Volvo 819 E - Page 44

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 al so yields a reduction in 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 late 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 well retarded and that a re l atively high proportion of the fuel does not. 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 it reduces the cyl 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 it 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 the form of thermal losses. As a result, a higher quantity of heat is
transferred
more quick ly to the cyli nder wall, exhaust gas port, intake manifo ld and coolant passages.
Timing compensation on cold engine
Temperature compensation of the timing on a cold engine usually involves advancing th e ti ming to shorten the
warm - up period. However, temperature-controlled functions which retard the timing in a co ld 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, accelerating the defrosting action of the cl imate control system.
The temperature sensor signals may also be used by the control unit to determine when the knock sensor signal
should be switched in. Bl ocking this signal when the engine is cold ensures that the control unit is unaffected by spu-
rious signals caused by the mechanical no ise 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-
ature. 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 syst em is pressurized). The timing advance is normally applied
only when idling, since the setting is normally fairly retarded under these conditions .
•
42