The increased requirements for ignition systems could
not be met by the conventional inductive ignition system since 1960. The introduction of new exhaust emission criteria in 1965 and the demand for improved fuel economy in 1975, have forced to use electronics into ignition system to meet the statutory requirements for a vehicle. Legislative requirements and driver demands for better engine performance, added to the manufacturer’s marketing strategy to offer a more sophisticated vehicle are the impetus for electronic innovation in this field.
Drawbacks of a Conventional System.
The basic principle of a conventional inductive ignition system has not changed for several decades till it became unable to meet the needs as regards energy output and contact breaker performance. In contrast to an ignition output of 10 – 15 kV used in earlier day, the modern high speed engine needs an output of 15 – 30 kV to ignite the weaker mixtures required to provide better economy and emission. To meet this requirement a low-inductive coil is often used. Due to much higher current flow in this coil, the erosive wear of the contact breaker is unacceptable. This reason alone is sufficient to adopt an electronic system in place of the mechanical breaker. The other drawbacks, however, of the breaker are :
(i) Ignition varies from specified value due to the change in speed because of (a) wear at the contact heel, cam and spindle, (b) erosion of the contact faces, and (c) contact bounce and the inability of the heel to follow the cam at high speed. («) Adverse effect on the dwell time as a result of dwell angle variation. (Hi) Frequent servicing.
The following descriptions cover the basic principles of the electronic ignition systems used during the period from the start of the changeover from the mechanical breaker to the latest.
Transistor Assisted Contacts (TA.C.)
This system incorporates a normal mechanical breakers, which drives a transistor to control the current in the primary circuit. Since a very small breaker current is used, erosion of the
contacts is eliminated so that good coil output is maintained. Also it provides accurate spark timing for a much longer period. When a low inductive coil and ballast resistor are used with this system, excessive contact arcing produced by the high primary current is also eliminated.
The basic principle of a breaker-triggered, inductive, semiconductor ignition system is illustrated in Fig. 16.25 where a transistor works as of the contact breaker, by acting as
Fig. 16.25. T.A.C. ignition system.
a power switch to make and break the primary circuit. The transistor performs as a relay, which is operated by the current supplied by a cam-operated control switch and thereby called as breaker-triggered.
A small control current passes through the base-emitter of the transistor when the contact breaker is in closed condition. This switches-on the collector-emitter circuit of the transistor and allows full current to flow through the primary circuit to energize the coil. The flow of current, at this stage, in the control circuit and transistor base is governed by the total and relative values of the resistors R\ and R2. These resistance values are chosen to provide a control current of about 0.3 A, which is sufficient to provide a self-cleaning action of the contact surfaces without overloading the breaker.
When the spark is required, the cam opens the contact to interrupt the base circuit, which causes the transistor to switch-off. With sudden opening of the primary circuit a high voltage is induced into the secondary, which produces a spark at the plug. This sequence is repeated to provide the required number of sparks per each revolution of the cam (Fig. 16.26). The T.A.C. arrangement provides a quicker break of the circuit compared with a non-transistorised system, and, as a result, a more rapid collapse of the magnetic flux takes place. Consequently a high HT secondary voltage is obtained. The components of this ignition system are similar to those used with a conventional system except for the extra control module containing the power transistor.
Extra refinements are needed on the basic circuit (Fig. 16.25) to protect the semiconductors from overload due to self-induction and to minimize radio interference. Also this circuit is unsuitable for use with a conventional contact breaker having a fixed-earth contact. An additional transistor is used (Fig. 16.27) to overcome this problem. In this layout the transistor T\ is connected in series with the contact breaker in the control circuit and it acts as a driver for the power transistor T%. Similar to the previous systems, resistors limit the base current in T\ and T2, and also the contact-breaker current.
Fig. 16.26. Primary current control (4-cylinder engine).
Fig. 16.27. TA.C. with driver and power transistors.
