parkland
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Joined: February 2015
Posts: 57 
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Post by parkland on Feb 2, 2015 14:30:53 GMT -5
If you have valves but no secondary form of compression you may be able to have more of a mismatch between turbine inlet and compressor outlet pressures, but they will still have to be fairly close (otherwise residual pressure on the turbine side will always be higher than the compressor outlet pressure and you won't be able to fill the combustion chamber with air) A free piston engine allows a much higher compression ratio, which can extract more energy from the fuel and give higher overall efficiency. If it's built like a two stroke engine, it can perform the valve functions with minimal parts, as well as increasing the compression ratio. I see what you're getting at. But consider this; if the timing of the fuel injection in the combustion chambers, is spread out enough, the pressure on the exhaust turbine could jump from full pressure, to none, so that the combustion chamber pressure could be 0 psi, and if spread out even more, could flow air through the non burning chamber as the compressor always has pressure, and the other chamber would be full of compressed air, and starting to burn. If the fuel injection was such, that it is ALWAYS spraying fuel to either of the chambers, then there would always be back pressure, and thus, always pressure in the chambers. However, if the injection events are set up, that there is enough time in between, for the burning chamber to completely empty all pressure on exhaust turbine, before the other fires, then there should be (i think) no pressure, maybe even venting from the compressor, if too much time gap was used. It would be a matter of pushing more fuel, quicker, and shortening the injection and burn, instead of using a steady burn like jet engines do. Think diesel injector, but with an ignition coil, or multiple spark igniters to burn it quick. The downside would be the short bursts of pressure on the turbine, but I think they can take that in stride, although I may be wrong. |
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Post by racket on Feb 2, 2015 15:29:23 GMT -5
Hi
Some of the very early gas turbines operated with sealed "combustion chambers " but overall efficiency was poor .
If we want fuel efficiency , go turbocharged diesel engine , if we want concentrated lightweight power, use a conventional gas turbine
Cheers
John
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Post by finiteparts on Feb 2, 2015 22:40:13 GMT -5
Hi, Just to correct a few statements...gas turbines operate ideally with constant pressure combustion...you can see this in the definition of the Brayton cycle. In the real world, there is a pressure loss through the combustor. The heat from the combustion, expands the gas and thus causes the compressor to have to do more work to push the same mass flow rate through the back end, which is a form of back-pressure...but the highest pressure exists at the diffuser exit and then continually falls from there. The purpose of burning all that fuel is to increase the internal energy of the gases, not the pressure of it. Now, there are a lot of papers and research out there on "pressure rise" combustors that use a shock wave to trap the combustion so as to get a constant volume process, but I don't like the idea of hammering a turbine wheel with detonation waves. Some have tried it with stated decent results...see the AFRL's work on slide 22 of: www.netl.doe.gov/publications/proceedings/10/utsr/presentations/wednesday/Paxon.pdfYou can see the "gain" in cycle (the area enclosed in the curves) on slide 25 between the Brayton and the Pulse Detonation Engine (PDE). A few more papers... www.princeton.edu/cefrc/Files/2012%20Lecture%20Notes/Richards/NewDevelopmentsGeoRichards-Part-2of3.pdfntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120009956.pdfGood luck! Chris |
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parkland
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Joined: February 2015
Posts: 57
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Post by parkland on Feb 3, 2015 11:35:00 GMT -5
Cool stuff you guys !! Thanks for the fun conversation, I can tell I will like it here  |
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parkland
Junior Member
Joined: February 2015
Posts: 57
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Post by parkland on Apr 13, 2015 18:33:49 GMT -5
OK one more question haha. I see a lot of places, using a turbocharger jet engine as the gas producer, driving a power turbine to have shaft horse power. Many times is it insinuated that the power turbine should be larger than the turbine on the gas producer. Is there a reason for this, other than the gearing advantage? Is there any reason to say not order 2 identical turbochargers, use one for a gas producer, and hack the 2nd up for the power turbine assembly? |
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Post by racket on Apr 13, 2015 20:07:09 GMT -5
Hi
The freepower turbine will be processing gases at no more than ~10psi generally , whereas the gas producer can be processing gases at 45 psi in a large unit , density of the gases is much lower in the freepower and the gas velocities will be much slower as well, so we need more space for the gases to flow through .
Without that increased flow area the gases simply can't get through the freepower quick enough and will cause excessive backpressure on the gas producer which will respond by increasing its temperatures and the compressor going into surge with the potential of wrecking the gas producer :-(
It is possible to bypass some of the gases coming from a gas producer of the same size as the freepower , but thats only wasting potential horsepower , much better to source a bigger freepower wheel and optimise the freepowers output , after all , thats the point of the exercise, makin HP .
Cheers
John
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parkland
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Joined: February 2015
Posts: 57
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Post by parkland on Apr 13, 2015 21:03:07 GMT -5
OK Awesome!
If the gas producer is a VVT turbo, I suppose that should make it easier to size the freepower turbine easier?
