Mazda Trucks 1972-1986 Repair Guide

General Information


See Figure 1

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Fig. Fig. 1: The rotary engine combustion cycle

The Mazda rotary engine replaces conventional pistons with three-cornered rotors which have rounded sides. The rotors are mounted on a shaft which has eccentrics rather than crank throws.

The chamber in which the rotor travels is roughly oval shaped, but with the sides of the oval bowed in slightly. The technical name for this shape is a two lobe epitrochoid.

As the rotor travels its path in the chamber, it performs the same four functions as the piston in a regular four cycle engine:

  1. Intake
  3. Compression
  5. Ignition
  7. Exhaust

But all four functions in a rotary engine are happening concurrently, rather than in four separate stages.

Ignition of the compressed fuel/air mixture occurs each time a side of the rotor passes the spark plugs. Since the rotor has three sides there are three complete power impulses for each complete revolution of the rotor. As it moves, the rotor experts pressure on the cam of the eccentric shaft, causing the shaft to turn.

Because there are three power pulses for every revolution of the rotor, the eccentric shaft must make three complete revolutions for every one revolution of the rotor. To maintain this ratio, the rotor has an internal gear that meshes with a fixed gear in a three-to-one ratio. If it was not for this gear arrangement, the rotor would spin freely and timing would be lost.

The Mazda rotary engine has two rotors mounted 60 degrees out of phase. This produces six power impulses for each complete revolution of both rotors and two power impulses for each revolution of the eccentric shaft.

Because of the number of power impulses for each revolution of the rotor and because all four functions are concurrent, the rotary engine is able to produce a much greater amount of power for its size and weight than a comparable reciprocating piston engine.

Instead of using valves to control the intake and exhaust operations, the rotor uncovers and covers ports on the wall of the chambers, as it turns. Thus, a complex valve train is unnecessary. The resulting elimination of parts further reduces the size and weight of the engine, as well as eliminating a major source of mechanical problems.

Spring loaded carbon seals are used to prevent loss of compression around the rotor apexes and cast iron seals are used to prevent loss of compression around the side faces of the rotor. These seals are equivalent to compression rings on a conventional piston, but must be more durable because of the high rotor rpm to which they are exposed.

Oil is controlled by means of circular seals mounted in two grooves on the side face of the rotor. These oil seals function to keep oil out of the combustion chamber and gasoline out of the crankcase, in a similar manner to the oil control ring on a piston.

The rotor housing is made of aluminum and the surfaces of the chamber are chrome plated.