Ignition Arc Monitor Circuit

1. Field of the Invention

This invention relates to electronic apparatus for testing automotive ignition systems, and more particularly relates to apparatus for monitoring the duration of ignition arcs across spark plugs in internal combustion engines.

2. Description of the Prior Art

When engine ignition systems are properly tuned, engine performance is optimized and fuel consumption is reduced. However, deterioration in ignition performance results in incomplete combustion which reduces engine efficiency and results in increased fuel consumption. Furthermore, as a result of incomplete combustion gaseous mixtures, rich in hydro-carbons are discharged adding to pollution by the increased emission levels. Prior art ignition test instruments such as the oscilloscope are too complex and costly for the average automobile owner. Prior art ignition testing devices are generally intended for use during engine test and are not readily integrated into an automobile to allow for continuous monitoring.

It is the object of this invention to provide apparatus for monitoring the duration of the ignition arc and for indicating improper functioning of the ignition system.

In accordance with this invention, electronic circuitry is provided which senses ignition coil primary voltage during the energy storage or dwell period and during the arc discharge period. A first capacitor is charged during the dwell period and a second capacitor is charged during the arc discharge period. The resulting voltage on the respective capacitors are compared during a specified time interval in the ignition cycle by means of a differential amplifier. In the event that the comparison shows inadequate charge on the second capacitor, a silicon controlled rectifier is enabled which is latched in the "ON" position and activates an indicator. The two capacitors are discharged after the compare period and the test is repeated independent of the state of the indicator. Since the dwell period generally will be longer than the discharge period, the capacitors are charged at different rates such that the charge on the second capacitor will exceed slightly the charge on the first capacitor, after completion of the arc discharge period under proper operation of the ignition system. If the arc duration is shorter than desired, the charge on the second capacitor will be inadequate.

FIG. 1 is schematic representation of one embodiment of an ignition arc detector in accordance with this invention.

FIG. 2A is a representation of a voltage waveform at input to the circuit, superimposed on a time diagram.

FIG. 2B is a time diagram representing voltage on a capacitor charged during the dwell period.

FIG. 2C is a time diagram representing voltage on a capacitor charged during the arc duration period.

Commonly, automobile ignition systems consist primarily of a battery, ignition coil, periodically opening and closing contact points (or output transistor in performing the function of the points), a distributor and spark plugs as depicted generally at 10 in FIG. 1. The points 11 are controlled by means of a mechanism such as a cam (not shown) to be opened and closed in a predetermined cycle. The ignition cycle is represented in FIG. 2A as the period from t1 to t1. The time period of the cycle that the points remain closed is referred to in the art as "dwell." This is represented in FIG. 2A as the period from t1 to t2. When the points 11 are closed, current flows in the primary winding 12 generating a magnetic field in the coil. When the points 11 are opened at the end of the dwell period (t2) the magnetic field collapses rapidly and a high voltage is induced in the secondary winding 12. The high secondary coil voltage is applied to spark plugs 15 by means of a known distributor 14. The well-known spark plugs, which are inserted in the cylinders of an internal combustion engine, have an air gap of a specified dimension. Initially, the spark plug acts as an insulator. When the voltage applied to the plug reaches the potential required for ionization of the gaseous mixture in the engine cylinder, a low impedance ionized arc is formed across the gap. The duration of the arc will depend upon the energy stored in the coil and is referred to as "arc duration." This is represented in FIG. 2A as the period from t2 to t4.

A wave form representing the high voltage induced in the secondary coil 13 is shown in FIG. 2A. The figure shows high voltage build-up at the end of the dwell period at 200 and a lower sustained voltage during the arc period shown at 201. The voltage in the secondary winding 13 is reflected to the primary winding 12 and the voltage wave form on conductor 18 may also be represented by the waveform of FIG. 2A. The arc begins time t3 which occurs at some time shortly after t2, upon ionization of the gas in the spark gap. The arc will continue until the energy build-up in the coil has been dissapated, at time t4. FIG. 2B represents voltage build-up on capacitor 25 during the dwell period (t1 to t2) and FIG. 2C represents voltage build-up on capacitor 57 during the arc period (t2 to t4).

