Which motor? Most Audi turbos already have oil coolers that are sufficient for all but full racing. The real difference is in the watercooling for reliability, NOT the oil regardless. Turbos need to be hot in order to work.

this thermal energy.

taken as:
pressure is force/area
force x time = work

a jet engine works on the principle of suck burn work blow
the burn or heat addition increases the pressure due the chemical/combustion generation of heat. The heat produces a higher pressure. The pressure works the turbine.

The oil system in a car is used to cool the bearing surfaces of the turbo and other components, and to keep the turbo temp down to prevent damage to the rotor components.

beside pressure being force/area
it is also the potential of velocity.
therefore the pressure delta from the exhaust across the turbine generates an acceleration of fluid velocity. The higher velocity imparta a higher lift profile, which in turn rotates the part.

I added an Oil cooler to my 1.8t. My peak to peak temp drop has been 40 degrees. Not only is the Oil cooler but the entire Motor is cooler.

My car now runs at 200 or so where it used to run 235. Running the car on the track I could see up to 280. 40 minutes of HARD road race laps with the cooler in place led to 240 or so. In 115 degree temps with the AC on in traffic I would see 245-250. With the cooler is 215.

A cooler engine is less prone to detonate. This = more timming advance. Power is more consistant.

During cool down after a hard run my turbo cools off MUCH faster. Especialy with my electric fan kit running pulling air through the oil cooler core. An amazing amount of heat radiates of my cooler.

I am working on a new unit with a lower price point. The one I have now was a full custom core with cost no object in mind. Due to this it is not cheap. I have located a core that will withstand the 110psi of oil pres this motor has. I will offer this kit with a Thermostat in the very near future.

Clark

There's a good write-up in the this month's European car that helps clarify this question. Check it out if you can.

Here's the intuition behind a turbo's benefit. No need to break out the thermodynamic and fluid-dynamics text book.

The typical 4 cycles of our car engine corresponds to (1) suck, (2) squish, (3) boom, (4) blow. Gasoline contains a great amount of energy. In burning the gasoline, energy is created to "run" the car. However, not all of the energy goes to run the car. A good portion of it is simply dissipated as heat (friction). A good amount of energy is also dissipated as exhaust. Exhaust heat contains potential energy and exhaust velocity stores kinetic energy.

The turbo aims to capture some of that exhaust energy and "recycle" the energy into the combustion process. However, the heat energy of the exhaust is effectively useless as far as a turbo (and engine) is concerned. Rather, the turbo uses the kinetic energy of the exhaust to cram more air into the combustion cycle. This in affect, "recovers" some of the "lost" energy in the exhaust.

The translation: turbo's run hot not because they need to. Rather the heat is simply a by-product of the "recycling" system that we've created.

Two identical turbo motors, one at operating temperature with the turbo hot, the other with a cold turbo. The hot turbo makes more power.

Why? Because a cold turbo will absorb the exhaust heat and not convert it to accelerating the turbine. Hot air hitting cold metal reduces the rate of expansion (or [rate of]velocity[delta]). The oil performs the job of lubrication of the center "floating" bearing. The oil/water cooler is simply an efficient means to reduce coking in conventional oil and improve warmup of the coolant on a cold engine.

It is the job of the water cooling to reduce the turbo temperature. The first generation of modern automotive turbo didn't have the water cooling and they had a very short service life (saab sobs). With water cooling turbo service life is equivalent to the engine and the transmission.

Note that the water cooling only brings the temperature down to the coolant temp 190-220. They could simply run an independant water/water cooler to the turbo if you gained any efficiency by running it cooler. Nobody does this.

Any decent (setrab/mocal, etc.) oil cooler has a thermostat setting because you don't want the oil temp to drop too low to volatize out the water and VOC's in the oil.

Turbos primarily recover the kinetic energy of the exhaust. It's true that turbo's (as with many systems) operates most efficiently around certain temperatures. Unarguably, exhaust gas contains energy in two forms, potential and kinetic. Imagine the following scenario:

1) the exhaust gas exists only as potential energy (only as heat). You pass it through a turbo and you get a melted mass of ceramic and metal.
2) the exhaust gas exists only as kinetic energy. You pass it through the turbo and you get energy transferred back into the system.

It's true that turbo's need to run at a certain temp to be efficient. But under no circumstances does it do most of it's "recycling" through exhaust heat.

velocity of the gases being forced out of the cumbustion chamber. That is why the exhaust gases are cooler at the turbo exit. Some of that thermal energy was converted to mechanical energy.

