Page 254 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Key fact
No matter how complex the
electronics may seem, a high-
voltage spark is produced by
switching a coil on and off.
Key fact
For optimum effi ciency the ignition
advance angle should be such as
to cause the maximum combustion
pressure to occur about 10° after
TDC.
Automobile mechanical and electrical systems
manual is essential when working on any vehicle. The main ignition components
are the engine speed and load sensors, knock sensor, temperature sensor and
ignition coil. The ECU reads from the sensors, interprets and compares the data,
and sends output signals to the actuators. The output component for ignition is
the coil.
Ignition systems continue to develop and will continue to improve. However,
keep in mind that the simple purpose of an ignition system is to ignite the fuel/air
mixture every time at the right time. And, no matter how complex the electronics
may seem, the high voltage is produced by switching a coil on and off.
2.6.1.2 Generation of high voltage
If two coils (known as the primary and secondary) are wound on to the same
iron core then any change in magnetism of one coil will induce a voltage in the
other (see Chapter 3 for more details). This happens when a current is switched
on and off to the primary coil. If the number of turns of wire on the secondary
coil is more than on the primary a higher voltage can be produced. This is called
transformer action and is the principle of the ignition coil.
The value of this ‘mutually induced’ voltage depends on:
the primary current
●
the turns ratio between primary and secondary coils
●
the speed at which the magnetism changes.
●
The two windings are wound on a laminated iron core to concentrate the
magnetism. This is how all types of ignition coil are constructed.
2.6.1.3 Ignition timing (advance angle)
For optimum effi ciency the ignition advance angle should be such as to cause
the maximum combustion pressure to occur about 10° after TDC. The ideal
ignition timing is dependent on two main factors, engine speed and engine load.
An increase in engine speed requires the ignition timing to be advanced.
The cylinder charge of fuel/air mixture requires a certain time to burn (normally
about 2 ms). At higher engine speeds the time taken for the piston to travel the
same distance reduces. Advancing the time of the spark ensures that full burning
is achieved.
A change in timing due to engine load is also required as the weaker mixture
used on low load conditions burns at a slower rate. In this situation further
ignition advance is necessary. Greater load on the engine requires a richer
mixture, which burns more rapidly. In this case some retardation of timing is
necessary. Overall, under any condition of engine speed and load an ideal
advance angle is required to ensure that maximum pressure is achieved in the
cylinder just after TDC. The ideal advance angle may also be determined by
engine temperature and any risk of detonation.
Spark advance is achieved in a number of ways. The simplest of these is
the mechanical system comprising a centrifugal advance mechanism and a
vacuum (load-sensitive) control unit. Manifold depression is almost inversely
proportional to the engine load. I prefer to consider manifold pressure;
although it is less than atmospheric pressure, the MAP is therefore proportional
to engine load. Digital ignition systems adjust the timing in relation to the
temperature as well as speed and load. The values of all ignition timing
functions are combined either mechanically or electronically to determine the
ideal ignition point.