I noticed in a video that a VVT turbo with the vanes in the tight position let's it run at really low exhaust temperatures,
but obviously less velocity.
OK, so, let's see if I can make sense of this.
I found 2 turbochargers locally for cheap.
One is a VGT turbo from a 6.0 liter ford diesel.
On the truck, guys have made over 40 PSI of boost, but about 30 PSI is a safe normal limit for extended use.
The other is off a 12.7 detroit diesel series 60.
From what I've read, those can also make 30 PSI.
Is it a fair assumption then, to conclude that since the big turbo is from an engine with roughly 2x the displacement as the small turbo's engine,
and that if the small turbo is the gas producer, making 30 PSI, then the pressure driving the large turbine could be 15 PSI?
And since the small turbo has VVT vanes in it, that should give some room to play with right?
I know it seems like the big turbo isn't big enough to turn into a freepower turbine, but would this work considering the 6.0 turbo is
a VVT turbo?
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Post by racket on Apr 13, 2015 21:26:16 GMT -5
Hi
It doesn't always "translate" from IC engine use to gas turbine use , they're different "animals" , a turbo on an IC engine can have higher pressures going into the turbine than what comes out of the compressor ,we can't , our turb inlet pressures will always be lower .
VVT for our use generally means fixing the vanes in a mid position and leaving them there until the engine is running and some minor "tuning" can be attempted with small movement of the vanes until optimum position is found and then fixing them at that point .
Theres no point running our engines at low temperatures , low temperatures mean an engine with less than potential power output .
Nope , just because the turbos are off roughly 2 times engine capacity doesn't mean the pressure into the freepower will be halved , you simply won't get that , generally it'll be ~1/3rd to 1/4 at best.
The best way to get a rough idea of matching is to measure the comp wheel inducer and turb wheel exducer ( in and out) of the 2 turbos , this will give you an idea of actual flow potentials , if the exducers ( outs) of the two turbos indicate the bigger has roughly twice the area of the small turbos turbine exducer then you're probably OK
Cheers
John
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parkland
Junior Member
Joined: February 2015
Posts: 57
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Post by parkland on Apr 13, 2015 22:41:06 GMT -5
The 6.0 diesel exducer is 82 mm . (~211 cm2)
The detroit turbo exducer should be 98 mm. (~301 cm2)
So with a VVT turbo, does that change this at all?
Should the freepower turbine still have 2x more surface are of exducer?
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Post by racket on Apr 13, 2015 22:52:27 GMT -5
Hi
I think you'll also need to check those compressor wheel inducer diameters , the 6.0 diesel exducer could be a bit "oversized" relying on the VVT to produce impulse power to the turbine for boost production with the vanes "tightened up" to produce a small throat area, but then have them fully opened to reduce backpressure in conjunction with the large exducer when needed.
We'll need those comp inducer sizes to be certain
Cheers
John
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parkland
Junior Member
Joined: February 2015
Posts: 57
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Post by parkland on Apr 14, 2015 10:49:10 GMT -5
I'll keep looking.
I don't know where you guys find these giant exducers lol.
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Post by racket on Apr 14, 2015 16:15:20 GMT -5
Hi
The 98mm exducered wheel could be big enough for the freepower .
But I need those compressor inducer sizes to be sure , there are thousands of differently configured turbos out there and depending on the combination of comp and turb stages some can be used successfully and others can't , each needs to be assessed individually .
Cheers
John
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Post by finiteparts on Apr 14, 2015 22:06:57 GMT -5
Hi The freepower turbine will be processing gases at no more than ~10psi generally , whereas the gas producer can be processing gases at 45 psi in a large unit , density of the gases is much lower in the freepower and the gas velocities will be much slower as well, so we need more space for the gases to flow through . Without that increased flow area the gases simply can't get through the freepower quick enough and will cause excessive backpressure on the gas producer which will respond by increasing its temperatures and the compressor going into surge with the potential of wrecking the gas producer :-( It is possible to bypass some of the gases coming from a gas producer of the same size as the freepower , but thats only wasting potential horsepower , much better to source a bigger freepower wheel and optimise the freepowers output , after all , thats the point of the exercise, makin HP . Cheers John Just a corrections here....John you must have been typing too fast! "...density of the gases is much lower in the freepower..." ----> The density of the gases in the free turbine will be higher, since the temperature is lower. Also, I think your description is sort of lacking a few points...I'm gonna take a stab at it and you can feel free to critique my comments!  When you stick a free turbine on the back of a gas producer, you are taking some of the available pressure drop away from the gas producer turbine to push the gas flow through the free turbine section. Since the temperature drops through the first stage turbine, the local speed of sound is lower for free turbine, so the free turbine chokes first, just like a nozzle on a thrust engine. On full size engines, the free turbine's flow capacity (or a thrust nozzle on a jet arrangement) establishes the compressor operating line and this should apply to our small engines too. If the effective area of the free turbines NGVs is too small, the compressor operating line (op-line) will move towards the surge line as opposed to where the operating line of just the gas producer itself sits on the map.  