In the closed position of the contact breaker, a small current flows in the control circuit. While most of this current passes through Ri, a very small proportion is passed through the base of T\ to switch-on the transistor. This sensitive transistor then supplies a current to the base of the power transistor T2 to switch-on this. Consequently, the collector-emitter of T2 conducts and completes the primary circuit to allow the build up of magnetic flux in the coil. At the time of the spark, the contact breaker is opened, which interrupts the current in the control circuit and base circuit of T\. With T\ switched-off the current is cut-off from the base of T% thereby breaking the primary circuity
The power transistor T<i has to be robust to handle the large current and high voltage due to self-induction. These conditions are more critical when a low-inductance coil is used, especially if the engine is started. In this case a current of about 9 A built up in the primary circuit during normal operation can increase to about 16 A for cold-starting. A conventional breaker can handle a maximum of only 5 A. Therefore, the high currents associated with this type of coil can be switched effectively only by electronic means. However, the reliability of a normal power transistor is reduced at this high load, hence a Darlington amplifier, a special dual-transistor is used with this coil. The substitution of a Darlington amplifier for the power
Fig. 16.28. Darlington amplifier.
transistor T2 in the system illustrated in Fig. 16.27 considerably improves the reliability of the system. A Darlington amplifier circuit (Fig. 16.28) with two transistors forms an integrated circuit (IC) having three terminals, E, B and C. When a small current is supplied to the base of T\t it switches-on and causes a proportionally larger current to flow to the base of T2. This in turn, switches-on T%, which allows the main current to flow through T2 from the collector to the emitter.
An electronic switch in place of the mechanical contact breaker offers the following advantages.
(i) Accurate spark timing is available throughout the operating speed range.
(ii) There is no erosion and wear due to absence of any contacts. This system is maintenance free in respect of constant replacement, dwell adjustment and setting of the spark timing. Also the timing remains correct for a very long period.
(Hi) Build-up time for the ignition coil can be varied by changing the dwell period to suit the conditions. This provides a higher energy output from the coil at high speed, but has no risk of HT erosion at low speed.
(iv) There is no bouncing of contacts at high speed and hence the chance of robbing the coil of its primary current is eliminated.
The main layout of a breaker-less, electronic ignition system is illustrated in Fig. 16.29. The distributor unit is similar to a conventional unit except that an electronic switch, called a pulse generator, replaces the contact breaker. The pulse generator generates an electrical pulse to signal when the spark is required. The solid state control module makes and breaks electronically the primary current for the ignition coil by amplifying and processing the signals received from the pulse generator. Additionally, the control module senses the engine speed from the pulse frequency and accordingly varies the dwell time to suit the engine speed.
The three main types of pulse generator are (i) inductive (ii) hall generator, and (Hi) optical.
Inductive Pulse Generator.
One design of this generator is shown in Fig. 16.30 where the permanent magnet and inductive winding are fixed to the base plate. The distributor shaft drives an iron trigger wheel. The number of teeth formed on the trigger wheel or reflector matches the number of engine cylinders. If a tooth comes close to the soft iron stator core, the magnetic path is completed causing a flux to flow. When the trigger
wheel is moved away from the position showed the air gap between the stator core and the trigger tooth increases due to which the magnetic resistance or reluctance also increases causing the flux in the magnetic circuit to decrease.
The change in the magnetic flux generates an emf in the inductive winding fitted around the iron stator core. The maximum voltage is induced when the rate of change in flux is greatest, which occurs just before, and just after, the point where the trigger tooth is closest to the stator core. Figure 16.31 represents the variation in voltage due to movement of the trigger wheel through one revolution. A positive and a negative peak are established due to the build up of flux and the decay of flux rspectively. In the trigger position of greatest flux, no emf is induced into the winding. The mid-point of change between the positive and negative pulses is used to signal the requirement of a spark.
