Abstract:
A capacitive discharge ignition system for an internal combustion engine including: a capacitor for storing electrical energy to produce a spark; a first switch having a trigger input for receiving a trigger pulse; a second switch connected to the trigger input of the first switch such that the trigger pulse may be selectively inhibited from triggering the first switch; and a speed sensor for detecting the speed of an engine and in communication with the second switch such that the second switch will not inhibit the trigger pulse when the speed of the engine is below a threshold speed and the second switch will inhibit the trigger pulse when the speed of the engine is above a threshold speed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an ignition system for a gasoline engine. More particularly, but not by way of limitation, the present invention relates to a capacitive discharge ignition system with integral over-speed governor for a gasoline engine. 
     2. Background 
     Electronic ignition systems are well known in the art. In particular, capacitive discharge ignition systems have become popular on gasoline engines ranging from a few horsepower up to engines of hundred, or even thousands, of horsepower. Generally speaking, in a capacitive discharge ignition system, electrical energy is first stored in a capacitor. Upon receiving a trigger signal, the energy stored in the capacitor is transmitted to a transformer, commonly known as a spark coil, which increases the voltage to cause arcing across the electrodes of a spark plug. Capacitive discharge ignition systems have proven to provide a number of advantages over other types of ignition systems and, thus, are finding their way into even small, inexpensive engines such as lawn mower engines, chain saw engines, and the like. 
     While capacitive discharge ignition systems store electrical energy in a capacitor, other types of ignition systems, i.e. breaker point or magneto, rely on the inductance of the spark coil to store the energy required for the spark. This significantly increases the size, weight, and cost of the spark coil. In addition, breaker points exhibit a relatively short useful life and traditionally, such systems require periodic maintenance. 
     A problem common to any gasoline engine operating in an unattended fashion, is the management of the engine in response to widely varying loads. It is a common practice to provide a governor, either to control the speed within a range of speeds or to prevent an over-speed condition. A variety of techniques have been developed for governing the speed of an engine and, for purposes of this application, a governor is any device which automatically regulates the speed of the engine. While some governors adjust the throttle, or fuel injectors, to maintain a fixed engine speed, other governors simply respond to an over-speed condition to prevent damage to the engine or damage to downstream equipment powered by the engine. 
     Perhaps the most common technique for governing speed is through the control of air or fuel supplied to the engine. This type of governor may be controlled either mechanically or electronically. Typically, an electronic control provides a higher degree of accuracy and reliability. Unfortunately, however, an electronic control also requires some type of electromechanical actuator to adjust the air or fuel. As a result, these governors require a non-trivial amount of electrical power, are relatively expensive, and often exhibit an unacceptable delay in responding to a sudden change in load. 
     Another known technique for governing the speed of an engine is through manipulation of the timing of, or the presence of, the ignition spark. This type of governor is typically used to prevent an over-speed condition. This may be particularly important where an engine operating under relatively heavy load may suddenly become unloaded. For example, U.S. Pat. No. 4,163,437 issued to Notaras et al, describes a magneto ignition system which uses transistors in lieu of mechanical breaker points. While effective, the governor circuit disclosed by Notaras is imprecise in regard to the governing speed and the device requires the transistors to absorb virtually all of the energy which would otherwise create the ignition spark. This requires the use of transistors which can tolerate significant amounts of energy, even if for relatively short durations. 
     Thus it is an object of the present invention to provide a capacitive discharge ignition system which includes an electronic governor for a gasoline engine. It is another object of the invention to provide a governor for a gasoline engine which responds rapidly to an over-speed condition and is relatively inexpensive. 
     SUMMARY OF THE INVENTION 
     The present invention provides a capacitive discharge ignition system for a gasoline engine which includes an electronic governor. Electrical energy is temporarily stored in a capacitor. At the precise time an ignition spark is needed, a trigger pulse is directed to the gate of a thyristor. The thyristor then becomes conductive to deliver the energy stored in the capacitor to a spark coil, which ultimately results in the generation of a spark across the electrodes of a spark plug. When an over-speed condition is detected, the trigger pulse is inhibited so that the thyristor is not triggered, and thus, energy is never delivered to spark coil to produce the spark, preventing any further increase in the velocity of the engine. Once the speed of the engine is below a threshold speed, the trigger pulse is once again allowed to trigger the thyristor and normal operation resumes. 
     In a preferred embodiment, a first set of magnets are placed on the flywheel of the engine to interact with a first set of coils placed proximate to the flywheel to generate a voltage for charging the capacitor of the capacitive discharge ignition system. A second set of magnets are attached to the flywheel to interact with a trigger coil to generate the trigger pulse. A speed detector is used to determine the speed of the engine and, upon detection of an over-speed condition, a switch is activated which prevents triggering of the thyristor by the trigger pulse. 
     In another embodiment, the governor includes an adjustment to allow a user to select the speed at which the trigger pulse will be inhibited. Preferably, the trigger pulse is directed to a frequency-to-voltage (f/v) converter. When the output of the f/v converter exceeds the threshold selected by the user, a transistor is activated to shunt subsequent trigger pulses to prevent triggering of the thyristor. When the output of the f/v converter falls below the user selected threshold, normal operation is resumed. 
     Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 provides a perspective view of the inventive capacitive discharge ignition system with integral governor. 
     FIG. 2 provides a perspective view of a gasoline engine employing the inventive capacitive discharge ignition system. 
     FIG. 3 provides a block diagram of the preferred circuitry for use with the inventive system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation. 
     Referring now to the drawings, wherein like reference numerals indicate the same parts throughout the several views, a preferred embodiment of the controller or control module  20  for the inventive capacitive discharge ignition system with electronic governor is shown in FIG.  1 . In one application, the ignition system is used with a gasoline engine  22  (FIG. 2) to provide a reliable, high energy spark to initiate combustion within the engine  22 . Module  20  is shown having connectors  70  and  72  to facilitate the connection of other components of the ignition system to module  20  as discussed further hereinbelow. 
