US2900575A - Electric ignition systems - Google Patents

Description

1959 F. R. F. RAMSAY 2,909,575

ELECTRIC IGNITION SYSTEMS Filed Sept. 4, 1956 I FIG. 8,

INVENTOR FRANK RF. Rams BY 406%, 4 4, M

Papa/a4 ATTORNEYS ELECTRIC IQNITION SYSTEMS Frank Raymond Faber Ramsay, East Sheen, London,

England, assignor to D. Napier & Son Limited, London, England, a company of Great Britain Application September 4, 1956, Serial No. 607,726

Claims priority, application Great Britain September 5, 1955 Claims. (Cl. 315-209) This invention relates to electric ignition systems for use with low resistance sparking plugs, that is to say, sparking plugs having a resistance of the order of a few hundred to a few thousand ohms and requiring a potential of only a few hundred to a few thousand volts to fire them.

Sparking plugs of the air gap type have a very high resistance and require an ignition system capable of producing a potential of the order of tens of thousands of volts to fire them. This potential is normally obtained from the secondary winding of a magneto or ignition coil, which has a very large number of turns and is of high impedance.

Sparking plugs of another kind, known as surface discharge plugs, have also been developed. In such plugs the gap between the electrodes is bridged by a poorly conducting solid substance, and when a potential is applied between the electrodes a current leaks across the surface of the bridge on one or more paths. The temperature along these paths grows rapidly to the point where the gas adjacent to them becomes ionized and permits an arc ot strike. The resistance of such plugs has usually been such that a potential of about 2000 volts is required to fire them. Ignition systems for use with such plugs have usually employed a large charged condenser as the source of potential, practically all of the energy for the discharge being stored in this condenser. Several different ignition systems of the kind have been devised. Such ignition systems have, in general, included a vibrator-operated step-up transformer fed from a source of direct current such as a battery, the secondary winding of the transformer charging the large condenser through a rectifier, and means such as a series spark gap for connecting the large condenser, when charged, to the plug.

Recently, surface discharge plugs of low resistance, as defined above, have become available, and ignition systems similar to those used with the higher resistance surface discharge plugs but modified to produce a lower potential have been employed.

It is an object of this invention to provide an electric ignition system suitable for use with such low resistance sparking plugs which does not require a large condenser (i.e. a condenser large enough to store virtually all the spark energy), nor a high voltage step-up secondary winding, nor a rectifier, nor a series spark gap.

Other and further objects will be apparent to those skilled in the art from the following disclosure.

These objects are achieved according to the present invention, by an electric ignition system in which both the ionizing potential and the main spark energy are produced by the collapse of flux in an inductive Winding of low impedance (i.e. an inductive winding whose number of turns and impedance are only a small fraction of those of the secondary winding of an ignition system for an air gap sparking plug), a condenser is connected across the plug to permit the establishment of an ionization path through the gas for the discharge of the main spark energy from the inductive winding, and the flux is built up be- 2,900,575 Patented Aug. 18, 1959 2 tween discharges by a charging circuit comprising an inductive winding connected across a source of direct current through a contact-breaker actuated by an oscillating sprung balanced mass of a natural periodicity of not more than about twenty oscillations per second. a

The invention may be performed in various ways and one particular form of ignition system embodying the invention, and some modifications, Will be described by way of example with reference to the accompanying drawings, in which: j a

Figure 1 is a side view of the inductive system and the oscillating mass assembly, shown partly in section,

Figure 2 is a cross-sectional view taken on the line IIII in Figure 1,

Figure 3 is a diagram of a simple form of circuit incorporating the inductive system shown in Figures 1 and 2; and

Figures 4 to 8 are diagrams of alternative forms of circuit.

The ignition system shown in Figures 1 to 3 of the drawings comprises an inductive winding 10 consisting of a large number of turns of relatively heavy gauge copper Wire wound round one limb of a laminated iron core 11, this core forming a closed circuit except for a narrow gap 12. Since it is desirable for the ohmic resistance'of the winding 10 to be as low as possible the ratio of weight of copper in the winding to weight of iron in the core 11 will generally be much higher than is usual in normal transformer practice. Mounted on the core 11 is a frame 13 carrying an oscillating mass 14 and a contact-breaker comprising a movable contact 15 and a stationary contact 16. The contacts are connected in series with the inductive-winding 10, to connect it to a source of direct current (e.g. a battery), indicated by positive and negative terminals in the drawing, when the contacts are closed.

