This invention relates generally to ignition systems and more particularly to capacitor discharge ignition systems for internal combustion engines.
Capacitor discharge ignition (CDI) systems are widely used in spark-ignited internal combustion engines. Generally, CDI systems include a main capacitor, which during each cycle of an engine, is charged by an associated generator or charge coil and is later discharged through a step-up transformer or ignition coil to fire a spark plug. CDI systems typically have a stator assembly including a ferromagnetic stator core having wound thereabout the charge coil and the ignition coil with its primary and secondary windings. A permanent magnet assembly is typically mounted on an engine flywheel to generate current pulses within the charge coil as the permanent magnet is rotated past the ferromagnetic stator core. The current pulses produced in the charge coil are used to charge the main capacitor which is subsequently discharged upon activation of a trigger signal. The trigger signal is supplied by a trigger coil that is also wound around the stator core, wherein the permanent magnet assembly cycles past the stator core to generate pulses within the trigger coil. Upon receipt of the trigger signal, the main capacitor discharges through the primary winding of the ignition coil to induce a current in the secondary winding that is sufficient to cause a spark across a spark gap of the spark plug to ignite a fuel and air mixture within a combustion chamber of the engine. The time and occurrence of CDI is of importance to startability, output power, and emissions performance of engines, including small two and four stroke engines. Optimum ignition timing varies, primarily as a function of engine speed and engine load factors. Secondary factors, such as emissions performance and fuel quality, also play a role in determining optimum spark timing.
Microprocessor electronic timing control systems have been proposed for large engine applications, such as automotive engines, but typically are not well-suited to small engine applications because of cost and packaging constraints. Specifically, it has been proposed to employ microprocessor ignition modules in small engine applications, in which engine timing factors and desired advance or retard timing characteristics are pre-programmed into the microprocessor. For example, a microprocessor may be used to create a timing advance with increasing engine speed. However, cost constraints associated with microprocessor ignition systems are prohibitive in most small engine applications.
Moreover, in many CDI systems a somewhat high engine speed must be obtained before sufficient current pulses are generated in the charge coil and transferred to the capacitor to charge the capacitor sufficiently such that when discharged, a spark is generated across the spark gap of the spark plug. Thus, these prior ignitions systems require the engine to attain a relatively high startup speed before the ignition system is capable of producing a spark across the spark gap of the spark plug to start the engine.
Furthermore, engine overspeed is a problem in many small engine applications, such as chainsaws. It is possible for an engine to accelerate to an RPM range at which engine components and a saw blade can become damaged, such as where a load on a chainsaw is suddenly removed when the engine is operating at full throttle. Mechanical and microprocessor speed governors are typically employed to alleviate this problem, but are space-consuming and/or expensive, and often lead to unburned fuel in the engine exhaust.
Finally, it is possible during engine startup for the engine to rotate in a reverse rotational direction and for such reverse direction to be sustained after startup. Reverse startup and sustained operation may result in damage to the chainsaw and may result in a startup “kick-back” condition.
Thus, prior ignition systems are not yet fully optimized to provide a comprehensive ignition system that includes the ability to start the engine at a relatively low engine cranking speed, does not require relatively expensive microprocessor circuits, does not succumb to engine over-speed conditions, does not suffer from startup kick-back, and is of relatively simple design.
According to one aspect of the present invention, a capacitor discharge ignition (CDI) system is provided for an engine having an ignition device. The CDI system includes an ignition coil having a primary winding and a secondary winding for coupling to the ignition device. An ignition capacitor is coupled to the primary winding, and a charge coil is coupled to the ignition capacitor for generating a charge signal in synchronism with operation of the engine in order to charge the ignition capacitor. A trigger circuit generates a trigger signal in synchronism with operation of the engine and is connected in circuit with the ignition capacitor and the primary winding for discharging the ignition capacitor through the primary winding. A timing circuit is connected to the trigger circuit for controlling the timing of the trigger signal. The timing circuit includes a timing coil for generating a timing signal in synchronism with operation of the engine, and further includes a switch having primary current conducting electrodes in circuit with the trigger circuit and further having a control electrode coupled to the timing coil for shorting the trigger circuit as a function of engine speed to advance engine timing.
In accordance with a second aspect of the present invention, the trigger circuit further includes a trigger coil that generates the trigger signal, which is phased from the timing signal generated by the timing coil. Furthermore, a capacitor is connected across the timing coil so as to provide skip-spark speed-governing at relatively high engine speeds. In other words, the capacitor selectively prevents a spark ignition event at engine operating speeds above a predetermined threshold speed.
In accordance with a third aspect of the present invention, the timing circuit includes a transistor as the switch to provide timing retard speed-governing at relatively high engine speeds. In other words, the switch selectively provides timing retard at engine operating speeds above a predetermined threshold speed.
In accordance with a fourth aspect of the present invention, the timing circuit includes a capacitor operatively connected to the charge coil and to the control electrode of the second switch for disabling the trigger segment to prevent reverse rotation of the engine. A third switch has primary current conducting electrodes connected across the capacitor and further has a control electrode coupled to the timing coil, whereby the third switch discharges the capacitor to permit forward rotational operation. Accordingly, the CDI system prevents startup kick-back and reverse rotation operation of the engine.
