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Thread: LOP and the 2 cycle engine…..

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    LOP and the 2 cycle engine…..

    LOP and the 2 cycle engine…..

    I am considering operating a mixture control on a 2 cycle engine. The 4 cycle world has lots of info on LOP operations but I have heard nothing about operating a 2 cycle LOP. Since this engine is not generally miserly on fuel is there a reasonable way to operate this type of engine more economically?

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    I am no expert, but a lean mixture in a 2-stroke is a recipe for a seize-up because it is fuel-lubricated. Besides, I don't see how, in most 2-stroke designs, you could regulate the mixture. Yes, there are a few crank-lubed 2 strokes, but there are also 2-stroke Diesels and a whole world of strange designs.

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    Aaron Novak's Avatar
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    Simply put, traditional 2 stroke engines are not run LOP due to the higher EGT causing a multitude of mechanical and thermal issues. Stratified charge designes are some of the most effecient designs however.

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    Hey Jim, if you want to tinker, check out installing a fuel/air ratio gauge on your engine. Map out the optimum mixture range - it will be well on the rich side - and run it there. You'll get a decent TBO out of the engine and avoid those pesky forced landings caused by engine seizures and burned pistons. Some racer types I use to know blended 10% nitro in their fuel so they could run a lean mixture and not burn a piston but in return they had poor engine life. Not a big deal if you rebuild the engine every couple races but no so practical on a flying machine.

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    My 2 strokes have pumper type carbies and you can tune the mixture for both high and low settings. i just lean them until i reach peak egt then richen up the mixture to best egt and rpms.

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    So, the current comments are varied. As I expected. The regular 2 cycle guys are mostly using Bing Carbs and there are lots of compromises throughout the RPM band with those carbs. Considerable depending on the airframe drag. I hadn't thought about the mixture meter at the time but it does make sense. Only question is the presence of lube oil and its effect on the sensor/readings. Adjusting for ROP seems the most KISS so far but is there a down side? I bet there is....
    Keep the ideas and facts coming...

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    All two-stroke fuel metering devices are a compromise. Too many variables for mechanical metering to compensate for. A computerized fuel injection offers what, a ~5% overall improvement?

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    Aaron Novak's Avatar
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    Quote Originally Posted by Jim Heffelfinger View Post
    So, the current comments are varied. As I expected. The regular 2 cycle guys are mostly using Bing Carbs and there are lots of compromises throughout the RPM band with those carbs. Considerable depending on the airframe drag. I hadn't thought about the mixture meter at the time but it does make sense. Only question is the presence of lube oil and its effect on the sensor/readings. Adjusting for ROP seems the most KISS so far but is there a down side? I bet there is....
    Keep the ideas and facts coming...
    The only down side is BSFC. The reason you need to run ROP is not due to lubrication, but to thermal distortion and combustion related issues. Remember, mixture "controls" burn rate, and burn rate "controls" EGT. The 2 real big issues you are watching for are power cylinder component temperature ( causing knock ), and exhaust port wall temperature causing cylinder distortion. Going lean slows the burn, raises piston crown and exhaust port wall temps, which leads to cylinder "sticking" and/or detonation. Stratified charge engines get away with their overall "lean" burn due to the fuel charge "plume" being rich and so burning quickly.

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    So, operating ROP maintaining manf suggested EGT/CHT temps - does that put the engine at say 12:1 or10:1? Is it possible to get a close mixture using standard temp probes or is a O2 meter needed? Is the Bing method - pick a best guess needle/jet combo and look at the plugs the better method?

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    Aaron Novak's Avatar
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    Quote Originally Posted by Jim Heffelfinger View Post
    So, operating ROP maintaining manf suggested EGT/CHT temps - does that put the engine at say 12:1 or10:1? Is it possible to get a close mixture using standard temp probes or is a O2 meter needed? Is the Bing method - pick a best guess needle/jet combo and look at the plugs the better method?
    Jim,
    The AFR vs. EGT varies with engine design. O2 is probably the least useful bit of instrumentation on a 2 stroke engine, stick to temperatures. Using the plugs as a mixture reference is also sketchy. Normally you calibrate your fuel system first, than adjust the heat range of the plug for the situation. Unfortunately 2 stroke calibration is not at straightforward as 4 stroke ( especially without the use of a dyno, indicating, and all the other instrumentation typical of an R&D lab ), which is why typically everyone errs on the rich side to be safe.

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