The circuitry represented schematically in FIG. 1 comprises a dwell sensing circuit 20, which charges a capacitor 25 during the dwell period, that is, the period between t1 and t2. The conductor 18 is substantially at ground potential when the points 11 are closed. This allows the PNP transistor 22 to conduct current from its emitter to collector causing a voltage drop across resistor 28. Consequently, the NPN transistor 24 goes into the conducting state. The current flow through the transistor 24 causes current flow in resistor 29, the diode 30 and through the PNP transistor 23. The parameters of the diode 30 and the transistor 23 are chosen such that the forward voltage drop of the diode 30 is very nearly the same as the base-to-emitter (VBE) voltage drop of the transistor 23.

It is well-known that when two transistors having substantially the same characteristics (e.g., VBE) have the same base and emitter voltages, their collectors will conduct the same currents. Such equivalent collector current is known in the art as "mirror current." The diode 30, being equivalent to a transistor having its base terminal connected to its collector terminal, will conduct very nearly the same current as the transistor 23. Since base current of the transistor 23 is very small compared to the current conducted in the diode 30, the current conducted by the transistor 23 is approximately the same as the current in resistor 29. Accordingly, the charging current which determines the rate of charge of the capacitor 25 may be controlled by selection of the appropriate value of the resistor 29. The voltage build-up may be represented by a ramp as shown in FIG. 2B if the charging current is relatively constant.

When the points 11 are opened, at time t2, the voltage on conductor 18 rises rapidly as represented at 200 in FIG. 2A. Consequently, PNP transistor 22 will switch to a non-conducting state and the charge current to capacitor 25 will be turned off. The voltage on conductor 18 typically may be anywhere from 30 volts to approximately 100 volts. A portion of this potential is applied to the base of the NPN transistor 51, by means of the input network 41. As a consequence, PNP transistors 52 and 56 will begin to conduct current from their emitters to their respective collectors. As described earlier herein with respect to diode 30 and transistor 23, the current flow in a diode connected between the base and emitter of a transistor is approximately the same as the emitter to collector current of the transistor if the forward voltage drop of the diode is approximately the same as the base to emitter voltage drop (VBE) of the transistor. Accordingly, assuming that the devices are selected such that the necessary voltage drops are nearly equal, the current flow to capacitor 57 will be approximately equal to the current flowing through diode 55 which is approximately equal to the current flowing in resistor 54 and transistor 52. Hence, the rate of charge of the capacitor 57 may be controlled by the proper selection of the value of resistor 54.

A positive potential will remain on conductor 18 during the duration of the arc, i.e., during the entire period between t2 and t4 indicated in FIG. 2A. When the voltage is reduced to near zero at time t4, transistors 51, 52 and 56 will be turned off, terminating the charge current to capacitor 57. For comparatively constant charge currents, the charge build-up on capacitor 57 may be represented by a ramp as shown in FIG. 2C.

Since the voltage on conductor 18 may vary from 0 volts to over 100 volts, protective circuitry is provided between transistors 22 and 51 and the conductor 18. The diode 21 protects transistor 22 from high voltage, and resistor 31 serves to provide a threshold. Resistors 44 and 42 comprise a voltage divider and resistor 47 serves to limit the current to the base of transistor 51. The capacitor 48 is provided to dampen any transient voltage and zener diode 45 serves to prevent excessively high voltages from reaching the base of transistor 51.

