Gasoline Diluted Oil Calculation

[427Z06]

Gasoline (Thinnest specs I found)
0.46 cSt @15.6°C
0.40 cSt @37.8°C

5w30 Oil
56.0 cSt @40°C
10.0 cSt @100°C

3% Gasoline Diluted Oil
48.3 cSt @40°C
9.0 cSt @100°C

 

[Pablo]

Does this jibe with UOA's? (Not many 3%'s out there, probably)

I would think the oil would not just thin, though. You would have other reactions going on, including oxidation which would keep it from hitting the calculated low number.

 

[427Z06]

Good points Pabs. I was just looking at a worst case scenario. Many worry about viscosity drop with fuel dilution and I was trying to quantify it in some way.

 

[JHZR2]

A lot of the gasoline components would volitilize and come out prior to hitting the 100C mark, just like when there are small amounts of water in the oil.
Or so I would think.

JMH

 

[GMorg]

Please, someone more knowledgeable than I should chime in, but my understanding is that the concern with fuel dilution is not the viscosity drop. The issue is that fuel is fairly volatile and you begin to get cavitation wear on bearings. I do not have a ready citation, but that point of view make sense to me. A 10% drop in viscosity is not a big issue in my mind. In addition, I think that as the particulates rise you begin to see more of an issue due to a nucleation of phase change. In other word, suspended particles begin to act like boiling chips for the fuel to move from liquid to vapor.

 

[JHZR2]

that would make sense to me, except that chances are you wont have localized high concentration areas to do this, the oil system should be very well mixed.

JMH

 

[GMorg]

From my understanding, one should think of fuel-in-oil a little different than just diluted oil when at operating temperature. Before fuel is "blown off" but after the engine nears normal operating temperature, it is appropriate to think of fuel as a dissolved gas (state of matter, not fuel). I agree that it will be evenly distributed, but it does not behave as a typical fluid.

A common example of a fluid with alot of dissolved gas would be carbonated drink. With just minor changes in pressure, these fluids can foam. Fuel in oil will never be as bad a Pepsi, but the phenomena are related.

 

[Gary Allan]

You can even take normal water, GMorg. It off gases as it heats. All your free oxygen exits before the rolling boil ever gets close to occuring. That's why they have deaerators ..to transition any make up water through this event.

Interesting ponderings here.


But, our main topic is viscosity impact. Fuel dilution can have its own liabilities that may or may not impact oil of any viscosity

 

[GMorg]

Since I am of the oppinion that the impact of fuel dilution on viscosity is not important in the 0-3% range of fuel (10% change in viscosity at best), I completely missed that the thread was about viscosity. I thought 427Z06 was making the point that the impact of fuel dilution was minimal and thus was implying that the issue of fuel dilution must be something other than viscosity. Sorry guys...

 

[Gary Allan]

A worthy point to dwell upon, no doubt. I think that the immediate issue is to show how much real impact can be expected with lighter oils, let's say a 5w-20, and just how much are we really talking here

..but..it's a point surely worth discussion ..somewhere on the board.

 

[427Z06]

GMorg, no apologies needed. You brought up some important and related points.

 

[GMorg]

Gary Allan wrote:
"I think that the immediate issue is to show how much real impact can be expected with lighter oils"


As the selected viscosity of the oil is reduced, the fuel dilution will have less impact on the viscosity of the mix, not more. Think about it by considering the extreme. If the oil started as thin as fuel, then fuel dilution would not change the viscosity at all. Or, just do some some calculations and you will see that as the oil's viscosity is lowered, the impact of fuel dilution on viscosity goes down.

 

[Gary Allan]

So, we can probably conclude that it would take extreme fuel dilution ...clearly in the realm of either 1) mechanical problem to the point of being detectable or 2) severe service that included a masked mechanical problem that may not be detectable.


