h a l f b a k e r yI CAN HAZ CROISSANTZ?
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A known characteristic of the wankel engine is its lack of torque at low rpms. I propose an interesting new design.
At low rpms: There is a compresson from a gas turbine that is hooked up to the shaft of the wankel with gearing such that the compressor provides around 20lbs boost at 1,500 rpm, and
the compressed gas exits via the inlet port of the rotary (just like any other supercharged wankel). The wankel runs on vegetable oil, so the compressor can feed in quite a bit of air. But once you reach the 4,500 rpm mark, there's no turning back (well, unless you release the throttle and let the rpms drop... but never mind that).
At high rpms: Well right about now the compressor would be just about useless in the boost department, so it is instead put to good use. Spring controlled sliding valves uncover ports where the compression "stroke" would be in the wankel. Since the vegetable oil would probably not be able to be ignited from the low boost from the compresor in the wankel, it is injected into the compressor to provide thrust (the "supercharger" is instantly converted to a gas turbine). Only since the compressor is still hooked up to the engine mechanically, the thrust is instead transferred to mechanical energy, which it shares with the wankel (in turn increasing its power). Now the way the compressor would be linked by gearing, it would be spinning over 100,000 rpm and would most likely be making more power than the wankel would be. But instead of just quiting like that, the exhaust of the turbine still exits via the inlet port of the wankel. Since there is now a valve where the compression stroke would occur, this is a new exhaust port for the turbine's spent gasses. The only reason they are flowed through the intake port of the wankel is to transfer heat. Then in the combustion "stroke" where there is now no gas whatsoever, the vegetable injector is used to pump water on to the now-hot rotor and create some good torque (much like on a crower 6-stroke, only with a more efficient design). And the best thing is, butterfly valves can make the wankel's exhaust port a high-rpm steam condenser since there would be no left-over gases.
Then once you're done with your acceleration run and just want to cruise, you have the quiet power and efficiency of a diesel wankel engine with a small "supercharger".
Oh yeah, and I immagine that when the left-over thrust is pumped through the compression-port exhaust of the wankel, it would give it some of the sound characteristics of a wankel exhaust (aka better than the vacuum cleaner whine of a gas turbine) that could be make even better with the right type of exhaust.
The next engine of the Mazda RX8 (or not). Oh, and when the car is stopped and in neutral, there is a fuel-cut to prevent it from being revved past 4,000RPM (just to prevent one from over-using the changover and creating more noise).
Much needed illustration
http://s210.photobu...Cycle_anim_en-2.jpg
I'll explain anything it doesn't explain itself. [acurafan07, Jul 21 2007, last modified Jul 22 2007]
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I am confuzled about some things.
Can you re-read through it and try to make it less confusing. |
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Poked it, prodded it, thought about it... I don't think it would be practical, but in principle I think it might work. Whether it would be better than a gas turbine on its own, I'm sceptical. |
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Sorry [BJS], this was another 3 a.m. posting. Any place I wrote diesel should be changed to vegetable oil. I think high rpm mode might be confusing you. There is a port where the compression stroke would happen at low rpm that opens and allows the newly sucked in exhaust from the turbine to be pumped out. So when the water (steam) occurs, there is no other gas in the chamber. |
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And [david_scothern], the only advantage this would possibly hold over a gas turbine alone is the waste heat recovery (steam) in the wankel. Sure you could run a steam turbine off the gas turbine, but they would both be low-torque, high-rpm machines. The steam wankel is designed more for torque, which the gas turbine certainly would lack. |
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How would you keep the vegetable oil form mingling with the conventional oil, gradually increasing the vegetable/conventional oil ratio during the changeover period? Over time this would corrupt the oil. This could cause a rather large explosion or engine fire. |
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I'm missing something maybe, but complexity aside, this seems a large issue. |
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I'm not sure what you mean. The vegetable oil is storred in a totally different tank and is ignited as soon as it is injected. Doesn't seem there's ever a point where the conventional oil and vegetable oil can "mingle". |
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I think I mostly get it now...
I'm trying to visualize it, but I don't know what configuration the components are in.
