Secondary Air System - APRILIA RS 125 Workshop Manual

ENGINE
4.2.

SECONDARY AIR SYSTEM

4.2.1. SECONDARY AIR SYSTEM
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.
WARNING
When adjusting the CO rate, disable the
Secondary Air System or exhaust gas dilution
caused by the system would give a wrong
reading of the carbon monoxide percentage.
See CO ADJUSTMENT for instructions on how
to disable the SAS.
WARNING
After setting the CO rate, remember to re-
enable the Secondary Air System. The vehicle
will operate regularly
disabled but in this case the harmful emissions
will be higher. Moreover, overheating might
derive due to air not circulating, thus causing
damages to SAS tubes and unions.
DANGER
Stop air from circulating using only suitable
equipment; using plugs to block off the air box
scoop might
secondary air system and, if left there for a
certain time, the plug might enter the intake
channel while the vehicle is running, thus
jamming the throttle and making the vehicle
impossible to control.
4 - 6
even with system
overheat and damage the
RS125