A comparator 80 comprising transistors 86 and 87 is connected to capacitors 25 and 57. Capacitor 25 is connected to transistor 86 by means of conductor 36 and resistor 81 and diode 82. Capacitor 57 is connected to transistor 87 by means of conductor 53, resistor 85 and diode 84. Transistors 86 and 87 are connected at their respective emitters and the connection from the emitters to ground is through transistor 88 and resistor 89. Thus the comparator circuit is enabled only when transistor 88 is conducting. As will be explained later herein, the transistor 88 will be placed in the conducting state, to temporarily enable the comparator after completion of the arc. If, after enablement of the comparator, the voltage on capacitor 57, as applied to the base of transistor 87, is greater than that of capacitor 25, transistor 87 will conduct and no substantial current will flow in transistor 86. The transistor 86 will conduct only if the voltage on capacitor 25 is greater than that on capacitor 57. In that case, transistor 94 will be placed in the conducting state, enabling the silicon controlled rectifier (SCR) 95, which, in turn will cause lamp 99, or like indicator, to be activated. Diode 93 is provided in the indicator circuit to prevent voltage from being conducted from the SCR 95 to the remainder of the circuit. Switch 96 represents a reset switch to allow the indicator to be reset by temporarily shunting current external to the SCR 95. Resistor 83 is provided to isolate the base of transistor 94 from the power supply conductor 66.

The ratio of the dwell period to arc duration varies depending on the operating characteristics of the engine, and may be readily established for most internal combustion engines. For optimum operation of the circuit described above, the values of resistors 29 and 54 should be selected such that the charge built up on capacitor 25 during the dwell period is slightly smaller than that built up on capacitor 57 during the arc duration period, under optimum engine operation. Any significant degredation of the arc duration will be noticed under such circumstances, since the comparator circuit 80 will readily detect the condition where the charge on capacitor 25 is larger than that on capacitor 57. The actual duration of the ignition cycle varies with engine RPM, since the spark plug firing frequency increases with RPM. The dwell and arc periods, however, vary in approximately the same manner with variation in RPM. Hence, the ratio of charging time of the capacitor 25 and 57 is substantially unaffected by change in RPM and the circuit provides a reliable ignition system test at any engine RPM.

The circuitry shown generally at 100, represents timing circuitry for temporarily enabling the comparator and for discharging the capacitors 25 and 57.

The base drive for the comparator enabling transistor 88 is provided from the supply conductor 66 via resistor 71 and conductor 75. During the dwell and arc duration periods, current will be diverted from conductor 75 primarily by means of diode 73 or diode 74, since during these periods either transistor 24 or transistor 51 will be in the conducting state. When neither of these last mentioned transistors is in the conducting state, current supplied to conductor 75 by resistor 71 will tend to divide between resistors 108 and 110, thereby further preventing the enabling of the comparator. When neither diode 73 or diode 74 is conducting, i.e., after completion of the arc at t4, transistor 104 is turned on by virtue of current flow in resistors 110 and 111. When transistor 104 is turned on, transistor 105 is turned off temporarily until such time as charge on capacitor 120 has been restored. During the period that transistor 105 is turned off, current flow through resistor 108 and diode 107 will substantially cease to flow causing transistor 88 to be switched to the conducting state, thereby enabling the comparator 80. During this same period, a charge will build up on capacitor 121. When transistor 105 is turned-on once again, capacitor 121 will be discharged causing transistor 106 to be turned off temporarily, until such time as the voltage at the base of transistor 106 once again reached the turn-on level. When transistor 106 is off, transistor 103 will be turned on causing the capacitors 25 and 57 to be discharged through diodes 101 and 102 respectively. It will be apparent to those skilled in the art, that the values of the resistors and capacitors may be readily selected to obtain the proper current division and time delay to allow for proper comparison and discharge of the capacitors 25 and 57. In FIGS. 2A through 2C, the discharge of the capacitors is shown to occur at time t5. Capacitors 33 and 59 provide for slow turn-off of transistors 24 and 51, respectively, to avoid untimely comparison or capacitor discharge.

The circuitry at 60 represent a voltage regulation circuitry comprising resistor 61, capacitor 62, zener diode 63, and diode 64.

It is to be understood that the above described arrangement is merely illustrative of one application of the invention, and that numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of invention. It will be recognized, for example, that the voltage on capacitor 57 may be measured against a fixed potential by connecting the base of transistor 86 to a reference voltage. In such a case, the capacitor 25 and its associated circuitry could be eliminated. The reference voltage should be selected to be smaller than the voltage level to be built-up on capacitor 57 at high RPM.