My inspiration for this idea comes from a design for an air compressor I've heard of.
In this compressor, there is a central hub with radial slots cut into it. Each slot is fitted with a spring-loaded vane. This central hub with vanes is located off-center within a larger chamber, which has a circular or elliplical cross-section. The hub rotates, and, as it does, the vanes slide against the walls. Because the hub is off-center, the distance from the center of the hub to the edge of the chamber varies as the hub rotates, so the vanes are pushed in and out over the course of a rotation. The volume between vanes (a "wedge" of air), therefore, increases over one half of a rotation and decreases over the other half. In the air compressors, there is an inlet port at the point of largest volume and an output port at the point of smallest volume, and that's all there is to it.
Half of a revolution, in other words, is intake/expansion, and the other half is exhaust/compression. So let's combine two of these compressor devices to perform a total of four "strokes": Air/fuel mixture enters the first device, which performs intake and compression functions, and then moves into the second, which performs combustion/expansion and exhaust functions.
Here's a more detailed description: Imagine two of these air-compressor-like devices stacked atop one another and attached to a common shaft. In the top half of the engine, the intake port extends over nearly an entire half of the rotation (the expansion half), and, as the vanes are pushed out against the walls and the volume of each "wedge" increases, air is sucked in. Once that half of the revolution is done, we get to the compression half. Naturally, the intake port ends at the beginning of the compression half. As the vanes are pushed in and the wedges are made smaller, the air is compressed.
Then, shortly before the point of greatest compression, there is a large port leading to the second half of the engine.
Now we're in the second half. As soon as rotation brings the current "wedge" of gas past the port from the first half, it comes to a spark plug, which ignites the fuel-air mixture. The gas then expands over this first half of the revolution. Then, beginning at the point of largest volume, and extending until shortly before the port from the first half of the engine, there is an exhaust port. During the compression half of the revolution, exhaust is forced out.
I'd expect this engine design to have many advantages over traditional piston designs. First, all strokes of the engine are always performed simultaneously, so combustion is always occuring, thereby eliminating the need for a heavy flywheel (though a small one may make sense to ensure smooth operation). Also, it is a relatively compact design with a large working volume, so it should have excellent power-to-weight characteristics. And with all these wedge-shaped chambers, I would expect pretty explosive torque characteristics.
I do have a few concerns though. First, there's the fact that these vanes are compressing and decompressing springs repeatedly and rapidly. At first I though that this would quickly cause fatigue. However, isn't this exactly what the valvesprings for the poppet valves in a traditional automobile are subjected to, without incident? So I think that concern may be a nonissue.
My second concern is sealing. The seals would need to be essentially the same as the apex seals used in Wankel-type designs, which have proved problematic in the past. However, those problems have been solved in Wankels, and this uses sealing which is nearly identical, so I suspect that this too is already a solved problem.
Now, this idea is also easily adapted to two-stroke operation using a single half of the 4-stroke version. Scavenging would be easy to achieve by overlapping the intake and exhaust ports slightly, and by placing the intake towards the center of the engine and the exhaust towards the edge; centrifical force would facilitate scavenging.
An external-combustion Stirling-like version would also be simple, again, using just a single rotor like the two-stroke version. One need only put the expansion half of the engine in contact with a hot reservoir and the compression half in contact with a cold reservoir. One could run such an engine using the temperature difference between an external burner and the atmosphere (using a heat sink), or between the atmosphere and liquid nitrogen.
In other words, the concept seems very versataile, and isn't even limited to four-stroke operation.
So now, my question is: What's the catch?
[Edits: Subsequent Changes to idea]
- Springs are unnecessary to press the vanes against the housing; since its spinning there is centrifical force.
