Too much oil.
 

Hello dear members [8D]


I’m going to start flying with r/c airplanes this summer, and now I have a question for you concerning fuel.

Almost every were I look, it is said, that a 2 stroke glow engine had to have at least 15 % of oil in the fuel.

When I raced with 1/8 Off-road cars in 1986 to 1993, we never used fuel with more 10 % of oil. We have NEVER ruined engines, crankshafts, or bearings due to low amount of oil. One year a friend and I, had the luck to buy some additive from Swedol. It was a prototype of additive for 2 stroke motocross engines. We mixed 1 % of the additive, 6 % Castrol MSSR oil, 25 % nitro and 68 % methanol. Our engines was running like a Swiss watch for the entire race season. The color on the top of the piston was gold, and there was a nice thin coat of oil (not wet) inside the engine.
For two years I used OS-MAX .21 engines with ABN cylinder, with 10 % oil, without any problems.

A normal airplane engine is running with ca 15.000 rpm, where a .21 car engine is turning >35.000 rpm. That must put the oil in a car engine on a harder job.

In my mind, too much oil will not make a better lubrication, than a correct amount of oil... And a higher % of methanol, instead of unused oil in the fuel, will make a more powerful engine-performance.

I would think, that 3 % castor oil, 7 % synthetics oil, (or 10 % Klotz Super TechniPlate) 5 % nitro and methanol for the rest, would be an ok mix.

Had any of you tried fuel in r/c airplanes with only 10 % oil?

And am I all wrong about glow engines? :eek:


Best regards

Bjarne, Denmark

RE: Too much oil.
 

yes, in fact, many have tried, and all went back to more oil.
In a car engine, the engine NEVER runs all out for longer than mere seconds, and then is throttled back again, relieving the loads on all parts. Due to the high rpm, lots of fuel passes the engine, so even with 10% mix lots of oil enters the engine as well.
In an airplane, the engine hardly ever runs at idle, but it uses full power most of the time at about 16,000 rpm.
A customer runs MVVS engines with 10% oil, and is happy with it. He does mini tractor pulling, revving up to 30,000. I checked his engines, and they look like they still have at least ten years of hard service to go before anything wears out.
When I inspect my combat plane engines, running at 22% oil mix, I see a lot more grueling use. I have tried to run them at 10%, and they survived, but did not feel good at all.

RE: Too much oil.
 

Proost pe reivers [sm=regular_smile.gif]

That was exactly what I was hoping to learn. Now it make sense, why you all recommended >15 % oil.

Bjarne (Big fan of Anouk and Vengaboys, and drove Serpent cars for 8 years) [sm=wink_smile.gif]

RE: Too much oil.
 

Pe, it's been my experience with hi-performance 2-stroke engines (motorcycle racing) that high revs on a trailing throttle is the *worst* possible thing for engines running a pre-mixed fuel.

The problem is that when you close the throttle the supply of essential oil is effectively stopped -- but the friction continues because the revs are high.

Just about every hi-performance 2-stroke I've seen that's suffered sudden failure due to lube failure has done so during or shortly after a period of running on a trailing throttle like this.

From this, you'd expect that model cars would actually need *more* oil in their fuel than a model aircraft. When you snap the throttle closed on a model car, the inertia of the car and the effectively direct connection between engine and wheels (ie: the clutch is engaged) means that the engine is being turned at high RPMs by the wheels, but with no fuel/oil passing through it. On an aircraft, prop-slip means that snapping the throttle close results in a very rapid decay in engine-revs -- pretty much in proportion to the amount of fuel/oil passing through the engine.

That may have been the case 20 years ago -- but today I'd wager that things have changed somewhat.

For a start, most non-competition engines these days are propped for 12K-14K RPMs and many types of application (scale, 3D, aerobatic, etc) don't actually require protracted periods of full-throttle operation.

What oil is he running?

It's as much (or more) about the performance of the oil as it is about the quantity.

Can you define "did not feel good"?