In the above image, the green line represents the op-line for the gas producer itself. The thin red lines represent lines of constant temperature ratio. I just threw these on there, so don't read a lot into their placement...they just represent the behavior for illustration purposes. The temperature ratio is the turbine inlet temperature over the inlet temperature. In full size engines, when the first stage NGVs and the exhaust nozzle (or power turbine stages) choke, this is how you control the engine, by changing the upstream conditions (temp and pressure). This changes the acoustic velocity and thus the mass flow passing through the engine at choked conditions. Full size engines that operate at higher pressure ratios (higher than ours! Maybe 5-8 and over) run in choked conditions in the NGVs and exhaust nozzle (or power turbines) over the majority of their operational space. So if you think that a choked flow means that you hit some sort of "wall" you aren't seeing the whole picture. So back to our topic, when you put a free turbine on the back end (or a thrust nozzle), the op-line is pushed over toward the surge boundary due to back pressuring of the cycle from the free turbine. You can see that in this representation, the op-line shows that the compressor is still away from any surge condition, so you might think you are ok...but you might not be. If we look at the engine behavior as you try to accelerate, we can see there are other things that affect the engine operability and maybe require more "surge margin".  In the above image, let's say we are operating at conditions that put us on the blue point and we want to accelerate the engine to full speed. When we add fuel to increase the gas energy, the gas temperature responds much faster than the rotor rpm, due to the rotors inertia. So from the blue point, we move towards the left roughly along a constant speed line (since the rotor is slowly accelerating). As we move to the left, we are moving across higher temperature ratio lines (the turbine entry temperature is increasing)...the mass flow that the engine can push is reducing...this makes sense since due to the increasing gas temperature, the gas density entering the NGVs is falling and thus the volumetric flow rate is going up...to push more volume through, it would take more pressure, but the rotor isn't accelerating and there isn't more pressure to push it through, so the flow rate falls. As the rotor starts to pick up some speed, the power imbalance between the compressor requirements and what the turbine is generating starts to reduce as the rotor moves back towards a steady state point at the black dot. Hopefully this happens without your engine crossing the surge boundary...but if you don't have enough surge margin, you will find your engine difficult or impossible to accelerate to full speed. Your only hope if your op-line is too close to the surge line is to slowly accelerate. The earliest jet engines experienced this behavior and thus you will often find "acceleration limiters" as a part of their fuel control hardware. So, what does this have to do with free turbines? If you guess and put a free turbine on that has too little effect area, it can push your compressor into an operating condition that renders your engine inoperable. The safe bet is to go with a larger effective area turbine. Now, don't confuse the large power turbine sections in full size engines as a indication that you must go really large. The reason that they are so large is because they are trying to drive the fan sections at a speed more suited to the fans operational efficiency. To go to the relative low speeds that the fans require, you have to have a very large diameter turbine so that your vectors make sense. Even with the large turbine diameters, the blading still needs to turn the flow by very large angles...this is a whole different animal that what we are trying to do. The final comment is that proper turbine matching requires you to calculate the flow velocities and angles..no "if's", "and's" or "but's". There is always a trade for any component design...the trade off on turbine diameter for reducing the flow control on the gas generator is as you get larger, the turbine nozzle effective area is getting larger...which means that for a given mass flow and pressure ratio, the turbine entrance velocity is falling. In the ideal case for a radial turbine (assume radial entry and no exit swirl), the power is equal to the mass flow times the tip speed squared, so if you are giving up the tip speed for what ever reason (pressure ratio, output speed, etc), you are loosing available power. There is no substitute for doing the design work. Good luck! ~ Chris |
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Post by racket on Apr 14, 2015 23:13:55 GMT -5
Hi Chris What , me make a mistake  Better have a look at my 10/98 engine calcs 180 deg C rise in the comp at a 3.75 PR , so a ~160 drop through the turbine , T I T of 900 C - 1173K , 5% pressure drop across the flametube , so 3.56 PR going into the turb stage at 900 C - 1173 K with a density of 0.0669 lbs/cubic foot . Coming out of the turb stage after a 2.11 PR across it theres a 1.685 PR at 740 C - 1013 K with a density of 0.0366 lbs/cubic foot Now if we have a density of 0.0669 lb/cu ft going into the gas producer and a density of only 0.0366 going into the freepower , thats roughly half the gas producers density . Nope, didn't make a mistake, what a relief  Cheers John |
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Post by racket on Apr 14, 2015 23:23:42 GMT -5
Hi Chris
One correction with your comments .......... " Even with the large turbine diameters, the blading still needs to turn the flow by very large angles...this is a whole different animal that what we are trying to do".
We do turn our gases at large angles , the freepower for both my 10/98 engine as well as Anders JU-01 engine has quite shallow NGV angles down near 20 degrees feeding gases into a freepower wheel with tip angles near 20 degrees in the other direction
Our engines are operating exactly the same as commercial engines .
Cheers
John
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