Effect(s) of changing compressor trim on map width/size/quality

[Hohn]

The PS66 thread got me thinking about compressor trim (well, turbine trim too, but there are no turbine maps that I have seen).



The way I think of compressor trim is basically the "slant" of the angle at the edge of the compressor. Since it's inducer^2/Exducer^2*100, that's the best way I can illustrate it. Is this correct? Cliff?



So, when you go to a higher trim wheel (like from say a 40 trim to a super 60), what effect does that have on the map?



Our CTDs seem to need a higher PR than a gasser engine would-- blame the crappy airflow in the heads and anemic stock cam.



Is a Higher trim more conducive to a higher PR or a lower PR?



Is it correct to say that optimizing a compressor for higher PR results in a center efficiency island that is both 1) smaller and 2) lower in overall efficiency?



It appears that a lower trim can make the map fatter and more efficient, but you lose PR and peak airflow.



I'm looking at this map:



#ad




It appears that the surge line moves to the right with the bigger, lower trim compressor.



Yet the smaller wheel (56 trim) appears to have a fatter middle section, at the cost of peak airflow and PR.



Doesn't the GT42 overall appear to be a potentially good candidate for a CTD??



Just curious. I'm educating myself on this, and getting more questions than answers. (like why is everyone using Holset or Schwitzer chargers and totally ignoring Garrett/Turbonetics?)





Justin

 

[Diesel Freak]

The GT42 IMO would be a terrible single. It moves some serious air, but it is just too heavy (laggy). Also the surge line is too far to the right to be used as a turbo on a daily driver that gets used as a TRUCK. However it would make one sweet Primary when coupled with the right top turbo.



Trim is basicly a ratio of area.



Remember back in High School geometry... .



The area of a circle πr² or πd²/4



With respect to compressor (and turbine trim)



It becomes



(πd²/4)/( πD²/4)



D=big diameter

d= little diameter



after some algebra we end up with



d²/ D²

 

[Hohn]

OK, now that this is established, how about some REAL discussion? Care to answer any of my other questions?

 

[Diesel Freak]

This is the way I look at it... . over simplified of course.



Inducer diameter gives you an idea of how much air the turbo can move. Exducer diameter gives you an idea of how much pressure it can generate.



Take for example an axial fan. inducer and exducer diameters are the same. Trim = 1 great air flow, but basicly no pressure. The rise in pressure in a turbo is from the radial acceleration of the air and subsequent slowing down in the housing when they don't move in the direction of the engine. It's a Kenetic to Potential energy conversion.



The smaller of the two compressors above has an inducer diameter of ~70mm, where the larger has a diameter of ~74mm.



The wheels are totally different.



Trim is just a ratio

 

[Diesel Freak]

I just did some quick looking at the 92mm map, and if you could get it lit off at a decent RPM (under 1800)..... it would surge and bark like a rabid dog when used as a single.

 

[Diesel Freak]

am I answering your questions adequately?



keep in mind that I am just a High School Graduate.

 

[Hohn]

Not really, you're just a HS graduate Just giving you the raspberries, Cliff!



It makes sense to me that a higher trim would allow for a higher Pressure ratio.



But I disagree that a regular axial fan cannot produce compression. True, it may not be able to produce a HIGH pressure ratio, but it would almost CERTAINLY have to be able to generate pressure above atmospheric, true? I mean, the air has mass, and therefore inertia. You spin that axial fan at 100K rpm, and you get compression, I guarantee it.



Consider the "birdcage" axial fan on most home HVAC systems. It MUST be able to produce a change in pressure, or it would NOT be able to create flow, true? I mean, if there is no pressure differential, then there is no flow, right?



When trying to understand things like air and water flow, it's sometimes useful to think from the perspective of one single molecule of air or water.



With this perspective, we see how the trim can effect pressure ratio because it determines how muich acceleration each molecule will see. The higher the rate of acceleration, the more compression involved.



It gets complicated to think about a dynamic system-- air is moving out and coming constantly. Then you have to account for the differences of wheel SIZE versus SPEED, vs TRIM. They all interrelate, and complicate things.



jlh

 

[swankmetal]

i'm confused. but please keep in mind that i have a 3rd grade edumucation. is the equation actually as simple as inducer diameter squared divided by the exducer diameter squared? i tried that with a 56mm inducer, 76mm exducer wheel and got . 54???? does that equate to a 54 trim? sorry for the basic math questions here guys.



dave

 

[Diesel Freak]

yep, it is that easy.



I am currious about wheel depth... . anyone know anything about that?

 

[swankmetal]

so where does the # of blades the wheel has come into this whole thing? and is it possible to somewhat accurately guess what a holset's wheel PR might be from all this info? i read through that link to the garret site you put up in another thread cliff, but couldn't find tooooo much. way too many questions.



dave

 

[Diesel Freak]

Holset does not let out it's compressor maps for public consumption.



But, you can generalize.



If a wheel shares all the same dimensions with another that you have a map for... ... they will act "simmilarly"

 

[Trashdawg]

here is one

 

[Diesel Freak]

LMAO..... yeah, that one I can use!!!

 

[Hohn]

Man, I coulda just made a Holset map with graph paper and a Sharpie!

 

[RustyJC]

Take for example an axial fan. inducer and exducer diameters are the same. Trim = 1 great air flow, but basicly no pressure.

Many/most gas turbines (including larger aircraft jet engines) use axial compressors. The Rolls-Royce Avon, for instance, has 17 stages of axial compression and a 3 stage axial turbine to drive the compressor.



Axial air and gas compressors are also used in some process/refining services.



Rusty

 

[Diesel Freak]

I realize that, but the cross section of the axial compressor decreases as you move along the flow path.



a typical single stage ventilation fan does not... . gues I should have been more specific

 

[CRoth - BD-Power]

Doesn't the GT42 overall appear to be a potentially good candidate for a CTD??

Garrett wanted for use to a modified GT42 a couple of years ago, although it was a BB it was very laggy. Two problems as DF eluded was the surge plus they didn't use a large enough turbine wheel.



Garrett has some great designs, but its there turbo matching or applications that need a little work.



Rememeber a gasoline engine loves to rev thats why the gasonline turbo map looks totally different than a fat and wide diesel map.

 

[CRoth - BD-Power]

Many/most gas turbines (including larger aircraft jet engines) use axial compressors. The Rolls-Royce Avon, for instance, has 17 stages of axial compression and a 3 stage axial turbine to drive the compressor.



Axial air and gas compressors are also used in some process/refining services.



Rusty

This has been tried for many years in the turbo industry for the turbine wheels, but there are pros and cons.



Radial provides more torque, hence starts the turbo spin quickly, Axial provides better top end efficiency.

 

[rbattelle]

I realize that, but the cross section of the axial compressor decreases as you move along the flow path.

Remember that the camber on an axial compressor will manifest considerable pressure gradient across the blade. There are some single-stage fans out there with pressure ratios greater than 4. They are the same diameter at the inlet and outlet.



Radials, of course, will always make much more pressure than axials... but I don't think they can flow as much air.



I've never heard the term "trim" before... thanks for the education, guys! There's a good book on the subject titled "Design of Radial Turbomachines" by A. Whitfield and N. C. Baines, but you probably need a solid math background to get anything useful from it.



-Ryan

 

[RustyJC]

This has been tried for many years in the turbo industry for the turbine wheels, but there are pros and cons.



Radial provides more torque, hence starts the turbo spin quickly, Axial provides better top end efficiency.

It's common on the turbochargers on engines in, say, the 2000 BHP and higher ranges to have radial compressors and axial turbines - that's what we build for our large industrial engines.



Rusty