The heat does not get translated into mechanical work. (would a match turn the turbo?) The entire exhaust system soaks up heat from the exhaust system. Keeping the oil cooler would help prevent localized coking of the oil.

A cooling effect at the surface of the turbo will not cool the air temp acting on the turbine. It does not have enough time to transfer the heat. (conduction heat transfer principle)

If the turbo cools the air behind the turbine you will actually get better performance as you will increase the flow rate and pressure delta across the turbine.

You now have two sources saying the same thing.
1. European car magazine
2. An aerospace engineer (me =)

when exhaust gas cools, its density increases, right? When its density increases, its volumetric flow rate decreases, right (mass flow rate hasn't changed)? When its volumetric flow rate decreases, its flow velocity decreases, right (flow area hasn't changed)? When its flow velocity decreases, its knetic energy decreases (a function of the square of the velocity)...

I think one must look at total internal energy, not just potential or kinetic energy in these matters.

the mass flow rate is constant (yes)

flow velocity is dependent upon pressure delta (again pressure being a potential of velocity). So if you have localized cooling of exhaust gas the pressure and density will drop, but will still be higher than the pressure/density downstream, so flow velocity is maintained/or accelerated. As the downstream exhaust is cooled the pressure delta will be sustained/accelerated in some exhaust systems until pressures match ambient.

Again the key thing with the turbo is the aerodynamic effects (lift profiles) on the turbines which depend on the velocity and the mass flow rate.

"flow velocity is dependent upon pressure delta"

Agreed.

"So if you have localized cooling of exhaust gas the pressure and density will drop."

Well, if pressure drops more than temperature, yes density will drop also. However, if pressure drops, so does velocity, per the statement above.

"but will still be higher than the pressure/density downstream"

Agreed.

"so flow velocity is maintained/or accelerated."

Disagree, flow velocity will be reduced (if heat is removed) either because density will increase (while pressure stays the same), pressure will decrease (while density stays the same) or a combination of both.

"As the downstream exhaust is cooled the pressure delta will be sustained/accelerated in some exhaust systems until pressures match ambient."

Disagree, as the exhaust gasses are cooled, the pressure delta is reduced until it gets to ambient at the exhaust exit.

"Again the key thing with the turbo is the aerodynamic effects (lift profiles) on the turbines which depend on the velocity and the mass flow rate."

I don't dispute this - only that removing thermal energy from exhaust flow does reduce the amount of energy which can be applied to make a turbo work.

This is what we need here at the Audiworld forums, more technical discussions based on actual engineering, not hearsay! :^)

"flow velocity is dependent upon pressure delta"

Agreed.

"So if you have localized cooling of exhaust gas the pressure and density will drop."

Well, if pressure drops more than temperature, yes density will drop also. However, if pressure drops, so does velocity, per the statement above.

"but will still be higher than the pressure/density downstream"

Agreed.

"so flow velocity is maintained/or accelerated."

Disagree, flow velocity will be reduced (if heat is removed) either because density will increase (while pressure stays the same), pressure will decrease (while density stays the same) or a combination of both.

"As the downstream exhaust is cooled the pressure delta will be sustained/accelerated in some exhaust systems until pressures match ambient."

Disagree, as the exhaust gasses are cooled, the pressure delta is reduced until it gets to ambient at the exhaust exit.

"Again the key thing with the turbo is the aerodynamic effects (lift profiles) on the turbines which depend on the velocity and the mass flow rate."

I don't dispute this - only that removing thermal energy from exhaust flow does reduce the amount of energy which can be applied to make a turbo work.

This is what we need here at the Audiworld forums, more technical discussions based on actual engineering, not hearsay! :^)

I hypothesize that the performance of a turbo has NOTHING to do with ITS(the turbo, manifold, or down-pipe) temperature. It is mainly a function of the temp of the EXHAUST GAS. It seems that this is in effect what everyone is saying here. The turbo gets hot when it is working more efficiently, but that is only because they are made of a material that absorbs energy and radiates it as heat. It would seem that you would get a greater return out of a turbo that absorbed no heat or very little. If this is true, it seems like coating the turbine and housing with a heat shielding substance would cause there to be less parasitic losses. When you convert energy to heat(radiating off the turbo), than that is energy that is in essence wasted. Am I way off on this?

mark

ok lets look at this system wide. My model was in a steady state condition with some assumption. Yours is a transient.

