US3871348A

US3871348A - Capacitive discharge ignition system for internal combustion engines

Info

Publication number: US3871348A
Application number: US330443A
Authority: US
Inventors: David T Cavil
Original assignee: Outboard Marine Corp
Current assignee: Outboard Marine Corp
Priority date: 1973-02-07
Filing date: 1973-02-07
Publication date: 1975-03-18
Anticipated expiration: 1992-03-18
Also published as: CA988995A; JPS49111050A

Abstract

The various circuits use the primary coil and breaker points of conventional inductive ignition systems (magneto or battery) as the charging and timing elements in a capacitive discharge conversion system. When the breaker points open, the stored energy is transferred from the coil to capacitor C1 which is discharged through a low impedance transformer to obtain a rapid voltage rise time to obtain improved performance of the spark plug connected to the secondary of the transformer. In one circuit the capacitor is discharged near peak voltage by triggering a thyristor in response to the inital discharge of the capacitor from its peak charge. In other circuits the thyristor is triggered to discharge the capacitor when the charge on another capacitor in an R-C circuit shunting capacitor C1 reaches the breakdown voltage of a trigger diode, the R-C time constant being selected to enable the charge capacitor C1 to reach its maximum charge voltage.

Description

United States. Patent [1 1 Cavil [451 Mar. 18, 1975 i 1 CAPACITIVE DISCHARGE IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINES [75] Inventor: David T. Cavil, Menomonee Falls,

[21] App]. No.: 330,443

[52] U.S. Cl. 123/148 E, 315/209 [51] Int. Cl. F02p 1/00 [58] Field of Search 123/148 E, 149; 310/153,

[56] References Cited UNITED STATES PATENTS 3,367,314 9/1965 Hirosawa et a1. 123/148 Primary ExaminerManuel A. Antonakas Assistant Examiner-J. W. Cranson Attorney. Agent, or Firm-Michael, Best & Friedrich [57] ABSTRACT The various circuits use the primary coil and breaker points of conventional inductive ignition systems (magneto or battery) as' the charging and timing elements in a capacitive discharge conversion system. When the breaker points open, the stored energy is transferred from the coil to capacitor C which is discharged through a low impedance transformer to obtain a rapid voltage rise time to obtain improved performance of the spark plug connected to the secon-' dary of the transformer. In one circuit the capacitor is discharged near peak voltage by triggering a thyristor in response to the inital discharge of the capacitor from its peak charge. In other circuits the thyristor is triggered to discharge the capacitor when the charge on another capacitor in an R-C circuit shunting capacitor C, reaches the breakdown voltage of a trigger diode, the R-C time constant being selected to enable the charge capacitor C to reach its maximum charge voltage.

PATENIEUMAR 81975 SHEET 1 0f 2 \1 Iii PATENYEB HA8 1 8l975 suinep z CAPACITIVE DISCHARGE IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION Inductive ignition systems of the type used on snowmobile engines and outboard motors have a tendency to foul spark plugs at low speeds, at which speeds the secondary voltage rise time is too low. In the case of snowmobile engines, the problem is more pronounced and no plugs offer an operating heat range suitable for both high power output and idle speed as well. The-reasons for this are related to permissible current across the breaker points and to the inductive limitations inherent in these systems. Proper selection of coil design can keep the switching current across the breaker points within reasonable limits but practical current'interrupting capacity of breaker points imposes a limitation on the available energy storage since a relatively large amount of inductance is thenrequired and results in the secondary voltage rise time being too slow for use with colder spark plugs because of their tendency to foul at low speeds.

Capacitive discharge (CD) systems use high frequency, .low impedance ignition coils which provide very rapid rise time on the secondary across the plugs improving'cold starting, operating heat range and ability to tire fouled plugs. Obviously, CD ignition is attractive in snowmobile engines and outboard motors but has been incorporated in original equipment only on a limited basis due to cost. Conversion CD (and transistor) systems are available for automotive engines but have not been compatible with magneto systems. Most automotive CD conversion systems use the breaker points. even though such use retains the problems of point wear, etc. albeit to a reduced extent since the current is reduced to unacceptable levels. for the uses contemplated herein since snowmobile engines and outboard motors generally require at least 3 amps on the points to operate properly in the oily conditions of these engines. Ideally, CD ignition is breakerless but the cost of such systems is unacceptable for conversion or for many original equipment situations.

