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I came across a question about wasted spark systems, and I was wondering whether there is a risk of prematurely igniting the incoming fresh mixture charge with that wasted spark.

At the top of the exhaust stroke, the inlet valve is opened already. An inlet valve timing of say, 20 degrees before TDC isn't uncommon, and ignition advance can be less than that. This means that these two overlap.

So with the right conditions concerning engine speed and engine load etc, I guess there's a chance that there's a bit of fresh mixture sucked into the cylinder already, when it isn't at it's TDC, nor at the time of ignition yet. Especially with forced induction engines. The wasted spark then could ignite that incoming charge. In the worst case, the flame could propagate throught the intake port.

These are all just guesses though, but i'm curious why it doesn't happen. Is it that the mixture isn't inflammable yet at this point? Or is it impossible for an intake charge to be forced into the cylinder so early, even when the inletvalve is already opening?

I'm curious about your theories!

Bart
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2 Answers2

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If the inlet valve opens 20 deg before TDC, will fresh mixture be going in or not? As there is still pressure in the cylinder - the exhaust leaving is helping to "draw" the fresh mixture into the cylinder, so a wasted spark has no "new" or very little mixture to fire so it doesn't happen.

Also, the engines designed with wasted spark ignition may not have had very high scavenge ratios... That is, overlap of exhaust & inlet valves.

Solar Mike
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The wasted spark will likely be 30 degrees or so before TDC. There would be no fresh fuel charge at this point.

TomO
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