Objects, features, and advantages of this invention include providing a capacitor discharge ignition system which improves starting of an engine, provides ignition spark at relatively low engine cranking speed, avoids use of relatively expensive microprocessor circuits, prevents over-speed operation of the engine, reduces delivery of unburned fuel to exhaust, retards engine timing at relatively high speeds, prevents ignition spark when the engine rotates in reverse to prohibit powered running in a reverse direction of rotation, is particularly well adapted for use in small two-stroke and four-stroke engine applications such as for chainsaws, is of relatively simple design and economical manufacture and assembly, and in service has a long, useful life.
These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiments and best mode, appended claims, and accompanying drawings in which:
Referring in detail to the drawings,
The stator assembly 26 includes a U-shaped ferrous armature or lamstack 28 that is composed of a stack of laminated iron plates. The lamstack 28 has first and second legs 30, 32 and is preferably mounted to a housing on an engine (not shown) leaving a measured air-gap between the stator assembly 26 and flywheel 22 on the order of about 0.3 mm/0.12 in. The stator assembly 26 further includes five coils or windings wound around the legs 30, 32 of the ferrous lamstack 28. Coil L1 is a charge coil and coil L2 is a trigger coil. Both coils L1, L2 are wound around the first leg 30 of the lamstack 28. Coil L3 is a timing coil for generating a timing signal and is wound around the second leg 32 of the lamstack 28, thereby creating a mechanical time delay between the coils L2, L3. In other words, the coils L2, L3 are preferably wound around the separate legs of the lamstack 28 to obtain a phase separation on the order of about 10 to 50 degrees, and preferably about 25 degrees. A transformer or ignition coil is defined by a primary winding L4 and a secondary winding L5, both of which are wound around the second leg 32 of the lamstack 28.
The flywheel 22 includes a permanent magnet 34 having pole shoes 36 that are rotatable in unison with the crankshaft 24. Because the flywheel 22 is preferably composed of a non-magnetic material such as aluminum, magnetic flux emitted by the permanent magnet 34 will be concentrated in the pole shoes 36 for magnetic coupling to the stator assembly 26. The permanent magnet 34 is located at a predetermined angular position relative to a key 38 that is located between, and couples, the crankshaft 24 and flywheel 22. Preferably the predetermined angular position is such that rotation of the permanent magnet 34 relative to the stator assembly 26 is in timed relation to a top-dead-center position of an engine piston (not shown) to control the timing of the ignition spark relative to the top-dead-center position of the piston. The timing of the ignition spark is preferably controlled by circuitry on a printed circuit board that is preferably carried along with the stator assembly 26.
In any case, as the engine crankshaft 24 rotates, the permanent magnet 34 rotates past the lamstack 28 and induces a magnetic field therein. This magnetic field induces a small amount of current and voltage in the coils L1, L2, L3, L4, L5 that, as will be described below, are leveraged for use in generating the ignition spark to ignite a fuel and air mixture in the combustion chamber of the engine (not shown). Typically, the energy output of a magneto apparatus is obtained in part as a result of a rapid rate of a change of magnetic flux through the ignition coil. The primary winding L4 has comparatively few turns of relatively heavy wire and the secondary winding L5 has many thousand turns of relatively fine wire, by way of example without limitation. One end of the secondary winding L5 is connected to an end of the primary winding L4 and is grounded. Circuitry is typically adapted to interrupt the primary winding L4 each time the magnetic flux therethrough is changing at its greatest rate. A resulting sudden collapse of current through the primary winding L4 tends to induce a very high voltage in the secondary winding L5, thereby creating the ignition spark.
The charge coil L1 has one end connected to electrical ground and another end in series through a diode D1, an ignition capacitor C1, and the primary winding L4 of the ignition coil. A resistor R1 is connected across the charge coil L1, and energy induced in the charge coil L1 during cranking at engine startup is used to charge the capacitor C1. The stored energy in the capacitor C1 is discharged into the primary winding L4 of the ignition coil upon receiving a discharge signal from a trigger circuit or sub-circuit 44. Accordingly, the capacitor C1 discharges the energy or voltage stored therein through primary winding L4 wherein the voltage gets transformed to a much higher amplitude voltage through secondary winding L5 of the ignition coil to create a voltage capable of jumping the spark plug gap 42 in the form of a spark.
The trigger sub-circuit 44 includes the trigger coil L2 having one end connected to electrical ground and another end operatively connected to a control electrode or gate of an electronic switch or SCR S1 through a diode D3 and a resistor R3. A resistor R4 is connected between the gate of SCR S1 and electrical ground. The primary current conducting anode and cathode electrodes of SCR S1 are connected to capacitor C1 and to electrical ground across the series combination of the capacitor C1 and the primary winding L4. A diode D4 is connected across SCR S1 and primary winding L4. The trigger sub-circuit 44 generates the discharge signal for discharging the capacitor C1 upon receiving a signal from a timing circuit or sub-circuit 46.
