US3728991A

US3728991A - Capacitive discharge ignition circuit

Info

Publication number: US3728991A
Application number: US00078519A
Authority: US
Inventors: M Montuschi; M Palazzetti
Original assignee: Fiat SpA
Current assignee: Fiat SpA
Priority date: 1970-10-06
Filing date: 1970-10-06
Publication date: 1973-04-24
Anticipated expiration: 1990-04-24

Abstract

A capacitive discharge electronic ignition circuit for an internal combustion engine, in which the capacitor is charged from the battery via a transformer with a saturable magnetic core. The transformer has a primary winding in a power circuit controlled by a transistor which is in turn controlled by one of the secondary windings of the transformer so that when the magnetic core reaches saturation the current to the primary switches off. This induces a surge of current in a further secondary winding of the transformer which charges the capacitor by a given amount since the energy stored in the saturable magnetic core of the transformer is always the same. The transistor is triggered repetitively from a contact breaker and a third secondary winding of the transformer supplies a firing signal for a controlled rectifier which controls the discharge of the capacitor into the high voltage coil to provide the spark.

Description

Unite States Patent 91 Mo ntuschi et a1.

[451 Apr.24,1973

[ 1 CAPACITIVE DISCHARGE IGNITION CIRCUIT [73] Assignee: Fiat Societa per Azioni, Turin, Italy 22 Filed: Oct. 6, 1970 [21] App1.No.: 78,519

[52] [1.8. CI ..l23/l48 E, 315/209 CD [5 l Int. Cl ..F02p 3/06 [58] Field of Search ..l23/l48 E;

[56] References Cited UNITED STATES PATENTS 3,549,944 12/1970 Minks ..3l5/209 CD Primary Examiner-Laurence M. Goodridge Assistant Examiner-Cort Flint AttorneySughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT A capacitive discharge electronic ignition circuit for an internal combustion engine, in which the capacitor is charged from the battery via a transformer with a saturable magnetic core. The transformer has a primary winding in a power circuit controlled by a transistor which is in turn controlled by one of the secondary windings of the transformer so that when the magnetic core reaches saturation the current to the primary switches off. This induces a surge of current in a further secondary winding of the transformer which charges the capacitor by a given amount since the energy stored in the saturable magnetic core of the transformer is always the same. The transistor is triggered repetitively from a contact breaker and a third secondary winding of the transformer supplies a firing signal for a controlled rectifier which controls the discharge of the capacitor into the high voltage coil to provide the spark.

5 Claims, 2 Drawing Figures CAPACITIVE DISCHARGE IGNITION CIRCUIT The present invention relates to capacitive discharge ignition circuit for an internal combustion engine.

Capacitive discharge ignition circuits find particular utility for the engines of motor vehicles. Capacitive discharge ignition circuits normally comprise a conversion circuit which supplies the charging energy to the capacitor, and a part which on firing of a controlled rectifier, provides for the discharge of the energy accumulated in the capacitor into the high voltage coil to supply the sparking plugs.

In the case of motor vehicles,- the conversion circuit is normally supplied by a direct current source of electricity such as the vehicles battery, the voltage of which varies between certain limits depending upon the state of the charge of the battery and the amount of current consumed by the various instruments with which the vehicle is provided.

With known conversion circuits, such variations of voltage influence the voltage to which the capacitor is charged and hence the amount of energy accumulated in the capacitor. Because of this, when a vehicle is starting up the voltage and charging energy of the capacitor are reduced because the voltage of the battery decreases. Thus, contrary to practical needs, at a time when a high sparking efficiency is required because of the likelihood that the sparking plugs may become fouled, in fact the stored energy is less and therefore the discharge less efficient. 1

Attempts have been made to remedy this situation b adjusting the conversion circuits so that they provide enough energy even when starting. This however, is done at the expense of overall efficiency since in such a case the energy supplied to the capacitor under normal driving conditions is excessive. It should be noted that efficiency is related to the amount of heat to be dispersed and hence to the bulk and cost of the apparatus.

With regenerative ignition circuits, part of this energy is recovered, however the voltage at the terminals of the capacitor still reaches relatively high values, which necessitates an increase in size of the controlled rectifier to the terminals of which the capacitor voltage is applied. Moreover, with this system, after the negative half-wave on the blocking diode, the controlled rectifier is suddenly subjected to the recovery voltage; this voltage variation may cause a second firing of the controlled rectifier and a consequent loss of recovered energy. In order to prevent this it is necessary to use carefully selected controlled rectifiers which are therefore more costly.

it should also be noted that in known circuits, in order to guarantee the turn-off of the controlled diode, it is necessary to utilize the inductance of the primary 'of the high voltage coil. This, however, constitutes a serious constraint since it is, in general, advantageous to keep this inductance small, in order to achieve a greater sparkefficiency.

