CITROEN DS series Technical Manual page 13

U
The Citroën Guide
sure that the amount of fuel injected as well as the timing
provide better combustion and lower pollution when the
engine is started and operated at low temperatures. They
do not have any effect once the engine reaches the normal
operating temperature.
Now that the correct amount of fuel is care-
fully determined and the necessary high pres-
HP fuel
return
sure generated by the pump, it has to be in-
1 jected into the swirl chamber. The pressurized
fuel entering the injector through a filter 1
3
tries to press the piston 2 upwards but a
spring 3 counters this force. As soon as the
2 pressure exceeds the force of the spring
(which can be adjusted by placing appropri-
ately sized shims behind it), the piston jumps
up and the fuel rushes into the swirl chamber
through the small orifice now opened. After the injection
pump closes its pressure valve at the end of the injection pe-
riod, the spring 3 pushes the piston 2 back, closing the ori-
fice until the next injection cycle.
Each swirl chamber has its own glow plug whose only
purpose is to heat up the chamber in cold weather. They
start to glow when the ignition key is turned into the first
position and stay glowing for some time afterwards unless
the starting was unsuccessful.
Turbo
More power requires
more fuel. An efficient
way to boost the perfor-
mance is to provide
both more air and fuel
to the engine. The ex-
haust gases rushing out
from the engine waste
a great deal of energy; a
turbocharger 4 spun
by the exhaust flow taps into this source of energy to pro-
vide added pressure in the air inlet. Diesel engines are partic-
ularly well suited for turbocharging. Gasoline engines may
not have the inlet pressure raised too much because the air
and fuel mixture may subsequently self-ignite when it is not
supposed to, and instead of burning controllably, detonate.
In a diesel such a situation is not possible because the fuel is
injected only when combustion should actually happen in
the first place. As a result, relatively high inlet pressures can
be used, considerably improving the power output of a die-
sel engine, and with proper attention to the subtleties of
the design, engine efficiency and fuel consumption.
On its own, once the amount and pressure in the ex-
haust manifold reaches a level high enough to power it,
with the engine fully loaded, the turbine would spin propor-
tionally to engine speed squared, because both the pres-
sure and the volume of the air pumped into the engine are
increasing.
Because the engine is required to deliver as much torque
as possible at the widest possible range of engine revolu-
tion, the requirements on the turbine are somewhat contra-
dictory. If the turbo is made very small and light, it will spin
up very quickly due to its low mass and inertia, ensuring its
full benefit already at low rpms. However, with a moderate
Fuel Injection: Diesel engines
4
5
2
3
air
1
exhaust
increase in engine speed, the rotational speed of the tur-
bine (note the quadratic relationship) would become exces-
sively high. When the turbine blade speed approaches the
speed of sound, a supersonic wave effect occurs that can
abruptly leave it without any load, at which point runaway
would occur, resulting in severe damage to the turbine.
On the other hand, if the turbine was dimensioned so
that even at the highest engine speed it is still operating
within safe limits, it would not be useful at all in the middle
range where the engine is most often used. A compromise
can be achieved using an overpressure valve, the waste-
gate valve 5. The turbo pressure is constantly monitored
by this valve opening above a set pressure limit, letting the
exhaust escape through a bypass. This avoids turbo run-
away by making the turbo rotational speed proportional to
that of the engine, once the limit pressure is reached. This
way the quick spin-up resulting from the quadratic relation-
ship can be preserved while the turbocharging effect is ex-
tended over a significant percentage of the usable engine
speed range—typically the higher 70-80%. But it comes at
a price: because of the simplicity of such a regulation, the
limit pressure is dictated by the maximum turbine speed,
which is usually calculated for maximum engine speed plus
a safety margin. The maximum pressure is already reached
at lower engine and turbine speeds, where the turbine
could conceivably still provide more pressure because of a
lesser demand for air volume. Although with a simple
wastegate a certain amount of the turbocharging potential
is lost, the increase in power output is still substantial.
Citroën is a pioneer in implementing variable wastegate
limit pressure using a controllable wastegate valve, to tap
into this previously unused turbo potential.
Essentially, a turbocharged diesel engine runs in two dif-
ferent modes: atmospheric pressure or turbo-charged. The
atmospheric pressure mode prevails while the exhaust gas
produced is not yet sufficient to power the turbine (below a
given engine speed and load). Once this limit is crossed and
the turbine starts generating higher than atmospheric pres-
sure, the engine is running in turbocharged mode.
turbo
pressure
ç
pedal
ß
9
Þ
Ý
ever, the amount of fuel injected during the atmospheric
mode of the engine—before the turbo kicks in—has to be
reduced in order to avoid crossing the smoke limit. The
turbo pressure drives a limiter in the injection pump: with
the increasing pressure the piston æ moves down. Its vary-
ing diameter forces the lever ç rotate around its pivot,
which then acts as a stop to limit the allowed range of oper-
ation of the regulator lever 9, limiting the amount of fuel to
be injected.
Intercooler
The injection pump regula-
tor needs to know about the
changes in the inlet pressure,
æ
because those changes mean
differences in the amount of
air entering the engine. And
this also means that the upper
limit of fuel injected needs to
be changed correspondingly.
These injection systems are
tuned for the turbo producing
the rated waste pressure (also
known as full boost). How-
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