First project: BudJet (the budget jet)

[turborico]

Hi everyone,

I'm glad to be here, there seems to be a wealth of knowledge, and it's great to be able to tap into that.

I don't have any experience with designing or building a homebuilt turbine. However, I have 8 years experience with maintaining some of the US Air Force's engines, mainly the T-56 turboprop engine, and now the F-108 (military CFM-56), with some APU's thrown into the mix.

I'd like to start my own small gas turbine project. I've had a few design points mind:

- Low project price: I'd like to spend as little money as possible. I already have a few turbochargers to work with, so that's a big head start. On the downside, I don't have much in the way of tooling or materials.

- Learn something new: Just building the engine is probably going to teach me a lot. I've only done small fabrication jobs. Also, the idea of learning programming, as well as hardware/software interfaces is very appealing. I think I'll probably use an Arduino board to do some simple monitoring. If that goes well, perhaps using it to automate engine controls.

- Do something useful: Making noise is probably fun enough, but powering a vehicle or generating electrical power would really sweeten the deal.

Any and all advice, criticism, or general input is greatly appreciated.

- Rico

[turborico]

Here's the turbochargers I have to work with:



I'd have to go look to see if I could get any part numbers from them. The largest came from a M35 "Deuce and a half" truck, the one on the right came from a Saab, and the little guy on the left came from some 1.2L diesel car.

Any thoughts about arranging the turbos into a multi-stage design? My gut instinct says that it probably wouldn't work out too well, but I guess I won't know until I try. I'm sure I'll worry about getting a single stage version up and running before I consider anything more complex.

[racket]

Hi Rico

I'd be making an engine out of the larger of the two small ones as it'd probably be about the right size if you wanted to try a 2 stage setup at a later date using the large turbo for the low pressure stage .

Cheers
John

[turborico]

Good call, John.

I was thinking I'd go with that one too, simply on the basis that it seemed more manageable as far as sorting out mounting and sizing a combustion chamber, etc.

I've seen some pretty neat designs using turbos in parallel, and some neat freepower turbines, but I haven't seen any sequential/staged designs. Do you know of any?

[racket]

Hi Rico
There have been some attempts at building a 2 stage engine but there hasn't to my knowledge been a serious attempt where the job has been done properly with interstage cooling between compressors and correct sizing of the low pressure turbine stage that will be experiencing higher than normal static pressures requiring it to be of a smaller size than "normal" unless an attempt is made to "pad out" the stage with water/steam and/or reheat between turb stages.

Unless the 2 stage engine is used for shaft power , where the greater expansion ratio than a single stager can be fully exploited , its not worth all the extra complication if its only a thrust engine , higher pressure ratios affect fuel consumption rates more than thrust rates , the extra expansion from the 2 stager will lower specific fuel consumption but doesn't do a huge amount for thrust increases over a single stage engine using the same low pressure turbo .

Cheers
John

[turborico]

Interesting... So, the main advantage to a higher PR is a better SFC? Probably not worth the complications then.

[racket]

Hi Rico

Nope, not worth the complications for a thrust engine , ....... our engines are the "same??" as internal combustion auto engines , high compression /high expansion makes more power per unit of fuel , basic thermodynamics that we can't change .

But our engines exhaust velocity is calculated with a square root component that makes it harder and harder to increase velocity ,unfortunately the thrust is only mass flow times velocity , but with a shaft power unit we can break down our pressure drop into a couple of increments and get a better result , thats one of the reasons jet engines started going over to fans once they had "too much" exhaust energy in just the pure thrust non fan configuration , the turbine driving a fan is more efficient at energy conversion.

eg, ....if we have a jetpipe with 5psi of total pressure ( 1.34 PR across the jet nozzle ) at 900deg K - 627 C - 1160 F and a mass flow of 1 lb/sec we'll have a temp drop of ~ 57 deg C across the 90% effic jet nozzle and a velocity of ~1185 ft/sec and a thrust of ~37 lbs

with 10 psi of total pressure in the jetpipe ( 1.69 PR) , double what we had before , and at the same temp there'd be a ~98 deg drop in our jet nozzle and a velocity of ~1557 ft/sec and a thrust of ~48 lbs .

so for a mere 11 lbs of extra thrust ( 30% increase) we need twice as much pressure in our jetpipe , this is why a 2 stage engine with all its complication doesn't give a big increase in thrust , but that extra 30% of thrust won't be using 30% more fuel , it'll be a lot less than that, same reason a diesel engine will give better fuel economy than a gasoline engine with a lower compression ratio .

Cheers
John

[turborico]

I'm not the best at visualizing the "math" that goes on behind the scenes, but that really put it all in perspective for me, thanks!

Since the most practical use of the power is in mechanical extraction (I've worked turboprops, so I can appreciate that. ), how would I begin sizing a free power turbine? Is there a "rule-of-thumb" I can go by when I'm sizing everything up?

I started to lean towards using the smallest turbo of the bunch for now. It's the one that seems like it'll require the least amount of work to get running... I'm quite sure this would all be easier with a mill & lathe, I'm having a hard time finding parts that go together without some real machining.

In any case, the first engine is just a proof-of-concept to my wife so she realizes that it REALLY is possible to build a jet engine so easily.

[racket]

Hi Rico

On the Yahoo DIY Turbines Site in Files Section > Starter Pack there are the Rules of Thumb , have a good read of them with regards freepower sizing, unfortunately some maths will be required :-(

Yep, that smaller turbo will be able to run on a BBQ bottle of propane, which greatly simplifies the build , the larger turbo will need kero/diesel .

Cheers
John

[turborico]

I found the files section at DIY last night after posting here, I spent quite some time going though there. Very helpful information.

Next up is to hit up the scrap yards for some parts, but the weather isn't really cooperating.

[racket]

Hi Rico

Reread the Rules of Thumb a couple more times before starting the build , there'll be things that didn't make sense the first time that will after a couple of readings :-)

Yeh, you guys in the northern hemisphere have been having a bit of a cold winter, its been on the news here in Oz .

[Johansson]

I managed to scan a picture of a RR Dart engine with a two stage compressor, not exactly twin spool but a nice pic anyway.

[turborico]

Very impressive machines indeed.

As far as the multi-stage centrifugal compressors go, the GTC-85 (at the least the variety that was used in the C-130's) have a dual sided compressor wheel (like two standard impellers, back to back) that fed into another single sided rotor for the 2nd stage.

It'd be pretty neat to work up a deal like that into a DIY turbine.

[Johansson]

Jan 9, 2011 12:03:53 GMT -5 turborico said:

It'd be pretty neat to work up a deal like that into a DIY turbine.

I have been thinking about something similar, but with two "separate" TV94 compressor wheels on the same shaft instead of a two stage compressor to double the mass flow instead of the pressure.

One problem is that it will most likely need a two stage turbine a well to cope with the flow, a single stage axial turbine will be difficult to make with revs reaching 70.000rpm since the blades would have to be very long and therefore too weak to power two compressor wheels.

[racket]

Hi Anders

Heh heh .........Just sent you a better project to make ;-)

Cheers
John