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Thread: LOP and the 2 cycle engine…..

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  1. #1
    Jim Heffelfinger's Avatar
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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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    Jim Heffelfinger's Avatar
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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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    Back to the original question (LOP)

    Quote Originally Posted by Jim Heffelfinger View Post
    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?
    A number of commenters, all more expert than I regarding 2-stroke engines, have gone off track a little here.

    1. The original question was about LOP, not just leaner or more economical.

    2. Yes, going to peak EGT is hotter and yes that is leaner than usual for a 2-stroke. However, actual LOP operations are, by definition, lower temperature than peak or rich of peak (ROP is where best power resides). Thus, the argument that LOP will kill the engine because it is too hot fails. It's the same erroneous argument that is often made against LOP in 4-strokes and it is equally wrong here.

    3. Based on my sad but limited experience, an extra lean mixture, if you could even do it with a 2-stroke and a carb., would provide insufficient lubrication with most fuel-oil mixes. This cannot be a hard+fast rule because the ratio of oil to fuel varies a great deal with various oil products and user practices. If you want proof, run the engine on pure gas. I can assure you from personal experience that no matter how rich you run it, it will seize. So try it the other way and run the oil-gas mix with more oil to compensate for the leaner fuel-air mix and try for LOP. Even if this worked, I doubt it would provide economy as it does in a 4-stroke because the oil is costly and because the mix is simply not as powerful with extra oil.

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    Jim Heffelfinger's Avatar
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    Howard, I am okay with the thread shift but still not quite sure of the dynamics that make running close to peak hazardous for the engine. If 12:1 ratio is the best mixture for a premixed 2 cycle engine and "4-cycling" is about 8:1 there is a big difference in efficiency between these two.
    If I use EGT ( the noted problem temp) as my baseline and keeping those readings below say - 1200 is that going to give me reasonable reliability, power and economy?
    I posed this idea to a number of well known EAA technical support people and even a 2 cycle expert and I got the Bing owners manual table quoted.

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