US 3704699 A
A capacitor discharge ignition system coupled to a source of electrical energy for discharging pulses of electrical energy into the transformer of a vehicle ignition system at a rate variable with engine speed. The capacitor discharge ignition system includes a revolution limiter which is responsive to the engine exceeding a predetermined speed for reducing the rate at which pulses of electrical energy are discharged into the transformer by the capacitor discharge system. With a reduced number of pulses being supplied to the transformer, the engine is rendered substantially incapable of exceeding the preselected speed.
Description (OCR text may contain errors)
I Umted States Patent 1151 3,704,699
Howard 1451 Dec. 5, 1972 [5 1 CAPACITOR DISCHARGE IGNITION 3,448,732 6/1969 Weiss ..123/148 15 SYSTEM WITH REVOLUTION 2,970,227 1/1961 Horton et a]. ..307/243 LIMITER 3,534,719 10/1970 Minks ..123/102 3,430,615 3/1969 Chavis ..l23/l02  Invent 223;" Hwam Costa Mesa 3,182,648 5/1965 Schneider ..123/102 3,402,327 9/1968 Blackburn ..123/102  Assignee: Howard Associates, Inc., Costa Mesa, Calif. Primary Examiner-Laurence M. Goodridge Assistant ExaminerCort Flint  June 1970 AttorneySmyth, Roston & Pavitt  Appl. No.: 49,26}
57 AB TRACT Related US. Application Data 1 S 63 A capacitor discharge ignition system coupled to a 1 fggg of 806998 March source of electrical energy for discharging pulses of electrical energy into the transformer of a vehicle ignition system at a rate variable with engine speed. The capacitor discharge ignition system includes a revolu-  Fie'ld 2 125/102 148 E tion limiter which is responsive to the engine exceeding a predetermined speed for reducing the rate at which pulses of electrical energy are discharged into  References cued the transformer by the capacitor discharge system. UNITED STATES PATENTS With a reduced number of pulses being supplied to the transformer, the engine is rendered substantially inl i capable of exceeding the preselected speed. lltSOl'l 3,356,082 l2/l967 Jukes ..123/102 3 Claims, 6 Drawing Figures PATENTED DEC 5 m2 SHEEI 1 [IF 3 PATENTED DEC 5 I872 SHEEI 2 0F 3 CAPACITOR DISCHARGE IGNITION SYSTEM WITH REVOLUTION LIMITER REFERENCE TO COPENDING APPLICATION This is a continuation-in-part of application Ser. No. 806,998, filed on Mar. 13, 1969, entitled Electronic Relay and Ignition System Utilizing same.
BACKGROUND OF THE INVENTION Automotive vehicles have for many years employed ignition systems which utilize the build-up and collapse of a magnetic field in a saturable reactor. The conventional saturable reactor ignition system includes a coil or transformer which is supplied with electrical energy from the battery when the ignition switch is closed and cam driven breaker points for controlling the discharge of the coil. Electrical energy from the coil is directed to the appropriate spark plug by the distributor.
This type of system performed satisfactorily many years ago when the typical engine had only four or six cylinders and was of the slow speed and low compression type. However, with the advent of the modern eight cylinder high-speed and high-compression engine, the saturable reactor ignition system is quite inadequate.
As speed of the internal combustion engine increases, the ignition system requires an increase in electrical power. The ignition system power demand is proportional to engine rpm. This creates a serious problem because as engine rpm increases, the time available for energization of the coil decreases so that the current or power actually received for the production of the spark is reduced as engine rpm increases. Stated differently, the current or power available for spark production varies inversely with the demand of the system.
Numerous attempts have been made to obviate this problem. One of the most common is the use of a 12- volt battery, a 6-volt coil and a resistor in series with the ignition switch to reduce the voltage applied to the coil to approximately 7 volts at low rpm. The voltage drop in this resistor is also an inverse function of engine speed so a higher voltage is applied to the coil as engine speed increases. This series resistor-is now utilized in most automobiles, and although it has alleviated the problem to some degree, it has not solved it.
