Rods are plain bearing in almost all engines so why can you use less oil for a BB engine?

Generally, a ball bearing engine produces less friction

Less friction = less heat.

Oil doesn't only lubricate, it also acts as our engine's coolant.

Also, plain bearings don't retain any oil,and they constantly need more.

Ditto

Not whole correct answered, the model diesel engine are using until 50% less fuel than glow plug engine therefore need more oil.

Don't roller bearing engines require just 8 or 9% oil to operate?

He's talking about the amount of fuel used in relation to the size.

Diesels = Less fuel used, but same amount of oil needed for engine size = richer oil mixture

What is the lowest percentage of oil you would feel comfortable running in plain bearing model diesel engine?

I believe I understand that.

But the bearing themselves are designed to need 8 to 9% oil to work.
At least in a gas motor.

The percentage of oil would remain the same no matter the fuel economy.
At least in my head anyway.

It's not the amount of fuel used per unit time, but the amount of fuel in the motor at any given time.

Say an engine is running at 60 RPM, firing once per second.

Every second, there is 1 cc of fuel pumped into it. At any given time, there is 1 cc of fuel in the motor. That is all the fuel required to keep it turning at that rate.

Now say the engine (this is just for simplicity's sake) needs .5cc of oil at any given time. .5cc of fuel, .5cc of oil.

The .5cc of oil must be maintained both in glow and diesel.

However, you need much more methanol than you do diesel to keep the engine running.

Now lets say the engine only needs .25cc of diesel to run at 60 rpm, but it still needs the same amount of oil, .5cc

In simpler terms, the same amount of oil is needed at all times in the engine, but not necessarily the same amount of fuel, due to the higher calorific value of diesel.

It's percent taken literally. Per 100, or per whole.





My values are ridiculous for a reason: I'm tired, but they still work.

Somebody will get around to this better than I can, but I figure ~20%

I just haven't had the time around to gain the experience most of the other people here have.

I dissagre somewhat about the amount of fuel determins the amount of lubrication. I see it as an oil film on all moving parts based on the % of oil in the fuel. More or less fuel going through the engine doesn't change the oil film on the parts. Once the oil film is on the parts it takes very little to maintain the same film.

Since the bottom end doesn't get very warm I don't buy the cooling argument either. Sorry to be hard headed but so far I can't see the logic behind less oil for a ball bearing engine because the rod should need the same oil film on either engine...

In fact I have some Davis ball bearing fuel and it is so thin I won't run it without extra oil even in a BB engine. The piston and cylinder need the same oil too. Oil also helps seal the piston. Better too much than not enough!

fiery,
I think 22% is generaly considered the min oil percentage in plain bushed engines for long life.
chevy,
Lubrication requirements vary with design sizes of the shaft and its clearances. There must be sufficient oil flowto maintain a full bush to shaft fill otherwise the shaft will begin to go into an oscilatory rotation which tends to squeese out the remaining oil film and cause increased wear. Imagine a shaft in a bush with a good deal of clearance , on the start of rotation(without oil)the shaft will try to climb around the circumference of the bush(cyclic rotation) as this is a crankshaft with vertical forces being applied it will be hammered up and down from this cyclic rotation - not pretty. Now if sufficient oil flow is present to fill the gap the shaft will rotate pretty much in the centre of the bush and be in a smooth rotation.
A ball race tends to trap enough oil in the groove of the race with very little resupply nessesary.

So how much oil, well that depends on the design and construction but generally 22% some PB will get away with 15% and be ok , but which ones ?
Now the little end generaly requires less oil as the film isnt being spun out, even the PB little ends in BB outboards and the like are ok at very low oil levels.
Stewart

I have a Fox .74 ABC, a Fox .60 ringed, a SuperTigre .51 and a couple of others I've been flying since 1991 or 1992 using Davis ABC mix, I would never think of adding any lube to it and my engines are all still as good as new. The .51 is a little tired but I suspect that is from a lousy break in when I was new to rc engines, ringed ones in particular. The .51 has always been flown with a 12x6 three blade since conversion to Diesel.

I run with 30% castor for plane bearings and 20% for ball bearings. I've always figured that the total area of the bearing surface in a bushed engine required the higher percentage of castor oil when compared to a an engine with two or even just one ball bearing ring.

i'm glad you explained that...i was very confused!

however, i think the dynamics of friction ranks higher than the volume fractions



i think you are right on the first statement - the % of oil in a drop of fuel from a gallon is the same as the % of oil in the whole gallon.

the cooling thing - i'm gonna go with it does cool. otherwise, it seems it wouldn't make the film and stick...it'd just burn up. less oil in a ball bearing - you make a valid point about the con rod. i'm just not sure if the ball bearing itself would require less oil, and then the "extra" oil could be used by the rod...especially true of sealed bearings.

