Overcoming The Rotary Engine’s Biggest Flaw
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Published 2023-06-24
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.
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All Comments (21)
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I would argue that the apex seal was totally resolved by Mazda and this was clearly not the biggest problem related to this engine. The large combustion chamber surface area with corners that prematurely quenched the combustion flame front resulting in high HC emissions was a far more fundamental problem than simply could not be resolved.
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Good video but I think it's missing some of the story though. Apex seals have gotten improved significantly by rotary shops after Mazda abandoned the rotary engine. Some rotary builders don't even consider them the most common source of the major issues with the engine.
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As a previous owner of more than one rotary powered vehicle ( A 12a and a Renesis ), I would argue that these days the biggest flaw with Wankel Rotary is that combustion is still occurring during the exhaust cycle. This results in poor fuel economy and really hot exhaust manifolds. If you've ever watched an underhood camera in a race rotary engine, you can see that the exhaust manifold is absolutely white hot. Edited for spelling
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The materials science in the apex seals is incredible
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A small correction: the Apex seal does not move uniformly across the epitrochoid, it instead moves at a variable speed throughout the sweep of the rotor, further complicating the lubrication and sealing issues.
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I've always thought that a reliable wankel engine would be a great boon to aviation because of the low weight of these engines. But "reliable wankel engine" is an oxymoron. My beloved RX-8 made it to 70k miles before I needed an engine. These seals are a fascinating engineering problem; thanks for a nice video!
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The biggest flaw is surface to volume ratio of the combustion chamber. This results in excess heat loss of the burning charge compared to a reciprocating piston engine. There is no cure for this basic geometric flaw. That's it, done, over, no recourse for the Wankel. I've followed this design from it's beginnings and also followed the history of NSU. NSU had some if the best engineers in the world and produced some world beating designs. These would include the VW Golf and the beginings of all modern Audis.
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Experimental aviation has many companies that take stock RX7 engines, and ceramic coat the inside and convert them to aircraft engines. They are perfect aircraft engines. Light weight, dual ignition built-in, minimum parts count, runs on regular unleaded gas, can get you home even if you've lost all oil pressure. This happened to a pilot. After landing they took the engine apart to find ceramic coatings had no wear whatsoever.
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NSU Wankel engines were reliable for most of the 10 years they were in production - only the first year of production had problems and it was mostly a reputation problem from then on. Also ignorance was a problem - a lot of engines replaced under warranty were not faulty because mechanics had no idea what they were doing…. NSUs approach to materials was equally interesting. They used cast iron apex seals and developed in partnership with BASF a silicon surface coating for aluminium called Nikasil. These coating is still commonly used as a bore coating in piston engines. It’s very popular in 2 strokes, as well BMWs for example.
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Great explanation! For years, the extreme challenges facing apex seals have intrigued me. Thanks!
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Would love a video about the new rotor-charged hybrid. The new engine uses exhaust recirculation and direct injection to increase efficiency by up to 25%, helping the other downside of rotary engines as far as mass consumer use. Pretty interesting, especially if you can get more info from Mazda itd be interesting to see a vid. Now if they develop a better AT, combined with Skyactiv and their other tech, they could have the best MPG and efficient stuff out there without a CVT, let alone their fun higher power Turbo CUVs and hatchbacks (or tiny hatch-cross-things, like the CX-30 lol) edit: go figure you cover the MX-30 at the end a bit lol, would still love a vid on the in depth stuff. They even pass strict emissions standards with the little engine now which probably wouldnt have before.
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Design Ideas has perfected the apex seal. These seals work at boost pressures not seen since the F1 turbo era of the 80's. They are not cheap, but worth it if you are trying to make real power with a rotary. Reliability is there too. I've heard of Design Ideas seals being so durable that they are not changed when rebuilding the engine-- and that is in drag racing applications!
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Fantastic video about the apex seals - thank you!
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in addition to the apex seal probs the side seals would stick due to carbon buildup from running at light loads, idling in traffic etc. this lead to blow by causing loss of compression and oil leakage causing the plugs to oil up, making cold starts sometimes impossible without changing or cleaning the plugs. most rotary mechanics came to realize that a poor running engine just needed a good 10 minute blast down the freeway and it was good as new.
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Pls review next the liquid piston engine!
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What seals the sides of the rotor and the walls of the housing? Is this area larger than on an equivalent piston engine?
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Liquid piston design is the rotary done right, seals are stationary and both the rotor and housing are oval - no edges. It has pre-chamber and CR of ~18:1
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The Wankel KKM is an evolutionary dead-end concept, it was a failure because it offers no advantages over reciprocating engines and has poor durability and reliability..
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The mechanic said, “You blew a seal.”. And the penguin said, “No, that is just ice cream.”…