I agree that the closer you are to some form of equalibrium, the change is reduced. I would probably anticipate the counter argument to be that regardless of the % of total that the starting line is somewhat closer to the goal line (although it's no goal - it can surely result in a "score").

..but if you apply the aforementioned qualifications ..it kinda indexes the unworth of the situation.

 

[GMorg]

I have again begun to see posts recommending 10W30 over 0W30 and 5W30 over 5W20 where the recommendation is being related to fuel dilution. I still have the oppinion that wear that is associated with fuel dilution is cavition related, not viscosity related. The idea that there is "less margin for error" with low winter viscosity ratings (or low operational viscosity ratings) just doesn't make sense to me when related to fuel dilution.

So, I thought I would bump this thread to see if the positons of others have changed...

 

[Gary Allan]

I can see your point of micro bubble formation (as I referenced in de-aerators). I'm not sure how it occurs in a dynamic environment like an engine.

One side effect that I'm interested in is any out of whack volume changes. That is, does fuel, when added to oil, impact volume in a manner that would create a higher volume then the individual amounts? Somewhat opposite what water and alcohol do.

The reason I ask is that I had a 2% full situation in my jeep. No consumption over 9k. I installed an exchanger ...Terry still saw the effects of fuel dilution ..but the results showed no fuel (2 quart every 6months 7k miles). I'm uncertain, but this may bolster your assertion here.

In short, I wonder if 2% fuel can result in a 10% added volume when mixed with oil.

I don't know if anyone has tested this.

 

[427Z06]

GMorg, I concur, and it was the reason for the original post. That is, until you reach unrealistically high levels of fuel dilution, changing the viscosity a grade or three is of little help.

 

[Turbodriven]

If you have say 5% fuel dilution and then you have a fair amount of shear on the oil too, how would this pan out on say a 0W-20/30 oil and the effects on the motor?
Is there a -Xw viscosity?

Im asking specifically, since I have a mate with and Audi RS4 which is still being run in, so oil consumption has not stabilised...currently it burns a litre per 5000km...Audi tell him the car deliberatly over fuels to cool the pistons (nice concept but sounds like a bit dodgy to me), this they say causes oil dilution and contributes to the consumption...it runs on Castrol SLX 0W-30 Longtec (known here as GC Gold I think) What are your thoughts?

 

[GMorg]

Turbodriven wrote: "If you have say 5% fuel dilution..."

Even if fuel had a magical viscosity of 0 cSt at every temperature, then the maximum effect would be only about 5% change in viscosity. For many oils, 5% won't even shift the product to the next grade. Fuel dilution can have a large effect on wear, but not due to viscosity changes.

 

[Gary Allan]

I just got an email back from Kristen Huff at B-S labs. They calculate fuel dilution based on flashpoint.

We determine fuel by the flash. We know what it should be (at or above), and
if it's lower than that, we calculate fuel by how low the flash point is. I
don't know if I can answer your question about weight or volume, since we
don't measure either when we calculate fuel.

One note--calculating fuel by the flash point, while a valid ASTM test, is
not a fool proof method. There are other things, like solvent-based engine
cleaners and some types of additives, that can lower a flash point and fool
us into thinking you have fuel present, when you don't. Let us know if
you've used anything like that, because it might change our thoughts on
whether you have fuel in the oil.

 

[427Z06]

Quote:

---

...We determine fuel by the flash. We know what it should be (at or above), and if it's lower than that, we calculate fuel by how low the flash point is...

---

Woah...that certainly sounds highly inaccurate.

In any event, you'll be hard pressed getting fuel dilution above a certain level in a correctly functioning gasoline engine because of gasoline's high vapor pressure (low flash point and a high autoignition temperature).

But to extend detrimental effects beyond GMorg's mechanism, the problem creating excessive fuel dilution may severely dilute the oil at the top and/or second piston rings. Again, changing the viscosity in the sump does little to help, but the loss in viscosity at this particular location is likely to cause problems.