Torbo Powered Ramjet

[syler]

Hi guys. I'm new to all this so sorry if this sounds like a silly idea.

I'm wondering why all the designs I see pass the full combustion products through the turbine. Is that necessary?  One would think it would only take a fraction of your combustion pressure to power the turbo. And, my understanding is that lean mixtures are used to keep temp down and not harm the turbine.

Is there a reason a ramjet design could be used with only as much pressure needed diverted to the turbine? One could even use a separate combustion chamber.  If the gasses passed through the turbine rejoined the ramjet output in the rear, the natural venturi in the back would even cause a low pressure on the back side enhancing the delta across the turbine.

The ramjet could then use optimum AF ratio and one would have a few degrees of freedom between the turbo and the overall thrust. I'm thinking nitrous injection could also be used in this setup with no ill effect on the turbine.

Is there a reason this wouldn't work? What do you guys think?

[racket]

Hi Syler

LOL.......theres never any "silly" ideas on this Forum, just opportunities :-)

Its not necessary to pass all the air/gases thru the gas producer turbine , but for a thrust engine it has certain benefits .

With a "normal" turbocharger based engine we use ~2/3rds of the pressure available to drive the compressors turbine wheel, leaving anywhere up to ~12psi of total pressure downstream of the turbine wheel , if that 12psi is fed into an afterburner and all remaining oxygen is "burnt", we end up with the best thrust outcome .

If we "split" the airflow from the compressor and used 2/3rds to power the comp turbine we would need full expansion of the gases within the turbine stage to provide sufficient energy to power the comp , those "exhaust" gases at atmospheric pressure could not be mixed with the remaining 1/3rd airflow which would be at much higher pressure , up to 45psi .

Now , theres a "problem" turning high pressure into velocity , theres a "square root" in the equation , this means we need ever increasing pressures for ever deminishing velocity increases , and because thrust is only made up of mass flow times velocity , the numbers would be less for the split system .

This need not be the case with a shaft power engine where that 1/3rd high pressure can be, and is in some cases, dropped in steps thru a couple of turbine stages where those smaller individual pressure drops are more "efficient" and produce less "losses" than a single large drop.

Its all in the maths :-)

Cheers
John

[syler]

I have a small turbo on my Nissan 240sx that can make 300HP at moderate boost. The exhaust from my 2.4L is more than enough to power the turbo. Would I be mistaken in presuming that the same 2/3 rule would apply meaning I use 200HP to drive the snail?

If it takes 2/3 of the total combustion product to drive the snail, it appears something is very wrong with the design. Unless a torpedoe heater is the objective. Obviously using 300HP worth of O2 to get 3HP indicates a major flaw.

I'm guessing the problem lies in a lack of any real compression. Have you considered 2 independent combustion chambers? One could be dedicated to opowering the turbo. The other, would run into something like a chromoly SCUBA tank fitted with a flame holder and running soic. I'm thinking the exit nozzel would be small, enabling dramatic pressure to build. This could also be fed with N2O essentially turning it into a hybrid rocket.

And why couldn't the "waste gas" from the turbine rejoin the thrust? It would be pulled in by a venturi. This would also provide extra air mass.

Am I really missing something here?

[racket]

Hi Syler

LOL.....yep , you're missing a couple of things :-)

Your car engines turbo probably has a compressor wheel with a 2 inch inducer and flowing ~30 lbs/min - 0.5 lbs/sec of air .

Assuming you're running 10 lbs of boost , thats a 1.68 pressure ratio , you'll have a temp increase in compressor of ~60 centigrade degrees , this equates to ~38 hp/lb of air , so your turbo needs half that for its 0.5 lbs/sec flow , so ~19 hp is being produced by the turbine wheel .

We need to use a whole different set of thoughts when playing around with DIY turbine engines compared to auto engines .

A venturi only reduces static pressure , it doesn't change total pressure ( neglecting losses of course) , some of the initial static pressure is turned into velocity/dynamic pressure , it can be restored by passing it into a diffuser , ............we need to work with total temperatures and pressures with our turbine engines due to the at times very high air/gas velocities .

The highest pressure in our engines is at the compressor wheel's exducer , its part static pressure and part dynamic velocity pressure which is converted into a static pressure rise in the comp scroll , at no point downstream of the exducer can the total pressure be any higher , otherwise the air would want to flow backwards out of the comp inducer , this is what surge is , it wrecks turbine engines and must be avoided at all costs, the only way to avoid surge is to have a reducing total pressure as we progress through the engine, irrespective of how the comp discharge air is used/divided/burnt etc etc .

The only time one would consider splitting the comps discharge air is if the compressor wheel is grossly oversized for its turbine , where the turbine stage can't flow the compressors optimum mass flow , but this is still a case of "two wrongs trying to make a right" ................for a pure thrust engine we get best results from passing all the comp air thru a suitably sized turbine stage and accelerate those lowish pressure "exhaust" gases in an afterburner nozzle .

I hope this helps explain things ...............LOL, if not , keep asking questions :-)

Cheers
John

[syler]

I'm not understanding what you are saying about the turbo. My understanding is that the compressor and exhaust have a 1:1 pressure ratio. My car's exhaust spools it up just fine without producing appreciable thrust in the process.

Are you sure about the pressure thing? I was under the impression that the sizes of the in and out as well as the flame holder prevented the surge. Specifically because it takes less pressure to go out the back than back to the compressor. I did however wonder.

If you are correct, why build one? None produce appreciable power. The building of pressure is a major factor in the combustion reaction. And automotive turbos aren't suited to flow massive amounts of air and compress to the extent of a jet.

