Patent Publication Number: US-7710229-B2

Title: Ignition coil and ignition coil system having the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2006-186900 filed on Jul. 6, 2006. 
     FIELD OF THE INVENTION 
     The present invention relates to an ignition coil having an ion current detector. The present invention relates to an ignition coil system having the ignition coil. 
     BACKGROUND OF THE INVENTION 
     An ignition coil provided with a sparkplug is mounted to an engine of a vehicle. The sparkplug generates spark to ignite mixture of fuel and air in each cylinder of the engine. When the mixture is burned in the cylinder, fuel contained in the mixture is ionized, so that an ion current flows between a pair of electrodes provided to the sparkplug. An ion current detector is provided to such an ignition coil to detect an ion current for monitoring misfire in the cylinder. 
     The ignition coil generates therein an inductive magnetic field by terminating electricity supplied to a primary coil of the ignition coil. The inductive magnetic field generates induced electromotive force in a secondary coil of the ignition oil, so that the pair of electrodes of the sparkplug generates spark. Immediately after generating the spark by forming the inductive magnetic field, residual magnetism remains in the ignition oil. The ion current detector may falsely detect noise, which is caused by the residual magnetism, as the ion current. Accordingly, the ion current detector detects the ion current by waiting a predetermined period after generating the spark. 
     According to U.S. Pat. No. 5,866,808 (JP-A-H9-195913), the ion current detector has a structure capable of stably detecting the ion current by reducing residual magnetism. Furthermore, according to the ignition coil in JP-U-3028977, the cross section of the outer core is set to be in a range between 75% and 100% of the cross section of the center core, thereby enhancing ignition energy of the ignition coil. 
     However, the above conventional structure of each ignition coil is not sufficient to stabilize detection of the ion current. Specifically, in the above conventional structure of each ignition coil, a relationship between axial end surfaces of the center core and the outer core on the axially opposite side of the spark plug is not considered. Accordingly, the above conventional structure is not sufficient to enhance accuracy in the detection of the ion current. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above disadvantage. According to one aspect of the present invention, an ignition coil for a sparkplug having electrodes, the ignition coil adapted to being electrically connected with a control unit, the ignition coil including primary and secondary coils each having an axial high voltage end adapted to connecting with the sparkplug. The ignition coil further includes an ion current detector for detecting an ion current flowing through the electrodes. The ignition coil further includes a center core provided on a radially inner side of the primary and secondary coils. The center core is formed of a magnetic material. The ignition coil further includes an outer core provided on a radially outer side of the primary and secondary coils. The outer core is formed of a magnetic material. The center core has an axial low voltage end defining a center low end surface. The outer core has an axial low voltage end defining an outer low end surface. The center low end surface axially protrudes toward an opposite side of the axial high voltage end relative to the outer low end surface. The center low end surface is located at a stagger distance from the outer low end surface with respect to an axial direction of the center core. The stagger distance is defined such that a detection period correlated to residual magnetic noise in the ion current detection output of the ion current detector is within a system requirement period of the control unit. 
     According to another aspect of the present invention, an ignition coil for a sparkplug having electrodes, the ignition coil including primary and secondary coils each having an axial high voltage end connectable with the sparkplug. The ignition coil further includes an ion current detector for detecting an ion current flowing through the electrodes. The ignition coil further includes a center core provided on a radially inner side of the primary and secondary coils. The center core is formed of a magnetic material. The ignition coil further includes an outer core provided on a radially outer side of the primary and secondary coils. The outer core is formed of a magnetic material. The center core has an axial low voltage end defining a center low end surface. The outer core has an axial low voltage end defining an outer low end surface. The center low end surface axially protrudes toward an opposite side of the axial high voltage end relative to the outer low end surface. The center low end surface is located at a stagger distance from the outer low end surface with respect to an axial direction. The stagger distance is equal to or greater than 3 mm, and is equal to or less than 12.5 mm. The center core has a center-core cross section perpendicularly to the axial direction. The outer core has an outer-core cross section perpendicularly to the axial direction. The center-core cross section and the outer-core cross section are in an outer-to-center cross-section ratio. The outer-to-center cross-section ratio is equal to or greater than 54.3, in a structure in which the stagger distance is equal to or greater than 3 mm and equal to or less than 6 mm. The outer-to-center cross-section ratio is equal to or greater than 7.11×(stagger distance−6)+54.3, in a structure in which the stagger distance is greater than 6 mm and equal to or less than 12.5 mm. 
