Turbocharger = shaft turbine power?

[parkland]

If I thought I knew anything before, now I don't lol.

I read Chris's comments a few times over, and might read over a few more times before understanding better.
So in essence, back pressure on the gas producer, either from a tight nozzle, or driving a power turbine, will cause
the gas producer engine to approach a stall or surge condition.
Shouldn't there be a large difference with how loaded the power turbine is loaded too?
For EG, a large power turbine stopped VS smaller turbine spinning fast?
Maybe a much smaller turbine would work if a clutch was used, so it isn't stalled?

On another thread, where a 6.0 ford diesel turbocharger was being used, it was stated that the VGT vanes needed to be open to get it to start.
That makes sense as the vanes in the closed position push the gas producer almost into a stall.

What would be really cool, would be to find a giant VVT/VGT turbocharger, and make a power turbine with the adjustable vanes!
Would it not make sense, that as the power turbine came up to speed, the vanes could be closed more, to extract more energy?
I mean, the faster the power turbine spins, the less back pressure is created, right?
Shouldn't that mean that as the RPM increases on the power turbine, the back pressure should drop, and vanes could be used to deliver more power
to the blades?

Asides all that talk, I was thinking a huge benefit of using a VGT turbo as the gas producer, was that power could be controlled better.
A control or spring could be used, to keep the vanes closed at idle.
As fuel increased, RPM would increase, and as the pressure hit around 15 PSI, the actuator could start opening, and changing the gas producer from
an idle mode, to power mode. So maybe the vanes would start opening at 15 psi, and be fully open at 30 psi.
Running it like that, I think it might be a better throttle response, as the turbine is spinning quite fast before any energy is harvested by the power turbine.
To me, this sounds like the equivalent of changing the idle on your car to 3000 RPM, so that when you want power, all it has to do is let in more air and fuel, not wait for the engine to speed up.

There would have to be a knob or switch, to open the vanes for starting.

I don't know, I'm just talking out my butt.

[racket]

Hi

Yep , too much backpressure and the comp can go into surge .

There isn't a lot of difference between when a freepower is stopped/stalled and with it a full rpm with regards backpressure if everything is in "harmony" , but we have experienced some increase in stalled temperatures compared to full freepower rpm temperatures exiting the gas producer due to the added backpressure from the more difficult passageway flow when stalled compared to the virtual "straight through" flow when at full rpm , to counter this "problem" a simple bypass/wastegate opened during idle conditions can be used , slowly closing off as freepower rpm rise .

Opening the vanes on a VGT gas producer turbo during start might make it a bit easier depending on the starter used , but its not normally necessary .

A VVT freepower does make a lot of sense and was used on some of the automotive gas turbine engines , but its not necessary for the gas producer as our relatively lightweight rotors accelerate very fast.

As a general rule , forget manipulating VVT for the gas producer it'll create more problems than its worth , once the best position is found for the vanes , lock them in that position.

Cheers
John

[finiteparts]

Hey John,

Whoops! Forgot about that pesky pressure term in the ideal gas equation! Sorry about that. A good example of why you should always work through the calculations.

~ Chris

[finiteparts]

Apr 14, 2015 23:23:42 GMT -5 racket said:

Hi Chris

One correction with your comments .......... " Even with the large turbine diameters, the blading still needs to turn the flow by very large angles...this is a whole different animal that what we are trying to do".

We do turn our gases at large angles , the freepower for both my 10/98 engine as well as Anders JU-01 engine has quite shallow NGV angles down near 20 degrees feeding gases into a freepower wheel with tip angles near 20 degrees in the other direction

Our engines are operating exactly the same as commercial engines .

Cheers
John

The central thesis of that paragraph was to dismiss the idea that you need the large diameter jumps in the power turbine stages that are seen in commmercial engines...that is what I was referring to as a whole different animal to what we are trying to do.  It was just a point of fact that the large power turbines, even with their large AN's still need to turn the flows at very large angles.

Thanks,

Chris

[mitch]

Apr 15, 2015 10:14:02 GMT -5 parkland said:

If I thought I knew anything before, now I don't lol.

I read Chris's comments a few times over, and might read over a few more times before understanding better.
So in essence, back pressure on the gas producer, either from a tight nozzle, or driving a power turbine, will cause
the gas producer engine to approach a stall or surge condition.
Shouldn't there be a large difference with how loaded the power turbine is loaded too?
For EG, a large power turbine stopped VS smaller turbine spinning fast?
Maybe a much smaller turbine would work if a clutch was used, so it isn't stalled?

