ROTARY-ENGINE BASICS

Since its first production in the late 1960s, the basic design principles of the Mazda rotary engine haven’t changed an awful lot. Although very different in design from a regular piston engine, the rotary shares many similarities — like a four-stroke combustion cycle — but with many fewer components. Over these two pages, we’ll get back to real basics and hopefully shed some light on the basic working of a Mazda rotary engine.

Design concept

As you can see by the pictures, rotary engines are very simple in their major engine components’ make-up. Compared with a simple four-stroke piston engine, a rotary has substantially fewer moving mechanical components. In fact, there are only three in a two-rotor (10A, 12A, or 13B) engine — the two rotors themselves and the eccentric shaft. In the piston engine, it’s easy to see there are many, many more, including the pistons, connecting rods, crankshaft, camshafts, valves, etc. All the rotary-engine parts move in one direction — unlike in the piston engine, in which components move up and down, and round and round, all at the same time — and this is basically where these engines get their unrivalled smoothness and free-revving nature.

In operation, just as in a piston engine, the rotary makes its power by using pressure that is created when the air/fuel mixture is combusted. Think about it like this: in a piston engine, combustion in the cylinders makes the piston move up and down; because the pistons join connecting rods, which in turn are connected to the crankshaft, this upand-down motion (or side-to-side in a Subaru boxer engine) is converted to a rotational motion, which is used to drive the car. In a rotary engine, the rotor is like the piston, the rotor housing like the cylinders, and the eccentric shaft like the crankshaft. It’s very similar, but also very different.

Operation

Mazda rotary engines use a four-stroke combustion cycle, just like a regular piston engine, but the way it works in a rotary is not the same. In its most basic form, the rotor (there are two of them in a 10A, 12A, and 13B and three in a 20B) spins around

1. INTAKE STROKE

The first part of the combustion cycle starts with the intake, in which the engine draws in the mixture of air and fuel. When the tip of the rotor passes the intake port and the port is exposed to the chamber, the chamber is at its smallest size, but then, when the rotor moves past the intake port, the chamber size grows as it draws in the mixture. Finally, as the rotor tip passes the intake port, the chamber is fully sealed off, and the compression stroke begins. inside the rotor housing. Each of the three peaks of the rotor is in contact with the housing at all times, and this creates three separate volumes of gas as the rotor moves around the chamber. Each volume of gas alternately expands and contracts. On the intake, it’s drawn into the engine; on

2. COMPRESSION STROKE

As the rotor continues its path around the housing, the chamber size gets smaller and the air/fuel mixture gets compressed as that’s all it can do — it has nowhere else to go. By the time the face of the rotor has made it around to the spark plugs, the volume of the chamber is again close to its minimum volume, and this is when the combustion stage of the four-stroke cycle starts.

3. COMBUSTION STROKE

the compression, it’s compressed; in the combustion, it ignites; and on the exhaust, it’s expelled. The beauty of the rotary engine is that each of the three faces of the rotor are always working on one part of the cycle at any given time. Let’s look at the four strokes. As the combustion chamber is long, Mazda uses two spark plugs for each rotor, as the flame would spread too slowly if there were only one. As the spark plugs ignite, the well-compressed mixture there is a combustion, which expands the gases and forces the rotor to move (the same as a combustion in a piston engine, which forces the piston downwards). This then provides the motive force until the rotor tip passes the exhaust port.

4. EXHAUST STROKE

Once the tip of the rotor passes the exhaust port, those combustion gases start to flow out of the exhaust port, which feeds into the exhaust manifold and exhaust system. As the rotor continues to move, the chamber volume decreases in size, again forcing the remaining exhaust out of the port. By this time, the tip of the rotor is once again passing the intake port, and the whole cycle starts again.