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Home How a Rotary Engine Works

No pistons, one spinning triangle

How a rotary engine works

Most car engines push pistons up and down. A rotary engine has none: a three-sided rotor spins inside a housing shaped like a fat figure of eight. Its three faces make three chambers that each draw in air and fuel, squeeze it, burn it and push it out, again and again.

Turn the rotor
3
chambers round each rotor
3:1
shaft turns to rotor turns
1
power stroke each shaft turn, per rotor
0
pistons, valves or camshafts
A twin-rotor rotary engine on a red cradle in a dark workshop, its front cut away: the grey triangular rotor inside the aluminium housing, a chamber of blue air and fuel on the left, a chamber glowing orange just after the spark on the lower right, the intake and exhaust above, the spark plug leads below.
A twin-rotor engine cut open at the front: one chamber filling with air and fuel, one burning.

Video

How a Rotary Engine Works - The Wankel Engine

Watch on YouTube

3D model · cut open · 360°

Start it, cut it, take it apart

A twin-rotor engine, cut open at the front. Open the throttle and watch the rotor turn, the shaft spin three times as fast, and each chamber change colour through its four strokes. Drag to turn the view, scroll or pinch to zoom, and click a part.

Try itPress Start, then open the throttle. The rotor turns once for every three turns of the shaft.
Loading the 3D model…

Meet the parts

Sixteen parts, one twin-rotor engine

The whole engine laid out on the workshop floor: the housings across the back, in order from front to rear; the gears, the two rotors, the eccentric shaft and the flywheel in the middle; and in front, the systems that feed it, fire it, oil it, cool it and let it breathe out. Tap any part or pick a name below.

The parts of a twin-rotor rotary engine laid out on a workshop floor: five housings in a row at the back (red side housings and aluminium rotor housings), the two triangular rotors, two small fixed gears, the eccentric shaft and the flywheel in the middle, and the intake, the fuel rail with its injectors, the ignition coils, the oil pumps, the water pump and the exhaust manifold in front.

Hover or tap the picture. Amber rings mark every part.

Parts diagram

From the intake to the exhaust

The front rotor seen face-on, with the front of the engine cut away. One chamber is filling with air and fuel on the left, one has just been fired at the bottom right, and the third is pushing out its exhaust at the top. The numbers follow the flow: in through the intake, round with the rotor, the spark, out through the exhaust, then the gears and the shaft that turn it all, and the housing.

The front rotor of a rotary engine cut open face-on: the intake runner and fuel injector at the top left, the exhaust stub at the top right, the triangular rotor with its apex and side seals, the internal gear round the fixed gear, the eccentric lobe and shaft in the middle, the spark plugs below, the ignition coils on the left, and the coolant passages in the aluminium housing, with seventeen numbered parts.

Blue: air and fuel coming in. Orange: the mixture just after the spark. Grey: burnt gas on its way out.

    How it works, step by step

    Eighteen steps round the triangle

    The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.

      The front of the engine cut away: the three-sided rotor inside its housing.
      Step 1 of 18

      No pistons, one spinning triangle

        Demo · the heart of it

        Turn the rotor

        Drag the rotor round, or press Run. Follow one face through its four strokes, watch its chamber grow and shrink twice in every turn, and count: the shaft goes round three times for each turn of the rotor.

        The rotor’s three tips never leave the wall, so its faces always make three separate chambers. Each chamber is smallest at a waist of the figure of eight and largest at a lobe. On its way round it passes the intake port, the plugs and the exhaust port, so it does all four strokes in one turn of the rotor.

        • Blue: air and fuel coming in, then being squeezed (paler).
        • Orange: burning, pushing the rotor round.
        • Grey: burnt gas being pushed out.
        –this face’s stroke
        –its chamber, cc
        –shaft turns · rotor turns
        –its chamber is open to

        Demo · how a push becomes a turn

        Push on the lobe

        The burning gas pushes on the rotor’s face. Whatever the face’s shape, that push always points straight through the rotor’s centre, and the centre sits on the lobe, 15 mm off the shaft’s axis. So the push turns the shaft like a hand on a crank.

        –push on the face
        –lever arm about the shaft
        –torque from this face
        –all three faces together

        Demo · what if a seal wears?

        Wear out an apex seal

        Each rotor tip carries a thin steel apex seal, pressed against the wall by a spring and by the gas behind it. Wear one out and the gas leaks past it into the next chamber. Run a compression test and see which faces it hurts.

        A rotary’s compression test reads every face: three pulses for each turn of the rotor. Mechanics crank the engine with the starter and compare the readings at about 250 rpm, because a leak loses more the more time it has. At running speed the same leak costs less, but it still costs power.

