Patent Publication Number: US-6700470-B2

Title: Ignition apparatus having increased leakage to charge ion sense system

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to an ignition apparatus for developing a spark firing voltage that is applied to one or more spark plugs of an internal combustion engine, and more particularly, to a system configured for ion current measurement within a combustion chamber of the engine. 
     2. Discussion of the Background Art 
     So-called ion sense systems are known for detecting a combustion condition (e.g., misfire, knock). The combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. Applying a relatively high voltage across, for example, the electrodes of a spark plug just after ignition is known to produce a current across the electrodes. Such current is known as an ion current. The ion current that flows is proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above, and consequently corresponds in some measure to the ionization throughout the entire cylinder as combustion occurs. The DC level or amount of ion current is indicative of a quantity of combustion, or whether in fact combustion has occurred at all (e.g., a misfire condition). An AC level of the ion current may be used to determine whether knock exists. The ion sense approach is effective for any number of cylinders, and various engine speed and load combinations. 
     Known ion current sensing systems generally include a capacitor or the like configured to store a voltage. The stored voltage is thereafter used as a “bias” voltage, which is applied to the spark plug to generate the ion current. One approach taken in the art involves using the voltage from a leakage inductance spike from the primary side of the ignition coil to charge a capacitor for biasing the spark plug, as seen by reference to U.S. Pat. No. 6,186,129 entitled “ION SENSE BIASING CIRCUIT,” issued to Butler. Because of relatively good flux coupling between primary and secondary windings in “pencil” coils (i.e., a relatively slender ignition coil configuration that is adapted for mounting directly above the spark plug), bias voltages of approximately 100 volts are about the maximum that can be achieved (i.e., the leakage inductance spike is limited by the relatively high coupling). While biasing at about 100 volts is adequate for most combustion conditions, it is nonetheless desirable to bias at higher voltage levels under certain other conditions, for example, in highly dilute or lean conditions. 
     U.S. Pat. No. 6,114,935 entitled “IGNITION COIL HAVING COIL CASE,” issued to Oosuka et al. disclose an ignition coil extending along an axis, where the longitudinal extent of a secondary coil is about the same as the longitudinal extent of the primary coil, which is generally conventional construction for coupling primary flux to the secondary coil. 
     There is therefore a need to provide an ignition apparatus and an ignition system that improves upon one or more of the configurations set forth above. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a solution to one or more of the problems set forth above. An increased leakage inductance spike would be required to charge the ion sense system for biasing at the increased voltage levels. One advantage of the present invention is that it provides such a configuration that increases a leakage inductance spike, which may be used by an ion sense system in providing an increased bias voltage level. This has the advantage of more effectively operating in highly dilute or lean conditions. Another advantage is that it provides an ignition apparatus having an increased, effective turns ratio (N S :N P ), thereby allowing a reduction in the amount of secondary wire used, which is typically the number one raw material cost in an ignition coil. This feature reduces cost. Still yet another advantage of the present invention is that as bias voltages increase, the invention decreases waste of potential spark energy. 
     In accordance with the present invention, an ignition apparatus is provided that includes a central core and primary and secondary windings. The central core extends along a main axis, and the primary winding is disposed about the central core. The secondary winding is also disposed about the central core. The primary winding is extended relative to the secondary winding. That is, the primary winding has a first axial length, and the secondary winding has a second axial length that is less than the first axial length. The primary winding extension decreases flux coupling, thereby increasing a leakage inductance spike. 