I have deliberately *tried* to destroy engines running with 12% lube and failed. This has included *very* lean runs (without castor) and protracted periods of hi-RPMs at full throttle.

While I'm sure there may well be a marginally increased rate of wear under such torture, no wear was measurable over the test period so I suspect that the amount of money you saved by using less oil would more than pay for the difference in engine life.

In fact, when you look at the price of oil, you'd probably find that the use of 22% is costing you the price of several *new* engines during the lifetime of one.

Just think -- if you ran less oil (and banked the money you'd have otherwise spent on that oil), you could probably afford replace that engine twice -- even though you probably wouldn't need to) over the same period of time.

What people seem to forget is that we used to run 20% oil back in the 1960s when engine design, construction and metalurgy were a *lot* worse than they are today.

How far did your car go between sets of rings, valve-grinds or new bearings back in the 1960s? You were probably lucky to get 80,000 miles before the engine needed reconditioning.

Now look at today's modern cars. It's not uncommon to go 200,000 miles+ before any significant engine work is required.

Why is that?

Better design, better manufacturing, better metalurgy and better lubrication.

Well guess what -- all that also applies to our model engines.

Use a *good quality* modern oil in your model engine and you *can* safely go down to 12% or lower without sacrificing a thing. Plus, you'll have more money in your pocket as well.

The only caveat to this is -- don't try and cut back on the percentage of oil if you're using one of the older US-made synthetics (such as those commonly found in US fuels). They're 1980s technology and not quite up to the task. There's no way I'd run 12% Klotz for instance!

The European ester-based synthetics and Coopers in the USA are a whole different (better) animal to the PAG-based oils used by most US fuel-blenders.

RE: Too much oil.
 

so what are you trying to tell me?
I know of sudden failures in full size MX engines (not glow engines) when throttled back. I also know why that happens. It cannot be compared with the issue at hand and is not a lubrication issue.
I also know of some Aerobatic pilots that went to 10% lube, because MVVS spares were cheaper than oil. They did not wear out their cylinder sets, yet returned to using more oil.
Even if you write 20 pages full of reasoning, you cannot prove the point that less oil is better in airplane glow engines, all of the time. Less oil means a price has to be paid one way or another. In the long run more oil is better in user satisfaction and engine power. The extra price to be paid for that is purely academical, and of no importance at all for practical purposes.

RE: Too much oil.
 

If it's not a lube issue, what is it?

Why?

If there were no negative effects, why go back to more oil?

Perhaps it isn't "all the time" -- but if the experience of our club is anything to go by, it has certainly been better for all of us over the past year or more.

That's at odds with my own experience.

What is that "price to be paid"? As you've pointed out -- it's not related to engine wear (on the MVVS engines anyway).

I've gained several hundred RPMs dropping my oil percentage from 18% ot 12% and have also found that my engines run cooler, idle lower, have better transition and I even get more minutes per tank.

That leaves me *very* satisified.

How do you define "user satisfaction"?

Well again, my practical experience seems to be at odds with your own. I'm really enjoying the benefits of a lower oil content without having seen any of the negatives after more than a year of very hard flying.

I guess that's what makes this hobby so much fun, the diversity of opinion and experience.

RE: Too much oil.
 

I used to wonder about the oil used in cars too until my son got into it and I saw how they were operated. I'd estimate that on his track the longest an engine would run at peak revs is around 4 seconds, the rest of the time it's mainly "brack brack brack" at fairly low revs. But I think there's one other thing involved and that's the amount of nitro used. Most of them think 25% is about as low as you can go but high nitro like that means horrific fuel consumption which means high fuel flow. But because the fuel carries the oil along with it then there's a higher rate of oil flow than what might be expected just going by the % of oil in the fuel.

We did a mod to his engine (raised the compression considerably) to make the most of zero nitro and use 20% all castor because that's what we use in our planes (true FAI fuel). Although it's now got exactly the same performance :) as the same stock engines using 25% nitro he gets 8 minutes per tank compared to 6 minutes for the others. That means his oil flow rate is roughly the same as the nitro fuel with 15% oil.

