Rotary Engine Mechanics: How the Wankel System Works

Rotary Engine Mechanics

How the Wankel Engine Functions

The triangular rotor rotates on an eccentric located on the motor shaft or output shaft. During its rotation, the three vertices of the rotor remain in constant contact with the inner surface of the epitrochoidal-shaped housing. The rotor describes orbits around a stationary pinion gear. The rotation of the rotor is transmitted to the drive shaft at a 3:1 ratio; for every three turns of the rotor, the shaft completes one revolution.

Each of the three chambers formed between the rotor and the housing undergoes a 4-stroke cycle. In one turn of the rotor, three complete cycles occur per rotor revolution. This means the engine receives a power pulse every 120 degrees, utilizing intake and exhaust ports rather than valves.

The Four-Stroke Cycle

• Admission: The air-fuel mixture enters as the rotor vertex uncovers the intake port. The rotor progressively increases the volume of the chamber, which fills with fresh gas until the vertex closes the window.
• Compression: The chamber remains closed, and the volume decreases, compressing the gases. Before reaching maximum compression, a spark plug ignites the mixture to start combustion.
• Expansion: The rapid increase in pressure from combustion drives the rotor while expanding the gases, lasting until the vertex reaches the exhaust port.
• Escape: Once the exhaust port is uncovered, the burnt gases are expelled at high speed due to residual expansion pressure. The rotor then decreases the chamber volume to complete the process.

Advantages and Disadvantages

Benefits

The main advantage is that rotation is generated directly, resulting in uniform operation with almost no vibration. These engines can attain high RPMs (up to 9,000 RPM) and use very few moving parts—eliminating the need for crankshafts, connecting rods, pistons, valves, and camshafts. This makes the engine simple, lightweight, and capable of high specific power (kW/L).

Drawbacks

One drawback is high fuel consumption at partial loads. Additionally, the engine faces challenges regarding the sealing of the chambers (apex seals) and their durability. While low-friction materials have improved wear resistance, the use of this engine type remains low, largely due to patent restrictions held by Mazda. Modern rotary engines now offer performance and reliability comparable to reciprocating piston engines.