Running two-stroke engines lean-of-peak

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

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?
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
Most two strokes use the fuel-air charge to cool the underside of the piston. That extra fuel in rich mixture is an important coolant. In a 4 stroke, a rich/lean mixture is primarily to limit gas temperature.
 

kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
That extra fuel in rich mixture is an important coolant.
Thanks, that's a good data point.

When the engine is making 50% power at altitude is cooling as important a concern? My instinct is that it's not. I flew my plane to 12k feet this weekend, and the CHT and EGT were both hundreds of degrees under the peak temp I saw during my sea-level takeoff.
 

jedi

Well-Known Member
Joined
Aug 8, 2009
Messages
2,796
Location
Sahuarita Arizona, Renton Washington, USA
You make a good argument with the high alt lean of peak and a 32:1 rather than a 50:1 oil mix.

I have a background in engineering and engine test but do not consider myself an expert in any respect.

I do know from former employment and personal experience that if you run a typical air cooled two cycle to fuel exhaustion the engine can be trashed. Examples follow:

Multiple 1975 era snowmobiles on a dino at less than 1,000 feet pressure altitude and 80 degree OAT; As fuel supply decreases rpm increases and engine has a sudden stop seizure in about one second.

MZ single cylinder at 1,000 feet PA and OAT less that 80 degrees. After flight with minimum fuel engine had scorred cylinder and no compression. Engine would not start or run. No problems were noted in flight. Suspect air in the fuel line leaned mixture at a low power setting causing lean running engine and scored cylinder.

Rotax at 10,000 feet PA and 60 degree OAT fuel exhaustion and engine promptly shut down at first indication of power loss with no damage. There was no rpm increase indicating lean mixture.
 
Last edited:

kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
You make a good argument with the high alt lean of peak and a 32:1 rather than a 50:1 oil mix.
Nice to hear! My rough calculation for 20% less fuel would mean 40:1 for the same amount of oil per engine cycle. Out of curiosity, why do you say 32:1?

MZ single cylinder at 1,000 feet PA and OAT less that 80 degrees. After flight with minimum fuel engine had scored cylinder and no compression. Engine would not start or run. No problems were noted in flight. Suspect air in the fuel line leaned mixture at a low power setting causing lean running engine and scored cylinder.
Oh, interesting. The engine in question is an MZ35 on an AC-5M motorglider. A little bit ago the magneto ground connection failed, and I had no choice but to shut it down by closing the fuel shutoff. It was at low idle so maybe that's what saved us from any damage.

We were discussing using the fuel shutoff to shut down the engine so that there wouldn't be a dangerous fuel/air charge left in the cylinder. Thanks to your input, the the answer is pretty clearly *don't*.
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
I too have trashed highly tuned 2 strokes by running them out of fuel. I was probably pushing the jetting, though. This is something to avoid at high power levels. At idle, I wouldn't worry about it.
At 50% power? Might be ok. Let us know, so we won't need to fix our engines, too ;)
 

MadProfessor8138

Well-Known Member
Log Member
Joined
Nov 3, 2015
Messages
1,041
Location
Ekron,Kentucky
I guess I'll be the bad guy here and ask a very straightforward and simple question.......
What are you trying to accomplish with LOP on a 2-stroke ?
1. Just asking out of curiosity ?
2. More hp ?
3. Better fuel economy ?
4. You dislike doing maintenance by changing plugs ?

You're walking a very fine line between an engine that runs and all is well & an engine that needs a rebuild,possibly aircraft damage and physical injury.
The jetting on a 2-stroke is not something to be taken lightly unless you are more than willing to pay deeply for your indiscretions.
Even the professionals with big money,tons of data and some of the best tuners in the world toast engines when they make a miscalculation.

Run it rich,pay for an extra half gallon of fuel,check your plugs routinely and live to ride/fly another day......
Recoup your money spent on running rich by not having to pay for engine rebuilds,aircraft damage and hospital charges.....you'll be money ahead in the end.

Kevin
 

kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
I guess I'll be the bad guy here and ask a very straightforward and simple question.......
What are you trying to accomplish with LOP on a 2-stroke ?
1. Just asking out of curiosity ?
2. More hp ?
3. Better fuel economy ?
4. You dislike doing maintenance by changing plugs ?