Could you walk me through both cycles of the engine? |
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"Since the vegetable oil would probably not be able to be ignited from the low boost from the compresor in the wankel, it is injected into the compressor to provide thrust" |
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So where does this particular vegetable oil end up in the end? |
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Can you explain it more depth here for us who also want to understand? |
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There, sorry it took a while. |
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Does the gas turbine not burn any fuel during the the lower RPMs? Or would it burn vegetable oil, and then the same exhausted would be re-burnt in the Wankel? |
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Right, at low rpms the turbine burns nothing and just acts as a supercharger. |
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I understand your idea, but I'm not sure it would work very well. |
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For one thing, it would be hard to make a diesel Wankel. Rotary engines have a very long and strangely shaped combustion chamber that could trap pockets of air and fuel in the corners and lead to incomplete burns. This is why most Wankels have low compression ratios. |
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It might be possible to use the shaft-driven gas turbine like you said, but I think a regular turbocharger might do better. |
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Vegetable oil might not be the best choice for fuel, especially in a Wankel. SVO tends to leave carbon deposits in the engine and gum up the seals, which are already temperamental in a Wankel. You might be able to clean all of that out with water injection, but like Giblet said, you might end up with water or vegetable oil in the engine oil. |
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You might want to re-write the description to make it clearer, but I was at least able to understand the basic idea. |
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Well I made the illustration better, and updated the idea. |
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You're right that a turbocharger might work better, but the whole point of this idea is the gas-turbine at high rpms. But I s'pose that could be linked somehow. As far as the compressor's efficiency, this would be kind of similar to centrifugal superchargers, which are a lot more efficient than any other. |
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A major problem is that turbine (or centrifugal) superchargers have terrible boost at low RPMs--if you are trying to improve low-end torque, they are exactly the wrong type of supercharger to use. You need a Roots or Twin-Screw SC, and I am skeptical that either of those can efficiently function as a power-producing engine. |
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If you want to extract more work from the gas stream of the turbine, what you should do is add another stage to the turbine - another set of blades through which the gas can be expanded a little further, cool a little more and give up a little more of its energy. Gas turbines get very shirty if there is a pressure build-up downstream of the turbine - the flow is liable to turn round and go back, with disastrous consequences. |
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Hmm not sure exactly what you mean with the extra set of blades. But as to the pressure build-up, there could be some sort of camshaft controlled exit port when the rotor(s) isn't/aren't sucking it in so that it doesn't build up. |
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By definition, if you're taking the exhaust and feeding it through anything at all, you're going to get a pressure build up, just like backpressure in an exhaust system. |
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To address an earlier comment, Wankels don't have low compression ratios because of the risk of incomplete burn; they have low compression ratio because their geometry can be varied over a small range only (epitrochoid) and hence there are limits on achievable CR. On that basis you're going to have to be quite clever to get a diesel to work - specifically, a supercharger is going to be a requirement. |
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And another thing: The output of a gas turbine is at atmospheric pressure, near enough. The mixture is taken in, compressed, burnt and expanded - the expansion is where you get power, like the power stroke of a piston engine. You'll have to tap the gas off immediately downstream of the compressor, and upstream of the turbine if you want compressed air. Obviously you can't get hot exhaust from the same place; the compressed gas will be hot but will only contain the energy you've put into it. |
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As I understand it the following is supposed to happen... |
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--- the wankle drives the turbine at low rpm |
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--- the turbine drives the wankle at high rpm |
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If that is the case then... |
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--- the turbine is plumbed to the induction stroke at low rpm |
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--- the turbine is plumbed to the expansion stroke at high rpm |
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Looks like you need a modification... |
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Well this idea is starting to seem less and less of a good one to me. |
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But actually, the turbine would still be feeding the exhaust to the intake (never the expansion, it would always be fixed to the intake) and then compressed out to the exhaust for the purpose of heat, not expansion and power. When the turbine "drives" the Wankel, it is through a shaft (driven by the blades of the turbine and connected to the shaft of the wankel), not its actual thrust (the shaft harnesses its power and is connected to the wankel, that way the thrust is harnessed before the exhaust gets to the wankel). That way by the time the turbine's exhaust gets to the wankel, it has no more thrust energy, only heat. That is why it is sucked in (rather than blown in) through the intake port always (rather than any expansion port). |
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I think a better way of doing this might just be to take away the low RPM part and just use the steam and turbine. Or maybe to just not do this at all. |
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It's certainly made for an interesting discussion though. |
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Indeed, and although it isn't practical, I personally happen to like it. And hey, maybe it might even work. |
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Oh yeah and [maddness], see anno above. |
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To be honest I like it --- and I have one other observation/question... |
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The low and high rpm modes of operation depend on the turbine having a wider range of operation that the wankel... |
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The wankel is a positive displacement pump and the turbine is centrifugal --- which one has the wider operating range? |
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[maddness], I'd sure assume the turbine.would have the wider range (if you're talking about RPM). Wankels typically have a redline of about 9k, where turbines have a redline of 10s of thousands. Now the wankel's usable RPM range is probably about 2,000-9,000. I don't know for sure where a turbine's would start and end but I can assure you that there is a whole lot more than a 7,000 opperating range (my guess would be either double or tripple that). |
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