Automotive ignition coil having a core with at least one embedded permanent magnet

In a spark plug ignition assembly for a spark plug, an ignition coil assembly has a steel laminated core, said core having a first core portion and a second core portion, and a primary winding around the first core portion and a secondary winding around the primary winding. A spark plug connecting member is provided for connecting the coil assembly to the spark plug. At least one of the first and second core portions has a slot therein with a magnet located in the slot.

BACKGROUND

The present disclosure relates to an ignition coil assembly for ignition of a spark plug, such as in an automobile. A known ignition assembly10for ignition of an automobile spark plug11is shown in prior artFIG. 1. The assembly10connects to the spark plug11via a boot12connected with an ignition coil assembly13having a steel laminated core14and a molded plastic cover15. The molded plastic cover15also houses a connector/electrical harness16at one side of the coil assembly13.

As shown in prior artFIG. 2with a portion of the cover15removed, a primary coil17is surrounded by a secondary coil18around what is known as an I core portion19adjacent ends of a C core portion20forming core14. The I core portion19and the C core portion20are each formed of a stack of individual electrical steel laminations19A and20A, respectively.

A magnet21is inserted between one leg of the C core portion20and a side adjacent one end of the I core portion19.

The laminations comprise electrical steel and the primary and secondary coils comprise copper magnet wire. An automotive battery provides a nominal 12 volt DC supply to the primary coil17as an inner winding around the I coil portion19. The number of copper turns are on the order of a few hundred for the primary coil, whereas the surrounding secondary coil18has several thousand turns of magnet wire. A diameter of the primary coil wire is larger as compared to a diameter of the secondary coil wire. Both primary and secondary coils are connected in series.

The coil assembly13of the overall ignition assembly10works together with a number of other components that make up the ignition system. The modern day system utilizes sensors placed around the engine in order to provide signals to the ECM (Electronic Control Module—also known as the computer). The ECM then determines where the engine crank and camshafts are in mechanical degrees. When the timing is right, the voltage is sent to the ignition primary coil which will then be transferred to the secondary coil. This high voltage is sent to the individual spark plug at the moment the engine piston is at the top of the stroke. The fuel and air mixture is then burned and the piston is forced downward. This downward stroke partially rotates the crankshaft which supplies power to the transmission. The transmission then converts this power to torque which is delivered to the wheels for mobility. This action repeats for every ignition coil and piston in the engine in timed intervals. The faster the automobile drives, the faster the ignition process.

When a switch is closed, current will build easily in the primary coil17because it has less inductance than the secondary coil18. Because the primary coil current is direct current, it creates a very small magnetic field with a constant magnitude which causes a minimal magnetic flux to flow in the steel laminations19A and20A. When the switch is opened, the current potential across the primary coil17collapses and dumps its energy into the secondary coil18which creates a much higher potential due to the many more turns of copper. Therefore this change of energy will add much more magnetic flux in the laminations. This exchange of energy happens within a few milliseconds. As the boost of current from the primary coil17decays down to zero, the magnetic flux completes a magnetic circuit loop and causes a large voltage potential across the secondary coil18. This secondary coil voltage potential is typically in a range between 40,000 to 70,000 volts depending upon the type of engine it was designed for. It also maintains the maximum amount of voltage potential transfer. This high potential then has the ability to jump across a gap11A of the spark plug11in order to ignite a fuel and air mixture in the cylinder at top dead center forcing the piston downward and thus causing a stroke which will help rotate the crankshaft of the internal combustion engine.

Design criteria requirements for an ignition coil are:

(1) Maximum transfer of energy without excess losses in the laminations. This is because losses are wasted energy and are converted to heat. Efficiency is reduced and the useable life of the product is compromised.

(2) Minimum decay time for transfer of energy. This is necessary for two reasons. First, the quicker the transfer of energy, the lower the losses on all the components. Secondly, tighter EPA (Environmental Protection Agency) emission regulations require recirculating the unburned gas mixture known as hydro carbons. These hydro carbons are not as easy to ignite as the pure fuel in the initial burn. Therefore the spark across the spark plug gap must have the highest potential.