SUMMARY OF THE INVENTION The invention provides a capacitive discharge ignition system for internal combustion engines, which system comprises a source of electrical potential, a charge capacitor in circuit with the source of electrical potential to be charged thereby, breaker points connected with the source of electrical potential and operative when opened to cause the source of electrical potential to charge the charge capacitor, an ignition primary winding in circuit with the charge capacitor, switch means in circuit with the primary winding and the charge capacitor and including a control terminal operative to turn the switch means on so as to discharge the charge capacitor in response to a predetermined trigger potential, and a circuit shunting the charge capacitor and including a resistance and a second capacitor, whereby the charge buildup on the second capacitor lags the buildup on the charge capacitor by the time constant of the shunt circuit the potential on the second capacitor being applied to the control terminal and the values of the resistance and of the second capacitor being selected so that the trigger potential is applied to the control terminal at approximately the time of maximum charge on the charge capacitor.

In one embodiment in accordance with the invention the source of electrical energy comprises means for storing energy in a coil.

In one embodiment in accordance with the invention the values of the resistance and of the second capacitor are selected so that the breakdown potential of the control terminal occurs when the potential on the second capacitor is less than the potential on the charge capacitor.

In one embodiment in accordance with the invention the switch means is a thyristor including a gate responsive to the potential on the second capacitor.

FIG. 1 is a schematic representation of a capacitive discharge conversion of a magneto ignition system.

FIG. 2 is the equivalent charging circuit of FIG. I.

FIG. 3 is the equivalent triggering circuit of FIG. 1.

FIG. 4 is the equivalent discharge circuit of FIG. I.

FIG. 5 is a schematic representation of a capacitive discharge conversion of a battery ignition system.

FIG. 6 is a schematic diagram of a phase control type of triggering system.

FIG. 7 is a schematic representation of a twin cylinder, single coil system utilizing two rotating magnets.

FIG. 8 is a schematic representation of a twin cylinder capacitive discharge system utilizing one rotating magnet (for graphic purposes, shown as two) and two coils.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. I is a schematic representation of a capacitive discharge conversion of a snowmobile magneto system. Here the flywheel (or other rotating part) is provided with a magnet 10 which rotates in proximity to the magneto primary 12 to store energy in the primary. The usual breaker points 14 are across the primary as is the usual capacitor C In the standard magneto arrangement the magneto secondary would have current induced therein and be connected directly to the spark plug(s). In a low tension magneto such as usually employed on snowmobiles, the

leads

16,18 would be connected to a separate ignition coil having a primary and secondary with the secondary being connected to the spark plug(s). The system about to be described can be utilized with either the standard magneto or low tension magneto. In connection with the low tension magneto, the entire ignition coil would be removed and replaced with the parts about to be described. In a standard magneto system, the secondary is discarded.

In a snowmobile engine there is provision for spark advance and as this occurs there is a reversal of polarity in the region of, say, 600 rpm. Due to this reversal of polarity, the rectifier bridge 20 is provided to maintain constant polarity on the system. The principle of a rectifier bridge is well understood. Suffice it to say that it operates to maintain junction 22 positive and junction 24 negative. When the points open, the energy stored in the magneto primary 12 will charge capacitor C through the bridge, junction 22 and line 26. Lead 26 is also connected to the primaries of the pulse transformers 28 which are then connected to anode 30 of the silicon controlled rectifier SCR. The cathode of the SCR is connected to the other side ofC through

leads

32,34 and is also connected to the anode of diode D the cathode of which is connected to the negative side of the bridge 20. The gate of the SCR is controlled by the Zener diode D Zener diode D in the gate circuit serves to desensitize the circuit to transients induced by point arcing and resistor R shunting capacitor C, is for the same prupose. Diode D and resistor R are removed for the sake of clarity in considering the equivalent charging circuit shown in FIG. 2. In this equivalent circuit the rectifier bridge is not shown as such but diode D represents the blocking action of the bridge and diode D 2 represents the reverse shunting action of the bridge. When the breaker points open, charge capacitor C, and the capacitor across the breaker points (the normal capacitor associated with the magneto and which may be omitted if desired) are charged exponentially from the primary coil along the path indicated by the dotted arrows in FIG. 2.

