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#4778075 - 06/05/18 11:18 AM Fuel dilution's effect on oil viscosity etc
nap Online   content


Registered: 04/27/18
Posts: 368
Loc: Canada

Download it while it lasts:

https://hrcak.srce.hr/file/75680

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#4778138 - 06/05/18 12:37 PM Re: Fuel dilution's effect on oil viscosity etc [Re: nap]
Cujet Offline


Registered: 02/15/03
Posts: 7164
Loc: Jupiter, Florida
The elephant in the room is not just the raw fuel dilution. It's not unusual for most of the the fuel in the oil to evaporate, leaving behind various additives and heavier components of fuel. It's entirely possible, for example, to start and end with 5 quarts in the sump. 5 quarts of oil at the start, and 4.5 quarts of oil at the end, along with 1/2 quart of contaminants.

A great example of this would be the diesel "veggie" fuel users. The sumps are seriously contaminated with waste veggie oil. Often leading to oil failure and engine failure.


Edited by Cujet (06/05/18 12:39 PM)
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#4778143 - 06/05/18 12:45 PM Re: Fuel dilution's effect on oil viscosity etc [Re: nap]
Cujet Offline


Registered: 02/15/03
Posts: 7164
Loc: Jupiter, Florida
FYI, aircraft operators in very cold climates used to dilute the oil with fuel intentionally. This reduces viscosity for easy starting in severe cold. The fuel evaporates rather quickly after start, and oil pressure and temperatures remain normal.

In fact, some aircraft had a built in system, from the factory, that added fuel to the oil for cold weather starting. The requirement was that the oil reach 100 degrees F prior to takeoff.
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#4778176 - 06/05/18 01:31 PM Re: Fuel dilution's effect on oil viscosity etc [Re: Cujet]
Johnny2Bad Offline


Registered: 05/20/13
Posts: 1810
Loc: Saskatchewan, Canada
Originally Posted By: Cujet
FYI, aircraft operators in very cold climates used to dilute the oil with fuel intentionally. This reduces viscosity for easy starting in severe cold. The fuel evaporates rather quickly after start, and oil pressure and temperatures remain normal.

In fact, some aircraft had a built in system, from the factory, that added fuel to the oil for cold weather starting. The requirement was that the oil reach 100 degrees F prior to takeoff.


First of all, an airplane engine is not an automotive engine. It is a mistake to assume they are similar enough that various conditions or systems are interchangeable.

The airplane engines that did use fuel to dilute engine oil have sumps that are much, much larger than a typical automobile, or at least a typical post-WWII automobile. As in sump capacity is measured in gallons, not quarts.

Aviation oils in general are higher viscosity than automotive oils, for example Aeroshell 100 is equivalent to SAE 50 automotive viscosity.

Aviation oils until relatively recently had no winter rated oils and no multigrade oils that were legal to use, so using fuel dilution was not a 'good practice", it was the only possible practice to adjust viscosity.

By far the more common method of starting in cold weather is to tarp up the nacelle and provide heat under the tarp. The use of the equivalent of a campfire was hardly unheard of. The automotive equivalent is to tarp up the engine compartment and run exhaust from another running vehicle under the tarp to heat up the engine / sump, ground clearance being what it is making open fires difficult.

If you've never had to do either to an aircraft or a vehicle, you haven't been in a cold start situation that would benefit from oil dilution via fuel either.
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#4778208 - 06/05/18 01:58 PM Re: Fuel dilution's effect on oil viscosity etc [Re: Johnny2Bad]
kschachn Offline


Registered: 12/26/05
Posts: 9420
Loc: Upper Midwest
Originally Posted By: Johnny2Bad
Aviation oils until relatively recently had no winter rated oils and no multigrade oils that were legal to use, so using fuel dilution was not a 'good practice", it was the only possible practice to adjust viscosity.

But multi-grade oils have been available to GA for over 30 years and manufacturers have approved their use in via service bulletins, right?
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#4778238 - 06/05/18 02:25 PM Re: Fuel dilution's effect on oil viscosity etc [Re: nap]
PimTac Offline


Registered: 03/04/17
Posts: 4338
Loc: Soviet State of Washington
Originally Posted By: nap

Download it while it lasts:

https://hrcak.srce.hr/file/75680





Why? Whatís going to happen to it?
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#4778242 - 06/05/18 02:29 PM Re: Fuel dilution's effect on oil viscosity etc [Re: nap]
nap Online   content


Registered: 04/27/18
Posts: 368
Loc: Canada
Internet rot. Interesting / useful info tends to disappear after some time.

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#4778358 - 06/05/18 04:36 PM Re: Fuel dilution's effect on oil viscosity etc [Re: nap]
JAG Offline


Registered: 10/23/05
Posts: 4841
Loc: Fredericksburg, VA
That is a good paper. Itís ironic that you posted it because I was searching on the same subject yesterday, and found that paper.

Another bad effect of fuel dilution is the oxidative stress that it puts on the motor oil. Itís particluarly bad in the case of bio-diesel due to its particularly poor oxidative stability.

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#4778627 - 06/05/18 10:01 PM Re: Fuel dilution's effect on oil viscosity etc [Re: Johnny2Bad]
Garak Offline


Registered: 12/05/09
Posts: 24620
Loc: Regina, Saskatchewan, Canada
Originally Posted By: Johnny2Bad
Aviation oils until relatively recently had no winter rated oils and no multigrade oils that were legal to use, so using fuel dilution was not a 'good practice", it was the only possible practice to adjust viscosity.

Well, you could use a heater, or even better, heated storage, but ask the Regina Flying Club what they charge for heated hangar space. wink
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#4779651 - 06/06/18 09:34 PM Re: Fuel dilution's effect on oil viscosity etc [Re: Johnny2Bad]
Cujet Offline


Registered: 02/15/03
Posts: 7164
Loc: Jupiter, Florida
Originally Posted By: Johnny2Bad


First of all, an airplane engine is not an automotive engine. It is a mistake to assume they are similar enough that various conditions or systems are interchangeable.

The airplane engines that did use fuel to dilute engine oil have sumps that are much, much larger than a typical automobile, or at least a typical post-WWII automobile. As in sump capacity is measured in gallons, not quarts.

Aviation oils in general are higher viscosity than automotive oils, for example Aeroshell 100 is equivalent to SAE 50 automotive viscosity.


Thin straight viscosity oils were allowed in cold weather ops. Lycoming aircraft engines can use a straight 20 below 10F.

Furthermore, many light aircraft hold 6 quarts, much like automotive applications.

The little bit of Alaska ops I've done include using thin oils, preheating and there was near universal talk of using fuel to dilute.

My point was simply that it was a procedure for cold weather ops. Not that it is good.
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