Air Injection: Description and Operation

Air Injection System (Typical):

This system is used to reduce carbon monoxide (CO) and hydrocarbon (HC) emissions by adding a controlled amount of air to the exhaust gases in the exhaust system. This addition of air causes further oxidation of the exhaust gases with an appreciable reduction of carbon monoxide and hydrocarbon emissions.

This system consists of an air pump, an air switching valve, a check valve, a vacuum solenoid, a coolant vacuum switch (CVSCC), an air manifold, and the necessary tubes and hoses to connect the various components. Cylinder heads used on engines with this system have drilled passages to conduct the air to the exhaust ports.

The air injection system is designed so that air injection will not interfere with the ability of the EGR system to control NOx emissions and to ensure air-fuel distribution for maximum fuel economy. Air is injected at the exhaust ports for a short period during engine warm-up, which assists in the oxidation process in the upstream catalyst. The air flow is then switched to a point downstream where it will assist the oxidation process in the main catalyst. The downstream location is in the main catalyst between the reduction and oxidation catalyst.

Air Injection Pump:

AIR PUMP
The belt driven air pump is mounted on the front of the engine. Intake air passes through a centrifugal fan at the front of the pump, where foreign materials are separated from the air by a centrifugal force. Air is delivered to the air injection manifold and tube assembly by a rubber hose through the air switching valve mounted on the rear of the pump.

Air Switching Valve (Combination Switch/Relief Valve):

AIR SWITCH RELIEF VALVE
This valve directs air injection flow to the exhaust ports or to the downstream injection point. The valve also regulates air pump output during high speed operation. When air pressure reaches a predetermined level, some of the air pump air is vented to the atmosphere through the silencer. Initially, air is injected at the upstream location as close to the exhaust valves as possible. As engine temperature increases, EGR and oxygen sensor systems will begin functioning. When this occurs, the air injection point will be switched to the downstream location. The valve is controlled by manifold vacuum and a vacuum solenoid. When the engine is cold, a manifold vacuum signal is sent to the valve which directs air injection to the exhaust ports. As engine temperature increases the manifold vacuum signal is closed to the valve. Without the vacuum signal, the air switch relief valve (combination switch/relief valve) directs air to the downstream location.



AIR SWITCH RELIEF VALVE SOLENOID
The air switch relief valve solenoid is an electrically operated vacuum switch. This switch vents the vacuum hose to the air switch relief valve under predetermined conditions.



CHECK VALVE
The check valve is located in the injection tube assembly that leads to the exhaust manifold and catalyst. This one-way valve allows fresh air to enter the exhaust system and prevents exhaust from escaping. The check valve will protect the system in the event of pump belt failure, excessively high exhaust system pressure, or air hose rupture.