Modern automobile ignition systems have advanced from the early days of one single ignition coil and distributor assembly for all cylinders to individual ignition coils, having one per cylinder, such as the single coil assembly13shown inFIG. 1. Different core geometric designs have been used such as the C core portion20and the I core portion19shown inFIG. 2, and C core portion7and T core portion22as shown in prior artFIG. 4also having primary and secondary coils8and9respectively. However, whether it is C and I or C and T core portions, the function remains the same. The basic building blocks are also the same including the introduction of a permanent magnet21shown inFIG. 2and also shown in the cross sectional view ofFIG. 3. In the C and T arrangement the permanent magnet23is disposed between a T end of the T core portion22and a side of one leg of the C core portion7. An example of the prior art magnet used in a C and T core portions is U.S. Pat. No. 5,241,941.

The advantage of the magnet21inFIG. 3or the magnet23inFIG. 4is a magnetic flux source within the steel core magnetic circuit. The permanent magnet flux opposes the previously described weak flux generated from the primary coil17when the switch is closed. This is to ensure that no voltage is generated on the secondary coil18until the switch is opened. At this moment the maximum voltage transfer is made.

SUMMARY

It is an object to improve upon the performance of ignition coils and improve the operational efficiency thereof.

In a spark plug ignition assembly for a spark plug, an ignition coil assembly has a steel laminated core, said core having a first core portion and a second core portion, and a primary winding around the first core portion and a secondary winding around the primary winding. A spark plug connecting member is provided for connecting the coil assembly to the spark plug. At least one of the first and second core portions has a slot therein with a magnet located in the slot.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to preferred exemplary embodiments/best mode illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and such alterations and further modifications in the illustrated embodiments and such further applications of the principles of the invention as illustrated as would normally occur to one skilled in the art to which the invention relates are included.

FIG. 5illustrates a first preferred exemplary embodiment in cross section similar to the cross section shown inFIG. 3. The C core portion26and I core portion27form the overall core29. A primary coil24and an outer secondary coil25are wound to surround the I core portion27. A magnet28is inserted in a through slot30in leg26A of the C core portion26adjacent to but spaced from an end face26B of the C core portion.

The magnet has a length which is at least 85%, preferably about 90%, and no greater than 95%, of a width of the leg at the slot. The 85% lower limit is chosen such that a neck region7A,7B at opposite ends of the magnet28is magnetically saturated to allow the MMF (Magneto-Motive Force) from the permanent magnet to travel the length of a magnetic path length. The necessity for the neck area to be saturated allows for a unidirectional flow of magnetic flux in one direction. Thus the function of the previously described magnet in the ignition coil is to act as a choke or filter so as to restrict the pulse of energy within the primary coil prior to rapid discharge through the secondary coil. The upper limit of 95% is chosen such that the lamination material at the opposite ends of the magnet28does not become too thin resulting in breakage and stamping problems.

The magnet has a width determined by a Magnetic Maximum Energy Product called MGOe. The higher the MGOethe thinner the width of the magnet can become. The magnet width is approximately in a range between ¼ to ⅕ the length of the magnet, but it is dependent on the magnet characteristics for maximum energy product and demagnetizing effect.

The material for the laminations is preferably electrical grade steel which may or may not have a silicon content in its chemistry.

The material for the magnet is preferably sintered based iron with other elements molded into the preferred rectangular shape. For example, the magnet may be specifically formed of, in one example, neodymium iron boron. In another example the material may in particular be iron based sintered ceramic.

The slot and magnet shown are rectangular but other shapes may be employed.

Although only one magnet is shown in this first preferred embodiment, one or more permanent magnets may be embedded within the C core lamination stack at different locations. Thus various other geometric shapes and placement locations for the magnet or magnets may be provided in order to optimize a function of the ignition coil. Additional possible preferred embodiments are shown hereafter although many other possible configurations are within the scope of the exemplary embodiments in this invention.

Although use for an automobile ignition system is described herein, the preferred embodiments may be employed in any type of vehicle or boat engine, or engines used in other applications.

