Sun Microsystems MCS 2000 SL - Page 73

L
DIESEL
OPTION:
THEORY
OF
A DIESEL
ENGINE
For
the
proper
funct'ion
of the
diesel
engine,
the
start
of
delivery
has
almost the
same
importance
as the
timing
of a
petrol
engine. In
a
petrol
engine the timing
of the ignition,
controls the
combustion
of the
fuel-air
mixture.
In a
d'iesel engine
the timing
of the injection,
controls
the
combustion. The
djesel engine is
more
sensitive
to
changes of
injection
timing
than a
petrol
engine
because the
air-diesel
mixture needs a certain
pressure
to
combust.
The
combustion, respectively
the
beginning
of injection,
can
be
investigated
by adequate
methods in
a
laboratory.
These
methods
are
appl'ied
for
the
development
of engines.
For the workshops,
however,
such extensjve
methods
are
not
appljcable
because
:
' At
a
given pump
adjustment
(ang'le
adjustment
of
the
pump
versus
the
crankshaft)
the
start of
injection
changes within
certain tolerances
depending
on
:
'
the tolerances
of the injection
system
.
the tolerances
of the nozzle
pressure
.
the
coking
condition
of the nozzle.
'
At
a
given
start of
injection
the
combustion
can
change
within
certain
limits,
depending
on
:
.
the
cetane number
of the fuel
.
the
air
pressure
of
the
engine
.
the
coking condition
of the
engine.
Therefore,
it is
suggestive
for
the workshop
to
use the
start of
delivery
as
an adjustment
value,
as
it
can be measured
and influenced
direcily.
Before
beginning
of
delivery,
there'is
a
certain
pressure
level
in the
injection
line
which
is
known
as residential
pressure.
At
beginning
of
delivery
the
pressure
at
the
pump
output rises.
The
pressure
wave
passes
through
the
injection
line
with
sonjc
speed. The
pressure
wave
needs
a
certain
period,
depending
on the length
of the lines,
to reach
the nozzle,
Therefore,
all
injection
lines
on a multi-cylinder
diesel
engine have
the
same length.
so the
start
of injection
is
delayed
with respect
to the
beginning
of
delivery,
due to the
delay in
the lines.
Expressed in
degrees,
this
delay rises
linearly
with
the
engine
speed.
At
a speed
of
'1000
RpM
the
delay
is
2
degrees,
at 2000 RPM
it is 4
degrees.
Because
the
air-diesel
mixture
needs
a
certain
time to
combust, the
injection
of
diesel
needs
to
be advanced.
If
the
speed
of the
eng.ine
increases
the
advancement
of
the
injection
must
also increase.
To
obtain
this increasing
advancement
of
injection,
the
diesel
pump
is
equipped
with
a
centrifugal
advance
control. This
centrifugal
advance
control
a'lso
compensates
the
delay in the
pressure
lines.
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