Air Injection: Description and Operation

Fig. 1 Air Guard system. 6-258 engine:

This system, Fig. 1 reduces the level of unburned hydrocarbons (HC) and carbon monoxide (CO) emissions by injecting secondary air from an air pump into the exhaust manifolds.
This system consists of a belt driven air pump which injects compressed air through the connecting hoses to a steel distribution manifold and into the stainless steel injection tubes in the exhaust port next to each exhaust valve. This air mixes with the hot and incompletely burned exhaust gases to permit further combustion in the exhaust ports and exhaust manifolds.

Fig. 2 Air injection pump with integral centrifugal air filter and pressure relief valve:

Fig. 3 Air injection pump with integral centrifugal air filter without pressure relief valve:

The air pump is a positive displacement unit that requires no maintenance, Figs. 2 and 3. The early units incorporated a replaceable element type air filter and pressure relief valve. Later units incorporate only a replaceable centrifugal type air filter. The pressure relief valve on early units is located in the exhaust cavity and encloses a preloaded spring, a seat and pressure setting plug.
The air delivery manifold distributes the air from the pump to each of the air delivery tubes in a uniform manner. A check valve which is an integral part of the air delivery manifold prevents the reverse flow of exhaust gases into the pump should the pump fail or the exhaust gas pressures exceed pump output.
The stainless steel air injection tubes are inserted into the machined bosses of the exhaust manifold. These tubes project into the exhaust port and direct the air into the vicinity of the exhaust valve seat.
The bypass valve used on some engines, is in the normally closed position to allow pump air to flow freely through the valve fittings and into the exhaust manifold. When the throttle is closed quickly and engine manifold vacuum rises, the sudden increase in vacuum (sensed through a vacuum connection at the carburetor base) overcomes the diaphragm spring tension and momentarily moves the valve to the open position. When the valve is in the open position, pump air flow is bypassed to atmosphere or ``dumped'' through a bronze silencer pressed into the outlet opening. This action is required to prevent a backfire in the exhaust system which could occur when the throttle is closed suddenly. This valve is operating normally when no exhaust system backfire occurs during rapid throttle closure.

Fig. 4 Diverter valve:

The diverter valve, Fig. 4, when triggered by a sharp increase in manifold vacuum, supplies the intake manifold with fresh air to lean out the fuel/air mixture and prevent backfire.
In addition to the above mentioned components, carburetors and distributors specifically calibrated for the Air Guard system must be used.