     According to another aspect of the present invention, an ignition coil system for a sparkplug having electrodes, the ignition coil system including a control unit. The ignition coil system further includes an ignition coil electrically connected with the control unit. The ignition coil includes primary and secondary coils each having an axial high voltage end adapted to connecting with the sparkplug. The ignition coil further includes an ion current detector for detecting an ion current flowing through the electrodes and transmitting an ion current detection output. The ignition coil further includes a center core provided on a radially inner side of the primary and secondary coils. The center core is formed of a magnetic material. The ignition coil further includes an outer core provided on a radially outer side of the primary and secondary coils. The outer core is formed of a magnetic material. The center core has an axial low voltage end defining a center low end surface. The outer core has an axial low voltage end defining an outer low end surface. The center low end surface axially protrudes toward an opposite side of the axial high voltage end relative to the outer low end surface. The center low end surface is located at a stagger distance from the outer low end surface with respect to an axial direction of the center core. The stagger distance is defined such that a duration period of at least one of a plurality of waves indicating residual magnetic noise in the ion current detection output is within a system requirement period of the control unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a sectional view showing an ignition coil according to a first embodiment; 
         FIG. 2  is a sectional view showing a connector portion of the ignition coil according to the first embodiment; 
         FIG. 3  is a schematic view showing an ion current detection circuit of the ignition coil, according to the first embodiment; 
         FIG. 4  is a time chart showing an output of the ion current detection circuit; 
         FIGS. 5 ,  6  are time charts showing residual magnetic noise; 
         FIG. 7  is a graph showing a relationship of a total duration Ta and a one-pulse duration Tp relative to a stagger distance X, between axial lengths of cores of the ignition coil, obtained in a verification experiment; 
         FIG. 8  is a graph showing a relationship between a cross-section ratio B/A and the total duration Ta, obtained in the verification experiment; 
         FIG. 9  is a graph showing a relationship between the cross-section ratio B/A and the one-pulse duration Tp, obtained in the verification experiment; and 
         FIG. 10  is a graph showing a relationship between the stagger distance X and the cross-section ratio B/A, obtained in the verification experiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiment 
     As follows, an ignition coil  1  is described with reference to  FIG. 1 . In this embodiment, as shown in  FIG. 1 , the ignition coil  1  includes a primary coil  41  and a secondary coil  42 . Each of the primary coil  41  and the secondary coil  42  has an axial lower end (axial high voltage end) with respect to an axial direction L in  FIG. 1 . The lower end is provided with a sparkplug  35  including a pair of electrodes  351 . The sparkplug  35  further includes an ion current detector for detecting an ion current flowing between the electrodes  351 . A center core  5  is provided on the radially inner side of the primary coil  41  and the secondary coil  42 . The center core  5  is formed of a magnetic material. An outer core  6  is provided on the radially outer side of the primary coil  41  and the secondary coil  42 . The outer core  6  is formed of a magnetic material. 
     As shown in  FIG. 2 , the center core  5  has an axial upper end (axial low voltage end) defining an upper end surface defining a center core upper end surface (center low end surface)  51  with respect to the axial direction L. The outer core  6  has an upper end surface defining an outer core upper end surface (outer low end surface)  61  with respect to the axial direction L. The center core upper end surface  51  upwardly protrudes relative to the outer core upper end surface  61  axially toward a low voltage side on the upper side in  FIG. 1 , i.e., toward an opposite side of the axial high voltage ends of the primary coil  41  and the secondary coil  42 . 
     In this ignition coil  1 , the center core upper end surface  51  is distant from the outer core upper end surface  61  by a stagger distance X with respect to the axial direction L. The stagger distance X is defined such that a detection period for residual magnetic noise in the ion current detection output of the ion current detector falls within a system requirement period of an engine control unit (ECU). The ECU is an electronic control unit for an engine  8 . 
     As follows, the ignition coil  1  is described with reference to  FIGS. 1 to 7 . As shown in  FIG. 1 , the primary coil  41  is constructed by winding a primary wire, which is applied with an insulative coating, around the outer circumferential periphery of a primary spool  411  for a primary winding number. The secondary coil  42  is constructed by winding a secondary wire, which is applied with an insulative coating, around the outer circumferential periphery of a secondary spool  421  for a secondary winding number, which is greater than the primary winding number. The secondary coil  42  is arranged on the radially inner side of the primary coil  41 . The center core  5  is arranged on the radially inner side of the secondary coil  42 . The outer core  6  is provided on the radially outer side of the primary coil  41 . The outer core  6  is inserted through a coil case  2 , which is a cylindrical resin member. Thus, a coil main body  11  is constructed by accommodating the primary coil  41 , the secondary coil  42 , the center core  5 , the outer core  6 , and the like in the coil case  2 . 
