US2236396A - Magneto for internal combustion engines - Google Patents

Description

March 25, 1941- w. BRYAN EII'ALV 36,396

IIAGNETD FOR INTERNAL COMBUSTION ENGINES Filed Sept. 14, 1938 2 Sheets-Sheet 1 Wait, 03 In l w a: mad.

March 25, 1941. w. BRYAN EI'AL IAGNETO FOR INTERNAL COMBUSTION ENGINES Filed Sept. 14, 1.938 Z'Sheets-Sheet 2 Patented Mar. 25,1941

UNITED STATES 'MAGNETO FOR INTERNAL COMBUSTION ENGINE Walter Bryan and Geoffrey Robson North, London, England, assignors to Simms Motor Units Limited, London, England, a British company Application September 14, 1938, Serial No. 229,869 In Great Britain November 26, 1937 3 Claims.

This invention relates to magnetos for multicylinder, internal combustion, aircraft engines of the high speed type comprising a large number of cylinders.

The trend in aircraft engine design is towards an engine of high power (for instance 1500 to 2500 horse-power) having a large number of cylinders (14 to 24) and a comparatively high crank shaft speed (2500 to 4000 revolutions per minute). Such engines must also be absolutely reliable under all-climatic conditions and may have to operate, for instance, at relatively high temperatures and in the rarefied atmospheres of altitudes of up' to 45,000 feet.

The design of a magneto for such an engine presents special .difllculties, since, on account of the large number of cylinders and high crank shaft speed of the engine, the magneto must be capable of working satisfactorily at a speed of 7000 revolutions per minute or more and at a temperature of at least 100 C.

To provide the measure of surface insulation which is required at the high altitude mentioned,

it is necessary to arrange the distributor electrodes at a considerable distance apart and also from earth. This, in engines with a large number. of cylinders, makes the use of a single distributor unsatisfactory, since by reason of its necessarily large dimensions, a single distributor tends under the vibration and high temperature to which it is subjected, to warp or crack.

The object of the present invention isto provide an improved magneto in which all these difiiculties are overcome and which is thoroughly reliable in operation even under the most rigorous physical conditions.

,According to the present invention a magneto is provided which comprises, in combination, a main driving shaft, a single rotor carried thereby, two primary and two secondary stationary windings co-operating with the rotor so as to produce a maximum high-tension current eight times per revolution of the rotor, two distributors of small diameter on separate spindles arranged parallel to and at equal distances from the main driving shaft and driven thereby through gearing, .a make-and-break device in the circuit of each primary winding, comprising two pairs of contacts operated by a cam, and electrical connections between the distributors and the secondary windings, whereby each distributor effects the current distribution from a different winding.

All these features are known per se but their combination into a single compact unit which will overcome all the difilculties mentioned above in connection with the design of magnetos for aircraft engines is new and forms a considerable advance in the art of magneto design.

High tension current is preferably taken alternately by the two distributors from the two coils of the generator respectively. In another arrangement, however, high tension current may be taken simultaneously by the two distributors from the two coils, thus providing for ignition by the duplex system. The high tension cur- 1 rent may be led to the two distributors from the generator through conductors arranged coaxially with the spindles of the two distributors respectively.

In the latter case, in addition to the reduction in the overall dimensions of each distributor, the use of external high tension leads, with consequent risk of insulation breakdown is avoided.

The make-and-break device is preferably constructed and arranged in the circuit of the primary coil that if one, but not necessarily both .of the contacts is closed, the primary circuit is closed, while both contacts have to be opened in order to break the circuit. Thus the two pairs of contacts are preferably connected in parallel with each other and in series with the primary winding, and are operated by a cam, rotating at half the speed of the crankshaft of the engine, which has upon its circumference a number of lobes equal to one quarter of the total number of cylinders and arranged with respect to the contacts in such a manner that one pair will be closed and opened again while the other pair remains open.

'In this way the effect of inertia upon the action of the contacts is reduced to a negligible amount.

A construction according to the invention is illustrated by way of example in the accompanying drawings, in which:

Figure 1 shows, in longitudinal section, a mag neto suitable for a twenty-four cylinder aircraft engine,

Figure 2 is a section on the line II-II of Figure 1,

Figure 3 illustrates the arrangement of the contacts within the two distributors, and

Figure 4 shows in detail one form of makeand-break device which may be employed with the construction shown in Figure 1.

In the construction illustrated the magneto comprises a twin-coil generator having a fourpole magnet rotor A driven by the main spindle A The rotating magnet A revolves between two pairs of laminated pole pieces A thus producing an alternating field in each of two cores B alternately, and giving eight sparks per revolution of the spindle A Each core B is associated with a primary coil B and a secondary coil B each primary coil being controlled by a make-and-break device, as hereinafter described, whilst each secondary coil is earthed at one end and is connected at its other end to a contact 1? against which bears a high tension brush 3*.

The main spindle A is furnished with a toothed wheel C which drives the two toothed wheels C each secured to a distributor body or spindle C furnished with an electrode C electrically connected through a conductor 0 to the associated brush 3*. The spindle C together with the conductors C brushes B and electrodes C are thus positively driven in synchronism with the rotor A of the generator. Each electrode C co-operates, on the jump-spark system, with twelve distributor contacts D arranged, as shown in Figure 3, within a casing D furnished with an opening D through which the high tension leads extend from the contacts D to the sparking plugs respectively.

For starting purposes each distributor is fur nished with an auxiliary contact B through which a high tension current can be supplied to an auxiliary electrode from a hand starter (notshown) Each distributor spindle is provided with a make-and-break cam E having six lobes E formed thereon as shown in Figure 4. The lobes E on each cam E co-operate simultaneously with two spring-loaded breaker arms F and G, the breaker arm F having a contact F which co-operates with a fixed contact F whilst the breaker arm G has a contact G which co-operates with a fixed contact G The contacts F and G are connected to earth and the breaker arms F, G are connected to one end of the associated primary coil so that the contacts F F and G G are arranged in parallel with each other and in series with the primary coil.

