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I was reading about a four stroke cycle. Here's what I understood:

  1. In the first stroke, the piston starts at the top and moves down.
  2. In the second stroke, the piston moves upwards.
  3. In the third stroke, the piston moves down due to the combustion by spark plug.
  4. In the final stroke, the piston moves up and the cycle continues.

I can understand why the piston moves down in third stroke due to the gasoline explosion. But, what moves the piston up and down in Step 1, 2, and 4?

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2 Answers 2

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As Manishearth says, for engines with more than one cylinder the firing of the other cylinders rotates the crankshaft. However, as any fan of vintage motorcycles will know, you can have four stroke engines with a single cylinder. In this case the engine has a heavy flywheel attached to the crankshaft and the momentum of the flywheel keeps the crankshaft turning while it's compressing the petrol/air mixture.

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Interesting point about motorcycles =D. I didn't know that.. –  Manishearth Mar 13 '12 at 9:57
    
an additional point, if one remember the vintage cars, one has to rotate a handle in the engine for it to start. This initial rotation is needed to start an engine. –  Vineet Menon Mar 13 '12 at 11:27
    
@VineetMenon: Exactly. Nowadays the car-battery does that, which is why we need the battery to start the car but not to keep it going.. –  Manishearth Mar 13 '12 at 11:30
1  
@VineetMenon: Exactly$^2$. On an old motor bike you have to use a kick starter, which on a vintage motor bike has a tendancy to kick back and smack you painfully on the calf. It can be a painful experience doing Physics. –  John Rennie Mar 13 '12 at 11:35
1  
@JohnRennie: bump starting is analogous to "rotating the crank" method... –  Vineet Menon Mar 14 '12 at 10:51

In an internal combustion engine, we have multiple cylinders. They are attached to a shaft in an alternating manner such that when one set of the cylinders have combustion, they drive the shaft to move down in the other set.

See http://commons.wikimedia.org/wiki/File:Cshaft.gif

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