Power questions

[arthur]

My ultimate goal for my little GTP30-150(f) gas turbine engine is to use it to power a very light street vehicle like a rail buggy. I want to use a hydrostatic transmission to take the power off. After talking to a couple of guys who have done this, it seems like the best way to use a single stage turbine that was designed to power a generator. In order to pick the best possible hydraulic pump and motor combination I have to know the real, usable power output.

My engine was new in the box when I got it. I have never been able to find anything like a technical manual for it despite my best efforts. It did come with an "acceptance test data sheet". This sheet has a ton of data about no-load and full load temperatures, pressures, fuel consumption amounts, and several rpm checks.
It gives the corrected "shaft output" as 134HP at full load and 980 lb-in of torque (or 81 lb/ft- my conversion). The torque data is just listed as "Output Torque". On the gear reduction box there are 2 outputs, one is 4.9:1 or 12000rpm at 100%, the other is 7.4:1 or 8000rpm at 100%. The data sheet does not list anything about the power outputs from the 2 different PTO's.

If I use this formula: Horsepower = (Torque * RPM)/5252

And if use the 81LB/ft and the 8000rpm of the higher reduction pto, I get a horsepower result of 123.

If I plug in the 134hp and the 8000rpm, I get a result of 88lb/ft of torque. Both of those numbers, 123hp and 88lb/ft, "seem" to be in the ball park but I don't have any way to know if this is a good way to determine the real power to drive a pump.

THAT is one question I am asking. Is my math reasonable? Does anyone have any different or better ideas about this?

Hydraulic pumps usually operate in the 2500-3500rpm range so I will have to have further gear reduction. If I use 3000 for the rpm value in the same calculations, I get 43hp and 234 lb/ft of torque. Is this math reasonable? Love the torque number-if it is real.

The next question: How does turbine power compare to piston engine power or electric motor power.
My brief research into hydraulics indicates that if 10hp is required, a 10hp electric motor 'might' do the job, but it will take a 20hp or higher piston engine to do the same thing as the 10hp electric motor. Is the power from a gas turbine more like an electric motor or a piston engine?

Again, this is to help in picking out the best hydraulic pump and motor combination to build a hydrostatic transmission and to determine if it can be direct drive or if I need a transmission like Ralphs turbine powered Honda CRX.

Thanks for reading,
Arthur

[racket]

Hi Arthur

With 134 hp at 8,000 rpm you have ~88 ft/lbs of torque as you correctly calculated , but you also mention that its "corrected" horsepower , the "corrected" might be the reason for the several ft/lbs difference between the given torque and the theoretical .

At 3,000 rpm you'll still have the same 134 hp but torque will have gone up to your 234 ft/lbs , torque changes with the amount of gearing whilst the horsepower remains constant , the difference is that we run out of rpm with the increased torque , its like using first gear in your car , lotsa torque with lotsa horsepower but we don't go very fast before we need to change the "gearing" by putting it up a gear , this then initially lowers the road torque and horsepower until we're again at redline on the tach , but then we'll be going a lot faster than when in first gear .

Horsepower is horsepower irrespective of the power provider , a 10 hp electric motor will have "more??" torque because it is rpm limited to modest rpm in most cases , here in Oz we might see a 10 hp electric motor spinning ~1,400 rpm , in which case it will be producing ~37.5 ft/lbs of torque , a 10 hp piston engine at 5252 rpm will have 10 ft/lbs of torque , but , assuming a linear power production , at the electric motors 1,400 rpm the IC engine will only have ~2.66 ft/lbs of torque , but if our 5252 rpm 10 HP piston engine were geared down by a 3.75:1 ratio gearbox with an output rpm of 1400 , then the output torque would be 10 X 3.75 = 37.5 ft/lbs exactly the same as the electric motors .

There will be variable with the way a motor produces power over its rpm range so my examples are a simplification , but I hope it helps explain things :-)

If you were using a 2 shaft turbine engine which has its own inbuilt automatic torque conversion characteristic you wouldn't need anything other than a fixed direct drive without any controls other than the engines throttle .

Single shaft engines are for generators or pumps , they have major shortcomings as a land based power plant due to them producing very limited power below 50% rpm , they need to be run at >70% to be a viable unit .

Could you please provide a Link to the turbine powered Honda CRX so that I can check it out and comment on its transmission

Cheers
John

[wolfdragon]

Provided you can keep the turbine running good and hot, a simple bypass on the hyd pump will let you keep the fluid pressure up along with the pump rpm at low output power, close the bypass to put more power at the wheels...

