Air Injection: Description and Operation

Secondary Air Injection Function

ENGINE 111.951 /952 /955 /956 /957 /958 /982 /983 with CODE (925) low emission EURO3
ENGINE 111.983 with CODE (494a) USA version




Illustration shows naturally-aspirated engine
126 Air shutoff valve (combination valve: check valve integrated)
128 Check valve (vacuum)
a Vacuum supply from intake manifold
B2/5 Hot film MAF sensor
B11/4 Coolant temperature sensor
G3/2 O2 sensor upstream TWC
K40/4k3 Air pump relay (model 202)
L5 Crankshaft position sensor
M16/6 Throttle valve actuator
M33 Electric air pump
N3/10 ME-SFI control module
N10/1kO Front SAM control module with fuse and relay module, secondary air injection relay (model 203)
Y32 AIR pump switchover valve

Secondary air injection warms up the catalytic converter more rapidly to its operating temperature and thus improves the emission levels in the warming-up phase.

The electric air pump (M33) is actuated through the air pump relay (K40/4k3) (model 202), or through the front SAM control module with fuse and relay module, secondary air injection relay (N10/1kO) (model 203)
The current consumption of the electric air pump (M33) is as much as 35 A.

Secondary air injection relay and air pump switchover valve (Y32) are actuated simultaneously by the engine control module for up to 150 seconds after engine start if:
- coolant temperature > 10 °C and < 60 °C
- engine speed < 3000 rpm
- throttle valve not fully opened
- After actuation, the secondary air injection remains inhibited until the coolant temperature is greater than approx. + 60 °C ist and then drops to below 40 °C

The pump draws in air through a maintenance-free filter and pumps it to the air shutoff valve. The valve at the same time prevents the exhaust gases from flowing back to the air pump (check valve discontinued).

The air pump switchover valve switches the intake manifold pressure through to the air shutoff valve (126). This opens and the air supplied by the air pump is forced into the exhaust manifold.
The injected air reacts with hot exhaust gases in the outlet port. An oxidation of carbon monoxide (CO) and hydrocarbon (HC) takes place and results in an additional increase in the exhaust temperature.

Engines fitted with supercharger
The injected air is drawn off on the pressure side downstream of the compressor. No air pump is required.