Volvo 819 E - Ignition Systems - Basic Parameters

Group 28 Ignition systems
Design and function - General
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Ignition systems - basic parameters
The ignition system control unit must be supplied with certain basic information in order to compute the frequency
or rate at w hich HT ignition pulses must be delivered to the spark plugs, when (in relation to the position of t he par-
ticular
piston before TOG) each pulse must be generated and to which plug the pulse is to be delivered. The signals
requi red for this purpose are derived from the engine speed (A), engine load (B) and crankshaft position (G).
Speed information
A. T he engine speed provides information on the number of ignition pulses which must be generated per unit of
time. The number of high-tension (HT) pulses delivered to the plugs must be increased as the engine speed rises.
The speed
signal is the most vital piece of information supplied to the control unit. For example, the engine cannot
be started in this signal is unavailable.
The
control unit a lso uses the engine speed to determine the point at which the ignition pulse must be delivered in
r
elation to the position of the piston before TOG. Ignition must take place earlier at higher speeds since the upward
and d ownward movement of the pistons is faster under these conditions. As a result, the time available for combus-
tion of the fuel/ air mixture is less and the timing must be advanced to ensure that the mixture is burned as comple te-
ly
as possible.
Load information
B. Except in turbocharged engines, the engine load varies with the vacuum in the intake manifold. At low loads, the
th r ott le is only partially open and the flow of induction air is low, resulting in a high vacuum. At higher loads, the
thro ttl e opens wider and more air is supplied to the engine. Under these conditions, the vacuum in the manifold ap-
proaches
closer to atmospheric pressure as the load is increased. It follows from this that a greater quantity of fuel/
air mixture is available for combustion as engine load increases and that the higher volume of gas produced rein-
forces the pressure
due to compression. Since the rate of combustion is accelerated by higher gas compression, the
t iming is retarded as engine load is increased. At low engine loads, on the other hand. the timing may be advanced
to
compensate for the low er rate of combus t ion and to improve fuel utilization.
Crankshaft po s ition information
C. The crankshaft position (i.e. angle ) provides information on the position of each piston in relat ion to TOC. This in-
formation is essential to the system control functions. which compute the timing continuously on this basis.
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