     In a preferred embodiment, the ignition apparatus is arranged so that first and second layers thereof extend approximately the same axial length as the secondary winding, with one or more additional layers being wound to extend beyond the secondary winding at the low voltage end of the secondary winding. 
     In another aspect of the present invention, the above-described ignition apparatus is coupled to an ion sense biasing circuit that is coupled to the primary winding for charging thereof and is further configured to bias a spark plug coupled to a high voltage end of the secondary winding to produce an ion current indicative of a combustion condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example, with reference to the accompanying drawings. 
     FIG. 1 is a simplified cross-sectional view of an ignition apparatus having a primary winding extension according to the present invention. 
     FIG. 2 is a simplified schematic and block diagram view of the ignition system shown in FIG.  1 . 
     FIG. 3 is a diagrammatic view showing an alternative embodiment of a primary winding extension according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, FIG. 1 illustrates an ignition apparatus or coil  10  in simplified, cross-sectional form. Ignition apparatus  10  may be coupled to, for example, a control unit  12 , which may contain primary energization control circuitry for controlling the charging and discharging of ignition apparatus  10 . The relatively high voltage produced by ignition apparatus  10  is provided to a spark plug  14  for producing a spark across a spark gap thereof, which may be employed to initiate combustion in a combustion chamber of an internal combustion engine. 
     Ignition apparatus  10  is adapted for installation to a conventional internal combustion engine through a spark plug well onto a high voltage terminal of the spark plug, which in turn may be retained by a threaded engagement with a spark plug opening in the above-described combustion cylinder. The engine may provide power for locomotion of a self-propelled vehicle, such as an automotive vehicle. In addition, ignition apparatus  10  may include ion sense capability integral therewith, and in particular, an ion sense system having means for biasing the spark plug gap immediately after sparking, and which is charged by a leakage inductance spike taken off of the primary side of the apparatus, for example, as disclosed in U.S. Pat. No. 6,186,129 entitled “ION SENSE BIASING CIRCUIT,” assigned to the common assignee of the present invention, hereby incorporated by reference in its entirety. It should be understood that the ion sense system may be separate from ignition apparatus  10 , and nonetheless have the same functionality. 
     FIG. 1 further shows a core  16 , an optional first magnet  18 , an optional second magnet  20 , an electrical module  22 , a primary winding  24 , a first layer of encapsulant such as an epoxy potting material layer  26 , a secondary winding spool  28 , a secondary winding  30 , a second layer  32  of encapsulant such as epoxy potting material, a case  34 , a shield assembly  36 , an electrically conductive cup  37 , a low-voltage (LV) connector body  38 , and a high-voltage (HV) connector assembly  40 . Core  16  is elongated, extending along a main axis designated “A,” and includes a top end  42  and a bottom end  44 . FIG. 1 further shows a rubber buffer cup  46 , annular flange portions  48 ,  50  of secondary spool  28 , a high voltage (HV) secondary terminal  52 , a boot  54 , and a seal member  56 . 
     As described in the Background, one area that can be improved relative to the known art relates to the voltage level at which biasing is conducted during ion sense operation (when using a leakage inductance spike from the primary side to charge a capacitor for biasing an ion sense circuit). One way to increase the leakage inductance spike produced off of the primary winding when the primary current is interrupted (i.e., when a spark is commanded), is to extend the primary winding relative to the secondary winding so as to decrease the level of flux coupling therebetween. As shown in FIG. 1, primary winding  24  has a first axial length, and secondary winding  30  has a second axial length that is less than the first axial length, by an amount designated “B.” In the embodiment shown in FIG. 1, the respective lowermost portions of the primary winding  24  and secondary winding  30  are substantially aligned, axially, with respect to longitudinal axis “A.” The primary winding extension is preferably implemented proximate the upper, low-voltage end of the ignition apparatus  10  (i.e., closer to upper end  42  of core  16  than to the lower end  44 ). Further, as shown diagrammatically in FIG. 1, in a first embodiment, the primary winding  24  comprises a plurality of layers, all of the layers being about the same axial length and ending at substantially the same axial position (i.e., relative to axis “A”). 
