Volvo 819 E - Page 39

Group 28 Ignition sys t ems
Des ign and fun ction Compensation functions
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Adaptive timing retardation
Background/ funct ion
If normal eng i ne load is exceeded for longer than usual ( for example , when the car is dr iven up a long incli ne , is pull -
ing a trailer or is running on low -g rade fuel), knock-controlled retardation wi ll intervene more often. The adaptive re-
tardat ion function , on the other hand , retards the t iming by a small amount for a longer per io d to redu ce the risk of
knock at specific combina t ions of engine speed and load.
Meaning
roughly 's elf-learning', th e term ' adaptive ' is used to describe the fact th at the con tr ol unit, af t er a time,
'tearns' that running condi tions have c hanged more or less permanently , The fun ctio n is applied individu ally to each
cylinder.
Conditions
On the upper curve (green), the upper posit ion on the vertical axis represents a combina tion of engine speed and
load which exper i ence has shown to be particularly susceptible to knock. The hor izontal axis is the t ime scale, cali-
brated in this case in tens of minutes .
If the
con tr ol unit detects that t he load/speed increase is of a 'p ermanent ' nature (Le. th at it has lasted for a specific
time ), it will adop t the adaptive retardat io n mode when kno ck occurs.
Activation
The lower curve illustrates the practical effect of adaptive tim ing retardation. Representing a per i od of approx , 10
minutes during wh ich the function is activated, interval lal is termed the 'active adaptive zone ',
If the control unit detects that the speedlload increase is of a permanent nat ure before the adaptive zone is reached,
the basic timing will be ret arded by 1°, The function is activated in the event of knock i.e. th e basic tim ing is retarded
as indicated by the start of interval (a) in the figure.
In
this case, the tim ing is retarded by 4° ( to -4 °, or 1° mo re than under 'nor ma l' kn ock conditions). following whi ch it
is
re -advanced in the usual manner (a t a rate which varies w i th engin e speed) to 1° below 't he basic setting' (0. _' 0 ).
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