As the rotational speed of the trigger wheel controls the rate of change of the flux, the pulse generator output varies from about 0.5 V to 100 V. This voltage variation, combined with the frequency change, is used as sensing signals by the control module for various purposes except spark triggering. Since the reluctance of the magnetic circuit varies with the size of the air gap, the output voltage also depends on the size of the air gap. Due to the magnetic effect, a non-magnetic feeler gauge, such as a plastics gauge is used to check the air gap.
A Bosch pulse generator operates on a similar principle but has a different construction (Fig. 16.32). This is consisted of a circular disc magnet with the two flat faces acting as the N and S poles. A soft iron circular pole piece is placed on the top face of the magnet which has fingers bent upwards to form four stator poles in the case of a 4-cylinder engine. A similar number of teeth are formed on the trigger wheel to make a path for the flux to pass to the carrying plate supporting the magnet. The inductive coil is wound concentrically with the spindle and
Fig. 16.29. Layout of Breaker-less electronic
rig. 16.30. Inductive pulse generator.
Fig. 16.31. Voltage output from pulse generator.
the complete assembly forms a symmetrical unit, which is resistant to vibration and spindle wear.
Fig. 16.32. Pulse generator (Bosch).
Some manufactures do not use a conventional distributor. Citroen use a single metal slug called a target, bolted on the periphery of the flywheel and a target sensor mounted on the clutch housing (Fig. 16.33). The target sensor uses an inductive winding placed around a magnetic core in such a way that the core is 1±0.5 mm away from the slug when no. 1 piston is just before TDC. The voltage output is similar to other pulse generators except that the control module (computer) in this case receives only one signal pulse per revolution. For control purposes, Citroen incorporate a second target sensor of identical construction to the other sensor and adjacent to the starter ring teeth on the flywheel. This sensor signals the passage of each flywheel tooth so that the computer can count the teeth and determine the engine speed to set the ignition advance suiting to the conditions.
Hall Pulse Generator.
The principle of operation of this type of pulse generator is based on the Hall effect. When a chip made of semiconductor material carries a signal current across it, and is exposed to a magnetic field, a small voltage called the Hall voltage is generated between the chip edges at 90 degrees to the path taken by the signal current. The Hall voltage is altered due to the change in the magnetic field strength and this effect can be used as a switching device to trigger the ignition point by varying the Hall current.
The principle of Hall generator is illustrated in Fig. 16.34. A semiconductor chip, retained in a ceramic support, has four electrical connections. An input signal current is supplied to AB and an output Hall current in delivered from CD. A permanent magnet is placed opposite to the chip, and is separated by an air gap. Switching action is carried out by vanes on a trigger wheel, which is driven by the distributor spindle. It is possible to generate a spark with the Hall generator when the engine is stationary, which is not possible with the inductive pulse generator. Care should be taken while handling this system as there is risk of receiving an electric shock.
Fig. 16.33. Pulse generator (Citroen)
Once the metal vane is clear of the air gap, the chip is exposed to the magnetic flux and the Hall voltage is applied to CD. The switch is now turned on and current flows in the CD circuit. Moving the vane into the air gap between the magnet and the chip blocks, diverts the magnetic flux away from the chip, which causes the Hall voltage to drop to zero. If the vane is in this flux-blocking position, the switch is off and no Hall current flows in the CD circuit. When the pulse generator’s trigger vane is passing through the air gap, the
control module used with this system switches on the primary current for the ignition coil. Therefore the angular spacing of the vanes governs the dwell period. If the space between the vanes decreases the time of closure of the primary circuit increases. When the Hall switch is closed i.e. when the vane leaves the air gap the closed period is terminated and the spark is produced.
The layout of a Hall generator used in a Bosch distributor is shown in Fig. 16.35. The semiconductor chip in this model is used in an integrated circuit, which also performs the functions of pulse shaping, pulse amplification and voltage stabilization. The number of vanes on the trigger wheel equals the number of engine cylinders. In this design the trigger wheel and the rotor arms form one integral part. A three-core cable connects the Hall generator with the control module, and the leads form signal input, Hall output and earth.
