Running two-stroke engines lean-of-peak

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jedi

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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?
.........
No real reason for the 32:1 reference just a common mix ratio that is less fuel / more oil than the otherwise standard manufacturers specification. I was not sure if you had referenced 40:1 or 50:1.

It would be much better to have an oil injection system so a fuel lean would not also lean the oil.

General Answer:

The limiting factors for two cycle operation are excessive temperatures and inadequate lubrication.

The lubrication issue is best addressed with the oil injection system.

The temperature issue comes from several variables. That is throttle position/manifold pressure, fuel air ratio, and RPM.

You have commented on the first issue and are attempting to counter with the second issue. It is possible to balance these two issues within narrow limits.

The application given has limited ability to control RPM as the load is a fixed pitch propeller. Heat rejection is directly tied to RPM if other variables are constant. If a variable prop could lug the engine to 1/2 the rpm (and half the power) the LOP option would likely work well. Set up for max power at takeoff and at altitude is probably not a good idea without an extensive test program.

I have many hours with a Rotax 582 or Hirth and electric IVO Prop. I prefer to take off WOT and leave the throttle open while bringing the rpm down with the prop pitch. If that were done to 10,000 feet I am certain the mixture lean would be worth while if oil injection were installed.

If power is reduced with throttle or rpm EGTs and CHT can be somewhat increased (safely) by a mixture lean.
 
Last edited:

Billrsv4

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NW Oregon
@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.



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.
I would caution everyone about point 1. in this response. two-strokes will show a temperture drop LOP, if you can get there. Since many two-cycles are running near their peak temp at WOT even with the correct mixture, as you lean the mixture you will pass through a very dangerous area just LOP. you can burn a piston or seize very quickly. If the pilot isn't expert at adjustment they can cause damage even if they manage to get to a LOP stable operation point. There are enough concerns that I would be reluctant to ever do it with my engine. The gains in fuel consumption for a short time don't make up for the need for a topend rebuild. Just doesn't make $ and sense. Your mileage may vary...
Bill
 

Billrsv4

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Messages
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Location
NW Oregon
No real reason for the 32:1 reference just a common mix ratio that is less fuel / more oil than the otherwise standard manufacturers specification. I was not sure if you had referenced 40:1 or 50:1.

It would be much better to have an oil injection system so a fuel lean would not also lean the oil.

General Answer:

The limiting factors for two cycle operation are excessive temperatures and inadequate lubrication.

The lubrication issue is best addressed with the oil injection system.

The temperature issue comes from several variables. That is throttle position/manifold pressure, fuel air ratio, and RPM.

You have commented on the first issue and are attempting to counter with the second issue. It is possible to balance these two issues within narrow limits.

The application given has limited ability to control RPM as the load is a fixed pitch propeller. Heat rejection is directly tied to RPM if other variables are constant. If a variable prop could lug the engine to 1/2 the rpm (and half the power) the LOP option would likely work well. Set up for max power at takeoff and at altitude is probably not a good idea without an extensive test program.

I have many hours with a Rotax 582 or Hirth and electric IVO Prop. I prefer to take off WOT and leave the throttle open while bringing the rpm down with the prop pitch. If that were done to 10,000 feet I am certain the mixture lean would be worth while if oil injection were installed.

If power is reduced with throttle or rpm EGTs and CHT can be somewhat increased (safely) by a mixture lean.
Something most people ignore about two-cycle operation is that most systems that inject oil are based on RPM of an engine driven injector pump. There are also pumps made that work off both (engine) RPM and throttle opening, similar to a diesel fuel pump. These pumps don't VARY the mix due to a lean mixture. (At least that is typical of engine driven pumps for most two-strokes.) It also isn't a simple equation, most two-cycle oils are formulated to burn as well as lubricate. Most two-cycle oils burn hotter than gasoline. I worked for Kawasaki in the two-cycle motorcycle era. All the road-going two-strokes used a seperate oil tank and injector pump. The public always believed that the factory set these pumps "high" for break-in. Not true. In fact as oils got better they lowered the amount of oil injected to prevent seizure. I'm not trying to be scary here, just to inform that most two-strokes are a very different animal than their four-stroke counterparts. Lastly if your engine runs on premixed fuel and oil there is no way to not lean the oil mix since it is delivered with the fuel. If you up the oil in the premix you are back to the problem of oil burning hotter than the gas possibly causing higher engine temperatures. Lets be careful out there.
Bill
 

Andy_RR

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Sep 29, 2009
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308
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Melbourne, Australia
There really is no such thing as "burning hotter" or "burning cooler". You have to be more specific about what is hotter and what is cooler. Oil can do many things to combustion including slow the burn rate which has a similar effect to retarding the ignition angle. It's also detrimental to the knock resistance of fuels so increasing the fuel:eek:il ratio can be a good thing for performance. Unlike four-stroke engines, piston temperatures are influenced by exhaust temperature because of the proximity and flow of exhaust over the ring land and skirt so if you lean without correcting (advancing) the ignition angle you can raise the exhaust temperature and overheat your piston even though the actual combustion chamber (head, piston crown etc) heat flux may actually be lower.
 

Werner

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Jun 13, 2012
Messages
33
Location
Ortisei Italy
maibe the major difficult is to know the exact amounth of possible leaning.
an other is the mixture enriching around the sparks and the avoiding to transport unburned fuel out of the combustion camber.
a feedback ist extreme important. A chemical feedbeck (lambda or flame color) is reliabler as a egt temperature. but the best coiche would be a field test. for exemple any 0.1 second a automatic progressive leaning to understand the possible leaning with real combustion. a piezo- knock sensor or a current tracing in the spark plugs can give a good feedback.
But this all is realistic in the year 2030, not now.

normally we see the 2 stroke as poor variant of the 4stroke. thaths the error.
 

Armilite

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Location
AMES, IA USA
Your 25hp Engine at Sea Level rating and Elevation of 8000 feet there is 6.0 HP loss. To get that 6.0hp back you can raise the CR 2-3 points and use a Tuned Pipe vs a Muffler. A lot depends on the original engine specs.

Running an Engine Lean is not going to make much hp difference or that much difference in gph use with these Small CC 2 Stroke Engines!

Going from the Ground to 8000ft at Continuous Max rpm, you not only want a Good Oil like Hirths Blue Max or Amsoil Saber Pro at 80/90:1 Ratio but also use 93+ Octane Gas, 100LL is best to fight Detonation.

To Help fight the Heat made, use the different Engine Coatings.

Example of a Rotax 277UL with R&D Aero's Tuned Pipe. Made Max [email protected] BTU's made at 6250rpm was 77,605. But if using all the Engines Coatings the Engine should run about 20% Cooler 62,084 BTU's about the same made as turning it 5250rpm with No Coatings!


ROTAX 277UL R&D TUNED PIPE.jpg
 
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