<div class="htmlview paragraph">For years, secondary air injection Systems have been used to reduce hydrocarbon exhaust emissions for a short period after engine cold start. In the beginning, passive secondary air systems were used, with the airflow driven by the pressure pulsations in the exhaust system. Since 1990, for most applications, active secondary air systems (i. e., systems where air is injected into the hot exhaust gases by a pump) have been employed. Secondary air injection into the hot exhaust gases is realized by a d-c motor driven turbine pump, i. e. a secondary air pump, and a control valve.</div> <div class="htmlview paragraph">Numerous factors, including raw engine emissions during cold start and warm up, driveability requirements and the need to adapt to different emissions legislation, dictate the use of secondary air injection systems. The development of other exhaust aftertreatment systems, e. g., close-coupled or heated catalysts as well as packaging and cost factors will influence the market penetration of secondary air systems.</div> <div class="htmlview paragraph">The transition from LEV to ULEV and from EU 3 to EU 4 will cause a considerable increase in the use of secondary air injection with port injection SI engines.</div> <div class="htmlview paragraph">The technical improvements of secondary air injection systems are mainly related to increased airflow performance and enhanced acoustical properties of the secondary air pumps. With a new type of secondary air valve, the number of system components and thus the system cost can be significantly reduced. A new demand-responsive secondary air pump allows further improvement in the efficiency of the exhaust after treatment and facilitates OBD by allowing direct verification of the secondary airflow rate.</div>