Electronic Control Unit - Lotus TURBO ESPRIT Service Manual

Page 18
Lowering the pressure beneath the diaphragms w i l l increase the pressure differential across
the metering slits and increase fuel delivery to the iniectors.
The variable throttle takes the form of a Frequency Valve, which continuously cycles
open/clored, with the ratio between open and closed periods being determined
by
the
electronic control unit
.
LD.
15
-
ELECTRONIC
CONTROL
UNIT
(E
.C .U
.)
The function of the electronic control unit i s to process information fed to i t by
various sensors including the exhaust gas Lambda sensor, and set an operl/close
ratio for the frequency valve.
This method of regulating the air/fuel ratio to Lambda
= 1 .O
by continuously
monitoring exhaust gas oxygen content, i s called Lambda closed loop control,
and may be represented by the following flow chart.
Rich mixture
Injected fuel N o Oxygen i n
quantity increases
ex ha u s t g ases
Regulated fuel
pressure decreases
Open/close ratio ~ ~ e n / c l o s e ratio
of Frequency Valve
of Frequency Valve
i s increasing i s decreasi ng
Low sensor Regulated fuel
voltage pressure increases
Injected fuel
a
Oxygen i n
ex haus t gases
quantity decreases
Lean mixture
Certain operating conditions require different air/fuel ratios,
and
i t i s necessary
to switch to an open loop control system with a preset frequency valve open/close
ratio.
The opeq/close ratio of the solenoid opemted frequency valve is conholled by an
electrical pulse sent to open the valve many times a second.
Under normal
runntng conditions, the valve i s openedfor about
50%
of the time, and the pulse
rate supplied to it i s referred to as
50%
duty cycle.
100%
duty cycle would mean
the valve I s held permanently open.
0