Inductive ignition trigger

Inductive pulse generator

Figures 2.11 and 2.12 show two different types of inductive trigger or pulse generator. Both types were located in the ignition distributor body and as such physically replaced those components originally used for contact breakers. On earlier generations of electronic ignition, both types were also connected to the mechanical/vacuum advance mechanisms (inherited from the contact breaker systems).

Both examples operate on the same principle of using a reluctor with the same number of reluctor teeth (or triggering lugs) as the number of cylinders, i.e. four reluctor teeth for four cylinders. A permanent magnet and inductive coil (coil of wire) are located adjacent to the reluctor. When the reluctor is rotating, each of the

reluctor teeth passes the magnet and inductive coil assembly, which will produce a small electrical pulsed signal. This electrical signal is an analogue signal. The pulsed signal is then passed to the ignition module (power or switching transistor), which uses the pulses as a trigger or reference point to switch the ignition coil primary circuit.

The example in Figure 2.11 shows a magnet and inductive coil assembly located to one side of the reluctor. The iron reluctor is mounted on the distributor shaft and therefore rotates with the shaft. The example shown in Figure 2.12 operates in much the same way as the example in Figure 2.11, but the construction is slightly different. The magnet is a circular disc which is located concentrically with the distributor shaft; the inductive coil is also concentric with the magnet. The stator, or pole for the magnet, consists of fingers (one for each cylinder) which protrude upwards. A rotor or reluctor, which also has one reluctor tooth for each cylinder, is located on the distributor shaft; the reluctor teeth are formed as fingers that protrude downwards and pass adjacent to the stator fingers.

In both examples, when the reluctor teeth or fingers pass the stator or stator fingers, this causes an electrical signal to be produced as explained below.

Generating the pulse

When a reluctor tooth is aligned with the permanent magnet or stator (as shown in Figure 2.11), it allows the magnetic flux to flow from the stator, across the reluctor and back again. When the distributor shaft is rotating, the reluctor teeth will inevitably move away from the stator, providing a gap between the reluctor teeth and the stator. This gap results in a greater reluctance of the magnetic field or magnetic flux, i.e. the flow will be less.

In effect, when the reluctor teeth approach the stator, the flow of magnetic flux will increase. The flow of magnetic flux will be at its maximum when the teeth and stator are in alignment, and it will reduce when the reluctor teeth move away from the stator. When the flow of magnetic flux changes, i.e. increases or

Figure 2.11 Inductive pulse generator with the magnet and inductive coil located at the side of the reluctor

decreases (owing to rotation of the reluctor), this causes a small electrical current to be produced in the inductive coil. The voltage generated is at its greatest when the change in flux flow is at its greatest; this occurs just as the reluctor teeth are approaching or leaving alignment with the stator.

Figure 2.13a shows the voltage output from the inductive sensor, or variable reluctance sensor as it is sometimes called. There are four electrical pulses (for a four-cylinder engine) produced during one complete rotation of the reluctor. A positive voltage is produced as the reluctor teeth approach alignment with the stator (magnet); as the reluctor teeth leave alignment with the stator, a negative voltage is produced. The output signal is therefore an analogue alternating current (AC).

As described above, when the reluctor teeth approach or leave the stator (Figure 2.13b) this causes a large change in magnetic flux, which therefore produces higher voltage (positive or negative). However, when the reluctor teeth are close to alignment and in alignment with the stator, the result is very little or no change in the flux, which means that less voltage is produced. When the reluctor is directly in alignment with the stator, there is no change in magnetic flux:

therefore the voltage produced is zero.

Note: The gap between the reluctor teeth and the stator is effectively set during manufacture. However, on some types of construction it is possible to alter the gap. If the gap is not correct, this will affect the magnetic flux and the strength of the signal produced. Reference should always be made to manufacturer’s specifications.

Reference point for ignition timing

It is normal practice to use the change or ‘switch over’

from positive to negative voltage, i.e. the zero voltage point, as the reference point for ignition timing. The ignition module will therefore use this ‘zero volt’ point of the electrical signal as the reference to switch off the ignition coil primary circuit, thus creating the high voltage and a spark at the plug.