According to the present invention there is provided a capacitive discharge ignition circuit for internal combustion engines in which a capacitor is cyclically charged from a direct current source via a conversion circuit and discharged into a high voltage coil by means ofa controlled rectifier, in which the conversion circuit includes a blocking oscillator comprising a transformer with a saturable magnetic core which, during each cycle of operation stores a given quantity of magnetic energy which is independent of the supply voltage and transfers this energy, on saturation, to the capacitor so that the charging voltage is constant and independent of the voltage of the direct current source, the blocking oscillator also being adapted to trigger the controlled rectifier, during that part of the cycle when magnetic energy is accumulating in the saturable magnetic core, in order to discharge the energy stored in the capacitor in the previous cycle.

In such a circuit the controlled rectifier may be arranged to operate at relatively low voltages and with relatively long non-conducting periods so that the problem of heat dissipation is minimized, the controlled rectifier does not have to be of selected quality and can thus be manufactured more cheaply.

Also, with this arrangement, the circuit has a good spark efficiency without imposing any limitation on the high voltage coil, a low consumption and, proportionate to the rate of rotation of the engine, low heating anda minimum number of components.

Preferably the transformer of the blocking oscillator oscillator comprises a primary winding and several secondary windings which operate to transfer energy to the capacitor at the instant of saturation of the magnetic core, to supply, at the appropriate moment, the firing signal to the controlled rectifier and to supply a reaction circuit which will be described in greater detail below.

In one embodiment, the primary winding of the transformer is connected in a power circuit controlled by a transistor device which is initially switched into a conduction state by a control impulse and subsequently kept in this state, until saturation of the magnetic core, by the signal supplied by the reaction circuit.

One embodiment of the invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an electrical circuit diagram of a capacitive discharge ignition circuit constructed in accordance with the invention; and

FIG. 2 shows curves of the current and of the voltage of various components of the circuit of FIG. 1, during operation.

In FIG. 1, the first and second parts of a capacitive discharge ignition circuit are generally indicated 1 and Part 2 comprises a rectifying diode D, a discharge capacitor C, a transformer (or spark coil) B designed to produce the high voltage for the ignition sparking plugs S, a silicon controlled rectifier SCR which controls the capacitive discharge, and a blocking diode D which serves to cancel the reverse voltage at the terminals of the silicon controlled rectifier SCR.

Part 1, or the conversion circuit comprises a blocking oscillator including a transformer Tr with a saturable magnetic core. The transformer Tr has a primary winding A and three secondary windings A A The primary winding A is connected in a power circuit which is fed from a battery or from a continuous electrical source (not shown in the drawing) and controlled by a transistorized device, shown diagrammatically as the transistor Ts.

Of the secondary windings A is connected into a charging circuit of the capacitor C, A is connected into the control circuit of the SCR, and A is connected to form a reaction circuit. The reaction circuit provides the signal to the base of the transistor Ts which maintains it in its conducting state after it has been switched to this state by an impulse produced by the contact breaker device R and supplied to it via a circuit which includes a capacity C Various waveforms which occur in some of the components during operation of the ignition circuit are illustrated in FIG. 2. FIG. 2a shows, in relation to the time T, the variation of the current I, in the primary winding A FIG. 2b shows the variation of the current i in the controlled rectifier SCR, FIG. 2c shows the variation of the voltage V at the terminals of the capacitor C, FIG. 2a shows the variation of the gate voltage Vg of the controlled rectifier, and FIG. 2e shows the variation of the voltage V at the terminals of the said controlled rectifier SCR.

In FIG. 2, T indicates the period of one operational cycle of the circuit, that is one opening and one closing of the contact breaker device R, T and T respectively indicate the times of charging and discharging of the capacitor C.

