Programmed ignition makes use of computer technology and permits the mechanical, pneumatic and other elements of the conventional distributor to be dispensed with. Figure shows an early form of a digital ignition system.
A digital ignition system.
The control unit (ECU or ECM) is a small, dedicated computer which has the ability to read input signals from the engine, such as speed, crank position, and load. These readings are compared with data stored in the computer memory and the computer then sends outputs to the ignition system. It is traditional to represent the data, which is obtained from engine tests, in the form of a three-dimensional map.
An ignition map that is stored in the ROM of the ECM.
Any point on this map can be represented by a number reference: e.g. engine speed, 1000 rpm; manifold pressure (engine load), 0.5 bar; ignition advance angle, 5Ž. These numbers can be converted into computer (binary) code words, made up from 0s and 1s (this is why it is known as digital ignition). The map is then stored in the computer memory where the processor of the control unit can use it to provide the correct ignition setting for all engine operating conditions. In this early type of digital electronic ignition system the ‘triggering’ signal is produced by a Hall effect sensor of the type shown in Figure.
A Hall type sensor.
When the metal part of the rotating vane is between the magnet and the Hall element the sensor output is zero. When the gaps in the vane expose the Hall element to the magnetic field, a voltage pulse is produced. In this way, a voltage pulse is produced by the Hall sensor each time a spark is required. Whilst the adapted form of the older type ignition distributor is widely used for electronic ignition systems, it is probable that the trigger pulse generator driven by the crankshaft and flywheel is more commonly used on modern systems. This is a convenient point at which to examine the type of system that does not use a distributor of the conventional form but uses a flywheel-driven pulse generator.
Optoelectronic Sensing Ignition System.
the electronic ignition photoelectronic distributor sensor used on a Kia. There are two electronic devices involved in the operation of the basic device. One is a light-emitting diode (LED), which converts electricity into light, and the other is a photo-diode that can be ‘switched on’ when the light from the LED falls on it.
the electronic ignition photoelectronic distributor sensor used on a Kia. There are two electronic devices involved in the operation of the basic device. One is a light-emitting diode (LED), which converts electricity into light, and the other is a photo-diode that can be ‘switched on’ when the light from the LED falls on it.
An Optical Sensor.
Another version of this type of sensor is shown in next figure. Here the rotor plate has 360 slits placed at 1Ž intervals, for engine speed sensing, and a series of larger holes for TDC indication that are placed nearer the center of the rotor plate. One of these larger slits is wider than the others and it is used to indicate TDC for number 1 cylinder.
An alternative form of optoelectronic sensor.
As the processing power of microprocessors has increased it is natural to expect that system designers will use the increased power to provide further features such as combustion knock sensing and adaptive ignition control.
