ENGINE MANAGEMENT – SPARK IGNITION
2.2 ELECTRONIC IGNITION SYSTEMS (EARLY GENERATIONS)
2.2.9 Ignition modules: later types
constant energy function
Improving the coil output at all engine speeds One of the main problems with early electronic modules was that the dwell period was a compromise (as was the case with contact breaker systems); this meant that the dwell period was too long at slow engine speeds and too short at high engine speeds (hence the use of a ballast resistor as described in the previous section).
The next generation of ignition modules therefore provided a facility to alter the percentage of dwell depending on the engine speed. In effect, when the engine was at low speeds and an ignition cycle lasted a relatively long time, the dwell percentage was a small percentage of the long ignition cycle time. When the engine was at high speeds and the cycle time was much shorter, the dwell percentage was increased.
Example of dwell control
As an example, to achieve a good quality spark, if we assume that a current that flows for 2.5 ms is needed to allow the coil to build up the required amount of energy (magnetic field strength), then this in theory would be the same irrespective of engine speed. At 1000 rev/min on a four-cylinder engine, the ignition cycle for one cylinder would last for 30 ms, so the required 2.5 ms would represent one-twelfth of this period i.e. 8.33%.
If the engine speed is then increased to 2000 rev/min (twice the speed), then one ignition cycle will now last for only 15 ms (half the time). However, the coil would still require 2.5 ms of charge up time, which represent one sixth of the total 30 ms i.e.
16.66%. In effect, the charge up time remains the same but the percentage of the whole cycle changes in proportion with the change in engine speed.
As a last part of the example, if the engine speed is now increased to 6000 rev/min, the ignition cycle for one cylinder will last for only 5 ms. The coil charge up time will remain at 2.5 ms, which now represents 50%
of the ignition cycle.
It is therefore possible with this type of dwell time control to operate an engine at high engine speeds and still provide a long enough dwell period for the coil to build up strong energy levels.
The actual control of the dwell period is not necessarily as precise as in the explanation above, and
there are several variations in the exact dwell time provided depending on the ignition system module design. However, the objective is to ensure that the dwell time is sufficient for all engine speeds, thus allowing the ignition coil to provide a reasonably consistent output at all speeds.
For engines with more than four cylinders, the dwell time will have to be slightly less because of the shorter time available for one ignition cycle for each cylinder.
However, on more modern ignition systems, developments have included one coil to provide a spark for two cylinders and more recently, systems now provide one coil for each cylinder. In both cases, the time available for each coil to charge up is considerably increased. These systems are explained in greater detail later in this section.
Controlling dwell for specific conditions Although the system can control the dwell to suit engine speed, there are some operating conditions where it is an advantage to enable the coil to provide greater energy levels than normal. The usual examples are to provide a slight increase in dwell time at starting and at low engine speeds. Starting inevitably requires a strong spark, and at idle speed where emissions are critical, a better quality spark helps to ensure improved combustion. This is especially true with engines operating on relatively weak mixtures, which then benefit from long spark durations.
Constant energy control and high energy coils It is obviously an advantage to use an ignition coil that has a rapid build or charge up time, and this can be achieved by using a coil with a low primary winding resistance (low inductance). Many modern ignition coils have a primary winding resistance that is as low as 0.5 ohms, which is one-sixth of the resistance of older contact breaker system coils. Therefore, the potential current flow through the primary winding on a modern coil could be as high as 24 A (12 volts through a resistance of 0.5 ohms). In fact, such potentially high current levels would be too high and could damage the wiring and the coil winding. However, another major benefit of the low resistance primary winding is that the build up of current flow is much quicker than with primary windings of higher resistance. With this fact in mind, it is then possible to allow an initial rapid build up of current flow but to then limit the current so that it does not reach levels that are too high; the coil will be able to produce high energy levels due to the rapid build up time but without the problem of high currents damaging the system components.