An improved form of ignition system is the capacitor discharge ignition system. Such systems typically include a transformer for increasing battery voltage, a capacitor for storing the high voltage current, and a suitable means for discharging the energy stored in the capacitor across the primary of the coil. A primary advantage of a capacitor discharge ignition system is that the energy content of each capacitor storage discharge is constant, and thus such a system draws power from the primary source at a rate which varies directly with the demand. Thus, a capacitor discharge system draws twice the power at 4,000 rpm that it does at 2,000 rpm.
There are numerous capacitor discharge ignition systems available; however, the installation of such systems on existing vehicles is most difficult. For proper operation, the capacitor discharge system should draw full battery power, and accordingly, the series resistor described hereinabove should be bypassed. This presents a most serious installation problem in that this series resistance is in the form of an elongated conductor intermediate the ignition switch and the coil. in order to bypass this resistance and still utilize the ignition switch to turn the engine on, a direct electrical connection must be made at the ignition switch itself. Because of the location of the ignition switch, this connection is difficult or impossible to make.
It is often necessary or desirable to limit engine rpm. For example, governors have been used to limit maximum engine speed and hence maximum engine velocity. In high performance engines it is necessary to limit engine rpm when shifting gears to prevent possible engine damage. Similarly it is necessary to limit the rpm of a boat engine when the propeller comes out of the water to prevent possible engine damage.
The above are merely illustrative of circumstances requiring a revolution limiting means. Heretofore the revolution limiting function has been performed by separate units such as governors which are relatively expensive and require separate installation. In addition, at least some speed limiters completely shut down the engine at a preselected speed and are therefore unsuited for numerous applications such as racing vehicles. If the revolution limiter prevents every cylinder from firing during the time the preselected speed is exceeded, the cylinders become loaded with fuel and subsequent detonation of this fuel may cause engine damage.
SUMMARY OF THE INVENTION The present invention provides a revolution limiter in association with a capacitor discharge ignition system. The revolution limiter of this invention does not completely shut down the engine at a preselected engine speed but merely causes it to lose sufficient power to substantially prevent acceleration above such preselected speed. As such it is adapted for racing, private, commercial vehicles and marine usage.
The revolution limiter of this invention is extremely inexpensive yet very reliable and adjustable to permit the operator to select the maximum engine speed. A capacitor discharge system of the type described and claimed in my above identified copending application can be provided with a revolution limiting function merely by the appropriate selection of one resistor for the system. To permit operator adjustment of the maximum permissible engine speed the resistor is preferably a variable resistor having an appropriate range of resistances. It should be understood, however, that this invention is not limited to this particular construction as the concepts hereof are applicable to other constructions.
A conventional vehicle ignition system typically includes a source of electrical energy such as a battery, a coil or transformer and a movable member such as breaker points movable between open and closed positions at a rate proportional to engine speed. A capacitor discharge ignition means can be coupled to the source of electrical energy for discharging a pulse of electrical energy into the transformer in response to substantially each time the breaker points reach one of said positions.
According to the present invention, the capacitor discharge ignition means includes revolution limiting means responsive to the breaker points travelling between the positions thereof at greater than a predetermined rate for reducing the number of pulses of electrical energy discharged into the transformer by the capacitor discharge ignition means. To prevent engine shut down and to prevent an accumulation of unburned fuel in the engine, the capacitor discharge ignition means is allowed by the revolution limiting means to discharge at least some pulses of electrical energy into-the transformer even at engine speeds above the preselected engine speeds.
The energy stored in the capacitor of the capacitor discharge system is normally discharged in response to a triggering signal. A more specific feature of this invention is toprovide revolution limiting means which allows or causes the triggering signal to be supplied at a rate proportional to engine speed so long as the engine is travelling at less than the preselected speed. However, at speeds in excess of the preselected speed, the revolution limiting means reduces the rate at which the triggering signals are provided.
