AUTOMOTIVE MAINTENANCE TECHNOLOGY SUBJECT: AUTOTRONIC 2 TITLE: BREAKER-TRIGGERED TRANSISTORISED IGNITION TI-B

Transcription

1 Breaker-triggered transistorized ignition TI-B The ignition distributor of the breaker- triggered transistorized ignition system (TI-B) is identical to the ignition distributor of the breaker-triggered coil ignition system (CI). However, the contact breaker no longer needs to switch the primary current but only the control current for the transistorized ignition system. The transistorized ignition system itself plays the role of a current amplifier and switches the primary current via an ignition transistor (generally a Darlington transistor). In order to facilitate understanding, the wiring of the contact and the function of a simple TI-B are compared below to a breaker- triggered coil-ignition system. Operating principle Figures that shown below clearly show that the breaker-triggered transistorized ignition system is a further development of the conventional, non-electronic coil-ignition system: the transistor T is used as the circuit breaker in place of the contact breaker and assumes its switching function in the primary circuit of the ignition system. However, since the transistor has a relay characteristic, it must be caused to switch in the same way as the relay. This can be done, for instance, as shown below, with a control switch. Such transistorized ignition systems are thus termed breaker-triggered. In Bosch transistorized ignition systems, the cam-operated breaker performs the function of this control switch. When the contact is closed, a control current Is flows to the base B and the transistor is electrically conductive between the emitter E and the collector C. In this condition, it corresponds to a switch in the "On" position and current can flow through the primary winding L1 of the ignition coil. However, if the contact of the breaker is open no control current flows through to the base and the transistor is electrically non-conductive. It thus blocks the primary current and, in this condition, corresponds to a switch in the "Off" position. Duration : 5 Page :Page 1 / 5

2 Advantages The breaker-triggered transistorized ignition system has two essential advantages over the breaker-triggered coil- ignition system: An increase in the primary current and Considerably longer service life of the breaker contact. The primary current can be increased if using a switching transistor since a mechanical contact can switch currents of only up to 5 A for long periods and with the required frequency. Since the stored energy is proportional to the square of the primary current, the power of the ignition coil increases and, thus, also all high-voltage data such as secondary available voltage spark duration and spark current. Thus, a breaker-triggered transistorized ignition system also re- quires a special ignition coil in addition to the ignition trigger box. A far longer service life of the TI-B results from the fact that the contact breaker is not required to switch high currents. Duration : 5 Page :Page 2 / 5

3 In addition, the TI-B is also not subject to two other problems which indefinably reduce the secondary available voltage of contact-triggered coilignition systems. Contact chatter and the contact-breaking spark which results from the inductance of the ignition coil. The contact-breaking spark reduces the available energy and delays the highvoltage rise, particularly at low engine speed and when starting. Conversely, contact chatter occurs at high engine speeds owing to the high switching frequency of the contact and is a disturbing influence. The contact bounces when closing and thus charges the ignition coil less intensely, precisely at a point in time at which the dwell period is reduced anyway. The first negative characteristic of the contact breaker is not applicable to the breaker-triggered transistorized ignition system, the second is. Duration : 5 Page :Page 3 / 5

4 Circuit On a breaker-triggered transistorized ignition system, the ignition trigger box (control unit) is connected between terminal 1 of the ignition distributor (i.e. the contact breaker) and terminal 1 of the ignition coil. In addition, the ignition trigger box requires one further terminal 15 for its power supply and a ground connection 31. The primary circuit of the ignition coil is powered via a pair of ballast resistors which are normally connected in series. When starting, the left-hand ballast resistor is by- passed by terminal 50 at the starter. This means that a higher supply voltage is applied to the ignition coil via the right- hand ballast resistor. It compensates for the disadvantages which result from the starting operation and the resulting reduction in battery voltage. Ballast resistors serve to limit the primary current in the case of lowresistance, rapidly chargeable ignition coils. They thus pre- vent overload of the ignition coil, particularly at low engine speeds, and thus protect the ignition contact breaker since the dwell angle is still determined by the distributor cam. Since the ignition coil actually requires a constant period for charging but does not operate with a fixed dwell angle, there is too much time available at low engine speeds for charging and too little at high engine speeds. Ballast resistors' and a rapidly chargeable ignition coil permit an optimum situation over the entire operating range. On older vehicles, the TI-B was an original equipment item. It has now been displaced by the transistorized ignition with maintenance-free trigger systems. However, as a retrofit-equipment set, the TI-B is still well-suited for substantially improving the ignition characteristics on vehicles with breakertriggered coil-ignition systems fitted as standard. This is why it is advisable to retrofit such a system in the case of general ignition problems, specifically in the case of starting difficulties, and if the vehicle is to be used largely for stop-and-go driving. Duration : 5 Page :Page 4 / 5

5 Duration : 5 Page :Page 5 / 5