US3738340A - Internal combustion engine limiter - Google Patents

Abstract

An RPM limiter for an internal combustion engine employs a silicon-controlled rectifier (SCR) as a switch to shunt the ignition system in response to an RPM limiting pulse from an RC timing circuit which is reset to initiate the pulse immediately upon the ignition breaker points opening. A monostable multivibrator is also set at that time to initiate the generation of a spark timing pulse of sufficient duration to allow the magnetic field of the ignition coil to break down, and thereby fire a spark plug. An inhibit gate receives both pulses and allows the SCR to be fired during the period of the RPM limiting pulse after the period of the spark timing pulse. A switch connects a second capacitor in parallel with the capacitor of the RC timing circuit to permit a potentiometer of the timing circuit to be set and tested for the desired RPM limit while operating the engine at only half of that limit when the two capacitors are of equal size.

Description

United States Patent 1191 Olson INTERNAL COMBUSTION ENGINE Primary ExaminerLaurence M. Goodridge LIMITER Assistant Examiner-R0nald B. Cox [75] Inventor: John Olson Oxnard, Calif AttorneySamuel Lmdenberg, Arthur Freilich,

Abraham Wasserman et al. [73] Assignee: Ikon Engineering, Inc., Oxnard,

Calif- [57] ABSTRACT [22] Filed: Jan. 10, 1972 An RPM limiter for an internal combustion engine employs a silicon-controlled rectifier (SCR) as a switch to [2]] Appl' 216695 shunt the ignition system in response to an RPM limiting pulse from an RC timing circuit which is reset to ini- [52] US. Cl. 123/118, 123/102 tiate h p l immediately upon the ignition breaker [51] Int. C1. F02p 9/00, F02d 11/00 p n p ning- A monostable multivibrator is also set [58] Field of Search 123/102, 1 18 at that time to itiate the generation of a spark timing pulse of sufficient duration to allow the magnetic field [56] References Cit d of the ignition coil to break down, and thereby fire a UNITED STATES PATENTS spark plug. An inhibit gate receives both pulses and al- 3 673 992 8 1972 w tb 123,102 lows the SCR to be fired during the period of the RPM 4601103 811971 81236331111:iiiiiijiiiijiiiiiiii: 123102 limiting Pulse after the Period ofthespark fimingpulse- 3 581 720 6/1971 Hemphill 123/102 A Swltch cmmects a second capacltor Parallel Wlth 315631219 2/1971 Mieras 123/102 the p tor of the RC timing cir to permit a P 3,356,082 12/1967 Jukes 123 102 n i m r of the tlmmg circuit to be set and tested for 3,346,771 10/1967 Sutton..... the desired RPM limit while operating the engine at 3,182,648 5/1965 Scheider..... only half of that limit when the two capacitors are of 3,402,327 9/1968 Blackburn 123/102 equal Sim 3,572,302 3/1971 Wollesen 123/102 3,665,903 5/1972 Harris 123/102 7 C 2 ing Figures F ;fi Te I l l u 1 I I 1 1 i a i I MONOSTABLE I MQLTWIBQATQQ B I I i ace 4.5

I l(=NlT|oN I SWITCH I THQESHOLD DETEQTOQ t i 5% 34 v i [3 12PM LlMiT l l 1 I I -25- M m 42 41 l i INTERNAL COMBUSTION ENGINE LIMITER BACKGROUND OF THE INVENTION This invention related to a limiter for an internal combustion engine of the spark ignition type.

High performance engines such as are used in racing cars are sometimes inadvertently operated at such high RPM that damage is very likely to occur. Therefore, to assure that the engine is not operated at such a high RPM, it is desirable to employ some means for limiting the RPM of the engine.

The most obvious technique for limiting engine RPM is to simply cut off the ignition system when a preset RPM limit is exceeded. However, if the fuel system is still in operation, the cylinders will have a tendency to load up with raw gas. That then creates another problem because damage may occur if large quantities of gas are present in a cylinder during a compression stroke. If this problem is also to be avoided, the RPM limiting system must also be able to shut off the fuel system, but then the result is slow recovery of full power after limiting occurs.

