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 — the ‘10A’, ‘12A’ and ‘13B’.
THE ROTORS
The rotor itself is a triangular-shaped component with convex (rounded outwards) 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 tips that constantly stay in contact with the rotor-housing edge as the rotor makes its way around the housing. That means it forms sealed chambers for each of the four strokes of the engine. These intricate blade-like strips are produced 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. 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 end or centre plate. 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 end plates. 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 tips 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, or exhaust. On one side of the rotor housing, you will see two holes for spark plugs and, on the other side, 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 and there are two of them, so that totals 982cc — close to 1000, 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 HOUSINGS
Sandwiching the rotor housings together are the front, intermediate (centre), and rear housings. These housings are also home to the intake ports, one on each of the end plates, and two on the centre one (two rotors to feed — one on each side). When assembled, the intake manifold directly bolts up to these three components, much as an inlet manifold joins up to the cylinder head on a piston engine. The faces of these three housings, which point in towards the engine internals, are very smooth in order 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 they are offset from the centre line 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 eccentric 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 centre line 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 one 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 of knots.