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  Secondary-air system


The first testing of Lambda-regulated engines was carried out at a starting temperature of +20°C. In the meantime, one begins, much more realistically, at sub zero temperatures. In view of the slight enrichment necessary in this phase, higher demands are made on the reaction time of the catalytic converter and the Lambda sensor. If an increase in the idling speed (mostly for 5 to 20 seconds) is not enough to improve this and to quickly sort out the, particularly often occuring, CO- and HC values during the cold starting phase, there is the alternative of the secondary air injection.


Directly after starting, filtered air is supplied to the exhaust system for up to two minutes. This is drawn from the intake by an electric blower (secondary air pump) and sent through an electric shift valve, which at the same time, must be opened by the control device. The (sometimes still electro-pneumatic) shift valve is necessary to protect the blower from the temperature and pressure of the exhaust gas. Not exhaust gas recirculation is the target, but fresh-air induction, which warms up the catalytic converter and the Lambda sensor distinctly. The already present enriching during cold running does help to reduce the unfavourable exhaust gases by introducing additional air. Within an OBD-system the secondary air injection is checked from time to time (sporadically).


Secondary air injection is also found in motorcycles. In this case, it is not necessarily the catalytic converter which is heated, but the exhaust gases which, for measuring purposes, are diluted. 09/11