This is growing into a huge discussion over the interpretation of fractured english. (No offense intended for Motorboy)

What my interpretation as to what he said is that the ratio of lubricant to high calorific material varies between glow and diesel.

Diesels and Glow need the same amount of oil, but less fuel. The ratio of lubricant increases in a diesel.


We are getting sidetracked here. The surface area of the crank bushing in a PB engine is very large, therefore a lot of oil is needed to keep it constantly coated. Ball bearings are naturally lower friction AND they have the benefit of containing oil very well.

How big is the connecting rod bushing on the biggest nitro airplane engine you can think of?

Also, being right in a pressurized vessel with a hole specifically to allow lubricant in, it is very easy to keep it sliding.

Oil IS coolant for the engine. If the film is maintained so well, why does an engine get hotter when you lean it out? (assuming you maintain the same RPM)

And also, consider this, in a full size diesel, they maintain such a large oil reserve simply for the purpose of heat absorption.









The most important point here.

Oil percentage in glowplug engines and diesel engines are difference.

Let us see at the operating instruction for the Enya engine:

Enya 06 Main bearing: Light alloy Oil percent: 20%
Enya 06D Main bearing: Light alloy Oil percent: 25%

Enya 15-II Main bearing: Bronze Oil percent: 20%
Enya 15D-II Main bearing: one ball bearing and Bronze Oil percent: 25%

Enya 41-4C R/C Diesel Main bearing: all ball bearing Oil percent: 25%

The diesel engine need more oil than glowplug engine. The 4 stroke diesel engine need more oil to give enough oil to lubricate moving parts in crankcase.

And other engine as PAW to example are oil percent difference between engine with plain bearing and engine with ball bearing:

25% for engines with plain bearing.
20% for engines with ball bearing.
The lowest are 15% for ball bearing if the engine are used as racing engine with high kerosene in fuel.

The racing diesel engine who are running at 15% oil can not last long as a normal operated diesel engine.

In generally the diesel engine need more oil due low fuel consumption and enough oil film against hi load at bearing.

The first portion there was close to my interpretation, and I'm surprised that nobody mentioned the second part, as it is fairly obvious.

The fuel of a glow engine has a lot to do with cooling - not as much the oil. When the fuel evaporates it absorbs heat. Also a lean mixture runs hotter in gasoline engines too.

If loss of lubrication beggins tempratures do go way up till the parts gall and seze.

The rod end carries all the load in an engine and has far less surface area than the main bushing does for the same load. The main buhsings should get just as much oil as the rod end on a front rotary valve engine since all the fuel and oil pass right there. Some engines of use an oil groove too. Crank case pressure should also be pushing a constant supply foward twards the prop. Most of my engines leak some oil out of the front bushing.

I may have to run my own low lubrication destruction tests to see what fails first......

Evaporative cooling may not always be the case. Engines don't run on liquids. They run on vapors. I think by the time the fuel hits the crank shaft, which should be plenty warm due to thermal conductivity from the piston, it will vaporize. That shouldn't do much for the rest of the engine.

Gasoline engines, in this particular example, your average 4 stroke, the reason they run hotter with a leaner mixture is the oil film is burned off by the thirsty combustion, creating additional friction.


I have the feeling that if you run a low or no oil destruction test (which is very fun, especially on a Chevy V8) chances are the piston will lock istelf up before much else happens. it shouldn't take very long. If you're lucky, you might break the connecting rod.

2 points here need refining
Oil IS coolant for the engine. If the film is maintained so well, why does an engine get hotter when you lean it out? (assuming you maintain the same RPM)

This is also true of gas engines that do not use premix.
The fact that oil is a coolant in model engines comes from the fact that it asorbs heat from the combustion chamber
and is expelled out the exhaust.

The large oil reserve in automotive engines does play a part in cooling.
But it is because it picks up heat from bearings while it is being pumped to those critical areas.
Of course it does pick up heat from other areas too.
But it is picked up ran through an oil cooler and then back to the bearings.

The larger resevoir , to a point, the cooler the oil will be.

Chevy and all the other good folks in here to date...

I'm surprised that no one has yet mentioned the effect of the basic fuel liquid in diluting the oil fraction...

Petroleum oils dissolve in gasoline (likely also in kerosene), vegetable oils in methanol. (Generally, and oversimplified...)

As to maintaining an oil film once established: the basic fuel component is solvent for the oil, and can as easily flush it off the wearing surfaces as help bring it to them. That, and the added area of plain, full-contact bearings make a good first point in why manufacturers often - if not always - recommend more oil% for plain bearing engines. The second point mentioned in here is that diesels generally burn much less fuel for a given time. With less fuel passing through the engine per microsecond, each increment's fuel plausibly needs a bit more oil to maintain adequate lubrication. Our glow engines slosh bunches of fuel through; our diesels do not.