Optical Pulse Generator.
This type works on sensing the spark point by using a shutter to interrupt a light beam projected by a light-emitting diode (LED) on to a phototransistor. This photoelectrical method of triggering was developed for the Lumenition system.
The principle of this type of trigger is illustrated in Fig. 16.36. An invisible light, at a frequency close to infra-red, is emitted by a gallium arsenide semi-conductor diode and its beam is focused by hemispherical lens to a width of about 1.25 mm at the chopping point. A steel chopper, having blades to suit the number of cylinders and dwell period, is fixed to the distributor spindle. This controls the time periods of the light falling on the silicon phototransistor detector. This transistor forms the first part of a Darlington amplifier, which builds up the signal and includes a means of preventing timing change due to variation in line voltage or due to dirt accumulation on the lens. The signal sent by the generator to the control module switches on the current for the primary coil. Therefore, when the chopper cuts the beams the primary circuit is broken and a spark is produced at the plug.
Fig. 16.34. Hall effect.
Fig. 16.35. Hall generator (Bosch).
Fig. 16.36. Optical pulse generator.
The control module, or trigger box, switches the current of the primary winding of the ignition coil in agreement with the signal received from the pulse generator. Both inductive
storage type and capacity discharge type control systems are in use. These two different types of control form two distinct electronic ignition systems.
Inductive Storage Ignition
The primary circuit of this system is similar to the Kettering system with the exception that a robust power transistor fitted in the control module makes and breaks the primary circuit in place of a contact breaker. A typical control performs four functions such as pulse shaping, dwell period control, voltage stabilisation and primary switching (Fig. 16.37) in four semiconductor stages.
Fig. 16.37. Inductive storage control module.
The full line in Fig. 16.38 represents the output voltage from an inductive-type pulse generator connected to a control module circuit. The full negative wave is obtained only when the generator is tested on open circuit. Once the AC signal is fed to the trigger circuit stage, the pulse is shaped into a DC rectangular form (Fig. 16.38). The rectangular pulse width depends on the duration of the pulse output from generator. However, the height of the rectangle, or the current output from the trigger circuits, is independent of engine speed.
Fig. 16.38. Pulse shaping.
Dwell Period Control and Voltage Stabilization.
The dwell period in this stage is normally varied by altering the start of the dwell period. The secondary output, therefore, reduces when the dwell period decreases. This control feature is used to control the period of time the current flows through the primary winding of the coil to suit the engine speed.
The voltage supplied to this resistor-capacitor (RC) circuit must remain constant irrespective of the variation in the supply voltage to the control module due to changes in charging output and consumer loads. This is met through a voltage stabilization section of the module.
The current to the primary circuit is normally switched by a Darlington amplifier. Pulse signals received from the dwell period control stage are passed to a driver transistor acting as a control current amplifier. At the proper times, current from the driver is switched-on or switched-off to control the heavy-duty power transistor of the Darlington output stage.
The sequence of events from the time of receiving the signal from the original pulse generator to the instant of the spark in the cylinder is illustrated in Fig. 16.39. A
Fig. 16.39. Pulse processing.
cathode ray oscilloscope (CRO), when connected to the output of an ignition coil forming part of an electronic ignition system, gives the image shown by the secondary output patterns. Vertical and horizontal axes of the CRO pattern represent voltage and time respectively. The main features of one secondary discharge are shown in Fig. 16.40.
If the primary circuit is broken, the secondary voltage increases until the spark is initiated. When this occurs the voltage needed to sustain the spark falls to a value, which is then maintained until the output energy is no longer sufficient to support the sparking process. At this instant the secondary voltage rises slightly before it falls and oscillates two or three times as the remaining energy is dissipated in the coil.
