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Jan 09, 2019, 12:09 PM
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Why do small 4 stroke engines need oil in their fuel?


Why do small 4 stroke engines need oil in their fuel and not just oil directly in the crankshaft? Large 4 stroke engines have a special oil area to put oil in (like in your car). Why is this not the same for RC engines?
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Jan 09, 2019, 12:22 PM
AMA 537620
ChillPhatCat's Avatar
An oil pan adds to the weight and complexity... needs an oil pump too. As with just about anything in the hobby I would say weight is the number one reason... but it is also more complex than a car or lawn mower because most RC aircraft are also aerobatic so you need a system that is designed to function in all orientations.
Jan 09, 2019, 12:34 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
The biggest reason is model aircraft can fly inverted and/or often have the engine mounted inverted. If our model 4-Stroke cycle engines had an oil sump, the oil would be in the wrong place when the engine is inverted. This besides the complexity of an oil pump and related added weight.
Jan 09, 2019, 02:01 PM
Registered User
Not to mention the difficulty in getting crankcase oil to the lifters, cam followers and so on. Just simpler to add oil to the fuel.
Jan 09, 2019, 02:22 PM
A Picture is Worth 1000 Words
Gary Cee's Avatar
Remember the Damo Twin?

http://sceptreflight.net/Model%20Eng...%20FS-436.html
Jan 09, 2019, 03:26 PM
The great Brutifier
Actually, it is none of the above…. You could theoretically put some oil in the sump, and it "would work" sort of... Provided some channels for the oil to pass, it would even get oil everywhere it would be needed. Because that is exactly how it works now as well.
It would work inverted, because basically, that is how current engines work: there is an amount of oil in the crankcase and it splashes around, without any pump or whatever. Because like it or not, that oil in the fuel is NOT passing the crankcase first but last, it drips down along the liner wall and is being misted into the crankcase. But take a minute to envision how an inverted mounted engine would be lubricated at first start up or any consecutive start up? It would not, if not for the splashing around of oil either injected by the user if it is a brand new engine started first time, or the oil left from the previous run.....

The main thing is: it is impossible to seal an engine that small totally up so it won't consume (lose) that oil, because it would be pretty hard to fit one or two pistonrings AND an oil scraper ring, and you can't make reliable valve guide seals either in this size, or reliable crankshaft seals. The slightest leak would cause the engine to lose oil, and because there is so precious little of it, without warning you would have an engine damage on your hands.

Aside from that, it would get very tricky to fit a dipstick somewhere… What level to use? A .60 engine has at best something like 2 ml (possibly even less) of oil in its internals at any given time. You could add an oil pan, but that would make the engine a lot more bulky and it still would not contain much oil.

Another thing would be the lifetime of the oil. What service interval to use? One flight? 5? 10? What a hassle!

So they opted for the simplest solution: no scraper rings or seals, and let the oil in the crankcase be continuously replenished by seepage past the pistonring. The crankcase vent would ensure the amount of oil remains more or less constant (Oil collects untill a certain amount is retained and excess being thrown out through the breather nipple), and the close embrace of the case around the crank ensures the conrod is for most of its rotation in touch with the oil clinging to the casing, forcing oil into the oiler holes, so that part is covered as well.....

Problem solved.
Jan 09, 2019, 03:31 PM
Registered User
Quote:
I received this beautifully made engine from Sweden that really got my attention due to one very unusual feature, it didn't require any oil in the fuel due to the lubrication qualities of methanol was sufficient. Yada Yada
Methanol had nothing to do with lubrication, in fact methanol doesn't aid in lubrication at all. If you read on further, the engine is full of ball / needle bearings. Then a recommended 1/2-5 percent oil was added to the fuel as a safety factor. Nice looking engine none the less.

The KAVAN 50cc twin boxer actually had a sump and oil pump!
Jan 09, 2019, 03:37 PM
The great Brutifier
Quote:
Originally Posted by ClubFlyer
The KAVAN 50cc twin boxer actually had a sump and oil pump!
I was very dissapointed back then, to find out that it did not, as expected, have a pressurized oil feed, rather the pump was a small plunger pump that jetted some oil to the top of the crankcase from where the oil would drip down and continue as splash lubrication. They could just as well have made an oil scoop on one of the conrods….
Jan 09, 2019, 03:45 PM
AMA 537620
ChillPhatCat's Avatar
Quote:
Originally Posted by Brutus1967
Actually, it is none of the above…. You could theoretically put some oil in the sump, and it "would work" sort of... Provided some channels for the oil to pass, it would even get oil everywhere it would be needed. Because that is exactly how it works now as well.
It would work inverted, because basically, that is how current engines work: there is an amount of oil in the crankcase and it splashes around, without any pump or whatever. Because like it or not, that oil in the fuel is NOT passing the crankcase first but last, it drips down along the liner wall and is being misted into the crankcase. But take a minute to envision how an inverted mounted engine would be lubricated at first start up or any consecutive start up? It would not, if not for the splashing around of oil either injected by the user if it is a brand new engine started first time, or the oil left from the previous run.....

