GB2043166A - Ignition systems - Google Patents

Abstract

An energy discharge ignition system for motor vehicles has an ignition coil C and battery B, current from the battery flowing in the primary winding P1 of a transformer T, the current being interrupted at a rate proportional to engine revolutions; a capacitor C1 is connected in series with a first diode D1 across the secondary winding S1 and is repeatedly charged by the induced voltage in the secondary winding following interruption of the primary current; the primary winding P2 of the ignition coil C is connected in series with a switch, (an SCR as shown), across the capacitor which is operable in response to engine revolutions. A second diode D2 is connected in parallel with the ignition coil primary winding so that when the switch is repeatedly opened and closed the capacitor discharges through the ignition coil primary winding, the polarity of the second diode being arranged to permit continued current flow through the ignition coil primary winding and the second diode, thereby inducing a sparking voltage of long duration in the secondary winding of the ignition coil. <IMAGE>

Description

SPECIFICATION Energy discharge ignition system The present invention relates to ignition systems for motor vehicles and, in particular, to an energy discharge ignition system for motor vehicles having an ignition coil and a battery.

It is well known that conventional ignition systems, in which a set of contact points interrupts current flowing in the primary winding of an ignition coil, suffer from the disadvantage that only a limited amount of energy can be transferred from the primary winding to the secondary winding of the ignition coil each time the contact points open to interrupt the flow of primary winding current. As engine revolutions increase, this disadvantage is particularly noticeable since the time required for the primary winding current to reach a predetermined value is not available before the contact points re-open again. Therefore the secondary sparking voltage is reduced at high engine revolutions.

In addition, when the primary winding current is interrupted, the voltage induced in the secondary winding changes polarity and therefore the current flowing in the spark created by the high induced secondary winding voltage changes direction of flow. Therefore the spark current magnitude must be decreased to zero in order to permit this current reversal and, in consequence, the maximum ignition advantage otherwise obtainable by a continuously flowing spark current of high magnitude is not achieved.

It is the object of the present invention to provide an ignition system which overcomes the above-mentioned defects. According to one aspect of the present invention there is provided an ignition system for the internal combustion engine of a motor vehicle having an ignition coil and a battery wherein current from said battery flows in the primary winding of a transformer and is interrupted at a rate proportional to engine revolutions wherein: a capacitor is connected in series with a first diode across the secondary winding of said transformer and is adapted to be repeatedly charged by the voltage induced in said secondary winding by the interruption to said primary current; the primary winding of said ignition coil is connected in series with a switch, operable in response to engine revolutions, across said capacitor; and a second diode is connected in parallel with said ignition coil primary winding whereby, when said switch is repeatedly opened and closed at a rate proportional to engine revolutions, said capacitor discharges through said ignition coil primary winding, the polarity of said second diode being arranged to permit continued current flow through said ignition coil primary winding and said second diode, thereby inducing a sparking voltage of long duration in the secondary winding of said ignition coil.

One embodiment of the present invention will now be described with reference to the drawings in which: Figure 1 illustrates the basic circuit of the preferred embodiment, and Figure 2 illustrates one realisation of the circuit arrangement of Fig. 1.

Referring now to Fig. 1, the basic circuit of the preferred embodiment of an energy discharge ignition system, suitable for the internal combustion engine of a motor vehicle having a battery B and an ignition coil C, is illustrated. The primary winding P1 of a transformer T is connected in series with a switch S across the battery B. The switch S is repeatedly openable and closable by means of a pulse generating circuit which is activated by an ignition trigger. In the preferred embodiment, an R.P.M. limit circuit is provided between the pulse generating circuit and the ignition trigger in order to prevent the revolution rate of the internal combustion engine exceeding a pre-determined safe rate. However, the R.P.M. limit circuit is not an essential requirement and may be removed as indicated by the dashed line in Fig. 1.

