Secondary Air Technology
The use of secondary air is a reliable method for reducing cold-start emissions from petrol engines.
Pierburg, who have many years of experience as OE supplier where secondary air is concerned, provide information on components, causes of failure and remedies.
The first secondary air systems were already used at the beginning of the seventies and although they are popular today, not much is known about the details.
Why use secondary air?
The reliable cold start of a petrol engine requires a "rich mixture" – a mixture with excess fuel. However, since the catalytic converter has not yet reached its working temperature, large quantities of monoxide and unburnt hydrocarbons are produced between a cold start and the activation of the lambda control – even on low-emission petrol vehicles.
To reduce these cold-start emissions there are only two practicable strategies, namely an upstream catalytic converter or blown-in secondary air.
The disadvantage of an upstream catalytic converter is that it has to be cooled down again during the warm-up phase. This is achieved by a mixture enrichment at full load – resulting however in an increase in fuel consumption and higher exhaust emissions at full load. Such disadvantages are avoided by blowing in secondary air.
Blowing high-oxygen, ambient air ("secondary air") into the exhaust manifold causes a post-oxidation ("catalytic post-combustion") of the pollutants.
Although the secondary air system is only activated for a maximum of 90 seconds after a cold start, HC and CO emissions are considerably reduced during the cold start phase.
Simultaneously the starting time of the catalytic converter is significantly reduced due to the heat released by the post-oxidation.
Note that because diesel cars always run with excess air, and thus a lean mixture, even during a "cold start", there has never been any necessity to blow in secondary air.
Below: Diagram of a secondary air system