RE: Too much oil.
 

This has been my experience as well.

@ Xjet.
I am not trying to make you unhappy with your good results. I get better results (like better power and engine response) with a bit more oil, and I am not alone in this. If it were not llike that, every model flier would use less oil because it is better.
About the (gas) two stroke seizures at part throttle:
Oil tends to accumulate at metal parts. A MX engine reportedly ran over half an hour on straight gas without damage. (because the rider needed to get home) Seisure of pistons mostly is caused by gas blow by past the ring, scorching the cylinder wall oil away, and limiting the piston heat transfer to the cylinder wall. Gordon Jennings reported about this in Cycle magazine during the mid 7-ties I believe.

RE: Too much oil.
 

Good points you all have. There is not only on answer.

Can any of you follow me, if I say, that a engine not have to be all dripping wet with oil inside. If you move your finger around inside the engine, and it is getting moist with oil, there is enough oil in the fuel?

/Bjarne

RE: Too much oil.
 

Probably. But if the engine feels dry after a run, I would use a bit more oil.

RE: Too much oil.
 

Exactly Pe. Thank you [8D]

RE: Too much oil.
 

I agree with you. As engine size goes up, cube inches increase with the third power of size, and bearing surfaces increase with the square power of size. Roughly calculated, power increases in line with cube inches, and/or rpm.
So larger engines, or engines running at high rpm get more oil than is realy needed and could do with a little less.
However, a side note seems in place here.
A small engine, running at full power at high rpm (turbo fan engines), only have the small bearing surfaces of their low rpm brethern, but have to deal with a high power loading per unit of bearing surface. These engines need more oil. If I am informed well, oilo content for fan engines is in the range of 25%, whilst also running at additional big end play to reduce friction and subsequent big end seisure.

RE: Too much oil.
 

The other MAIN thing that has not been mentioned here is that the oil also carries away excess heat and acts as a conductor between the internal parts and the outer casing. Up to a point, more oil helps your engine run cooler until it starts to inhibit complete combustion.

Higher nitro racing engines alsh have a lot more cooling from the incoming fuel and air. Look at the old control line speed planes. The engines were enclosed in the fuselage with only a tiny slit for cooling air. They pumped so much fuel through them they didn't need much air!

RE: Too much oil.
 

Your last example is not realy sound in it's reasoning. The racers have a well designed cooling duct that is very effective. They do not rely on fuel to get rid of the heat, but use every single molecule of air that enters that tiny duct to transfer heat from the engine. That way, drag is minimized, and even can be positive, because air exit speed is higher than air entry speed, thus using the engine heat to provide extra thrust.

RE: Too much oil.
 

Pe,

Not a whole lot of engineering went into them in the provater ranks that I flew in. All we knew was that we had to limit the amount of air cooling because so much fuel and oil was passing through them. One even slightly lean run on those engines spelled instant disaster. The intakes of those engines were 1/3 the diameter of the pistons or larger and they ran wide open all the time. If the engine was run without the shrouds, they would run too cool and were hard to tune. The OS F1 engines were the same way. The intake was about 1/2 inch square.

RE: Too much oil.
 

[link=http://www.supercoolprops.com/articles/ducted_cooling.php]Here's[/link] what I thought was a very good and interesting article about the extremes at least one guy has gone to getting the ultimate cooling for a pylon engine but built into the engine itself. He's an Aussie so of course he's a bit crazy :D.
Actually it's worthwhile going back to the [link=http://www.supercoolprops.com/articles.php]index[/link] page for all his articles.

RE: Too much oil.
 

While it's true that oil may carry *some* heat away, simply by virtue of the fact that it enters the engine at ambient temperature, gets heated by combustion and then leaves the engine at a higher temperature, I have my doubts that this is a particularly significant factor in engine cooling.

