A Wankel rotary engine is an internal combustion engine that uses a triangular rotor orbiting inside an epitrochoid-shaped housing instead of reciprocating pistons. The rotor's three faces simultaneously perform intake, compression, combustion, and exhaust as the rotor turns on an eccentric shaft. Felix Wankel designed it to eliminate the reciprocating mass, valve train, and vibration of a piston engine. The result is a compact, smooth, high-RPM powerplant — the Mazda 13B-REW in the FD RX-7 made 280 PS from just 1.3 L of rotor displacement.
Wankel Rotary Engine Interactive Calculator
Vary shaft rotation time and rotor count to see rotor speed, timing ratio, and power-event rate.
Equation Used
The worked example states the Wankel speed ratio: the eccentric shaft completes 3 rotations while the rotor completes 1. Therefore rotor period is three times shaft period, rotor rpm is one-third shaft rpm, and a single rotor produces one power event per shaft revolution.
- Rotor turns once for every three eccentric-shaft revolutions.
- One power stroke occurs per shaft revolution per rotor.
- Ideal port timing and no combustion losses are assumed.
How the Wankel Rotary Engine Works
The geometry is what makes the Wankel work. The housing bore is not a circle — it is an epitrochoid, a two-lobed curve that looks like a fat figure-8 with rounded ends. Inside that bore sits a triangular rotor with slightly curved faces. As the eccentric shaft turns, the rotor orbits and rotates at one-third the shaft speed, and each of the three rotor faces traces around the trochoid bore creating three working chambers that constantly change volume. One face is taking in fresh charge through the intake port while another is compressing, another is firing, and the geometry guarantees a power stroke on every shaft revolution per rotor — three power events per rotor revolution, or one per shaft revolution. No valves. No camshaft. No reciprocating piston. Just gas-flow timing dictated by the rotor sweeping past fixed ports in the housing wall or end plates.
Why build it this way? Because reciprocating mass is the enemy of high RPM and smoothness. A piston has to stop and reverse twice per revolution, which limits redline and generates the secondary vibration that makes inline-fours buzz. The Wankel's rotor only orbits — it never stops — so a 13B-REW happily spins to 9,000 RPM and a race-prepped 4-rotor will see 11,000+ without distress. The trade-off is sealing. The three apex seals on the rotor tips must maintain line contact against the trochoid wall through every chamber, and the side seals on the rotor faces must seal against the end plates. If apex seal tip clearance opens past about 0.13 mm, compression bleeds between chambers, idle goes lumpy, and cold starts get hard. If the trochoid surface chrome plating wears through — a classic high-mileage 12A and 13B failure — apex seals chatter, score the housing, and the engine eats itself.
Lubrication adds another wrinkle. Because the apex seals run directly on the trochoid bore, the oil metering pump injects 2-stroke-style oil into the intake charge to lubricate the seal/housing interface. Disable that pump or run it dry and the apex seals weld to the bore in minutes. Coolant temperature swings also matter — the iron rotor housings and aluminium end plates expand at different rates, so chronic overheating warps the side housings, kills side-seal contact, and you lose compression on the leading face first.
Key Components
- Rotor: The triangular rotor with curved Reuleaux-style faces. In a Mazda 13B it weighs around 5.5 kg per rotor and carries the apex seals, side seals, corner seals, and an internal ring gear that meshes with the stationary timing gear at a 3:2 ratio, forcing the rotor to turn once for every three eccentric-shaft revolutions.
- Epitrochoid Housing: The two-lobed bore the rotor orbits inside. The trochoid surface is typically electroplated with hard chrome or coated with a Nikasil-type running surface to a thickness of roughly 80–120 µm. Surface finish below Ra 0.4 µm is what keeps apex seal wear in spec over a 100,000 km service life.
- Eccentric Shaft: The Wankel's equivalent of a crankshaft. The rotor rides on an eccentric lobe with a typical eccentricity of 15 mm in a 13B, converting the rotor's orbiting motion into pure rotation at 3× the rotor speed. Bearing clearance on the eccentric journals must hold 0.04–0.07 mm or oil pressure drops at idle.
