Sliding friction is an issue with efficiency, I suspect. The flat sides of the rotor are holding back hot, compressed, abrasive gas. That means the faces need to be very flat and very rigid.
Unlike the face of an engine block, you can't get away with a gasket to mask imperfections. You need a hard, smooth bearing surface. That means expensive materials, which makes them even harder to flatten. Then you need the actual sealing elements, and they'll be at the worst spots possible- far away from the shaft. The sealed edge is in fact the fastest-moving part of the engine.
Bearing surfaces are rated on their PV- pressure times velocity. Both pressure and velocity create heat, and the life of a bearing surface is very roughly inversely proportional to the 4th power of the PV it's run at. Naturally, you want to put bearings as close to the shaft as possible.
> They seem to think the apex seal wear issue will be better with the apex seals stationary.
They're almost certainly right, although the actual details of implementing the seal are very complicated. They have to seal the corners as well as the sliding edge, so you need multi-part seals that are cut so that spring tension pushes them all outwards, and they still have to seal against each other[1]. They also need to be pressed sideways into their groove and kept from being dragged diagonally.
However, normal Wankels burn oil to keep the seals lubricated. Since these are stationary, they can be fed from oil in the crankcase. They can also be fenced in with scraper rings like a normal piston has, which keeps oil on the bearing surface and prevents it from being burnt off. Normal Wankel rotors lift off too much to keep the oil contained.
> I also wonder if they have a good solution to having more than two rotors? Mazda made a 3-rotor but due to the way the engine fits together the eccentric shaft had to be in two pieces that bolt together.
IIRC they'll have pretty bad second-order vibration, but either way the exhaust/intake is in the way to a significant degree.
Unlike the face of an engine block, you can't get away with a gasket to mask imperfections. You need a hard, smooth bearing surface. That means expensive materials, which makes them even harder to flatten. Then you need the actual sealing elements, and they'll be at the worst spots possible- far away from the shaft. The sealed edge is in fact the fastest-moving part of the engine.
Bearing surfaces are rated on their PV- pressure times velocity. Both pressure and velocity create heat, and the life of a bearing surface is very roughly inversely proportional to the 4th power of the PV it's run at. Naturally, you want to put bearings as close to the shaft as possible.
> They seem to think the apex seal wear issue will be better with the apex seals stationary.
They're almost certainly right, although the actual details of implementing the seal are very complicated. They have to seal the corners as well as the sliding edge, so you need multi-part seals that are cut so that spring tension pushes them all outwards, and they still have to seal against each other[1]. They also need to be pressed sideways into their groove and kept from being dragged diagonally.
However, normal Wankels burn oil to keep the seals lubricated. Since these are stationary, they can be fed from oil in the crankcase. They can also be fenced in with scraper rings like a normal piston has, which keeps oil on the bearing surface and prevents it from being burnt off. Normal Wankel rotors lift off too much to keep the oil contained.
[1]: https://i.ebayimg.com/images/g/gRYAAOSw8-tWVSxv/s-l1600.jpg
> I also wonder if they have a good solution to having more than two rotors? Mazda made a 3-rotor but due to the way the engine fits together the eccentric shaft had to be in two pieces that bolt together.
IIRC they'll have pretty bad second-order vibration, but either way the exhaust/intake is in the way to a significant degree.