Volvo 819 E - Page 38

Group 28 Ignition sys t ems
Desi gn and funct io n - Compensa t ion f unctions
(lI 0
Q'°' F== == =
_8 °
Q'-15° { _
R EX-I 147 DIll
Load - controlled retardation
Function
Since a sudden increase in load greatly increases the risk of knock, the control un it in corpo rates a fun ction which re-
tards the tim ing sharply for a brief period u nder these condit ions. The fun c tion of load - controlled ret ardation is to re-
duce th e li ke lih ood of knock when the accelerator is suddenly depressed.
Backgro und
High engine load (equival ent to the injection of a large quanti ty of mixture) is a prerequis ite to the occurrence of the
more com mon type s of kn oc k. Thus, for examp le, when the accelerator is s uddenly depressed, the co ntrol uni t may
be unab le ( by virtue of its design) to retard the ignition quick ly enough by an amount equiva lent to the l oad increase.
For this reason, certain cars
may be subject to what is known as ' transitional ' knock if driven in too high a gear with
the accelerat or to the fl oor.
Sharp retardat i on
To overcome this problem , th e con trol unit incorporates a program -co ntr olled fun c tion wh ich retards the norma l
t
iming (oc) by SO in response to a load signallfrom the fuel co ntrol unit) indic ating a s udden pr essure drop in the in -
take
manifold.
Rapid re ·a dvan ce
Th e con trol uni t then restores t he t imi ng very rapidly to th e normal value, 0.. This is car ried ou t in steps of 1" every
five engine revolut ions (bo th values approx imate). regardless of whether th e higher load condi t ion is of longer dur-
ation .
Should the rap id pressure drop be fo llow ed by full openi ng of the throttle , th e con tr ol unit will have sufficien t t ime to
again retard
the basic tim ing by a sufficien t amount w i th ou t any significant ri sk of fu rther knock.
• EZ -K systems 8re equipped wi th 8 similar knock contr ol function.
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