SMALL ENGINE FOR HAND-HELD WORK MACHINES

Abstract: A small engine suitable for hand-held work machines such as cleavers and chain-saws which is characterized in that the engine is started by driving a self-starting motor (7) with electromotive force of a secondary battery (4) and that the size and weight of the engine is reduced by eliminating the use of a generator as the secondary battery (4) is charged by uti­ lizing electromotive force of a magnet (1, 13, 23) in reverse direction that is not normally used for igniting the engine.

Description: The present invention relates to a small engine suitable for hand-held machines such as cleavers and chain saws. Conventional large engines for work machines are usually started with a self-starting motor. Engines of this type are provided with a separate small generator driven by the rotation of the engine for compensating for the power in the battery consumed at the time of starting the engine. However, provision of a generator is extremely inadequate in engines intended for hand-held work machines such as cleavers and chain saws as the size thereof must be reduced by minimizing the weight and volume and ease in handling must be improved. EP-A-0 043 891 discloses an ignition system for a combustion engine, which system includes an ignition transformer, whose secondary winding is connected to a spark plug, and whose primary winding is connected via a diode to a battery. The windings of the ignition transformer are carried by the stator of a magneto, whose rotor comprises four permanent magnets. The rotor magnets induce positive and negative voltage pulses in the primary winding. The positive pulses are used to charge the battery, and one of the negative pulses is used for spark ignition. However, as can be seen from the induced voltages shown in Fig. 2 of the cited reference, so-called monopolar magnets are used, i.e. the magnetic poles of "N" and "S" are arranged alternately at equal intervals. In this arrangement the peaks of the negative and the positive voltage pulses are of equal height. EP-A-0 049 101 discloses a combustion engine in which additional coils (41 and 43 in Fig. 2) are provided to charge the battery. This makes the device more complicated and increases weight and production costs. It is the object of the invention to provide a combustion engine as outlined in the preamble of claim 1 which is simple in construction and more effective. This problem is solved by the means indicated in claim 1. The small combustion engine for hand-held work machines according to the present invention is started by driving the self-starting motor with electromotive force of the rechargeable battery. The charging mechanism charges the battery by effectively utilizing the output voltage of the magneto in the other polarity, that is, the electromotive force of the magneto in reverse direction which is generally not used for igniting the engine while the engine is in rotation. In this manner, electricity of the battery consumed for starting the engine can be compensated while the engine is in rotation without using a power generator. Because of the engine size, the capacity of the self-starting motor for the small engine need not be very large. The battery may also be small in capacity as the charging current is not exceptionally high. It is therefore possible to construct the charging mechanism with, for example, a rectifier which is directly connected in between the output terminal of AC voltage of the magneto and either one of the positive and negative terminals of the battery, to effect charging of the battery with the magneto output without control. Said magneto usually comprises a rotor connected to the crankshaft of the engine and a stator including an ignition coil opposed to the rotor, and the rotor is fixed at a predetermined position with a magnet which generates AC voltage for spark discharge at the ignition coil of the stator. The magnet induces voltage at the ignition coil by passing across the front of the stator at a suitable timing during the compression stroke of the engine. In order to further secure electromotive force for charging, at least one more magnet is fixed to the rotor at a position with a rotational angle which allows generation of AC voltage at the ignition coil at a timing other than during the compression stroke of the engine. The electric power for charging increases with the increase in the number of magnets. Although plural spark discharges occur in one rotation of the crankshaft, the additional magnet(s) is provided at such a position as to induce the voltage at a timing other than the compression stroke of the engine, so that the spark discharge of the additional magnet(s) does not cause ignition in the cylinder and does not affect the engine performance. According to the present invention the magnet which generates AC voltage for spark discharge at the ignition coil and the additional magnet(s) are so attached to the rotor that the flux changes caused in the stator by the respective magnets are in reverse directions with each other while the rotor rotates in one direction. This suppresses inadvertent sparks at the ignition plug at a timing other than during the ignition stroke and at the same time increases the charging current of the battery. Fig. 1 is a circuit diagram of a preferred embodiment of a small engine for hand-held work machines. Fig. 2 is an explanatory view to show the construction of the engine magneto according to a first