     In a constructed embodiment, the primary winding  24  contained  210  turns of 24 AWG copper, insulated wire, arranged in 2 layers. The secondary winding  30  contained about 15,660 turns of 46 AWG copper, insulated wire, arranged in a progressively wound manner. The axial length of the secondary winding was about 45.5 mm, while the axial length of the primary winding was about 57.9 mm, yielding a 14.4 mm extension. 
     FIG. 2 is a simplified schematic and block diagram view of the ignition system of FIG.  1 . In addition to the components illustrated in FIG. 1, FIG. 2 further shows a switch  58 , which may comprise conventional switching components (i.e., IGFET, MOSFET, bipolar transistor, or the like), and an ion sense system  60 . Ion sense system  60  includes means or circuit for biasing spark plug  14  that is coupled to primary winding  24 , and is configured to capture a leakage inductance spike therefrom for charging a capacitor or the like, as described in U.S. Pat. No. 6,186,129 entitled “ION SENSE BIASING CIRCUIT” issued to Butler, referred to above and herein incorporated by reference. The ion sense block  60  is further configured to bias spark plug  14  which is coupled to a high voltage end of secondary winding  30  so as to produce an ion current indicative of a combustion condition, as known by those of ordinary skill in the art. Control unit  12 , as known, is configured to generate an electronic spark timing (EST) signal that determines when charging is to commence (i.e., when the EST signal transitions from a logic low, to a logic high state), the duration of charging (i.e., how long the EST signal is asserted), and when the spark is to occur (i.e., when the EST signal is discontinued). 
     FIG. 3 shows an alternative embodiment according to the present invention wherein primary winding  24  is shown having a different configuration. Structure  62  may be a primary winding spool, or may be a core  16 . As shown in FIG. 3, in order to obtain an increased leakage inductance spike, a section of the primary winding turns are placed outside of the main flux path with the secondary winding  30 . In the illustrated embodiment, two layers, designated L 1  and L 2 , are wound so as to extend a first axial length. The secondary winding  30  extends a second axial length that is less than the first axial length. Further layers, such as a third and a fourth layer, designated L 3  and L 4 , are then wound so as to have a third axial length that is foreshortened relative to said first axial length layers. L 3  and L 4  are also axially spaced apart from the low-voltage end  63  of secondary winding  30 . In this embodiment, the extension is designated by an axial distance B′. Additional layers, such as a fifth and a sixth layer, designated L 5  and L 6 , may be further added depending on the level of the leakage inductance spike desired for any particular design. As with the first embodiment in FIG. 1, the primary winding extension B′ in this embodiment occurs at the low voltage end  63  of the secondary winding, with respect to longitudinal axis “A.” As with the embodiment of FIG. 1, the flux created by the primary winding  24  (by way of layers L 1 -L 6 , in the illustrated embodiment) would only be partially coupled to secondary winding  30 , and a predetermined portion of the energy stored in this flux would be delivered as a leakage inductance spike to charge a capacitor (or other storage element) contained in ion sense system  60 , as described above. 
     In addition, another advantage of the present invention relates to an effective increase in the turns ratio (N S :N P ), which is beneficial in a variety of different respects. First, the wire used for the secondary winding  30  is typically one of the most significant, if not the most significant, raw material cost in an ignition coil. Thus, the higher the turns ratio, the higher the cost (due to more copper). If one could increase the effective turns ratio without actually increasing the number of turns in the secondary, a cost savings would be realized. 
     In addition, in many design situations, long burn times are specified, therefore requiring a high turns ratio. 
     The following is an analysis of the burn time relationship to the turns ratio. The energy available to the secondary (hereinafter “Ea”), is given by equation (1) below. 
     
