Diesel Secondary air system - Function and Trouble Shooting

Why is a secondary air system used? 

This system is used to further reduce the HC and CO values during the cold-start phase before the catalytic converter becomes active.

In petrol engines with stoichiometric operation, 3-way catalytic converters are used to achieve a conversion rate of over 90 percent. On average, up to 80 % of emissions of a driving cycle are produced during the cold start. But because the catalytic converter only starts working effectively from a temperature of approx. 300 - 350 °C, other measures must be used during this time to reduce emissions. This is the task of the secondary air system. Provided there is sufficient residual oxygen available in the exhaust gas system and the temperature is high enough, the HC and CO react in a subsequent reaction to produce CO2 and H2O. To make sure there is enough oxygen available for the reaction in the cold-start phase when the mixture is extremely rich, additional air is added to the exhaust gas flow. In the case of vehicles fitted with a three-way catalytic converter and a lambda control, the secondary air system is switched off after approx. 100 seconds. Due to the heat produced by the subsequent reaction, the working temperature of the catalytic converter is quickly reached. The secondary air can be added actively or passively. In the case of the passive system, pressure fluctuations in the exhaust gas system are exploited. Through the partial vacuum produced by the flow speed in the exhaust gas pipe, the additional air is suctioned in through a timed valve. In the case of the active system, the secondary air is blown into the system by a pump. This system enables a better control.

Design and operation of the active secondary air system

Structure and function of the active secondary air system

The active secondary air system usually comprises an electrical pump, control relay, a pneumatic control valve and combination valve. Control of the system is taken over by the engine control unit. While the system is working, the electrical pump is switched on by the engine control unit via the control relay. At the same time, the pneumatic control valve is triggered. This opens and allows the partial vacuum from the intake pipe to act on the combination valve. The partial vacuum opens the combination valve and the additional air transported by the pump is pumped into the exhaust gas pipe downstream from the outlet valves. As soon as the lambda control becomes active, the secondary air system is switched off. The engine control unit deactivates the electrical pump and the pneumatic control valve. The combination valve is also closed and thus prevents hot exhaust gases getting to the electrical pump and causing damage.

Fault symptoms when the secondary air system fails

The lack of “afterburning” leads to increased exhaust gas values during the cold-start and warm-up phase. The catalytic converter only reaches its working temperature later. Secondary air systems that are monitored by self-diagnosis of the engine control unit cause the engine warning light to come on if faults occur.

Reasons for failure of the secondary air system

A frequent reason for failure is a defective pump. Humidity penetration leads to damage to the pump, which in turn causes pump seizure. Lack of ground and voltage supply can also lead to pump failure. Blocked or leaking pipes also cause system failure or malfunction. The control and combination valves fail due to blockage, damage or lack of control.

Troubleshooting and diagnosis work on the secondary air system

As with all other troubleshooting and diagnosis work, a visual inspection and additional acoustic test should be carried out first. The electrical pump can be heard during the acoustic test with the cold engine in idling. Even when the engine has been switched off, the run-out noise of the pump is clearly audible. During the visual inspection, all components must be checked for damage. Particular attention should be paid to the pipes and hose connections. They must be inserted correctly onto the components and must not be chafed. They must not be bent either or blocked by radii being too tight. Fuses must also be checked to make sure they are present and correct and not damaged. If no faults are found during these tests, a suitable diagnostic unit can be consulted for further diagnosis. The basic pre-condition is that the system is diagnosis-capable from the vehicle manufacturer's side.