How Rotary Engines Work, Step by Step

A rotary (Wankel) engine makes power by sweeping a triangular rotor around an oval-like housing to perform intake, compression, combustion, and exhaust in sequence. As the rotor orbits an eccentric (off-center) shaft, three separate chambers change volume and create one power stroke per output-shaft revolution per rotor, delivering smooth, compact power with few moving parts.

What “rotary engine” means in this context

Historically, “rotary engine” also referred to early aircraft radials whose entire engine spun with the propeller. Today, the term almost always means the Wankel rotary engine used in cars, motorcycles, UAVs, and range extenders. This article explains the Wankel, the design popularized by Mazda and others.

Core parts you need to know

To understand the step-by-step cycle, it helps to know the main components and what each one does inside the engine.

• Rotor: A roughly triangular (Reuleaux-style) rotor with three apexes that seals three working chambers.
• Housing: An epitrochoid-shaped cavity that the rotor sweeps; ports for intake and exhaust are machined into the housing.
• Apex and side seals: Spring-loaded seals at the rotor’s tips and flanks that keep chambers separate as volumes change.
• Eccentric (output) shaft: An off-center shaft with lobes that the rotor rides on; it converts the rotor’s orbital motion into shaft rotation.
• Phasing gear set: A stationary gear on the housing meshing with an internal gear on the rotor, keeping the rotor correctly oriented and establishing the 3:1 shaft-to-rotor speed relationship.
• Spark plugs and ignition: Usually two plugs per rotor housing to light the long, thin combustion chamber efficiently.
• Lubrication and cooling: Metered oil to seals and bearings; coolant passages in the housing; oil control rings on rotor sides.
• Ports: Intake (side or peripheral) to admit air-fuel, and exhaust (usually peripheral) to discharge gases; timing is set by port shape and position.

Together, these parts create three moving combustion chambers whose volumes expand and contract as the rotor orbits, allowing all four “strokes” of a conventional cycle to occur without poppet valves.

The step-by-step cycle inside a Wankel

Each face of the rotor forms a chamber with the housing. As the rotor orbits, each chamber passes the intake and exhaust ports and goes through four phases. The rotor turns at one-third the speed of the output shaft; in a single-rotor engine that yields one power stroke per shaft revolution (two per rev in a twin-rotor, three in a triple-rotor).

1. Intake: As one chamber sweeps past the intake port, its volume increases. Air-fuel mixture (via port fuel injection or direct injection) is drawn in until the trailing edge of the chamber passes the port and closes it.
2. Compression: With the chamber sealed by apex and side seals, continued rotor motion reduces the chamber volume, compressing the mixture as it approaches the top of the epitrochoid.
3. Ignition and power: One or two spark plugs ignite the mixture. Rapid combustion raises pressure, pushing on the rotor face and turning the eccentric shaft via the rotor’s orbit. This is the expansion (power) stroke.
4. Exhaust: As the chamber approaches the exhaust port, it opens to the outlet. The hot gases flow out as the chamber volume continues to decrease, clearing the cylinder for the next cycle.

Each rotor face completes this four-phase sequence once per rotor revolution, and because the shaft spins three times per rotor revolution, the engine delivers a steady series of power pulses—in a twin-rotor, two pulses per output-shaft revolution.

How motion becomes usable torque

The rotor rides on an eccentric lobe of the output shaft. As combustion pressure pushes the rotor, its orbital motion is constrained by the phasing gears so the rotor maintains orientation while the eccentric lobes force the output shaft to turn. The result is direct rotary output without the reciprocating masses and valvetrain found in piston engines, contributing to notable smoothness and high rev capability.

Port timing and combustion details

Because intake and exhaust ports are fixed in the housing, timing comes from geometry: the rotor’s leading and trailing edges open and close ports as they pass. Peripheral intake ports favor high power and revs but can allow more overlap (worse emissions and idle), while side intake ports reduce overlap for cleaner operation. Two spark plugs shorten flame travel across the long, thin chamber to improve efficiency and reduce knock; modern systems may add direct injection and exhaust gas recirculation to cut hydrocarbons and fuel use.

