Each combustion chamber of the Wankel Rotary engine is sealed by a "sealing grid" comprising eight different seals: two apex seals, two side seals and four button (or corner) seals (click here for the US3,064,880 patent of Felix Wankel).
With eight gaps to leak from (one per button seal and side seal, and one per button seal and apex seal), the leakage of air / mixture / burned gas from a combustion chamber towards its neighbor combustion chambers is several times more than the leakage in a reciprocating piston engine.
The working surface, whereon the seals abut and slide sealing a combustion chamber, comprises two side flat surfaces and a cylindrical surface ending on the two side flat surfaces at an angle of 90 degrees forming a corner wherein the curvature gets infinite.
With a different design, the "gerotor" rotary engines can have a sealing quality comparable to that of the reciprocating piston engines.
Instead of being cylindrical, now the working surface that connects the two side flat surfaces is 3D curved and ends smoothly / tangentially on the two side flat surfaces.
There are no "corners", any longer, nor need for matching several sealing means at each corner.
The engine can operate even with a single seal surrounding each combustion chamber.
The "non-Wankel" (or reverse Wankel, click here for the US8,523,546 patent of "Liquid Piston"), wherein the working surface is formed on the inner body, has similar sealing issues as the Wankel rotary engine:
The "sealing grid" for each combustion chamber comprises two "peak seals", two "side seals" and four "button seals" (which means eight gaps for leakage per combustion chamber).
In comparison to Wankel's architecture wherein all seals are mounted on the rotor, now some seals are "stationary" (they are on the immovable "tri-lobe" casing) while some others are "movable" (they are on the "two-lobe" rotor), and because the motion of the rotor is far from being geometrically perfect (there is, inevitably, a backlash / play in the synchronizing gearing, there is also a "play" in the bearings between the cooperating parts, there is also a deflection of the eccentric shaft due to the high pressure load and to inertia load, etc), the overall leakage cannot help being problematic as in the Wankel rotary.
With a different design, this kind of "gerotor" rotary engines can have a sealing quality comparable to that of the reciprocating piston engines.
The working medium inside each combustion chamber is sealed by one only "closed" seal, just like in the reciprocating piston engines.
There is only one gap per combustion chamber, just like in the reciprocating piston engines.
The working surface (whereon the seal abuts) is on the inner body.
The groove wherein the seal is mounted is in the outer body.
Rotating both, the inner and the outer, bodies, interesting thinks result.
A single spark plug mounted on the one lobe of the inner body can serve all the three combustion chambers.
Worth to mention: the hole for the spark plug passes over the seal at the end of the expansion, i.e. way later than in the Wankel rotary engine, reducing several times the relative leakage.
The outer body needs not to have ports.
There is no eccentric shaft, leaving the centre of the engine "free" for ducts etc (optionally, adding an eccentric shaft and keeping the outer body immovable, a more conventional design results).
The casing can be no more than an open frame that keeps the bearings of the two rotating bodies at their proper position.
With five combustion chambers per "rotor" (click here or here for the US3,872,838 and US3,985,476 patents of VW for five "cylinder" rotary engines) and a curved working surface matching smoothly / tangentially the two side flat working surfaces, only five seals, in total, are required for all the five combustion chambers.
Each combustion chamber can have its own / exclusive seal, just like in the reciprocating piston engines.
The outer body has grooves for the seals, cavities for the combustion:
and cooling fins:
The outer body is the slower one.
The seals mounted into grooves of the outer body perform a pure rotary motion at constant speed and at constant eccentricity, undergoing a constant centrifugal force during the complete cycle.
In comparison, the inertia force an apex seal of the Wankel rotary engine applies on the epitrochoidal wall of the casing varies substantially, in magnitude and direction, during a cycle:
When an apex seal passes through the long axis of the working surface, the inertia pushes it strongly outwards; when the same apex seal passes through the short axis of the working surface, the inertia pushes it inwards, away from the wall, making necessary a spring to push the apex seal outwards; 270 eccentric-shaft degrees later, the spring force is added to the inertia force pushing the apex seal even harder towards the wall.
The instant inertia forces on the two apex seals of the same combustion chamber are substantially different: when the one tends to take-off from the working surface, the other pushes hardly the working surface.
Similarly for the side seals: the one end of a side seal undergoes substantially different acceleration than the other end of the same side seal.
In a LiquidPiston engine each "side seal" on the "rotor" undergoes a substantially variable (in amplitude and direction) acceleration around the seal, and around the cycle.
The following two plots show the required acceleration in order a point on the outmost edge (top plot), and another point on the innermost edge of the "side seal" of a LiquidPiston engine to follow the motion imposed by the spinning-and-orbiting rotor (R1 is the "crank-arm" of the eccentric shaft, R2 is the distance of the "point" in question from the center of the rotor):
Springs under the seals (as in the Wankel rotary engine) can be used to preload the seals of the PatWankel engine pressing them against the working surface on the inner body.
At higher revs the centrifugal forces acting on the seals reduce the total force on the working surface (and the relative friction).
The 4:5 transmission ratio of the synchronizing gearing allows big diameter gearwheels, enabling big ducts to be formed inside the shaft of the inner body.
The inner body has intake and exhaust ports and passageways, it also has ducts for the installation of the spark plugs and/or injectors, etc.
There is plenty of space enabling the installation of the ignition system (including the generation of the electric current required for the ignition) inside the inner body, near the intake ducts (to avoid overheating).
The spark plug holes on the working surface of the inner body pass over the seals of the chamber at the end of the expansion (i.e. way later than in the Wankel rotary engine) reducing several times the relative leakage.
If desired, a turbocharger can be mounted at the center of the inner body (no intercooling; for Diesels, for instance):
A 3D-net open frame is where the bearings of the two rotating bodies are mounted on, as in the following "pusher" propulsion unit for airplanes, gyroplanes etc:
There is no eccentric shaft.
The shaft / pipe of the inner body is wherefrom the energy / torque is taken.
No balance webs are required.
With the inner body perfectly balanced as it rotates alone on its bearings, and with the outer body perfectly balanced as it rotates alone on its bearings, the overall balancing quality of the engine is perfect (zero free inertia force, zero free inertia moment, zero free inertia torque).
A silencer can be mounted in the exhaust side of the inner body shaft, with a small gap (thermal isolation) between them.
The outer body (that with the cooling fins) spins at 4/5 (80%) of the speed of the inner body.
A throttle valve mounted on the immovable frame can control the air or mixture flow towards the engine.
The seals can be like those of the famous NR750 Honda engine with the oval pistons (oval in order to accommodate eight valves per cylinder). In the rotary engine the inner side is the working side of the seal. For the rest, a seal like that of the NR750 (bottom center) is bend at the proper curvature:
In the following the rotation angle of the inner body proceeds at 15 degrees steps, the rotation angle of the outer body proceeds at steps of (4/5)*15=12 degrees.
Only a thin slice of the outer body is shown.
Two combustions happen per compete rotation of the inner body (as in a two-rotor Wankel rotary).
Each chamber completes each "stroke" (of the four strokes per cycle) of operation into 225 degrees of rotation of the inner body (and 180 degrees of rotation of the outer body).
Here is how the curved / smooth working surface of the PatWankel rotary engine derives:
In the following two animations the step is 10 degrees.
There are 180 degrees from TDC (wherein the volume in the chamber is the minimum) to BDC (wherein the volume in the chamber is the maximum):
Click here for another (per 5 degrees step) 2D animation.
In the following animation the step is 5 degrees (as in the reciprocating piston engines, 180 degrees separate the TDC, where the volume of a chamber is minimum, from the BDC, where the volume of the same chamber is maximum):