The oscillating mass 14 comprises a cylindrical body 17 which is mounted on a bearing member 18 which is rigidly fixed to the frame 13. The cylindrical mass 17 is free to rotate on the bearing member 18 but is constrained by alight spiral spring 19 towards a central position in which the spring is unstressed. The cylindrical mass 17 is balanced about its axis of rotation. It is made largely of non-magnetic material such as brass but is provided with a segmental insert 20 of magnetic mate rial such as soft iron adapted to be attracted towards the limb 21 of the iron core 11 by stray magnetic flux when the winding 10 is energized. This attraction imparts an unbalanced force to the mass to maintain it in oscillation. As the mass swings in the counter clockwise direction as seen in Figure 1 through the center of its amplitude of oscillation, i.e. when it is moving with its maximum velocity, a hammer 2.2 which it carries strikes a light leaf spring 23 carryng the movable contact 15 and separates the contacts as shown in dotted lines in Figure 1. This disconnects the inductive winding 10 from the direct current supply and causes the flux to collapse. One efiect of the collapse of flux is that the magnetic attraction on the ion segment 20 of the mass 14 ceases, thereby permitting the spring 19 to cause the mass to swing back in the clockwise direction. As the mass swings back through the centre of its amplitude of oscillation the contacts close again, so that the inductive winding 10 is again connected to the source of direct current and the flux builds up again.

When the contacts 15 and 16 separate and the flux in the inductive system collapses, a potential is produced across the inductive winding 10 which is applied across a condenser 24 (Figure 3) and across a low resistance surface discharge sparking plug 25. Although the condenser 24 limits the peak potential to a voltage of some hundreds of volts this is sufficient to initiate ionisation of the sparking plug, and the collapse of flux is retarded sufficiently to ensure that the ionisation is effective to produce an adequate low resistance gas path between the plug electrodes through which a spark can jump. Energy is thus transferred from themagnetic field of the inductive winding to the plug 25. As the collapse of flux proceeds, the potential across the inductive winding and across the plug decreases, but the spark continues to pass until'the magnetic field of the inductive winding 10 has practically completely collapsed.

Too 'low an initial rate of discharge due to having too small a condenser 24 would result in the flux dying away before the resultant energy could discharge across the sparking plug 25. Conversely, too large a condenser 24 would lower the initial voltage below that necessary for ionisation, and no spark would result. This would be the case with a condenser large enough to store a substantial portion of the total energy, as in certain prior art proposals. As an example, in a particularcase, a condenser of 2 'microfarads is used in conjunction with an inductive winding providing a spark with an energy of about 4 joules. If the same inductive winding and circuit were used in a conventional manner, storing practically all the energy in the condenser before ionisation, a condenser of about 40 microfarads would be required.

The condenser 24, being connected across the contacts and 16, also serves to suppress arcing at the contacts.

The moment of inertia of the mass 14 and the charaeteristic of the spring 19 are so chosen that the natural periodicity of oscillation is not more than about twenty oscillations per second, for instance about five oscillations per second. If the flow of direct current through the inductive winding 10 were interrupted at a higher frequency, the sparking rate at the plug 25 would be such that the heat generated would fuse the surface discharge material and ruin the plug. Moreover, a relatively low frequency of oscillation gives sufficient time for an ample build-up of flux to take place in the periods when the contacts 15 and 16 are closed, so that adequate energy is available for producing fat sparks.

Since the spring 19 controlling the mass must be relatively light in order that the natural periodicity of the oscillating sprung mass 14 shall be low, the mass would be affected by linear accelerations and by changes in attitude of the apparatus unless the mass were balanced about its centre of oscillation. It will be apparent to those skilled in the art, however, that the mass 14 need not necessarily be of cylindrical form. For instance it could be made in the form of a lever balanced about its fulcrum. It will also be apparent that the magnetic field for maintaining the oscillations need not necessarily be the stray flux from the iron core 11 adjacent the gap 12, since a separate electromagnet could be used for this purpose, connected either in series or parallel with the said inductive winding 10.

In the circuit shown in Figure 3 the current surge produced by the collapse of flux in the inductive winding 10 passes through the battery. Also, if by some mischance the sparking plug 25 should be short-circuited, the inductive winding 10 will remain in series with the battery and the short circuit current will not flow through the contacts 15 and 16.

