Two- vs. Four-Stroke Engines: What’s the Difference?

Anton Want/Getty Images

Anton Want/Getty Images

The modern four-stroke car or motorcycle engine is a marvel of engineering, making smooth, predictable power with great reliability. But if the four-stroke is an upstanding businessman with two and a half kids and a white picket fence, then the two-stroke is his unruly brother with a rap sheet and a penchant for anarchy.

It’s no doubt that two-strokes are now an endangered species in everything but chainsaws and weed whackers, so some gear heads may not know much about them. Most car and motorcycle enthusiasts are familiar with the suck, squeeze, bang, and blow (also known as intake, compression, combustion, and exhaust) sequence of the classic four-stroke. In this engine type, each phase of the process has its own unique stroke. This means that there’s just one power stroke for every two turns of the crankshaft. In a two-stroke, the intake and compression phases take place during the piston’s upward stroke. Combustion and exhaust occur during the downward, or power, stroke.

Source: Yamaha

Source: Yamaha

Four-stroke engines can be either liquid or air-cooled, depending on design goals. They also feature complex lubrication systems where the crankcase rotates in the oil-filled crankcase and sump. Not so with the humble two-stroke. This incredibly simple iteration of the internal combustion engine features air-cooling and lubrication courtesy of the fuel. Due to the location of the intake and exhaust ports, an oil-filled case is an impossibility for a two-stroke. As such, the only way keep all the moving parts from grinding against each other is to mix oil in with the fuel and burn it in the combustion chamber.

This important difference is one of the key reasons for the two-stroke’s unique character. It’s also the source of some of the design’s key problems. Burning oil creates not only the signature two-stroke smell that’s as beloved by moto-crossers as it is hated by lumberjacks, but it’s a noxious mix of gases that don’t do the environment any favors. Constantly mixing oil into the gas is also a bit of a pain and necessitates having both oil and gasoline on hand every time fuel is needed. Up until the 1970s, many gas stations had separate pre-mixed fuel pumps, but these disappeared as the two-stroke’s popularity plunged during the 1980s.

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