On another thread, where a 6.0 ford diesel turbocharger was being used, it was stated that the VGT vanes needed to be open to get it to start.
That makes sense as the vanes in the closed position push the gas producer almost into a stall.

What would be really cool, would be to find a giant VVT/VGT turbocharger, and make a power turbine with the adjustable vanes!
Would it not make sense, that as the power turbine came up to speed, the vanes could be closed more, to extract more energy?
I mean, the faster the power turbine spins, the less back pressure is created, right?
Shouldn't that mean that as the RPM increases on the power turbine, the back pressure should drop, and vanes could be used to deliver more power
to the blades?

Asides all that talk, I was thinking a huge benefit of using a VGT turbo as the gas producer, was that power could be controlled better.
A control or spring could be used, to keep the vanes closed at idle.
As fuel increased, RPM would increase, and as the pressure hit around 15 PSI, the actuator could start opening, and changing the gas producer from
an idle mode, to power mode. So maybe the vanes would start opening at 15 psi, and be fully open at 30 psi.
Running it like that, I think it might be a better throttle response, as the turbine is spinning quite fast before any energy is harvested by the power turbine.
To me, this sounds like the equivalent of changing the idle on your car to 3000 RPM, so that when you want power, all it has to do is let in more air and fuel, not wait for the engine to speed up.

There would have to be a knob or switch, to open the vanes for starting.

I don't know, I'm just talking out my butt.

In regards to what you said about the 6.0 diesel vgt thread, (which I assume was in my build thread lol) I never tried to start the engine with the vanes in any position other than fully open or fully closed position. fully closed caused the engine to whistle very loudly when I tried to spool it up and start it, and even got to the point where flame was coming out of the compressor inlet when I would pull the leaf blower away from the inlet. I used a 12v power supply to actuate the vane system. As for using the VGT as you described, you would want to open the vanes more as the power turbine came up to speed. The way the VGT is used in a car is as follows: the vanes are in the furthest closed position in order to help accelerate the exhaust gasses and spin the turbine faster, then the vanes open up when the engine is up to a heavy load and the turbine is spinning at a high enough rpm to spin the compressor fast enough to supply the engine with the desired boost level. The vanes open up once the turbo is spooled and the engine is under a heavy load as to allow exhaust gasses to flow better, and the VGT isn't needed as much to keep the turbine spinning as the engine is now producing enough exhaust gas to keep it up to speed without the aid of the tight vane configuration.
For the turboshaft engine I am making, I will be keeping the vanes in the fully open position on the gas producer, because I don't wanna mess with varying voltages to the VGT solenoid and I figure that the free power turbine ( a Cummins st-50) will provide just the right amount of backpressure needed to keep Turbine inlet temps at the gas producer in check. (as of right now, I am having a problem with high TIT/TOT temps, along with an injector that likes to leak!)

Hope this helps a bit! 

[racket]

Hi Mitch

If you have high T I Ts/TOTs with the vanes fully open then you'll need to close them up a bit .

With variable vanes the turbine wheel can be a tad oversized so that with the vanes fully open theres minimal backpressure on the IC engine , but when made into a gas turbine we need to control the compressor mass flow , the IC engine swept volume and rpm do that for a turbocharger , so we need to tighten up the vane throat area .

Its generally best to fix the vanes at around mid position and only adjust that position once the engine has been run a few times and some parameters have been noted that may indicate a different position is required.

Cheers
John

[parkland]

Mitch,

How are you controlling the actuator with voltage? I thought they used PWM signals to change position...
I would use your ECM to change the vane position. Even if there is one best position, it would be cool to do tuning through the ECM, but thats my opinion.

www.youtube.com/watch?v=4O2y5cjqPWY

At about 3:00, you can see what the temperature does as he starts closing the VVT nozzle.
It would be nice, IMHO, to be able to utilize the closed vane position to cool down or idle the motor.

[mitch]

I was controlling the actuator by assembling the turbo with the unison ring/vanes fully closed, then supplying Either 9v or 12v to the actuator solenoid. I found that 9v opened it about half way, and 12v opened the vanes all of the way.

He has the advantage of being able to simply and manually adjust the vanes on his setup. I guess you could monitor the egt or boost with an arduino, then adjust vane position using a program and pwm from the arduino. There are a ton of options! But as racket has said, it is probably best to leave the vanes in a single position.

