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Cozmik Mezzenger
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From Wikipedia, the free encyclopedia
Four-stroking is an operating condition of two-stroke engines, where they instead begin to fire every four strokes or more, rather than every two strokes. This firing is uneven, noisy and may even, in cases where it doesn't occur normally, damage the engine if allowed to continue unabated.
However, in some circumstances, four-stroking is normal. When idling most two stroke engines will four-stroke, as well as when letting off the throttle.
Four stroking will also occur in a correctly adjusted two stroke engine at full throttle without load. In the latter case this happens because the air-fuel mixture becomes overly rich and prevents the engine from running faster. The engine is intentionally constructed by the manufacturer for this to happen, as a too lean mixture will cause the engine to over-rev as well as overheat, and in engines running on premixed fuel a too lean mixture will cause poor lubrication. Running the engine at full throttle without load is not normally done in most applications, but in a chain saw the full throttle mixture is actually adjusted for the engine to four stroke at a given rpm set by the manufacturer. This is done by adjusting the high rpm screw on the carburetor while the engine runs at full throttle until the correct rpm level can be read on a tachometer.
Causes
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 flammable again. The engine thus begins to 'fire-and-miss' every second cycle (every four strokes), rather than correctly on every cycle. Four-stroking begins gradually, so the engine first starts to run with an unpredictable mixture of two- and four-stroke cycles. When severe, this may even become six- or eight-stroking.
Scavenging of small two-stroke engines relies on inertial scavenging through the Kadenacy effect. At low rpm and low gasflow velocities, this effect is reduced. Scavenging thus becomes less effective when idling, and so it is when idling (at either low rpm or low throttle) that four-stroking is most likely to become a problem. Schnuerle or loop scavenging is considered to be less prone than the simpler cross-scavenging.
Four-stroking is not caused by an over-rich mixture, as is widely believed, although this can make it worse. Nor is it caused by excessive oil/fuel lubrication mixtures.
Four-stroking is an operating condition of two-stroke engines, where they instead begin to fire every four strokes or more, rather than every two strokes. This firing is uneven, noisy and may even, in cases where it doesn't occur normally, damage the engine if allowed to continue unabated.
However, in some circumstances, four-stroking is normal. When idling most two stroke engines will four-stroke, as well as when letting off the throttle.
Four stroking will also occur in a correctly adjusted two stroke engine at full throttle without load. In the latter case this happens because the air-fuel mixture becomes overly rich and prevents the engine from running faster. The engine is intentionally constructed by the manufacturer for this to happen, as a too lean mixture will cause the engine to over-rev as well as overheat, and in engines running on premixed fuel a too lean mixture will cause poor lubrication. Running the engine at full throttle without load is not normally done in most applications, but in a chain saw the full throttle mixture is actually adjusted for the engine to four stroke at a given rpm set by the manufacturer. This is done by adjusting the high rpm screw on the carburetor while the engine runs at full throttle until the correct rpm level can be read on a tachometer.
Causes
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 flammable again. The engine thus begins to 'fire-and-miss' every second cycle (every four strokes), rather than correctly on every cycle. Four-stroking begins gradually, so the engine first starts to run with an unpredictable mixture of two- and four-stroke cycles. When severe, this may even become six- or eight-stroking.
Scavenging of small two-stroke engines relies on inertial scavenging through the Kadenacy effect. At low rpm and low gasflow velocities, this effect is reduced. Scavenging thus becomes less effective when idling, and so it is when idling (at either low rpm or low throttle) that four-stroking is most likely to become a problem. Schnuerle or loop scavenging is considered to be less prone than the simpler cross-scavenging.
Four-stroking is not caused by an over-rich mixture, as is widely believed, although this can make it worse. Nor is it caused by excessive oil/fuel lubrication mixtures.
gary55
Well-Known Member
Interesting definition, but you can still all but eliminate it with proper air fuel mixture. To me it doesn't make sense to say the af mix has nothing to do with the cause of four stroking when I can stop most of it by leaning it out.
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