ROTARY ENGINE COMPONENTS

THE BASIC PRINCIPLES APPLIED TO ALL THE MAZDA PRODUCTION ENGINES

ARE VERY SIMPLE INDEED, BUT FOR THE SAKE OF THIS FEATURE, WE WILL TALK ABOUT THE TWO-ROTOR MAZDA ROTARY ENGINES THAT YOU HAVE PROBABLY HEARD REFERRED TO WITH NAMES LIKE ‘10A’, ‘12A’, AND ‘13B’. A ROTARY ENGINE IS ASSEMBLED IN LAYERS. THE TWO-ROTOR ENGINE HAS FIVE MAIN LAYERS THAT ARE HELD TOGETHER BY A RING OF LONG BOLTS. COOLANT FLOWS THROUGH PASSAGEWAYS SURROUNDING ALL THE PIECES. The rotors

The rotor itself is a triangular-shaped component with convex (rounded outward) faces. On each of these faces, you will notice a dip inwards. How deep this dip is directly relates to the compression ratio — just like the dish on the crown of a regular piston. Each rotor has special seals on each of its three apexes that constantly stay in contact with the rotor housing. That means it forms three sealed chambers for each of the four strokes of the engine.

These intricate blade-like strips are produced from special metals (or other composites depending on spec) and are referred to as the ‘apex seals’. On the two side faces of the rotor, there are also other metal ring seals that seal up the rotor sides to either the front, rear, or centre.

In the centre of the rotor, you will notice gear teeth. These feature on only one side and mate directly with gear teeth that are part of the stationary gears that press and bolt into each end plate. When it’s all joined together, this gives the rotor its direction and path as it rotates around its housing. The rotors themselves are mounted on a lobe of the output, or ‘eccentric’, shaft.

The rotor housings

The housing is the component that the rotor spins around. While it looks kind of like an oval, it’s actually called an ‘epitrochoid’, and its strange internal shape is designed so that the three apexes of the rotor will always stay in contact with the wall of the chamber — in turn forming three sealed volumes of gas at the same time. Each part of the housing is dedicated to one part of the combustion process — intake, compression, combustion, and exhaust. On one side of the rotor housing, you will see two holes for spark plugs and on the other, the peripheral exhaust port through which the burned gases leave the engine. The thickness of the rotor housings directly relates to the cubic capacity (in imperial terms measured in cc) of the engine. On a 10A engine, each housing measures 491cc. There are two housings making for a total of 982cc (close enough to 1000cc) and that’s where the ‘10’ in 10A comes from. In a 12A engine, each housing displaces 573cc (1146cc) and in a 13B, 654cc (1308cc).

The front, intermediate, and rear plates

Sandwiching the rotor housings together are the front, intermediate (centre), and rear plates. These are home to the intake ports, one on each of the end plates, and two on the centre plate (two rotors to feed — one on each side). When assembled, the intake manifold directly bolts up to these three components. The faces of these three iron plates, which point in towards the engine internals, are very smooth to allow the sides of the rotor to slide over as they make their way around their rotor housings.

The eccentric shaft

The eccentric, output, or ‘crankshaft’ (as it’s often referred to) has round lobes mounted ‘eccentrically’, meaning that they are offset from the centreline of the shaft. Each rotor fits over one of these lobes, with the lobe acting sort of like the crankshaft in a piston engine. As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted eccentrically to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it to spin. The lobe that the rotor is attached to is offset from the centreline of the shaft, and gives the rotor the leverage it needs to turn the output shaft. As the rotor runs around inside the housing, it pushes the lobe around in tight circles, turning three times for every single revolution of the rotor. So, while the tacho shows the 9000rpm the engine is pulling, the rotor itself is actually only spinning at 3000rpm, and it’s under nowhere near as much stress as a piston engine going up and down at the same rate.