RotorThe three-sided part that does the work of pistons. Each of its faces runs its own cycle.
Rotor housingThe casing the rotor turns in. Its pinched-oval bore is called an epitrochoid.
Eccentric shaftThe engine output shaft. The rotor rides on its off-center lobe and turns it.
Intake portThe opening that lets the fuel and air mixture in. The rotor uncovers it as it passes.
Exhaust portThe opening the burnt gases leave by. A rotor face pushes them out through it.
Spark plugIt fires the squeezed mixture. This housing has two, one above the other.
Combustion chamberThe space where the mixture is squeezed and fired. It moves round with the rotor.
Apex sealA blade at each of the three tips. It seals one chamber from the next.
Side sealA strip along each edge of the rotor. It seals against the flat end housing.
Internal gearA ring of teeth fixed in the rotor. It rolls round a smaller gear that does not turn.
Coolant passageA channel through the housing wall. Cooling liquid flows along it.
How this list was compiled: Every name checked on 2026-10-01 and kept only where two independent sources use it. (1) US Army TM 9-8000, Principles of Automotive Vehicles (1985), chapter 2, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 . (2) Wikipedia, text only: Wankel engine, https://en.wikipedia.org/wiki/Wankel_engine . (3) A Kansas State University thesis surveying rotary engines, https://archive.org/details/surveyofrotaryty00chan . (4) NASA's 1984 overview of its rotary engine research, https://archive.org/details/NASA_NTRS_Archive_19840020722 . Rotor, housing, eccentric shaft, ports and spark plug are in sources 1 and 2; apex seal in 2, 3 and 4; side seal and coolant passage in 3 and 4; internal gear in 1 and 3; combustion chamber in 2 and 4. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry (the bore is computed from the epitrochoid's formula); no maker's product, nothing traced. Licence: own work
How it works
A rotary engine has no pistons and no valves. A three-sided rotor turns inside a housing and does both jobs. The drawing cuts across the housing, looking along the shaft.
Read the full guide
One rotor does the work of pistons and valves
The rotor turns inside the rotor housing. Each of its three faces draws mixture in at the intake port, squeezes it, has it fired by a spark plug in the combustion chamber and pushes the burnt gas out of the exhaust port.
Seals, a gear and an off-center shaft
An apex seal at each tip and a side seal along each edge keep the three spaces apart. The internal gear guides the rotor round, and the rotor turns the eccentric shaft: three turns of the shaft for one turn of the rotor.
The ports need no valves. The rotor covers and uncovers them as it turns.
Coolant passages run through the housing wall, since one side is always the hot side.
Questions and answers
What are the main parts of a rotary engine?
The rotor, the rotor housing it turns in and the eccentric shaft it drives. The housing carries the intake port, the exhaust port and the spark plugs. The rotor carries apex seals, side seals and an internal gear.
What do apex seals do in a rotary engine?
An apex seal is a blade set in a groove at each of the rotor's three tips. It presses against the housing as the rotor turns and keeps the gas in one chamber from leaking into the next.
Does a rotary engine have pistons or valves?
No. The rotor takes the place of the pistons, and its faces open and close the ports as it passes them. With no pistons going up and down and no valve train, the engine runs smoothly at high speed.
How does a rotary engine work?
It runs the same cycle as a piston engine: intake, compression, power and exhaust. Each face of the rotor goes through all four in one turn of the rotor, and the eccentric shaft gets one power stroke for each of its turns.