AUTOMOTIVE MAINTENANCE TECHNOLOGY SUBJECT : AUTOTRONIC 2 TITLE: TRANSISTORISED IGNITION WITH HALL GENERATOR TI-H

Transcription

1 Transistorized ignition with Hall generator TI-H In addition to the breaker-triggered transistorized ignition system (TI-B), there are two other versions of transistorized ignition with Hall triggering system (TI- H). On one version, the dwell angle is determined by the shape of the rotor in the ignition distributor. The other version contains a control unit incorporating hybrid circuitry and which automatically regulates the dwell angle. An additional current limiter with a highly efficient ignition coil makes this version a particularly high-performance ignition system. Hall Effect If electrons move in a conductor to which the lines of force of a magnetic field are applied, the electrons are deflected perpendicularly to the current direction and perpendicularly to the direction of the magnetic field, an excess of electrons occurs at A1 and a deficiency of electrons occurs at A2 i.e. The hall voltage occurs across A1 and A2. This so-called Hall Effect is particularly prominent in the case of semiconductors. Duration : 5 Page :Page 1 / 7

2 Hall generator AUTOMOTIVE MAINTENANCE TECHNOLOGY When the ignition-distributor shaft turns, the vanes of the rotor move through the air gap of the magnetic barrier without touching it. When the air gap is unobstructed the incorporated IC and the Hall layer are subjected to the magnetic field. At the Hall layer, the magnetic flux density B is high and the Hall voltage UH is maximum. The Hall IC is activated. As soon as one of the rotor vanes enters the air gap, most of the magnetic flux runs through the vane area and is thus largely prevented from reaching the Hall layer. The flux density at the Hall layer is reduced to a virtually negligible level, resulting from the leakage field. Voltage UH is at minimum. The Hall generator is accommodated in the ignition distributor. The magnetic barrier is mounted on the movable carrying plate. The Hall IC is located on a ceramic substrate and is potted in plastic together with one of the conductive elements in order to protect it against moisture, dirt and mechanical damage. The conductive elements and trigger wheel are of a soft magnetic material. The trigger wheel and distributor rotor comprise one component on the retrofit version. The number of vanes is equal to the number of cylinders. The width b of the individual vanes can determine the maximum dwell angle of the ignition system, dependent upon the ignition trigger box. The dwell angle consequently remains constant throughout the entire service life of the Hall generator; thus, there is no need to set it. The mode of operation and design of the Hall generator permit the ignition system to be set with the engine switched off providing peak-coil-current cutoff is incorporated. If the technical prerequisites are fulfilled, and the installation instructions are observed precisely, it is an easy matter to convert from conventional ignition to breaker less ignition. Bosch service stations will be able to provide you with further information. Duration : 5 Page :Page 2 / 7

3 Current regulation and dwell-angle closed-loop control High-performance ignition systems operate with ignition coils which charge very rapidly. For this purpose, the ohmic resistance of the primary winding is reduced to less than 1 ohm. The information content of the signal of a Hall vane switch in the ignition distributor corresponds to the signal of an ignition contact breaker. In one case, the dwell angle is determined by the distributor cam and, in the other the pulse duty factor is determined by the rotor vane. A rapidly chargeable ignition coil cannot operate with a fixed dwell angle. Duration : 5 Page :Page 3 / 7

4 This is why two measures must be taken to protect the ignition coil: a primarycurrent regulation system and a dwell-angle closed-loop control system. Current-regulation function The primary-current regulation system serves to limit the current through the ignition coil and, thus, to limit the build-up of energy to a specific amount. A certain lead time is required in order to cope with the dynamic conditions applicable when the engine accelerates. This means that the ignition coil should reach its nominal value before the ignition point. During this current regulation phase, the ignition transistor operates in its active range. More voltage than in the pure switch mode drops across the transistor. Duration : 5 Page :Page 4 / 7

5 This means a high power loss which may lie between 20 and 30 W. In order to minimize this, and in order to set the appropriate dwell angle, a dwell-angle closed-loop control system is required (which is actually a dwell period closed loop control system since the coil is charged as a function of time). Function of the dwell-angle closed- loop control system Since control processes in analog systems are carried out simply by shifting voltage threshold values, the square wave signal of the Hall generator is first converted to a ramp signal by charging and discharging capacitors. The pulse duty factor of the Hall generator is 30:70 between two ignition points. The ignition point determined by adjustment of the ignition distributor lies at the end of the vane width corresponding to 70 %. The closed loop control system is set such that the current control time t1 corresponds precisely to the required dynamic lead. A voltage is formed on the basis of value t1 and is compared with the trailing ramp of the ramp voltage. The primary current is switched on at the intersection point "ON" and the dwell angle starts. In this way, the switch-on point of the dwell angle can be varied as required by shifting the intersection point on the ramp voltage and by varying the voltage derived from the current control time. This means that the correct dwell angle is always formed for every operating range. Since current regulation and dwell angle closed-loop control are de- pendent directly upon current and time, this eliminates the effects of varying battery voltage and temperature effects or other ignition-coil tolerances. This renders these ignition systems particularly suitable for cold starting. Since primary current can flow owing to the wave form of the Hall signal with the engine switched off and with the ignition-starting switch switched on, the control units can be equipped with an auxiliary circuit which switches off this "peak-coil current" after a certain period. Duration : 5 Page :Page 5 / 7

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7 Control unit (ECU) AUTOMOTIVE MAINTENANCE TECHNOLOGY Transistorized ignition systems with current regulation and dwell angle closed loop control, virtually all comprise hybrid circuits. This makes it possible to combine the compact and lightweight control units for instance with the ignition coil, to form one assembly. Owing to the power loss which occurs in the ignition coil and in the transistorized ignition control unit, adequate cooling and good thermal contact with the bodywork are required. Duration : 5 Page :Page 7 / 7