Fundamentals of
Fundamentals of
PowertrainPowertrain
Control strategies & OBD II
Control strategies & OBD II
Diagnostics
Diagnostics
Secondary Air System Monitoring:
Secondary Air System Monitoring:Secondary air system is used to improve the performance of the Secondary air system is used to improve the performance of the catalytic converter (Three way) by providing extra catalytic converter (Three way) by providing extra
oxygen rich air to either the converter itself or to the exhaust
oxygen rich air to either the converter itself or to the exhaustmanifold. The catalyst temperature must be above about 200manifold. The catalyst temperature must be above about 200o o C to efficiently oxidize HC and C to efficiently oxidize HC and reduce
reduceNOxNOx. During engine warm. During engine warm--up when the catalytic converter is cold, HC and CO are oxidized up when the catalytic converter is cold, HC and CO are oxidized in the exhaust manifold by routing secondary air to in the exhaust manifold by routing secondary air to the exhaust manifold in controlled quantify by the PCM. This cre
the exhaust manifold in controlled quantify by the PCM. This creates extra heat to speed warmates extra heat to speed warm--up of the converter and EGO sensor, enabling the PCM to up of the converter and EGO sensor, enabling the PCM to go into closed
go into closed--loop mode more quickly.loop mode more quickly. During open
During open--loop control (cold converter) the converter is liable to be damaloop control (cold converter) the converter is liable to be damaged if excessive heat is applied to it, to warm it up. This can ged if excessive heat is applied to it, to warm it up. This can happen if happen if
excessive amounts of HC and CO are oxidized in the exhaust manif
excessive amounts of HC and CO are oxidized in the exhaust manifold during periods of heavy loads which call for fuel enrichmentold during periods of heavy loads which call for fuel enrichment, or during severe , or during severe deceleration. During start
deceleration. During start--up and such heavy loads, the secondary air is not let into exhauup and such heavy loads, the secondary air is not let into exhaust manifold but directed into the air cleaner where it has no st manifold but directed into the air cleaner where it has no effect on exhaust temperatures.
effect on exhaust temperatures.
After warm
After warm--up, during closedup, during closed--loop operation, the secondary air is used to supply oxygen to thloop operation, the secondary air is used to supply oxygen to the second chamber of the threee second chamber of the three--way catalyst, in dualway catalyst, in dual-- chamber converter system. In a dual
chamber converter system. In a dual--chamber converter, the first chamber contains rhodium, palladiumchamber converter, the first chamber contains rhodium, palladium, and platinum to reduce, and platinum to reduceNOxNOxand to oxidize HC and to oxidize HC and CO. The second chamber contains only platinum and palladium.
and CO. The second chamber contains only platinum and palladium.The extra oxygen from the secondary air improves the converter’The extra oxygen from the secondary air improves the converter’s ability to oxidize s ability to oxidize
HC and CO in the second converter chamber. The control of the se
HC and CO in the second converter chamber. The control of the secondary air is done by using two solenoid valves similar to the condary air is done by using two solenoid valves similar to the EGREGRpintlepintlevalve. One valve. One valve switches air flow to the exhaust manifold or to the air cl
valve switches air flow to the exhaust manifold or to the air cleaner (atmosphere). The other valve switches air flow to the exheaner (atmosphere). The other valve switches air flow to the exhaust manifold or to the aust manifold or to the
catalytic converter. The air routing is controlled based on engi
catalytic converter. The air routing is controlled based on engine coolant temperature and Air/Fuel ratio, indicated by the lambne coolant temperature and Air/Fuel ratio, indicated by the lambda sensor. If the control is da sensor. If the control is
open
open--loop and if the coolant temperature is below threshold and Air/Floop and if the coolant temperature is below threshold and Air/Fuel ratio is not too rich, then the air flow is directed to the uel ratio is not too rich, then the air flow is directed to the exhaust manifold. If exhaust manifold. If coolant temperature is higher than threshold and the Air/Fuel ra
coolant temperature is higher than threshold and the Air/Fuel ratio is rich (lambda < 1) then the secondary air is directed totio is rich (lambda < 1) then the secondary air is directed tothe air cleaner which exits the air cleaner which exits to the atmosphere. If the control is closed
to the atmosphere. If the control is closed--loop, then the lambda sensor is monitored for correlated deviatiloop, then the lambda sensor is monitored for correlated deviations when the secondary air flow is changed ons when the secondary air flow is changed
from exhaust manifold, or catalytic converter, or air cleaner, d
from exhaust manifold, or catalytic converter, or air cleaner, depending on coolant temperature, and lambda value. The OBD II reepending on coolant temperature, and lambda value. The OBD II requirement is that the quirement is that the
secondary air system shall have the diagnostic system monitor th
secondary air system shall have the diagnostic system monitor the proper functioning of the secondary air delivery, and any air e proper functioning of the secondary air delivery, and any air switching valve switching valve (solenoid).
(solenoid).
The critical parameters of the secondary air system are monitore
The critical parameters of the secondary air system are monitored and if found to be out of permissible range of values, the faud and if found to be out of permissible range of values, the fault code is set. The MIL is lt code is set. The MIL is
illuminated.
illuminated.
Secondary air diagnostics are described in more detail in a late