Outboard Turbine Project

[racket]

Hi Mark

Theres very little difference between a single or two stage freepower for us , other than a lot more complication , we don't have sufficient pressure going into our freepower to warrant using two stages , a single stage can easily handle the pressure drop and still produce acceptable efficient power extraction , there'd probably only be a few horsepower difference, not worth the trouble .

Cheers
John

[turbochris]

When our home made afterburners start making really good diamonds, then we need multi stage freepower! some day.......

[boatmedic]

Feb 9, 2013 16:12:43 GMT -5 turbochris said:

When our home made afterburners start making really good diamonds, then we need multi stage freepower! some day.......

Chris, I am not sure what this means. I was asking about a second stage for horsepower and a way to decrease the diameter of the freepower unit. I was simply thinking that two smaller fans and stators might solve a packaging problem I will have.

[racket]

Hi Mark

What Chris was referring to was pressure downstream of the gas producer turbine wheel , it takes more pressure to make a supersonic exhaust with shockwave diamonds than what a DIY can produce .

With freepower turbine stages you'll find that a 2 stage setup will be larger in diameter than a single stage due to the slower gas velocities through the 2 stages requiring greater flow areas .

We CANNOT force the gases through a small freepower stage , the gas producer will simply go into surge and potentially wreck itself :-(

The only way you can use "smaller" turbines is to have them in parallel rather than series , but a pair of small wheels will probably take up more room than a slightly larger single stage ,........ then theres the gearing to couple them together , .........................stick with a single stage wheel of adequate proportions and you'll have an easier build with less problems .

Cheers
John

[boatmedic]

John,

Thanks for clearing that up, I wasn't sure if I was supposed to laugh or explain that I wasn't planning on using an afterburner.
I am not sure if I should pursue this or just buy or rebuild a turbine ( I have an allison C20 that needs a compressor section). The efficiency and low pressure issues are going to be hard to ignore. I also just visited a company in Pa that has three Solar T62's for sale 6.5 K for the three plus spares.
I would like to explore opportunities to make these engines more practical and less of a novelty on a test stand .

Mark

[racket]

Hi Mark

Checkout T62 info on this Site
www.technologie-entwicklung.de/Gasturbines/gasturbines.html

But fuel burn rates won't be any better than our DIY engines , I've owned a T62 and have a manual with that data in it .

Cheers
John

[boatmedic]

Hey John,

Thanks for the information. I read your thread about the Twin Shaft Cart and I have a few questions. What approx shaft HP was achieved with that type of set up?
I used JetSpecs and it would suggest a much larger Combuster set-up than you have there, what size did you end up with?
I am still interested in achieving a higher PR ratio in a GT engine in your opinion how could this be done safely. I am stuck thinking in convential IC thermodynamics where one would increase the size of the compressor relative to the size of the engine.

[racket]

Hi Mark

As it only ran a small freepower and at very reduced rpm due to the direct chain drive , horsepower was limited to ~30hp at 15,000 rpm and probably ~40hp at 20,000 rpm , the rpm limit of the racing chain is ~22,000 on a direct drive kart engine.

The combustor on the kart was an ~7" dia stainless fire extinguisher with an ~140mm dia flametube for the 3.5" inducer on the comp.

The combustor was left overly long so that it could have a mounting at the fuel inlet end so that the outer can acted as a "torque arm" to stop the overhung turbo at the other end from bouncing around , it stiffened things up .

There was a longish snout on the outlet end of the flametube to allow the incoming air to be fed into the plenum surrounding that snout .

To achieve a higher pressure ratio we need a higher tip speed , but that is limited by safety considerations as well as overal compressor efficiencies which drop off considerably at high tip speeds , the drop in efficiency makes it unproductive to force the rpm/tip speed past a certain point as the engine will start to produce less power as the compressor starts consuming more than the extra pressure ratio can produce at the turbine , we end up on the "wrong side of the curve" .

An "oversized" compressor wheel will only go into surge if its flow can't be "willingly" swallowed by the turbine wheel at the pressure produced by the comp wheel , unlike an IC engine that can force gases through the turb stage at pressures well above what the comp puts out

Cheers
John

[boatmedic]

Do you think it would be possible to make a Power Turbine from the hot side of a large turbo? It would seem with a proper gear box it would work well. and solve a packaging problem as well.

[racket]

Hi Mark

It would be possible but power output is limited by the mass flow when using gases at relatively low densities , we physically can't get enough gas through a turbo wheel when used as a freepower , on the two shaft kart the scroll housing I made had a large A/R to match up with the relatively low rpm/tip speed of the wheel due to the chain drive , the large A/R allowed a greater mass flow into the wheel , if I'd wanted to run higher rpm on a gearboxed redux then I'd have needed to have the gases at a higher speed and a shallower tangent to the wheel which would have reduced mass flow , the setup I used was a comprise specific to using a "slow" freepower .

