The air injection system used on the Mazda rotary engine differs from the type used on a conventional piston engine in two respects:
Air is supplied to the system by a normal vane-type air pump. The air flows from the pump to the air control valve, where it is routed to the air injection nozzles to cool the thermal reactor/exhaust manifold or, in the case of a system malfunction, to the air cleaner. A check valve, located beneath the air control valve seat, prevents the back-flow of hot exhaust gases into the air injection system, in the event of air pressure loss.
Air injection nozzles are used to feed air into the exhaust ports, just as in a conventional piston engine.
On 1979–80 RX-7s, an air pump feeds fresh air into the hot exhaust gas as the gas leaves the exhaust ports. This burns the HC and CO in the exhaust gas. The system works as follows: The air pump draws in fresh air from the air cleaner and sends it to the air control valve, which routes it through the heat exchanger and into the exhaust ports. Not all of the air from the pump follows this path, however; some of the air is, at times, sent through the outer shell of the thermal reactor to keep the reactor from reaching destructively high temperatures. At other times, excess air from the pump is fed back into the air cleaner by the air control valve.
When the air from the air pump passes through the heat exchanger, it is pre-heated so that cold air is not pumped into the exhaust ports, which would lower the basic temperature in the exhaust system and affect the thermal reactor's ability to consume noxious gases. The fresh air injected into the exhaust ports adds oxygen to the exhaust gases, and they begin to burn as they pass into the thermal reactor. By the time the gases pass out of the thermal reactor, the previously unburned hydrocarbons and the carbon monoxide have been brought down to legal emission levels.
On 1981–89 RX-7s, the system used on these models replaces the thermal reactor with two catalytic converters (No.1 monolith and No. 2 monolith) and a reactive exhaust manifold. The system retains the air pump and the air control valve.
Air is pulled in from the air cleaner by the air pump and sent to the air control valve, where, according to engine operating conditions, the air is either sent into the exhaust ports or directed down to the dual bed-type catalyst. Excess air is sent back to the air cleaner. The air-burned hydrocarbons and carbon monoxide ignite these unused gases in much the same way the thermal reactor does on 1979–80 RX-7s. The air control valve sends air to the exhaust ports, mainly during deceleration and low engine speeds when HC and CO tend to be produced in large amounts. During this phase, the catalysts act as backup units to insure that fewer noxious gases are produced.
At the middle engine speeds, the air control valve routes air down to the two-bed catalyst. Air is injected through a nozzle between the two pellet beds of the rear catalyst. When the exhaust port air is stopped, the front bed of the rear catalyst processes oxides of nitrogen (NOx), while the rear bed, with the help of the injected fresh air, oxidizes hydrocarbons and carbon monoxide. The monolithic catalyst, located in front of the two-bed catalyst, acts as a backup system for the two-bed unit.