In operation the rotor A of the generator produces maximum current alternately in the two primary coils B. When the current reaches its maximum value in one primary coil the corresponding cam E opens the primary circuit and thus induces a high tension current in the corresponding secondary coil 3'. The high tension current is then supplied from the brush 3 through the conductor 0*, and electrode C to one of the distributor contacts D and thence to one of the sparking plugs. This sequence is then repeated from the other pair of coils B and B and thence to acontact D (and sparking plug connected thereto) of the other distributor. The high tension current is therefore taken alternately by the two distributors from the two coils of the generator respectively. I

From Figure 4 it will be seen that the two pairs of m'ake-and-break contacts controlling each primary circuit are so arranged that their opening andclosing periods are overlapped. Thus, as shown in the drawings the contacts F and F are about to commence to open whereas the contacts G and G2 are already open and will remain open until the cam E has rotated through the angle indicated at K, twelve sparks being obtained for each revolution of each cam E. The two pairs of contacts are thus simultaneously open for the angular period K at the termination of which the contacts G and G close to establish the primary circuit. When the contacts G G again open, the contacts F F are still open, but subsequently close and also open again while the contacts G G remain open. Thus the contacts F, F and G, G alternately efiect the closing of the same-primary circuit so that they require to oscillate more slowly than a single contact performing the same function. In this way, even with the engine operating at maximum speed the inertia of the breaker arms F and G will be ineffective and accuracy of the timing of the make-and-break device associated with each primary coil B will be maintained.

With the arrangement illustrated in the drawings a magneto is arranged to supply high tension current to the cylinders of a twenty four cylinder engine. As the rotor is arranged to produce eight sparks per revolution of the rotor and as twelve cylinders are fired during each revolution of the crank shaft, the rotor must be rotated at one and a half times the speed of the crank shaft. It will be appreciated that the relative speeds of the rotor and the crank shaft depend upon the design of the magneto and the'number of cylinders there are to the engine to be supplied from the magneto.

The use of a twin-coil generator combined to form a single unit with two distributors, in accordance with the invention, has the advantage that distributors of small dimensions can be employed thus reducing risk of disintegration even with high engine speeds. Moreover, in addition to enabling the use of distributors of small dimensions, the spacing of the distributor contacts can be such as to avoid all risk of sparking-over if the engine is used at high altitudes, for example, 45,000 feet. As will be evident from the example described the necessity for external high tension leads between the generator and the distributors is obviated as also the necessity for. concentric high tension leads whose insulation is liable to break down and cause faulty operation or failure of the magneto.

By suitably modifying the relative timing of the two distributors and make-and-break de-, vices, and ensuring that the primary coils produce their maximum current simultaneously, high tension current may be taken simultaneously by the two distributors from the two coils of the generator, thus providing for ignition on the duplex system, that is each cylinder can be provided with high tension current simultaneously from each of the two distributors thereby ensuring that if the ignition from one distributor should fail the cylinder would receive its ignition charge from the other distributor.

What we claim as our invention and desire to secure by Letters Patent of the United States is:

1. For a multi-cylinder, internal combustion aircraft engine, a magneto-formed as a single integral unit, and comprising, in combination, a

main driving shaft, a single rotor carried there- 7 by, two primary and two secondary stationary windings co-operating with the rotor so as to produce a maximum high-tension current eight 'the crankshaft of the engine, which cam has upon its circumference a number of lobes equal to one quarter of the total number of cylinders and is arranged with respect to the contacts in such a mannerthat one pair of contacts will be closed and opened again while the other pair remains open, and electrical connections between the distributors and the secondary windings whereby each distributor effects the current dis= tribution from a difierent winding.

2. For a multi-cylinder, internal combustion aircraft engine, a magneto formed as a singleinte ral unit, andcomprising, in combination, a main driving shaft, a single rotor carried thereby, two primary and two secondary stationary windings co-operating with the rotor so as to produce a maximum high-tension current eight times per revolution of the rotor and alternately in each primary winding, two distributors of small diameter on separate spindles arranged parallel to and at equal distances from the main driving shaft and driven thereby through gearing, a n ake-and-break device in the circuit of each primary winding, comprising two pairs of contacts connected in parallel with each other and in series with the primary winding and operated by a cam rotating at half the speed of the crankshaft of the engine, which cam has upon open, and electrical connections between the dis-- tributors and the secondary windings whereby each distributor effects the current distribution from a different winding.

p 3. For a multi=cylinder, internal combustion aircraft engine, a magneto formed as a single integral unit, and comprising, in combination, a main driving shaft, a single rotor carried there by, two primary and two secondary stationary windings co-operating with the rotor so as to produce a maximum high-tension current eight A times per revolution of the rotor and alternately in each primary winding, two distributors of small diameter on separate spindles arranged parallel to and at equal distances from the main driving shaft and driven thereby through gearing, a make-and-break device in the circuit of each primary winding, comprising two pairs of contacts connected in parallel with eachother and in series with the primary winding and operated by a cam rotating at half the speed of the crankshaft of the engine, which cam has upon its circumference a number of lobes equal to one quarter of the total number of cylinders and is arranged with respect to the contacts in such a manner that one pair of contacts will be closed and opened again while the other pair remains open, and a conductor arranged coaxially with the spindle of each distributor for efiecting the electrical connection between the distributor and the corresponding secondary winding.

WALTER BRYAN. GEOEY ROBSON NORTH.

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