Fuel dilution and viscosity

[moribundman]

Does anybody know how many percent fuel dilution each of the following viscosities can safely handle while staying in grade?


0W-20
5W-20
5W-30
5W-40
5W-50
10W-30
10W-40
10W-60
15W-40
15W-50
20W-50

 

[427Z06]

IIRC, on average, 6% fuel dilution will lead to a 25% drop in viscosity.

 

[Doug Hillary]

Hi,
in heavy diesel engines the typical (Engine Manufacturer's) maximum allowable fuel dilution is 2.5%
Oil Companies may tolerate a little more according to the lubricant being used and the engine family

At the same time Manufacturer's typically allow the following viscosity change @ 40C
Maximum decrease = 15%

Regards
Doug

 

[moribundman]

Thanks guys.

Here is part two of my question. Which oils tend to pick up more fuel over time, the thinner ones, or the thicker ones?

 

[moribundman]

I'm going somewhere with this, so please bear with me.

 

[427Z06]

Thicker ones.

 

[Quick_lude]

Not sure if this will help you but I've had 3-4% every time.


Oil Analysis

 

[MolaKule]

quote:

---

Which oils tend to pick up more fuel over time, the thinner ones, or the thicker ones?

---

Neither have more affinity toward fuel than the other.

Thicker oils would "tolerate" fuel dilution better and have a safer HTHS after dilution.

 

[427Z06]

quote:

---

Originally posted by moribundman:
Which oils tend to pick up more fuel over time, the thinner ones, or the thicker ones?

---

Mola is, of course, correct. But I read that question as "in applications with high fuel dilution, is thick or thin oil used?"

 

[moribundman]

quote:

---

Thicker oils would "tolerate" fuel dilution better and have a safer HTHS after dilution.

---

Alright, now let's see when fuel dilution occurs in modern engines.


1. Short distance driving: even FI engines run rich(er) while the engine is cold. I see a problem here regarding the use of thin oil in engines that never get a chance to burn off execess fuel.

2. Driving a lot at WOT or at full throttle: By monitoring O2 sensor output, I can tell how my driving style influences whether my motor runs lean, aproximately stoichiometric, or rich. Let's say I flog the engine for an hour. While I can expect more fuel to get into the oil when the engine is running rich, would the fuel burn off quickly enough to not constitute an issue, even with thin oil?

 

[moribundman]

quote:

---

Mola is, of course, correct. But I read that question as "in applications with high fuel dilution, is thick or thin oil used?"

---

I actually did want to know if fuel had an affinity towards thicker or thinner oil.

As for what you thought I was asking, I can answer that qustion by looking at race oils. In applications with higher fuel dilution (ralley, uphill, endurance) thicker oils are used than in applications with lower fuel dilution (qualifying, short tracks).

 

[427Z06]

quote:

---

Originally posted by moribundman:

quote:

---

Mola is, of course, correct. But I read that question as "in applications with high fuel dilution, is thick or thin oil used?"

---

I actually did want to know if fuel had an affinity towards thicker or thinner oil.

As for what you thought I was asking, I can answer that qustion by looking at race oils. In applications with higher fuel dilution (ralley, uphill, endurance) thicker oils are used than in applications with lower fuel dilution (qualifying, short tracks).

---

That is how I arrived at my answer. An extreme example is a nitromethane fueled Top Fuel Dragster when they loose a cylinder or two.

As far as it happening in a street driven vehicle, I believe it would be highly dependent on the vehicle design and it's state of tune and/or mechanical condition. I guess the only way to know for sure is to flog it hard and immediately take an oil sample for a UOA.

 

[moribundman]

What I find interesting is that while Audi motors (at least the 12v engine) tend to run rich, as evidenced not only by the common black tailpipe, my UOA showed no fuel dilution at all with a 5W-40 oil.

 

[Quick_lude]

quote:

---

Originally posted by moribundman:
What I find interesting is that while Audi motors (at least the 12v engine) tend to run rich, as evidenced not only by the common black tailpipe, my UOA showed no fuel dilution at all with a 5W-40 oil.

---

I'm not sure if you can associate running rich with fuel dilution.. I've seen a few analysis from H22a powered Honda Preludes which run very rich, especially at VTEC and they didn't show any fuel dilution. The bumpers on the Preludes are almost always black from the rich exhaust.

 

[MolaKule]

Let's say you have a fuel injector which is leaky and by that I mean it's pintle doesn't close and it dribble's fuel after partially closing, especially at shutdown, then until the fuel rail pressure went to zero, then fuel might get past the rings and into the oil.

If an engine is running rich but has good ring sealing, then you may not see fuel dilution.

Also it would also seem that if the oil was always getting up to operating temperature, then it might be evaporated before it has had a chance to mix with the oil.

I think a thicker oil might tolerate fuel dilution better. I also think that unless the fuel is evaporated right away, the fuel is going to accelerate sludge creation because it adds hydrocarbons to the oil.

 

[moribundman]

There is evidence that the Audi 12v motor does run rich. It ALWAYS happens on decelleration. I can see it by monitoring O2 sensor out put, and I can hear it when the unburnt fuel then ignites in the cats, producing the 12v-typical burbling on decelleration.

Whether or not running rich necessarily favors fuel dilution I can't say. I would have figured that at over 135k miles, the rings on my engine wouldn't be that good anymore. However, I do almost never drive the car without letting the engine reach operating tmperature and then some, so odds are that unburnt fuel will vaporize.

Judging from all that's been said, I'd guess it'safe to conclude that someone who drives only short distances, or drives a car that suffers from fuel dilution, may be better off not using the thinnest available oil.

 

[theguru]

The Motul oil website (www.motul.com.au) list their 300v oils with reference to fuel dilution (although they have changed their terminolgy to exhaust smoke). The heavier oils being more suited to heavier fuel dilution, whilst grades below 30 are rated as suitable to low or no fuel dilution.

 

[moribundman]

The French/English Motul site lists next to their racing oils specific uses and acceptable fuel dilution. They list 0W-20 for qualifying and drag racing and mention "low fuel dilution." All their other racing oils are increasingly thicker and for short tracks, uphill racing, ralley, and endurance racing. The thicker the oil, the higher a tolerance for fuel dilution is listed. Their race oil viscsities are 0W-20, 5W-30, 5W-40, 10W-40, 15W-50, and 20W-60. The thinnest non-race oils Motul makes are 0W-30, 0W-40, and 5W-30.

 

[bulwnkl]

quote:

---

Originally posted by moribundman:
Driving a lot at WOT or at full throttle: By monitoring O2 sensor output, I can tell how my driving style influences whether my motor runs lean, aproximately stoichiometric, or rich.

---

Mori, do you see your engines run richer at (or near) WOT when the FI system goes to open-loop? Every FI car I've had supposedly works this way, but I noticed something interesting about our Neon starting ~90mph during a recent trip. It made me think that this car actually runs a little leaner when it goes open-loop. Just curious.

 

[AEHaas]

Dragsters use the thickest oils as high as 70 wt. The reason is because of extreme fuel dilution from just one run.

aehaas