The timing sub-circuit 46 includes the timing coil L3 having one end connected to ground and another end operatively connected to a control electrode or gate of an electronic switch or SCR S2 in series through a diode D2. A resistor R2 is connected between the gate of SCR S2 and electrical ground.
Moreover, the polarity of the coils L1, L2, L3 and the polarity of the diodes D1, D4 are such that the first positive pulse 56 of coil L2 is of the correct polarity to be applied to the gate of SCR S1 to trigger the SCR S1. The negative pulse 58 of coil L2 is not of the correct magnitude or polarity to trigger the SCR S1, but the positive pulse 50 of coil L1 is of the correct magnitude and polarity to be applied through the diode D1 to charge the capacitor C1, during which time the SCR S1 must be non-conducting for normal ignition operation. Upon continued rotation of the permanent magnet 34, the second positive pulse 60 of coil L2 is again of the correct polarity to trigger the SCR S1 rapidly enough to discharge the charged capacitor C1.
Referring again to
Accordingly, the ignition charge is thus further retained in the capacitor C1 until the next signal cycle of the trigger coil L2. This suppression of the second positive pulse 60 of the trigger coil L2 by SCR S2 tends to alter the leading edge of the next succeeding first positive pulse that appears on the next cycle of operation, such that a successive first positive pulse 56″ has an increased width, as reflected by graph Vb of
The timing advance of the present invention is illustrated in
As shown in
Skip-spark speed-governing may be provided with the present invention if desired. Referring now to
In operation, as long as engine speed remains below a predetermined threshold that is determined by the component values of the capacitor C2 and the resistors R2, R5, there is sufficient time after the timing signal to allow the capacitor C2 to discharge through the resistors R2, R5 before generation of the trigger signal in the trigger coil L2 to allow closure of the SCR S1. However, when engine speed exceeds the predetermined threshold, there is insufficient time for the capacitor C2 to discharge between operating cycles and residual charge therefrom gates operation of the SCR S2 during at least the initial portion of the trigger signal in the trigger coil L2. This effectively short-circuits the first and second cycles of the trigger signal to prevent any closure of the SCR S1, which prevents discharging of the capacitor C1 and thereby prevents ignition at engine speeds above the threshold. Further discussion on speed governing is included in U.S. Pat. No. 5,245,965, which is assigned to the assignee hereof and incorporated in its entirety by reference herein.
In addition to the speed governing function of the previously described circuit, a timing retard function may be provided for excessively high engine speed operation, if desired. Referring now to
In operation, as long as engine speed remains below a predetermined threshold that is determined by the component values of the capacitor C2 and the resistors R2, R5, there is sufficient time after the timing signal to allow the capacitor C2 to discharge through the resistors R2, R5 before generation of the trigger signal in the trigger coil L2 to allow closure of the SCR S1. However, when engine speed exceeds the predetermined threshold, the capacitor C2 does not have time to fully discharge through the resistors R2, R5 between operating cycles. Thus, the control voltage across the capacitor C2 continues to operate the transistor T1 during the beginning of the trigger pulse of the next operating cycle, thereby delaying or retarding the spark ignition signal. When the transistor T1 finally shuts off, such as when the control voltage from the capacitor C2 decays below the predetermined threshold value of the transistor T1, the trigger pulse is allowed to increase in voltage to once again initiate an ignition.
The high-speed timing retard feature of the present invention is illustrated in
Finally, the present invention may also include circuitry for preventing operation of the engine during reverse rotation of the engine crankshaft at startup. Referring now to
The timing sub-circuit 346 of the present embodiment includes the SCR S2 that includes an anode connected to the trigger sub-circuit 44 previously described with reference to
During reverse operation of the engine crankshaft 24, positive pulses from the charging coil L1 are rectified through the diode D7 and the resistor R2 and a voltage is stored on the capacitor C2. The voltage stored on the capacitor C2 is applied to the SCR S2 through the resistor R5 and the diode D2. The SCR S2 is held on for a length of time necessary to prevent the trigger pulse from the trigger coil L2 to be applied to the SCR S1 (by grounding coil L2), thereby preventing ignition in reverse. However, the additional combination of the diode D5 and the transistor T3 permits ignition in forward operation of the engine crankshaft 24. During forward operation, when the timing coil L3 generates a voltage pulse through the diode D5 to the transistor T3, the transistor T3 is put into a conductive state, thereby discharging the capacitor C2 therethrough and, thus, preventing the voltage stored on the capacitor C2 from reaching the SCR S2. Accordingly, the timing sub-circuit 346 permits ignition to occur in a forward rotation of the engine crankshaft 24 but prevents ignition from occurring in a reverse rotation.
From the above, one of ordinary skill in the art will recognize that the present invention provides a simple and cost-effective ignition system that covers a comprehensive range of features that are desirable to incorporate into a two-stroke engine, particularly for a chainsaw.
While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. Also, while similar reference numerals have been used amongst several different embodiments, it is to be understood that various electrical components described herein may have different values within and between the several embodiments. It is not intended herein to mention all the possible equivalent forms or ramifications of the invention. It is understood that terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention as defined by the following claims.
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