Another form of circuit is shown in Figure 4. In this circuit the positions of the inductive winding 10 and of the contacts 15 and 16 are in effect reversed from the positions they occupy in the circuit shown in Figure 3, and when the plug 25 has been ionized the surge of current from the inductive winding 10 passes directly to the plug without passing through the battery. In this circuit the condenser 24 is not connected across the contacts 15 and 16, so a separate arc-suppressing condenser 26 is provided.

In certain cases the voltage of the available direct current source might be such that if the values of capacitance and inductance of the system remained unchanged the potential produced across the inductive winding would be either too high or too low for the particular sparking plug. One way of getting over this difficulty is by arranging the inductive winding as an auto-transformer. Two different arrangements of this kind are shown in Figures 5 and 6.

In Figure 5 the inductive winding 27 has a tapping 28 to which the movable contact 15 is connected. In this arrangement the inductive winding 27 acts as a voltage step-up auto-transformer, since the potential applied across the condenser 24 and the sparking plug 25 will be developed by a larger number of turns than are connected across the source of direct current when the contacts are closed.

In Figure 6 the inductive winding 29 is in the form of a voltage step-down auto-transformer. In this case a tapping 39 is connected to the condenser 24 and the sparking Plug 25. V

Instead of arranging the inductive winding as an autotransformer, a transformer having separate primary and secondary windings 31 and 32 respectively could be employed, as shown in Figure 7, the numbers of turns in both windings being selected to suit requirements.

In order to prolong the spark if this is desired, for instance for igniting a heavy fuel oil or solid powdered fuel, an inductance connected in series with the sparking plug beyond the connection to the condenser 24 may be provided. One such arrangement is shown in Figure 8, in which a series connected inductance 33 is formed by a few additional turns on the inductive winding 34 beyond the tapping 35 which leads to the condenser 24.

What I claim as my invention and desire to secure by Letters Patent is:

1. An electric ignition system for use with a sparking plug having low resistance of a few hundred to a few thousand ohms comprising an inductive winding whose number of turns and impedance are only a fraction of those of the secondary winding of an ignition system for an air gap sparking plug, a charging circuit for said inductive winding including a source of direct current and a contact breaker, an oscillating spring balanced mass having a natural periodicity of not more than about twenty oscillations per second adapted to actuate said contact breaker to close said charging circuit to build up flux in said inductive winding and to interrupt said charging circuit to cause said flux to collapse, a condenser, connections permanently connecting said condenser across said sparking plug, and connections permanently connecting said inductive winding across said sparking plug whereby energy released by each collapse of flux in said inductive winding first ionizes said sparking plug and then provides a spark discharge across said ionized sparking plug.

2. An electric ignition system according to claim 1 which includes electromagnetic means for maintaining said mass in oscillation.

3. An electric ignition system according to claim 2 in which said electromagnetic means comprises stray flux from said inductive winding.

4. An electric ignition system according to claim 2 in which said mass is made mainly of non-magnetic material and has a part made of magnetic material disposed eccentn'cally with respect to the axis of oscillation.

5. An electric ignition system according to claim 1 in which said mass is disposed in a partial relationship to said contact breaker such that said contact breaker is actuated by said mass when said mass is in the vicinity of the centre of its amplitude of oscillation.

6. An electric ignition system according to claim 5 in which said contact breaker comprises a fixed contact, a movable contact and a flexible carrier for said movable contact, and said mass is adapted to strike said flexible carrier to separate said contacts.

7. An electric ignition system according to claim 1 in which said connections between said inductive winding and said sparking plug include at least a part of another inductive winding.

8. An electric ignition system according to claim 7 in which said other inductive winding is separate from said first mentioned inductive winding and the two windings are coupled by a common iron core.

9. An electric ignition system which said inductive winding is in the form of an auto transformer with difierent numbers of turns connected in the charging circuit and to the sparking plug respectively.

10. An electric ignition system according to claim 7 in which said other inductive winding is connected in series with said first mentioned inductive winding between said sparking plug and one of said connections to said condenser.

according to claim 7 in 5 References Cited in the file of this patent UNITED STATES PATENTS Apple June 6, Falge et a1. May 12, Lodge May 11, Ruben Nov. 27, Kasarjian Feb. 4, Ramsay May 17, Debenharn May 1,

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