What is Variable Turbine Geometry and how does it work?

[ka mano]

What is Variable Turbine Geometry?

Elsewhere in this forum the question was asked: how can the vanes of the VTG turbo's turbine be small at low boost and then somehow open up to a larger size at higher boost levels?

While nothing changes in size, the centripetal angles of exhaust-gas flow are modified by a set of adjustable vanes encircling the turbine wheel. By changing their pitch, they vary the manner in which the radial flow of the exhaust gasses in the volute is redirected at the turbine's blades. VTG (aka VGT) articulates these vanes optimally at various rpms, presenting maximum fin surface low down and next to nothing up top, potentially eliminating the need for a wastegate.

Diesels have been using this technology for some years. In 1989, Shelby was among the first to try it in a gasoline-powered production car in his CSX-VNT, using a Garrett VNT-25 (Variable Nozzle Turbine). Production was halted due to the materials' inability to handle the gasoline-generated heat. The Chrysler Lebaron and Dodge Shadow CSX also used VNT in 1989, but it improved boost so much they were blowing head gaskets right and left! The EGT hurdle proved to be too great for Honda, who gave VTG a shot before backing away from turbochargers altogether.

Thanks to aerospace R&D, the creation of high-nickel-content, temperature-resistant, superaustenitic alloys such as Inconel has changed all that. Exploiting the arrival of these new metals in cooperation with Borg Warner Turbo Systems, Porsche has re-introduced VTG in the 2007 911 '997 Series' Twin Turbo (yes, VTG + tt). There are rumblings about Audi and Hyundai following suit.


How does VTG work?

At low rpm -- since getting low velocity EGs to spin the impeller is a challenge and getting rid of EGs isn't -- VTG vanes are pitched flat in order to a) maintain maximum velocity prior to the wheel, and b) direct EGs at the wheel in an approximate right angle, maximizing the force against it.




As exhaust-gas velocity & volume climb with rpms, the relation between these two objectives (energy absorption vs. gas evacuation) reverses by degree, with the mechanism's task becoming less how to spin the wheel and more how to increase combustion efficiency by raising knock thresholds in the combustion chambers, helping to rid them of post-combustion gasses through minimal backpressure. Each VTG vane, then, pivots on its central axis, matching rpm with a designed balance between capturing power and letting it go.




The above diagrams show 13 vanes for the VTG mechanism and 9 for the turbine's wheel. In a one-for-one design, VTG vanes can close against the impeller fins, creating a series of single vanes and adopting the function of a wastegate.




VTG allows turbo engineers to vary the geometry used when diverting the circular movement of EGs entering the hotside into the linear movement of their exit. The more the turbine wheel is allowed to perform this diversion, the more kinetic energy it captures and converts to mechanical energy. The more the vanes of the VTG preempt that diversion, the less the rotating assembly will drag on the exhaust gasses' effort to escape.

Also know as Variable Vane Technology.

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[Ozer]

I have been reading about these VNT turbos, but before the new Porsche turbo, there was NONE available. Do you guys know if Garrett will be coming up with its own VNT turbo for Gasoline-powered engines? Keeping everything same in my rotated set-up and replacing my GT30R .63 with a lag-free equivalent would be awesome! : )

[Master2192]

I don't know when they'll start coming out with aftermarket versions but you can bet that quite a few drag teams are waiting for these to come out. Super fast boost response + lowering back-pressure needed to spin the turbine at high engine speeds will see good gains on all turbo engines.

They'll probably be more expensive than the current versions because they are much more complex and use special materials. If they pair them with electronic actuators instead of the diaphram like wastegate actuators, you'll see much more precise boost control. Caution though, as fitting too small of a exhaust housing on these turbos can result in boost creep. The VW TDI guys are having this problem when putting on bigger exhaust and turning up the boost on their diesel engines. I think they fix that by switching to a bigger turbo though.

[TMessick]

One of the typical issues on VNT is that having all them moving parts in the hot exhaust gas stream makes mechanical reliability pretty hard to achieve. On diesel applications they are used frequently, but there haven't been a lot of terribly successful petrol-engined applications.

On a similar front, the new Acura RDX has an interesting new application to achieve a similar function. Basically, they run a twin-scroll turbo, but run all 4-cylinders to both scrolls. At low RPM, a bypass valve (like a wastegate) closes the secondary scroll and increases boost response. Higer revs progressively open the second scroll. Not quite as trick as "true" VNT, but seems like a better reliability option in the long-term.

RDX info:
http://hondanews.com/catID3000?mid=2...43956&mime=asc
http://www.acura.com/index.aspx?init...owTurbocharger

[Capt Crunch]

So is this done only on the hot side or can this be done on the cold side too in reverse?

[ka mano]

Just in the hotside, which is why we've had to wait for the new alloys.

In the compressor housing, what would this be for... maybe a tricked-out anti-surge mechanism? I guess there are easier ways to do that.

That's thinking outside the box, though!

[modaddict]

Originally Posted by ka mano

Just in the hotside, which is why we've had to wait for the new alloys.

In the compressor housing, what would this be for... maybe a tricked-out anti-surge mechanism? I guess there are easier ways to do that.

That's thinking outside the box, though!

I thought you were asking a question initially. but then i read you knew. So i guess i don't get to explain it to you

[bboy]

Originally Posted by ka mano

Just in the hotside, which is why we've had to wait for the new alloys.

In the compressor housing, what would this be for... maybe a tricked-out anti-surge mechanism? I guess there are easier ways to do that.

That's thinking outside the box, though!

Actually Garrett Turbo has developed variable compressor technology but it's not out of the box yet.