Starting from the rest condition of the circuit, at the moment t= 0. As soon as the contact breaker R opens a control impulse is sent via capacitor C to the base of the transistor Ts. The current 1, (FIG. therefore starts to flow in the winding A growing steadily in value as the conduction state of the transistor Ts is maintained by the signal supplied by the reaction winding A,. I

The rate of increase of the current 1,, (FIG. 2a) will vary, as shown by the lines V V V depending upon the battery voltage, but it will increase steadily until it ultimately reaches the saturation current ls of. the magnetic core. The time taken for this to occur is T for the case when the current increases at the rate shown in FIG. 2a by the line V This rate ofincrease may be considered to correspond to the maximum battery voltage.

Upon saturation, the current in the reaction winding A drops to zero and this switches the transistor Ts to its non-conducting state. The current in the winding A therefore cuts out sharply, inducing transference of the magnetic energy accumulated in the magnetic core to the discharge condensor C, through the winding A Charging ofthe capacitor after T occurs in a small portion of the time T as shown in FIG. 20. At this point the first cycle of operation is concluded.

At the beginning of the second cycle, that is upon the opening of the contact breaker R, the cycle is repeated as before with this difference, that during the time T when magnetic energy is accumulating in the magnetic core of the transformer Tr, the capacitor C, which was charged in the previous cycle is discharged. Discharge of the capacitor C starts from the beginning of the period T of the second cycle, from this moment there is applied to the gate of the controlled rectifier SCR the control voltage Vg (FIG. 2d). The time of discharge and applying the control voltage is very short, so that the discharge is effected completely before the moment of saturation of the magnetic core of the transformer. This ensures that at the beginning of each time period T the voltage at the terminals of the controlled rectifier is nil. The length of time during which this voltage is nil is relatively long so as to ensure the extinguishing of any type of SCR even one with a relatively long turning off time. I

The third cycles of the circuit and those following, are repeated identically to the second.

To summarize, during every cycle, as soon as the magnetic core of the transformer reaches saturation the current in the primary winding A of the transformer is interrupted and this induces transferring of the magnetic energy stored in the magnetic core to the capacitor C. This energy is constant since it depends not upon the battery voltage but on the magnetic characteristics of the transformer, therefore the capacitor will always be charged with a constant quantity of energy.

Thus the maximum voltage at the terminals of the controlled rectifier is the same whatever the frequency at which the contact breaker R is opened, that is whatever the speed of rotation of the engine.

This stabilization of maximum voltage makes it possible to use a less costly controlled rectifier because there is no danger of overload from the peak voltages which can occur in conventional circuits with variations of engine speed.

Moreover the controlled rectifier SCR is in a particularly favorable operational position. Because of the diode D the controlled rectifier is never subjected to substantial reverse voltage and moreover due to the relatively complete discharge of the accumulated energy of the capacitor C, the controlled rectifier remains for most of the cycle with no voltage acrossits terminals.

From the description above it will be appreciated that the electrical input of the circuit is very low, and proportional to the speed of the engine. The components do not have to conform to a vigorous specification and the circuit is not wasteful in that there is accumulated only the energy necessary for sparking, whilst during inactive periods, between one spark and another, the oscillator is blocked and does not use current. Efficiency is therefore high at all engine speeds.

It will be apparent to those skilled in the art that various modifications and alterations can be made to the circuit shown as a specific embodiment without nevertheless going beyond the scope of the invention as defined in the following claims.

For example reference has been to an oscillating discharge but the operating principle remains the same even in the case of an aperiodic discharge except for minor changes to the circuit. The circuit has been described with the initial control impulse being given by the opening of a contact breaker but it will be apparent to those skilled in the art that the impulse could be given with the contact breaker closed. The contact breaker could for example be of a mechanical type, as shown in the drawing, or else of the electronic type. Although shown and described as a single transistor the transistor device Ts may consist of a group of transistors if specially high performance is required.

What is claimed is:

I. A capacitive discharge ignition circuit for an inter nal combustion engine comprising a first capacitor, a direct current source, control impulse generating means coupled to said direct current source, said impulse generating means including a resistor in series with said direct current source, a second capacitor in series with said resistor, and circuit breaker means connected between the point of connection of said resistor and second capacitor and ground whereby the opening of said circuit breaker allows said second capacitor to charge through said resistor, thereby producing a control impulse, a conversion circuit connecting said first capacitor and said direct current source and operating to charge said first capacitor from said direct current source during successive cycles of operation, a high voltage coil, means for discharging said first capacitor into said high voltage coil during said successive cycles of operation, said conversion circuit including a blocking oscillator coupled to said control impulse generating means and turned on thereby, said blocking oscillator comprising a transformer, a saturable magnetic core on said transformer, and a transistor coupled directly to said second capacitor for receiving said control impulse, said saturable magnetic core storing a given quantity of magnetic energy in each said cycle of operation, said given quantity of magnetic energy being independent of the voltage of said voltage source, said blocking oscillator transferring said given quantity of magnetic energy to said first capacitor upon saturation of said magnetic core, said blocking oscillator triggering said means for discharging said first capacitor at a time when magnetic energy is accumulating in said saturable magnetic core whereby said energy stored in said first capacitor in the previous cycle of operation of said circuit is discharged into said high voltage coil.

