How a Rotary Engine Works: No Pistons, Apex Seals and Burning Oil (Redlined 101, Ep 19)

Your engine slams every piston up and down a hundred times a second. This one has no pistons. It just spins, and it out-revs your car. But rotary guys won't tell you the catch. Here's how a rotary works, what makes it scream, and what it costs you. (For how the piston layouts compare, see V6 vs V8 vs inline-six.)

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Your engine: up, stop, down, stop

That hundred-times-a-second figure isn't an exaggeration. At 6,000 rpm, the crank turns a hundred times every second, and every piston goes up and down once per turn. Each time, it stops dead and reverses. So do the connecting rods, the valves and the valve springs. All of that stopping and starting is what limits how fast a piston engine can safely spin.

A rotary doesn't do any of it.

How it works: a spinning triangle

Inside a rotary, a triangle-shaped rotor spins around a shaft, inside a housing shaped a bit like a pinched oval. The shaft isn't straight: it has an offset lobe, which is why it's called the eccentric shaft. As the rotor orbits around that lobe, its three tips stay in contact with the housing wall the whole time.

Each tip seals off a pocket, the space between one face of the rotor and the housing. As the rotor turns, each pocket grows and shrinks, and that's how it breathes. Every pocket goes through the same four jobs a piston engine does, in order:

1. Suck: the pocket grows and draws in air and fuel.
2. Squeeze: the pocket shrinks and compresses it.
3. Bang: the spark plug fires and the burning mix pushes on the rotor face.
4. Blow: the pocket shrinks again and pushes the exhaust out.

Three pockets, all firing in turn. While one is sucking, another is squeezing and another is banging. No valves either: the rotor itself uncovers the intake and exhaust ports as it passes them.

The timing works out neatly. The rotor turns at one third the speed of the shaft, so the shaft spins three times for every one turn of the rotor. In that one rotor turn, each of its three faces fires once. Three firings in three shaft turns means every rotor fires once per shaft turn, and a two-rotor engine gets two power pulses every turn, overlapping smoothly. Compare a four-cylinder piston engine, which also gets two per turn, but from pistons slamming back and forth to make them.

Guess how many moving parts

So guess how many moving parts it has.

Three. Two rotors and one shaft. That's a two-rotor engine like Mazda's, counting the main rotating parts (the seals and the oil pump aside).

Your four-cylinder has pistons, rods, cams, valves and springs: around a hundred moving parts, all stopping and reversing, thousands of times a minute.

Nothing in a rotary reverses. The rotors spin one way and the shaft spins one way, all the time. That's why the RX-8 redlines at nine thousand rpm. Most daily cars quit around six and a half.

High revs are where horsepower comes from, as the S2000 showed in horsepower vs torque: power is torque times rpm, and a rotary makes its power by spinning.

The catch: apex seals

Now, the catch. Remember those three tips sealing off the pockets? Each tip is sealed by a thin strip called an apex seal, and it scrapes the housing every single turn. At nine thousand rpm, that's a lot of scraping.

They wear. And when they go, the pockets can't seal any more. Compression leaks past the tip into the next pocket, compression's gone, and it's rebuild time. It's the rotary's version of a worn-out piston ring, except a piston ring rubs a round bore and an apex seal rides a sharp tip across a curved wall.

It burns oil by design

To keep those seals alive, the engine has to lubricate them, and they live inside the fire. So the rotary squirts oil into its own combustion chambers, a little at a time, on purpose. That oil gets burned along with the fuel.

It burns oil by design. That's not a fault; it's how the engine is built. It does mean the oil level drops between changes, and a rotary that runs low on oil is a rotary heading for a rebuild. Rotary owners check the dipstick often and top it up.

And if your rotary buddy swears his doesn't burn oil? Check his dipstick.

Bonus: 1.3 liters that breathes like 2.6

Oh, and Mazda calls it 1.3 liters. But per turn, it breathes like a 2.6.

Here's why. A four-stroke piston engine's displacement is the air it breathes over two crank turns, because each cylinder only takes in air every other turn. A rotary breathes its full 1.3 liters on every single turn of the shaft. Over the same two turns a piston engine needs, the rotary moves about 2.6 liters of air.

So comparing a 1.3-liter rotary with a 1.3-liter piston engine isn't fair to either of them. On air moved per turn, it's closer to a 2.6.

The verdict

Pistons last. Rotaries scream.

A piston engine has more parts, but they're built to last a long time with ordinary maintenance. A rotary has very few parts, revs to the moon and runs incredibly smooth, but its seals wear and it drinks oil to keep them alive. Which side you're on comes down to what you value more.

Recap: how a rotary engine works

1. No pistons: a triangle rotor spins around an eccentric shaft inside the housing.
2. Each tip seals a pocket, and each pocket goes suck, squeeze, bang, blow. Three pockets, firing in turn.
3. Three main moving parts: two rotors and one shaft, against around a hundred in a four-cylinder.
4. Nothing reverses, so the RX-8 redlines at 9,000 rpm.
5. The catch: apex seals wear, and when they go it's rebuild time. It burns oil by design to keep them alive.
6. 1.3 liters, breathes like 2.6 per turn of the shaft.

Piston gang or rotary gang: which side are you on?

Next episode: anti-lag vs two-step vs pops and bangs.

Frequently asked questions

How does a rotary engine work?

A triangular rotor orbits around an eccentric shaft inside an oval-shaped housing. Each of its three tips seals off a pocket, and each pocket goes through intake, compression, combustion and exhaust in turn, pushing on the rotor to turn the shaft.

Do rotary engines burn oil?

Yes, by design. The engine injects a small amount of oil into the combustion chambers to lubricate the apex seals, and that oil is burned. Rotary owners check the dipstick often and top up between oil changes.

What are apex seals?

Thin strips at each tip of the rotor that seal the pockets against the housing. They scrape the housing every turn and wear over time. When they wear out, compression drops and the engine needs a rebuild.

Why can a rotary engine rev so high?

Nothing in it stops and reverses. A piston engine has pistons, rods, valves and springs that change direction thousands of times a minute; a rotary only spins, so it can rev to 9,000 rpm in the RX-8.

Why is a 1.3-liter rotary compared to a 2.6-liter engine?

A four-stroke piston engine takes two crank turns to breathe its full displacement. The rotary breathes its 1.3 liters every single shaft turn, so over the same two turns it moves about 2.6 liters of air, like a 2.6-liter piston engine.