the connecting rod that connects the two. There are no valves to open or close, thus no need for timing gear. Since the engine powers every revolution, the two-stroke can in theory be twice as powerful and the engine can fire twice as often. Fuel and oil are combined and fed into the engine. Due to the fact that they do not contain a separate lubrication system, two-stroke engines wear out faster and are not particularly efficient.
The four-stroke dirt bike engine is similar to typical gasoline car engines; the powerplant's spark plug fires once every other revolution. There is more engine hardware since a four-stroke has engine intake and exhaust valve fuel lines and timing equipment. Four-stroke engines have become very popular with increased technological advances, particularly over the past decade. The separate lubrication system provides greater engine life and efficiency.
Since oil and fuel are mixed in the crankcase of a two-stroke engine, a two-stroke engine produces far more pollution than does a four-stroke engine, which separates fuel and oil.
Since four-stroke engines are more complex, maintenance and manufacturing costs of a four-stroke engine are higher compared with those of the simple two-stroke engine.
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