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 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 (Cooley, 1901) 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 a 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 / eliminating 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 Reverse_Wankel / 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 Reverse_Wankel / 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 / eliminating 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 operation (of the four strokes per cycle) into 225 degrees of rotation of the inner body (and 180 degrees of rotation of the outer body).
A common characteristic of the rotary engines in use is that some of the seals, and some of the seal grooves, are shared between neighbour working chambers.
In comparison, in a reciprocating engine the top-ring of the piston is the main seal of the combustion chamber (or cylinder) and it utilises a groove (ring groove, formed on the piston) that relates exclusively with one only combustion chamber (or cylinder).
This is a significant characteristic because the pressure inside the combustion chamber and inside the groove (which communicates with the combustion chamber) assists the piston ring to minimize the gas leakage keeping the friction low.
The pressure in the cylinder and at the "top" side of the groove presses the piston ring to expand radially and to abut tightly (during the high pressure period of the cycle) onto the cylinder liner; it also presses the ring to seat tightly on the "bottom" of the groove ("bottom" of the groove: the groove flank away the combustion chamber).
The same could be achieved by preloading the piston ring (for instance by springs, as is the case with the oil scrapper rings), but in such a case the heavy preloading would multiply the relative friction (a heavy preloading during the low pressure period of the cycle is useless, whilst it increases the friction loss, the wear of the piston ring and the wear of the cylinder liner).
In comparison to a reciprocating piston engine, an RX-8 Wankel rotary engine uses two apex seals per working chamber, with the one apex seal (and its groove) shared with the leading working chamber, and with the other apex seal (and its groove) shared with the trailing working chamber.
The apex seal "plays" inside its groove on the rotor, bouncing between the two flanks of its groove.
For instance, the "leading" apex seal of a chamber, when the exhaust starts in the leading chamber (and the pressure in the leading chamber drops abruptly), leaves the "trailing flank" and moves towards the leading flank of its groove, allowing a significant leakage towards the exhaust. At the end of its "stroke" it slaps the "leading flank" of its groove.
There are similar problems in the Reverse_Wankel / LiquidPiston rotary engine wherein each "peak seal" with its groove is shared between two neighbouring working chambers.
A "peak seal" cannot help bounching between, and slapping on, the two flanks of its groove.
The converging of the grooves enables the two different seals at the specific apex of the rotor to abut closer to the geometrically correct point on the epitrochoid working surface (the converging of the two grooves reduces the required motion of the seals inside their grooves in order to remain permanently in contact with the working surface on the casing, it also reduces the "dead" volume, i.e. the volume between neighbouring apex seals, it also reinforces the "backside" of the groove at its "foot").
With the similar design, each peak seal of a Reverse_Wankel / LiquidPiston engine relates with the combustion in one only working chamber.
Here is a PatWankel wherein the working surface is on the outer body (not shown). Each working chamber has its own seal and its own groove. The inner body (actually the rotor) is shown sliced, with the one seal in place and the other two seals disassembled:
In the following PatWankel, wherein each seal and each groove serve one only working chamber, the working surface (whereon the seals abut and slide) is the external surface of the inner body:
Here is the inner body alone, with the three seals on it:
In the following drawing the two, of the three, seals have been removed.
The working chamber at left is at its TDC with its seal surrounding it and sealing it.
The inner body is like a "piston" pushed deaply into the working chamber (an unconventional piston that needs neither a connecting rod, nor a crankshaft).
At operation the "piston" (i.e. the inner body), remaining permanently in contact with the seal, is pushed outwards from the chamber and the volume increases, then the "piston" is pushed inwards and the volume decreases, and so on:
Every point of the inner periphery of the seal remains permanently in contact with the external surface of the inner body.
And if, instead of keeping the outer body (and the seals with it) immovable, the outer body is spinning at constant speed about a fixed axis (and the inner body with the working surface is also spinning at constant speed about another fixed axis), the elimination of the eccentric shaft comes with many other advantages:
Among the advantages of the discrete grooves for each working chamber is the independence of the sealing of neighbouring chambers, also the elimination of the leakage towards the leading and trailing chambers: each seal seats onto the right side of its groove and uses the pressure in its own chamber to tightly abut on the working surface during the high pressure period of the cycle, i.e. as in the reciprocating piston engines.
Significant advantage is also that only the one face of each seal relates with high temperature gas; its other face abuts on the cool "bottom" of its groove ("bottom" of the groove: the flank away the combustion chamber); this way, the thermal load on the seal reduces substantially (the number of combustions it participates is half of those of a conventional apex seal), the mechanical stress of the seal is reduced substantially (there is neither bouncing of the seal among the flanks of the groove, nor slapping of the seal on the flanks of the groove when the one of the neighbouring chambers fires), the cooling of the seal is improved, etc.
All these improve the long-term reliability.
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):
The pitch circle diameter of the ring gear (blue) is six times the distance between the rotation axis of the outer body and the rotation axis of the inner body.
Click here for another (per 5 degrees step) 2D animation.
The pitch circle diameter of the ring gear (blue) is ten times the distance between the rotation axis of the outer body and the rotation axis of the inner body.
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):
In the following animation the step is 10 degrees (there are 180 degrees from the TDC to the BDC):
The pitch circle diameter of the ring gear (blue) is fourteen times the distance between the rotation axis of the outer body and the rotation axis of the inner body.
Timing / Combustions per "rotation" / GearWheels loading etc
In the following Wankel they are required three eccentric-shaft-rotations for three combustions (one per working chamber).
The synchronizing gearwheels are not loaded by the combustions.
In the following LiquidPiston rotary engine (or alternatively: in the following section of a PatWankel wherein the working surface is the external surface of the inner body and wherein the outer body is immovable) :
they are required only two eccentric-shaft-rotations for three combustions (one per working chamber).
While the engine "appears" more powerful than a similar capacity Wankel (for instance: at the same 10,000rpm of the eccentric shaft, they happen 5,000 combustions per minute in the LiquidPiston and only 3,333 combustions per minute in the Wankel), this phenomenal (but not real) "superiority" comes together with several and severe side effects:
* The synchronizing gearwheels are heavily loaded during each combustion. This causes wear, noise, friction, need for better lubrication of the gearwheels, "play" of the rotor, need for bigger gaps (i.e. increased gas leakage) between the spinning and orbiting side seals of the LiquidPiston and the immovable "corner" and peak seals of the LiquidPiston, etc.
* Every time the rotor passes from a combustion TDC (it happens 1.5 times per eccentric shaft rotation), the load on the teeth of the inner gearwheel changes direction; the inevitable backlash is a problem (impact loads, noise, wear etc)
* The bearing of the rotor "runs" at 50% higher r.p.m than the eccentric shaft.
* The force on the rotor bearing is substantially heavier than the force the high pressure gas applies onto the rotor during the compression / combustion / expansion. The "super-over-square" design causes a heavy force onto the rotor, and an even heavier force onto the fast revving rotor bearing.
* There is an extra load on the rotor bearing (also in a continues basis, because every combustion causes this loading) by a strong pair-of-forces caused by the side location of the heavily-loaded synchronizing gearwheels. Due to this pair-of-forces, the side-walls of the rotor stop being exactly parallel to the side-walls of the casing: the seals suffer and the gas leakage increases.
In the following PatWankel:
they are required three rotations of the power shaft (which is secured on the inner body) for three combustions (one per working chamber).
As in the Wankel, the synchronizing gearwheels are not loaded by the combustion.
Reverse PatWankel shown with intake and exhaust ports: