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I know that when the gas combust it do so in millisecond. But I find it very hard to believe it goes though that entire process that fast. The piston is still a mechanical part and will add a lot of friction. So how long to it take to usually for a piston to go down and back up? I know this maybe hard so can you also give certain conditions as well or at least some type of guide to help figure this out?

  • This is a fairly reasonable speed. It's not hard to understand. Sometimes people find it hard to believe if they don't actually understand the engineering behind it. – cloudnyn3 May 29 '16 at 14:48
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math it out. we will use 3 RPM readings that will math out easily, and give a good idea of how fast things are happening at different engine speeds.

600 RPM - This is about idle speed. 600 revolutions per minute (RPM) = 10 revolutions per second (RPS). Assuming this is a 4 cycle engine, so 2 revolutions per cycle, so 5 complete cycles per second. Each cycle is 4 strokes, so 20 strokes per second. The combustion stroke takes 1/20th of a second or 50 milliseconds

3000 RPM - This is about cruise speed. 50 RPS, 25 cycles, 100 strokes. combustion stroke takes 1/100 seconds or 10 milliseconds

6000 RPM - This engine speed will be reached at wide open throttle. 100 RPS, 50 cycles, 200 strokes. combustion stroke takes 1/200 seconds or 5 milliseconds

  • You are talking for the combustion stroke itself. I would submit that the combustion itself can occur completely before the end of the combustion cycle if the engine is moving slower, or longer than the combustion stroke if, for instance, there is more air/fuel than can be burned. This is one reason you can get flames out the exhaust, like with top fuel dragsters or even the Wenkle (rotary) engine. Combustion is occurring outside of the combustion chamber and cylinder. – Pᴀᴜʟsᴛᴇʀ2 May 29 '16 at 15:11
  • Yes, there are a couple factors that effect how long the actual combustion takes, and I'm really not sure the best way to calculate that. A couple that come to mind are the volume or air/fuel mixture (Why timing is advanced when under less load/higher vacuum) and air/fuel ratio (why timing is pulled when the engine is cold). I do remember hearing once that the combustion is only pushing the piston for the first 10% of the stroke. After that the piston is moving faster than the combustion can expand. – rpmerf Jun 1 '16 at 12:25
  • @rpmerf you forgot 10 degrees ish BTDC to nearly 35 BTDC as well so it would be slightly longer since it starts that much sooner. and yes during teh middle of the stroke the piston can move faster than combustion but near the end of the stroke it slows and you have pressure again but usually you will have something burning even at low rpm's – Cc Dd Nov 14 '16 at 5:19

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