Bypass motor

[syler]

Any of you metal shop guys dare to embark on a journey of discovery?

The idea is simple. Fabricate a ducted fan / pre-compressor that produces a modest pressure increase pre turbo and slips just a little bypass air. IDK, something 18" wide or so. It wouldn't even need to be dead nuts center and use all the fancy shaft stuff. A free power turbine could power a shaft on the outside of the main motor, sneak up the side and power the fan. I'm no machinist but it seems a lot easier and more efficient than redesigning the compressor housing and all that business. Bulkier of course, but so what.

[racket]

Hi Syler

I was going to use the first stage axial compressor wheel from a T700 engine , 10 lbs/sec mass flow ..............powered by a 4th stage Allison C20 turbine wheel , with the gases from one of my "micro engines" .

Calculations indicated better thrust if the fan didn't "supercharge" the micro engine .

Rather thirsty when afterburned though

Cheers
John

[racket]

Hi Syler

Theres been a number of turbo designs proposed with an axial component in front of the "standard" centrif comp wheel , but despite decades going by , nothing has come of it :-(

Instead the turbo manufacturers have gone to higher pressure ratio centrif comp wheels , either titanium or cooled aluminium , the large ABB turbos are up in the high 5:1 PRs with aircraft grade efficiencies ............but theres no need for high pressure ratios other than for heavy duty diesel applications , ordinary spark ignition engines will be "bustin' their guts" at a lot less boost .

The Allison/RR heli turbines are running 9.2:1 PRs using single stage titanium centrif comp wheels, 6.1 lbs/sec , with a respectable SFC of 0.58 lbs/shp/hr at 650 rated horsepower in a package weighing just 270 lbs.

The "large" Garrett GT55 range of automotive turbos have pressure ratios up in the 5:1 range , but as with all "small" compressor wheels its difficult to get good efficiencies due to their size , gas turbines are easier to scale up rather than down as its impossible to scale the required clearances, and the time frames for compression are reduced with the smaller units, .............theres nothing wrong with our turbo compressors , after all , they're designed by the same people that design the larger aero units , its just we can't beat certain Laws.

Cheers
John

[syler]

Does the average Joe have access to anything like that? What I'm thinking is that the physical restraints of anything we can make on a shoestring budget requires us to some Miyaging. I think one of the biggest power disadvantages comes from the fact that unlike a piston motor, these don't transfer potential energy into kinetic energy efficiently. using one of our motors in a bypass design to move say 10X the air mass should do that. That's how commercial jet engines work. Do the math for how much air a well designed 12" axial turbine or ducted fan would move. Forget the additional compression. Plus, we get our energy from the number of moles of O2 put through the motor. If we half the temp(C) of the compressed air entering the motor, we get twice the joules of energy upon combustion. If we can keep everything cool using things like bypass air, we can push the motor harder.

The math is simple. If Diesel gives of 100KJ per gram and motor A can burn 1g per sec at stoic AF ratio it gives 100KJ. If motor B passes 2X the O2 and can burn 2g of fuel per second and be stoic, you get 200KJ. Temperature is a whole other matter. But, if we design stronger components with more heat dissipating ability, we keep them stronger. How about a couple old air cooled Harley cylinders reamed out for chambers instead of a chunk of thin wall tubing? Then maybe run nitrous through the system now that you have greater strength and heat dissipating ability.

[Johansson]

Feb 16, 2014 14:39:26 GMT -5 syler said:

If we can keep everything cool using things like bypass air, we can push the motor harder.

Hi Syler,

I don´t want to sound like an ass, but do you even read the replies people here post? How many times have you been told that a gas turbine cannot be cooled down to produce more power? It is against the nature of gas turbines, the hotter they run the more power they produce.

I am really curious about your background, are you actually working with combustion and thermodynamics so you have some real experience or are you just flinging wild theories around after passing the high school physics class?