The main thing is: it is impossible to seal an engine that small totally up so it won't consume (lose) that oil, because it would be pretty hard to fit one or two pistonrings AND an oil scraper ring, and you can't make reliable valve guide seals either in this size, or reliable crankshaft seals. The slightest leak would cause the engine to lose oil, and because there is so precious little of it, without warning you would have an engine damage on your hands.

Aside from that, it would get very tricky to fit a dipstick somewhere… What level to use? A .60 engine has at best something like 2 ml (possibly even less) of oil in its internals at any given time. You could add an oil pan, but that would make the engine a lot more bulky and it still would not contain much oil.

Another thing would be the lifetime of the oil. What service interval to use? One flight? 5? 10? What a hassle!
You really need a filter and oil tank of some sort so that you don't immediately wear out the oil... and so weight is again going to creep into your equation and become a major detractor.
Jan 09, 2019, 04:09 PM
The great Brutifier
Quote:
Originally Posted by ChillPhatCat
You really need a filter and oil tank of some sort so that you don't immediately wear out the oil... and so weight is again going to creep into your equation and become a major detractor.
Why? Know these little 20 cc fourstroke chinese generators? No filter, no pump, not even a particle magnet, no oil tank (just a very small and shallow pan) and an easy 100 hours of oil life time...

But that is what I hinted at when I said
Quote:
Another thing would be the lifetime of the oil. What service interval to use? One flight? 5? 10?
I can assure you however, that, say, 1,5 ml of a decent 2-stroke oil in the crankcase of a 91, has a lifetime of at the very minimum 15 minutes, probably a few times that….
Jan 09, 2019, 06:40 PM
AMA 537620
ChillPhatCat's Avatar
I don’t doubt that a couple ml will set you up for a half hour... or some small amount of time. But if you ever wanted people to use it you would want a larger reservoir of oil so it could be changed after something more like 10 hours of run time. Nobody would ever buy into the tiny oil system that you are describing because it would be far more of a hassle than running mixed fuel. There’s no point in describing a completely impractical system like that... and claiming that people are wrong for citing weight concerns because of it.
Jan 09, 2019, 08:10 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
I want to poke the bear a bit here. Hypothetically, if we had a 4-Stroke cycle engine in a model airplane with a full oiling system and oil sump containing more than a “splash” of liquid oil in it (think like 2 fluid ounces or about 60mL)... Is someone going to tell me that oil isn’t going to cause problems when the engine is run inverted? In this hypothetical situation, the oil is pumped via a wet sump pump. So if inverted, the pump would also suck air, not to mention the oil would be on the bottom of the piston going up and down and not where it should be.

Bah. To the OP: model 4- Stroke cycle engines are made the way they are (the majority of anyway) because it’s the most efficient way to do it. Now a Conley 609 v8 stinger engine is a completely different ball of wax - even though I believe they run those on mixed fuel too just like our airplane engines.
Jan 09, 2019, 08:33 PM
Registered User
Quote:
Originally Posted by Brutus1967
I was very dissapointed back then, to find out that it did not, as expected, have a pressurized oil feed, rather the pump was a small plunger pump that jetted some oil to the top of the crankcase from where the oil would drip down and continue as splash lubrication. They could just as well have made an oil scoop on one of the conrods….
Yeah it was a fairly primitive system, but it got the oil to where it needed to go.

http://sceptreflight.net/Model%20Eng...20Mk%20II.html
Jan 10, 2019, 01:02 AM
The great Brutifier
Quote:
Originally Posted by ChillPhatCat
Nobody would ever buy into the tiny oil system that you are describing because it would be far more of a hassle than running mixed fuel.
I think you really missed my point, because that was exactly what I was getting at.... All others being impractical and this one as well....

Qwk hit the nail on the head, that larger oil volumes (large enough to give decent runtime) will get you into trouble elsewhere, although that problem would mainly be overloading the oil scraper ring and losing the oil. Small glow engines simply aren't big enough, and 1 or 2 oz of oil not large enough a quantity to cause too much issues due to sloshing or windage or whatever.
Jan 10, 2019, 02:10 AM
AMA 537620
ChillPhatCat's Avatar
I think you are missing my point. You can’t just take a Saito or some off the shelf RC engine and make some simple mods to it and expect anything useful to happen. It takes a complete engine redesign and that redesign is going to add weight and complexity, and you then need a real inverted oil system which will further exacerbate the weight equation. i.e: https://www.extraaircraft.com/docs/t...MM300/CH79.pdf


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