Diesel Engines - CITROEN DS series Technical Manual

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The Citroën Guide

Diesel engines

Diesel oil has been a contender to gasoline for
many decades. Earlier diesel engines were not re-
fined enough to win the hearts of many drivers
but recent advances in technology made these en-
gines not only a worthy competitor in all areas
but in some features—fuel economy or low end
torque, to name just two—even exceeding the
characteristics of their gasoline counterparts. And
in addition to the general technological advan-
tages, Citroën's diesel engines have a widely ac-
cepted reputation—even among people blaming
the quirkiness of its suspension or other fea-
tures—of being excellent and robust.
As it is widely known, diesel engines have no ignition to initi-
ate their internal combustion, they rely on the self-combus-
tion of the diesel oil entering into a cylinder filled with hot
air. Due to this principle of operation, the supply of the fuel
has to comply with much more demanding requirements
than it is necessary in the case of gasoline engines.
Unlike in the gasoline engine, not a mixture but air en-
ters into the cylinders via the inlet valves. During the adia-
batic compression all the energy absorbed is used to in-
crease the temperature of the gas. The small droplets of
fuel will be injected at high velocity near the end of the com-
pression stroke into this heated gas still in motion. As they
start to evaporate, they form a combustible mixture with
the air present which self-ignites at around 800 °C.
This self-ignition, however, is not instantaneous. The lon-
ger the delay between the start of the injection and the ac-
tual ignition (which depends on the chemical quality of the
Mechanical injection
Although the basic principles of fuel injection are similar to
what we have already discussed for gasoline engines, there
are some notable differences. First of all, diesel engines op-
erate without restricting the amount of air entering the en-
gine: there is no throttle, the only means of regulating the
engine is to vary the amount of fuel injected.
The fuel is injected into the engine, creating a combusti-
ble mixture in the same place it is going to be burned. Be-
cause the forming of this mixture results in its self-combus-
tion, the diesel injection system is, in essence, an ignition
control system. Unlike on the gasoline engine, fuel injection
and ignition cannot be separated in a diesel engine.
The complete mechanical injection system is built into a
single unit which can be divided into five individual—al-
though interconnected—subsystems:
U a low pressure fuel pump to deliver the fuel for the
rest of the injection system;
U a high pressure pump and distributor that routes
the fuel to the appropriate cylinders in firing order
(similar in purpose to the distributor on gasoline en-
Fuel Injection: Diesel engines
diesel oil, indicated by the cetane number), the more fuel
will enter the cylinder, leading to harsher combustion, with
the characteristic knocking sound. Only with the careful har-
monization of all aspects—beginning of injection, the distri-
bution of the amount injected in time, the mixing of the
fuel and air—can the combustion be kept at optimal level.
Small diesel engines suitable for cars were made possible
by a modification to the basic principle, that allowed these
stringent parameters to be considerably relaxed. It includes
a separate swirl chamber connected
to the cylinder via a restrictor orifice.
The air compressed by the piston in
the cylinder enters this chamber
through the orifice, starting to swirl in-
tensively. The fuel will then be injected
into this swirl, and the starting igni-
tion propels the fuel-air mixture still incompletely burned
into the cylinder where it will mix with the air, continue and
finish the combustion process. Using a prechamber results
in smaller ignition delay, softer combustion, with less noise
and physical strain on the engine parts, but introduces
some loss of energy because of the current of air having to
pass between the chambers. Citroën engines of this type
use a tangentially connected spherical prechamber.
As diesel engine evolution continued, better simulation
and modeling techniques became available, which, to-
gether with the improvements in fuel injection technology,
lessened or removed the problems initially solved by the in-
troduction of the prechamber. The direct injection engines
of today have no prechamber, instead, the piston has a spe-
cially formed swirl area embedded in its face.
gines) and generates the high pressure needed for the
injection as well;
U a regulator that determines the amount of fuel to be
injected in relation to the engine speed, modified by
additional factors like idle speed, cold starting, full
load, etc.;
U an injection adjuster to compensate for the higher
engine speed by advancing the start time of the injec-
tion;
U a fuel stop valve to cut off the fuel supply when the
ignition has been switched off.
The diesel fuel is drawn—through a filter—from the tank by
the low pressure pump 1 operated by the engine. A pres-
sure regulating valve 2 ensures that the fuel pressure will
not exceed a preset limit; when the pressure reaches this
value, the valve opens and lets the fuel flow back to the pri-
mary side of the pump.
The piston 6 of the high pressure part is driven through
a coupling 4 consisting of a cam disc and four cam rollers.
The piston rotates together with the shaft coming from the
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