       
           Ea =Estored−switch loss−core loss  (1) 
       
     
     Assuming a linear system, the energy available to the secondary is dissipated in two principal places, namely, across the spark plug gap, and through a zener diode conventionally employed in the secondary circuit of an ignition coil, as set forth in equation (2) below. 
     
       
           Ea =( V   ZENER   ×I   S PEAK   ×T   BURN )+( I   2     S PEAK     ×R   S   ×T   BURN )/3  (2) 
       
     
     The burn time can be solved for by rearranging equation (2) to yield equation (3) set forth below. 
     
       
           T   BURN   =Ea /(( V   ZENER   ×I   S PEAK )+( I   2   S PEAK   ×R   S )/3)  (3) 
       
     
     The peak secondary current set forth in equations (2) and (3) is provided for in equation (4) below. 
     
       
           I   S PEAK   ≈K   COUPLING   ×I   P PEAK /(TurnsRatio)  (4) 
       
     
     Therefore, as a natural consequence of equation (4), as the turns ratio (N S :N P ) goes down, the peak secondary current goes up, assuming the same coupling. As the peak secondary current increases, the burn time decreases. Therefore, to obtain increased burn times, conventionally, the turns ratio would have to be increased. If one could increase the effective turns ratio without actually adding turns, then increased burn times could be obtained without cost penalties. 
     In addition, lower clamp voltages with respect to switch  58  also drive higher turns ratios. Specifically, the secondary output (i.e., voltage output) is limited to approximately the primary side clamp voltage times the turns ratio. If one could increase the effective turns ratio, the output voltage could be increased. To obtain any of the foregoing, with the secondary and the primary windings at the same length, the only practical way to increase the turns ratio is to increase the actual number of turns in the secondary winding. This increases cost. 
     However, if you include a primary winding extension according to the invention, you can increase the effective turns ratio without actually increasing the number of secondary winding turns. Let P=permanence, the Φ=flux, N=Turns, and AT=amp-turns. From the foregoing, equations (5), (6) and (7) are set forth below. 
     
       
           P≡Φ/AT   (5) 
       
     
     
       
           AT=Φ/P   (6) 
       
     
     
       
           I   S   =AT/N   S   (7) 
       
     
     Therefore I S ∝1/P, and P∝Φ. Using the magnetic vector potential (A) to get a relative value for the flux normalized per turn by the following equation, Φ∝(ΣA×N/ΣN), and multiplying the ratio of these values for a conventional design (i.e., where the primary length is equal to the secondary length), and the extended primary by the secondary current expected from the turns ratio, this calculated value for secondary current substantially approximates the measured current. 
     EXAMPLE 
     For a conventional design where the axial length of the primary winding is substantially equal to the axial length of the secondary winding, the quantity (ΣA×N/ΣN) is determined over the axial length was 1.5478×10 −3  wb/m. The same calculation was made for an ignition apparatus according to the invention having an extended primary winding, and the quantity (ΣA×N/ΣN) over the axial length was 2.037466×10 −3 . Given the equations referred to above, the peak secondary current for the conventional design where the primary winding axial length was substantially equal to the secondary winding axial length, was approximately 63.6 ma (average). See equation (8) for the calculated level when lp≈ls. 
     
       
           I   S =(0.93)( I   P   /N )=(0.93)(7.2)/105=0.0638 A   (8) 
       
     
     The calculated (expected) secondary current for the extended primary ignition apparatus according to the invention is 51.8 mA for the peak secondary current as shown by equation (9). The measured average for a constructed embodiment was 50.3 ma (average). 
     
       
           Is =63.6×(105/98)×(1.5478/2.037466)=51.8 mA  (9) 
       