After the charge capacitor C, has been charged to peak voltage, it begins to discharge along the path indicated by the dotted arrows in FIG. 3 which illustrates the equivalent triggering circuit. This results in the gate of the SCR going positive with respect to the cathode to render the SCR conductive and discharge C, (and C into the pulse transformers as indicated in FIG. 4. Diode D (the-rectifier bridge) clamps the capacitor in the reverse direction and provides a free-wheeling path for the transformer primaries. Two transformers are shown and this is typical of snowmobile engines having opposed cylinders fired in unison.

FIG. 5 shows the conversion of a battery system to capacitive discharge. Again, only the primary coil 36 is utilized, the secondary being discarded in the conversion. When the breaker points 38 are closed, the coil is energized from battery 40 through ballast 42. Capacitor C normally provided, may be used or not, as desired in the conversion. When the breaker points open, the charge capacitor C, is charged through line 44 which also leads to the primary of the ignition transformer 46. The other side of the primary is connected to the anode of the SCR, the cathode of which is connected to junction 48 and diode D, leading to line 50. Resistance R is connected between line 50 and the gate of the SCR. This system is also automatically triggered upon the capacitor C, reaching maximum charge. As the charge starts to leak through the primary coil 36, line 50 and resistance R, the gate of the SCR goes positive, fires the SCR, and discharges capacitor C, through the primary of the ignition transformer 46. Diode D, provides a free-wheeling path for the transformer and clamps the capacitor. The source of energy for the primary coil 36 in this case is a battery. In the prior circuit it was a magneto. The source is not the important part of this invention. The use of the primary coil as an energy storage device and the use of the breaker points to control the charging and timing of a capacitive discharge system is the crux of the invention.

Another method of timing the discharge of the charge capacitor is illustrated in FIG. 6. Here again there is a rotating magnet 10, a magneto primary 12 with the breaker points 14 and the optional (original) magneto capacitor C across the primary. In this arrangement the charge capacitor C, is shown across the

lines

16,18 although it will be understood that if polarity reversal is a factor, then a rectifier bridge would be interposed. When the breaker points open in this circuit. the capacitor C, is charged. The capacitor C is also charged through resitance R. The charge on C is applied to the Zener diode D, (which could be a diac or any other trigger diode) and when the Zener diode breaks down, the voltage will be applied to the gate of the SCR to fire the SCR and discharge capacitor C,

through the primary of the ignition transformer 52, and the thyristor (SCR) to the grounded side of the circuit. Diode D again functions to provide the free-wheeling path for the primary of the ignition transformer and to clamp C, in the reverse direction. In this circuit it will be noted that the Zener diode D, breaks down when the voltage on C reaches the breakdown value. The time required to reach this breakdown value is determined by the time constant of R and C The values of R and C are adjusted (selected) so the voltage across C reaches the breakdown voltage of the Zener diode D, at a time sufficiently delayed in time to enable the charge capacitor C, to reach its maximum charge voltage. The resistor-capacitor combination R,C,, which shunts the charge capacitor is a phase control type circuit. This type of phase control is used in AC. power control but, to my knowledge, has not been used in the present type of environment.