2. The ignition circuit of claim 1 wherein said means for discharging said first capacitor includes a controlled rectifier and said first capacitor is completely discharged well before said saturable magnetic core of said transformer is saturated so that there is no voltage across said controlled rectifier at the beginning of each said charging cycle,

3. The ignition circuit of claim 1 wherein said transformer with said saturable magnetic core comprises,

a primary winding connected in a power circuit with said electrical source of supply, said transistor controlling said primary winding,

a first secondary winding connected in said charging circuit of said first capacitor, operating to transfer said magnetic energy accumulated in said magnetic core to said first capacitor when said core is saturated,

a second secondary winding connected in said control circuit of said controlled rectifier and supplying a firing signal to said controlled rectifier,

a third secondary winding coupled in said power circuit to form a reaction circuit, said reaction circuit providing, during each cycle, a control signal to maintain said transistor in a conduction state until saturation of said magnetic core, said transistor being switched to said conduction state by said control impulse generating means.

4. The ignition of claim 3, said output of said transistor controlling said power circuit, said output of said transistor being connected to said primary winding of said transformer, said transistor receiving the said control signal subsequently to said control impulse 5. The ignition circuit of claim 4, wherein said control impulse is produced by a mechanical breaker device controlled by said engine.

Claims

1. A capacitive discharge ignition circuit for an internal combustion engine comprising a first capacitor, a direct current source, control impulse generating means coupled to said direct current source, said impulse generating means including a resistor in series with said direct current source, a second capacitor in series with said resistor, and circuit breaker means connected between the point of connection of said resistor and second capacitor and ground whereby the opening of said circuit breaker allows said second capacitor to charge through said resistor, thereby producing a control impulse, a conversion circuit connecting said first capacitor and said direct current source and operating to charge said first capacitor fRom said direct current source during successive cycles of operation, a high voltage coil, means for discharging said first capacitor into said high voltage coil during said successive cycles of operation, said conversion circuit including a blocking oscillator coupled to said control impulse generating means and turned on thereby, said blocking oscillator comprising a transformer, a saturable magnetic core on said transformer, and a transistor coupled directly to said second capacitor for receiving said control impulse, said saturable magnetic core storing a given quantity of magnetic energy in each said cycle of operation, said given quantity of magnetic energy being independent of the voltage of said voltage source, said blocking oscillator transferring said given quantity of magnetic energy to said first capacitor upon saturation of said magnetic core, said blocking oscillator triggering said means for discharging said first capacitor at a time when magnetic energy is accumulating in said saturable magnetic core whereby said energy stored in said first capacitor in the previous cycle of operation of said circuit is discharged into said high voltage coil.

2. The ignition circuit of claim 1 wherein said means for discharging said first capacitor includes a controlled rectifier and said first capacitor is completely discharged well before said saturable magnetic core of said transformer is saturated so that there is no voltage across said controlled rectifier at the beginning of each said charging cycle.

3. The ignition circuit of claim 1 wherein said transformer with said saturable magnetic core comprises, a primary winding connected in a power circuit with said electrical source of supply, said transistor controlling said primary winding, a first secondary winding connected in said charging circuit of said first capacitor, operating to transfer said magnetic energy accumulated in said magnetic core to said first capacitor when said core is saturated, a second secondary winding connected in said control circuit of said controlled rectifier and supplying a firing signal to said controlled rectifier, a third secondary winding coupled in said power circuit to form a reaction circuit, said reaction circuit providing, during each cycle, a control signal to maintain said transistor in a conduction state until saturation of said magnetic core, said transistor being switched to said conduction state by said control impulse generating means.

4. The ignition of claim 3, said output of said transistor controlling said power circuit, said output of said transistor being connected to said primary winding of said transformer, said transistor receiving the said control signal subsequently to said control impulse signal.

5. The ignition circuit of claim 4, wherein said control impulse is produced by a mechanical breaker device controlled by said engine.