The most simple type of internal combustion engine, it is frequently used in situations where not much power is needed. These typically include small motorcycles, lawnmowers, radio-controlled vehicles etc. It is called a 2-stroke to distinguish it from the 4-stroke engine. While a 2-stroke is actually about 50 per cent more powerful than a 4-stroke of the same displacement, they do not scale well to greater displacements.

It works, as the name suggests, through 2 phases of movements. It's kinda hard to explain without a diagram ... let's see, time for some ASCII ...

      |   |
      |PPP|===== <-- intake pipe
      |PPP|===== --> ejection pipe
      | | |   
      | \ |
      | C\|

S = spark plug
P = piston
C = crankshaft
Above is a simplified diagram of a 2-stroke (there's a complicated tube at the side that ensures that more fuels is burnt, but we'll forget about it for now).

The first stroke, the downward stroke is when the piston is below the ejection pipe. This pushes out burnt gases. It also brings in new fuel.

The second stroke is on the way back up again. As the cylinder goes up, it compresses the mixture of fuel and air coming in from the intake pipe. At the top, the mix is ignited by the spark plug, which pushes the cylinder back down again, simultaneously taking us back to the first stroke and also making the crankshaft turn. The crankshaft turning is the useful side effect we use (usually moderated by a flywheel) to make wheels turn, blades spin, etc.

Update (special thanks to Starrynight): The problem with large 2-stroke engines isn't so much the issue of power, but the issue of emissions. Because of the way they work, the lubricant must be included in the fuel; and when lubricants burn, they create nasty stuff for the environment. Apparently, there are 2-strokes with displacements of more than 1000cc.

One of the ways internal combustion engines work. Alternatives include the four stroke engine cycle (Otto cycle), and the one used in rotary engines (Wankel engines, though they technically use the Otto cycle, but in an odd way).

Like the four stroke engine cycle, the 2-stroke engine cycle -using engine is of the reciprocating type, with pistons moving back-and-forth in a column. Fuel mixed with air is burned, and the increase in pressure and volume is used to generate useful work.

The cycle is:

  • intake/compression - the piston moves into the combustion chamber, with fuel and air being introduced at the beginning of the stroke. At maximum compression, the fuel/air is ignited, usually by a spark plug
  • exhaust/power stroke - the piston is moving out of the combustion chamber, pushed out by the force of the burning fuel. At near the end of the cycle, the exhaust gasses leave the combustion chamber

Because there are twice as many power cycles compared to the Otto cycle, more power should be generated by two cycle engines. However, the inefficient exhaust and intake phases cancel out the benefits. However, there are some benefits to two cycle engines, with simplicity being perhaps the most important.

Due to the construction of two cycle engines, they usually have lubrication mixed in with the fuel.

Two cycle engines are used in lawn mowers, some motorcycles and snowmobiles, and other light vehicles.

A two stroke internal combustion engine, like the name implies, has only two strokes - compression and expansion (power). Since power is made once per revolution (as opposed to once per two revolutions with a four-stroke engine), a two-stroke has the potential to make twice as much power per unit displacement or weight as a four-stroke. Inefficiencies inherent to the two-stroke system reduce the power boost to 1.8 times more, but it is still substantial.

Instead of valves, two-stroke engines use ports on the side of the cylinder to let in air and expel exhaust. Since there is no intake stroke, the incoming charge must be either compressed by the crankcase or a supercharger. If the crankcase is used, then there can be no dedicated oil system, and oil must therefore be added to the fuel. Two-stroke engines are commonly found in mopeds and chainsaws, because weight is important and mopeds usually have displacement limits. Two-stroke engines are very polluting; some of the fuel goes unburned, and the burning of the added oil causes blue smoke.

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