Diesel Engine Fuel Dilution 1

[KBasson]

I have been told by the OEM's that fuel dilution in the oil to within certain specified limits is acceptable. Any comments as to what you believe these acceptable limits should be and what the possible consequences of excessive fuel dilution could be.

Ken

 

[Aquaduct]

You'll usually hear that none is acceptable, but realistically about 1% is liveable. The problem is that as the oil is diluted, is loses it's ability to do what oil is supposed to do, which means the engine will grind itself apart.

Oil dilution happens whenever fuel actually hits the cylinder walls before combusting, so it's a major concern with diesel misfire or things like post injection.

I've seen some test cell work in a research lab using post injection for some things on an engine that had a cylinder that would misfire when cold. After 100 hours of testing, they'd gotten about 9% oil dilution which I think translates into about a 25% loss of lubricity. The only thing that kept the engine from being severely damaged was the testing had only run at low speed/loads up to that point.

 

[Aquaduct]

Oops. Meant viscosity.

 

[gbent]

Check with your oil supplier as to the acceptable dilution. There is no hard rule, as some oils or engines are more forgiving than others.

As to the cause of dilution, my experience has been with faulty injectors or fuel lines under the valve covers rather than misfires. Some engines (such as Cat 3406) have no return line from the injectors. Any fuel leakage from worn injectors goes into the oil. There are many other ways fuel can leak into the engine oil that are more likely than misfires.

 

[icrman]

I agree gbent. Also some multiple pump systems, past worn pumps, seals, orings.
It also depends on viscosity of oil, bearing loads, clearances etc.
I have a question, how come diesel has enough lubricating qualities for the injection equipment, but supposedly not enough for the rest of the engine?
Isn't lubrication a fascinating subject?

 

[GregLocock]

That is an interesting question, but mainly, the fuel injection system is designed to use fuel as a lubricant, and the engine is designed to use oil as a lubricant. Sometimes the easy answer is the obvious one!


Cheers

Greg Locock

 

[patprimmer]

As Greg says.

Also, the loads bearing surfaces in the engine are much larger than the loads in the fuel pump.

Regards
pat pprimmer@acay.com.au
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[rmw]

In the for what it is worth department, during my college years, I bought an old volkswagen (36 hp) that I suspected had not had the oil changed in years, (my clue was that nothing would come out of the drain plug) so imagine the surprise when this dumb college kid asks the fuel pump attendant at the gas station (yes it was that long ago) to pump a gallon of diesel into the engine at the oil filler cap.

(He kept trying to tell me that it was a gasoline car, and that the fuel filler was up front, not in the rear.)

I ran it (not too hard) for several miles, whereupon I then dropped the oil, which would now flow freely, put fresh oil in, and ran that engine for many, many years thereafter.

That did not seem nearly as bad to me, with respect to dilution of the lubricating oil, as the time I got stuck in the creek, engine partially submerged, and I failed to remember that the crank shaft oil seal on a VW is a internally threading screw labrynth type seal, which promptly 'screwed' my engine full of water as it sat in the creek idling.

So much for dilution of the lubricating oil.

rmw

 

[Aquaduct]

Yeah, there are a number of mechanical reasons for oil dilution. I'm just stuck in the development world trying to get aftertreatment dosing to work. That's why my answer was so narrow.

As for the VW with oil dilution, it becomes a much bigger problem if your customers are are going 35,000 miles between oil drains and are hauling 80,000 lbs. of stuff up and down the interstate daily. Like I said, our prefered spec is 0%.

 

[CCycle]

Rmw, Interesting VW story. I had a VW in college years too. As I recall it was 32 hp, up from the previous 28 hp (wow!!). I kept good care of it however since I could not afford anything else. But the engine had a habit of seizing. You would be cruising along, probably at full throttle and all a sudden the rear wheels would start skidding. You had to be fast on the clutch to keep from doing a quick 180. I would sit at the side of the road for 5 min then start it up and drive off. This must have happened a half dozen times. I ran that car 100K miles and never had the engine apart.

On the subject of oil dilution; Consider the two stroke boat engines. They use 50 to one dilution. Forty years ago McCullough brought our a series of engines that used 100 to one. They were great performers and did not have crank problems. The rod bearings were needle bearings however.

I like to run jet boats. It is an all too common occurrence to fill the engine with water. This is due to the cooling water being supplied by the main jet pump. The operator gets the valving screwed up which pressurizes the water jacket to 100 psi or so. This blows through the gaskets of the old large blocks and fills the crankcase with water. The engine usually survives but it is a real mess to clean up.

 

[Fredbert99]

Hi all,

The reason diesel is ok for FIE and not for engines is, as stated above, the loading. In nearly all diesel FIE the lubrication of the high pressure parts is done by the fuel, and all the motion is effectively sliding. The lower parts with high contact forces or rubbing tend to have an oil feed or oil bath of normal lub oil.

I cant go into the specifics of what leaks where, as different applications have different needs.

 

[NeilDS]

I was a fuel system engineer for Caterpillar and many of our fuel components used engine oil as hydraullic oil, like the HEUI injectors. Our system specification limit was 7% fuel dilution in the oil and we did all of our endurance testing with oil at that level or higher. I believe the engine specification was the same fuel dilution spec.

Neil