why do two-stroke and four-stroke engines have such different power outputs?

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why do two-stroke and four-stroke engines have such different power outputs?

In: Engineering

A two-stroke produces a power stroke every revolution of the crankshaft. A four-stroke produces a power stroke every other revolulution. For the same displacement and RPM, a two-stroke is making roughly twice the power.

There’s some other effects involved like pumping and incomplete exhaust and valve efficiency that mean it’s not exactly twice, but it’s close.

A stroke is either the up or down movement of the piston. In a 4 stroke engine the first stroke is the power stroke. The fuel is ignited and the piston is propelled down by the explosion. The next stroke the piston moves back to the top of the cylinder and vents the exhaust gases. The 3rd stroke the piston fills with fuel/air mixture as it moves down again. The 4th stroke is the compression stroke where the piston moves up again to compress the fuel in preparation for ignition, then boom. In a 2 stroke engine the first stroke is a power stroke. The exhaust fill and compression strokes are all combined on the next upstroke. While you are producing power on each stroke, you end up exhausting fuel and not getting as complete of a burn. Therefore 2 stroke is much less fuel efficient and runs dirtier.