[sjburt]

The other challenge vs reciprocating engines is that a piston engine cylinder has two volumes; the area above the piston where combustion occurs, and the area below, where there isn't any combustion. So during the stroke, you can spray the cylinder walls with lubricant, the bottom of piston, etc. So the seals can run on a layer of lubricant, and the piston can be cooled by the lubricant. In a rotary, you don't have that. Both sides of each seal are filled with fuel-air mixture, so to lubricate, you have to spray oil into the mixture. It's the same issue as with two strokes. They have to burn oil, and that is really tough on emissions, and it's really difficult to get heat away from the seal (the only place it can go is to the rotor).

[Grishnakh]

That's an excellent point; I mentioned squirting oil before, but I overlooked that it serves another purpose in cooling the piston and the seals. Lots of modern engines now have oil squirters underneath the pistons to do this.

However, since, as you say, they have to burn oil to cool and lubricate the seals, that does make me wonder if this kind of engine wouldn't work much better with a lubricating fuel, such as diesel or kerosene (jet fuel), rather than gasoline. Kerosene is used in jet turbine engines largely because it is a lubricating fuel, so it serves the purpose of both fueling and oiling. The engine in TFA appears to be a rotary derivative, but they do say that it's designed to run on JP-8 jet fuel, not gasoline.