Another very significant problem with RPM limiters is that it is necessary to exceed the RPM limit of the engine at least momentarily to adjust the limiter for proper operation, thereby incurring some risk of damage. Since a test of the limiter should be conducted from time to time, there is further risk of damage and a greater risk of damage from the cumulative effects of such tests.

SUMMARY OF THE INVENTION An object of the invention is to provide an improved system for limiting the RPM of an internal combustion engine of the spark ignition type.

Another object of the invention is to provide a system for limiting the RPM of an internal combustion engine of the spark ignition type which will permit adjusting and testing the proper operation of the limiting system without exceeding the RPM limit of the engine.

Still another object of the invention is to provide an RPM limiting system for an internal combustion engine of the spark ignition type which alleviates the problem of loading cylinders with raw gas during RPM limiting without shutting off the fuel system in order that there be fast recovery of full power after the RPM limiting.

These and. other objects of the invention are achieved in an internal combustion engine having an ignition system comprised of a coil, condenser, breaker, distributor and at least one spark plug in each cylinder of the engine connected to a different point in the distributor. A timing means is triggered by the action of the ignition coil and condenser each time the breaker points open to produce a spark timing pulse of sufficient duration to allow the action of the ignition coil and condenser.

to induce a spark in an engine cylinder across points of a spark plug. The leading edge of the spark timing pulse is used to reset an RPM limit timer having an RC timing circuit. That circuit produces a limiting pulse of a period selected for the RPM limit desired. The limiting pulse is applied to an ignition shunt switch through a gate which is inhibited by the spark timing pulse. In that manner, the output of the inhibit gate actuates the ignition shunt switch to effectively turn off the ignition system from immediately after the spark timing period until after the RPM limiting period to prevent at least the next ignition coil and condenser cycle from firing a spark plug when the breaker points open again when the RPM limit has been exceeded, but allowing subsequent ignition cycle (normally the next cycle) to fire a spark plug on the next opening of the breaker points after the limiting period. To set and test operation of the RPM limiter, a switch is closed to increase the capacitance of the RC timing circuit by a known factor, thereby increasing the RPM limiting period, and reduce the RPM limit proportionately.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a preferred embodiment of the present invention.

FIG. 2 is a timing diagram helpful in understanding the RPM limiting operation of the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, an ignition system 10 is illustrated diagrammatically for a typical eight-cylinder engine of the internal combustion type. The ignition system is comprised of an ignition coil 11 and condenser 12 connected in series to a battery 13 by an ignition switch 14. The ignition system also includes a distribution unit (not shown) housing breaker points 15 and a distributor 16. In a four-stroke cycle engine, the distributor 16 is driven by the engine cam shaft at half engine speed to distribute the output of the ignition coil 11 to the cylinder spark plugs.

The ignition coil consists of a waterproof case containing a primary P of approximately 200 turns wound upon an iron core and a secondary S of approximately 15,000 turns. While the points are closed, current flows through the primary winding P, thus building up a magnetic field around the coil. A back emf induced in the primary while the magnetic field is being built up opposes the battery potential and thereby limits the building up of the magnetic field. During this build-up period, a voltage is induced in the secondary S, but the voltage is too low to produce a spark across a gap in a spark plug because of the relatively slow build-up of the magnetic field. When the points are opened by a breaker cam 18 driven synchronously with the distributor 16, the magnetic field about the ignition coil starts to collapse. A primary current is immediately induced by the collapsing field which tends to prevent the field from breaking down. This induced current initially charges the condenser 12 with the polarity shown. When the voltage of the condenser is greater than that of theinduced current flowing in the primary, the condenser discharges back into the primary thereby breaking down the magnetic field completely.

