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Running two-stroke engines lean-of-peak

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kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
I have a 25hp single-cylinder two-stroke engine and I'm wondering about lean-of-peak operation. I've flown extensively LOP in a Piper Arrow, and am well versed in the (AFAIK now-settled) debate on the relative merits of ROP vs LOP. Suffice to say that we are very pleased with our operations.

The arguments against LOP have some merit when having unbalanced fuel/air mixtures in the cylinders, however in a single-cylinder engine that obviously can't happen.

It seems based on my reading that people's negative experiences with running 2-cycles lean happen on the ground, when the engine is making max engine power. However, typical aviation LOP operation is only at steady-state at high altitudes where the engine can only make 50-70% of peak power. This is significantly different from a dirt-bike racing around a track, constantly opening and closing the throttle.

I have read claims that LOP operation starves an engine of lubrication, because there is less oil passing through the engine per revolution. However I've never seen any empirical evidence to back this up and seems a little off to me. To wit, on the face of things there's only a 20% difference fuel-flow between max-power ROP and max-efficiency LOP. That 20% difference might sound big, but it's easily achieved by accident when we try to measure out a 50:1 ratio for a 2.5gal tank and miss by a tablespoon. So I find it somewhat hard to believe that the system is so sensitive to lubrication that I'd suffer an early demise. If that were the case, we'd be measuring fuel by weight, much like when mixing two-part epoxy.

Couple the fact that better lubrication is required for higher power and higher temperatures, but we know that LOP operations result in lower temperatures and pressures, and it really sounds like the lubrication argument is somewhat lacking. Now, we might find that if someone misses their fuel/oil mix by 10-20% and they run LOP that they are indeed starving the engine of lubricant. However, this is easily solved by using a precise method to weigh out lubricant and gas.

Does anyone have any experience running two-cycles LOP?
 
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