2-Cycle vs 4-Cycle Engines: Understanding the Difference

You may have heard the terms 2 stroke and 4 stroke engines before, but do you really all of the differences in their operation?

Throughout the history of automotive design, there has existed two main types of gasoline powered combustion engines, the 2 cycle (2-stroke) and 4-cycle (4-stroke) engine. As you may have guessed, the names of each engine suggest exactly how they operate. For an engine to complete combustion, there needs to be combustion of the gas-air mixture and expulsion through the exhaust. The way this process is repeated varies between these engine types.

A 2 cycle engine completes a combustion and exhaust cycle in only 2 strokes of the piston, with a 4 cycle taking 4 strokes of the piston.

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4-Stroke Engine

For a 4 cycle engine, there is a power stroke (piston down) which is caused by combustion of the fuel mixture. Then there is an exhaust (piston up) stroke that pushes the gas exhaust out of the engine. The following two strokes include an intake (piston down) to draw in the fuel and a compression stroke (piston up) to start the process all over again. Examining this process closer yields the observation that the whole cycle is powered by the initial combustion stroke. This means that the combustion of the fuel mixture needs to impart enough power into the piston to allow it to rotate the crankshaft twice. 

A 2-cycle engine has much simpler mechanics, as it basically consolidates the different processes of a 4-stroke engine. There is power stroke which also releases the exhaust after a certain amount of travel, then an intake and compression stroke that draws in new fuel and completes combustion.

These are the basic mechanics of each engine type, but both have significant advantages and disadvantages, which have ultimately led to a more widespread use of the 4-stroke engine.

Looking first at efficiency, a 4-stroke engine achieves great efficiency due to the fact that nearly no fuel is wasted in the intake cycle, i.e. the fuel is drawn into a closed system. In a 2-stroke, since the fuel is drawn in during a combined intake and compression stroke, the mechanics needed for this also mean that some fuel can escape while being drawn into the cylinder, decreasing efficiency.

2-Stroke Engine

Modern 2-strokes use fuel injection which increases their efficiency to levels close to the 4-stroke, but generally, a 4-stroke can be much more efficient. 

Efficiency isn’t everything, though. A 4-stroke is certainly more efficient, but they also weigh upwards of 50% more than a comparable 2-stroke engine. This weight is due to more complexity within the motor’s mechanism, which also leads to significantly more moving parts in a 4-cycle.  In terms of simpleness and easiness to fix, a 2-stroke easily wins out. 

The last main difference between these different engine types is how oil is added to the engine. In a two-stroke engine, the oil is typically mixed with the fuel, usually somewhere in a 50:1 to 20:1 range. This makes oil application fairly reliable, but it also means increased fuel costs and fuel modification. A four-cycle engine uses normal gasoline, like your car (which also happens to be a 4 stroke engine), and the oil is injected from a separate reservoir. This eases the burden of fueling the engine up, but of course, there is always the possibility that the fuel pump will break.

All things considered, even with the drawbacks of the 4-stroke, four-cycle engines maintain higher efficiency over 2 strokes and can produce far more power than comparable 2-strokes. There have been cars made with 2-stroke engines, but they mostly died off in the early 1980s due to efficiency constraints. Modern 2-stroke engines are still produced for small watercraft and gardening equipment.

Sometimes simple design, like the 2-stroke, doesn’t always beat out more complex mechanics.

Sources: Pro Tool ReviewsQuoraDeep ScienceSBT

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Trevor is a civil engineer by trade and an accomplished internet blogger with a passion for inspiring everyone with new and exciting technologies. He is also a published children’s book author whose most recent book, ZOOM Go the Vehicles, is aimed at inspiring young kids to have an interest in engineering.


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