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The New 911 turbo has a new feature called Variable Geometry turbo's, Does anybody any info about this new feature?

This technology allows the angle of the compressor's turbine blades to continually adjust. While some diesel engines have enjoyed this technology since the Nineties, the higher exhaust gas temperatures created by gasoline engines necessitated the creation of new heat-resistant materials to handle the hotness. Porsche and Borg Warner Turbo Systems were able to overcome the heat issue and have developed a VTG turbo system that will be incorporated into the next 911 Turbo. The VTG turbo will allow Porsche's flat-six to mimic a twin-turbo setup with a much broader torque curve and more flexible powerband than a standard single turbo could provide on its own. Power ratings for the new VTG turbo engine haven't been released and probably won't be until the new 911 Turbo surfaces sometime next year.

the "hotness" of the exhaust gas in question is said to be well over 1000deg C.

On the picture above, there are two impellers. The one on the left is the exhaust, and receives the hot exhaust gas. It spins the impeller on the right, which compresses the intake air. It also has the variable geometry blades, which can have their angle changed.
In the older style turbos, with fixed blades, as the engine RPM went up and down, the turbo changed speed too, relative to RPM. So the engineers had to design the turbo size based on max RPM. Obviously, when the engine was spinning at maximum RPM, you had to have the boost level within acceptable levels. But at lower RPM, you wound up with low boost, low pressure, and you had to wait until RPM climbed enough to start delivering that sweet turbo power. Back then, turbo lag and the violent onset of lots of power worked against making this power delivery easy for the driver to deal with.
But with variable geometry, you can change the intake blade angles to deliver lots of boost while the engine RPM is still low, and then when RPM's do climb, reduced the angle of the blades, and thus reduce boost pressures to a safe level. This delivers a much broader power band, and the power does not come on with a violent bang. Also, turbo lag is virtually non-existant.

I've made animated gif out of those two pics showing max and min position of blades so that it becomes more understandable how geometry of blades is changed depending on regime of the engine.

DaveKillens wrote:On the picture above, there are two impellers. The one on the left is the exhaust, and receives the hot exhaust gas. It spins the impeller on the right, which compresses the intake air. It also has the variable geometry blades, which can have their angle changed.

The compressor is on the left and the turbine is on the right. The turbine converts the exhaust gas energy into rotational motion. The compressor uses rotational motion to compress the incoming air. It is a variable turbine geometry. Not a variable compressor.

The compressor is on the left and the turbine is on the right.

And here I was wondering why the exhaust gas plumbing is so small... thanks for clearing that up.

ginsu wrote:The compressor is on the left and the turbine is on the right. The turbine converts the exhaust gas energy into rotational motion. The compressor uses rotational motion to compress the incoming air. It is a variable turbine geometry. Not a variable compressor.

Thank you ginsu, for this correction. I stand corrected. I was thrown off by manchild's quote that mentioned a compressor's blade adjustment. I was also wondering on what I assumed was the hot exhaust impeller having o-rings. But it now all falls into place now, these changing blades function in the very hot environment of the exhaust system. It makes sense, you can vary their pitch depending on mass of exhaust flow, and control the boost by turbo impeller RPM.
It's a nice way for good engineering and materials to overcome one of the big negative factors against turbos in a racing application, lag, and low boost at low engine RPM. And even sweeter with two turbos, they can be smaller than one, have less mass, and accelerate quick as you can wink.
As a side note, this is a very good example of why Max's present engine technology lockdown should be canned, and more innovation allowed. Under present f1 rules, this turbo technology cannot be used, yet it is a solid, quality solution to a real world problem.

I guess the main drawback of this technology though is the increased mass of the rotating components in the turbo. The variably vanes must have some form of mechanical actuation, which can only add weight I assume. I must say as well that the unit looks pretty hefty compared to a traditional turbo. I wonder if they had to increase the amount of material on the components to retain structural integrity at these temperatures.