This can be advantageously provided by supplying input pulses to the revolution limiter with the: duration of each of the input pulses and the length of time between successive input pulses varying inversely with engine speed. Specifically, one of such input pulses may begin when the points open and terminate when the breaker points close. As engine speed increases, the length of time between pulses progressively decreases.
A capacitive network is coupled to receive these pulses with the capacitive network providing a triggering signal in response to eachof such input pulses with the triggering signal having a predetermined amplitude necessary for discharging the system capacitor. The capacitive network includes a triggering capacitor and means such as a variable resistor for adjusting the time constant of the network. The triggering capacitor charges when the points are open and discharge when the points close and if the points do not remain closed sufficiently long, the capacitor may not be completely discharged when the points open again. If the triggering capacitor is discharged less than a predetermined amount when the points open, the pulse of energy passing therethrough will have less than the predetermined amplitude and as such it will not constitute a triggering pulse capable of discharging the system capacitor. This occurs when the engine exceeds the maximum speed set by the revolution limiter. However, the system has means for permitting the next input pulse to generate a triggering signal so that approximately one triggering pulse is provided for every other input pulse so long as engine speed is above such maximum speed. This assures that the engine will not shut down. Such a system may find application as a control apparatus in other environments.
The present invention can be used with an electronic relay of the type described in my copending application; however, the present invention is not limited for use with a capacitor discharge system having such an electronic relay therein.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of an ignition system constructed in accordance with the teachings of this invention.
FIG. 2 has a schematic wiring diagram of a preferred form of the capacitor ignition discharge system and the electronic relay.
FIG. 3a is a plot of input signal vs. time at tie point GEIl7 FIG. 3b is a plot of voltage signal at tie point F" vs. time.
FIG. 4 illustrates capacitor discharge rate for various values of resistance for the associated resistor.
FIG. 5 is a typical plot of maximum allowable engine rpm vs. resistance of the resistor of the revolution limiter.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a DC power source in the form of a battery 11 having a positive terminal and a negative terminal that supplies electrical energy to a capacitor discharge ignition system 13 through a pair of leads l5 and 17. The capacitor discharge ignition system 13 is turned on and off by an ignition switch 19 which is connected between the positive terminal of the battery 11 and a control lead 23 from the capacitor discharge ignition system 13. A resistor 25 is provided in series with the ignition switch 19 for the purpose of improving the spark when the capacitor discharge ignition system 13 is not utilized. The resister 25 is typically in the form of an elongated cable extending away from the ignition switch 19 and is found on substantially all of the existing automotive vehicles.
When the ignition switch 19 is closed, the capacitor discharge ignition system periodically supplies electrical energy to a coil or transformer 27 through leads 29 and 31. The coil 27 steps up the voltage supplied thereto and supplies it to a distributor cap 33 of a distributor which supplies the energy to a spark gap 35. Although only one spark gap 35 is shown in FIG. I, there will, of course, be one of such spark gaps for each cylinder of the engine with which the ignition system is being utilized.
The distributor also has the usual breaker points 37 and 39 which are driven by the cam shaft 39a of the engine with which the ignition system is utilized. The breaker points 37 and 39 are connected to the capacitor discharge ignition system by a lead 40. When the points 37 and 39 are closed, the capacitor discharge ignition system 13 is building up its charge, and when the points are opened, the system 13 discharges the energy stored therein to the primary winding of the coil 27. In this manner, the breaker points 37 and 39 control the rate at which pulses of energy are supplied to the coil 27.
A condenser 41 is connected in parallel with the breaker points 37 and 39. This is the standard condenser which is provided for most, if not all, conventional ignition systems.
