Magneto having transistor ignition circuit for engines
BACKGROUND OF THE INVENTION 
     The present invention relates to a magneto which has a transistor ignition circuit for engines and which is adapted to prevent the engine from rotating reversely on starting. 
     Magnetos of this type comprise a rotor, such as a flywheel, provided with a magnet and a pair of poles, and a stationary assembly including a U-shaped iron core having a pair of legs and an ignition coil wound on one of the legs. The rotation of the rotor produces in the primary winding of the ignition coil a current, which is suddenly interrupted by a transistor, inducing a high voltage in the secondary winding to fire a spark plug. When the magneto is to be incorporated, for example, into a two-cycle engine, the air gap between the pair of core legs and the pair of poles must have a specified dimension. If the air gap is dimensionally inaccurate, there is the likelihood that the engine will start backward. Thus the magneto must be assembled with extremely high accuracy. More specifically stated, the ignition coil primary current has approximately the same wave form whether the rotor rotates positively or reversely, so that when the engine starts backward instantaneously, i.e. in the event of a kickback, a high voltage will be induced in the ignition coil at the very moment. If sparking occurs, the engine will continue reverse rotation. 
     Accordingly various devices have been proposed for preventing such reverse rotation. For example, a device is known which is adapted to electrically bring the magneto out of igniting operation upon detecting reverse rotation. Although the known device does not require high accuracy in assembling, the device necessitates an increased number of parts and is costly to make due to the use of the electric means. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved magneto of the type described above wherein the magnetic path through one leg of a U-shaped core opposite to the coil carrying leg thereof is made to have an increased magnetic resistance without using any additional part to ingeniously utilize leakage flux for greatly inhibiting the ignition coil primary current in the event of reverse rotation of the engine and preventing the reverse rotation on starting. 
     Another object of the invention is to provide a magneto of the type described wherein the magnetic resistance of the path is increased by a simple structure without using any additional electric or other part and which can be manufactured at a low cost. 
     Still another object of the invention is to minimize the reduction of the igniting performance of the magneto during the normal rotation of the engine despite the increase of the magnetic resistance. 
     Other objects, features and advantages of the invention will become more apparent from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view showing a conventional magneto; 
     FIG. 2 is a diagram showing the ignition circuit included in the same; 
     FIG. 3 is a diagram showing the wave form of current produced by the magneto during the normal rotation of its rotor; 
     FIG. 4 is a diagram showing the wave form of current during reverse rotation; 
     FIG. 5 is a front view showing a magneto embodying the invention; 
     FIG. 6 is a diagram showing the wave form of current produced by the magneto of FIG. 5 during reverse rotation; and 
     FIGS. 7 to 9 are front views showing other embodiments of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a conventional magneto having a transistor ignition circuit incorporated therein. With reference to FIG. 1, a rotor 1 is rotatable with the crankshaft 2 of an engine and is provided with a magnet 3, a pair of poles 4a, 4b and a flyweight 5. A U-shaped iron core 6 has a pair of legs 7a, 7b and is supported by a stationary member with its legs 7a, 7b opposed to the poles 4a, 4b. An ignition coil 8 is mounted on the leg 7a of the core 6. As seen in FIG. 2, the ignition coil 8 comprises a primary winding 9 and a secondary winding 10 and is molded integrally with a circuit unit 11 with synthetic resin or the like. As shown in FIG. 2, the circuit unit 11 comprises diodes 12, 13, transistors 14, 15, resistors 16, 17, 18, 19, thermistor 20, etc. 
     When a recoil starter is pulled to start the engine and rotate the rotor 1 in the normal direction L, current Il, as shown in FIG. 3, flows through the primary winding 9 of the ignition coil 8. Upon the current reaching a value If, the transistor 15 is turned off abruptly to interrupt the primary current, consequently inducing a high voltage in the secondary winding 10 to cause a spark across a plug gap 21 for ignition. 
     However, magnetos of the type described involve the likelihood of permitting the engine to rotate reversely on starting. When the rotor 1 rotates in the normal direction, ignition is set about 20° in advance of the top dead center (T.D.C.) as illustrated in FIG. 3. When the engine is about to stop upon cranking, the rotor is likely to rotate reversely (in the direction of arrow R) instantaneously due to a kickback or like phenomenon, so that if sparking occurs across the spark gap 21 near the top dead center at this very moment, the engine will continue reverse rotation. In the case where reverse rotation occurs with the magneto shown in FIG. 1, ignition takes place about 10° in advance of the top dead center as shown in FIG. 4 because the current produced in the ignition coil 8 has substantially the same wave form whether the rotor rotates in the normal direction or in the reverse direction. This phenomenon inevitably occurs in the above arrangement wherein the magnet 3 for the rotor 1 has two poles, i.e. poles 4a and 4b, opposite to the two legs 7 a and 7b of the core 6 having the ignition coil 8. 