     The center core  5  is constructed by stacking substantially flat electromagnetic steel plates perpendicularly to the axial direction L of the ignition coil  1 . The substantially flat electromagnetic steel plates are, for example, silicon steel plates each applied with an electrically insulative coating. The outer core  6  is constructed of multiple substantially cylindrical electromagnetic steel plates such as silicon steel plates having at least one silt (gap) with respect to the axial direction L. The electromagnetic steel plates are stacked with respect to the radial direction to construct the outer core  6 . The center core  5  and the outer core  6  define therebetween a magnetic path (magnetic circuit) through which magnetic flux is formed by supplying electricity to the primary coil  41 . An insulative tape  55  is wound around the outer circumferential periphery of the center core  5  for relaxation of stress. 
     The upper ends of the primary coil  41  and the secondary coil  42  with respect to the axial direction L are provided with a connector portion  12  for electrically connecting the ignition coil  1  with the engine ECU. The coil case  2  is constructed of a case main body  21 , a connector case  22 , and a plug case  23 . The case main body  21  accommodates the primary coil  41 , the secondary coil  42 , the center core  5 , the outer core  6 , and the like The connector case  22  is connected with the upper end of the case main body  21  with respect to the axial direction L. The plug case  23  is connected with the lower end of the case main body  21  with respect to the axial direction L. Each of the case main body  21 , the connector case  22 , and the plug case  23  is formed of resin. 
     The connector case  22  includes a mount portion  221  and a connector portion  222 . The mount portion  221  is provided with an igniter  223  having an electric power circuit and the like for controlling the ignition coil  1 . The connector portion  222  electrically connects the igniter  223  with the engine ECU. The connector portion  222  is integrated with a plus-power pin, a minus-power pin, a plus-spark signal pin, a minus-spark signal pin, and the like by, for example, insert-molding. Each pin of the connector portion  222  is connected with corresponding pin of the igniter  223 . 
     As shown in  FIG. 3 , the igniter  223  includes a power control circuit C 1  and an ion current detection circuit C 2 . The power control circuit C 1  supplies electricity to the primary coil  41  by receiving a signal from, for example, the ECU. The ion current detection circuit C 2  detects the ion current passing between the pair of electrodes  351  of the sparkplug  35 . The ion current detector of the ignition coil  1  is constructed of the ion current detection circuit C 2  in the igniter  223 . 
     The ion current detection circuit C 2  includes an amplifier circuit for amplifying the detection signal of the ion current. 
     Referring to  FIG. 1 , the plug case  23  is provided with a plug cap  31  formed of rubber for electrically insulating the ignition coil  1  and protecting the ignition coil  1  against water intrusion. An insulator  352  of the sparkplug  35  is fitted into a plug fitting hole  32  of the plug cap  31 . The lower end of the secondary spool  421  with respect to the axial direction L extends to define an extended portion  422 . A high voltage terminal  33  is provided in the extended portion  422  on the radially inner side of the plug case  23 . The high voltage terminal  33  is electrically conductive with a high voltage winding end of the secondary coil  42 . The high voltage terminal  33  is provided with a coil spring  34  conductive with a terminal portion  353  of the sparkplug  35 . 
     The ignition coil  1  has a stick-type structure. Specifically, the lower end of the coil main body  11  with respect to the axial direction L is inserted into a plughole  81  of a cylinder head cover of the engine  8 , together with the sparkplug  35 . The connector portion  12  provided with the igniter  223  and the upper end of the coil main body  11  with respect to the axial direction L are located outside the plughole  81 . The sparkplug  35  mounted to the ignition coil  1  is screwed with a bottom portion of the plughole  81 . The pair of the electrodes  351  of the sparkplug  35  protrudes into a corresponding combustion chamber  82  of the engine  8 . Each gap in the coil case  2  is charged with electrically insulative resin  15  such as epoxy resin. Specifically, a gap surrounded with the coil main body  11 , the connector case  22 , and the plug case  23  is are charged with the electrically insulative resin  15 . 
     As shown in  FIG. 3 , a voltage signal V 2  is obtained from electricity flowing in the secondary coil  42  through the pair of electrodes  351  of the sparkplug  35 . As shown in  FIG. 4 , the ion current detection output is obtained as a pulse signal P by performing a digital processing to the voltage signal V 2 . 
