APRILIA SR 50 Workshop Manual page 105

SR 50
SAS
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 that raises the
temperature of the exhaust gasses for a quick light-off of the
catalyst.
The secondary air system is especially useful when the
engine is idling and during warm-up, since the catalyst alone
is not capable of triggering post-combustion under these
operating conditions.
DESCRIPTION OF THE SYSTEM
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 can
complete the combustion.
During overpressure stages, the secondary air reed valve
cuts air supply to prevent backflow.
On 4-stroke engines, the reed valve is equipped with a cut-
off device 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.
DETAILS OF SAS
• The system consists of a rubber hose that takes filtered
air in through a scoop on the casing.
•
Secondary air flow is controlled by a reed check valve,
on the vehicle right-hand side, on the flywheel cover.
• The secondary air tube is connected to a flanged metal
fitting attached directly to the head exhaust duct.
ENGINE
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