Looking at the above I did sound like an ass, but I am sitting here clenching my teeth from the way you totally dismiss the very thorough answers people here are giving you. Just the fact that you think that a high bypass gas turbine can be built on a shoe string budget by the "average Joe" tells me that you have absolutely no idea what you are talking about.

Hugs and kisses /Anders

[racket]

Hi Syler

Yes , there is stuff available if you look around .

The only thing that could benefit from cooling in a gas turbine is the turbine stage bits , NGV/scroll and wheel , a good quality turbocharger has the same material for its turbine wheel ( Inco 713C) that a lot of full sized commercial engines use and we can run the same turbine inlet temperatures that they do.

The very complex, high pressure ratio, high bypass fan engines can run hotter temps due to very sophisticated and very expensive cooled blades in their wheels, but they can still only run ~25% hotter .

As for cooling the outside of our engines ..............why ?? .............they don't run very hot ( <200 deg C)

Yeh , take Anders advice and read our replies and/or purchase the books I've recommended :-)

Cheers
John

[gidge348]

Hi Syler,

I have stopped posting to your questions as I found myself loosing my temper a few times and don't want to do that here.

John, Anders and a host of others have spent a lot of time explaining how turbines work and why your theories wont work.... But that being said it's a free world so why not do some small scale experiments and test your theories and post the results?

Build your "cooling air bypass engine" or "water cooled combustion chamber" or "thermite powered turbine" and see what happens? You may be right? Some of the greatest advances is science came when people said it could not be done...... and someone "just did it."

Good luck
Ian

[Richard OConnell]

Agreed. Its great that you are thinking outside the box. Thats what got us past the wheel. People are offering advice based on what they know out of experience. Your ideas seem outlandish, but to be fair, a cell phone would probably seem outlandish to an inventor in the renaissance era. Maybe you should get together a local build team consisting of people with a variety of experiences and learn together? You can theorize all day but you cant prove a thing till you make it happen

[enginewhisperer]

The thing is a lot of the air in a turbine engine is not used to burn the fuel - but the overall ratio is there to keep the temperatures under control.
You can't burn more fuel in the same amount of air without things getting too hot.
Increasing the pressure ratio means you can get the same power out of less fuel - but you can't get a lot more power from the same sized engine / mass flow.


The easiest (and only) way to get more power is to increase the mass flow - which means a larger engine.

In theory if you could cool the turbine and combustor parts you could run higher turbine inlet temps, and maybe make more power - but all that heat you have to remove from the metal is lost energy, and therefore efficiency.

[turbochris]

perforated hollow blades cooled w air helped get the power up a long time ago.

[enginewhisperer]

bit hard for us to make our own like that

[racket]

Yep , the Germans used cooled turbine blades back in the 1940s.

We could possibly use liquid cooling during our limited run times , water plumbed to behind the heat shield , it exiting around the shaft , then centrifuged up the back of the wheel to meet with the hot gases and hopefully be spread over the blades/hub before exiting the turbo , we'd get quite a power advantage if it'd work .

[rexhunt]

Hi John,

I like the idea of putting the water through like that, I guess though it would probably take some fine tuning to ensure there are no thermal shocks to the turbine and that the coverage was even enough

Perhaps for testing if there was some kind of coating that dis-colours easily could be applied to see if the water is being dispersed evenly enough.

Cheers,
Rex

[racket]

Hi Rex

To negate any thermal shocks you could always plumb P2 air into the heat shield instead of water , the supply would automatically increase as P2 increased .

If using water it'd need to be used immediately the engine fired up, but even a 2-300 deg C increase would make a big difference , I've heard that some drag race turbo run T I T s of 1,300 C for their several second pass ...............it takes time to melt a turbine blade even when using an oxy torch , something I had to do once to disassemble my FM-1 engine when the blades "creeped" :-(

Cheers
John

[rexhunt]

Hi John,

If you're saying that 2-300 deg C would be a big improvement then I imagine for that you could probably get that from P2 bleed air.(Guessing with no practical experiance)

Might be something for me to look at once I get an engine together.

Cheers,
Rex