- Apex Seals: Three spring-loaded strips at the rotor tips that maintain line contact against the trochoid wall. Modern 2-piece carbon-composite apex seals tolerate roughly 7,000 RPM continuous; race-spec ceramic or steel seals push past 9,000 RPM but accelerate housing wear.
- Side and Corner Seals: Side seals are thin strips around the perimeter of each rotor face sealing against the end plates; corner seals are small cylindrical pins that connect the apex seals to the side seals at each rotor corner. If any one corner seal sticks in its bore, that chamber loses compression and the engine runs on 'one and a half' rotors.
- Intake and Exhaust Ports: Cut directly into the rotor housing (peripheral port) or the side end plates (side port). Port timing is fixed by geometry — there is no cam to adjust — so the only way to change overlap is to grind the port shape. A bridge port adds about 15–20% peak power but kills idle quality.
- Oil Metering Pump: A small mechanical injection pump that meters engine oil into the intake stream at roughly 1 cc per minute at idle, scaling with RPM and throttle. Without it, apex seals run dry and gall the housing within minutes of startup.
Real-World Applications of the Wankel Rotary Engine
Wankels never displaced piston engines in mainstream automotive because they drink fuel and oil and struggle with modern emissions, but their power-to-weight ratio and smoothness keep them alive in niche applications where a compact, vibration-free, high-RPM powerplant matters more than fuel economy. You see them in sports cars, aircraft, range-extenders, and unmanned platforms.
- Automotive — sports cars: Mazda 13B-REW in the FD RX-7 (1992–2002) and the Renesis 13B-MSP in the RX-8, the only mass-production rotary cars.
- Light aircraft and UAV: Austro Engine AE50R and the Rotron RT600 used in long-endurance UAVs where vibration would otherwise crack airframe avionics mounts.
- Range-extender hybrids: Mazda MX-30 R-EV uses an 830 cc single-rotor 8C as an electrical range extender, exploiting the rotary's compact packaging to fit transversely beside the e-motor.
- Motorcycles: Suzuki RE5 (1974–1976) and the Norton Classic and Commander police bikes, which used air-cooled twin-rotor Wankels.
- Snowmobiles and personal watercraft: Arctic Cat used the Sachs KM914 single-rotor in production sleds; experimental Yamaha and Bombardier prototypes ran twin-rotor units.
- Stationary and auxiliary power: KKM 502-derived APUs developed by Wankel SuperTec for hybrid drivetrains and gensets where low frontal area matters.
The Formula Behind the Wankel Rotary Engine
The single most useful Wankel calculation is figuring out the engine's effective displacement and then using it to estimate brake power at a given speed. Effective displacement matters because Wankel chamber displacement Vc is the volume swept by one rotor face — but each rotor produces three power events per rotor revolution, and the eccentric shaft turns three times per rotor revolution, so the engine produces one power event per shaft revolution per rotor. That makes a Wankel behave more like a 2-stroke than a 4-stroke for displacement-equivalence purposes. At the low end of the practical street operating range — say 2,000 RPM in a 13B — the engine is making barely 30 kW and the rotor housings are still warming up. At the nominal 6,000 RPM cruise/power band the 13B sits in its sweet spot, with apex seal velocity, port flow, and combustion chamber filling all aligned. Push to 9,000 RPM redline and you get peak power, but apex seal tip loading climbs with the square of speed, and time-to-overhaul shortens dramatically.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Pb | Brake power at the eccentric shaft | W | hp |
| Vc | Single-face chamber displacement (per rotor face per revolution of rotor) | m³ | in³ |
| nr | Number of rotors | — | — |
| N | Eccentric shaft speed | rev/s | RPM |
| pme | Brake mean effective pressure (BMEP) | Pa | psi |
Worked Example: Wankel Rotary Engine in a Mazda 13B-REW twin-rotor swap into a kit aircraft
You are estimating the brake power available from a Mazda 13B-REW twin-rotor turbocharged Wankel that you plan to install in a 2-seat experimental kit aircraft as a direct-drive prop engine through a 2.85:1 PSRU. Each rotor has a single-face chamber displacement Vc of 654 cc (0.000654 m³), there are 2 rotors, and you want to know power output at 2,000 RPM (taxi/idle), 6,500 RPM (cruise), and 9,000 RPM (redline takeoff). Take BMEP at a healthy turbo Wankel as roughly 950 kPa cruising and 1,300 kPa peak.