All of the components of the ignition shown in FIG. 1 except for the capacitor discharge ignition system 13 may be conventional. In addition, the typical conven' tional system would include a conductor (not shown) joining a point A adjacent the resistor 25 to a point B adjacent the primary winding of the coil 27 and a second conductor joining a point C on the coil with a point D near the breaker points 37 and 39. To install the capacitor discharge ignition system, it is only necessary to disconnect the points A and B and the points C and D and to connect the leads 15, 17, 23, 29, 31 and 40, as shown.
With further reference to FIG. 1, the capacitor discharge ignition system 13 includes an electronic relay 43 to which DC current is supplied by the leads l5 and 17. The electronic relay 43 receives a small signal current from the battery 11 through the lead 23 when the ignition switch 19 is closed. The electronic relay 43 normally blocks the flow of current from the battery to the remainder of the capacitor discharge ignition system 13, but is responsive to receiving the signal current through the ignition switch 19 and the lead 23 to permit conduction of the battery current therethrough to an oscillator or inverter 45 which converts DC to AC. This permits stepping up of the voltage in a transformer 47. By way of example, if the battery 11 is a 12 volt battery, the transformer 47 may increase the voltage to about 300 volts.
The alternating current from the transformer 47 is converted back to DC in a rectifier 49. The current from the rectifier 49 charges a capacitor 51 with the discharge of the capacitor being prevented by a silicon control rectifier 53 which is nonconductive when the breaker points 37 and 39 are closed. A trigger circuit 55 which receives direct current from a lead 56 is responsive to opening of the points 37 and 39 to make the SCR 53 conductive to permit the capacitor 51 to discharge the energy stored therein across the primary winding of the coil 27. This energy in the coil 27 is then utilized to produce a spark across the spark gap 35 to initiate combustion of the fuel-air mixture within the cylinder of the engine.
Referring to FIG. 2, the electronic relay 43 includes a power transistor 57- and a complementary control transistor 59. In the embodiment illustrated, the transistor 57 and 59 are of the PNP and NPN types, respectively. The power transistor 57 and the control transistor 59 are cascaded with the power transistor 57 operating as a collector-follower and the control transistor operating as an emitter-follower.
As shown in FIG. 2, the lead 17 is connected to the emitter of the transistor 57 and extends through the relay 43 while the grounded lead also extends through the relay 43. A conductor 61 electrically connects the base of the transistor 57 to the collector of the control transistor 59 while a second conductor 63 connects the emitter of the transistor 59 to the grounded lead 15. A conductor 64 is connected between the conductor 61 and the lead 17 around the emitter of the power transistor 57 and a biasing resistor 65 is provided in the conductor 64 to control the voltage in the conductor 61. A load resistor 67 is connected between the emitter of the transistor 59 and the grounded lead 15. Control current is applied to the base of the transistor 59 when the ignition switch 19 is closed through the resistor 25 and a resistor 69.
The transistor 59 may be very small and draws only a very small current which may be of the order of 0.005 amp. Thus, when the ignition switch 19 is closed, a very small amount of current acts on the base of control transistor 59 to render it conductive. The resistor 69 is preferably large in comparison with typical values of the resistor 25 and may be of the order of 100 ohms so that variations of the resistance of the resistor 25 from one vehicle to the next will have little effect. The resistor 69 serves to assist in establishing the desired current which is applied to the base of the transistor 59.
With the ignition switch 19 open, a positive 12 volts is supplied through the conductor 64 to the base of the power transistor 57; however, because the transistor 59 is nonconductive with the ignition switch 19 open, there is no current flow in the conductors 61 and 64, and accordingly, the power transistor 57 is turned off and prevents supply of current to the remainder of the capacitor discharge ignition system. With the closing of the ignition switch 19, the control transistor 59 becomes conductive so that there is a small current flow in the conductors 61 and 63 and through the transistor 59 to the grounded lead 15. The power transistor 57 is responsive to this current flow to become conductive to thereby permit the supply of current therethrough to the remainder of the system 13.