The action of the ignition coil 11 and condenser 12 will produce an alternating signal of rapidily decaying amplitude each time the breaker points 15 open, as shown by the wave form diagram A of FIG. 2. This induced voltage in the primary winding is not limited to the voltage of the battery 13, but rather by the number of turns in the primary and the rate of change of the flux in the magnetic field breaking down. Accordingly,

the induced primary voltage may be as high as 250 volts.

The signal of the wave form A in FIG. 2 appearing at point A of FIG. 1 is employed to initiate operation of the RPM limiting circuit the function of which is to trigger a silicon-controlled rectifier (SCR) 20 for a period of time to set an RPM limit on the engine. Since the SCR is in parallel with the ignition system 10, it shunts the ignition system when it conducts, but it is not caused to conduct until after a spark timing period of approximately 400 microseconds (sufficient to allow the magnetic field of the ignition coil to break down) set by a monostable multivibrator 21.

The signal of wave form A is coupled to the monostable multivibrator 21 by a low-pass filter 22. A zener diode D is connected to the input of the monostable multivibrator to limit the excursions of the ignition pulses to between approximately 0.5 v and 3.9 v. The leading edge of the filtered and limited burst of ignition pulses triggers the monostable multivibrator to deliver a positive-going spark timing pulse to an inhibit gate 23, and a negative-going pulse to an RPM limit timer 25. The duration of the spark timing pulse applied to the gate 23 is set by the monostable multivibrator 21 to a period of approximately 400 microseconds, as noted hereinbefore, a period sufficiently long to allow breakdown of the magnetic field in the ignition coil. Then the monostable multivibrator resets to allow the gate 23 to transmit a pulse from the RPM limit timer to the gate of the SCR, via a transistor Q which functions as an emitter follower to provide sufficient current to drive the SCR into conduction. A load resistor 26 limits the SCR gate drive current.

A spark timing pulse applied to the inhibit gate 23 from the monostable multivibrator (appearing at point B in the circuit diagram) is shown as waveform B in FIG. 2. The negative pulse produced by the RPM limit timer 25 (appearing at point C in the diagram of FIG. 1) is shown as waveform C in FIG. 2.

The inhibit gate 23 is preferably implemented as a resistor-transistor-logic (RTL) NAND gate such that only when both inputs are at ground potential will the output to the transistor Q, be positive. If either the input B or input C is positive, the output to the transistor Q is zero and the SCR will not conduct. In that manner shunting the ignition system is delayed by a spark timing pulse until the action of the ignition coil and condenser has produced a spark across the gap of a spark plug. Thereafter, when both waveforms B and C are at ground potential, the output of the inhibit gate 23 is positive and the transistor Q, conducts sufficiently to provide drive current to the gate of the SCR to shunt the ignition system for the remainder of the period set by the RPM limit timer 25. The SCR drive current pulses (which appear at point D in the circuit of FIG. 1) are shown in waveform D of FIG. 2.

If the engine is running below the RPM limit set by the timer 25, the breaker points will initiate another ignition system cycle each time they open and another set of pulses from the monostable multivibrator 21 and RPM limit timer 25. As the RPM of the engine is increased, the breaker points open earlier in time following the termination of an RPM limiting cycle, but will still produce a spark in the gap of a spark plug until the engine RPM is increased to where the points open again before the period of the RPM limit timer 25 ends. Then the action of the ignition coil and condenser will be inhibited by the conducting SCR, and the spark plug then being selected by the distributor 16 will not fire. However, some subsequent breaker point opening (normally the next) will occur after the RPM limiting pulse has terminated and a spark plug then being selected by the distributor will fire. The normal condition of a spark plug being fired by the very next point opening is shown by the second half of the waveform diagrams in FIG. 2, thus illustrating the normal operation of the RPM limiting circuit for operation of the engine above the desired RPM limit.

From the foregoing it isevident that when the engine exceeds the RPM limit set by the timer 25 under normal conditions, every other point opening will fail to fire a spark plug. That immediately reduces power in the engine so that the high RPM level will not continue, but rather will be decreased to that RPM selected by the timer 25. A distinct advantage of this arrangement for limiting the RPM without completely shutting off the ignition system is that, since every other point opening fires a spark plug during the RPM limiting action, there is no tendency for the cylinders to load up with raw gas, thus avoiding the possibility of damage due to large quantities of gas present in a cylinder during a compression stroke.

