Two stroke oil question- how does it separate?

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1/7/2019 8:35 AM
Edited Date/Time: 1/7/2019 8:46 AM

You know how when you pull the cylinder off, there is that nice sheen of oil in
the bottom end? How does the oil separate from the gas and stay behind?

One would think that as it is fully mixed with the gas, it would just continue on
and get burnt. Does it fall out when it reaches a certain temperature? Or is
there a density/weight difference at the micro level that causes it to separate
with the turbulence?

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1/7/2019 9:27 AM

Ever put vegeteble oil in water? Notice how its like bubbles of oil sitting on the water?

Thats exactly what oil in gas does. It doesnt “mix” with the gas. It gets dispersed evenly in the gas container - totally seperate from the gas.

The oil lubricates the cylinder by sticking to the walls. Thats the entire purpose of it. Over time it will just run down the walls to the crank and pool up. Its not supposed to burn off completely. If that happens youll seize your motor.

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1/7/2019 9:38 AM

Oh no kidding? So it never actually, fully integrates itself with the gas? I mean, even the vegetable oil and water can be mixed, but they will separate when left to sit. Oil and gas remain mixed but (apparently) separate in the crank.

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1/7/2019 9:39 AM

It's a function of temperature. The gasoline is simply boiled off leaving the 2-stroke oil as the residue in the crankcase.

If the gas and oil do not mix, then the oil and gas would separate out into two phases in the container which should not happen. Assuming the oil and gas are compatible to begin with, then you should never see it separate in the container and/or the engine.





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1/7/2019 7:52 PM

Thanks, guys!

Anyone have a more tech heavy answer to this?

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1/7/2019 8:50 PM

Not sure what you are looking for but as EXBMXMX said, the fuel is burned off leaving behind a sheen/residual oil. 2 stroke oil/fuel isn't like water/vegetable oil as stated above, it does mix. Here is a link if you just want some reading material: http://www.klemmvintage.com/oils.htm

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1/7/2019 9:04 PM

It does mix, and separates partly due to the flash point. That's why a lot of times, all else being equal, some oils you get more spooge because the flash point is higher.

Nothing at all like guy said about vegetable oil. If that were the case, your fueling would be inconsistent and engine would seize up very quickly. It's probably partly why they call it premix.

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1/7/2019 9:40 PM
Edited Date/Time: 1/7/2019 9:42 PM

exbmxmx wrote:

It's a function of temperature. The gasoline is simply boiled off leaving the 2-stroke oil as the residue in the crankcase.

If the gas and oil do not mix, then the oil and gas would separate out into two phases in the container which should not happen. Assuming the oil and gas are compatible to begin with, then you should never see it separate in the container and/or the engine.





"It's a function of temperature. The gasoline is simply boiled off leaving the 2-stroke oil as the residue in the crankcase."

The majority of the oil is burned with the gas. The majority of the unburned oil is blown out the exhaust.


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1/7/2019 9:42 PM

A little of the mixture is left behind to lube the bottom end. Think of how transfers close off when the piston rises, if all the fuel mixture got pulled into the cylinder the bottom end would cook.

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The older I get, the faster I was.

1/7/2019 11:12 PM

dkurtd wrote:

Not sure what you are looking for but as EXBMXMX said, the fuel is burned off leaving behind a sheen/residual oil. 2 stroke oil/fuel isn't like water/vegetable oil as stated above, it does mix. Here is a link if you just want some reading material: http://www.klemmvintage.com/oils.htm

That was a cool article, thanks!

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1/8/2019 3:30 AM

Yz229 wrote:

It does mix, and separates partly due to the flash point. That's why a lot of times, all else being equal, some oils you get more spooge because the flash point is higher.

Nothing at all like guy said about vegetable oil. If that were the case, your fueling would be inconsistent and engine would seize up very quickly. It's probably partly why they call it premix.

