Patent GB-1382959-A (GB)
Title: Electronic engine speed sensing circuit apparatus
IPC: G01P1/10
Filed: 1973-08-07
Abstract: 1382959 Measuring engine speed DANA CORP 7 Aug 1973 [24 Aug 1972] 37477/73 Heading G1N [Also in Divisions F1 and H3]  Current pulses in the primary winding of an engine ignition coil are used to measure the engine speed so that an exhaust emission control system may be activated when a predetermined speed is reached. Speed measurement: in Fig. 5 the coil is shown at 26 and a varying current flows through it as the contact breaker (not shown) opens and closes. This pulsating current is passed from point 110 or 112, depending on the nature of the engine ( which is allowed for by switch SW1-SW2), to transistor  Q1 and to an integrated circuit timer shown within the dotted lines. This timer, type NE555, has an input terminal U2 and an output U3; when a pulse is delivered to U2 the output voltage at U3 goes to a high level for a time defined by resistor R4 and capacitor C5, after which it drops immediately to zero. The timer will not respond to another input at U2 until the output at U3 has dropped back to zero; however a reset terminal U4 is provided and a pulse here will initiate a new timing cycle. In Fig. 1 (not shown), the terminals U2 and U4 are connected together. When the engine speed is low the pulses arrive at U2 and U4 at a relatively low frequency and R4, C5 are chosen such that the output at U3 goes to a high value and falls back to zero before the next pulse arrives. However, at a predetermined faster engine speed the next pulse arrives and resets the timer before the output drops to zero. As a result the timer output is a constant voltage for faster engine speeds but is pulsating for lower speeds. The transition from pulse to constant output at a given speed is detected by transistors Q2, Q3 and their associated components, and when the output is constant transistor Q4 is switched off to de-energize a solenoil valve 84 and hence bring the emission control system into oper- - ation. A degress of hysteresis is introduced into the system to prevent hunting by feeding back the increase in collector voltage of Q3 which occurs when the constant output is present at U3. Feedback is to terminal U5 and effectively modifies the voltage sensed by the timer at the junction of R4, C5 to make their time constant appear longer.  Emission control: this operates by passing exhaust gases back to the inlet manifold when solenoid valve 84 is de-energized by the speed measurement circuit. When valve 84 is closed by de-energizing it a vacuum controlled by the valve opens an exhaust gas valve to pass the exhaust back to the inlet; the vacuum may also be used to advance the ignition at this time if desired. The exhaust gas valve has a return spring chosen so that during full, e.g. emergency, acceleration the vacuum in the inlet manifold prevents the valve opening and allows full power without emission control. A typical arrangement is shown in Fig. 2 where solenoid valve 84 controls the application of inlet vacuum via pipe 86 to the ignition advance 34 and to the exhaust control valve 72. The latter, when open, passes some gas from exhaust pipe 64 to the inlet manifold 44 along pipe 100. Pipe 70 is arranged at right angles to the exhaust pipe, and acts as a heat exchanger to cool the gases. It is preferably stainless steel.