2-Stroke vs. 4-Stroke Engines

All internal combustion engines have something in common; they follow the four step process (suck, squeeze, bang, blow) to converting gasoline into forward (or backward) momentum. It all starts by mixing fuel and air in the appropriate ratio, compressing it,  igniting it either with a spark plug or by self-ignition, and allowing the explosion of combusting gasses to force a piston to move downward, expelling exhaust gas.

2 strokeThat piston’s vertical movement is converted into rotary motion in the crankshaft by using connecting rods. The crank then goes out tot he gearbox via a flywheel and clutch, and that gearbox then sends the rotary motion to the differentials and then to the wheels, driving the vehicle forwards.

This is the general layout of a 4 stroke engine. However, there are other types of layouts that you can find in engines that drive other kinds of vehicles. For example, single cylinder engines are generally all that’s necessary for a motorbike, snowblower, or chainsaw. A triple is enough for a Triumph motorbike. Inline fives used to be common in Audis but now are more prevalent in Volvos. V5 is a VW thing, V6 is pretty common because it is smoother than an inline-four but has better fuel economy than V8 engines.

Ok but now it’s time to talk about 4 stroke and 2 stroke engines. Know that almost every car sold today uses a 4 stroke engine. Plenty of motorbikes, lawnmowers, snowblowers and other mechanical equipment follows suit. Smaller motorbikes, lawnmowers etc might still use a 2 stroke engine.

What’s the difference? The difference is the number of times the piston moves up and down in the cylinder during a single combustion cycle.

So in the case of 2 stroke engines, the combustion cycle is completed within a single piston stroke as opposed to two piston strokes (which happens in a 4 stroke engine). Another important difference: the lubricating oil used by a 2 stroke engine is mixed with petrol or fuel, either before being put inside the engine or by a pump that mixes the fuel in the right proportions.

4 strokeThe reason that a 2 stroke engine can complete the combustion cycle in a single piston stroke is that has a reed valve and its piston is designed to be taller with two slots cut into the inside of it.

2 stroke engines are typically more powerful than 4 stroke engines with the same cylinder capacity. The issue is that the oil-petrol mixture causes the exhaust to be more contaminated with pollutants.

4 stroke engines are more complex and do not use reed valves. They generally have valves on the top their combustion chambers, one of which is intake and one is exhaust. More complex engines have multiple of each.

You’re not going to find a lot of single-cylinder 2 stroke engines because it’s hard to get a decent start out of one cold piston. More cylinders mean more pistons which means a smoother engine that’s more in balance. That’s the general rule anyway. Be sure to check back in for more ill informed and poorly explained auto advice!

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