The phase control type triggering system shown in FIG. 6 can be made bi-directional as in FIG. 7. In this instance the flywheel is provided with two rotating magnets having opposite polarity. The points 14 are operated by a cam which opens the points each time a magnet passes the primary coil 12. Because the magnetization is opposite, the primary current in coil 12 will alternately be positive and negative and the points will open on alternately positive and negative current peaks. This will cause capacitor C, to be charged alternately positive and negative with respect to ground. The original and optional capacitor C, will also be charged positive and negative with respect to ground. In this circuit the trigger device D, is a diac which is a bi-directional trigger diode. It will be conductive when the voltage across it reaches the breakdown voltage regardless of polarity. One anode of the diac D, is connected to the capacitor C and the other anode is connected to the gate of the thyristor which in this case is a triac. Anode 1 of the triac is connected to ground and anode 2 of the triac is connected to junction 54. The primary coil 56 of ignition transformer 58 is connected between line 16 and junction 60.

The primary 62 of transformer 64 is connected between junction and junction 54. When a positive voltage across C reaches the breakdown voltage of the diac D,, the diac breaks down to trigger the triac, and C, is discharged through diode D leading from line 16 to junction 66 to junction 60 and then through coil 62 of ignition transformer No. 2 which fires the spark plug connected to the secondary of the transformer. Diode D, acts under these conditions to clamp the capacitor and provide free wheeling for transformer No. 2. N616 the triac acts the same as the SCR in the previous circuit. When C, is charged in the negative direction, C, is also charged in the negative direction and when the voltage across C reaches the breakdown voltage of the diac (in the opposite direction to that just described), the diac conducts and fires the triac gate (the gate becomes negative with respect to its anode No. 1). This discharges C, through the triac and diode D and the primary 56 of ignition coil No. 1 while diode D acts to clamp the capacitor. Thus this system can be utilized in conjunction with converting an alternate firing twin cylinder ignition system.

If it is desired to reduce the rotating mass, a single rotating magnet can be used in conjunction with two magneto primary coils and two breaker point sets as in dicated in FIG. 8. While at first blush this drawing appears to have two rotating magnets, bear in mind but one magnet is used and is denoted 68. The two coils are denoted 70,72. The breaker point sets are denoted 74,76. The two original capacitors C ,C., may be optionally left in the system. One breaker point set will be closed when the other is opened so the coils 70,72 are discharged alternately. When points 74 open, assuming that line 78 is charged in the positive sense, a positive charge is put across charge capacitor C, and across capacitor C The junction 80 of the diac is now going negative and when the breakdown voltage of the diac is reached, the gate of the triac will fire the triac to allow the charge capacitor to discharge through diode D the triac, and the primary coil 82 of the ignition transformer 84 for cylinder No. 1 while diode D clamps the capacitor. When breaker points 76 open, the line 86 goes positive to charge capacitor C in the opposite direction and also to charge C in the opposite direction, thus having junction 80 going positive and,

when the breakdown voltage of the diac is reached, it

triggers thetriac which tires to allow the capacitor C, to discharge through diode D the triac, and the primary coil 88 of the ignition transformer 90 for cylinder No. 2 while diode D now functions to clamp the capacitor C.

The time constant factors described relative to FIG. 6 apply in conjunction with the circuit of FIG. 8. The phase control type triggering system, whether it be the single cylinder type shown in FIG. 6 or the twin cylinder type shown in FIGS. 7 and 8, is not as precise as the system first described insofar as triggering at the peak charge is concerned. At low engine speeds, when the charge voltage is slightly lower, thetime delay is longer and this may allow the charge capacitor to partially discharge back through the primary coil. By way of compromise, the trigger point can peak at low speeds while accepting a slight loss of charge voltage at higher speeds due to triggering before peak charge voltage is reached. This triggering system is'also more temperature sensitive than the first system.

This invention has primary application to conversion of snowmobile engines and as original equipment in snowmobiles where costs are critical. Application to outboard motors is feasible but not as attractive since the cost relative to present breakerless capacitive discharge systems for outboard motors offers no significant advantage and does have some disadvantages due to retention of breaker points. The CD systems described herein do offer the possibility of application to existing outboard motors, however, and would offer an upgrading of performance.

