Air Injection: Description and Operation
To ensure compliance with the EPA regulations (I/M 240) for exhaust emissions, the M73 engine is equipped with a secondary air injection system. The air injection system is an output control function of both DME control modules.
The components of this system include:
- Electric Air Injection Motor and Pump
- Non-Return Valves (One Way Valves)
- Stainless Steel Air Injection Pipes
- Secondary Air Vacuum/Vent Valve
- Vacuum Reservoir and Check Valve
- Two system relays (slow and fast)
- In-line resistor
The injection of air into the exhaust manifolds accomplishes two things:
- Helps oxidize the CO and HC content
- Decreases the warm up time of the catalytic converter
The benefit this system provides greatly reduces the exhaust gas pollutants during the period of operation which is normally high in emission pollutants.
The 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 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.
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 I 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 0.8 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 test 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 seconds after 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 start-up.
The injected air causes the exhaust gas to become oxygen enriched. The induced lean exhaust gas is detected by the ~2 sensors and relayed back to DME.
The function is activated as follows:
- Cold Engine start-up 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.
If 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.