The resistor 67 limits current flow to ground and therefore prevents burnout of the small control transistor 59. Although the particular resistance of the resistor 67 must be selected depending upon the current capacity of the transistor 59, in the embodiment illustrated, it is of the order of 50 ohms.
When the ignition switch 19 is opened, the current bias on the base of the transistor 59 is removed and this transistor shuts off to thereby open the circuit from the base of the transistor 57 to the grounded lead 15. Although the resistance of the resistor is dependent upon the design parameters for the relay 43, in the embodiment illustrated, it is of the order of 100 ohms.
With the transistors 57 and 59 cascaded as shown, the total gain across the relay is the product of the gains of both of the transistors. Thus, assuming gains of 40 and 50 for the transistors 57 to 59, respectively, a control current of 0.005 amp applied to the base of the transistor 59 can control a current of 10 amps in the lead 17. As the lead 23 conducts only minimal current, the power loss there is negligible. Physically the lead 23 can be very small, and only the leads 15 and 17 between the relay 43 and the battery should be heavy in order to handle full battery current.
It can be seen, therefore, that the relay 43 is responsive to the application of a small control current to permit substantially full battery current to be supplied to the load. Although the electronic relay 43 may be utilized in various environments to control the supply of the current to many different kinds of loads, in the embodiment illustrated, it is utilized as a portion of a capacitor discharge ignition system to control the supply of current thereto.
In FIG. 2, the oscillator 45, the transformer 47 and the rectifier 49 are in the form of a DC to DC converter. The lead 17 supplies a positive voltage to a center tap of a primary winding 71 of the transformer 47 and the direction of current flow from the center tap through the primary winding 71 will depend upon which of two transistors 73 and 75 are in a conductive state. As shown, the emitters of the transistors 73 and 75 are connected to the opposite ends of the primary winding 71 by conductors 77 and 79, respectively, with the collectors of the transistors being connected by a conductor 81. The lead 17 and the conductor 81 are interconnected by a conductor 83 having resistors 85 and 87 therein. The transistors 73 and 75 are controlled by a control winding 89, the opposite ends of which are connected by conductors 91 and 93 to the bases of the transistors 73 and 75, respectively, and the center of which is connected to the conductor 83 intermediate the resistors 85 and 87 by a conductor 95.
The transformer 47 also includes a secondary winding 97 in which a high voltage current is induced by the primary winding 71. The efficiency of the tranformer is enhanced by the use of ferrite cores for the windings 71, 89 and 97, and this also permits a high conversion frequency in order to obtain high power from relatively small components. In the embodiment illustrated, the frequency of the alternating current may be approximately 6,000 cycles per second. In operation of the DC to DC converter, a positive current is supplied to the center of the coil 71 and the oscillator functions as a feedback oscillator to induce a high frequency voltage in the secondary winding 97 with the secondary voltage alternating between positive and negative values.
In the embodiment illustrated, the rectifier 49 is a full wave diode bridge rectifier. The rectifier 49 provides substantially a DC voltage through the lead 29, the primary of the transformer 27 and the lead 31 to charge a capacitor 51. The capacitor 51 cannot discharge electrical energy stored therein because the SCR S3 is in a nonconductive state.
A positive voltage is supplied from the battery through the lead 56 and a resistor 101 to a tie point When the breaker points 37 and 39 are closed, the tie point E" is coupled to ground potential through the breaker points 37 and 39. When the breaker points 37 and 39 are open, the positive voltage is provided at the tie point E" with such positive voltage being substantially equal in amplitude to the positive 12 volts supplied by the battery 11. Consequently, as the breaker points 37 and 39 open and close, an input signal in the form ofa pulse train is developed at the tie point having individual input voltage pulses as shown in FIG. 3a. The duration of the input pulses is proportional to the length of time the breaker points 37 and 39 are open and the period of time between the input pulses is proportional to the length of time the breaker points 37 and 39 are closed.
