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Why does ignition occurs during the power stroke in spark ignition engines and not during other strokes in the four-stroke cycle?

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    Burning fuel is what produces energy (or converts / releases its chemical potential energy.) Whenever you trigger that, that's when it's going to be doing mechanical work, so it makes sense to call that part of the cycle the "power" stroke. Mar 3 at 0:20

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While I agree for the most part of @WeatherVane's answer, it doesn't really answer your question:

Why does ignition occurs during the power stroke in spark ignition engines and not during other strokes in the four-stroke cycle?

Ignition occurs to fire off the air/fuel charge which is in the cylinder. Here are the reasons it doesn't happen at any other time then when it does:

  • Intake: Why would you fire the incoming charge? The intake valve is open. If you fired the air/fuel charge as it's coming into the cylinder, it would backfire out through the intake. If it's a direct injected engine, it would be wasted spark because there's just air in the cylinder.

  • Ignition: While there would be an air/fuel charge at this point, it's too late. The flame front would be chasing the piston down the cylinder and you'd be losing all kinds of power.

  • Exhaust: Nothing there to light off. Exhaust is spent fuel. Nothing there (or rather there should be) to burn.

  • Compression: Well, actually it does fire at the end of the compression stroke, a few degrees before TDC. You can't fire it anywhere else and make any reasonable power.

Why the air/fuel mixture will only combust explosively when under pressure? Can the air/fuel mixture combust explosively at the beginning of the compression stroke?

It isn't that it can't combust explosively when under pressure, it combusts the best when under pressure. The static compression ratio is a good way of looking at this because it helps to explain something. The higher the compression ratio, the greater the squeeze on the air/fuel, and the greater the pressure. As a rule of thumb it's stated that for each point of static compression, the power output is increased by 3%, and that's with no other changes. It creates more power.

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  • What do you mean " the greater the squeeze as the air/fuel", why is the word "as" in there? What do you mean by "As a rule of thumb it's stated that for each point of static compression...", what "point" are you talking about? Thanks. Mar 2 at 1:12
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    The "as" should be probably be "on the" ... change incoming. A "point" of compression is the difference between 9:1 CR and 10:1 CR. From "9" to "10" is one point. This is common speech in engine building. Mar 2 at 3:03
  • Also, what is "air/fuel charge"? Is "Ignition" the same as "Power (Compression)" stroke? Mar 2 at 4:24
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    The "air/fuel charge" is what goes bang during ignition. It is the combination of air and fuel ... who'da'thunk it? Ignition is when the "air/fuel charge" inside the combustion chamber goes boom. Has nothing to do with the individual strokes. The strokes are: 1) Intake; 2) Compression; 3) Power; 4) Exhaust. I would think you'd already know this considering your question. Mar 2 at 4:30
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In normal running conditions, the ignition does not happen during the power stroke, but just before, at the end of the compression stroke, because it takes time for the burn to propagate through the fuel/air mixture.

There is no point sparking at any other time in the cycle, because the fuel is only injected/drawn in during one of the four (Otto) cycles, and will only combust explosively when under pressure. In carburettor engines the fuel is drawn in on the intake stroke along with the air. On more modern systems the fuel is injected during the intake or the compression stroke, depending on the injection system.

Sparking at or after TDC (top dead centre) is less efficient. The faster the engine is rotating, the earlier the spark needs to happen, because the propagation time is more or less constant.

On early engines with a manual ignition timing control, one would retard the timing when starting, because a) the engine is rotating very slowly, and b) to avoid backfiring. Modern engine management systems control the timing automatically, mainly based on engine temperature and rpm.

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    It's a little bit more than just the propagation time. While that is important, you make more power and have a cleaner burn. The tighter the squeeze, the bigger the bang. Yes, there is a point of diminishing returns, but it makes a difference. Mar 1 at 22:19
  • Why the air/fuel mixture will only combust explosively when under pressure? Can the air/fuel mixture combust explosively at the beginning of the compression stroke? Mar 2 at 0:48
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    That would be pointless, because it would try to drive the piston in the wrong direction. And if sparking after TDC, the piston is already moving downwards and the volume of air-fuel mixture is already expanding. The burn is most powerful when the volume is at its smallest, so with the spark happening just before TDC and the burn continuing (it is not instantaneous) after TDC is most efficient. Mar 2 at 0:52

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