05-07-2013, 02:38 PM
Diesel distributor fuel-injection pumps
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The diesel engine
Diesel combustion principle
The diesel engine is a compressionignition
(CI) engine which draws in air
and compresses it to a very high level.
With its overall efficiency figure, the diesel
engine rates as the most efficient combustion
engine (CE). Large, slow-running
models can have efficiency figures of as
much as 50% or even more.
The resulting low fuel consumption,
coupled with the low level of pollutants in
the exhaust gas, all serve to underline
the diesel engine’s significance.
The diesel engine can utilise either the
4- or 2-stroke principle. In automotive
applications though, diesels are practically
always of the 4-stroke type (Figs. 1
and 2).
Power stroke
Following the ignition delay, at the beginning
of the third stroke the finely atomized
fuel ignites as a result of auto-ignition
and burns almost completely. The
cylinder charge heats up even further
and the cylinder pressure increases
again. The energy released by the ignition
is applied to the piston.
The piston is forced downwards and the
combustion energy is transformed into
mechanical energy.
Fields of application
Diesel engines are characterized by their
high levels of economic efficiency. This is
of particular importance in commercial
applications. Diesel engines are employed
in a wide range of different versions
(Fig. 1 and Table 1), for example as:
– The drive for mobile electric generators
(up to approx. 10 kW/cylinder),
– High-speed engines for passenger
cars and light commercial vehicles (up
to approx. 50 kW/cylinder),
– Engines for construction, agricultural,
and forestry machinery (up to approx.
50 kW/cylinder),
– Engines for heavy trucks, buses, and
tractors (up to approx. 80 kW/cylinder),
– Stationary engines, for instance as
used in emergency generating sets (up
to approx. 160 kW/cylinder),
– Engines for locomotives and ships (up
to approx. 1,000 kW/cylinder).
In-line fuel-injection pumps
All in-line fuel-injection pumps have a
plunger-and-barrel assembly for each
cylinder. As the name implies, this comprises
the pump barrel and the corresponding
plunger. The pump camshaft
integrated in the pump and driven by the
engine, forces the pump plunger in
the delivery direction. The plunger is returned
by its spring.
The plunger-and-barrel assemblies are
arranged in-line, and plunger lift cannot
be varied. In order to permit changes in
the delivery quantity, slots have been
machined into the plunger, the diagonal
edges of which are known as helixes.
When the plunger is rotated by the movable
control rack, the helixes permit the
selection of the required effective stroke.
Depending upon the fuel-injection conditions,
delivery valves are installed between
the pump’s pressure chamber and
the fuel-injection lines. These not only
precisely terminate the injection process
and prevent secondary injection (dribble)
at the nozzle, but also ensure a family
of uniform pump characteristic curves
(pump map).
Pressure-control valve
The pressure-control valve (Fig. 5) is
connected through a passage to the
upper (outlet) kidney-shaped recess, and
is mounted in the immediate vicinity of
the fuel-supply pump. It is a springloaded
spool-type valve with which the
pump’s internal pressure can be varied
as a function of the quantity of fuel being
delivered. If fuel pressure increases
beyond a given value, the valve spool
opens the return passage so that the fuel
can flow back to the supply pump’s
suction side. If the fuel pressure is too
low, the return passage is closed by the
spring.