- Rather than a two-part engine, one can achieve four strokes from a one-part engine consisting of a rotor centered in an ellipse!-- TerranFury, Dec 12 2003 Sliding Vane Compressors http://www.cashflo.co.uk/Vane.htmlBackground [TerranFury, Oct 05 2004] "Vading Engine" http://www.mtf.ntnu...ivarse/vadart_x.pdfMore research reveals its not as original as I'd thought... [TerranFury, Oct 05 2004] Like this but more complicated? www.starrotor.comA rotary "external" combustion engine, no sliding vanes. [elhigh, Jul 12 2005] QuasiTurbine http://quasiturbine.promci.qc.ca/A similar idea but with elliptical chamber and fixed shape rotor [Carmi, Aug 22 2011] Baked. Or nearly. look at the wankel engine. Its the same convept, but without moving vanes.-- EdZ, Dec 12 2003 The wankel rotary moves the entire rotor to produce the various chamber volumes. I see this as a circular rotor spinning in a non-circular chamber, with "undulations" in the shape of the chamber wall which produce the appropriate cycles. an offset circular chamber would allow one full cycle for every two rotations, but would require valves. A two-lobed chamber (a distorted ellipse) would allow the full four cycles to occur every revolution.
With the Wankel rotor, the apex seals project a fairly short distance from each apex, and as such experience relatively little force during the cycle. One major drawback I see in this design is that the chamber size variations are entirely dependent on the extension of these seals, and I fear they may experience large loads. Solve this problem, and I think you'll have a winner on your hands. (I think you're right that this could work reasonably well as an external combustion device)
Nice job! (+) Welcome to the 'bakery!-- Freefall, Dec 12 2003 Thanks for the comments so far!
Freefall: Actually, the two-part engine I was imagining wouldn't require any valves. I really do like your one-part four-stroke idea though. I had considered a two-lobed housing shape and had initially ejected it, since the "humps" would prevent the vanes from reaching points on the wall that they would "occlude" (for lack of a better term, a shadows/raytracing metaphor), but that's only for an off-center rotor. I hadn't thought of a rotor in the center. Now that you mention it, with the rotor in the center, it would work!! Thanks for the suggestion. I'm going to try to come up with a nice function in polar coordinates for the desired shape. I'll post back...
[Later] Not sure if its ideal, but a rotor centered in an ellipse will do the trick!-- TerranFury, Dec 12 2003 I have a nifty variation on your concept: Make the housing circular, with a centered rotor in the middle. The stator has fixed vanes, the rotor had retractable vanes. You don't want the vanes to hit each other, ever.
The rotor vanes are NOT spring-loaded. They move according to a cam profile, roughly circular, with appropriate bumps, which is part of the housing, but on the outside of the combustion ring. The advantage of using an outside profile is that you can use rollers, and lubrify the profile, without worrying about seals or excess temperature there. The retractable vanes part would have a slitted U shape, with 1 branch inside the engine proper, and the other branch "outside". The outside Cam profile circle could be of smaller diameter than the main casing, or be of the same diameter.
The fixed vanes and the retractable vanes would both have rounded, lobular shapes, sliding by each other while maintaining seal.
The cam profile would be "desmodronic", both a push and a pull cam. A pair of rollers each side of a curved blade profile would do the trick.
Now, if I could make a drawing...
_Arthur-- _Arthur, Jan 07 2005 <adds 'lubrify' to her vocabulary>
I haven't a good mental image of this or I might try to draw it. I don't think the term desmodromic is right (isn't that a crankshaft-like affair?) but it gets that part of the image across.-- bristolz, Jan 07 2005 That's why I added Push-Pull.-- _Arthur, Jan 08 2005 I'm not exactly sure what you're emagining... but here's a website that I think is similar to the compressor you discribed http://www.starrotor.com/ this company is also developing basically a form of a jet engine meant for cars.-- BJS, Jul 12 2005 The star rotor engine has already been noted (back in 2004) on other ideas here.
Also, there is a facility for adding web links to a page (over there to the left beneath the idea text).-- bristolz, Jul 12 2005 [bristolz], I added the starrotor link.
I first saw a sliding-vane compressor for an air-based (instead of refrigerant-based) AC system in a 70's issue of Pop Sci. It was interesting, but it looked, well, a bit much. Too many moving parts. I get the feeling that may be more of the same case here. Has Mazda really gotten a handle on apex seals? Wouldn't that be a problem here, too?-- elhigh, Jul 12 2005 I actually remember reading research about the QuasiTurbine several years ago. The concept is almost identical, though they were doing it with an elliptical chamber and a fixed-shape rotor. Identical physics though, and saves you the trouble of maintaining the springs and vanes, which would wear and cause sealing issues. Anyway, Link added. Apparently, they're in production already.-- Carmi, Aug 22 2011 random, halfbakery