If the turbo pressure output can't be exceeded, shouldn't the plan be to fabricate an axial fan for an initial compression stage?

Anyway, my design isn't that different from the ordinary. The difference is that my afterburner receives fresh air and burns stoic. In case I wasn't clear, the turbine exhaust would be routed to the read convergent nozzle and be injected into the fast moving, lower pressure stream as dense cool air. The two would then enter a small divergent cone to benefit from the expansion of the cool air.

afterburners as I understand them open wide in the back and work like the divergent cone on a rocket. Why do all the ones I see converge?

What I see are essentially poorly designed ramjets that run on leftover, high temperature and therefore very sparce air as opposed to the far more O2 rich air of my design.

[syler]

I guess it's time for a pvc mock up.

[racket]

Hi

The pressure ratio (PR) produced by the compressor doesn't usually match the pressure ratio across the turbine stage , with our DIY turbine engines the compressor might produce a 4:1 PR but it will only require a 2.2:1 PR across the turbine stage to power the compressor, this then leaves a ~1.8:1 PR in the jetpipe for making thrust .

Automotive turbos when still on a car engine often have a higher PR across the turbine stage than the PR from the compressor , as I said in an earlier email we need to "forget" our automotive turbo knowledge when making jet engines .

LOL........I'm pretty certain about my maths, I've been doing it for a while :-)

Our DIY turbo based jet engine can turn out equivalent specific thrust to the early jet engines from the 1950's , and in some cases even better pounds of thrust per pound of mass flow,...............even better Pressure Ratios from the turbo compressor than the large radial compressors in the early jet fighter aircraft .................they ain't toys.

I wouldn't say none of our DIY turbine engines produce appreciable power , my TV84 powered turbine bike had 115 RWH , our latest TV94 based engine should produce 150 - 170 HP .

We don't need convergent-divergent ( CD) jet nozzles on our afterburners because we don't have sufficient pressure within them to require anything other than a convergent section , the divergent section on full sized engines is to allow higher gas velocities from their higher pressures . ...............just to confuse you even further , commercial civilian aircraft jet engines with high jetpipe pressures only use convergent jetnozzles even though they could use CD nozzles , they prefer to run choked convergent nozzles with "pressure thrust" .

As for our afterburners being "poorly designed jamjets than run on leftovers" they work exactly the same as your best fighter aircraft afterburner :-(

Yep , I think you need to give us a mock up of your design so that we can better understand where you want to go ...............it'll be easier to explain any potential problems rather than me hypothesizing :-)

Cheers
John

[syler]

I'm getting it now. I wasn't seeing the backpressure factor. When driving a shaft it's obvious. If I'm understanding this right, these motors are basically at idle when the afterburner isn't on. Essentially, the concept I imagined is the same as the conventional design. I just wasn't seeing how much unused O2 sneaks through the turbine. All these motors are basically ramjets fed from the turbine with a little bit of combustion upstream to keep the pump pumping.

If one were to carefully match the AB pressure to the combustion chamber pressure, couldn't the pressures go extremely high? As long as there is a delta across the turbine, it should do it's job at 15psi or 1000psi shouldn't it? If I'm putting this in a boat I could keep the steel cool and strong.

[gidge348]

I think you may be associating pressure with thrust?

Thrust is a function mass velocity over time with pressure playing only a very small part, get onto the NASA website it explains this all very well. www.grc.nasa.gov/www/k-12/airplane/thrust1.html

All that is done here is experimentation, maybe a good time to put your theories into action and see how it goes, please post pics as you go it is always interesting.

Cheers
Ian...

[Johansson]

Jan 26, 2014 0:26:05 GMT -5 syler said:

If one were to carefully match the AB pressure to the combustion chamber pressure, couldn't the pressures go extremely high? As long as there is a delta across the turbine, it should do it's job at 15psi or 1000psi shouldn't it? If I'm putting this in a boat I could keep the steel cool and strong.

For 1000psi in the afterburner you would need a compressor capable of a 70:1 or so pressure ratio in a single radial stage, good luck finding one... *LOL*

[racket]

Hi Syler

As I mentioned in an earlier email on the 22nd Jan ,

................"The highest pressure in our engines is at the compressor wheel's exducer , its part static pressure and part dynamic velocity pressure which is converted into a static pressure rise in the comp scroll , at no point downstream of the exducer can the total pressure be any higher , otherwise the air would want to flow backwards out of the comp inducer , this is what surge is , it wrecks turbine engines and must be avoided at all costs, the only way to avoid surge is to have a reducing total pressure as we progress through the engine, irrespective of how the comp discharge air is used/divided/burnt etc etc ".

There is no pressure rise when we burn the fuel, unlike in a turbocharger IC piston engine , with a gas turbine when the fuel burns , there is a velocity increase , but there is also a pressure DROP during combustion , this is necessary so that the "working fluid" keeps heading in the right direction .

Generally the oil and fuel pressures are much higher than the gas pressures in our gas turbines , as they were in most full sized commercial jet engines up to the 1960's .

Cheers
John

[gidge348]

Jan 26, 2014 14:48:18 GMT -5 Johansson said:

Jan 26, 2014 0:26:05 GMT -5 syler said:

If one were to carefully match the AB pressure to the combustion chamber pressure, couldn't the pressures go extremely high? As long as there is a delta across the turbine, it should do it's job at 15psi or 1000psi shouldn't it? If I'm putting this in a boat I could keep the steel cool and strong.

For 1000psi in the afterburner you would need a compressor capable of a 70:1 or so pressure ratio in a single radial stage, good luck finding one... *LOL*

Maybe a 50 stage compressor..... Boeing would be fascinated ............ LOL