Table of Contents
Advertisement
U
The Citroën Guide
tentions of the driver. The ECU uses this sensor to learn
about special conditions like idle speed or full load as well.
Air temperature is measured by a sensor in the inlet mani-
fold (but if the air mass is measured by a heated platinum
wire sensor, this already provides the necessary air tempera-
ture correction, thus there is no need for an additional sen-
sor).
The ECU stores the basic engine characteristics, the intrin-
sic relationship between the air intake and the engine
speed (plus the manifold pressure if a turbo is fitted). The
values obtained from this table are corrected according to
the inputs of the various sensors, in order to arrive at a basic
timing and smoke limit value. The actual amount of fuel in-
jected and the accurate timing are a function of these
results and the position of the accelerator pedal.
The final amount of fuel calculated will be used to con-
trol the electric actuator 8 which—by moving a lever Û—
changes the position of the regulating collar 5. To ensure
the necessary precision, the factual position is reported
back to the computer using a potentiometer.
As we have already mentioned, the exact timing of the in-
jection is of utmost importance in a diesel engine. The elec-
tronic system uses a needle movement sensor built into
one of the injectors (the other are assumed to work com-
pletely simultaneously) notifying the computer about the
precise time of the beginning of the injection. Should there
be any time difference between the factual and designated
opening times, the electro-valve ß of the injection ad-
juster à will receive a correction signal until the difference
disappears. If the electro-valve is completely open, the injec-
tion start will be delayed, if it is closed, the start time will be
advanced. To achieve the timing required, the valve is
driven with a modulated pulse signal, with the duty cycle
(on-off ratio) determined by the ECU.
The input from this sensor is also used for compensating
calculations on the amount of fuel injected, and to provide
the on-board computer with the exact amount of fuel used
up so that it can calculate the momentary and average con-
sumption.
The computer has extensive self-diagnostic functionality.
Many sensors can be substituted with standard input val-
ues in case of a failure (serious errors will light up the diag-
nostic warning light on the dashboard). Some sensors can
even be simulated using other sensors—for instance, the
role of a failing engine speed sensor might be filled in by
the signal generated from the needle movement sensor.
As there is no standalone ignition in a diesel engine, the
only way to stop it is to cut off the fuel supply. The mechani-
cal default position of the actuator 8 is the position where
no fuel enters the injectors at all; this is where it returns
when the computer receives no more voltage from the bat-
tery, the ignition switch having turned off.
As it has already been mentioned, the inlet pressure is
one of the principal EDC parameters for a turbocharged en-
gine. Later Citroën turbocharged diesels—starting with the
2.5 TD engine of the XM—pioneered variable turbo pres-
sure technology. The wastegate on these turbines has sev-
eral actuators, fed with the turbo pressure through electric
valves. The ECU, based on the relevant engine operation pa-
rameters obtained from the sensors, controls these actua-
tors in various combinations, providing a selection of two
or three different wastegate limit pressures. This lets the sys-
Fuel Injection: Diesel engines
tem ease the compromise between the turbo pressure and
turbine speed: the pressure is kept at the usual value for
higher engine speeds (limited by the maximum turbine
speed) but is allowed to go higher than that in the middle
rpm ranges, adding a significant amount of torque in the
range where it is most needed.
Green versus Black
Diesel oil, just like gasoline, is a mixture of various hydrocar-
bons (C
n
the air, transforms to carbon-dioxide (CO
(H
O). However, as the combustion is never ideal, the ex-
2
haust gas also contains various byproduct gases: carbon-
monoxide (CO), various unburned hydrocarbons (C
trogen-oxides (NO
sel engine is operating with reduces the hydrocarbon and
carbon-monoxide content to 10–15%, and the amount of
nitrogen-oxides to 30–35% of the corresponding figures
measured in gasoline engines without a catalytic converter.
The sulphur content of the fuel—drastically reduced during
the recent decades—is responsible for the emission of sul-
phur-dioxide (SO
Conversely, these engines emit 10–20 times more par-
ticulates—or black soot—than gasoline engines. These are
unburned or incompletely burned hydrocarbons attached
to large particles of carbon. These substances are mainly al-
dehydes and aromatic hydrocarbons; while the first only
smells bad, the second is highly carcinogenic.
The much higher amount of particulates is due to the dif-
ferent combustion process. The various aspects of mixture
formation, ignition and burning occur simultaneously, they
are not independent but influence each other. The distribu-
tion of fuel is not homogenous inside the cylinder, in zones
where the fuel is richer the combustion only takes place
near the outer perimeter of the tiny fuel droplets, produc-
ing elemental carbon. If this carbon will not be burned later
because of insufficient mixing, local oxygen shortage (large
fuel droplets due to insufficient fuel atomization, caused by
worn injectors) or the combustion stopping in cooler zones
inside the cylinder, it will appear as soot in the exhaust. The
diameter of these small particles is between 0.01 and
10 mm, the majority being under 1 mm. Keeping the
amount of fuel injected below the smoke limit—the
lambda value where the particulate generation starts to rise
extremely—is essential.
Similarly to gasoline engines, the exhaust gas can be
post-processed to reduce the amount of pollutants even fur-
ther. There are two different devices that can be used:
U
Soot burning filter: as the diesel engine always oper-
ates with excess air (its lambda is above 1), there is
enough oxygen in the exhaust gas to simply burn the car-
bon soot present. The burning filter is manufactured
from ceramic materials that can withstand the resulting
high temperatures (up to 1200 °C). As the diesel engine
is very sensitive to excessive back pressure, the filter has
to be able to self-regenerate. This is solved by the addi-
tion of organic metal substances.
U
Catalytic converter, identical to the simpler ones used
on gasoline engines before the proliferation of three-
way, controlled converters. It reduces the carbon-monox-
ide and hydrocarbon content of the exhaust gas.
H
), and burned together with the oxygen (O
m
). The relatively high lambda value a die-
x
) and sulphuric acid (H
2
16
) of
2
) and water vapor
2
H
), ni-
n
m
SO
).
2
4
Hide quick links:
Advertisement
Table of Contents