You're walking a very fine line between an engine that runs and all is well & an engine that needs a rebuild,possibly aircraft damage and physical injury.
The jetting on a 2-stroke is not something to be taken lightly unless you are more than willing to pay deeply for your indiscretions.
Even the professionals with big money,tons of data and some of the best tuners in the world toast engines when they make a miscalculation.

Run it rich,pay for an extra half gallon of fuel,check your plugs routinely and live to ride/fly another day......
Recoup your money spent on running rich by not having to pay for engine rebuilds,aircraft damage and hospital charges.....you'll be money ahead in the end.

Kevin
Good questions to ask!
  1. Just asking out of curiosity?
    1. Yup, for the most part. LOP has been a minor revolution in airplane operation, and I am curious about it in a context which is relevant to my plane.
  2. More hp?
    1. Running LOP will always give less power than ROP (when the engine is run properly)
  3. Better fuel economy?
    1. The times I'll be high enough to think about LOP is when traveling a large distance. In this context, fuel = safety and LOP operations could increase cruise distance by 20%.
  4. You dislike doing maintenance by changing plugs?
    1. LOP operation has been shown to reduce overall maintenance. The reduced pressures and temperatures greatly extend engine life.
You mention thtat jetting is delicate and I agree. Fortunately, in a self-launching sailplane it's a lot easier than pretty much any other place, There's no mid-range, you're either running WOT to climb or your engine is off.

My baseline jetting is for ROP operation at my airport's elevation. If i were to want to run LOP, it would only be after climbing 5,000+ feet. I've got a CHT and EGT which are the essential tools for running LOP safely.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,836
Location
Memphis, TN
Practical questions.
What carb would you use to do this?
How do you balance oil need? You may not need as much oil for cooling, but RPM will be the same, so you will have to figure out friction need, but without over oil the plug.
My model airplane experience has power dropping very fast if over lean. Them being premix, usually shows damage if not enough oil.
 

MadProfessor8138

Well-Known Member
Log Member
Joined
Nov 3, 2015
Messages
1,041
Location
Ekron,Kentucky
This will probably sound like disorganized babbling,but here goes....

The problem is :
No 2 engines,even though identical,will run the same be they 2-stroke or 4-stroke.
What one engine likes...the other won't.
And then there's the whole difference between 2 & 4 strokes.

Let's use a motocross 2-stroke as an example.....
Each team has their own tuner and he's a very smart guy.
He knows what that one particular engine likes and what it doesn't.
He also knows the riding style of that one particular rider....does the rider row the gears or does he lug the engine by running a gear or two high or even over-rev by running the gears low to always be in the powerband.
He also knows how to jet according to elevation,humidity,temperature and fuel used.
Then there is the layout of the track....is it long straight aways or is it technical with short burst of power from corner to corner.

No matter what engine,rider or track....the tuner is always trying to get the most power out of the engine by running it on the ragged edge of seizure without actually seizing the engine.
This generally means running lean,for the particular conditions, and picking the rpm up.
Running lean on a 2-stroke will generally give you more rpm & hp but it does nasty things to the engine....increased temps,reduced lubrication and holes in pistons when pushed too far.

4-strokes are different and like to be ran rich.....
Just go to any drag strip and watch the guys tuning to run as rich as possible to make hp.
They are actually hydro-locking the cylinders,to an extent,to bump the compression.
Ever watch the diesel guys "role the coal" and smoke everyone out ?
They are making hp by flooding the cylinders with fuel to bump the compression....the smoke you see is the unburnt fuel from the cylinders being burnt in the exhaust system...like a smoke system on a plane.

When you start playing with fuel flow....you have to know that particular engine and understand what it likes and doesn't like.
So......there probably isn't a "one size fits all" answer to the discussion of LOP or ROP.

Kevin
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
Running 'lean' on a 2 stroke is actually something of a misnomer. 'Lean' is actually still richer than stoichiometric, is just not as rich as the safe zone. Fuel is used to internally cool two strokes as well as keep gas temperatures down, to the detriment of power. That's why running less rich gives a nice power increase, right up to when you take it too far and it melts...
At greatly reduced power, running leaner than stoichiometric may be possible.
 

kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
Practical questions.
What carb would you use to do this?
How do you balance oil need? You may not need as much oil for cooling, but RPM will be the same, so you will have to figure out friction need, but without over oil the plug.
My model airplane experience has power dropping very fast if over lean. Them being premix, usually shows damage if not enough oil.
The carb I've got is a Tillotson 194A, however in my case (single cylinder) I don't believe there is particular sensitivity to the carb. If I have a mixture lever that I can use in the air, then I'll be able to set any fuel setting I desire. Am I not thinking about this correctly?