The secondary winding S1 of the transformer T is connected in series with a diode D1 and a capacitor C1. The primary winding P2 of the motor vehicle ignition coil C is connected in series with a switch, comprising a silicon controlled rectifier (SCR), across the capacitor C1. A diode D2 is connected in parallel with the primary winding P2. The secondary winding S2 of the ignition coil C is connected to the spark plug(s) of the internal combustion engine in conventional manner.

During operation of the ignition system, the ignition trigger momentarily closes switch S and triggers the SCR into conduction simultaneously at a rate proportional to engine revolutions. Therefore a current flows from the battery B through the primary winding P1 and the magnitude of this current increases in time towards a pre-determined level. When the switch S opens, the current flowing in the primary winding P1 is interrupted thereby inducing a voltage in the secondary winding S1. This induced voltage charges the capacitor C1 via the diode D1.

When the SCR conducts, the energy stored in the capacitor C1 is discharged in the form of a current flowing through the primary winding P2 and the SCR. The capacitance of the capacitor C1 and the inductance of the primary winding P2 together constitute an LC resonant circuit. Therefore the voltage appearing across the primary winding P2 rises rapidly to a maximum voltage and then falls to zero in sinusoidal fashion, at which time the current flowing in the primary winding P2 reaches a maximum value. The voltage across the primary winding P2 continues to fall below zero, thereby permitting diode D2 to conduct the primary winding current. In consequence, the current flow through the SCR is reduced to zero and the SCR turns off.

It will therefore be seen that energy is transferred via the transformer T to the capacitor C1 and the entire energy stored in the capacitor C1 is discharged through the primary winding P2 in the form of a uni-directional current which flows for a relatively long period of time. As a result, the voltage induced in the secondary winding S2 is substantially unipolar and does not change polarity. In consequence, the spark produced in the spark plug(s) connected across the secondary winding S2 is a unipolar spark and therefore the magnitude of the spark current is high during the entire period of the spark. This is not the case with a bipolar spark in which the direction of current flow is reversed one or more times during the sparking period.Under such circumstances the magnitude of the spark current must be reduced to zero, and therefore the effectiveness of the spark current is diminished. It is believed that unipolar sparks of relatively long duration (approximately 500 microseconds) produce more reliable firing on leaner mixtures and give rise to lower pollution levels than those associated with conventional bipolar sparks of alternating current flow and polarity which have a relatively short duration (approximately 60 microseconds).

The circuit diagram of the preferred embodiment of the ignition system of the present invention is illustrated in datail in Fig. 2. The transformer T, ignition coil C, diodes D1 and D2, capacitor C1 and silicon controlled rectifier SCR, are as before. The primary winding P1 is connected to a vehicle battery B, as is a resistor R1, and the positive rail A is derived from the vehicle battery B. A set of contact points S, which may be conventional breaker points, are operable so as to be repeatedly opened and closed at a rate corresponding to engine revolutions. When the contacts S open, the capacitor C2 is charged via resistor R1 and diode D3 from the vehicle battery B.

The voltage appearing across the capacitor C2 is applied to a voltage comparator formed by the operational amplifier Al, thereby discharging the capacitor C2 via resistor R2. In consequence, the output of the operational amplifier Al comprises a series of positive pulses of relatively long duration having a pulse repetition rate proportional to engine revolutions.

It will be seen that the above-described arrangement overcomes problems of contact bounce since the capacitor C2 must be charged to a pre-determined level before the operational amplifier Al will be triggered. In addition, the above-described arrangement constitutes an R.P.M. limit circuit, since the capacitor C2 must have discharged by a predetermined amount via resistor R2 before opening of the contacts S can produce another pulse at the output of the operational amplifier Al. In this way the output of the operational amplifier Al is limited in terms of its maximum pulse repetition rate. Thus the ignition system prevents the engine being operated above a specified R.P.M. limit which may be dangerous to the engine and/or cause excessive engine wear.