[parkland]

Feb 1, 2015 18:50:33 GMT -5 racket said:

For a "vehicle" its best to use the 2 shaft system with a freepower turbine to produce the shaft power , single shaft engines aren't really suitable .


Cheers
John

Sorry to beat a dead horse,

I am just curious why a single shaft isn't suitable to extract power.
Is it because the turbocharger exhaust vanes are not good at extracting kinetic energy from the exhaust flow?
Or is it because of the hardware required to gear down from such high speed?
It seems to me, like if there is enough velocity in one of these engines to drive a freepower turbine, why can't that energy be available from the main shaft?

Just trying to understand better....

[racket]

Hi

LOL.............it just isn't , its the nature of the beast .

Thats not entirely correct , there are some very good single shaft engines around , a lot of APUs are single shaft , the very successful Garrett TPE331 turboprop engine is another , but single shaft engines are more of a single speed engine with very little power to be had below 80% rpm, it all happens in the last 10% of N1 , whereas a freepower engine will produce 80% power at 50% N2 rpm.

To get the best from a single shaft engine the turbine wheel needs to be large enough , this does present some problems when using turbocharger based components, but its not impossible if combining parts from a "family" , ............in the Garrett range there are "4 inch turbos " in a family where parts can be interchanged , it would be possible to use a small comp flow on a larger turb wheel with power extraction .................but then we come to power takeoffs ..............how do you intend constructing that ??

Its just easier to use a freepower :-)

Cheers
John

[finiteparts]

I think you missed one of the key advantages of the two shaft engine...you can stall it! Think of the free turbine as an air coupled torque converter. Just like in an automatic car, you can stop the car with the engine still whirling around and the hydraulic torque converters allows the fluid to swirl around inside making torque, but not turning the wheels...you don't need a clutch to mechanically decouple the drivetrain.

Additionally, the torque curve is more ideal for use on road vehicles....you get peak torque at stall...



So when you are sitting still and need to get going, it's torque that gets you going...with the free turbine, you have peak torque when you are sitting still. As you roll out your torque falls but if you sized everything to have a final torque value, you will be on par with all the other arrangements.

Now, if you look at the single shaft, the torque curve is reversed. You can't run it at stall! You would kill the engine. So you might think "no big deal right...we'll just put a clutch on it." The problem with a clutch is that when you engage the clutch you are suddenly putting a load on the engine and the engine might get dragged down quick enough to flame out or surge. On the other end of it, your at the end of the run and you get off the throttle and push the clutch, decoupling the engine load from the shaft. If the inertial loading of the rotor is high enough or you have a delay in the fuel control system, you could have a case where the turbine overspeeds due to the sudden drop in load with there still being a high amount of energy in the turbine. Or heaven forbid you accidentally engage the clutch while your at load. This is the reason that the large multishaft commercial engines have things to stop the rotor if it suddenly becomes unloaded (like a shaft break or the fan sucks in a bird and looses all it's blades).

Like John said, single shaft engines are great for constant speed stuff...APU's, turboprops that run constant speed props, generators, etc...since you usually get your peak efficiency at the top rpms, these engines run at 100% rpms all day and don't do very well when throttled down.

I hope that helps make it more clear.

Chris

[parkland]

Thanks for the further explanation.

I think I am over thinking things above my knowledge level.
I do agree the free power turbine is practical for these types of builds.

Imagine trying to make an engine from this guy:
www.internationalpowerstroke.com/sst.html

LOL
I do agree the free power turbine is a better idea haha.

[mitch]

parkland,
That is the turbo I referred to the other day in a different thread on this website!
That would be really cool to use, I wonder what the mass flow for that turbo is rated at, along with what boost pressures it is designed for?

One thing to keep in mind when using these large turbochargers for a turbojet/turboshaft, is that it requires quite a bit to get it spooled up. For example, my first turbojet, which was a smaller garrett t25, could be spooled using a vacuum cleaner blower. My GT3788VA turbojet requires a large Stihl backpack leafblower running full throttle to bring it up to idle speeds, which is about 5-8 lbs of "boost".

[parkland]

Haha, today I just got to thinking what if something like this was built, but with jet power haha


www.youtube.com/watch?v=hipkH1qm1Lg


(And the exhaust not so close to the ears lmao)

[parkland]

Was just thinking again and have another question.

Does a turbocharger jet engine require heavy oil like a diesel engine, or can you get away with something much thinner?
I was thinking maybe the lighter the better, as it will flow faster, and cool better.
Straight 5 weight?
I would assume straight weight would be better, so it flows quicker when hot?