oil cooled engines?

[fuzz!]

i was wondering if an engine could be cooled by oil alone.. as in, oil running through the water galleries in the block as well as the radiator. just one big oil pump supplying the moving parts with lubrication while also stripping the engine of heat.

does oil not conduct heat as well as water? i thought oil was good at that. it doesnt turn to gas as quickly as water, and its good for metal - no more rusty water galleries.

there must be a reason its a bad idea as cars arent built like that, and i have to assume its because oil doesnt transfer heat as well as water. but i wonder if anyone has heard of an attempt at such an idea.

edit - actually, now that i think about it.. if you were suddenly unable to kill the heat (lets say, your thermos fail) it would be preferrable to blow steam all over the engine bay than near-flashpoint oil all over your headers..

 

[Anaema]

ya, thats true. not to mention oil is alot heavier than water, and thus you would need a stronger pump to run it around the engine.

 

[Jake Breyck]

last time i dropped some oil into water the oil was lighter

oil costs more this could be a factor

 

[fuzz!]

yeah good points, not heavier perhaps, but probably doesnt flow as easily..

 

[rarson]

Oil-cooled engines have obviously been a reality in past cars such as Porsches and Volkswagons. Now, to simply put oil into the cooling system and run a 1UZ, yeah... I don't think it's a good idea. If the engine were designed around an air-cooled oiling system (remember the typical Porsche engine uses a lot more than your average 5 quarts of oil), then sure it would be possible.

The thing about heat transfer is that water is much less viscous than oil. As such, water can travel through the cooling passages much faster. The faster the water travels, the better the cooling. Also, localized boiling of the water actually benefits heat transfer.

Along the walls of a water jacket, you'll have what is called nucleate boiling. That is, very tiny amounts of steam forming directly on the wall of the coolant channel. This steam breaks off the wall and mixes back into the coolant, transferring all the latent heat of vaporization with it into the coolant within the middle of the channel, which is cooler than the coolant at the wall. Increasing coolant flow also increases turbulence, which works in a similar way to nucleate boiling. The more chaotic the water molecule mixing, the less time individual water molecules will stay near the wall. This allows more even coolant temperature through the channel and better heat transfer to the coolant.

I hope I explained this adequately; all this information comes directly from my Heat Transfer and Fluid Flow training in the Naval Nuclear Power training program. Basically the heat capacity of water and lower viscosity make it a better choice for cooling medium than oil. That's not to say that oil cooling isn't beneficial though, of course.

 

[fuzz!]

thanks for the writeup rarson, you have explained it quite well - and it makes perfect sense.. the fast flowing turbulent water creates a larger 'surface area' if you will..

 

[hi-lex]

vw's only use 2.5l of oil and get hot very quick without it or when low,from what Ive read the airflow doesnt take much of the heat away its mainly the oil, on the other hand I remember the last oil change on my 69 911, 12l of oil !!! but they hav dry sump, you would use a thin hydraulic oil like tellus as it as thin as water an has high combustion temp, prob is oil compat wit pump seals and pos cavitation of pump an damage it may cause as cavitation with oil causes small explosions that eat away metal surrounding it

 

[Zuffen]

Whilst 911 Porsches use oil to cool the crowns of the pistons (after 1972!) the oil does very little to cool the engine. They use big oil coolers because the oil get very hot due to the engine operating at quite high temps.

If you run a 911 without the fan belt when the engine is hot for more than 30 seconds the rings and valve guides are toast.

I killed a Porsche engine about 12 years ago when it dropped the fan belt in traffic. By the time I turned into a side street it was knackered. I put a new belt on and it ran but was way down on compression.

One other thing with oil due to its nature it does not have the same heat transfer rate as water.

 

[Anaema]

thanks for the writeup rarson, you have explained it quite well - and it makes perfect sense.. the fast flowing turbulent water creates a larger 'surface area' if you will..

thats not always true, if you take the therostat out of an engine it will overheat because the water is moving too fast too cool the engine.

 

[fuzz!]

say what? if you take the thermostat out of an engine it runs waaaay too cold.

thats not always true, if you take the therostat out of an engine it will overheat because the water is moving too fast too cool the engine.

 

[MaxPower]

Thermal dynamics is way out of my league, but i agree with Anaema having no thermostat can cause overheating issues. The engine takes longer to heat up but often its harder to keep the temp under control due to the higher flow.

As an example a friend installed a davis craig electric pump and has no thermostat (it is in a drag car), the pump was meant to be the thermostat by cycling on and off. We had more overheating problems than with the standard belt driven pump, which was also causing problems hence going to an electric pump thinking more flow would help. With the engine idling we could watch the temp rising with pump and fan running. I built a PWM speed controller for the fan and ran it at about 40% duty cycle, it slowed the water flow down considerably and now has now cooling problems - temp will drop until either the fan or pump turns off.

 

[jibby]

Yeah, the Porsches are cooled with like 12 quarts of oil or more on average, and are also known for having oil leaks happen regularly... Had a friend who worked at a porsche dealership and he had said to me the oil cooling is a bad idea on these cars and the porsches are always being brought into the dealership for oil leak repairs....however I did get the opportunity to roast a C4 turbo for a full day courtesy of my friend and I must say the C4 really hauled, and also had incredible traction off the line and the handling was sick, but honestly it felt like a $100,000+ go-cart to me. I remember getting alot of looks in that black Porsche...Chicks loved it, I did not...

 

[Nick M]

thats not always true, if you take the therostat out of an engine it will overheat because the water is moving too fast too cool the engine.

That is correct. You need the thermostat to hold coolant in place so it can absorb the heat. A temp gauge will only show what the sensor is touching. And the coolant being cool doesnt mean the actual cylinder wall is cool.

 

[340i]

with no thermostat, the water isn't in the radiator long enough to transfer the heat to the atmosphere. thats why they overheat instead of being cooler like some would expect.

 

[rarson]

No guys, sorry but that is wrong.

What happens is, there is a passage that allows coolant to bypass the engine. Most people when removing the thermostat improperly leave this passage open when removing the thermostat. This allows the coolant to bypass the block and boom, there you go, overheating.

If you seal the passage up and run no thermostat, with the increased flow the temps WILL be cooler. The faster the fluid flow, the greater the heat transferred.

 

[hi-lex]

what motors have a bypass system? if they do then why do motors overheat when thermostat sticks shut? heat transfer takes time, if coolant doesnt have enuf time to absorb heat from block then the block stays hot and coolant stays cool effectivly cooking your motor without temp warning as coolant is not overheated, fact...

 

[slyalfa]

rarson is deadon. almost all cars have the bypass even my 72 Alfa spider.
The bypass is there to let the car heat up fast.(helps with smog for one)
and many other things as to why you want it at temp as soon as you can.
also with out the bypass the temp will oscolate. FI system do not like haveing the temp go up and down.

The thermostat is not just open and closed. it is a 3 way system. if it is stuck the coolent just goes around and around never geting to the rad.

see this for more info on how a bypass system works
http://www.are.com.au/feat/techt/thermostat.htm

 

[hi-lex]

maybe Im just used to lazy old v8s an 6s that take half an hour to warm up