Patent GB-1448373-A (GB)
Title: Electronically controlled ignition systems
IPC: F02P5/155
Filed: 1973-10-08
Abstract: 1448373 Ignition systems ROBERT BOSCH GmbH 8 Oct 1973 [7 Oct 1972] 46856/73 Heading F1B An electronically controlled ignition system for an internal combustion engine comprises an electronic switching device 50 determining the ignition point which is arranged to be switched on over a fixed crank-shaft angle and after the expiration of the discharge time of a storage capacitor C, dependent on at least one operating parameter of the engine, to be switched off to initiate an ignition operation, the switching device including a constant current source for charging and a constant current discharge circuit for linearly discharging the storage capacitor, the constant current source being arranged to be maintained active during a fixed angle of rotation of the crank-shaft. As shown the system includes a speed and angle detector 11 which consists of four permanent magnets fixed at 90 degree intervals on a rotating part of the engine and a fixed inductance coil 12 in which four trigger pulses 13 are induced by the permanent magnets on each rotation of the engine crankshaft. These pulses are fed to the input of a pulse forming stage 15. The pulse forming stage 15 contains a monostable multi-vibrator with an input transistor 17 and an output transistor 18. The two transistors 31, 32 are of the npn type . With each of the positive half-cycles of the trigger pulses 13 the amplifying transistor 27 becomes conducting. This has the result that one of the two transistors 31, 32 which was previously conducting now passes into its blocked state, whereas the other becomes conducting. At the collector of the transistor 32 a switching voltage 45 is produced which is used for the control of the electronic switching device 50. This includes the storage capacitor C which is charged with a constant charging current Ja during an angle of rotation of 90 degrees of the crank-shaft. Immediately following the charging process the discharge of the storage capacitor C takes place with a discharge current Je the intensity of which depends on the speed of the engine and the induction pressure in the induction manifold of the engine. During the charging processes for the storage capacitor the output transistor 60 is conducting and keeps the input transistor 62 blocked. However as soon as the switching voltage 45 goes back from positive potential values to zero at the end of a charging process the output transistor 60 obtains via a differentiating capacitor 64 a negative voltage jump so that it is blocked. At the same time the input transistor 62 is made conductive. The output transistor 60 is blocked by the charge on the storage capacitor C until the charge is linearly reduced by the discharge current Je to zero. An output stage 80 of a transistor-coil ignition device can be rendered operative with the trailing edge of the pulse 66 and can release an ignition process. In order that sufficient time is available for building up the magnetic field in the ignition coil (not shown in detail) at high speeds the output stage 80 is controlled by way of an OR- member 70 and is conducting during a time which corresponds to an angle of rotation of 90 degrees of the crank-shaft associated with the half-period of the switching voltage 47 and the period of the pulse 66.