An engine that's 4 stroking will have more torque in that rpm range. when it breaks into a 2 stroke the torque drops out so there is less HP until the rpm comes up into a higher range. Generally speaking of course.
Sort of. What is definitely true is that each individual firing in a 4-stroke is more energetic than a 2-stroke at essentially the same mixture and rpm. So, the peak torque during the firing rev is substantially higher when 4-stroking than 2-stroking. This is more than compensated by the fact that when 2-stroking, it fires twice as often. So the measured torque and horsepower is generally higher in a 2 than in a 4. How much more if a function of a lot of other things.
This is a finding from the rather remarkable paper by Scott Bair, "THE TWO-CYCLE STUNT ENGINE", wherein the cylinder pressure is measured during operation.
To summarize, when a stunt engine is four-cycling there is a misfire on alternate revolutions. However, the peak pressure when the cylinder fires while four-cycling is about 50% higher than when two-cycling. This should be interpreted as meaning that a good portion of the charge is retained (not ejected from the exhaust port) after the misfire and is added to the charge that will burn during the next revolution.
Integrating over two full revs gives you substantially more torque/HP when it is 2-stroking than 4-stroking.
What happens *in flight* when it goes from a 4 to a 2 is more complex - since the engine doesn't just change modes while keeping everything the same as in the test. As measured, the engine goes into a 2-stroke when the load on the engine has dragged the RPM lower enough for engine to fire on a overly-rich charge for level flight load conditions (due to thermal effects, another observation by Bair:
This may be the explanation for why a two-stroke engine will four-cycle with a rich mixture. The rich mixture cools the head and glow plug and a fuel and air charge that is diluted by the exhaust that has not been swept away will not fire. But, on the next revolution more fuel and air mixture are added raising the concentration of combustible material to where ignition can occur. If the engine temperature increases, either by leaning the mixture or by loading the engine, even the diluted charge will fire and the engine two-cycles.
The RPM when it finally breaks into a 2, and how much torque difference there might be between the two modes is a function of other engine parameters including the compression ratio.
Brett
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