     Referring next to FIGS. 2 and 3, typically, engine  22  includes a flywheel  24  which is non-rotatably mounted to the crankshaft  26 . The inventive capacitive discharge ignition system includes: a first set of magnets  28   a-c , mounted to flywheel  24  to interact with coils  30   a-c , to generate a charging voltage as flywheel  24  rotates; a second set of magnets  32   a-b , mounted to flywheel  24  to interact with coil  34  to generate a trigger pulse as flywheel  24  rotates; control module  20  receives the outputs of coils  30   a-c  and  34  to produce a relatively high energy output; and a spark coil  40  which receives the output of module  20  to provide a high voltage pulse to spark plug  36  through wire  42 . Preferably, magnets  32   a-c  are placed on flywheel  24  such that the trigger pulse will be generated such that a spark is produced by spark plug  36  at the proper time in relation to the position of the piston in engine  22 . 
     It should be noted that, typically, coils  30   a-c , coil  34 , and spark coil  40  connect to module  20  through connectors  70  and  72  (FIG.  1 ). However, it should be noted that connectors  70  and  72  are merely provided to simplify installation of the ignition system on an engine, such as engine  22  depicted in FIG. 2, and do not form a part of the present invention. 
     Continuing with FIG. 3, preferably controller  20  comprises: a full-wave bridge rectifier  44  for converting the electrical energy generated by the interaction of coils  30   a-c  and magnets  28   a-c  from alternating current to direct current; a capacitor  46  for temporarily storing the electrical energy so produced; and thyristor  48  for controlling the discharge of capacitor  46  into spark coil  40  upon receiving a trigger pulse from coil  34 . Thyristor  48  is preferably a triac, SCR, or other similar device. As will be appreciated by those skilled in the art, thyristors are known to latch in the conducting or “on” state, once triggered, until electrical current ceases to flow. Thus, once triggered, thyristor  48  will remain in a conducting state until virtually all of the electrical energy stored in capacitor  46  has been discharged into spark coil  40 . Upon the discharging of capacitor  46 , electrical current will cease to flow through thyristor  48  causing thyristor  48  to return to the non-conducting or “off” state until the next trigger pulse is received. When the voltage stored in capacitor  46  is discharged into the primary winding  50  of coil  40  a much higher voltage is produced across the secondary winding  52  of coil  40 , sufficient to result in a spark across the electrodes of spark plug  36 . 
     In addition to the capacitive discharge circuitry discussed above, control unit  20  also includes governor circuitry  54  comprising: a frequency to voltage (f/v) convertor  56 ; a potentiometer  58  for setting a threshold speed; a comparator  60  for comparing the output of f/v convertor  56  to the threshold voltage selected with potentiometer  58 ; and a transistor  62 . Preferably the output of comparator  60  is binary in nature such that, when the output of f/v convertor  56  is below the threshold set with potentiometer  58 , the output of comparator  60  will be low and transistor  62  will be in its non-conducting state. On the other hand, when the output of f/v convertor  56  is higher than the threshold selected with potentiometer  58 , the output of comparator  60  will be high, turning on transistor  62 . As can be appreciated by those skilled in the art, when transistor  62  is conducting, trigger pulses will be shunted to prevent triggering of thyristor  62 . 
     As will be apparent to those skilled in the art, the f/v converter  56  and comparator  60  simply detect the speed of the engine and provide a binary output indicative of an over-speed condition, or lack thereof. Many alternative method exist to perform this function, and any such method is within the scope of the present invention. For example, centrifugal switches are available which provide a contact closure at a predetermined speed, retriggerable one-shot logic devices may be configured to detect and indicate whether pulses are received above or below a predetermined rate, or the trigger pulse may be directed to a microprocessor which is programmed to detect an over-speed condition and inhibit the spark upon such an occurrence. As will be further apparent to those skilled in the art, if the trigger pulse is directed to a microprocessor, an output of the microprocessor may then be used to fire the thyristor. In such an embodiment, rather than shunting the trigger pulse, a trigger is simply never issued to the thyristor. Furthermore, the microprocessor could be programmed to advance or retard ignition timing based on the speed of the engine, acceleration of the engine, operating temperature, etc. 
     As will also be apparent to those skilled in the art, thyristor  48  is effectively a controllable switch. A variety of devices are available to perform a similar function and the inventive capacitive discharge ignition system could be adapted to use such devices. For example, transistors, MOSFETs, IGBTs, and the like could be used with minor modification of the circuitry to ensure the output to the spark coil is of sufficient duration to make effective use of the energy stored in capacitor  46 . 
     In a similar vein, transistor  62  is also used as a controllable switch. A variety of devices are also available which would perform satisfactorily in place of transistor  62 . For example, MOSFETs, IGBT&#39;s, relays, and the like would provide adequate shunting or opening of the circuitry associated with the trigger pulse to inhibit a spark when an over-speed condition is detected. 
     It should be noted that engine  22  depicted in FIG. 2, is merely an example of an engine which is suitable for use with the present invention. It should also be noted that installation of the inventive ignition system onto an engine is identical to the installation of prior art capacitive discharge ignition systems onto such engines and is easily performed by a person of ordinary skill in the art. 
     Finally, it should also be noted that the inventive device is equally well suited to work with engines of any number of cylinders as long as charging voltage for the capacitor is provided between sparks and magnets are appropriately placed to provide triggering at the appropriate position of the crankshaft. Furthermore, is should be noted that the inventive apparatus is equally well suited for use on 2-cycle, as well as 4-cycle, engines. 
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the appended claims.