APRILIA Pegaso 650 2005, Manual

Page: 103 / 139

ENGINE
4 - 26
PEGASO 650
4.2. SECONDARY AIR SYSTEM
4.2.1. SECONDARY AIR
The Secondary Air System (SAS) has been developed to
reduce carbon oxide and unburnt hydrocarbon emissions in
vehicles that are not equipped with a lambda sensor. Natural
air (which is rich in oxygen) is channelled into the exhaust
stream to trigger a post-combustion process.
The secondary air system is especially useful when the
engine is idling and during warm-up, as the catalyst alone is
not capable of triggering post-combustion under these
operating conditions.
The post-combustion process is activated just outside of the
combustion chamber and raises the temperature of the
exhaust gasses for a quick light-off of the catalyst.
The secondary air system operates on pressure fluctuation
in the exhaust system.
During depression stages, the exhaust takes in oxygen-rich
air from the secondary air system so unburnt gasses in the
exhaust stream -hot and active- can complete the
combustion.
During overpressure stages, the secondary air reed valve
cuts air supply to prevent backflow.
The reed valve is equipped with a cut-off device -controlled
by a vacuum pick-up point on the intake manifold- that shuts
down additional air during cut-off stages, as exceedingly lean
exhaust gasses would lead to exhaust blowing, resulting in
exhaust valve and catalytic converter damage.
Valve stays closed when engine is running.
ProCarManuals.com