     As shown in  FIG. 5 , the total residual magnetic noise falsely appears as a pulse-voltage signal in the ion current detection output for a total duration Ta. As shown in  FIG. 6 , one pulse of the residual magnetic noise falsely appears a pulse-voltage signal in the ion current detection output for a one-pulse duration Tp. 
     The system requirement period includes the total duration Ta and the single duration Tb, and is a performance requirement to the ignition coil  1  for detecting the ion current. 
     When the total duration Ta and the one-pulse duration Tp of the residual magnetic noise become long, the total duration Ta and the one-pulse duration Tp exert bad influence to detection of the ion current. Therefore, the performance requirements are defined by the total duration Ta and the one-pulse duration Tp correlated to the ion current detection performance of the ignition coil  1 . In this example, the system requirement period is defined by Ta≦1000 μs and Tp≦416 μs. When the total duration Ta is greater than 1000 μs, or the one-pulse duration Tp is greater than 416 μs, residual magnetic noise may be falsely detected as the ion current, and may cause misevaluation. In this condition, even when misfire occurs, the misfire may not be detected. 
     Referring to  FIG. 3 , when the ECU transmits a pulse-shaped spark generating signal to the igniter  223 , the power control circuit C 1  of the igniter  223  is activated to flow electricity through the primary coil  41 . Thus, the magnetic field is formed to pass through the center core  5  and the outer core  6  ( FIG. 1 ). Subsequently, the ECU terminates the electricity supplied to the primary coil  41 , so that the center core  5  and the outer core  6  form therebetween an inductive magnetic field opposite to the magnetic field. The inductive magnetic field generates induced electromotive force (counter electromotive force) in the secondary coil  42 , so that the sparkplug  35  provided to the ignition coil  1  sparks. The spark ignites mixture of fuel and air, thereby burning the mixture in the combustion chamber  82  in each cylinder of the engine  8 . 
     When the mixture is properly burned in the engine  8 , ingredients contained in fuel are ionized. In this condition, the ion current flows between the pair of electrodes  351  of the sparkplug  35 . Thus, referring to  FIG. 3 , the ion current detection circuit C 2  detects generation of the ion current. 
     The ion current detection circuit C 2  amplifies the detection signal of the ion current, and transmits the detection signal to the engine ECU. The engine ECU includes a processing circuit and a microcomputer for detecting and monitoring combustion of the engine  8  in accordance with the detection signal transmitted from the ion current detection circuit C 2 . 
     In  FIG. 4 , the generation of the ion current is detected when an ion current detection waveform, which indicates the ion current, upwardly passes beyond an ion current detection reference. 
     Immediately after forming of the inductive magnetic field to generate spark in the sparkplug  35 , residual magnetism remains in the magnetic circuit constructed of the center core  5  and the outer core  6 . As shown in  FIGS. 4 to 5 , when the ion current detection circuit C 2  detects the ion current, the engine ECU instructs to wait for a predetermined period before detecting of the ion current, in order to avoid false detection of residual magnetic noise caused by the residual magnetism. This predetermined period is defined by Ta≦1000 μs and Tp≦416 μs as the system requirement period of the engine ECU. 
     Referring to  FIG. 2 , the center core upper end surface  51  protrudes from the outer core upper end surface  61  by the stagger distance X with respect to the axial direction L. In the ignition coil  1  of this example, the stagger distance X is defined possibly small to reduce magnetic flux, which outwardly leaks without passing through the center core  5  and the outer core  6 . Specifically, the stagger distance X is defined in a range between 3 mm and 12.5 mm. Furthermore, a cross-section ratio B/A (%) between a cross section A of the center core  5  and a cross section B of the outer core  6  is defined in a range between 90% and 120% in the cross section of the ignition coil  1  perpendicular to the axial direction L. 
     That is, in the ignition coil  1  of this example, the stagger distance X is set possibly small, and the cross-section ratio B/A is appropriately defined. In this structure, the leaking magnetic flux can be restricted from remaining as residual magnetism around the ignition coil  1 , immediately after generating spark in the sparkplug  35 . Thus, the detection period for residual magnetic noise can be defined within the system requirement period of Ta≦1000 μs and Tp≦416 μs. Thereby, in the ignition coil  1  of this example, detection of the ion current can be protected from influence caused by residual magnetism. Thus, detection accuracy of the ion current can be enhanced. 
