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The Citroën Guide
EMS MONOPOINT
see whether this is a cold start or a warm one. If the coolant
fluid is measured cold, a special warm-up sequence will be
started.
The engine needs significantly more fuel, a richer mix-
ture during this period. This extra fuel is used for two pur-
poses: first, part of the fuel injected is condensed on the
cold walls of the engine, second, to ensure better lubrica-
tion, the engine should run at an elevated revolution during
this period.
There are two ways to provide more fuel: through the
usual injectors, making the computer inject more gas than
normal, or by using an additional cold start injector
(CSV)—there is only one such injector even in multipoint
systems. This injector is fed through a temperature-timer
switch, protruding into the coolant just like the CTS, plus it
is heated by its own electric heater. The injector operates as
long as the ignition key is in the starting position but its be-
havior later on is governed by the timer switch. The colder
the engine initially is, the longer it stays closed to let the
cold start injector do its job. In a warm engine (above 40 °C)
it does not close at all.
Without a cold start injector, the computer itself adds
about 50% extra fuel initially and drops this surplus to
about 25% until the end of a 30-second time period.
From that point, the surplus is dictated by the warming
of the engine, communicated by the CTS to the computer.
EFI systems without an idle speed control device often use
an electromechanical auxiliary air valve (AAV). This
valve, which is fully open when the engine is still cold but
will close gradually as it warms up, lets an additional
amount of air measured by the AFS pass through the sys-
tem. Because it is measured, it tricks the computer into pro-
viding more fuel. The valve is heated by its own heating ele-
ment as well as the engine, thus it closes shortly.
The injectors are electrovalves. As with any electromag-
net, there is a small time delay between the arrival of the
control signal and the actual opening of the valve due to
the build-up of electromagnetic fields. The length of this de-
Fuel Injection: Electronic Fuel Injection
ECU
OS*
CAS
ATS*
MAP
fuel
pump
fuel
engine
distri-
butor
CTS
TP
throttle
ISCM
As the computer has already calculated the exact
amount of fuel to be injected, there is only one task left: ac-
tually injecting it. There are two possible ways: to inject the
fuel into the common part of the inlet, still before the throt-
tle butterfly, or to inject them close to the inlet valves, indi-
vidually to each cylinder. Depending on the solution cho-
sen, the system will be called monopoint or multipoint.
Monopoint fuel injection requires a single common injec-
tor; the smaller cost and simpler setup makes it more com-
mon on smaller engines (in the case of Citroëns, the 1380
ccm ones). In all cases, the computer actually calculates the
half of the fuel amount required as it will be injected in two
installments, once for each revolution of the engine.
The injectors of the multipoint system can be operated si-
multaneously or individually. Previous Citroëns on the road
today still use simultaneous operation. Individual cylinder in-
jection, however, holds great potential—just to name one,
some of the cylinders of a larger engine can be temporarily
shut off by cutting off their fuel supply if the car is operat-
ing at partial load, saving a considerable amount of fuel—,
so we are sure to meet this sort of fuel injection systems in
the future.
All systems—regardless of the number of injectors—use
a similar fuel supply layout. The fuel is drawn from the tank
by a continuously operating fuel pump, transported via a fil-
ter to the injectors, then back to the tank. There is a pres-
sure regulator in the circuit as well to keep the pressure of
the fuel at a constant pressure above that in the inlet mani-
fold (this regulator is a separate unit on multipoint systems
while integrated into the injector on monopoint ones). As
the pressure difference between the two sides of the injec-
tors are constant, the amount of fuel injected depends
solely on the opening time of the injectors. The pressure
used in contemporary EFI systems is 3 to 5 bars.
This is practically all there is to it, there are only a couple
of safety and economy features in addition. If the engine
revolution exceeds a certain limit (between 1,200 and
1,500 usually) and the throttle is closed—this is called decel-
lay depends heavily on the
voltage the injectors are fed
fuel
exhaust
with. The same pulse width
air
would result in shorter open-
coolant
ing time, hence less fuel in-
*
not present
in all systems
jected if the battery voltage
drops below nominal (which
is often the case on cold morn-
ings). The injection computer
therefore has to sense the bat-
tery voltage and to lengthen
the injector pulse width if nec-
essary.
The final, total pulse width
(also called injector duty cy-
cle) is calculated by summing
up all these values received:
injector &
the base pulse width from the
pressure
regulator
RPM/AFS table lookup, the
ATS*
various
correction
based on the temperature sen-
sors, throttle position and the
like, plus finally, the voltage
correction.
7
factors
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