The input signal as represented in FIG. 3a is for an engine which is accelerating, and accordingly the duration of each of the input pulses and the period of time between successive pulses becomes successively shorter. That is, because the breaker points 37 and 39 are engine driven, the rate at which the input pulses are provided at the tie point B is proportional to engine speed. FIG. 3a is provided to illustrate in a general way the effect of engine acceleration on the input signal, it being understood that in actual practice an engine may not be capable of accelerating at the rapid rate shown in FIG. 3a. For purposes of illustration, the input signal at tie point E varies from zero to a positive voltage V,, which may be, for example, 12 volts.
At the initiation of each of the input pulses at the tie point E" such as at time I a positive voltage spike (FIG. 31)) having an amplitude related to the amplitude of the input pulse is produced across a control capacitor 105 to a tie point F. The voltage spike produced at the tie point F forward biases a diode 103 and is received at the gate of the SCR 53. If the amplitude of the voltage spike is approximately equal to or greater than V which is the required trigger voltage level for the SCR 53, the SCR 53 will switch into condition and effectively couple the anode of the SCR 53 to ground potential. When the gate of the SCR 53 is triggered, a charge current flows through the capacitor from tie point E" to tie point F, through the diode 103 which is forward biased, through the SCR S3 gate to ground potential. As current is drawn when the SCR 53 conducts, the voltage spike at T1 (FIG. 3b) will not substantially exceed V The charge current through the capacitor 105 charges the capacitor 105 to an electrical energy potential related to the amplitude of the input pulse provided at the tie point E. For example, with no previous charge on the capacitor, a 12 volt pulse at the tie point E will charge the capacitor 105 at tie point E to an electrical energy potential of a positive 12 volts with respect to ground potential.
When the SCR 53 switches into conduction, the capacitor 51 discharges the stored electrical energy thereon, in the form of an electric current, through the SCR 53 from the anode to the lead 29. When the capacitor 51 discharges, an electrical current is developed in the primary of the transformer 27 which produces a corresponding current in the secondary of the transformer 27.
As the capacitor 51 discharges, the electrical signal on the lead 31 will momentarily swing to a negative voltage from the positive voltage due to back emf and overshoots. When the electrical signal on the lead 31 goes negative or is zero volts, the SCR 53 switches off or into a nonconducting state. A capacitor 109 which is connected between the leads 29 and 31 functions as a safety device in delaying application of the recharge cycle to within the safe limits of the SCR 53 dv/dt rating. Subsequently, the capacitor 51 will recharge due to the voltage provided at the lead 31 from the rectifier A capacitor filters the high-frequency components of the oscillator from appearing at the gage of the SCR. As a precaution against the false triggering of the SCR 53 due to any breaker point bounce which might occur, the diode 113 clamps the SCR gate negative for the period during which the capacitor 51 is being charged.
When the breaker points 37 and 39 close, an input pulse (FIG. 3a) is instantaneously coupled to ground potential. However, due to the physical phenomenon that a capacitor cannot be instantaneously discharged, a voltage of equal amplified but of opposite polarity to the voltage at the tie point E will be produced at the tie point F as shown by way of example at times T T T etc. in FIG. 3b. Thus, V equals V The negative voltage provided at the tie point F will discharge to ground through a variable resistor 11] and a diode 113. For example, if a positive 12 volts charge was at the tie point E" during the application of the input pulse, a negative 12 volts will be developed at the tie point F when the breaker points 37 and 39 close. The negative 12 volts developed at the tie point F" will discharge through the resistor 111 and the diode 113 to the ground potential. The time required to discharge the capacitor 105 is dependent upon the RC time constant including the resistance of the resistor 111 and the capacitance of the capacitor 105.
With this invention the resistor 111 and the capacitor 105 are used to limit the maximum number of times the capacitor 51 can be discharged in a given time interval to thereby place an upper limit on the number of energy pulses which the transformer 27 can supply to the engine in such time interval. Thus, the capacitor 105 and the resistor 111 serve as a revolution limiter to limit engine rpm. So long as the engine rpm is below the maximum set by the revolution limiter, the capacitor discharge ignition system functions as described above.
