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Old 04-26-2011, 06:23 PM
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Cooling oil with fuel?

So I was sitting around thinking today (AKA avoiding doing real work), and I wondered if anyone had thought about designing a fuel/oil cooler to replace the coolant/oil cooler and in lieu of going with the air cooler.

I think it might be useful for a number of reasons:
1. Fuel is much much cooler than your coolant and would therefor cool the oil much faster.
2. You don't need to be moving for it to be effective, useful if you idle a lot.
3. It would heat the fuel up, aiding atomization and better combustion.

Any thoughts? This certainly isn't something that I'm going to go do any time soon as I'm no McGyver, but it seemed way better than trying to cool oil with something that's already 190!

-Ed

P.S. Only other consideration I'd think would be to ensure that in the cooler the fuel was pumped at a higher pressure than the oil, so if it suffered a leak, you'd get fuel in your oil instead of oil in your fuel...
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Old 04-26-2011, 07:22 PM
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Has someone done it? YES. There are plenty of cooling systems that in fact use FUEL as the coolant - notably most spacecraft engines including the venerable Space Shuttle orbiter's three main engines. The fuel (liquid hydrogen and liquid oxygen) cools the engine nacelles themselves before combustion for propulsion also cooling the lubrication system (not oil but rare metals and composites) that keeps the engines' gimbals and bearings from seizing.

Has someone done it for the 6.0L? I don't know specifically but in practice there are propane and water-meth injection systems for fuel delivery to keep the engine from running hotter - which by second order effect slows excessive oil temperature buildup.

Ultimately, another cooler in the engine - even as an alternative - strikes me as impractical (and potentially cost prohibitive to engineer). As for mixing the fluids, while undesirable in practice (fuel dilution of the oil is not good for main bearings), oil in the fuel seems the less dangerous or at least the less undesirable condition...diesels are referred to as oil burners after all! and more than a few even put a drop (or more) in their fuel tank for lubricity enhancement.

I couldn't resist an Aerospace moment...

Jonathan

1994 graduate of the Georgia Institute of Technology as an Aerospace Engineer - and lifelong space nut.
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Old 04-26-2011, 08:32 PM
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Quote:
Originally Posted by OBJDOG View Post
So I was sitting around thinking today (AKA avoiding doing real work), and I wondered if anyone had thought about designing a fuel/oil cooler to replace the coolant/oil cooler and in lieu of going with the air cooler.

I think it might be useful for a number of reasons:
1. Fuel is much much cooler than your coolant and would therefor cool the oil much faster.
2. You don't need to be moving for it to be effective, useful if you idle a lot.
3. It would heat the fuel up, aiding atomization and better combustion.

Any thoughts? This certainly isn't something that I'm going to go do any time soon as I'm no McGyver, but it seemed way better than trying to cool oil with something that's already 190!

-Ed

P.S. Only other consideration I'd think would be to ensure that in the cooler the fuel was pumped at a higher pressure than the oil, so if it suffered a leak, you'd get fuel in your oil instead of oil in your fuel...
I see a few issues...
1.) yes, the fuel may be cooler than the coolant, but once you run fuel through the cooler and back into the tank you warm the remaining fuel in the tank with no way to cool it, except through the plastic/metal of the tank into the air. Thus you would quickly heat the fuel beyond useful heat transfer delta.
2.) the more fuel you use, the less coolant you have hence enhancing the problem stated above.
3.) cooler diesel fuel is actually better for combustion and atomization. Just like air, you get more power and cleaner combustion from cooler fuel, hence why some engines (Detroit, Cat...etc) have fuel coolers as well.
4.) having The fuel pressure higher than the oil pressure would happen just from the operating pressures of both systems. But, fuel dilution of crankcase oil is a big concern in diesel engines. You dilute and therefor lose the lubrication of the oil which increases wear and heat stress which results in potentially massive failures. Oil in the fuel would be the lesser of two evils, but even then you would be on borrowed time as the oil in the crankcase disappears.

Just my two cense.
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Old 04-26-2011, 09:09 PM
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cooling

common practice in aircraft, cools the oil and stops the fuel icing up by warming it
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Old 04-27-2011, 12:55 PM
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You might want to consider that part of the reason for using the coolant is to help warm the oil when it is cold out. As for cooling the oil with 190* coolant that really isn't a big problem. I have been told by more than one engine builder that the ideal temp for the oil is 215*.
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Old 04-27-2011, 01:39 PM
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Originally Posted by howell_jd View Post

1994 graduate of the Georgia Institute of Technology as an Aerospace Engineer - and lifelong space nut.
Oh your one of them people huh!

I have a few buddies that are in Aero E, they are a little out there, at least compared to us ME's lol.
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Old 04-27-2011, 04:27 PM
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Interesting points! I thought about it mainly because I know that some GE turbine engines do this, but I guess there are some additional considerations for reciprocating engines and vice-versa.

Would diesel fuel in the oil create enough dilution to cause issues in the crankcase? I figure that diesel can still lubricate, just not to the same standard as a 15w50 oil. I do get the oil in the fuel issue... I was thinking of it from an aircraft engine perspective of if you foul the gas, you come out of the sky; whereas I'll gladly take the risk of an engine shutdown due to excessive oil in the fuel over my engine seizing due to lack of oil.

As far as heating the fuel in the tank up to 190... I was thinking that the fuel would cool the oil on it's way to the injector and the vast majority of that heated fuel would never make it back to the tank...

Either way, I like theoretical engineering discussions... mainly because it will never happen, so we're all right

-Ed
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