On current production variable geometry turbos, we control the flow of air by manipulating the vanes on the turbine side of the turbo to reduce lag and improve drivability. Now we are developing variable vanes for the turbine and the compressor wheel.

We call it the Variable Geometry Compressor, or VGC, and it gives drivers 10 percent more power, 20 percent more torque and 700 more rpm than today’s state-of-the-art variable geometry turbodiesel. Its first application will be on the road in Europe shortly… The next trick will be to bring variable geometry to gasoline. It’s not easy given the higher temperatures of gas engines. But we are working with our aerospace colleagues at Honeywell to develop high performance alloys that are affordable for automotive use.

Late in the F1 "turbo era" they had gasoline variable vane turbos but they were racing team expensive. Another interesting technology they used was to place a throttle before the turbo inlet. When shut, the compressor spun in a vacuum and could maintain more of it's momentum.

[SaabTuner]

Originally Posted by bboy

Actually Garrett Turbo has developed variable compressor technology but it's not out of the box yet.

Variable Vane Diffusers? A simple electronic bypass valve would be simpler and more effective.

[TurboDXsti]

subscribed

[boozy04]

Haven't driven the Porsche, but the RDX was awesome! Made me want to get an SUV again. NO noticible lag, and lots of power down low and up top.

[tmarcel]

Originally Posted by Master2192

Since Garrett = Borg-Warner,

Did they just merge or something, I hadn't seen it in the news.

[SkyRealms]

Originally Posted by ka mano

Just in the hotside, which is why we've had to wait for the new alloys.

In the compressor housing, what would this be for... maybe a tricked-out anti-surge mechanism? I guess there are easier ways to do that.

That's thinking outside the box, though!

The EGT from a gasoline engine is relatively low for the metals that have been around for many years; therefore I think that it is more of a cost issue with the materials than a capability limit.

Theoretically there should be variable geometry in both the turbine and compressor. Both serving the same purpose which is to align the relative velocity (post vane) to the inlet of the rotor blade.



The best way to avoid stall/surge is to ensure that your impinging compressor flow is aligned with the blade entrance. Of course, cavitations and secondary flow effects become more of an issue at lower wheel speeds.

It can all be done fairly easily however and has been done on jet engine applications for 40+ years. However, there is little ROI on gasoline engine applications which is more than likely why we have not seen to many out there.

[Master2192]

Mr. Misinformation

[SaabTuner]

Originally Posted by SkyRealms

The best way to avoid stall/surge is to ensure that your impinging compressor flow is aligned with the blade entrance.

Or to use an electronically controlled bypass valve, which lets the turbo think it's pushing more air than is actually going into the engine.

Most jet engines use those too between the low-pressure and high-pressure compressor stages, though Rolls Royce seems to prefer using three-stage compressor sections to reduce the need for the bypass valves and other flow-control devices.

[Turn in Concepts]

These turbos have been around for quite a while. My step dad had one on his 84 Supra back in the day. Constant reliability issues with it though.

Still think it is the way to go as it just makes so much more sense.

[tmarcel]

Originally Posted by Master2192

http://www.honeywell.com/sites/ts/tt...us_history.htm

I still don't see the relationship between Borg-Warner and Garrett/Honeywell.

[Master2192]

Originally Posted by tmarcel

I still don't see the relationship between Borg-Warner and Garrett/Honeywell.

lol, oops. Got my turbo companies mixed up.

[cdclark]

VNT's unreliable? Just because you've never heard of it doesn't mean its unreliable. I know of several 300K+ mile TDI's rolling around with original turbos. VW and Audi have been using them in the TDIs for years.

Those that swap up to a bigger turbo, usually swap up to a bigger VNT. The stock one can still take quite a bit of abuse, up to about twice stock HP and torque.

[Ozer]

Diesel applications are different. The exhaust gas temperatures are much lower with diesel engines, which allow for ordinary materials to be used for VNT applications.

Gasoline VNT turbos, however, demand exotic materials for reliability, which is evidenced by the fact that the only modern application is the new 911 Turbo. I heard somewhere that the turbo of that car is $6000 alone.

- Ozer

Originally Posted by cdclark

VNT's unreliable? Just because you've never heard of it doesn't mean its unreliable. I know of several 300K+ mile TDI's rolling around with original turbos. VW and Audi have been using them in the TDIs for years.

Those that swap up to a bigger turbo, usually swap up to a bigger VNT. The stock one can still take quite a bit of abuse, up to about twice stock HP and torque.

[SaabTuner]

Originally Posted by Ozer

Diesel applications are different. The exhaust gas temperatures are much lower with diesel engines, which allow for ordinary materials to be used for VNT applications.

Gasoline VNT turbos, however, demand exotic materials for reliability, which is evidenced by the fact that the only modern application is the new 911 Turbo. I heard somewhere that the turbo of that car is $6000 alone.

- Ozer

IIRC, the only "exotic materials" were Silicon Nitride vanes. Low thermal expansion. High temperature tolerance. Great material!

The problem is that the high cost, if that $6,000 figure is accurate, is totally artificial: it does NOT cost that much to make those.

[flycaster]

Originally Posted by SaabTuner

...The problem is that the high cost, if that $6,000 figure is accurate, is totally artificial: it does NOT cost that much to make those...

I'd have to agree - all Porsche parts (even the plastic ones) are priced like they are made out of "unobtanium."

[ka mano]

...available for mining only on Pluto, so you can't even get it anymore!

[Keshav]

Anything in the aftermarket yet?

[rattmann316]

Originally Posted by Keshav

Anything in the aftermarket yet?

Five year old thread. Try googling it.