Figure 2.12 Inductive pulse generator with the magnet and inductive coil located concentrically around the reluctor

Figure 2.13 Rotation of the reluctor

a One rotation of the reluctor produces four pulses as an analogue signal

b Voltage levels produced when the reluctor teeth are in different positions relative to the magnet (stator)

The ignition coil will produce its high voltage output when each reluctor tooth is aligned with the stator: on a four-cylinder engine there will be four high voltage outputs from the coil for every rotation of the distributor shaft. The output from the coil must therefore be distributed to the appropriate spark plugs at the correct time (when each of the cylinders is close to TDC on the compression stroke); this is achieved by passing the high voltage from the ignition coil to the centre of a rotor arm located within the distributor.

When the distributor shaft and rotor arm are rotating, the rotor arm will direct the high voltage to the contact segments in the distributor cap, which allows the voltage to pass to each of the spark plug leads and spark plugs in turn (Figure 2.14). It is therefore important to note the exact location of each spark plug lead on the distributor cap to ensure that the voltage is directed to the correct spark plug at the correct time.

Wiring circuit for an inductive pulse generator Inductive pulse generators generally have two wiring connections to the ignition module. Effectively, these two wires provide a positive and a negative path for the electric current. However, remember that the current is an alternating current which means that the flow alternates within the wiring; each wire therefore alternately carries positive and negative flows.

Figure 2.15 shows the wiring for a typical inductive sensor and ignition module.

The wiring diagram (Figure 2.15) shows two wires carrying the pulsed signal from the inductive pulse generator to the ignition module. Because the module forms part of the earth circuit for the ignition coil primary circuit, the power supply from the ignition switch passes to the coil positive terminal (usually marked terminal 15) and then through the coil primary winding to the ignition module. The module functions as the switch for the primary circuit; therefore the circuit must pass through the power or switching transistor in the module before it is connected to earth.

If the ignition module contained only simple passive electronics, no power supply would be required for the module, but it contains active electronic components that require an additional power supply and earth connections.

On some applications, a third wire has been used which is wrapped around the two signal wires. The third wire is connected to earth or ground, acting as a screen or shield against interference from other electrical systems. Also note that when only two wires are used, one of the wires may be wrapped around the other, which provides a form of screening.

The inductive sensors can usually be classified as self-generating, which means that no additional power

Figure 2.14 Rotor arm and distributor cap (allowing a high voltage to be directed to the correct spark plug)

a Distributor cap

b Rotor Figure 2.15 Wiring for a simple inductive trigger ignition system

(a)

(b)

supply is required to enable the pulsed signal to be produced. Therefore the two wiring connections simply provide a complete circuit for the inductive coil. There are however some examples where an electric current is used to enhance or create the magnetic field; in such cases the same wiring that provides the current to the sensor also carries the pulsed signal.

Advantages and disadvantages of inductive pulse generators

Inductive pulse generators are relatively inexpensive to produce and generally reliable and accurate, even when working in the harsh environment of a vehicle’s engine.

Inductive pulse generators produce an analogue signal, which, when passed to an ECU that is operating with digital electronics, will require an analogue to digital converter (A/D converter). When the analogue signal is supplied to the early types of ignition modules (a simple power transistor switch), some simple circuitry within the ignition module is used to reshape the pulse so that the applicable reference points are recognised, i.e. the zero volt timing reference point.

The inductive pulse sensor operates using the same principles as a conventional electrical generator and, as is the case with a conventional generator, the voltage produced increases with the increase in rotational speed of the rotor (or reluctor). Therefore, when the engine speed increases, the voltage produced by the inductive pulse generator also increases, ranging from around 0.5 volts at slow speeds to a possible 100 volts at high speeds. If, therefore, there is any deterioration in the strength of the magnetic flux, and the engine is turning over very slowly during starting, it is possible that no signal will be produced.

Also of note is that, on the example shown in Figure 2.11, where the magnet and inductive coil assembly are located to one side, it is possible for an erratic or unusable signal to be produced if wear exists in the distributor shaft bearings. If a distributor shaft bearing is worn, the shaft can wobble during rotation; this in turn can result in the air gap between the reluctor teeth and the stator changing. The usual problem encountered is that the air gap for one reluctor tooth is too small whilst the gap for the opposite tooth is too large. It is not uncommon in these cases for the pulse signal to be missing a trigger pulse for one and sometimes two cylinders. This will mean that one or two cylinders may not receive a spark at the spark plug.