The output of the operational amplifier Al is passed directly to the gate of the SCR to trigger same and is also passed to operational amplifiers A2 and A3 which are interconnected so as to form a monostable multivibrator having a sawtooth voltage output. The sawtooth wave output of the monostable is amplified in operational amplifier A4 and the output of this operational amplifier is applied to the base of transistor T1 which acts as an emitter follower.

Transistor T2 is a further emitter follower, and therefore a base current having a sawtooth waveform is applied to the transistor T3 which controls the current flowing in the primary winding P1. Transistor T3 acts as the switch S of Fig. 1. A sawtooth waveform for the base current of T3 is desirable since the current flowing in the primary winding P1 cannot be increased rapidly because of the inductance of the primary winding P1. Therefore, transistor T3 need only be turned on at a controlled rate so as to accommodate the controlled current increase in the primary winding P1. In addition, the interconnected operational amplifiers A2 and A3 privde a monostable multivibrator having an output pulse width which varies in response to variations in the voltage of the vehicle battery B.

For lower battery voltages the pulse width is longer and therefore, even though the current flowing through the primary winding P1 will be reduced below its normal value, this current flows for a longer period of time before interruption so that the same amount of energy is transferred from the primary winding P1 to the secondary winding S1 of the transformer T.

The base current of sawtooth waveform of the transistor T3 results in interruption of the current flowing in the primary winding P1 at the monostable pulse repetition rate. Thereafter, the operation of the remainder of the circuit is as described above in connection with Fig. 1. Therefore, during operation of the circuit, for a given opening of the contacts S a pulse is produced at the output of operational amplifier Al which results in capacitor C1 being charged via the transformer T. However, it is the subsequent opening of the contacts S which triggers the SCR so as to discharge the energy stored in the capacitor Cl by the previous closure of the contacts S.

Therefore, although the SCR is triggered every time the contacts S open (within the permitted engine revolutions range), the SCR discharges energy stored within the capacitor C1 as the result of the previous opening of the contacts S. In addition, the capacitor C1 is completely discharged with each triggering of the SCR and therefore the maximum possible amount of stored energy is transferrecd to the ignition coil C.

Since all the high current paths involve only solid state switches or energy storage devices, the energy losses of the circuit are minimal and the ignition system efficiency reaches approximately 85 per cent compared with approximately 50 per cent for conventional ignition systems utilising a capacitor discharge into the primary winding of an ignition coil. In such conventional systems the capacitor is not fully discharged. The result is that less heat is generated in the circuit of the present invention in the electronic devices and therefore the circuit has higher reliability due to reduced thermal fatigue. In addition, the cost of the provision of the circuit may be reduced by utilisation of electronic devices having lower power ratings.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the art, may be made thereto without departing from the scope of the present invention.

Claims (6)

1. An ignition system for the internal combustion engine of a motor vehicle having an ignition coil and a battery wherein current from said battery flows in the primary winding of a transformer and is interrupted at a rate proportional to engine revolutions wherein: a capacitor is connected in series with a first diode across the secondary winding of said transformer and is adapted to be repeatedly charged by the voltage induced in said secondary winding by the interruption to said primary current; the primary winding of said ignition coil is connected in series with a switch, operable in response to engine revolutions, across said capacitor; and a second diode is connected in parallel with said ignition coil primary winding whereby, when said switch is repeatedly opened and closed at a rate proportional to engine revolutions, said capacitor discharges through said ignition coil primary winding, the polarity of said second diode being arranged to permit continued current flow through said ignition coil primary winding and said second diode, thereby inducing a sparking voltage of long duration in the secondary winding of said ignition coil.

2. An ignition system as claimed in claim 1, including a trigger circuit operable in response to engine revolutions and adapted to simultaneously cause interruption of said primary winding current and operation of said switch.

3. An ignition system as claimed in either of the preceding claims, including means associated with the interruption of said primary winding current to prevent the revolution rate of the internal combustion engine from exceeding a pre-determined limit.

4. An ignition system as claimed in any one of the preceding claims wherein said switch is a gated, unidirectional silicon controlled rectifier.

5. An ignition system substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

6. An ignition system substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.