Another advantage is that this alternate firing of cylinders during RPM limiting will provide fast recovery of full power after the limiting action occurs since there will be less of a problem with respect to raw gas in the cylinders.

It is possible for the RPM of the engine to be so accelerated that the limit is exceeded at a rate high enough to cause more than one opening of the breaker points to occur during an RPM limiting period, but obviously that condition will not continue for long and as the RPM decelerates, the normal limiting action will occur to fire every other cylinder until the RPM is below the limit set. If the throttle setting and road conditions which caused this high RPM condition are maintained, the ignition system could continue firing every other cylinder, and with an even number of cylinders, the same ones would continually be cleared of raw gas while the other ones load up. However, the set of conditions which have caused the high RPM condition will not remain constant so that the ignition system will begin to fire intermittently, and at random, the next cylinder in order, thereby tending to clear all cylinders of raw gas equally.

The preferred embodiment of a circuit for the RPM limiter 25 shown in FIG. 1 will now be described. The negative-going pulse from the monostable multivibrator 21 is differentiated by a capacitor 30 and resistor 31 to supply, from the leading edge of that negative going pulse, a sharp negative pulse to the base of a PNP transistor 0,. This pulse causes the transistor O to conduct sufficiently to completely discharge an RC timing capacitor 32. After this rapid discharge, the capacitor 32 begins charging through diodes D and D and through a resistor 33 and a potentiometer 34. The latter is used to adjust the charging rate of the capacitor 32, thereby adjusting the desired RPM limit, while the resistor 33 serves to limit the maximum charge rate, i.e., the highest RPM limit possible. The diodes D, and D provide temperature compensation for variations in the saturation voltage of transistor 0, with temperature variations to stabilize the RC time constant for charging the capacitor 32 through the resistor 34.

A second capacitor 35 is connected in series with a switch 36 to enable it to be connected in parallel with the capacitor 32 for setting and testing the RPM circuit. When that is done, the RC time constant of the timing circuit is increased, thereby reducing the desired RPM limit proportionately. This important feature of the present invention permits adjustment of the resistor 34 for the desired high RPM limit, and testing the operation of the circuit for that high RPM limit while running the engine at a fraction of that limit. For example, assuming the capacitor 35 equal to the capacitor 32, if the desired limit is 9,000 RPM, the switch 36 can be closed and the potentiometer 34 adjusted for a limit of 4,500 RPMs. Once the circuit is operating to limit the engine at 4,500 RPMs, the switch 36 can be opened and the circuit will operate properly to limit the engine to the desired level of 9,000 RPMs. After the potentiometer 34 has been set, the operation of the circuit can be tested from time to time by simply closing the switch 36 and running the engine up to the test limit of 4,500 RPMs. In that manner, the RPM limit of the circuit is set and can be tested repeatedly without ever having to operate the engine at the high RPM limit where there is some risk of damage.

The choice of having the two capacitors equal is simply a matter of convenience for selecting the RPM for the set up and test operation at just half the desired RPM limit. The choice of size for the capacitor 35 relative to the size of the capacitor 32 could be selected for any other desired ratio of test RPM to operating RPM such as one-third or two-thirds. For example for the ratio of l to 3, to decrease the test RPM by a factor of 3, the capacitor 35 is selected to increase the total capacitance of the capacitors 32 and 35 in parallel by a factor of 3. The size of the capacitor 35 should then be twice the size of the capacitor 32 so that when connected in parallel with the capacitor 32, it increases the total capacitance in the RC timing circuit to three times that of capacitor 32. By thus increasing the RC time constant by a factor of 3, the RPM limit is decreased by a factor of three for the test operation.

