Two- vs. Four-Stroke Engines: What’s the Difference?
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.
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.
Despite its practical and environmental drawbacks, this engine type does have some very real advantages over four-strokes. Perhaps the largest is its power-to-weight ratio. With twice as many power strokes, two-strokes are able to produce two times the power of a similar sized four-stroke. Two-strokes, with their lack of complicated and heavy cooling and lubrication systems, are also much lighter and more compact than their four-stroke counterparts. This simplicity also makes them cheaper and easier to manufacture.
Though these advantages are significant, they can’t outweigh the two-stroke’s less desirable traits. In addition to needing pre-mixed fuel and harming the environment, two stokes are also extremely noisy. The simplistic cooling and lubrication systems also cause the design to be less reliable and shorter lived than a comparable four-stroke power plant. In other words, they tend to live fast and die young. They also get significantly worse fuel economy and suffer from a peaky and unpredictable power band, which makes them less than ideal for anything bigger than a motorcycle or dirt bike.
The reality is that two-strokes, with all their faults, would likely still be with us if it weren’t for the EPA’s stringent emissions requirements that make it all but impossible for a two-stroke to pass. That’s a shame because modern motorcyclists may never get to experience the joy of leaving the competition behind in a cloud of blue smoke.
Like classics? It’s always Throwback Thursday somewhere.