Given
- Vc = 0.000654 m³
- nr = 2 rotors
- Ncruise = 6500 RPM
- pme,cruise = 950000 Pa
- pme,peak = 1300000 Pa
Solution
Step 1 — convert nominal cruise speed to rev/s. The factor of 2 in the formula accounts for one power event per shaft revolution per rotor (a Wankel fires every shaft turn per rotor, like a 2-stroke):
Step 2 — compute brake power at nominal 6,500 RPM cruise with cruise BMEP of 950 kPa:
That is the sweet spot — the 13B is breathing cleanly, apex seal velocity is around 18 m/s, intercooler load is manageable, and oil temperature stabilises in the 95–105 °C window the iron rotor housings prefer.
Step 3 — at the low end of the operating range, 2,000 RPM taxi:
BMEP drops to roughly 600 kPa here because turbo boost has collapsed and port overlap hurts low-speed scavenging — that's barely enough to taxi a loaded experimental, and the engine feels lumpy because corner seals haven't reached their thermal seating temperature.
Step 4 — at the high end, 9,000 RPM redline takeoff with peak BMEP 1,300 kPa:
This is the published 13B-REW peak figure territory, but you cannot live there. Apex seal tip loading scales with N², so seal life at 9,000 RPM continuous is measured in tens of hours, not thousands. The certified Austro and Wankel SuperTec aviation rotaries cap continuous operation around 7,500–8,000 RPM for exactly this reason.
Result
At nominal 6,500 RPM cruise the 13B-REW produces about 134. 6 kW (180 hp) at the eccentric shaft — comfortably enough to cruise a 2-seat experimental at 140 knots through a 2.85:1 PSRU. The low end (2,000 RPM, 26 kW) feels weak and lumpy because corner seals haven't seated thermally and turbo boost has dropped out, while the 9,000 RPM redline figure of 255 kW is real but not sustainable — apex seal tip loading scales with the square of RPM and TBO collapses. If your dyno number comes in 15–20% below the predicted cruise figure, suspect three things first: a leaking oil-cooler thermostat letting oil run cold and thickening apex seal drag, a worn or stuck oil metering pump nipple causing localised seal scuffing on one rotor, or an exhaust port leak at the rotor housing-to-end-plate joint where the gasket distortion drops effective expansion ratio.
Wankel Rotary Engine vs Alternatives
Wankels solve a specific engineering problem — power density and smoothness in a small package — but they pay for it in fuel economy, emissions, and apex seal life. Compare against a turbocharged inline-4 piston engine and a 2-stroke piston engine on the dimensions that actually matter to a builder choosing a powerplant.
| Property | Wankel Rotary (13B-REW) | Turbo Inline-4 Piston (4-stroke) | Two-Stroke Piston |
|---|---|---|---|
| Peak RPM (production) | 8,500–9,000 RPM | 6,500–8,500 RPM | 8,000–13,000 RPM |
| Power per litre of effective displacement | ~100 kW/L turbo | ~75–110 kW/L turbo | ~80–100 kW/L |
| Vibration (primary + secondary) | Near-zero, fully balanced | Secondary imbalance present | Significant |
| BSFC at cruise | 280–320 g/kWh (poor) | 210–240 g/kWh (good) | 300–400 g/kWh (poor) |
| Time between overhauls | 80,000–150,000 km / ~1,500 hrs | 200,000–400,000 km / ~3,000+ hrs | 20,000–60,000 km |
| Cold-start emissions (HC) | High — port overlap and oil injection | Low with modern cat | Very high |
| Frontal area / installed volume | Smallest for given output | Largest | Compact |
| Cost to overhaul | Apex seals + housings: $3,000–6,000 | Rings, bearings, valves: $2,000–4,000 | Pistons + rings: $500–1,500 |
Frequently Asked Questions About Wankel Rotary Engine
The front rotor housing runs hotter than the rear because coolant flow enters at the front and exits at the rear in a Mazda series-flow cooling layout, but more critically the front rotor sees less oil from the metering pump on a worn OMP because the front feed line is the longer run and bleeds back to tank during shutdown. Apex seals on the front rotor consequently see more dry-start cycles.
