Overcoming The Rotary Engine’s Biggest Flaw

From New Mind.

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The inspiration for the Wankel rotary engine is derived from the geometric principle that when a circle is rolled on its own circumference along another circle that has double the radius, a curve known as an Epitrochoid is created. This curve forms the shape of the inner walls of the rotor housing. The rotor housing hosts all stages of the rotary engine’s combustion cycle, much like a cylinder in a conventional engine.

In order to keep compression in the chamber of a Wankel engine, the three tips of the rotor must form gas-tight seals against the inner walls of the rotor housing. This is accomplished by seals at the three apexes of the triangle, known as apex seals. These seals are usually made of metal and are pushed against the wall housing by springs. Since the seals are in contact with the housing’s inner case, in order to reduce friction they’re covered in engine oil. Because the exposure of engine oil to the combustion process, a rotary engine burns oil by design. The amount of oil used is metered by a throttle-controlled metering pump.

The three apexes of the triangular-shaped rotor move uniformly along the inside walls of the rotor housing, dividing the cavity between the rotor and the interior walls of the housing into three continually changing regions of volume. Because of the unique configuration of a rotary engine, they’re classified as variable-volume progressing-cavity systems. Each rotor has three faces and each face has three cavities of volume per housing. In effect, each face of the rotor «sweeps» its own volume as the rotor moves in an eccentric orbit within the housing.

Each side of the rotor is brought closer to and then further away from the wall of the internal housing, compressing and expanding the combustion chamber. A rotor is effectively akin to a piston.

Starting in the early 1960s, Mazda has released a slew of unique, Wankel rotary powered models such the Cosmo, RX-3 and three generations of the Mazda RX-7. The iconic history of Mazda and the evolution of the Wankel rotary engine began with a joint study contract between Mazda and the German car firm NSU. Which came equipped with a water-cooled single-rotor engine and standard front disc brakes, which differentiated it from other similar cars of the period. Early cars required an engine rebuild only after 50,000 kilometers or 31,000 miles. Many of these failures were attributed to poorly designed apex seal tips, a common weak point later realized in rotary engines.

Since the seals are in contact with the housing’s inner case, in order to reduce friction they’re covered in engine oil. Because the exposure of engine oil to the combustion process, a rotary engine burns oil by design. Because of the direct contact of apex seal, the biggest obstacle engineers faced in initial designs were the chatter marks on the rotor housing’s sliding surfaces. To an extent, these carbon seals were self-lubricating, addressing the issues facing the rotor housing wall surface.

They were also used in conjunction with an aluminum rotor housing, in which the walls were chrome-plated for durability. What made this possible was the new porous chrome plating on the interior walls of the rotor housing. Ths surface finish of this plating improved the effectiveness of the lubrication between the apex seal and the rotor.

From 1975 -1980 it was discovered that the current apex seal version was subjected to high thermal and centrifugal loads during high RPM operation and under periods of high engine load. To rectify this issue, Mazda implemented a slight crown of . This additional crowning compensated for the rotor housing’s slight deformation under high loads, ensuring sufficient contact with the rotor housing walls. Mazda also improved the corner pieces by incorporating a spring design to keep the clearance of the rotor groove at a minimum.

By the early 1980s, further refinements by Mazda led to the adoption a top-cut design that extended the main seal. The purpose was to reduce gas leakages at one end of the apex seal, where it would segment into two pieces. From 1985 to 2002, the apex seal had been further reduced in size to 2mm. Additionally, Mazda filled the center cavity of the spring corners with a heat-resistant rubber epoxy, adding additional sealing properties.

This latest iteration of the apex seal design was used in Mazda’s iconic high output, low weight twin turbocharged 13B-REW engine. Made famous by the 3d generation RX-7, it was used until the engine was finally dropped from production and replaced with the Renesis engine which used its own apex seal design. The apex seal in the Renesis engine was now a two-piece design made from cast iron with a low carbon content.