yes if the heat is pulled out of the system the pressure will drop for that constant volume pv/t=pv/t v=constant

so lets look at the heating/cooling of the turbo. A metal has an exponential heat soaking characteristic. It will absorb a large thermal heat unit/time and then will tapper off until it reaches a steady state for the system. This usually doesnt take too long.
Now lets look at what transfer mechanisms are availble in the turbo, the housing/sidewall and the turbine blades. The sidewall will heat soak due to the conductive heat transfer characteristics. Boundary layer effects will exclude convectional transfer. The turbine blades are convectional and will absorb heat quickly there. Given the area of the top surface to the area of the bottom surface of the fin more heat would be absorbed by the top. In turn this would generate even more lift effect (minimal per blade, but measurable combined possibly). Now we have heat being pulled out of the turbo by the water and oil cooler. The water and oil cooler is pulling heat from the base of turbine. We can assume the housing will maintain a steady state.

so if were to look at the temperature and pressure across the system lets assume 200 at the inlet. It may drop slight from heating the port and sidewall until heatsoaked. Due to turbulent flow the air at the sidewall that was cooled will mix with the center section flow and heat up somewhat. That mix will hit the turbines, loose temp and pressure across the fins.

Downstream of the turbo you have a bunch of thin walled constant section tubing that will cool the exhaust gases as they pass and reach the exhaust tips and ambient pressure. Along the total exhaust length, including the turbo the pressure will equalize with ambient, some will be from heat loss in the exhaust other from increase of velocity. Thus sucking the air from upstream.

*wheh* ok other things which are in here that need to be considered:
1. Turbo housing is thick walled, conductive and cooled by engine bay air
2. Exhaust system is thin walled, conductive and cooled by passing under car air.

Geza I fully agree...educated input helps and is stimulating

see my post below "fully agree"
there is heat transfer going on, but you reach a steady state quickly.
Once there you will not have the same loss.

The turbo is not a straight energy recycler, pesky laws of thermodynamics.

also the turbo is cooled by radiation

I dont have my book in front of me. But if memory serves that radiation and conduction, together, are less than convection in terms of heat transfer capabilty.

with an automobile...once you have heat produced it is loss. There is no way to reclaim it. once hot gas leaves the engine block... it is a lost cause.

I still think that we are thinking on the same wavelength. The original statement made that a turbo had to be hot, just bothered me. I knew(99%) that it wasn't correct, but I wanted to hear the justification for making the statement.

mark

the mass flow rate is a constant. Therefore it doesnt matter what the temp is. The hotter the gasses the lower the higher the pressure. The colder the gasses the lower the pressure for a certain volume of gas. You still have the same amount of mass flowing through the turbo.

the aerodynamics are dependent on the total mass flux (mass flow rate) times velocity. The velocity component is dependent upon the effects downstream of the area of interest.

this helps explain why aftermarker exhausts help spool up of the turbo by lowering backpressure/increase the velocity.

It is in response to ARK's posting.
heat does not work the turbine.

On to your point though...the oil gets heated up pretty quickly by the engine as well as the turbo cooling lines.

I had to wait until I got into work; I don't own a computer :^)

I, too am refering to a steady state condition. Heat is being removed continuously via the modes you described; namely, conduction, convection and radiation and removing heat from the exhaust gas steam reduces its internal energy, thus its energy available which can be converted to mechanical energy. This is indisputable.

I understand and agree with your position that it is the mass flow and velocity (momentum) of the exhaust flow that actually causes this transfer of mechanical energy. The point I am trying to make is that this momentum is affected by exhaust gas temperature (EGT) in such a way that if temperatures are reduced (by some extent), this momentum will be reduced too.

All I am saying is that a balance is required in maintaining a high enough EGT, while maintaining a low enough turbocharger temperature. If you do something that lowers the EGTs too much, the exhaust gases ain't gonna be able to do the job, for the reasons I described in previous posts.

Not that this means anything, but APR is talking up how their custom Inconel exhaust manifold has 4 times the insulating properties of the stock cast iron manifold. Why, because by maintaining high EGTs, the internal energy, thus energy available for conversion to mechanical energy, is maximized.

BTW, I dispute APRs claim regarding the fact that Inconel is a 4X better insulator - based on my review of material properties, it seems that cast iron is 4X better than Inconel. If someone from APR is reading this, please correct me if I'm wrong.

Ah, this was stimulating.

By the way. This has been on of the more thought provoking threads that I've seen lately. Before all of you start making jokes about eggheads, I'll quickly mention that these types of discussions provide some background to why we do things that we do - at least in terms of our cars.

Thanks for everyone's input!