My own experiences indicate that lowering the oil ratio from 18% to 12% has actually resulted in cooler-running engines here. Perhaps that's because we've also changed rom PAG-based synth/castor blend to an ester-based synth/castor blend -- I don't know.

What I do know however, is that one click on the needle will make a lot more difference to the engine's operating temperature than the addition or subtraction of a single percent of oil.

One also has to query why (if oil is such a critical part of the cooling system), larger engines (which suffer from a vastly inferior volume/surface-area ratio for their cylinders) tend to be run with lower oil percentages than the little ones.

RE: Too much oil.
 

Hey Xjet

I will agree with you.

For some years ago some of my racecar colleges started flying helicopters. They were told to use 20 % oil and a bit nitro. Their engines ran hot no matter what they did. They tried different mufflers, glow plugs etc... Nothing helped until they lowered the oil %. I don’t know what oil they were using.
Perhaps this issue has not only one answer. The world champignons drivers from Kyosho in 1992 used 20 % of oil in their OS engines, and Curtis Youngblood is using 23 % oil in his heli-fuel…

Pease

RE: Too much oil.
 

I am with Brian here.
These engines run so much power, and the cooling fins are so small, that internal fuel cooling only will just not cut it. A well designed shroud with intakes and exits will keep your engine healthy IF run at the right mixture. Running lean, and no amount of cooling will help you keep the engine alive.
Oil does hardly contribute to cooling. There is too little of it, and it does not evaporate where it counts. The amount of BTU carried away by the oil will maybe account for 10% or less of the total required cooling, with the fuel evaporation guestimated at about 40%. That leaves 50% of the required cooling to the cooling air, so the speed of the air through the scanty fins of the engine must be very high, only to be accomplished by a well designed cowl, if engine air resistance is to stay within limits.

RE: Too much oil.
 

Some time ago I did a very quick and rough guesstimate concerning the rate of flow of oil through an engine to get some idea of how much heat it might carry away. Obviously, oil out can only equal oil in which comes with the fuel flow. The method was guesstimated by taking a 10cc engine turning 10,000revs and emptying a 300cc tank in 10 minutes. With 20% oil in the fuel that meant there would be 60cc of oil in the tank. I didn't actually run an engine with that size tank but it would be very easy to get more accurate figures by doing so than my guesstimates.

But based on those figures, in 10 minutes that engine would have turned 100,000 times using up 60cc of oil. So every rev it would get .0006cc (60/100,000) of fresh oil coming in with the fuel. Basically, in round figures, that implies that any engine using around 20% oil will have a flow rate of oil that's close to 1/15,000 the swept volume of the engine. Seeing that virtually 100% of this oil comes out the exhaust as a liquid then there's no latent heat of evaporation to take into account. I didn't bother going into any further research into heat because I figured it was already down to negligible heat rejection.

RE: Too much oil.
 

I was thinking:
Could it be that manufactures of fuel, puts too much oil in their products to be sure, that it wouldn’t destroy the user’s engines, if the mixture is set too lean?
“My engine broke down, when I used fuel from the company X” – that is not a positive publicity manufactures wants, even when it is not their fault.

RE: Too much oil.
 

If you mix your own, you also put in a safe amount of oil.
FWIW: No oil amount will protect your engine from lean runs. You will cook your engine, unless the engine is so forgiving as to stop, or very much reduce power before it cooks.

RE: Too much oil.
 

Interesting discussion. i felt the need to point out one thing that was said concerning motorcycle 2 strokes vs rc cars. When you get off the gas in an rc car (decell) the engine is NOT spinning at high rpm with less oil. As soon as you get off the gas, the centrifical clutch releases and the engine speed drops to somewhere near idle.

RE: Too much oil.
 

are you sure?
As long as the engine is driven by the wheels at high rpm, the clutch will not disengage because the shoes are still pressed outward against the drum by centrifugal force.

RE: Too much oil.
 

Yep, the clutch will stay locked up until the engine gets below the rpm at which it locked up.