Secondary Output Control. Except for the changes due to mechanical defects, a breaker-triggered system has a constant dwell over the whole speed range. As a result at high speed the dwell period is too short, due to which the secondary output is poor because of the comparatively low primary current. However, a low inductance coil improves the output in the upper speed range, but causes erosive wear at the lower speed range. Use of a constant energy system overcomes this problem. This energy system incorporates a high output coil and is electronically controlled to vary the dwell period suiting to all speeds. At low speed the percentage dwell is kept relatively small, which is increased progressively with the increase of speed.
As shown in Fig. 16.40, dwell starts at point (1) and ends at point (2) at low speeds. With the increase of engine speed, the start of the dwell period (i.e. the point at which current starts
Fig. 16.40. Dwell in relation to secondary voltage.
to flow in the primary winding) gradually shifts towards the extreme limit (3). Any increase in dwell start beyond point (3) reduces the spark duration because this limit represents the end of the sparking discharge period.
The variation in percentage dwell with engine speed is shown in Fig. 16.41. At idling speed the percentage dwell is set large to give a high energy spark to control exhaust gas emissions. However between idling and 4000 rpm the increase in percentage dwell prevents a reduction in the stored energy. Consequently this provides a near-constant secondary voltage up to the system’s maximum, which is considered at about 15000 sparks/min.
SPARKING RATE, SPARKS/MIN 4 CYLINDER
Fig. 16.41. Alteration in dwell to suit engine speed.
When the system is incorporated on 6- and 8-cylinder engines it becomes necessary to decrease the percentage dwell at speeds beyond 5000 rpm, otherwise the start of the dwell would occur before the end of the spark discharge period. This problem is overcome by using a transistor in the control system to switch on the primary current at a given time after the spark has been initiated. Duration of 0.4 millisecond is normally sufficient to meet most combustion requirements. Figure 16.42 shows an output given by constant-energy sys tem using dwell-angle control.
Control Module Circuit.
Figure 16.43 represents a simplified control module circuit indicating the four main sections A, B, C and D discussed as follows.
Fig. 16.42. Output from constant energy system.
Fig. 16.43. Control module circuit (simplified).
Voltage Regulation (A).
Use of the Zener diode (ZD) ensures application of constant voltage to control sections B and C and is not affected by voltage variations occurring in other circuits of the vehicle. Voltage drop across a diode is constant and this feature is used to provide the regulated voltage to drive the control circuit.
Pulse Shaping (B).
In this section, the two transistors, T\ and T2, produce an arrangement called a Schmitt Trigger, which is a common method used in an A/D converter for forming a rectangular pulse while converting an analogue signal to a digital signal. Transistor Ti is switched-on when the pulse generated by the external trigger opposes current flow from the battery to the trigger via the diode D. This causes current to flow through the base-emitter of T\, which switches-on the transistor and diverts current away from the base of T% The action of the Schmitt Trigger causes T2 to be ‘off when T\ is ‘on’ and vice versa. The voltage at the time of switching is controlled by the threshold voltage required to switch-on T\. The switching of Ti occurs at a very low threshold voltage so, for practical purposes, the switching is considered to take place when the trigger potential changes from positive to negative.
Dwell Control (C).
Primary current in the coil flows when the pnp transistor T\ is switched-on, which is controlled by T3. The switching of T3 is controlled by the current supplied through i?5 and the state-of-charge of the capacitor C. During the charging of the capacitor with current from R5 no current passes to the base of T3, and so T3 is switched-off. Once the capacitor is fully charged, current passes to the base of T3 and switches it on to start the dwell period (i.e. to initiate current flow in the primary winding of the coil). The time taken to charge the capacitor governs the dwell period. The RC Time Constant, in this case, is determined by the amount the capacitor is discharged prior to receiving its charge from R5.
At low engine speeds, the transistor T2 is switched-on for a comparatively long time. This allows the capacitor plate, adjacent to T2, to pass the charge to earth that it received from Ra when T2 was switched-off. At this slow speed, there is sufficient time for the capacitor to fully discharge to a point where the plate potential becomes similar to earth. This causes the capacitor to attract a large charge from R5 when the transistor T2 switches off. Since the time taken to provide this charge is long, the switch-on point of T3 is delayed and a short dwell period results.