     
     Therefore, N EFFECTIVE =(0.93)I P /I S =(0.93)(7.2)/(0.0518)=129.3. 
     Note, that the actual turns ratio decreased from 105:1 to 98:1, thereby reducing the amount of secondary wire needed for the ignition apparatus. However, the effective turns ratio was increased to approximately 129:1, which saved approximately 25% of the secondary wire cost, over adding turns to yield the same effect. 
     Referring again to FIG. 1, further details concerning ignition apparatus  10  will now be set forth configured to enable one to practice the present invention. It should be understood that portions of the following are exemplary only and not limiting in nature. Many other configurations are known to those of ordinary skill in the art and are consistent with the teachings of the present invention. Central core  16  may be elongated, having a main, longitudinal axis “A” associated therewith. Core  16  may be a conventional core known to those of ordinary skill in the art. As illustrated, core  16 , in the preferred embodiment, takes a generally cylindrical shape (which is a generally circular shape in radial cross-section), and may comprise compression molded insulated iron particles or laminated steel plates, both as known. 
     Magnets  18  and  20  may be optionally included in ignition apparatus  10  as part of the magnetic circuit, and provide a magnetic bias for improved performance. The construction of magnets such as magnets  18  and  20 , as well as their use and effect on performance, is well understood by those of ordinary skill in the art. It should be understood that magnets  18  and  20  are optional in ignition apparatus  10 , and may be omitted, albeit with a reduced level of performance, which may be acceptable, depending on performance requirements. 
     Module  22  may be configured to perform a switching function, such as connecting and disconnecting an end of primary winding to ground. Additionally, the module may include the ion sense system  60  described above. 
     Primary winding  24  generally may be wound directly onto core  16  in a manner known in the art. Primary winding  24  includes first and second ends and is configured to carry a primary current I P  for charging apparatus  10  upon control of control unit  12  of module  22 . Winding  24  may be implemented using known approaches and conventional materials consistent with the foregoing principles. Although not shown, primary winding  24  may be wound on a primary winding spool (not shown) in certain circumstances (e.g., when steel laminations are used). In addition, winding  24  may be wound on an electrically insulating layer that is itself disposed directly on core  16 . 
     Layers  26  and  32  comprise an encapsulant suitable for providing electrical insulation within ignition apparatus  10 . In a preferred embodiment, the encapsulant comprises epoxy potting material. The epoxy potting material introduced in layers  26 , and  32  may be introduced into annular potting channels defined (i) between primary winding  24  and secondary winding spool  28 , and, (ii) between secondary winding  30  and case  34 . The potting channels are filled with potting material, in the illustrated embodiment, up to approximately the level designated “L” in FIG.  1 . In one embodiment, layer  26  may be between about 0.1 mm and 1.0 mm thick. Of course, a variety of other thicknesses are possible depending on flow characteristics and insulating characteristics of the encapsulant and the design of the coil  10 . The potting material also provides protection from environmental factors which may be encountered during the service life of ignition apparatus  10 . There is a number of suitable epoxy potting materials well known to those of ordinary skill in the art. 
     Secondary winding spool  28  is configured to receive and retain secondary winding  30 . In addition to the features described above, spool  28  is further characterized as follows. Spool  28  is disposed adjacent to and radially outwardly of the central components comprising core  16 , primary winding  24 , and epoxy potting layer  26 , and, preferably, is in coaxial relationship therewith. Spool  28  may comprise any one of a number of conventional spool configurations known to those of ordinary skill in the art. In the illustrated embodiment, spool  28  is configured to receive one continuous secondary winding (e.g., progressive winding) on an outer winding surface thereof, between upper and lower flanges  48  and  50  (“winding bay”), as is known. However, it should be understood that other configurations may be employed, such as, for example only, a configuration adapted for use with a segmented winding strategy (e.g., a spool of the type having a plurality of axially spaced ribs forming a plurality of channels therebetween for accepting windings) as known. 