The output of the RC timing circuit is coupled by a current limiting resistor 37 to the base of an NPN transistor Q which functions as an emitter follower to couple the high impedance of the RC timing circuit to the low input impedance of a threshold detector 38. The latter is essentially an inverting amplifier which provides an output signal at ground potential once the charge of the capacitor 32 (or the capacitors 32 and 35 in parallel) has been discharged. That occurs immediately upon the transistor Q being turned on to clamp the junction of the capacitor 32 and diode D to the positive potential of the power supply.

The output signal of the threshold detector remains at ground potential until the capacitor 32, or the capacitors 32 and 35 in parallel, have been charged to a predetermined level, at which time the output of the threshold detector increases to a predetermined positive potential as shown by the waveform C of FIG. 2.

A conventional way of implementing the function of the threshold detector 38 is with a high-gain operational amplifier connected as a voltage detector or comparator such that the output is at a low cut off potential when the input signal is more positive than a threshold or reference voltage, and at a positive (saturation) level when the input signal is less than the reference voltage.

In the absence of a positive spark timing pulse from the monostable multivibrator 21 the negative-going RPM limiting pulse from the threshold detector would produce a positive output pulse to turn on the transistor Q the moment the transistor Q; is turned on to discharge the RC timing circuit, thereby firing the SCR. That would, of course, have the effect of immediately shunting the ignition system 10 after the breaker points open and no spark plug would ever fire. The function of the spark timing pulse from the monostable multivibrator 21 is clearly to delay the action of the RPM limiting pulse for a predetermined period (about 400 microseconds) sufficient to allow the magnetic field of the ignition coil to break down and cause a spark plug to fire. In that manner the monostable multivibrator effectively increases the dwell time of the ignition system,

resulting in greater coil saturation and higher output voltages to the spark plugs.

This delay in firing the SCR could be achieved by turning on the transistor Q with the trailing edge of the negative-going pulse applied to its base. That could be done simply by substituting an NPN transistor for the PNP transistor shown. However, it is preferred to begin the RPM limiting pulse immediately and to delay its effect by applying an inhibit pulse to the gate 23 in order that the 400 microsecond period of the inhibit pulse be included in the period of the RC timing circuit. Otherwise, upon closing the switch 36 to double the RC timing period, (assuming capacitors 32 and 35 of the same size), the desired result of reducing the RPM limit by exactly half, would be in error by z 400 microseconds. If the desired RPM is very low, this error of z 400 is quite small, but at higher limits the error of z 400 microseconds is a greater percentage of the RC timing period and may therefore introduce an intolerable error in setting the desired RPM limit.

To further provide accuracy and stability in the operation of the RPM limiting circuit, the power supply derived from the battery 13 is decoupled from the ignition system by a capacitor 40 coupled to the battery 13 through a resistor 41 and an on-off switch 42. A zener diode D in parallel with the capacitor 40 establishes the stable level for the power supply.

Although a particular embodiment of the invention has been described and illustrated herein, it is recog nized that modifications and variations may readily occur to those skilled in the art, such as substitution of other active elements for the SCR and transistors, or the implementation of the RPM limit timer 25 as an adjustable-period monostable multivibrator in an analogous manner with a switch to place a second capacitor in parallel with the normal RC timing capacitor. Consequently, it is intended that the claims be interpreted to cover such modifications and variations.

What I claim is:

1. An RPM limiter for an internal combustion engine of the spark ignition type having an ignition system including a breaker with points which open to produce a transient electrical signal in the primary winding of an ignition coil to fire a spark plug comprised of an electronic switch connected in parallel with said ignition system, said switch having a control terminal for receiving an electrical pulse in response to which current flows through said switch to shunt said ignition system, and

means for producing said control .pulse for a selected period after a predetermined delay period following the opening of said breaker points, said means including an RC timing circuit comprised of a first capacitor and a resistor for RPM limiting operation by shunting said ignition system for a period of time following breaker point opening set by said RC timing circuit, and a second capacitor in series with a test switch, said series circuit of said capacitor and said test switch being in parallel with said first capacitor, whereby upon closing said test switch, the

period of said control pulses set by said RC timing circuit is increased to permit testing proper RPM limiting operation at a lower RPM.