        • Red: high pressure. Blue: low.
        • Amber jets: gas leaking past a worn seal.
        –face A, peak
        –face B, peak
        –face C, peak
        –power, running

        Demo · one rotor, two rotors, four cylinders

        Power pulses

        Every face gives one power stroke per turn of the rotor, so one rotor gives a push every turn of the shaft. Two rotors half a turn apart give two, as many as a four-cylinder piston engine, but each push lasts three quarters of a turn instead of half. See how smooth the turning force is, and how much the flywheel has to do.

        –power strokes per shaft turn
        –each push lasts, in degrees of the shaft
        –turning force: peak ÷ average
        –speed ripple at 3,000 rpm

        More to know

        Kinds, history and care

        Six ways to build a rotary

        One rotorSingle rotorThe simplest: one rotor on one shaft. Mazda’s MX-30 e-Skyactiv R-EV of 2023 uses a single 830 cc rotor to drive a generator that charges its battery.
        Two rotorsTwin rotorThe engine on this page: two rotors half a turn apart, with the proportions of Mazda’s 13B, 654 cc a chamber and 1.3 litres in all. It powered the RX-7 and, as the Renesis, the RX-8.
        Three rotorsMazda 20BA three-rotor engine, used in only one production car: the Eunos Cosmo, built from 1990 to 1995.
        Four rotorsMazda R26BFour rotors and 2.6 litres. Held to about 700 hp at 9,000 rpm to last the distance, it won the 24 Hours of Le Mans in the 787B in 1991.
        Side portsRenesisThe ports on this page are cut through the rotor housing, so the rotor’s tips open and close them. Most production 13Bs took their air in through the side housings instead, where the rotor’s flanks open and close the ports, and in the RX-8’s Renesis the exhaust ports moved there too.
        Where it beganA turning housingFelix Wankel’s first running engine, the DKM 54, spun its housing as well as its rotor. The KKM that followed held the housing still, as almost every rotary since has done.

        From a workshop idea to Le Mans

        1. Felix Wankel, a self-taught German engineer, starts work on rotary engines. His first patents follow.
        2. The DKM 54 runs for the first time at NSU. Both its rotor and its housing turn.
        3. The KKM 125 runs: a still housing and a rotor on an eccentric shaft, the layout of the engine on this page.
        4. Mazda and NSU sign a contract to develop the Wankel engine together.
        5. The NSU Spider, the first car on sale with a Wankel engine. By July 1967, 2,375 are built.
        6. Mazda’s Cosmo Sport goes into full production in May, and NSU launches the Ro 80, voted Car of the Year for 1968.
        7. The Mazda RX-7 arrives in March, in Japan.
        8. A Mazda 787B with a four-rotor engine wins the 24 Hours of Le Mans: still the only winner without pistons.
        9. The RX-8, with the Renesis engine and its side exhaust ports.
        10. The last RX-8 is built.
        11. The MX-30 e-Skyactiv R-EV brings a single-rotor engine back, as a generator.

        Keeping a rotary healthy

        • Check the oil often. A rotary is meant to use a little: the metering pump feeds oil to the apex seals, and it burns with the fuel.
        • Warm it up before you switch it off. Started from cold and stopped straight away, the extra cold-start fuel can flood the plugs. RX-8 owners know this one well.
        • Test compression face by face, with a tester made for rotaries. It gives three readings for each rotor, compared at about 250 rpm of cranking.
        • Change the spark plugs as a set. Each rotor has a leading and a trailing plug, and on many engines, the RX-8’s among them, the two are different types.
        • Look after the cooling. The fuel always burns on the same side of the housing, so that side runs hot all the time. Keep the coolant topped up and the radiator clear.
        • Drive it properly once it is warm. Like any engine, it is happiest at a steady working temperature.

        Things worth knowing

        Three times as fastThe shaft turns three times for every turn of the rotor. The fixed gear has 34 teeth and the ring inside the rotor 51: two teeth for every three.
        One push per shaft turnEach face fires once per turn of the rotor. Three faces make three pushes per rotor turn, which is one for every turn of the shaft.
        Three quarters of a turnEach stroke takes a quarter of the rotor’s turn, which is three quarters of a turn of the shaft. A piston engine’s strokes take half a turn each.
        The gears don’t do the pushingThe gas pushes on a face straight through the rotor’s centre, so the push goes into the lobe. The gears only keep the rotor in step and carry little load.
        How big is a 13B?Its chambers each sweep 654 cc, so it is called 1.3 litres. But every turn of its shaft it breathes as much as a 2.6 litre four-stroke piston engine, which is why racing rules argue about it.
        Chatter marksEarly rotary engines wore chatter marks into the housing wall as their apex seals vibrated. Mazda’s cure was lighter seals: its first production engines used aluminium-impregnated carbon.
        Quick check

        Four questions