Turbocharger efficency

[Ron Earp]

Okay, you automotive engineers here please help me clarify something. The term, adiabatic efficiency, when used to describe compressors, refers to what?

To me 100% adiabatic efficiency would be that our theoretical compressor when compressing air imparts no additional heat to the discharge air. That is, the air heats up but no more than what is described via PV=nRT. A compressor that is 74% efficient heats the air up in addition to what happens during normal compression.

Is this correct?

And, if so how can I use the efficiency number on a compressor map to make some calculations on discharge air temp?

Part of this came up on a discussion on the Lotus board about compressors. I maintain that a more efficient compressor will not heat the air as much at a given boost level when compared to a compressor with lower efficency. I know I'm right because I've swapped turbos and with no other change measured with thermocouples a lower discharge temp at a given level of boost with a more efficient compressor. But, I'm not sure how to put this in correct terms.

R

 

[StuartB]

Just tell `em the`re a bunch of thick twats and never return to that forum, jobs a good`en mate. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif

 

[Adam C.]

You're correct Ron. If a compressor is 70% efficient, then 30% of the shaft work that you put in to the compressor is lost to irreversibilities (heat). If you want, I can write something up.

 

[John]

related question...

will a ball bearing type turbo be more or less efficient inherent to its design? It seems to me as though it would be. I love turbos... such a neat invention. /ubbthreads/images/graemlins/grin.gif

John

 

[Ron Earp]

Hey Adam,

Thanks for the information, this should be received okay.

In this situation there are folks that believe that regardless of the compressor, if you take the a given volume of air to a higher pressure then the temp increase is the same, regardless of method. That definitely isn't true, otherwise we'd have no R&D on compressor wheels etc. to be more efficient at given flow levels.

 

[Ron Earp]

On the ball bearings - on the other forum I posted a post on this. I had a Mustang SVO and had a hybrid T3/04 on the car. Made good power.

Buddy of mine was into selling parts got me a T3/04 ceramic ball bearing unit at cost. We wanted to know the answer to that question too. Put it on. Dynoed the same, quarter mile the same, didn't notice any difference in response.

I think they will last longer, but, I've never worn out an oil bearing unit.

 

[Trevor Booth]

Ron, your post re adiabatic efficiency. Adiabatic refers to a change of state in a gas with no loss of heat to the surroundings ie the energy is conserved. You take a given volume of at a given pressure and raise it to a higher pressure by compressing it into a smaller volume the heat produced is the same no matter the method for compressing the air. The process is reversible and is the basis for refrigeration ( the carnot cycle) In terms of compressing air Adiabatic efficiency means nothing. You cannot express the adiabatic process in terms of efficiency. You can express the heat loss after the adiabatic process but that is what is desired with an air compressor, get rid of the heat. The more correct term for an air compressor is Polytropic due in part to the fact that the air compressor is cooled thereby modifying the adiabatic process. What is more relevant is the mechanical efficiency of the compressor - frictional losses, its ability to breathe- porting etc, which all comes down to the energy input to drive the compressor relative to the work energy contained within the compressed air. That is the true measure of efficiency of an air compressor.
Try water cooling a supercharger (the compressor) some time.

 

[Ron Earp]

Is there a term that describes mechannical efficency then? Basically what I want to know is what is the percentage figure shown on the turbo compressor maps? Take a look here:

http://www.turbocalculator.com/compressor-maps/t04e-50.jpg

 

[Adam C.]

Now that we are all wrapped up in semantics... /ubbthreads/images/graemlins/tongue.gif Ron, what you were looking for is the isentropic efficiency. The conditions for an isentropic process (well insulated and reversible) are somewhat more restrictive than those for an adiabatic process (well insulated). Isentropic processes are by definition adiabatic. Anyway, I knew what you were talking about.

Well you asked for it, here are both barrels.

 

[Adam C.]

2

 

[Adam C.]

3

 

[Adam C.]

4

 

[Adam C.]

Oh and the real answer to your question.

Eta sub c in my sample calcs is the % value in your plot (isentropic efficiency). Just find the efficiency for a certain pressure ratio and mass flow rate, and plug it into the equations.

Adam

 

[Ron Earp]

Adam,

nice post! Thanks for the help, that makes things clearer now and I can do some calculations. I'll definitely post a link to this over on the Lotus forum too so we'll have some visitors.

 

[Jools]

Somedays I think that I really should have spent more time listening in class /ubbthreads/images/graemlins/crazy.gif and less time messing with cars and girls /ubbthreads/images/graemlins/tongue.gif... but only somedays /ubbthreads/images/graemlins/grin.gif !!

 

[Trevor Booth]

And I thought I would keep it simple!!
You have studied well Adam. The isentropic efficiency of good turbos does not change a lot from manufacturer to manufacturer and it is all academic in the event that the turbo is too large or too small for the engine. The other problem is they operate in less than ideal conditions, have a constantly changing speed of rotation and variable ambient temperatures. I am fortunate enough to have had my own Dyno for some 30 yrs and have played around with forced induction for a similar period. I have done pages and pages of calcs and then when you run the engine the results are not the same as calculated, more often not even close. I have found that paying a lot of attention in getting the free air into the turbo reaps huge benefits ( isentropic efficiency goes up)as does getting the exhaust to the turbo. To just consider the isentropic properties alone will not necessarily give a good result. These days the turbo manufacturers can give you reasonably accurate figures of what you can expect for the correct engine/turbo match.
Trevor

 

[Adam C.]

Not a big deal. This is 2nd year engineering undergrad stuff. This table will make it easier to try other combinations.

 

[Jim Rosenthal]

This is fascinating although I understand very little of it. I do have a question (don't laugh): since ideally what you want coming out of the turbo is air that's compressed but not hotter, doesn't the material of the compressor wheel and housing play into this? specifically, the ability of the compressor to absorb heat from the air it is compressing? just curious.
I don't recall ever seeing a GT40 with a turbocharged engine, but it seems to me that the plumbing does somewhat lend itself to that; has anyone done it?

 

[Ron Earp]

There isn't much there to absorb heat, and, there isn't a whole lot to carry heat away in a turbocharger. They are, most are, water cooled and oil lubricated but these things are designed to live at high temps, especially on the turbine side. I imagine the compressor housing gets to a stable temp under load and stay there. The design of the compressor wheel definitely plays a part as to how much hotter the air gets compared to another wheel, but neither are going to be heak sinks to any appreciable extent.

But, that is what we have aftercoolers for!! Either water to air, or air to air, they accomplish the same thing and bring down the temp of the compressed air. This is where you can see some serious differences in efficency of setups.

R

 

[John]

I also love turbos! They are incredible little devices.

I know of a guy who is currently building a Chevy small block engine for a '57 Chevy that will be force-fed by 8 turbos! It looks like a plumbing nightmare... and is more for show, but very interesting. If you're interested, you can find some pics of it by going to www.pro-touring.com and looking in the "forums" and looking in the "power adders" forum. The name of the thread is "why stop at 2?"

My experience with turbos consists primarily of my Nissan 300ZX twin-turbo. I'm amazed that some people with these cars are putting out nearly 1,000 horsepower on a 3 Litre V6 that is stock except for fuel, turbo, and some other "bolt-on" upgrades (on stock short block and cylinder heads). All while being very driveable on the street.

Someday, I'll be strapping a pair of turbos on to the small block in my '68 Camaro. /ubbthreads/images/graemlins/grin.gif

John