Of the oils: the synthetics available in the 1970's, when Dave Gierke did his classic engine test articles for Flying Models mag, burned off at combustion chamber temperatures indirectly measurable on glow engines run leaned out (ST 40, possibly a G21-40?). They burned off at 485°-490°F, IIR. Castor didn't, it held its mud to 500°-510°F, IIR. Measured head temps were between those two ranges... Implication? Castor is heated by conditions as it goes through an engine, and by staying intact, unburnt, carries the heat it absorbed out the exhaust. That is a cooling advantage. Synthetics, unless they have raised the flash point 20°-30°F since Dave G's tests, burn away - adding heat inside the engine instead of taking it with them as intact, hot droplets, out the exhaust port.

(The lubrication effects of synthetics burning above the piston on the power stroke seem to be insignificant. Nothing needing lube is in there, and the generous transfer porting in today's schneurle layout engines douses the upper sleeve with ample fresh, cooler fuel/air mix to lube the piston on the way up. A cyl/piston pair which, btw, is most often cut to fit best at operating temperatures, thus is a bit less dependent on the oil for survival...)

Empirically, glow fuels DO cool the lower crankcase very effectively as the methanol - at least - vaporizes out the spraybar/throttle nozzle. You can check this on the test stand by sliding a finger under the case, below the venturii/carb as it runs. It is cooler, if not actually cold to the touch, unless the engine is run extremely hot. Even then it is cooler than the outer case from the base of the exhaust stack up.

Diesels, and the few spark engines, I've run do not have the 'heat of vaporization' advantage of methanol-based fuels, and soak hotter throughout. (I've noticed that diesels do not get as hot up top as glows, but soak to a more uniform temp, overall.) Ether does chill on vaporization, but there's usually about half as much by percentage as there is methanol in glow fuels. Kerosene does not chill much on vaporization, nor does gasoline.

We have the ability, with modern metals and manufacturing techniques, to produce inexpensive engines that require a very low oil% in pre-blended fuels. Traditional model engines, and even our most modern-technology engines, usually require much more oil than those (weed-whacker, lawnmower, etc.) engines, partly to absorb and eject heat. Some few modern model engines are fitted and metal-matched so that they DO NOT run well on castor, or castor+synthetic fuels. Part of that may be the tendency of castor fuels to deposit a varnish-like coating on the hot wear surfaces they are exposed to. That is great for 'porous' cast-iron pistons in steel sleeves, but not so nice for chromed, or nickeled, softer metal sleeves running high silicon aluminum alloy pistons. ANY varnish depth ruins the designed and intentionally developed operating temperature fits.

As to diesel manufacturers recommending lower oil% for racing purposes - the general presumption seems to be that:
1) a good break-in comes before competition use, so that heat cycling, fit burnishing, and any wear-tempering processes are well established.
2) Control-line racing, at least, is NOT at an engine's flat-out ultimate output conditions. It is eased a bit so that pitting restarts are dependable. Overheated engines do not restart well... And, laps per tank fill are critical - the models have NO airspeed while on the ground in the pits...
3) CL Racing (and Combat) operation is a pretty constant-RPM condition. Loads conditions dn not vary drastically as they would in throttled (CL Scale or Carrier), or maneuver loaded (CLPA) use. Load and speed changes vary heat generation more than steady load and RPM conditions. (In CL Combat, the engines seem optimized for the energetic maneuvering involved, and audibly go over the top in straight and level flight.)(Straight and level flight in a match is suicidal self-sacrifice...)
4) To be competitive, CL racing engines must be in that flat plateau of their wear lifespan where they perform well and dependably. No one wants a racer that gets a 5 lap lead on the first tankful through sheer speed, then takes 10 laps to restart and rejoin the fray... The first sign of falling off the end of the performance plateau means either accepting loss of competitive performance, or a new engine at the beginning of its useful lifespan.

Further note: PAW, for one, suggests breaking in ALL their engines on an oil-rich fuel for the first several minutes, then reducing to about 25% castor for plain bearing engines, 22%-23% for single BB engines, and 20% or so for general use 2BB engines - and less oil% for well broken-in engines used in racing.

Great post Lou, Thanks, one of my earliest Diesel conversions was a Fox .74 ABC and at first I thought it ran hotter than it did as a glow engine. Then, I realized as you say, the whole engine was the same temperature but cooler over all.

very awesome Lou. the flash points are something i'm very curious about. i prefer castor or a blend of some sort. i thermodynamically know why now..

Back in the day when we were flying ST G-15 glow's in CL Goodyear racers, the oil thinking went like this. Too little oil and the bottom end of the conrod blows. Too much oil and the ball bearings hydroplane rather than rolling, and one or both bearings blow. So, according to this thinking, too much oil in a ball bearing engine is not good. However, other than the race situation mentioned above, I don't recall ever blowing a bearing, too much oil or not.