Downloaded from www.Manualslib.com manuals search engine Volumetric efficiency depends mainly on the:
a) Inlet valve and camshaft design
b) Inlet manifold design
c) Engine speed
The volumetric efficiency is normally less than one (1) and
has the effect that the actual air mass entering the cylin-
der will be less than the measured value.
Peak volumetric efficiency co-insides with the engine speed
where the maximum torque is developed.
7.4 FUEL METERING
Now that a method has been established to determine the air
mass entering the engine it is possible to calculate the
quantity of fuel to be injected to achieve the required A/F
ratio. An electric fuel pump delivers fuel at a constant
pressure to the injector which is opened for the required
injection duration. The fuel pressure is regulated by a
mechanical pressure regulator.
The fuel mass to be injected is:
Fm = Am/(A/F) Where Fm = Fuel mass
Am = Air mass
Since the fuel is injected in bursts it is necessary to know
the fuel flow rate of the injector, pump and regulator
system, as well as the injector opening and closing delays
at various battery voltages to enable calculation of the
injection duration.
Downloaded from www.Manualslib.com manuals search engine 8 IDLE SPEED CONTROL
Idle speed is controlled by means of the IACV (Idle Air
Control Valve) mounted on the throttle body assembly. The
IACV is driven by the IACSM (Idle Air Control Stepper Motor)
which is controlled by the ECU.
The IACV maintains constant idle speed (temperature depend-
ent) under all engine loads.
The ECU makes use of closed loop control algorithms to
ensure the best idle conditions for warm and cold engines.
Whenever the ignition is first switched the ECU selects a
pre-set idle position which is temperature dependent, there-
after it operates in the closed loop control mode.
Idle speed control is only possible with the throttle in the
closed position.
Timing advance support is utilised in the idle range to
enhance idle speed control. Spark timing is advanced by up
to 10ø if the idle speed drops below the set point, result-
ing in increased engine torque to offset the drop in speed.
If the idle speed rises above the set point the timing will
be retarded up to 10ø to reduce engine torque.
Correct idle speed control is not possible when the battery
voltage is below 9 volt.