It may be further noted that in the first embodiment inFIG. 5the leg26A progressively widens as it approaches the end face26B with the slot30, and the magnet28is located at this widened portion.

FIG. 6shows a second preferred exemplary embodiment in an I core portion34and a C core portion35wherein the C core portion has progressively widening mirror image legs35A and35B approaching the end faces35C,35D. A magnet31is located in a through slot32in a central region33of an intermediate part of the core portion35substantially halfway between the legs35A and35B.

The length and width of the magnet in the intermediate part are similar to the length and width previously described for the magnet when the magnet is in a leg of the core portion.

In a third preferred exemplary embodiment shown inFIG. 7an I core portion36and a C core portion37are provided wherein the C core portion37has progressively widening mirror image legs37A and37B as they approach end faces37C,37D. In a central region38in an intermediate part approximately half way between the legs37A,37B first and second magnets39A and39B are provided in respective through slots40A and40B forming a V configuration with the apex of the V being substantially halfway between the legs and legs of the V being slanted such that the opening end of the V points toward the leg37B. The magnets39A and39B are aligned mirror symmetrically about a longitudinal center line passing through the central portion38between the legs.

In the second preferred embodiment ofFIG. 6a higher flux linkage is obtained compared to the first embodiment ofFIG. 5. And in the third embodiment ofFIG. 7a higher flux linkage is obtained compared to the second embodiment ofFIG. 6. This supports the relationship that the secondary voltage equals the number of turns times the change in flux linkage over the change in time. This also helps the primary coil achieve the proper inductance.

In a fourth embodiment as illustrated inFIG. 8, the core is formed of a T core portion41and C core portion40with a shorter leg and a longer leg. A T part41A of the T portion41abuts against a side of the longer leg of the C core portion40. An end face of the shorter leg of the C core portion40abuts against a portion of the side face at an end of the T core portion41opposite the T part41A. Adjacent this abutment a magnet44is located in a through slot45in the shorter leg.

Although C and I and C and T portion cores have been shown, other types of cores may be employed to create other embodiments.

FIG. 9is a graph showing primary coil flux linkage and current versus time for the prior art core shown inFIG. 4.FIG. 10shows the primary coil flux linkage and current versus time for the third preferred embodiment with mirrored legs and magnets aligned in a V shape as shown inFIG. 7. Thus the graphs ofFIGS. 9 and 10depict simulations of the original prior art design versus the third preferred embodiment. TheFIG. 9graph for the prior art core shows a low flux linkage of zero Henries of inductance when the switch is closed. TheFIG. 10graph of the third preferred embodiment illustrates a flux linkage of 7.5 millihenries of inductance when the switch is closed. This difference is due to the magnetic flux or Magneto-Motive Force (MMF) emanating from the permanent magnets. This flux then travels within the magnetic circuit known as the steel laminations and couples with the copper windings of the primary coil. This MMF is in opposition to the Electro-Motive Force (EMF) emanating from the energized primary coil. The result is impedance across the primary coil which yields higher inductance. This can be described as a “choke” effect. This is necessary to have no leakage prior to the switch opening.

With the various exemplary embodiments shown, there are the following benefits:

(1) the quality, energy transfer, and duration of the spark is enhanced;

(2) the embedded magnet or magnets will allow for a smaller size of ignition coil because they create a minor restriction to the flux flow and do not cut off the flux as in the sandwich-type design of the prior art with the magnet located in the air gap between one of the legs of the C core portion and a side at one end of the I core portion;

(3) having the additional flux source close to the primary coil can reduce the turn count and allow for a larger wire (lower cost); and

(4) provision of the magnet or magnets embedded as illustrated has the effect of lowering the primary coil inductance such that the total ignition coil will work with a lower energy input, as well as reducing the magnetic path length. In short, the size (and cost) of the ignition coil is reduced since it will operate more efficiently.

Although preferred exemplary embodiments are shown and described in detail in the drawings and in the preceding specification, they should be viewed as purely exemplary and not as limiting the invention. It is noted that only preferred exemplary embodiments are shown and described, and all variations and modifications that presently or in the future lie within the protective scope of the invention should be protected.