Volvo 819 E - Contact-Breaker Versus Electronic Systems

Group 28 Ignition systems
Design and function - General
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Contact-breake r versus electronic systems
Figure {AI shows the tim ing curve for a con tact -breaker ignition system, wh ile (BI illustrates the settings with wh ich
an
electronic system is programmed , in the form of a three - dimensional map _In each case, both engine speed and
load are taken into account when determining the setting . However , in co ntrast with the electronic system, which
computes the t iming, the con ta ct-breaker system controls it w i thin a specified range.
The timing must be con trolled to a high degree of accuracy to meet the demands of the modern engine for the clean-
es t possib le exhaust emissions, maximum fuel economy and high performance under fluc tuating condi tions.
N
either may the se requirements be permitted to vary, even following an extended period of driving .
Timing curve symbols
Both figures illustrate the manner in whi ch the t iming angle {al varies w i th the engine speed (n) and the va cu um (pi
in the intake man i fo ld. The ang le (a) increases with speed; in other word s, igni t ion of the fuel/air mixture takes place
earlier
as the speed rises. The angle also increases w i th the vacuum in the intake manifold i.e. as the eng ine load falls
and
the fuel/air charge supplied to the engine becomes smaller .
Contact-breaker ignition systems
A. Co nt ac t-breaker ignition systems con trol the timing with the aid of a centrif ugal governor (1) and a vacuum ad-
vance
un it (2). (Being a speed - dependent device , the governor advances the ignition as the speed increases, while
the vacuum advance unit retards it as the engine load increases in response to the vacuum signal from the intake
manifold .) T he two devices operate independently w i thin a tol erance band of approx . 2-4 ". Overall, this means that
the system controls the tim ing with in a to l eran ce range of approx. 4-8" , since the setting is the sum of the angles in-
dicated
by the two curves. Furthermore , the variat i on wh ich results from wear of the breaker points affects the over-
all t
iming.
Electronic ignition systems :
B. The co ntrol unit ( 3) of an electronic ignit i on system, by contrast, is programmed w i th a series of specific settings
f or a number of defined speeds and engine loads. This means that the timing can be controlled to an accuracy as
h igh 8S 8 fraction of a degree and that each ignit ion pulse is delivered as close as pos sible to the op ti mum point.
( Thi s high level of accuracy is achieved partly by the fact that the con trol unit interpola tes between the programmed
speed and load points . In other words , it is capable of computing an intermediate setting.) Furthermore , the timing
can be com put ed over a wider range and is not restricted by the type of linear functions typ ical of a centrifugal gov -
ernor .
The t iming is computed with th e aid of sensors wh ich determine the engine load and speed. Th is in format io n is
transmitted electrically to the control unit electronics, in wh ich details of a number of different speed and load com-
binations , together with the appropriate t iming se tt ings, are s tored . Pr i or to each igni ti on pulse , the control unit com-
putes a setting wh ich is perfectly mat ched to the engine running co nd i tion s prevailing at the precise instant .
Adjustments required by the various timing co mpensation fun c tions are added to the three - dimensional map.
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