As a guess, I'd say a turbo turbine stage freepower would limit power to ~100 hp ..........UNLESS , heh heh , theres always another solution , .....................if you used a couple of large gas producers and bled air from each to feed a combustor that then fed a turbo turbine with high pressure gases then your output could easily go up into 200 hp range without problems .

Cheers
John

[boatmedic]

Hey John,

Thanks again for all your answers, you are very helpful as I am trying to absorb as much information as I can. In another life ( before retirement) I designed torque converters for large IC industrial engines for a company which I used to own but now do design consulting. The point I am getting at is, I am traveling to Taiyuan, China to a investment casting company that builds the stators in our torque converters. In their catalog the also make turbine wheels and turbine blades. Is there an opportunity here to get a custom set of turbine / compressor wheels or even the perfect sized turbine for a power turbine wheel. I can get one of my AutoCad techs to draw most anything up for 5 axis cutting.

What are your thoughts on this?

Mark

[racket]

Hi Mark

Mmmm, sounds like you can do something more complicated than the usual DIY build, so we'd better start from scratch and define what you'd like to build and whats required to achieve it .

Freepower turbine wheels have a very wide "power band" , we can achieve 80% power potential at 50% rpm , 40% power at 20% rpm ...................and maximum torque at stalled rpm , ~2 times the torque at 100% rpm , something you would be familiar with your background in torque convertors , freepowers are "pneumatic" torque convertors using the hot gases from the gasproducer

Now, from my rather limited understanding of boats and their power requirements, they require modest power at low speeds but far greater power as the speed builds up , I'm just thinking perhaps a single shaft engine might do the job , you don't need heaps of low rpm torque/power because all that will do is "spin the prop" unnecessarily fast until the hull speed and drag catches up with the power being produced.

A single shaft engine will produce very little power below ~40% rpm , a graph in one of my turbine books indicates at 50% rpm the power is ~10% of potential whilst torque is ~30% , at 70% rpm they have climbed to 35% for power and near 60% for torque .

You may need to get a, horsepower required vs speed graph , for the hull you intend using, this will indicate the required "steepness" of the horsepower , it may well fall into the single shaft range .

The problem with single shaft engines would be "gearing" , which mightn't be such a big issue for you as it is with most of us DIY'ers , gas producers and single shaft engines generally spin around twice the rpm of a freepower wheel

As a summary , we need your desired power and some idea of the hulls power requirements vs speeds .

This scenario hasn't been fully explored by us DIY'ers , .......................thanks for giving us the opportunity to do so :-)

Cheers
John

[boatmedic]

Hey John.

I like your enthusiasm but I am not sure I have the emotional resources to do a from scratch build. Here is what my parameters are, The boat is a 16 ft Ebbtide Mustang and is currently powered by a 115 hp Johnson outboard. The boat is currently a little over powered as the hull requires 90 hp via factory specs.

I was pursuing a free power turbine because of the torque curve it provides and a very convienant packaging solution overall. If a single shaft turbine would work in your opion then I am all for it. I will do some research while on my travels.

The very important thing with a boat is getting it on plane as fast as possible.

My goal is to refine a high pressure pump, injector and electronic fuel control for this engine and design a recuperator to improve fuel burn rates to make these engines more usable.

Overall it would appear that folks such as yourself have done most of the heavy lifting already and I thank you in advance for your advice and suggestions

Mark

[racket]

Hi Mark

Lets say 100 hp for the sake of the execise , easily achieved using a DIY turbo based engine for the gas producer and an Allison C20 third or fourth stage wheel for the freepower .

I used a high pressure pump and injection ( ~750 psi) on my turbine bike build ,I used an IC engine for powering it , but easy to do using a 24 volt scooter motor with electronic control , lotsa bits available .

Some more info on the hulls power requirements will soon determine if we need a 2 shaft engine , it would be simpler but a bit bulkier .

Cheers
John

[boatmedic]

Hi John,

Thank again for the information, I have included a link to show how the turbine base freepower would almost be perfect. in you opinion can we get the same orientation from the axial fan of the C20? I really like your idea of the single shaft turbine connected to a planetery reduction would actual be perfect. Has anyone ran these engines in a vertical position?

I will take a look at your turbine bike build to see what you have done with the high pressure system and I will do some research on the hull power requirements the boat is 30 years old so it will take a little digging

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