Group
28
Ignition systems
Design and
fu
nction
Compensation functions
Theory of temperature-compensated timing
advance
Advancing
the
ignition
timing
increases the cylinder temperature while reducing
the
exhaust gas temperature. Un-
der
certain conditions, this a
l
so yields a
reduction
coolan
t
temperature.
The
higher
cylinder
t
emperature is due to
the higher
pressure of
the fuel
/
air mixture as it is ignited, while the lower exhaust gas
temperature
is due
to the
rela-
tively
l
ate scavenging of the gases at
the
end of the combustion process.
Th
e lower coolant
temperature
achieved by advancing
the timing
when
idling is
partly
due to the
fact
that the
setting
is already
retarded and that a re
l
atively high proportion of the fuel does
as a
result.
produce mechanical
work, the
energy being dissipated in the form of heat losses. Advancing the timing
under
these conditions grea
tl
y
improves the efficiency of combustion, increasing the amount of energy converted
into
mechanical work and reduc-
ing
the
amount of
heat
discharged
to
the coolant.
Theory of temperature-compensated timing retardation
Although
reduces
the
cy
l
inder temperature,
retarding the
ignition increases the exhaust gas and coolant
temper-
atures.
The lower
cylinder temperature is due to
the
reduced pressure of the mixture when
is ignited, while the
rise
in exhaust gas
t
emperature is due
to
scavenging of
the
gases closer
to the
exhaust valve opening
point.
The
higher
coo
l
ant
temperature is
attributable
to the
fact
that less
of
the
energy content of
the
fuel is converted into mechanical
work,
a higher
proportion
being dissipated in
t
he form of
thermal
losses. As a
result,
a higher quantity of heat is
transferred
more quick
l
y to
the
cyl
i
nder
exhaust gas
port,
intake manifo
l
d and coolant passages
.
Timing compensation on
cold
engine
Temperature
compensation of the timing on a cold engine usually
i
nvolves advancing
th
e
ti
ming to shorten
the
warm
-
up period. However,
temperature-controlled functions which retard the timing
in
a co
l
d engine are also
used
.
One of the effects of this is to bring the catalytic converter up to working temperature more quickly, while another is
to
increase
the
coolant temperature, accelera
t
ing
the
defrosting action of
the
c
l
imate con
t
rol system.
The temperature
sensor signals may also be used by
t
he control unit
to
determine when
t
he
knock
sensor signal
should be switched in. B
l
ocking this signal when
the
engine is cold ensures that the control unit is
unaffected
by spu-
rious
signals caused by
the
mechanical
no
i
se
typically
emilled by the engine as it
warms
up.
Timing compensation on hot
engine
Temperature
compensation of the timing on a hot engine means advancing the timing
to
reduce
the
coolant
temper-
a
t
ure.
This
prevents the engine from boiling (although
the
cooling water will not begin
to
boil
while
the
t
emperature
is
below
approx.
120-125
°
C, due to the fact that the sys
t
em is pressurized)
.
The timing
advance is normally applied
only when
idling,
since
t
he setting is normally fairly retarded under these conditions.
•
42
