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Secondary-air system

Design, components

Why is air only injected during cold starts and what influence does this have on pollutants?
Read on for information about which components are fitted in the secondary air system and why.

All vehicles with spark-ignition engine and secondary-air system
Secondary-air valve, secondary-air pump, solenoid switching valve

With spark-ignition engines, the greatest pollution occurs on cold starting. Secondary-air systems have been successfully employed to reduce such cold starting emissions.

A “rich mixture” (λ<1), i.e. a mixture with excess fuel, is required for starting a cold spark-ignition engine.
Until the catalytic converter reaches operating temperature and Lambda control action starts to take effect, large quantities of carbon monoxide and unburnt hydrocarbons are produced.
To reduce the level of these pollutants, ambient air with a high oxygen content (“secondary-air”) is injected into the exhaust manifold directly downstream of the exhaust valves during the cold starting phase. This results in post-oxidation (“afterburning”) of the pollutants to form carbon dioxide and water.
The heat generated in this process additionally warms the catalytic converter and speeds up the onset of Lambda control action.

  1. Air filter
  2. Secondary-air pump
  3. Secondary-air valve with shut-off and nonreturn function
  4. Solenoid switching valve
  5. Cylinder
  6. Lambda sensor
  7. Catalytic converter

Secondary-air system components

The secondary-air pump draws in ambient air and injects it into the exhaust manifold downstream of the exhaust valves. There is a separate integrated air filter if the air is not drawn in from the intake system but rather directly from the engine compartment.

Secondary-air valves are fitted between the secondary-air pump and the exhaust manifold.
Use is made of different versions. The secondary-air non-return valve stops exhaust gas, condensate or pressure peaks in the exhaust system (e.g. misfiring) causing damage to the secondary-air pump.
The secondary-air shut-off valve ensures that secondary-air is only routed to the exhaust manifold in the cold starting phase.
Secondary-air valves are either actuated by a vacuum controlled by a solenoid switching valve or open in response to the pressure of the secondary-air pump.

With more recent generations, the shut-off and non-return functions are combined in one single “secondary-air valve”.

The latest development is the solenoid secondary-air valve, featuring shorter opening and closing times than pneumatically actuated valves.
Thanks to higher actuating forces, this type of valve is less susceptible to clogging by soot or dirt.

Solenoid secondary-air valves may be equipped with an integrated pressure sensor for on-board diagnosis (OBD) monitoring.

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