embodiment of the invention. Fig. 3a through 3d show the operational waveforms of the primary coil which would be found if the two magnets of the embodiment shown in Fig. 2 were given the same polarity. Figs. 3e through 3h show the operational waveforms of the primary coil of the embodiment shown in Fig. 2, wherein, according to the present invention, the two magnets have opposite polarities. Fig. 4 is an explanatory view to show the construction of the engine magneto according to another embodiment of the invention. A preferred embodiment of a small engine for hand-held work machines will now be described in detail referring to the accompanying drawings. Fig. 1 shows the electric circuit of a small engine for hand-held work machines according to the present invention, in which a magneto 1 generates AC voltage on the primary side N₁ of an ignition coil by the rotation of the engine. When the AC voltage is negative, the primary coil N₁ is short-circuited by an ignitor 2 comprising a transistor ignition circuit connected to the primary coil. As the short-circuit current in the primary coil N₁ substantially reaches the peak, the primary short-circuit current therein is rapidly cut off by the ignitor 2, whereby a high voltage is induced in a secondary coil N₂ of the ignition coil to discharge sparks in an ignition plug 3 connected to the secondary coil N₂. Said short-circuit is controlled in the ignitor 2 only for the period when the AC voltage induced in the primary coil N₁ is negative. For the period when the voltage is positive, current in the ignitor 2 can be led outside. In order to supply the current in the positive period to a battery 4 as a charging current by rectifying the current in non-control manner, a rectifier 5 having the polarity as shown is connected in between the primary coil N₁ and the battery 4. The battery 4 may, for example, be a small and sealed type accumulator of lead, nickel or cadmium. A self-starting motor 7 is connected between the terminals via a starter switch 6. An engine switch 8 is connected between the terminals of the ignitor 2 which is to be closed for stopping the engine by short-circuiting the induced power of the magneto 1 to the grounding; otherwise the switch is normally open. As the starter switch 6 is closed while the engine switch 8 is in the open state, the current from the battery 4 is supplied to the self-starting motor 7 only for the time when the switch is closed, whereby the self-starting motor is rotated to actuate the engine. In this case, since the switch 8 is open, AC voltage generated at the primary coil N₁ of the magneto 1 is applied to the ignitor 2. While the voltage is in its negative period, the primary short-circuit current passing in the primary coil N₁ is rapidly cut off by the action of the ignitor 2 when the current is substantially at its peak, to induce high voltage in the secondary coil N₂ of the ignition coil. This causes spark discharge at the ignition plug 3 connected to the secondary coil N₂. As the engine is started in the manner as described above, the positive current of the AC voltage generated at the primary coil N₁ of the magneto 1 which is not utilized in the ignition stroke passes through the rectifier 5 to flow into the battery 4 as the charging current. Since the charging current is not very large as mentioned earlier, non-control type charging by the rectifier alone is effected, minimizing the number of components necessary for charging. Fig. 2 shows an embodiment of the invention wherein two magnets 13 and 23 and poles 14 and 24 are fixed to the rotor. One of these magnets is fixed to the rotor at a rotational angle which allows generation of AC current at the ignition coil at timings other than during the compression stroke of the engine for securing electromotive force for charging. When the magnetic polarities of the magnets 13 and 23 are in alignment, magnetic flux changes are caused in the stator 12 in the same direction as the direction the rotor 11 rotates. In this case, no-load voltage is induced at the primary coil N₁, as shown in Fig. 3a. Fig.3a shows the waveform of the AC voltage under no load induced at the primary coil N₁ of the ignition coil. Fig. 3b shows the waveform of the current of the primary coil N₁ in short-circuit. Figs. 3c and 3d show the voltage and current waveforms respectively of the primary coil N₁ in the circuit connection as shown in Fig. 1. The voltage induced at the primary coil N₁ which is in the negative period is utilized in igniting the plug, while the short-circuit current which is in the negative period is cut off at its substantial peak, as shown in Fig. 3d. The current which is in the positive period flows into the battery 4 via a diode 5 as charging current. In the circuit connection shown in Fig. 1, the voltage/current waveforms are obtained as shown in Figs. 3c and 3d. Charging waveforms appear 4 times during the positive period for one rotation of the rotor, increasing the charging current by ca. 2 folds by simple calculation. However, if the magnets 13 and 23 are identical in intensity, then sparks occur at a position directly opposite the position of ignition. When the engine is in operation, sparks at the opposite position are in no way a problem. At the time of starting the engine (at low speed rotation) on the other hand, the sparks may cause abnormal combustion within the cylinder. When the sparks at the opposite plug are suppressed by using a magnet 23 which is weaker in magnetization