Good point wow, I guess that's why they run two turbos, to make each individual one smaller, less mass to overcome.

Also, it looks like the turbine blades are forced to be flat in order to work properly with the variable turbine geometry. After taking apart a few turbos you realize how intricately designed the turbine blades are (sorta like a spoon shape blade). So, obviously, the blades are not optimal.

wowf1 wrote:I guess the main drawback of this technology though is the increased mass of the rotating components in the turbo. The variably vanes must have some form of mechanical actuation, which can only add weight I assume. I must say as well that the unit looks pretty hefty compared to a traditional turbo. I wonder if they had to increase the amount of material on the components to retain structural integrity at these temperatures.

This technology has been used for ages now in diesel engines! Till now these turbos couldn't be used in petrol engines because of its higher EGT's that would damage the blades, locking them in place... Now that at least porsche ones can withstand the petrol engine's EGT's they are obviously the best choice possible!

Notice that the variable vanes do not rotate, they are fixed to the core of the turbo and are actuated by the same type of actuator that moves the wastegate of the common integrated wastegate turbo. The actuator is connected to only one of the vanes, and all the others are connected to that one.
Weight is not an issue as it is pretty much the same. Although it has the vanes it dispenses the wastegate, so it kinda balances out.





More on:
http://www.autozine.org/technical_schoo ... _3.htm#VTG

DaveKillens wrote:Good point wow, I guess that's why they run two turbos, to make each individual one smaller, less mass to overcome.

I think the 911 turbo uses a twin turbo setup like all previous turbo models because it's a boxer engine and it makes sense to have a turbo for each bank of cylinders (just like a V type engine) so that you don't have exhaust pipes going everywhere on the engine bay running under the engine, just like on the scoobie engines! The efficiency of the turbos benefit from being closer to the exhaust valves and being two cuts the sizes down giving a better spool up time due to the lower moving masses.

Currently the BMW 535d (3.0 diesel - 272bhp / 413 lb ft) is using two variable geometry turbos, one smaller one for low down power and a bigger one for higher revs to give that huge horsepower figure that with a single turbo would mean that the engine would be completely dead below ~2500rpm...

in pure racing it actually isnt that big a deal if they have one or two turbos because of the wastegate. the biggest problem is raising the pressure, as it drops during gear changes and is lower a low revs. this has been overcome in racing and also in normal cars by designing the size not by max revs but about half the max revs, so that there is max pressure at low revs and the peak pressure is achieved at maybe 4000 rpm from whereon it stays constant as the wastegate opens little by little bleeding out the growing pressure and keeping the power stable. an example of this is the audi originated 2.0t, also found on vw and seat. with 280nm of torque from 1900-5000rpm and max power 200hp from 5000-6100 rpm and a consumtion of 7.8l/100km average, it is quite a nice piece of turbo engineering. on racing, as in wrc, the turbo lag has been overcome by passing gasoline into the exhaust at low revs and during gear changes. the gas ignites when it hits the hot turbo and keeps it spinning at maximum revs so the boost is istantly present.

Hello all.

I'm new to the forum, you could also use NOS to spool the turbo up to speed,
Regards Senna1

joseff wrote:the "hotness" of the exhaust gas in question is said to be well over 1000deg C.

.it reaches this temps at times.. ...
I cannot be entirely specific about this but 1000 is too high and if you look closely to the setup you will see there is a exhaustgas temperature probe at the entrance to the turbo just because of this.. and this will not last long at temps above 1050..the VTG turbos in porsche cost an arm and a leg each if you fancy to buy a set..i think around 2500€ each... and then they are adjusted by actuators(included in that price) .so you need for each VTG a PWM signal in and the unit feeds back a PWM signal.. and you need of course an electronic throttle control as well as VTG is not really the equivalent of a waste gate.. so the boost pressure is largely controlled by throttle . ..
I think on the Borgwarner-BWTS web site is something about VTG even in english and about the Porsche development to be seen..unfortunately the real details are not available on the web ..