Fuel dilution and viscosity

[teamDFL]

quote:

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

aehaas

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While you are correct, I think that there is also more to it than that. The fuels that the drag racers use, alcohol and nitromethane, have a tendency to washdown the cylinder walls of oil very quickly. Partially this is due to the air fuel ratio that they run (on a passenger car it is 14.7:1 while they can run as rich as 1:2 which also goes along with the fuel dillution you pointed out) but is also due to the chemical properties of the alcohol which break up oil films. A thicker oil is used in order to stop this process from happening as quickly and ensure that there is some lubrication present at the rings.

Perhaps someone can actually add the chemistry to the interaction, as my info is from reading and talking to racers who use different fuels. Or, maybe I am wrong and someone will prove it!

 

[moribundman]

quote:

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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.

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The transition from closed to open loop seems pretty smooth, but yes, it does run richer. It must be a safeguard to prevent the engine from running lean under all circumstances at higher RPM.

 

[427Z06]

quote:

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Originally posted by moribundman:
It must be a safeguard to prevent the engine from running lean under all circumstances at higher RPM.

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Yep. Tuners that remove this rich condition for maximum horsepower remove the built in factory safety net.

 

[Geoff]

moribundman, doesn't your Audi use 2 qts per OCI? I submit your black tailpipe is mostly from the oil consumption, not fuel.

 

[moribundman]

Geoff, my Audi, and probably all Audis with the 12v motor, have black tailpipes. Mine had it when it was new and when needed only 1/3 qt oil per 10k miles! Trust me, each time I get off my gas, the engine runs momentarily rich. As I said, I can monitor the Voltage output of my O2 sensors while driving. I get off the gas, and there is ALWAYS a rich condition for a moment. What do you suppose is crackling in my cats? It's unburnt fuel igniting. It's a known fact in the Audi community and it makes no sense to debate this particular issue.

 

[the_oil_dealer]

most modern fuel injection systems have a decel fuel cut. with my car the idle switch has to be closed and the RPMs above 1400ish. below 1400 fuel resumes. takes about a full sec after letting up on the pedal and coasting in gear to hear the exhaust note change from the fuel cut

 

[Dr. T]

mori correct me if I'm wrong, but what I think you're getting at is...thicker oils are: 1) more resistant to fuel dilution and 2) directly inhibit it by providing a better 'sealing' effect in the piston area.

 

[moribundman]

Dr.T, I am convinced fuel dilution is less of a problem with thicker oil. My '96 Audi may not have what constitutes a modern, fuel-injected engine, and it does not cut off fuel immediately when I get off the gas. At least when driving spirited, getting suddenly off the gas will result in some fuel getting unburnt into the exhaust, as evidenced by "exhaust burbling" (fuel droplets blowing up when getting in contact with the cats). Fuel dilution is anon-issue with my engine, though -- but that's likely due to my driving habits, which do not include a whole lot of short distance driving.


As for ring seal and viscosity, I really don't know what's better, thin or thicker (if we reduce the oil's properties to viscosity). Based on the information that I've read, ring and cylinder lubrication is very complex and depends on a multitude of factors. I can't even guess what viscosity would offer the best compromise -- and I'm convinced this is an area where compromises have to be made.

 

[AEHaas]

There is a previously discussed reference to a Shell oil document that shows that the thinner the oil the better the piston / ring seal. If you think about this it makes sense.

aehaas

 

[moribundman]

Yeah sure, engines can achieve such fantastic ring/pistion seal that the engine seizes. It's a routine occurance on German Autobahn at the beginning of the summer vacation in August as evidenced by broken down cars with blown engines. Pappy takes his family car out on the Autobahn for the trip to Italy, and after 200 km at high speed the commuter car's engine seizes due to fuel diluted oil. Maybe they should go even thinner. And I'm not talking about clapped-out beater cars here.

 

[AEHaas]

"Maybe they should go even thinner. And I'm not talking about clapped-out beater cars here. "

Several things come to mind. I think that Europeans usually run oils longer than Americans on the average. And they seem to use thicker oils to begin with. Now if they are getting oxidative thickening then running down the high speed European highways may in fact result in even more heat and thickening of the oil and finally seizing. There are many assumptions but may this be one of the causes?

aehaas