It will be noted the breaker points control both the charging of the charge capacitor and the timing in that the time of discharge of the capacitor is ultimately related in time to the opening of the points. Systems have been proposed in the past where the points control charging, and timing is controlled by voltage responsive means which merely look for a minimum voltage. Other systems have the breakers control timing (by controlling a trigger) while other means control charging. The present system using energy stored in an induction coil can control both charging and timing with the points.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A capacitive discharge ignition system for internal combustion engines, said system comprising a coil, means for storing energy in said coil, a charge capacitor in circuit with said coil to be charged thereby, breaker points connected across said coil and operative when opened to cause the energy stored in said coil to charge said capacitor, an ignition primary winding in circuit with said charge capacitor, switch means in circuit with said primary winding and said charge capacitor and including a control terminal operative to turn said switch means on so as to discharge said charge capacitor in response to a predetermined trigger voltage, a circuit shunting said charge capacitor and including a resistance and a second capacitor, whereby the charge buildup on said second capacitor lags the buildup on said charge capacitor by the time constant of said shunt circuit, the voltage on said second capacitor being ap plied to said control terminal and the values of said resistance and of said second capacitor being selected so that said trigger voltage is applied to said control terminal at approximately the time of maximum charge on said charge capacitor.

2. A capacitive discharge ignition system according to claim 1 in which said switch means is a thyristor including a gate responsive to voltage on said second capacitor.

3. A capacitive discharge ignition system in accordance with claim 1 wherein the values of said resistance and of said second capacitor being selected so that the breakdown voltage of said control terminal occurs when the voltage on said second capacitor is less than the voltage on said charge capacitor.

4. A capacitive discharge ignition system for internal combustion engines, said system comprising a source of electrical potential, a charge capacitor in circuit with said source of electrical potential to be charged thereby, breaker points connected with said source of electrical potential and operative when opened to cause said source of electrical potential to charge said charge capacitor, an ignition primary winding in circuit with said charge capacitor, switch means in circuit with said primary winding and said charge capacitor and including a control terminal operative to turn said switch means on so as to discharge said charge capacitor in response to a predetermined trigger potential, a circuit shunting said charge capacitor and including a resistance and a second capacitor, whereby the charge buildup on said second capacitor lags the buildup on said charge capacitor by the time constant of said shunt circuit, the potential on said second capacitor being applied to said control terminal and the values of said resistance and of said second capacitor being selected so that said trigger potential is applied to said control terminal at approximately the time of maximum charge on said charge capacitor.

5. A capacitive discharge ignition system in accordance with claim 4 wherein the values of said resistance and of said second capacitor being selected so that the breakdown potential of said control terminal occurs when the potential on said second capacitor is less tha the potential on said charge capacitor.

6.A capacitive discharge ignition system according to claim 4 in which said switch means is a thyristor including a gate responsive to the potential on said sec-

Claims

1. A capacitive discharge ignition system for internal combustion engines, said system comprising a coil, means for storing energy in said coil, a charge capacitor in circuit with said coil to be charged thereby, breaker points connected across said coil and operative when opened to cause the energy stored in said coil to charge said capacitor, an ignition primary winding in circuit with said charge capacitor, switch means in circuit with said primary winding and said charge capacitor and including a control terminal operative to turn said switch means on so as to discHarge said charge capacitor in response to a predetermined trigger voltage, a circuit shunting said charge capacitor and including a resistance and a second capacitor, whereby the charge buildup on said second capacitor lags the buildup on said charge capacitor by the time constant of said shunt circuit, the voltage on said second capacitor being applied to said control terminal and the values of said resistance and of said second capacitor being selected so that said trigger voltage is applied to said control terminal at approximately the time of maximum charge on said charge capacitor.

5. A capacitive discharge ignition system in accordance with claim 4 wherein the values of said resistance and of said second capacitor being selected so that the breakdown potential of said control terminal occurs when the potential on said second capacitor is less than the potential on said charge capacitor.

6. A capacitive discharge ignition system according to claim 4 in which said switch means is a thyristor including a gate responsive to the potential on said second capacitor.