The time constant of the RC circuit which includes the capacitor 105 and the resistor 111 can be varied in any suitable manner to change the maximum number of energy pulses which the transformer 27 can supply to the engine. Although this can be accomplished in different ways, in the embodiment illustrated, the resistor 111 is variable. Increasing the time constant such as by increasing the resistance of the resistor 111 decreases the maximum energy pulse discharge rate of the capacitor discharge ignition system.
The discharge of the capacitor 105 is represented in FIG. 3b by line segments D D D and D, with the slope of such line segments representing the rate of discharge of the capacitor 105. In FIG. 3b, the slope of the line segments D through D, are equal thereby indicating that the time constant of the RC circuit is constant. However, as engine speed increases, the period of time during which the points 37 and 39 remain closed decreases and the period of time between successive input pulses decreases. Accordingly at time T the points 37 and 39 open but the capacitor is not yet fully discharged. This is shown in FIG. 3b by the line segment I) which terminates at time T at a negative voltage amplitude.
When the breaker points 37 and 39 open before the capacitor 105 is fully discharged such as at time T (FIG. 3b), the voltage at the tie point E will increase in amplitude an amount substantially equal to the algebraic sum of the voltage applied to the tie point E and the remaining voltage charge at tie point F." For example, if a positive 12 volt pulse is applied to the tie point at time T and if the capacitor 105 at the tie point F has discharged to a negative 8 volts at time T the voltage at the tie point B will be a positive 4 volts. Accordingly, the maximum amplitude of the voltage spike passing through the capacitor 105 at time T will also be a positive 4 volts. As shown in FIG. 3b, the voltage spike at time T is equal to V to thereby render the SCR 53 conductive.
However, if the capacitor 105 has not discharged sufficiently when the breaker points 37 and 39 open, the voltage spike applied to the gate at the SCR 53 will not have sufficient amplitude to trigger the SCR 53 into conduction as shown, for example, at time T (FIG. 3b) where the amplitude of the voltage spike is less than V It has been found that when this occurs the voltage at the tie point F remains at some positive value V, which is greater than zero as shown, for example, between the times T and T in FIG. 3b. When the points 37 and 39 close at time T the negative voltage at the tie point F has a value V; which is approximately equal to the algebraic sum of V and V and accordingly the voltage at the tie point F" has a lesser negative value following a voltage spike which does not trigger the SCR 53.
Between the times T and T the capacitor 105 discharges at the same rate as between the times T and T However, because the voltage at the tie point F" at time T has a lesser negative value than at the time T the voltage at the time T, following discharge of the capacitor has a correspondingly lesser negative value than the voltage at the time T Accordingly, when the next input pulse is applied at the time T the resulting voltage spike has an amplitude of at least V which is sufficient to trigger the SCR 53. Typically this operation will continue with every other spark to the engine being cut off. This causes the engine to lose power until engine speed is reduced to increase the length of time between input pulses sufficiently for the capacitor 105 to discharge the requisite amount.
It has been found through actual tests that the circuit of FIG. 2 will produce a voltage waveform at tie point F of the type shown in FIG. 3b. Specifically, such tests showed, among other things, that the voltage at tie point F would have a positive value such as V, after the failure of the preceding voltage spike to trigger the SCR. Such tests also showed that the voltage at time T had a lesser negative value than at time T, with the result that the subsequent voltage spike at time T was normally sufficient to trigger the SCR 53.