Agreed 100% on the RPM. I should be careful in thinking what that means. RPM is a sign of how much air and oil is passing through the engine, but the oil needs are a function of speed and power. I don't know of any data on the ratio between these two. This is going to be the driving factor for oil lubrication.

BTW, lower air densities will mean less fuel per revolution. So there might already be low lubrication if the primary driver of oil consumption is RPM.

@MadProfessor8138 Thanks for the description. I couldn't agree with you more about the differences between two seemingly identical engines. This was the topic of my graduate research, where we were identifying SFC curves for individual engines, capturing the differences between age, wear, and usage patterns.

I feel that motocross racing is a completely different beast from steady-state cruise, though. Those guys have a really hard problem and they're looking for the last % of power. I don't envy them the task.

At greatly reduced power, running leaner than stoichiometric may be possible.
Yes, I think that's exactly the thrust (pardon the pun). At 8000 feet pressure altitude, there isn't enough oxygen to make more than ~75% power. And the air temperature is 15-20C cooler, which gives a 10% boost in cooling. Climb to 12,000' PA and the engine makes less than 65% of peak power, and you get another 5% boost in cooling. And then running LOP would mean a further 10% drop in power.

Out of curiosity, what makes fuel cooling more important in a two-cycle than a four-cycle? My understanding was that it's critical to both types of engines.
 
Last edited:

MadProfessor8138

Well-Known Member
Log Member
Joined
Nov 3, 2015
Messages
1,041
Location
Ekron,Kentucky
I think it was Green Sky that offered a altitude compensation kit for carbs when they were in business......
If I remember correctly,his name was Gerald Olenick and he has passed away now.
Anyways.....I always wondered what air/fuel ratio was figured when developing the kit.
I've never ran across anyone using it to test it out.....

Kevin
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,836
Location
Memphis, TN
On the carb, I was wondering if you had one that could be changed on the fly off the shelf?

LOP, if you do not get the EGT drop after peak, you are not there. The only two planes I have been in flown LOP was one friend’a SR-22 and another friend’s RV 7. The Cirrus will tell you the target EGT to get to. The RV was all manual. Same idea, as you lean past detonation, you watch EGTs go down to your target. You do have to do it fast and you don’t want to go to far.

Two strokes, leaning past detonation, is just odd. Leaning a two stroke air/fuel to let’s say 17 to 1 to be LOP, I’m not sure the mixture consistent enough.
 

Billrsv4

Well-Known Member
Joined
Sep 29, 2016
Messages
152
Location
NW Oregon
I guess I'll be the bad guy here and ask a very straightforward and simple question.......
What are you trying to accomplish with LOP on a 2-stroke ?
1. Just asking out of curiosity ?
2. More hp ?
3. Better fuel economy ?
4. You dislike doing maintenance by changing plugs ?

You're walking a very fine line between an engine that runs and all is well & an engine that needs a rebuild,possibly aircraft damage and physical injury.
The jetting on a 2-stroke is not something to be taken lightly unless you are more than willing to pay deeply for your indiscretions.
Even the professionals with big money,tons of data and some of the best tuners in the world toast engines when they make a miscalculation.

Run it rich,pay for an extra half gallon of fuel,check your plugs routinely and live to ride/fly another day......
Recoup your money spent on running rich by not having to pay for engine rebuilds,aircraft damage and hospital charges.....you'll be money ahead in the end.

Kevin
Kevin,
I wouldn’t recommend LOP on a 2 cycle. The reason is that you are carrying your lube oil, either in pre-mix in the fuel, or in some engines injected from a separate oil tank. Since that oil lubes your big end bearings you don’t want to starve them. You can sometimes run a 2 cycle out of fuel without seizing or damage, but it still isn’t a good idea.
Bill
 

Andy_RR

Well-Known Member
Joined
Sep 29, 2009
Messages
308
Location
Melbourne, Australia
LOP only makes any sense when you have a fixed ignition angle.