At high speed, the T2 is switched-on for a short period thereby allowing only partial discharge of the capacitor. Consequently, the time taken to charge the capacitor is shorter and the dwell commences at an earlier point providing a longer period. Interruption for the primary coil occurs when T2 is switched on. This is dictated by the trigger signal due to which the end of the dwell period always occurs at the same time. At the instant T2 switches-on, the capacitor starts to discharge, which causes T3 to switch-off to trigger the spark.
Darlington Output (D).
A Darlington pair, a common power transistor array, is used for switching large currents. The pair uses two robust transistors, T5 and Tq, which are integrally put in a metal case having three terminals-base, emitter and collector.
If a forward-biased voltage is applied to the base-emitter circuit of T5, the transistor is switched-on. This increases the voltage applied to the base of T& and if it exceeds the threshold value, T% is also switched-on. When t5 and Tq are switched-on, the coil primary is energized. If T5 is switched-off by the switching-off T4, the primary circuit is broken and a spark is generated. To make the system suitable for a vehicle, additional capacitors and diodes are used in the circuit
shown in Fig. 16.43, which prevent damage to the semiconductors due to high transient voltage and also reduce radio interference.
Alternative Method of Dwell Control.
Another method of achieving dwell-angle control is to superimpose a reference voltage on to the output signal supplied by the pulse generator (Fig. 16.44A). In this layout the triggering of the spark at the end of the dwell period occurs at the change-over point between the positive and negative waves, but the start of the dwell period is signalled when the pulse voltage exceeds the reference voltage. A reference voltage of 1.5 V acts on the dwell control stage at a low speed, which rises to 5 V at high speed. The stronger pulse signal, combined with the higher reference voltage, provides a longer dwell period (Fig. 16.44B). No pulse signal is generated when the engine is stationary, so no current can flow through the coil and hence the dwell control cannot operate.
Fig. 16.44. Use of reference voltage to control dwell.
Fig. 16.45. Distributor with integral amplifier.
Ford Escort Electronic Ignition.
Ford 1300 and 1600 engines used electronic ignition systems from 1981. The control module is mounted on the side of the distributor assembly. The supply to the control module is provided through a four-pin multi-plug built into the distributor body. External LT cables from distributor are limited to two leads, connecting with the coil and ignition switch (Fig. 16.45). A tachometer, connected to the’-’ side of the coil, uses the LT charge pulses of the coil to sense the engine speed.
After installation, the distributor is accurately set for the engine and, because it has a breaker-less construction, no further check of the timing is necessary during servicing of the vehicle. Since the dwell angle is governed by the control module, no check or adjustment is required
Honda Electronic Ignition.
This system, installed on the Accord, contains an inductive-type pulse generator and a control module called an igniter (Fig. 16.46). The switching of the coil’s primary current is executed by two transistors, namely a driver transistor Ti and power transistors, T% The pulse generator uses a reluctor, which is shaped in the form of a saw tooth to produce the AC wave form.
Fig. 16.46. Electronic ignition circuit (Honda).
If the ignition switch is closed with the engine stationary, R2 applies a voltage to the base of T\. This voltage is above the trigger voltage and, since the resistance of the pulse generator’s winding is above 700 Q. the transistor T\ is switched-on. At this stage, T\ conducts current ‘a’ to earth instead of passing to the base of T2. Consequently T2 is switched-off and the primary circuit is open.
During cranking of the engine an ernf is generated by the movement of the reflector. If the polarity of the generator’s emf at the T\ end of the winding is negative, the resistor R2 supplies current to earth through the winding and diode D\. At this stage, voltage applied to the base of T\ is less than trigger voltage, and hence T\ is switched-off. Current ‘a’ from R3 is now diverted from T\ to the base of T2, so T2 is switched-on and current passes through the primary winding. If the emf from the pulse generator is reversed, the combined effect of the voltage from R2 and the emf from the pulse generator triggers and switches-on Ti, and switch-off T2 to interrupt the primary current and give a spark at the plug.