     The depth of the secondary winding in the illustrated embodiment may decrease from the top of spool  28  (i.e., near the upper end  42  of core  16 ), to the other end of spool  28  (i.e., near the lower end  44 ) by way of a progressive gradual flare of the spool body. The result of the flare or taper is to increase the radial distance (i.e., taken with respect to axis “A”) between primary winding  24  and secondary winding  30 , progressively, from the top to the bottom. As is known in the art, the voltage gradient in the axial direction, which increases toward the spark plug end (i.e., high voltage end) of the secondary winding, may require increased dielectric insulation between the secondary and primary windings, and, may be provided for by way of the progressively increased separation between the secondary and primary windings. 
     Spool  28  is formed generally of electrical insulating material having properties suitable for use in a relatively high temperature environment. For example, spool  28  may comprise plastic material such as PPO/PS (e.g., NORYL available from General Electric) or polybutylene terephthalate (PBT) thermoplastic polyester. It should be understood that there are a variety of alternative materials that may be used for spool  28  known to those of ordinary skill in the ignition art, the foregoing being exemplary only and not limiting in nature. 
     Features  48  and  50  may be further configured so as to engage an inner surface of case  34  to locate, align, and center the spool  28  in the cavity of case  34  and providing upper and lower defining features for a winding surface therebetween. 
     Spool  28  may have associated therewith an electrically conductive (i.e., metal) high-voltage (HV) terminal  52  disposed therein or in contact therewith configured to engage cup  37 , which cup is in turn electrically connected to the HV connector assembly  40 . The body of spool  28  at a lower end thereof is configured so as to be press-fit into the interior of cup  37  (i.e., the spool gate portion). 
     FIG. 1 also shows secondary winding  30  in cross-section. Secondary winding  30 , as described above, is wound on spool  28 , and includes a low voltage end and a high voltage end. The low voltage end may be connected to ground by way of a ground connection through LV connector body  38  in a manner known to those of ordinary skill in the art. The high voltage end is connected to HV terminal  52 . Winding  30  may be implemented using conventional approaches and material known to those of ordinary skill in the art. 
     Case  34  includes an inner, generally enlarged cylindrical surface, an outer surface, a first annular shoulder, a flange, an upper through-bore, and a lower through bore. 
     The inner surface of case  34  is configured in size to receive and retain spool  28  which contains the core  16  and primary winding  24 . The inner surface of case  34  may be slightly spaced from spool  28 , particularly the annular features  48 ,  50  thereof (as shown), or may engage the features  48 ,  50 . 
     A lower through-bore is defined by an inner surface thereof configured in size and shape (i.e., generally cylindrical) to accommodate an outer surface of cup  37  at a lowermost portion thereof as described above. When the lowermost body portion of spool  28  is inserted in the lower bore containing cup  37 , a portion of HV terminal  52  engages an inner surface of cup  37  (also via a press fit) as shown. 
     Case  34  is formed of electrical insulating material, and may comprise conventional materials known to those of ordinary skill in the art (e.g., the PBT thermoplastic polyester material referred to above). 
     Shield  36  is generally annular in shape and is disposed radially outwardly of case  34 , and, preferably, engages an outer surface of case  34 . The shield  36  preferably comprises electrically conductive material, and, more preferably metal, such as silicon steel or other adequate magnetic material. Shield  36  provides not only a protective barrier for ignition apparatus  10  generally, but, further, provides a magnetic path for the magnetic circuit portion of ignition apparatus  10 . Shield  36  may be grounded by way of an internal grounding strap, finger or the like (not shown) well know to those of ordinary skill in the art. Shield  36  may comprise multiple, individual sheets  36 , as shown. 
     Low voltage connector body  38  via module  22  is configured to, among other things, electrically connect the first and second ends of primary winding  24  to an energization source, such as, the energization circuitry (e.g., power source) provided by control unit  12 . Connector body  38  is generally formed of electrical insulating material, but also includes a plurality of electrically conductive output terminals  66  (e.g., pins for ground, primary winding leads, etc.). Terminals  66  are coupled electrically, internally through connector body  38  to module  22  and other portions of apparatus  10 , in a manner known to those of ordinary skill in the art. 
     HV connector assembly  40  is provided for establishing an electrical connection to spark plug  14 . Assembly  40  may include a spring  68  or the like. Contact spring  68  is in turn configured to engage a high-voltage connector terminal of spark plug  14 . This arrangement for coupling the high voltage developed by secondary winding  30  to plug  14  is exemplary only; a number of alternative connector arrangements, particularly spring-biased arrangements, are known in the art.