Lean in a 2S works fine as long as you're not also leaning your lubrication! Lean operation will lower piston temperatures but also power.
 

TiPi

Well-Known Member
HBA Supporter
Log Member
Joined
Aug 25, 2019
Messages
480
Location
Peeramon (AUS)
2-strokes have a power stroke at every revolution and that puts a double heat-load into the piston. At the same time, there is very negligible fresh charge cooling of the piston top. That is the main reason to run richer than desired. Most 2-strokes will die very quickly from lean mixtures either by seizing (over-expansion of the piston as well as lack of lubrication) or a hole burnt through the piston top.
 

AdrianS

Well-Known Member
Joined
Jul 5, 2014
Messages
748
Location
Australia
The carb I've got is a Tillotson 194A, however in my case (single cylinder) I don't believe there is particular sensitivity to the carb. If I have a mixture lever that I can use in the air, then I'll be able to set any fuel setting I desire. Am I not thinking about this correctly?

Agreed 100% on the RPM. I should be careful in thinking what that means. RPM is a sign of how much air and oil is passing through the engine, but the oil needs are a function of speed and power. I don't know of any data on the ratio between these two. This is going to be the driving factor for oil lubrication.

BTW, lower air densities will mean less fuel per revolution. So there might already be low lubrication if the primary driver of oil consumption is RPM.

@MadProfessor8138 Thanks for the description. I couldn't agree with you more about the differences between two seemingly identical engines. This was the topic of my graduate research, where we were identifying SFC curves for individual engines, capturing the differences between age, wear, and usage patterns.

I feel that motocross racing is a completely different beast from steady-state cruise, though. Those guys have a really hard problem and they're looking for the last % of power. I don't envy them the task.



Yes, I think that's exactly the thrust (pardon the pun). At 8000 feet pressure altitude, there isn't enough oxygen to make more than ~75% power. And the air temperature is 15-20C cooler, which gives a 10% boost in cooling. Climb to 12,000' PA and the engine makes less than 65% of peak power, and you get another 5% boost in cooling. And then running LOP would mean a further 10% drop in power.

Out of curiosity, what makes fuel cooling more important in a two-cycle than a four-cycle? My understanding was that it's critical to both types of engines.
Are you taking density into account when calculating your 10% cooling boost at altitude?
Remember it's air mass flow that takes the heat away.
 

kubark42

Well-Known Member
Joined
Oct 19, 2020
Messages
90
@TiPi, I think the point confusion about "lean" was addressed well in Running two-stroke engines lean-of-peak. Running LOP allows for lower temperatures, which means reducing the possibility of over-expansion or burning a hole.

Are you taking density into account when calculating your 10% cooling boost at altitude?
Remember it's air mass flow that takes the heat away.
This is a very good point. It is mitigated by the fact that true airspeed goes up as density goes down. So we have fewer molecules of air per unit volume, but we have more unit volumes of air. It's not a perfect balance, but for reasonable altitudes for normally-aspirated engines the airflow mass stays roughly the same.

-----------------------

I think summing up where we are right now:
  1. We can dismiss the risk of cooking the engine at LOP. That fear seems to be born largely out of the multiple meanings the word "lean" can have. Engine tuners who talk about "lean" mean "less rich than max power but richer than peak temps", but LOP means leaner than peak temps. We know that in true LOP operation pressures and temperatures drop significantly, so there's no heat risk at high altitude where lack of O2 already substantially reduces power output.
  2. On the other hand, there is a very real risk of lubricant starvation and careful attention has to be paid to the fuel/oil mix. I don't believe there's any clarity on this.
    1. Without firm knowledge of all the factors which affect fuel flow-- e.g. at high altitude, lower manifold pressure means less fuel per revolution--, and without a firm understanding of the engine's oil requirements-- e.g. how much oil consumption is driven by speed and how much by combustion--, it becomes a research effort.
To get an authoritative answer, it would take a fair amount of lubrication testing. It's not something I'm personally interested in doing, because the only real way to do testing correctly is to intentionally abuse/damage/destroy some engines and compare their wear patterns to a known baseline.
 
Last edited:
Top