If you look at an image, you can see the three apex seals on the stator, which is shaped the same way as a wankel rotor. The rotor of the liquidpiston engine is

If you look at an image, you can see the three apex seals on the stator, which is shaped the same way as a wankel rotor. The rotor of the liquidpiston engine is shaped similarly to the wankel stator and has no seals. It's almost the same just turned inside-out.