Diagnostic check: pull the leading and trailing spark plugs from the front rotor and compare to the rear set. If the front plugs are dry and grey while the rear are slightly oily-tan, your OMP front feed is starved. Convert to premix as a stopgap and rebuild the OMP.
This is a regulatory artefact, not engineering reality. SAE and most motorsport bodies historically rated a Wankel by single-face chamber displacement times the number of rotors (so a 13B is 654 cc × 2 = 1,308 cc), ignoring that each rotor completes three power strokes per rotor revolution. Because the eccentric shaft turns 3× per rotor revolution, the engine actually produces one power event per shaft revolution per rotor — equivalent to a 2-stroke of double the rated displacement.
For honest power-density comparisons against 4-stroke pistons, multiply the rated displacement by 2. A 13B compares to a 2.6 L 4-stroke piston engine, which is why it makes 280 PS naturally and why FIA equivalency factors for rotaries sit around 1.8–2.0×.
Peripheral ports cut intake and exhaust openings directly through the trochoid wall, which gives you huge port area and much longer effective duration — peak power climbs 20–40% over a stock side-port engine. The cost is that the apex seal sweeps directly across the open port edge twice per revolution, which chips seals and roughens the trochoid chrome. Idle is also unusable below about 1,500 RPM because port overlap dumps fresh charge straight out the exhaust.
Side ports cut openings in the end housings instead, so the apex seal never crosses a port edge — only the side seals do, and they're far less stressed. Pick peripheral if it's a dedicated race engine that lives above 4,000 RPM and gets torn down annually. Pick side or bridge if it has to idle, pass tech, or last a season.
Almost always intake runner length or exhaust resonance, not the engine itself. Wankel ports are fixed-geometry — you cannot change duration with a cam — so all your tuning happens in the manifolds. A stock 13B intake runner is tuned for a torque peak around 5,500–6,500 RPM, and above 7,000 the standing-wave timing goes out of phase and volumetric efficiency collapses 10–15%.
Quick check: log manifold pressure against RPM. If MAP rises smoothly to 7,000 then plateaus or dips while throttle stays wide open, it's a runner-length problem. Shortening intake runners by 30–50 mm typically moves the peak up by 800–1,200 RPM. The other suspect is exhaust — a too-small turbine A/R or a collapsed cat will choke a Wankel hard above 7,000 because exhaust mass flow is enormous on a rotary.
Alcohol fuels wash oil off the trochoid surface much more aggressively than gasoline because they carry more liquid fuel into the chamber per unit energy and have higher latent heat of vaporisation. The standard OMP curve is calibrated for gasoline; on E85 you want roughly 1.4–1.6× stock OMP delivery, and on M85/M100 closer to 1.8–2.0×.
The rule of thumb most rotary builders use: target 1 part oil per 150–180 parts fuel by volume on E85, versus the 1:300 the stock OMP delivers on pump gas. The cheap insurance is to add premix to the fuel tank at 1:200 on top of the OMP, which guarantees seal lubrication even if the OMP injector nozzles partially clog — a common failure on alcohol fuels because they dissolve varnish that then plugs the nozzles downstream.
Two reasons. First, the Wankel's expansion ratio is geometrically tied to its compression ratio and ends up around 9:1 in most production rotaries, versus 12–14:1 effective expansion in a modern piston engine. That means combustion gases are still hot — typically 950–1,050 °C EGT under load — when the exhaust port opens, versus 750–850 °C on a comparable piston engine.
Second, the long, narrow combustion chamber (the rotor face is shaped like a shallow bathtub) burns slowly, so combustion is still completing as the chamber moves toward the exhaust port. This is also why Wankels are so emissions-hostile and why they need oversized turbos, oversized exhausts, and serious header insulation — under-bonnet temperatures will cook nearby wiring and brake fluid lines if you don't shield aggressively.
References & Further Reading
- Wikipedia contributors. Wankel engine. Wikipedia
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