Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 128

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OTHER CONTROL SYSTEMS - EGR Cut-off Control System, Fuel Octane Judgmen t
EGR CUT-OFF CONTROL
SYSTEM
This system actuates the VSV, which therefore
causes atmospheric air instead of intake
manifold vacuum to act on the EGR (exhaust gas
recirculation) vacuum modulator . This shuts off
the EGR to maintain drivability when the engine
coolant is cold or during high-speed driving etc .
OPERATIO N
The Engine ECU actuates the VSV, shutting off
the EGR when the coolant temperature is below
a predetermined temperature or when the
engine speed is above a set speed (roughly
4,000 to 4,500 rpm), to maintain drivability . The
ECU also actuates the VSV to shut off the EGR
when the intake air volume is above a
predetermined level or when the fuel cut-off
function is on in order to maintain EGR valve
durability .
Thro
tt
le valv e
l
EGR
vsv
~-
.n, .+~ ~ i ~ . .,~
EGR valve
Air
intake
chamber
i
Sensors Exhaust ga s
EGR
'
ECU
S
Battery
OHP 8 1
REFERENCE
Some recent models use stepper motor type
EGR valves . An EGR vacuum modulator and
VSV are not provided in this system . The
Engine ECU controls EGR volume and cut-off .
In addition, when current is not applied to the
EGR valve, the valve is fully closed by the force
of a return spring .
Exhaust ga s
FUEL OCTANE JUDGMEN T
The Engine ECU in some engine models
determines the octane rating of the gasoline
being used (whether it is "premium" or
"regular") according to engine knocking signals
from the knock sensor .
OPERATIO N
The ECU judges whether the gasoline is
"premium" or " regular" based on the retard
angle of the ignition timing, which is determined
by the strength of engine knocking when the
coolant temperature is above a predetermined
temperature . It judges the gasoline to be
"regular" when the engine knocks severely an d
the retard angle is larger than a predetermined
value . It judges the gasoline to be "premium"
when the engine knocks only mildly and the
retard angle is smaller than a predetermined
value . The ECU stores the result of this
judgment until it judges that the octane rating of
the gasoline has changed .
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