Durability

Seals Development

The adjacent figure identifies the major components of the Legacy Engine discussed below. The roton tip seal design requirements represent the major known technical problems associated with the development of the Legacy Engine.   A tip seal will be developed that meets the operating requirements so the Legacy engine will be able to achieve the anticipated performance gains. The design parameters include geometric concepts, material selection, tribology, lubrication, reliability, etc.   Since this engine concept involves such a radically new configuration and combustion concept, the seal configuration cannot be related well to, and thus extrapolated from, seal geometry design solutions in reciprocating or existing rotary engines.   The tip and side seals perform the same function as piston rings in a reciprocating engine and the rotor apex and side seals in the existing Wankel rotary engine.   In the Legacy Engine, however, the tip seal must provide separation between the combustion and compression volumes in adjacent sections of the housing lobe, which are divided by the roton tip. A study of the Wankel engine apex seal design results in establishing a design starting point, but design differences between the two engines require a completely new approach.

Perhaps the major barrier to the seal design is that the seal is not in contact with the housing during the complete rotational cycle.   The seal alternates between being in contact with the housing, non-contacting, and in contact with the rotor, as shown in the figures on the right from top to bottom.   Designing a seal configuration that will allow for breaking and making contact with the housing and the rotor will prove to be a very difficult design challenge.   When not in contact, the seal will extend from the roton because there is no force from a contact surface to hold the seal in the slot.   If the seal extends from the roton after separating from the housing and then reestablishes contact with the rotor, there will be interference with the rotor cavity in the amount of the extended length.   Compromises to accommodate this extension involving a radius relief of hard corners represent leakage paths, and must be used sparingly.

The engine geometry requires that the roton have concave sides as opposed to the convex sides of the Wankel rotor.   This difference limits the possibility of fitting side seals, tip seals, and corner seals in this application.   The unique operating cycle requires that the tip seal operate against the combustion pressure for one-half of the cycle and the reversed compression pressure for the second half of the cycle.   This adds complication to the seal design because the pressure will be very high on one side of the seal during the first stages of expansion (while the combustion gas pressure is very high), and on the other side of the seal during the latter stages of compression (while the compression gas pressure is very high).  This "bilateral loading" illustrated in the adjacent figure would cause the Wankel-type seal to "wobble" in the slot and cause considerable additional wear if a design similar to the Wankel seal approach is used in this engine.

Since the lubrication system should be isolated from the combustion and compression volumes, the seal configuration and lubrication system design represents a very difficult design task. Mazda experienced this problem during the development of the single side loading of the seals in the Wankel engine, and in fact this was perhaps the main obstacle to the success of the Mazda rotary engine for more than 50 years.   The design of the Mazda rotary engine provides for a small oil injection port for apex seal lubrication.   This allows for a small quantity of oil consumption that is accepted as reasonable for that engine.   The Legacy Engine design will incorporate materials known to exhibit very low friction coefficients and that will operate reliably with minimum lubrication requirements.   It is true that the diesel fuel provides improved lubrication characteristics over gasoline engines, which represents an advantage over the gasoline burning Wankel engine.

Side seals for both the roton and the housing complete the chamber sealing requirements.   These seals are similar in requirement and configuration, differing only in length and curvature.   Both seals experience pressure from only one side during both the combustion cycle and the compression cycle.   The current design uses a wave spring and a rectangular cross-section wire very similar to the side seal in the Wankel engine.   The design of the side seals is not anticipated to represent a severe problem.   There remain the questions regarding the optimum seal width and seal pressure against the mating surface (spring design parameter), but these will not represent severe design challenges to determine and mechanize.

Design considerations for the seal ends remain unsolved. There are leakage paths at the ends of both the tip seals and the side seals corresponding roughly to the gap in the circumference of a conventional piston ring.   The Wankel engine uses a "corner seal" consisting of a short round plug slotted to accept the apex seal, but the problem is similar in both engines.   There has been limited effort in the determination of the requirement for a similar corner seal or for a practical configuration for that seal.   The Wankel engine uses a three piece apex seal assembly backed by two springs to maintain seal length.   The side seals in the Wankel engine extend to the perimeter of the corner seals, but necessarily leave a small gap between the side seal and the corner seal. The Legacy Engine does not have a corner seal, but the side seal end points extend farther toward the housing surface.   One critical difference between the two engines, and in favor of the Legacy Engine, is that the Wankel is a four-stroke engine, and leakage past the seals enters the exhaust volume.   This means that the energy contained in the compressed gas is lost and any fuel in the gas mixture is lost to the exhaust gas stream.   In the Legacy Engine, any leakage past the seals enters the next compression volume. This means that compressed gas energy would be lost only during the first half of the cycle and that fuel in the compressed gas mixture will be compressed additionally for the following combustion cycle.   Fully combusted gas passing into the next compression volume represents a small amount of Exhaust Gas Recirculation (EGR), which is quite desirable, but of course it is not cooled, except by expansion.