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The Citroën Guide
Both changes have to do with the fact that the enhanced
system, whose new task is to generate the ignition signals
as well, cannot at the same time build on them as inputs.
This new sensor—practically a replacement for the induc-
tion magnet in the distributor of earlier systems—informs
the computer of both engine speed and camshaft position.
The flywheel has steel pins set into its periphery. As it ro-
tates, the inductive magnet of the CAS sends pulses to the
computer. Two of the pins are missing and this hole passes
before the sensor just as the first piston reaches its TDC posi-
tion. The missing pins cause a variance in the sensor output
that can be read by the ECU easily.
The rest is the same: the base pulse width is calculated
based on the CAS and AFS/MAP sensors. The correction fac-
tors—air temperature, idle or full load, starting, warming
up, battery voltage—sum up into an additional pulse
width. Besides, the same input signals (AFS, CAS, CTS and
TS/TP) are used for another lookup in a table, yielding the
correct dwell time and timing advance for the ignition. The
dwell period remains practically constant but the duty cycle
varies with the chaging engine speed. The ignition signal is
amplified and sent to a distributor containing only second-
ary HT components: it does not create the ignition signal
only routes the HT current to each spark plug in firing order.
Some systems also have a knock sensor (KS), sensing
the engine vibration associated with pre-ignition (so-called
pinking). If this occurs, the ignition timing is retarded to
avoid engine damage.
Think green
As we saw, fuel injection and engine management systems
are capable of determining the ideal amount of fuel to be in-
jected, depending on the conditions of operation and sev-
eral other factors in the engine. It is capable of deciding on
lean mixture for general, partial load to save fuel, or on rich
mixture when performance considerations call for this.
Unfortunately, this is not what such systems are used for
today. With the proliferation of catalytic converters, the
only concern of our systems is the welfare of the converter.
Ideal combustion would not generate polluting materi-
als in the exhaust gas. Fuel is a mixture of various hydrocar-
bons (C
H
), which when burned together with the oxygen
n
m
(O
) of the air, should transform to carbon-dioxide (CO
2
and water vapor (H
O). However, combustion is never ideal,
2
besides, fuel contains many additives: the exhaust gas, in
addition to the products mentioned, has various byprod-
ucts as well, some of them toxic: carbon-monoxide (CO),
various unburned hydrocarbons (C
(NO
) and lead (Pb) in various substances coming from the
x
anti-knock additives found in the fuel.
%
NO
x
C H
n
m
CO
1.3 V
0.8
1.0
1.2
Fuel Injection: Electronic Fuel Injection
H
), nitrogen-oxides
n
m
The relative amount of these
byproducts depend on the
lambda ratio of the air-fuel mix-
ture burned. As shown on the di-
agram, a value between 1.2 and
1.3 would give a relatively low
percentage of toxic byproducts
while, as we can recall, being a
lean mixture would be in the
right direction towards fuel
economy.
By using platinum (Pt) or rhodium (Rh) as a catalyst—a
catalyst is a substance whose presence is required to enable
(or to boost) a chemical transformation while it does not
take part in the process itself, remaining intact—the follow-
ing processes can be carried out:
W 2 CO
2 CO + O
2
W 4 CO
2 C
H
+ 7 O
2
6
2
2 NO + 2 CO W N
These precious metals are applied in a very thin layer to the
surface of a porous ceramic body with thousands of holes
to make the surface contacting the exhaust gases much
greater. Actually, a converter does not contain more than 2
or 3 gramms of these metals.
If you compare this diagram
with the previous one, you will
see that the real gain is the
supression of nitrogen-oxides.
CO and C
H
will be reduced as
m
n
well, although to a much lesser
extent. Nevertheless, the overall
reduction in polluting byprod-
ucts is quite high, amounting
up to 90 percent. Lead sub-
stances are not considered as
lead must not reach the converter anyway, it would clog the
fine pores of the converter in no time. The fuel used in cars
equipped with a catalytic converter has to be completely
free of lead.
But there is something of even greater consequence de-
picted on the diagram: to keep the amount of pollutants
down, the lambda has to be kept inside a very small value
range, practically at l=1 all the time. If the lambda drops
just a fraction below 1, the CO emission rises sharply, while
a small step above 1 skyrockets the NO
task of the fuel injection is therefore to ensure that the air-
fuel mixture sticks to the stochiometric ratio all the time.
This means higher consumption than the one of a car with
fuel injection without a converter to start with.
There are situations where this lambda cannot be ob-
served. A cold engine will simply stall without a much richer
mixture, thus the cold start mechanism does not obey the
lambda control. The catalytic converter does not work at all
below 250 °C, so this is not a significant compromise (its
)
normal operating temperature is 400 to 800 °C, above
2
800 °C is already harmful; unburned fuel getting into the ex-
haust and detonating inside the converter could cause over-
heating, thus ignition and similar problems has to be recti-
fied as soon as possible in catalytic cars).
Dynamic acceleration (full throttle) is also something not
observing the welfare of the converter. Reducing pollution
might be a noble cause but to be able to end an overtaking
is even more important...
The system uses an oxygen sensor (OS, also called
lambda sensor) which measures the oxygen content of the
exhaust gas. It is located between the engine exhaust and
the catalytic converter. Similarly to the converter, it is not
functional below 300 °C, hence it has its own heating ele-
ment to make it reach its operating temperature faster.
The computer uses the input from this sensor to keep the
mixture injected always as close to l=1 as possible. If the
sensor is still too cold to give accurate input, the computer
can ignore it safely.
(oxidation)
2
+ 6 H
O (oxidation)
2
2
+ 2 CO
(reduction)
2
2
%
CO
C H
n
m
0.9
0.99
1.0
emission. The main
x
9
NO
x
V
1.1
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