Secondary Air Injection System
ECM controlled secondary air injection Systems were first introduced on 1994 E34 530 with M3.3 to meet federal emission requirements. Variations of the system were subsequently added to other vehicle through the model years.
The secondary air injection system helps the catalytic converter to function more efficiently at its most inefficient period of operation. Catalytic converters need heat and oxygen to operate effectively. When first starting and idling a cold engine, heat and oxygen are at their lowest levels in the exhaust stream.
The Secondary Air Injection system injects fresh air into the exhaust manifolds. The air mixes with the exhaust gas as it leaves the engine. The additional air helps the catalytic converter to function earlier and further reduce the exhaust emissions during the critical higher emission warm up period.
SYSTEM COMPONENTS
Example System M73/M5.2: The mechanical components of the system are as follows:
AIR PUMP: The air pump is located in the front right inner fender well. It is a 12 volt DC, two speed, electric motor pump that is controlled by the DME control modules through two relays. Operation is based on the programmed requirements (see M5.2).
The pump draws filtered air from the air filter housing of cylinder bank 7-12 and supplies it to two non-return valves.
NON-RETURN (ONE WAY) VALVES:
Located on the front of the engine are the two Non-Return Valves, The Non-Return valves serve the following functions.
- When activated by a controlled vacuum source the valves open and allow the supplied air from the electric motor pump to inject air into the exhaust manifolds.
- In the event of an engine backfire they will check the back pressure from damaging the pump, hoses or secondary air solenoid vacuum valve.
STAINLESS STEEL INJECTION PIPES: Outlet of each Non-Return Valve is a stainless steel injection pipe. Each pipe forks into two outlets for connection to each of the four exhaust manifolds (two per bank). The pipes are connected to the exhaust system to inject the air directly into the exhaust gas stream.
SECONDARY AIR VACUUM/VENT VALVE: The Vacuum/Vent Valve is located in the main harness conduit tray on top of the engine. It is energized by DME II simultaneously with the Air Injection Pump.
When the air injection pump is switched off the Vacuum/Vent valve disconnects the vacuum supply from the Non-Return Valves leaving them in a closed position.
VACUUM RESERVOIR AND CHECK VALVE: Also located in the main harness conduit tray are the vacuum reservoir and check valve. The vacuum reservoir stores a vacuum supply for the Non-Return Valves from the throttle housing. The Check Valve ensures the vacuum reservoir has an ample vacuum supply for operation.
OPERATION
The secondary air injection is controlled by both DME control modules to provide two pump speeds. After engine start up, DME II simultaneously switches the Stage/ relay control circuit to ground and activates the secondary air vacuum/ vent valve allowing engine vacuum to open the Non-Return valves.
The air pump is energized through the closed contacts of the stage I relay. This circuit has an .8 ohm in-line resistor which causes the pump motor to run at a slow speed. The supplied air pressure is directed through the Non-Return Valves to the air injection inlet pipe directly into the exhaust manifolds.
After approximately 5 seconds, DME I switches the Stage II relay control circuit to ground and closes the contacts of the relay The Stage II relay supply circuit bypasses the resistor causing the pump to run at a fast speed for up to 85 seconds. The faster speed of the motor injects additional air volume into the exhaust manifolds.
Three control factors alter the state of system operation:
- Engine Temperature
- Engine Speed
- Load Signal (Ti)
Sequence of Operation Based on Engine Temperature
< - 10 °C = 10 seconds after the engine starts, DME II activates stage I relay for 2.5 seconds. Then DME I activates stage II for 2.5 seconds.
- 10 °C to 40 °C = 10 secondsafter the engine starts, DME II activates stage I relay for 2.5 seconds. Then DME I activates stage II for 85 seconds.
> 40 °C = Immediately after engine start, DME II activates stage I relay for 28 seconds.
System Cut Out
The DME control modules will de-energize the relays within the programmed period of operation if:
- Engine speed exceeds 2720 RPM and/or
- Engine load (Ti) exceeds 5.4 ms
These cut out parameters protect the system from damage due to over pressure in the exhaust system feeding back into the air injection system.
SECONDARY AIR SYSTEM MONITORING
EPA regulations require that the DME M5.2 monitor the function of the secondary air injection system. Secondary air injection operation is monitored by the DME during cold engine startup.
The injected air causes the exhaust gas to become oxygen enriched. The induced lean exhaust gas is detected by the O2 sensors and relayed back to DME.
The function is activated as follows:
- Cold Engine startup between -10 ° and 40 °C.
- O2 sensors must reach a stabilized temperature.
- No faults detected from the air injection system components or from DME.
- DME maintains the stable millisecond injection time without oxygen sensor influence (O2 sensor signals ignored with regard to A/F ratio).
- The O2 sensors are monitored only for the change in the O2 content to confirm the operation of the air injection system. The monitoring duration lasts up to 16 seconds.
It the system detects an increase in oxygen within a predetermined period it recognizes the air injection system to be functional. If the additional oxygen is not detected for two consecutive cold starts, DME determines a general fault with the function of the secondary air injection system.