Zener diodes ZD\ and ZD2, fitted at each end of the primary winding, conduct to earth the high-voltage oscillatory current caused by self induction and thereby protect both transistors from high-voltage charges.
Capacity Discharge (CD) Ignition
This system stores high voltage electrical energy in a capacitor until the trigger releases the charge to the primary winding of a coil. The coil in this case is a pulse transformer instead of normal energy-storage jdevice (Fig. 16.47). To provide a voltage of about 400 V to the capacitor, the battery current is inverted to AC and then voltage is raised through a transformer. When the spark is required, the trigger releases the energy to the coil primary winding by ‘firing’ a thyrister, which is a type of transistor switch. Once the thyrister is triggered, it continues to pass current through the switch even after the trigger current has ceased. Due to sudden discharge of the high voltage energy to the primary winding, a rapid rise in the magnetic flux of the coil takes place, which induces a voltage in excess of 40 kV in the secondary circuit to produce a high intensity, short duration spark.
Fig. 16.47. Capacity discharge electronic ignition layout.
Advantages of a CD system are :
(i) It reserves high secondary voltage.
Hi) It provides a constant input current and constant output is available voltage over a wide speed range.
(Hi) It causes a fast build up of output voltage. Since the speed of build up is about ten times faster than the inductive type of electronic ignition, the CD system reduces the risk of the HT current shorting to earth via a fouled plug insulator or taking some path other than the plug electrodes.
Although the CD system is specifically suited for the high performance engines, a spark duration of about 0.1 ms given by this system is normally too short to ignite reliably the weaker mixtures used in many modern engines. To offset the problem of short spark duration, advantage of the high secondary output is sometimes used to provide a larger spark by increasing the spark plug gap.
The system can be triggered by a mechanical breaker, but a pulse generator, either of inductive or Hall effect type is used to make the system more attractive. The AC signal from the generator is applied to a pulse shaping-control circuit, which converts the signal into a rectified rectangular pulse and then changes it to a triangular trigger pulse to fire the thyrister when the spark is required.
A voltage transformer capable of providing either a single- or multi-pulse output is used to charge the 1 uF capacitor to the voltage of about 400 V. A diode is installed in both cases, between the charging stage and the capacitor to prevent the flow of current from the capacitor. Single-pulse charging of the capacitor enables the build up to maximum voltage in about 0.3 ms, whereas the oscillatory charge provided by the multi-pulse is much slower (Fig. 16.48) and hence the former is preferred. This short charge-up time overcomes the need for dwell-angle control since the charge time of a CD system is independent of engine speed. As the primary winding of the ignition transformer (coil) always receives a similar energy discharge from the capacitor, the secondary available voltage is constant throughout the engine speed range (Fig. 16.49).
Fig. 16.48. Capacitor charging.
Fig. 16.49. Secondary output from CD system.
The external appearance of an ignition transformer of a CD system is similar to a normal ignition coil, but internally it is quite different. It is robust to withstand higher electrical and thermal stresses. Also the inductance of the primary winding is only about 10% of that of a normal coil. Because of low impedance of about 50 k£l, the CD coil readily accepts the energy discharged from the capacitor, due to which the rise in secondary voltage is ten times faster. This feature reduces the risk of misfiring because of the presence of HT shunts, for example a leakage path through a fouled spark plug, which has a resistance of 0.2-1.0 M£2.
During replacement only the recommended type of transformer must be fitted. A standard coil in place of an ignition transformer, however, operates without damaging the system, but many of the advantage of a CD system are lost. On the other hand, if an ignition transformer is used with a non-CD system, damage to control module and transformer occurs immediately after switching on the system. The CD principle is also adopted in some small engines fitted to motor cycles, lawn mowers, etc. As battery is not used in these cases, the energy needed by the CD system is supplied by a magneto.