than the magnet 13, the voltage waveform of the magnet 23 shows a slightly lower value than that of the magnet 13, as shown in Figs. 3a through 3d, achieving a current which is ca. 1.5 times the case as shown in Fig. 2. When the magnetic polarity of the magnet 13 is in the direction opposite to the polarity of the magnet 23, magnetic flux changes in opposite directions are caused in the stator 12 by the magnets 13 and 23, resulting in the waveforms as shown in Figs. 3e through 3h. In other words, no-load voltage induced at the primary coil N₁ assumes a waveform as shown in Fig. 3e, and the short-circuit current as in Fig. 3f. When the magnet 23 is thus magnetized in the opposite direction, the voltage peak in the negative period induced by the magnet 23 would not be sufficient for causing sparks at the plug 3. On the other hand, the positive voltage induced by the magnet 23 increases to provide charging current which is higher than when the magnets 13 and 23 are magnetized in one direction, as is clearly shown by the voltage waveform in Fig. 3g and the current waveform in Fig. 3h with the circuit connection as shown in Fig. 1. The embodiment shown in Fig. 4 is constructed with plural additional magnets and magnetic strips (23a, 24a, 23b, 24b). In the case of embodiments shown in Figs. 2 and 4, the electric power for charging increases in proportion to the increase in the number of these magnets. Plural spark discharges also occur during one rotation of the crankshaft. It is noted, however, that the additional magnets 23, 23a and 23b are provided at such positions that they would induce voltage at timings other than during the compression stroke of the engine. By designing the magnetic field at an intensity which would not cause inadvertent sparks at the plug during the starting operation of the engine with lower speed rotation, spark discharges by the additional magnets 23, 23a and 23b would not cause ignition in the cylinder and thus would not affect the engine performance. Thus, by providing additional magnets 23, 23a and 23b at suitable positions on the rotor 11, it is possible to increase the capacity of battery charging without affecting the engine performance. As is stated in the foregoing, the present invention engine supplies electricity to the battery while the engine is in rotation for the amount consumed by the self-starting motor without providing a separate generator for battery charging, and is therefore highly practical as a small and lightweight engine for hand-held work machines.

Claims: A small combustion engine for hand-held work machines, comprising: a magneto (1) generating an AC voltage by the rotation of the engine; an ignition mechanism (1, 2) causing spark discharge at an ignition plug (3) for the engine with a first part of said AC voltage having a first polarity; a self-starting motor (7) for starting the engine; a rechargeable battery (4) for driving said self-starting motor (7); and a charging mechanism (2, 5) with means for charging said battery (4) with a second part of said AC voltage having a second polarity opposite said first polarity; said magneto (1) comprising rotor means (11) mounted so as to be driven by rotation of said engine and stator means (12) including ignition coil means (17) opposed to said rotor means (11); said rotor means (11) including first magnet means (13) fixed thereto for generating said AC voltage at said ignition coil means (17) of said stator means (12); said ignition coil means (17) including a primary coil (N1) in which said AC voltage is induced when said first magnet means (13) sweeps past said primary coil (N1) during the rotation of said rotor means (11), and a secondary coil (N2) which is electromagnetically coupled to said primary coil (N1) and electrically connected to said ignition plug (3); said ignition mechanism (1, 2) including ignitor means (2) connected to said primary coil (N1) to control a short circuit of said primary coil (N1) for a period when said first part of the AC voltage is induced in said primary coil (N1); said rotor (11) further comprising second magnet means (23; 23a, 23b) fixed thereto for generating an AC voltage at said primary coil (N1) at a timing other than during compression stroke of the engine, said second magnet means (23; 23a, 23b) being deviated in rotation angle from said first magnet means (13), characterized in that said first magnet means (13) and said second magnet means (23; 23a, 23b) are bipolar magnets having opposite polarity, said second magnet means thus inducing voltage peaks having opposite polarities with respect to the voltage peaks induced by said first magnet means and that said ignitor means (2) comprises a transitor ignition circuit. A small engine according to claim 1, characterized in that said charging mechanism (2, 5) further comprises a rectifier (5) directly coupled between said primary coil (N1) and either of a positive or negative terminal of said battery (4). A small engine according to claims 1 and 2, characterized in that said second magnet means comprises a plurality of magnets (23a, 23b) fixed to said rotor at such positions that each of said magnets (23a, 23b) induces an AC voltage at said primary coil (N1) at an individual timing other than compression stroke of the engine, said magnets (23a, 23b) being deviated in rotation angle from each other.

Inventors: ISHIDA, SHIGETOSHI
Applicants: TANAKA KOGYO CO., LTD.
Application year: 1988
Main classification: F02P1
IPC codes: F02P1, F02P3