     As follows, an experiment for verification of the above structure is described. Specifically, in this experiment, a relationship among the stagger distance X, the total duration Ta, and the one-pulse duration Tp of residual magnetic noise is obtained. In this experiment, the number of stacked the electromagnetic steel plates, which construct the outer core  6 , is altered for three the cross-section ratios B/A. In  FIG. 7 , as the stagger distance X becomes large, both the total duration Ta and the one-pulse duration Tp become large. 
     In a case where the number of the electromagnetic steel plates is four and the cross-section ratio B/A is 91%, the stagger distance X is preferably equal to or less than 12.5 mm to satisfy the condition where Ta≦1000 μs and Tp≦416 μs. In a case where the number of the electromagnetic steel plates is three and the cross-section ratio B/A is 68%, the stagger distance X is preferably equal to or less than 8 mm to satisfy the condition where Ta≦1000 μs and Tp≦416 μs. In a case where the number of the electromagnetic steel plates is four and the cross-section ratio B/A is 91%, and the stagger distance X is greater than 10 mm, the one-pulse duration Tp (waveform distortion) becomes large. In a case where the number of the electromagnetic steel plates is three and the cross-section ratio B/A is 68%, and the stagger distance X is greater than 6 mm, the one-pulse duration Tp (waveform distortion) also becomes large. 
     As shown in  FIGS. 8 ,  9 , as the stagger distance X becomes large, the total duration Ta and the one-pulse duration Tp become large. By contrast, as the cross-section ratio B/A becomes large, the total duration Ta and the one-pulse duration Tp become small. 
     In this experiment, the cross-section ratio B/A is obtained with respect to each stagger distance X in a condition where the total duration Ta is 1000 μs. In addition, the cross-section ratio B/A is also obtained with respect to each stagger distance X in a condition where the one-pulse duration Tp is 416 μs. Thus, the relationship between the stagger distance X and the cross-section ratio B/A is obtained. As shown in  FIG. 10 , as the stagger distance X becomes large, the cross-section ratio B/A is increased, so that the detection period for residual magnetic noise can be defined within the system requirement period of Ta≦1000 μs and Tp≦416 μs. 
     Thus, the relationship between the stagger distance X and the cross-section ratio B/A can be defined within a proper region S in  FIG. 10 , by increasing the cross-section ratio B/A adaptively to increase in stagger distance X. 
     As follows, the relationship between the stagger distance X and the cross-section ratio B/A is linearized, so that the following equation is obtained to define the relationship satisfying the system requirement period. The one-pulse duration Tp is strict, i.e., effective to the system requirement period, compared with the total duration Ta. Therefore, this relationship is obtained on the basis of the one-pulse duration Tp. In this relationship, when the stagger distance X is equal to or less than 6 mm, the cross-section ratio B/A is set to be equal to or greater than 54.3. When the stagger distance X is greater than 6 mm, the cross-section ratio B/A is set to be equal to or greater than 7.11×(X−6)+54.3. Thus, the relationship satisfies the system requirement period. 
     In the structure of the ignition coil  1 , it is difficult to set the stagger distance X to be less than 3 m. By contrast, when the stagger distance X is greater than 12.5 mm, It is difficult to maintain the one-pulse duration Tp to be small. Therefore, when the stagger distance X is equal to or greater than 3 mm and equal to or less than 6 mm, the cross-section ratio B/A is preferably set to be equal to or greater than 54.3, i.e., B/A≧54.3. In addition, when the stagger distance X is greater than 6 mm and equal to or less than 12.5 mm, the cross-section ratio B/A is set to be equal to or greater than 7.11×(X−6)+54.3, i.e., B/A≧7.11×(X−6)+54.3. 
     Preferably, the cross-section ratio B/A is set possibly large, in order to possibly reduce the total duration Ta and the one-pulse duration Tp. Practically, the cross-section ratio B/A is set to be equal to or greater than 90%. When the cross-section ratio B/A is set excessively large, the thickness of the outer core  6  becomes large, and the outer diameter of the ignition coil  1  becomes large. Therefore, the cross-section ratio B/A is preferably set to be equal to or less than 120%. 
     The above controls and processings such as calculations and determinations are not limited being executed by the ECU and the engine ECU described in the above embodiment. The control system may have various structures including a control unit such as the ECU, the engine ECU, and combination thereof. 
     Various modifications and alternations may be diversely made to the above embodiments without departing from the spirit of the present invention.