Advantages and limitations

Advantages

Designers choose Wankel rotaries when compactness, smoothness, and simplicity outweigh efficiency constraints. Key upsides include:

• High power density: Compact, lightweight package with few moving parts.
• Smooth operation: No reciprocating pistons; evenly spaced power pulses per rotor.
• Mechanical simplicity: No camshafts, valves, or complex valvetrain.
• Fuel flexibility: Adaptable to gasoline and, in research/limited applications, hydrogen or LPG.

These traits make rotaries appealing for sports cars, UAVs, and range-extender gensets where space and vibration matter.

Limitations

Trade-offs stem from chamber shape, sealing challenges, and port timing. Common drawbacks include:

• Seal wear and oil consumption: Apex and side seals need careful lubrication; some oil is intentionally metered into the intake.
• Emissions: Large surface-to-volume ratio and port overlap can raise unburned hydrocarbons.
• Fuel economy: Typically lower thermal efficiency than comparable modern piston engines.
• Thermal management: Hot spots near the exhaust and uneven heat distribution stress materials.
• Part-load efficiency: Less effective at light loads than optimized piston designs.

Modern controls—direct injection, side-port layouts, improved seals and materials—mitigate but don’t fully eliminate these issues.

Where you’ll find them today

As of 2024, mainstream automotive use is limited. Mazda reintroduced a single-rotor engine as a generator in the MX-30 R-EV range-extender to leverage compactness and smoothness without driving the wheels directly. Rotary engines also appear in UAVs and specialty applications where low vibration and power density are prized. Manufacturers continue to explore hydrogen-fueled rotaries and advanced injection/port strategies for cleaner operation, and concept vehicles periodically showcase potential future uses.

Summary

A Wankel rotary engine works by sweeping a triangular rotor around an epitrochoid housing, creating three chambers that sequentially perform intake, compression, combustion, and exhaust. Phasing gears keep the rotor oriented while an eccentric shaft converts the rotor’s orbital motion into output torque. The geometry yields one power stroke per output-shaft revolution per rotor, delivering compact, smooth power with minimal moving parts—along with characteristic challenges in sealing, emissions, and efficiency.

How does a rotary engine work step by step?

All working to create power in harmony. Now if only there was a YouTube channel where you could learn all about this stuff in great detail. One can dream.

How do rotary engines make so much power?

Rotary engines produce significant power for their size because they achieve more power strokes per engine revolution compared to a traditional four-stroke piston engine, have fewer moving parts, and can operate at higher RPMs. Their compact design, high power-to-weight ratio, and smooth, vibration-free operation also contribute to their reputation for high performance in vehicles like sports cars.
 
This video explains the differences between rotary and piston engines: 48sCar ThrottleYouTube · Nov 10, 2017
More Power Strokes 

• Continuous Power Generation: Opens in new tabUnlike a four-stroke piston engine, where a power stroke only occurs once every two crankshaft revolutions, a rotary engine’s rotor performs intake, compression, combustion, and exhaust processes continuously. 
• Higher Power Density: Opens in new tabThis results in a more frequent application of power to the output shaft, meaning a rotary engine delivers more power relative to its displacement and size. 

Fewer and Smoother Moving Parts 

• Reduced Reciprocating Mass: Opens in new tabRotary engines lack the heavy, up-and-down motion (reciprocating mass) of pistons, which dramatically reduces internal friction and vibrations. 
• Higher RPMs: Opens in new tabThis smoother, balanced design allows rotary engines to spin at higher speeds, further contributing to their powerful output. 

Compactness and High Power-to-Weight Ratio 

• Smaller Footprint: With fewer components, a rotary engine can be made significantly smaller and lighter than a comparable piston engine. 
• Improved Vehicle Dynamics: This high power-to-weight ratio allows engineers to design more agile and performance-focused vehicles by placing the engine lower and further back in the chassis, improving weight distribution. 

Why were rotary engines banned?

The rotary has never been explicitly banned, the alignment to F1 was the only reason it wasn’t allowed, much like many of the piston engines that had been racing at the time were no longer allowed.

What is the biggest problem with rotary engines?

First, they are neither efficient nor clean to run due to limitations in the chambers and rotors giving relatively poor gas mileage and poor emissions. Additionally there have always been problems with the rotor seals wearing prematurely reducing engine longevity and increasing oil burn.