GB1403711A - Ignition test instrument - Google Patents

Abstract

1403711 Testing ignition waveforms T I F INSTRUMENTS Inc and B S GOLDSMITH 15 May 1972 54133/74 Heading G1U In an arrangement for testing the ignition system of an I.C. engine, a first probe 10, Fig. 2, is attached to the ignition coil, a second probe 11 is attached to a particular sparking plug, and the voltage across the plug is utilized, via a bistable 13, to activate a test circuit 12 which then analyizes a particular characteristic of the ignition waveform, the characteristic examined relating either to the initial firing potential A, Fig. 1, (in which the vertically upwards axis corresponds to negative potential) or to the spark potential B, or to the oscillatory waveform C following termination of the spark. The result of the analysis is indicated on a meter having an appropriate time constant to average the result over many engine cycles. By moving the probe from plug-to-plug, each can be examined singly. As described the arrangement has four modes of operation selected by a switch 38, Fig. 5, having ganged wipers 38A, 38B, 38C. In each mode of operation, the bi-stable 13, Fig. 2, which is constituted by SCR's 34, 35, Fig. 5, is switched from its normal rest state, in which SCR 34 is conducting and SCR 35 is non- conducting, to its 'on' state, in which SCR 34 is conducting and SCR 35 is non-conducting, by the large initial negative firing pulse A, Fig. 1, which is applied to the cathode of SCR 35. Two things then happen: firstly the right hand plate of a capacitor 37 rises from-9v towards +9v, and secondly the upper plate of a capacitor 40 rises from zero towards +9v, but more slowly. Eventually, a uni-junction transistor 36 conducts and applies a positive pulse to the gate of SCR 35 thereby causing SCR's 34, 35 to revert to their normal rest conduction state. Capacitors 37 and 40 then discharge. Capacitor 40, resistor 42 and uni-junction 36 are so arranged that, having been switched 'on', the bi-stable 13 normally remains on for a duration which embraces all three portions A, B, C, of the waveform Fig. 1. The four modes of operation are as follows:- Mode (a) In mode (a) a measurement of engine speed is made so that it can be adjusted to a standard value. This is effected by shorting the pulses picked up from the coil to earth, and by passing a pulse to a meter 39 each time the bistable 13 is set 'on', this pulse resulting from the rising voltage on the right hand plate of capacitor 37, which causes a positive pulse to be applied, via a capacitor 44, to the base of a transistor 48. Mode (b) In mode (b) the peak firing voltage A, Fig. 1, is measured. This is effected by a conventional peak detector 49, 50 which receives the negative peak voltage from the coil over contact b. Whenever the bi-stable 13 is set 'on' by a pulse from the plug lead, charging of capacitor 37 causes, via capacitor 44, potential to be applied to the drain of an FET 51. The FET does not conduct however until the negative potential on its gate from the peak detection capacitor 49 falls sufficiently. It then conducts until the bi-stable is reset by uni-junction 36. Thus, the meter 39 receives a train of pulses each having a duration dependant upon the peak firing voltage - the higher the voltage the shorter the length. Mode (c) Mode (c) measures the duration of portion B, Fig. 1, of the voltage waveform by using the positive pulse that demarcates the end of portion B to reset the bi-stable earlier than it would normally be reset by uni-junction 36. Thus, in this mode the meter receives a train of pulses each having a length equal to the length of portion B. Mode (d) During mode (d) a measure of the integral of the positive half-cycles of the damped oscillations in portion C, Fig. 1, of the waveform is derived. This is effected by passing the signal from the coil direct to the gate of FET 51 and applying a bias to its source over a resistor 53 and contact d of bank 38B. Pulses representative of the areas of the positive peaks are then passed to the meter 39. The entire circuit can be housed in a case Fig. 3, having a metal coated pistol grip 16 which forms an earth connection via the operator.