This system further lowers the HC and CO values during the cold start phase when the catalytic converter is not yet active.
A conversion rate of over 90 percent is achieved using a 3-way catalytic converter in stoichiometrically operated gasoline engines. On average, up to 80 percent of the emissions of a driving cycle are emitted during a cold start. However, because the catalytic converter only works effectively from temperatures of approx. 300°C – 350°C, the emissions must be lowered during the cold start phase using different measures. This is the task of the secondary air system.
If there is sufficient residual oxygen in the exhaust system and the temperature is high enough, the HC and CO react in a secondary reaction to form CO2 and H2O.
To ensure there is enough oxygen for the reaction during the cold start phase, when the mixture is very rich, air is added to the exhaust flow. For vehicles with a three-way catalytic converter and a lambda control, the secondary air system is switched off after approx. 100 seconds. The operating temperature of the catalytic converter is quickly reached through the heat generated in the secondary reaction.
The secondary air can be supplied actively or passively. In the passive system, fluctuations in pressure in the exhaust system are utilised. Additional air is drawn in via a timed valve due to the vacuum created through the flow speed in the exhaust pipe. In an active system, the secondary air is blown in by a pump. This system allows better control.