The logic is the same reason you shake your gas can(after its sat for a while with oil) and bike before you ride. Oil will sit on top. Once youre riding yes it stays mixed - obviously. But the individual particles are seperate. Oil particles stick to everything and what doesnt stick of oil and gas gets burned off or spooged out the pipe. What i meant is it doesnt become 1 new bonded liquid.

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1/9/2019 7:36 AM

exbmxmx wrote:

It's a function of temperature. The gasoline is simply boiled off leaving the 2-stroke oil as the residue in the crankcase.

If the gas and oil do not mix, then the oil and gas would separate out into two phases in the container which should not happen. Assuming the oil and gas are compatible to begin with, then you should never see it separate in the container and/or the engine.





barnett468 wrote:

"It's a function of temperature. The gasoline is simply boiled off leaving the 2-stroke oil as the residue in the crankcase."

The majority of the oil is burned with the gas. The majority of the unburned oil is blown out the exhaust.


...with the minority of oil being left in the crankcase.

There's a reason why you have to "clear it out" when you first start up or let a 2 stroke idle for too long and it's because there is not sufficient heat or intake charge velocity to keep the gasoline/oil vapor moving through the engine as a vapor cloud.

Definitely the oil and gasoline mix and go into solution with each other if they are compatible. Once they are in solution it requires some form of energy (eg distillation) to separate the solution back into its starting components.

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1/10/2019 7:50 AM
Edited Date/Time: 1/10/2019 7:51 AM

Update!

Gentlemen, thank you for the replies. I finally wrote to Maxima and got the piece of info
that helped clear it up in my pea brain. You guys that said it separates due to a higher flash point
are correct, according to the folks at Maxima. .

The part I couldn't understand was- the fuel charge enters the crank area, then
transfers up to the combustion chamber. So while the charge is in the bottom end, some of
the oil stays behind. There is no burning taking place in the crank area so how does the oil drop out at that point?

But the deal is it separates in the combustion chamber then literally flows down to the bottom end. "Migrates" is the term they use. Some of the oil is burnt, some gets spit out the exhaust
port and some stays behind coating the cylinder and migrates to the bottom end. I suppose y'all were trying to tell me that so I appreciate it but it was the migration explanation that finally cleared it up for me. Thank you!

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1/10/2019 8:21 AM

"But the deal is it separates in the combustion chamber then literally flows down to the bottom end. "Migrates" is the term they use. Some of the oil is burnt, some gets spit out the exhaust
port and some stays behind coating the cylinder and migrates to the bottom end. I suppose y'all were trying to tell me that so I appreciate it but it was the migration explanation that finally cleared it up for me. Thank you!"

I'm not completely convinced they are correct with the migration from the combustion chamber theory. Every top end I have pulled off I have always observed "clean" two stroke oil pooled in the crankcase area, not black or oil that has come into contact with any of the combustion products.

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1/10/2019 9:10 AM

Think about how the air/fuel mixture enters the combustion chamber...through the crankcase. Some of that fuel is vaporized before it gets to the top end, and the higher flash point oil sticks around at the bottom.

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1/10/2019 5:25 PM

Moto Tribology needs to come in here and clear this up, but the way I understand it the fuel holds the oil in solution, the same way salt is held in saltwater. It is literally "dissolved" in the gasoline. The exception is for oils with different properties, like castor - this oil will separate in cold temps or after long periods of sitting.
The fuel/oil droplets which are in the air charge coming through the crank will absolutely collect on the surfaces over time, and that is why you often see a small pooling of that product on the crank.
Most of the oil is vaporized in the combustion process and sent out the exhaust port. What remains is pushed out in subsequent burn cycles (remember that 2-strokes aren't terribly efficient per piston cycle).

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Braaapin' aint easy.

1/11/2019 1:18 PM

When oil separates does/can it always be mixed back in by shaking, regardless of brand/type of oil?

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1/11/2019 2:57 PM
Edited Date/Time: 1/11/2019 5:02 PM

Jrewing wrote:

When oil separates does/can it always be mixed back in by shaking, regardless of brand/type of oil?

Have you actually read this thread? Modern oil doesn't separate, are you still running castor bean oil?

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50