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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?
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.
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.
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.
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.
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...
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?
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.Quote:
Originally Posted by Jim Heffelfinger
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,Quote:
Originally Posted by Jim Heffelfinger
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.
Hey Jim,
Looks like numerous people have already weighed in on this one with lots of good information and advice. Nonetheless, I'll offer my two cents worth.
We all know that a 2-stroke engine's strongest suite has never been economy. If our primary purpose in wanting to control mixture on a 2-Stroke is economy, we would do well to respectfully remind ourselves that this can be a very slippery slope and one often met with catastrophic results. Two-stroke engines, and especially air-cooled smokers tend to fare poorly when subjected to lean mixtures. As already said in other posts, not so much from a lack of lubrication but from the increased heat generated from leaner mixtures. Heat is the enemy of any internal combustion engine and directly impacts longevity (TBO, etc.)
If on the other hand you're hoping to lean your engine out for increased altitude operation, you may want to give Mike Jacober of Arctic Sparrow a call. Mike was the first person to every fly an ultralight over Alaska's Mt. McKinley and he modified Bing 54's for just that purpose. He offers the same modification to your carbs (no modification to the carb body itself) for approximately $100 each and it allows you to dial-in the mixture from a dash mounted control. Might want to read about his modification (link provided above) and then give him a call. He can be reached at:
Arctic Sparrow Aircraft Inc.
7321 Rovenna St.,
Anchorage, AK 99518-2177
Phone (907)272-7001
Hope that points you in a direction that can help.
Sadly the owner of Arctic Sparrow was killed in a crash in 2003.
I have a MZ 201 engine which uses a Tillotson HR Carb. It has an accessible idle jet screw and Main Jet screw. The carb body can be realigned to have the adjustments facing the firewall and allow for in cockpit adjustment.
A number of commenters, all more expert than I regarding 2-stroke engines, have gone off track a little here.Quote:
Originally Posted by Jim Heffelfinger
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.
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.
Howard,Quote:
Originally Posted by Howard Handelman
LOP running with a 2 stroke is indeed dangerous on an engine designed for decent power output. Please do not confuse lower EGT values with lower component temperatures. Your second comment is based on the premis that the engine is using a pre-mixed fuel. Inlet inject or direct oiling engines are not subject to what you describe.
Quote:
Originally Posted by Jim Heffelfinger
Aaron is, of course, correct that my remarks are not applicable to direct oiling engines. I think they probably do apply to inlet inject, but I'm not sure.Quote:
Originally Posted by Aaron Novak
The burn temps in a 4-stroke while LOP are lower so there is no way that the components could be hotter than at some other setting. The burn is also longer and less peaked in terms of BMEP pressure. I am not the expert on this, but I got this understanding from conversations with the Advanced Pilot guys. There is no question, though, that when operating LOP within the recommended percent of power - usually not over 65% - not only are EGT's lower, but so are CHT's.
The air-fuel mix, by weight, when LOP is about 15.4. It's about 12.5 or 12.6 at best power and around 14.7 at peak which is also stoichiometric (a fancy term for the perfect mix to use all the oxygen and all the fuel in a perfect ratio). The reason for operating this way is to get the lowest SFC which is the measure of pounds of fuel per horsepower per hour. There is excess oxygen. Carbon monoxide is not formed.
When I had a KFM 2-stroke engine, the prescribed oil mix was 4% which translates to about 25:1. I have seen fuel-oil mixes as high as 50 or even 100 when, for example, Amsoil is used. I'm not endorsing that, just pointing it out.
I don't know if Jim is talking about air-fuel or fuel-oil when he talks about 12:1 and 8:1. I don't think a normal 4-stroke engine can even run at 8:1 air-fuel mix. Even 12:1 is rich in the 4-stroke world.
I don't know if fuel with oil in it (no matter when it is mixed in) has the exact same ratios (15.4, 14.7, 12.6) as straight gas. I rather doubt it, but I simply don't have any information on that.
No matter the reasoning (lubricity or temperature or both) I don't think it's wise to pursue LOP operations in a fuel-lubricated 2-stroke.
As for other kinds, well the Delta-Hawk Diesel is a 2-stroke and Diesels run with excess air on purpose. It's just not the same concept.
I think you'll find that once you lean to peak temp, the mixture will be too lean for proper engine operation and engine damage is very likey (engine damage is likely before reaching peak temp). As you can imagine, continuing to lean past peak will be difficult if you want the engine to continue running and producing power. On a conventional two-stroke, the incoming fuel/air charge controls combustion chamber temp. Too lean means too hot and you end up with a melted piston, in a matter of a few minutes. Too rich and the engine won't operate properly. What I said earlier, some racer types mix nitro in their fuel because of it's cooling effects in the combustion chamber, allowing a leaner mixture without burning a hole in a piston. Conventional two-strokes by design are simply not models of efficiency.Quote:
Originally Posted by Jim Heffelfinger
The unfortunate thing with a carburetor on a two-stroke is they never provide optimum mixture from idle to full power. That's why I think a fuel air ratio gauge has a huge advantage over a temperature gauge. It enables the operator to "see" when the mixture is too lean (and too rich for that matter). Then he can take action to remedy that condition, which may be something as simple as changing throttle position. EGT on the other hand fluctuates with throttle position, engine load, etc and is not necessarily a reliable indication of fuel air mixture. Actually, having both, F/A and EGT would probably be the best setup.