     The reverse rotation phenomenon will be described below in greater detail. Even if an engine is equipped with the magneto of FIG. 1, reverse rotation occurs rarely or very infrequently. Usually the recoil starter is pulled strongly to start the engine in the normal direction to fire the spark plug about 20° in advance of the top dead center as shown in FIG. 3 and cause explosion within the cylinder. The engine thereafter rotates in the normal direction at a rapidly increasing speed. However, in the case where the engine will not start smoothly with the recoil starter pulled lightly, there is a very small likelihood that the first ignition occurs the moment the engine rotates backward, i.e. upon a kickback. The engine then rotates continuously reversely, and hence, is hazardous. The momentary speed of reverse rotation due to a kickback is very low; it is up to 1500 r.p.m. for about 10 to 50 cc two-cycle engines. Accordingly the continued reverse rotation of the engine can be prevented if the magneto is so adapted that it will not cause ignition at low speeds, for example, of up to 2000 r.p.m. when the engine rotates reversely. 
     A description will be given of the ignition characteristics in the event of reverse rotation. With reference to FIG. 1, the air gaps Ga, Gb between the rotor 1 and the core 6 are set within a specified range, for example, of 0.25 mm±0.01 mm when the magneto is incorporated into an engine. Naturally the magneto is designed not to permit rotation insofar as the gaps are thus set as specified. In other words, the adjusting resistor 19 of the circuit unit 11 is so determined as not to permit ignition at low speeds of up to 2000 r.p.m. However, if the air gaps are set to about 0.05 mm, a high current will be induced in the ignition coil 8 to cause ignition even at 1000 r.p.m. in the event of reverse rotation, consequently entailing continued reverse rotation although rarely. 
     In view of the foregoing problem heretofore encountered, the present invention provides a magneto which is adapted not to cause ignition at low speeds of reverse rotation of up to 2000 r.p.m. even when the air gaps are set, for example, at about 0.05 mm. 
     Throughout the accompanying drawings, like parts are referred to by like reference numerals. 
     FIG. 5 shows a preferred embodiment of the invention. The magneto comprises a U-shaped iron core 6 having a leg 7a provided with an ignition coil 8 and another leg 7b opposed to the leg 7a and cut out at its forward end to provide a gap portion G. The magnetic path through the leg 7b is made to have an increased magnetic resistance by the gap portion G. With the exception of this feature, the magneto has substantially the same construction as the one already described. 
     With this structure, the magnetic flux from the pole 4b to the leg 7b has difficulty in passing across the gap portion G during reverse rotation, greatly reducing the current through the primary winding 9 of the ignition coil 8 as indicated at I3 in FIG. 6. Thus the gap portion G formed serves to reduce the current from I2 in FIG. 4 to the smaller value I3 in FIG. 6 to thereby prevent ignition during reverse rotation. The value I3, which increases with the increase in the rotational speed of the rotor 1, reaches an ignition level at a speed of above 2000 r.p.m. However, since the momentary speed of reverse rotation is about 1000 r.p.m., the above structure prevents continued reverse rotation in any case. Since the value I3 is small, no particular trouble will result even when the magneto as incorporated into an engine involves some dimensional variation in the air gaps. The magneto is therefore easy to assemble. Further because the gap portion G in the leg 7b is in the form of a cutout, the gap portion is easy and inexpensive to make without necessitating any other additional part. Especially when the core 6 is an assembly of blanked steel plates joined together in layers, the gap portion G can be formed simultaneously when the plates are blanked out, so that the core with the gap portion can be made at the same cost as the conventional ones without necessitating any additional machining step. 
     If the above structure greatly impairs the characteristics of the magneto during normal rotation, the magneto will not be fully useful, whereas we have found that the reduction of the characteristics during normal rotation is about 6%. The reduction can be compensated for to such an extent as to be comparable to the characteristics of magnetos having no gap portion by varying the ratio in the number of turns between the windings of the ignition coil 8. 
     The very small reduction in the characteristics during normal rotation despite the presence of the gap portion G is attributable to the following reason. With reference to FIG. 5 showing the position of the rotor 1 in normal rotation relative to the ignition coil 8, etc., the gap portion G increases the magnetic resistance between the leg 7b and the pole 4b while increasing the leakage flux from the D portion of the core 6 to the pole 4b, with the result that the leakage flux acts as an effective flux for the ignition coil 8 during normal (forward) rotation. 
     While FIGS. 3, 4 and 6 show current wave forms all at 1000 r.p.m. for comparison, I2 is smaller than I1 because there is a leakage flux from the D portion to the pole 4b although the gaps Ga and Gb are dimensionally identical in FIG. 1. The present invention ingeniously utilizes the effect of the leakage flux. 
     The gap portion G, which is formed asymmetrically in the end face of the leg 7b as shown in FIG. 5, may alternatively be formed symmetrically in the center of the end face of the leg 7b as seen in FIG. 7. 
     FIG. 8 shows a gap portion G formed in the leg 7b of the core 6 and positioned away from the lengthwise midportion of the leg 7b closer to its forward end. The gap portion G is formed in the shape of a cutout in one side of the leg 7b opposite to the other side thereof which faces the leg 7a to reduce the area of the magnetic path through the leg 7b and give an increased magnetic resistance. 
     FIG. 9 shows gap portions G each in the form of a cutout and formed in the end face of one of the pair of poles 4a, 4b, i.e. the pole 4b, which face is opposed to the leg 7b. Although one or a plurality of gap portions may be provided in this case, it is more preferable to form a plurality of small separate gap portions than to form a single large gap portion.