Later generations of electronic ignition modules and modern ignition systems use a method of current control or ‘current limiting’ in conjunction with low resistance ignition coils. The process of current control is generally carried out in one of two ways, both methods relying on a ‘feedback’ or ‘closed loop’ system. In effect, the system monitors the current flow in the primary circuit and Electronic ignition systems (early generations) 57
when it rises to a predetermined level, the current in the primary circuit is then either restricted or the dwell time is reduced; both methods will prevent overheating that would otherwise be caused by an excessive current level that lasts for long periods.
Feedback system
The feedback systems are relatively simple in operation and rely on a resistor that forms part of the primary circuit, i.e. it is in series with the primary winding of the ignition coil. The resistor (referred to as a sensing resistor) is located within the ignition module, which is of course functioning as a switch on the earth path for the primary circuit. The voltage drop across the resistor will change with changes in current flow and, by passing the voltage signal from the resistor to other circuitry in the module, it is then possible to either control the current or control the dwell time accordingly.
With the current limiting systems, the ‘Darlington pair’ within the module is used to switch the primary current through to earth (Figure 2.22). When the voltage signal from the resistor indicates that the current has reached the predetermined level, the input voltage to the Darlington pair is reduced, which in turn reduces the primary circuit current flowing from C to E (collector to emitter). When the current is reduced, the voltage at the sensing resistor is also reduced and this voltage is then again used as the reference to control the input voltage to the Darlington pair. In effect, the process is a continuous action of monitoring and adjustment of voltages and current flow (i.e. it is a closed loop).
With dwell control systems, the same principle is used as for current limiting systems but the voltage from the sensing resistor is passed to the dwell control circuitry within the module. Dwell control therefore depends on the voltage at the sensing resistor.
A complete constant energy system
Figure 2.23 shows a simplified layout of a constant energy ignition system. The process is as follows.
● A trigger signal is passed from an inductive or Hall effect trigger to the ignition module.
● The signal will be processed by the pulse shaping device; if the trigger signal is provided by an inductive trigger, it will also be converted from analogue to digital.
● The processed signal is then passed through the dwell control device and peak coil current cut off device.
● The signal is then passed to the ‘driver’ which is effectively a low current/voltage switching transistor that is directly responding to the processed trigger signal. The driver responds to the trigger signal and in turn switches on and off the Darlington pair, which contains the main power transistor.
● The Darlington pair forms the final switching element of the ignition module. The primary current passes through the Darlington pair, so when the Darlington pair switches on the primary circuit, current will flow through the primary circuit thus enabling the ignition coil to build up a magnetic field. When the Darlington pair switches off the primary current, the magnetic field in the coil will collapse, thus providing a high voltage to the spark plug.
● Note that the voltage either side of the current sensing resistor is passed to a comparator, which then passes an appropriate signal either to the driver (which controls the Darlington pair), or the signal is passed to the dwell control device.
Different locations for ignition modules
There are several variations in the design of the electronic ignition systems so far discussed. Whilst in principle the systems will generally all function in the same way, the different versions produced for different vehicles have constructional variations that are either design preferences or are dictated by installation requirements.
In general, there are three basic physical layouts:
● remote module – the ignition module is remotely located away from the trigger mechanism
● integrated with the distributor body – the module is either located inside the distributor or mounted on the outside of the distributor body
● located on the ignition coil – the module is mounted on the ignition coil casing.
Figure 2.24 shows some examples of different module locations.
Figure 2.22 Voltage feedback control
Electronic ignition systems (early generations) 59
OR
Inductive pulse generator Darlington pair
Comparator
Reference voltage
Current sensing resistor Ignition coil primary Battery
Hall generator Pulse shaper Dwell
control
Peak coil current
cut-off Driver
Figure 2.23 Layout of constant energy ignition system
Figure 2.24 Different physical layouts for ignition systems a Ignition coil mounted retrofit system
b Remote mounting
c Integrated distributor body mounting
(a)
+
Heat sink assembly
Four heat sink mounting points for self tapping
screws Ignition switch
Existing wire or block ballast resistor (if fitted)
Distributor A
Coil +
Positive A
HT cable cap
–
(b)