2. An RPM limiter as defined in claim 1 wherein a given control pulse is initiated by said means immediately upon said breaker points opening, said means further including a spark timing means for producing a pulse of a predetermined period sufficient for a spark plug to be responsive to said spark timing pulse, and means responsive to said spark timing means for inhibiting said control pulse from being applied to said electronic switch to shunt said ignition system, thereby enhancing ignition system performance while providing for RPM limiting by shunting said ignition systemfor an RPM limiting period only after a spark plug has been fired, whereby another spark plug may be fired upon said breaker points opening again only after said control pulse has terminated.

3. An RPM limiter as defined in claim 2 wherein said second capacitor is equal to said first capacitor, whereby test RPM limiting may be conducted at half the desired RPM limit.

4. An RPM limiter is defined in claim 3 wherein said resistor is comprised of a potentiometer for setting said desired RPM limit while operating said engine at half the desired RPM.

5. An RPM limiter as defined in claim 1 wherein said electronic switch is a silicon controlled rectifier having an anode, a cathode and a gate, and said control terminal of said switch is said gate.

6. An RPM limiter as defined in claim 2 wherein said spark timing means is comprised of a monostable multivibrator.

7. A RPM limiter as defined in claim 1 wherein said ignition system includes a DC battery and power supply for said control pulse means is derived from said bat tery through a decoupling circuit comprising an isolating resistor, a filter capacitor, and voltage stabilizing zener diode in parallel with said filter capacitor.

Claims (7)

1. An RPM limiter for an internal combustion engine of the spark ignition type having an ignition system including a breaker with points which open to produce a transient electrical signal in the primary winding of an ignition coil to fire a spark plug comprised of an electronic switch connected in parallel with said ignition system, said switch having a control terminal for receiving an electrical pulse in response to which current flows through said switch to shunt said ignition system, and means for producing said control pulse for a selected period after a predetermined delay period following the opening of said breaker points, said means including an RC timing circuit comprised of a first capacitor and a resistor for RPM limiting operation by shunting said ignition system for a period of time following breaker point opening set by said RC timing circuit, and a second capacitor in series with a test switch, said series circuit of said capacitor and said test switch being in parallel with said first capacitor, whereby upon closing said test switch, the period of said control pulses set by said RC timing circuit is increased to permit testing proper RPM limiting operation at a lower RPM.

2. An RPM limiter as defined in claim 1 wherein a given control pulse is initiated by said means immediately upon said breaker points opening, said means further including a spark timing means for producing a pulse of a predetermined period sufficient for a spark plug to be responsive to said spark timing pulse, and means responsive to said spark timing means for inhibiting said control pulse from being applied to said electronic switch to shunt said ignition system, thereby enhancing ignition system performance while providing for RPM limiting by shunting said ignition system for an RPM limiting period only after a spark plug has been fired, whereby another spark plug may be fired upon said breaker points opening again only after said control pulse has terminated.

3. An RPM limiter as defined in claim 2 wherein said second capacitor is equal to said first capacitor, whereby test RPM limiting may be conducted at half the desired RPM limit.

4. An RPM limiter is defined in claim 3 wherein said resistor is comprised of a potentiometer for setting said desired RPM limit while operating said engine at half the desired RPM.

5. An RPM limiter as defined in claim 1 wherein said electronic switch is a silicon controlled rectifier having an anode, a cathode and a gate, and said control terminal of said switch is said gate.

6. An RPM limiter as defined in claim 2 wherein said spark timing means is comprised of a monostable multivibrator.

7. A RPM limiter as defined in claim 1 wherein said ignition system includes a DC battery and power supply for said control pulse means is derived from said battery through a decoupling circuit comprising an isolating resistor, a filter capacitor, and voltage stabilizing zener diode in parallel with said filter capacitor.

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