Sure you can throttle the turbine to get better fuel consumption while waiting at a light, the issue is how long does everything take to spool? For normal driving I would expect a happy medium could be found for an on demand throttle, whereas if you want a "sport" button to haul ass off the line, you would want everything hot and wanting to make you take off...

[arthur]

www.turbinefun.com/TurbinePoweredHondaCRX.asp

www.cardomain.com/ride/2358097/1990-honda-crx

www.youtube.com/watch?v=ZE1HnJmUTVY&feature=related


I have been reading a lot on hydraulicspneumatics.com and have recently joined the forum there to ask some of these same questions. I want to understand why they say there is a difference in the power, or power curves, on the types of power, piston or electric. I think I understand how the rpms are involved and that is probably the difference.

Another man described the way the fuel control works on turbine generator set. According to him, when the turbine is at 100% without load, the fuel is just enough to keep it at full rpm but when the load is detected, it can react and go to full power in 30-40 milliseconds. Since it is already at full rpm all it has to do is open the nozzle and there is the full power. I am not sure if this is exactly what happens but I can imagine the requirements for a military generator are pretty demanding. The best possible hydrostatic transmission setup will involve electronic control of variable displacement pump and motor($$$). The accelerator pedal in the vehicle will control the displacements and it will be much like a constant velocity transmission. This arrangement can react pretty quickly. If I have to change gears on a mechanical automotive transmission that is ok. If the whole works can deliver 200lb/ft of torque, the little 1500lb sand rail/street rail will have plenty of pep:)

Thanks so much for your help.

[arthur]

I think Ralph's CRX setup uses some kind of fluid bypass valve as the throttle.

[racket]

Hi Arther

Yeh, the gen sets run 100% rpm and vary the fuel flow to vary the turbine inlet temperature ( TIT) to vary the power production depending on load ................its not a very good system for a road vehicle as the engine is flowing full air flow at all times requiring lotsa fuel even when theres little load .

Ralphs setup seems to work OK :-)

Cheers
John

[racket]

Hi Arther

I did a bit of checking in my Solar T62 manual ( its a single shaft genset) for fuel flows , on a 60 deg F day and nil power output the engine is consuming ~60 pph of fuel or ~1 pound of fuel per minute , or a gallon in a bit over 6 minutes , when at full power of 150 horsepower on the same 60 deg F day it consumes ~140pph or 2.33 lbs/min or a gallon of fuel in a tad under 3 minutes.

The engine is burning ~43% of max horsepower fuel when "idling" ,........................ if you're also turning a hydraulic pump rather than a "frictionless" generator , you'll have an even higher "idling" fuel burn rate , probably close to 50% of your full bore rate , ......... she'll be a thirsty bugger :-(

Cheers
John

[arthur]

Hi John,
My Acceptance Test Data Sheet fuel consumption reads 60.9 lb/hr-no load and 135.1 lb/hr at full power. Not much different from your Solar. It will need a large tank if I am going far. I am predicting that with the noise, it will not be a machine that anyone would want to drive for more than 20 or 30 minutes at a time and then it will be time to refuel:) I think Ralph told me his little CRX is good for about 6 mpg:(

Luckily I am not doing this for an economical means of transportation. I am doing it to see if I can. I am doing it for the novelty, the sound and flames and generally being obnoxious. It will be excellent in those respects:)

Thanks

[racket]

Hi Arthur

Heh heh , they sound like perfectly reasonable reasons to burn kero :-)

Have you considered using a high rpm stall torque convertor and auto gearbox on your setup ?

With some engine rpm throttle control , say from 60 - 100% and the torque convertor stalling at 70% - 5,600 rpm on your 8,000 rpm output , you may have a more civilised beast to drive

Cheers
John

[thunderbirdsarego]

might get some ideas here
www.popsci.com/diy/article/2009-06/popsci%E2%80%99s-own-jet-powered-atv

or my own project

www.youtube.com/watch?v=tXRKCN-TFqw

with limited budget I used a 10hp electric motor powered off a VSD which was supplied by the AC from a 3 phase 400 Hz generator. I chose the generator because it fitted the spline drive pad PTO. I would have used hydraulics if any were available. Some of the GTP30 had hydraulic pumps attacthed in Chinooks I think, but I dont live next to an aircraft boneyard

A better drive system would be a 20 kw DC motor and i could easily build an AC - DC speed controller for this but such a motor is hard to find......

there is 25 kVA available from the generator - continuous.

[wolfdragon]

you can always DIY a brushless DC motor, of course finding a suitable speed controller may take some DIY effort as well