GB1095533A - - Google Patents

Abstract

1,095,533. Electric tachometer. P. GOSSEN & CO. G.m.b.H. March 7, 1966 [Sept. 10, 1965], No.9796/66. Heading G1N. A tachometer for measuring the rotational speed of an I. C. engine comprises a measuring circuit connected in parallel with the make and break device 12, Fig. 2, of the engine, through a self-saturable choke 10 of high permeability and a resistance 11, an indication of the frequency of the voltage pulses across the make and break device and therefore of the ignition speed being given by the measuring circuit. When the ignition is switched on and the device 12 is open, current from the engine battery 14 flows into the measuring circuit, saturating choke 10 and charging a measuring capacitor 5 to a level dependent on current limiting Zener diodes 2, 3. At a time t 1 , the device 12 is closed, the current from the battery is effectively cut off and flows only through the sparking circuit. When this occurs the capacitor 5 discharges exponentially through a resistance 4 and the choke, so that the latter becomes unsaturated, Fig. 5 (not shown). At a subsequent time t 3 when the device 12 opens, sparking takes place and high frequency oscillations occur in the voltage across the device 12, Fig.4 (not shown). These oscillations are blocked by the choke due to its high inductive resistance, thereby protecting the measuring circuit from them. During the time t 3 -t 4 , while the oscillations are gradually dying away, the voltage across the device 12 gradually builds up resulting in the re-saturation of the choke and the re-charging of the capacitor. As this occurs a pulse is transmitted to the measuring circuit where it is detected by a moving coil meter 9. Finally at a subsequent time t 4 , the device 12 is again closed and the cycle of operations is repeated. As the meter 9 has a long time constant, it averages out the constant voltage pulses to give a reading which is indicative of the frequency of the pulses and therefore of the ignition speed.