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Many years ago, I was involved in a hobby of building and flying model aircraft. The aircraft used alcohol-fueled 2-stroke engines with glow plug ignitions. When these engines ran rich (and always at idle), they produced a distinctively different sound to when they ran with the throttle mostly or fully opened and the mixture leaned to deliver the highest RPM. That difference in sound was informally described as "four-stroking". I've heard a similar effect in larger two-stroke engines as well - weed trimmers and motorcyles - so it seems to be a characteristic of two-stroke engines in general, not just the small, model aircraft variety.

My question is: what is going on in these engines when they "four-stroke"? It seems as though combustion is only occurring on every other top-dead/power stroke. Is this true? If so, why? What prevents the charge from igniting on one stroke while allowing it to occur on the next, and repeat the alternating pattern? In the case of the glow plug engine, how is it that the glow plug can fail to ignite a charge on one cycle, and still remain hot enough to fire on the next cycle?

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Wikipedia describes four-stroking as:

Four-stroking is an undesirable operating condition of two-stroke engines, where they instead begin to fire every four strokes, rather than every two strokes. This firing is uneven, noisy and may even damage the engine if allowed to continue unabated. Four-stroking was often a cause of poor idling in two stroke engines.

It then describes what's happening:

Two stroke engines rely on effective scavenging in order to operate correctly. This clears out the combustion exhaust gases from the previous cycle and allows refilling with a clean mix of air and fuel. If scavenging falters, the mixture of unburnable exhaust gas with the new mixture may produce an overall charge that fails to ignite correctly. Only when this charge is further diluted, by pumping through a second volume of clean mixture, does it become inflammable again. The engine thus begins to 'fire-and-miss' every second cycle (every four strokes), rather than correctly on every cycle.

So, yes it's true that it is only igniting every other intended power stroke. In some worse cases an engine can six- or eight-stroke, igniting every third or fourth power stroke.

The reason seems to be that the exhaust isn't purged completely and leans out the mixture on the next stroke and isn't rich enough to ignite leaves a mixture on the next stroke that cannot be ignited until the following stroke which then produces exhaust that again isn't purged completely. The more exhaust left in the cylinder, the more skipped strokes.

Model engines are affected differently because of their small scale:

Four-stroking is a common and expected behaviour with model engines, both glow fuel and diesel. These small engines run at extremely high rotational speeds and their scavenging relies upon this. When started, they run as inertially-scavenged four strokes and have a distinctive change in engine note when they accelerate past the point at which they begin to operate as two strokes. Owing to the scaling laws of such small engines, this four-stroking is an unavoidable consequence of limitations on their scavenging at slow speeds. However the same scaling laws also make the effects of four-stroking less severe and so the engines can idle happily in this mode, without damage.

I suppose that the glow-plug in these engines remains hot enough to ignite the fuel but, every other stroke is just too lean to ignite not an ignitable mixture. If it started six- or eight-stroking maybe it would cool too the point where it could no longer ignite the fuel on the strokes with an appropriately combustible mixture.

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This is a fantastic answer, and I definitely learned something new. However, I feel your intuition about the miss being due to the mixture being "too lean to ignite" may be incorrect. The presence of exhaust gasses rather than fresh air/fuel means there's a deficiency of oxygen, which would be a rich condition, rather than a lean condition. –  mac Sep 6 '13 at 15:38
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...Then again--the exhaust gasses might be completely inert, and therefore the air-to-fuel ratio of the non-inert, new charge would still be correct (neither rich or lean) as metered by the carb, there just might not be enough fresh air and fuel to make a meaningful power stroke. –  mac Sep 6 '13 at 15:44
    
@mac I think you're right. Lean is probably the wrong term. On Wikipedia the author just uses the word "unburnable", (I don't think unburnable is actually a word). –  Seminecis Sep 6 '13 at 17:13
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"not an ignitable mixture" –  mac Sep 6 '13 at 17:30
    
@mac thanks, I used that phrase to edit my answer. –  Seminecis Sep 6 '13 at 17:36

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