Although I do not wish to be limited to any particular theoretical analysis, I believe that when the SCR 53 does not conduct during the application of one of the input pulses to the tie point E, substantially no current will flow through the capacitor 105. When no current flows in the capacitor 105, the charge thereon will not increase. The voltage at tie point F just prior to time T having an amplitude V, may be developed due to the increase in the forward bias resistance at the diode 103 as the voltage spike applied thereto decreases in amplitude. The amplitude of the certain voltage amplitude is related to the time varying resistance formed by the resistor 111 and the diode 103. For example, when the diode 103 is driven in forward bias, the major component of resistance is the low for ward bias resistance of the diode 103. However, as the voltage spike to the diode 103 decreases in amplitude, the resistance of the diode increases until the resistance of the diode 103 is much larger than the resistance of the resistor lll. Consequently, as the resistance of the diode 103 increases, the resistance of the resistor lll becomes the major component of resistance. Accordingly, this may cause the voltage at the tie point F of amplitude V just prior to the time As explained hereinabove, it is believed that this voltage of amplitude V assures the triggering of the SCR 53 during a subsequent input voltage pulse at the tie point L6E",
FIG. 4 illustrates the effect of varying the resistance of the variable resistor 111 on the discharge time of the capacitor 105. For illustrative purposes, the capacitance of capacitor 105 is constant and resistance of the resistor 111 varies in steps as indicated from 5,000 ohms to 50,000 ohms. It will be appreciated that the slope of the line segments D D D and D (FIG. 3b) can be changed by varying the resistance of the variable resistor 11] as shown in FIG. 4. Obviously by increasing the resistance of the resistor 111, the slope of the line segments D -D decreases with the result that engine speed is limited to a lower rpm.
FIG. 5 shows by way of illustration a plot of maximum rpm versus resistance of the resistor 11 1 in ohms.
It should be appreciated that the plot shown in FIG. will vary depending upon the number of cylinders in the engine, the interval between input pulses, and the capacitance of the capacitor 105. For the particular engine tested in connection with FIG. 5, the rpm can be limited to 3,000 when the resistor 111 has a resistance of approximately 50,000 ohms. Similarly, engine speed can be limited to approximately 10,000 rpm by adjusting the resistor 111 to approximately a 10,000 ohm resistance. it has been found that a resistance of from 5,000 to 50,000 ohms for the resistor lll covers many engines,although other values of resistance for the resistor 111 can be selected depending upon the maximum speed desired. Also, other means may be used to vary discharge rate of the capacitor 105.
Although exemplary embodiments of the invention have been shown and described, many changes, modifications, and substitutions may be made by one having ordinary skill in the art without necessarily departing from the spirit and scope of this invention.
1. A capacitor discharge system for use with an ignition system of an engine wherein the ignition system includes a source of electrical energy, a coil and means for generating timing signals at a rate proportional to engine speed, said capacitor discharge system comprismg:
a first capacitor coupled to the coil;
means for coupling said first capacitor to the source of electrical energy to permit said first capacitor to be charged by the electrical energy from said source;
trigger means responsive to a triggering signal for discharging said first capacitor to provide a pulse of electrical energy to the coil each time said triggering signal is received by said trigger means;
control means including a control capacitor chargeable by the source of electrical energy for providing said triggering signal to said trigger means in response to substantially each of said timing signals;
revolution limiting means responsive to the engine exceeding a preselected engine speed for preventing said control means for supplying said triggering signal to said trigger means in response to substantially each of said timing signals whereby the first capacitor is not discharged in response to each of said timing signals, said revolution limiting means allowing said control means to supply said triggering signal to said trigger means in response to some of said timing signals when the engine exceeds said preselected speed whereby the revolution limiting means causes a power loss at engine speeds greater than said preselected speed but does not shut down the engine; and
said revolution limiting means including a discharge path for the control capacitor, said discharge path including a variable resistor and a diode in series, said preselected engine speed being variable by varying the value of the resistance of said variable resistor.
2. A capacitor discharge system as defined in claim 1 wherein said variable resistor is variable from at least about 5,000 ohms to at least approximately 50,000
3. A capacitor discharge system as defined in claim 1 wherein the revolution limiting means provides approximately a fifty percent decrease in the rate at which said triggering signals are provided in response to the engine speed exceeding said preselected speed.