Abstract:
A spark plug, including an insulator embedding a first metallic electrode axially extending therethrough from a high voltage outer end terminal to the center of the inner end of the insulator from which it protrudes; a metallic ground electrode isolated from the first electrode and having an extended inner termination facing toward the first electrode extending from the insulator tip for defining therebetween a spark gap, a resistive element connected to the ground electrode such that upon mounting the spark plug in an internal combustion engine, the ground electrode electrically connects to the engine body through the resistive element; and to a second outer termination of the ground electrode, adapted to constitute an accessible sensing terminal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of Italian patent application number MI2011A001896, filed on Oct. 19, 2011, which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to detection of ionization current in spark plugs of internal combustion engines and more particularly to a method of sensing ionization current and to a spark plug structure allowing an extended sensing during the different phases of an engine cycle. 
         [0004]    2. Discussion of the Related Art 
         [0005]    In internal-combustion engines, the fuel-air gaseous mixture ignition is activated by a HV electric discharge across the spark plug electrodes. During combustion, molecules in the combustion chamber ionize and a ionization current flows in the electrodes of the spark plug. 
         [0006]    The classic structure of a spark plug is shown in  FIG. 1 . 
         [0007]    Commonly, spark plugs have a metal casing  1  fitted over an insulator  2 , usually molded or cast around a first metal electrode  3 , axially extending from a high voltage terminal  4  at the outer end of the insulator to the inner tip  5  from which the first electrode  3  protrudes. The insulator  2  isolates the first electrode  3  from the threaded metal casing  1 , permanently fitted over the isolator, adapted to mount the spark plug into a threaded hole of a cylinder head of the engine, and from a ground electrode  6 , integral to or shorted to the metal casing  1  and having an end extension that is bent toward the tip of the first electrode  3 , protruding out of the inner end of the insulator in order to form a discharge gap  7 . 
         [0008]    When the spark plug is mounted, the threaded metal casing  1  is driven tight into the threaded hole of a cylinder head of the engine and thus the ground electrode  6  is shorted to the electrical systems ground node. A high voltage generated by the ignition coil (supplied thought an Electronic Ignition Controller circuitry), is applied to the terminal of the first electrode  1 . A spark then occurs in the discharge gap  7 , the air/fuel mixture in the combustion chamber is ignited and a ionization current flows in the electrodes. 
         [0009]    It is known in the art that the ionization current may be processed to provide early detection of plug fouling, for example, and more generally to monitor the combustion process. In particular, the sensed ionization current may be used in control loops to adjust ignition timing, valve timing, fueling, and/or airflow, for example, to better manage the combustion process. 
         [0010]      FIG. 2  shows an exemplary graph of ionization current versus crank angle, in which to three main phases of the combustion process are highlighted, namely the ignition phase, the flame-front phase and the post-flame phase. The ionization current is characterized by high frequency oscillations during the ignition phase (generating high frequency spectral components), then by a rapid increase and decrease during the flame-front phase and finally by a slow increase and decrease during the post-flame phase. 
         [0011]    Ionic currents in spark plugs may be measured at the secondary of the ignition coil, substantially by connecting a sensing circuit between the low-voltage terminal of the secondary winding of the ignition coil and ground, as schematically shown in  FIG. 3 . Such sensing circuits are used to monitor the ionization current after the combustion has occurred. 
         [0012]    It would be desirable a technique of sensing ionization currents crossing the electrodes of a spark plug during all phases of the combustion process. 
       SUMMARY 
       [0013]    The applicants observed that the traditional sensing scheme is unsuitable for sensing ionization currents during the first two phases, namely the ignition phase and the flame-front phase. High frequency components of the ionization current cannot circulate in the secondary winding of the ignition coil, thus they flow through parasitic capacitances towards the supply node of the first electrode  1  and then to ground and/or are dissipated as heat losses in the magnetic core of the ignition coil. Therefore, the sensing scheme of  FIG. 3  is useful to sense ionization currents only during the post-flame phase thereof, thus potentially useful information derivable from reading the ionization current during the ignition and the flame-front phases is not gathered. 
         [0014]    Novel architectures of spark plugs for implementing a method of sensing ionization currents also during the ignition and flame-front phases in a very simple way have been devised by the applicants. 
         [0015]    The novel spark plugs have a first electrode and a ground electrode insulated from one another by the dielectric material of the spark plug insulator in order to form a discharge gap at the inner tip of the insulator. 
         [0016]    Differently from conventional spark plugs, a resistive element is electrically connected to the ground electrode such that when the spark plug is mounted in an internal combustion engine, the ground electrode results electrically connected to the engine body through a resistive element interposed there between in the flow path of the ionization current toward the to ground node of the electrical system. Moreover, the ground electrode constitutes or is electrically connected to a second outer connection terminal, providing an accessible sensing terminal outside the combustion chamber. In this way, it is possible to monitor the ionization current even during the ignition and flame-front phases by sensing the voltage between the added sensing terminal end the ground electrode and the potential relative to the ground node of the engine body. 
         [0017]    According to an embodiment, the resistive layer is substantially cylindrical and is placed around a tract or substantially the whole length of the outer surface of the ground electrode and contacts the metal casing of the spark plug, whilst an outer end portion of the ground electrode has an electrical connection termination constituting the sensing terminal. 
         [0018]    The claims as filed are integral part of this description and are herein incorporated by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a classic structure of a spark plug. 
           [0020]      FIG. 2  is an exemplary graph showing the typical three phases of the ionization current during an engine cycle. 
           [0021]      FIG. 3  shows a classic scheme for sensing the ionization current. 
           [0022]      FIG. 4  shows an ignition control system and a circuit implementing the method of this disclosure for sensing ionization current. 
           [0023]      FIG. 5  shows an embodiment of a spark plug of this disclosure. 
           [0024]      FIG. 6  shows another embodiment of a spark plug of this disclosure. 
           [0025]      FIG. 7  shows yet another embodiment of a spark plug of this disclosure with an electrically resistive threaded ferrule in contact with the metal casing of the spark plug. 
           [0026]      FIG. 8  shows the equivalent circuit of the structure of  FIG. 7 . 
           [0027]      FIG. 9  shows yet another embodiment of a spark plug with an electrically insulating threaded ferrule and a sense resistor Rext connected between the metal casing of the spark plug and the engine body. 
           [0028]      FIG. 10  shows the equivalent circuit of the structure of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    A basic electric scheme implementing the novel method of sensing ionization currents to crossing the electrodes of a spark plug is shown in  FIG. 4 . 
         [0030]    Accordingly, a spark plug is equipped with a resistive element, Rsense, in the flow path of the ionization current toward the ground node of the electrical systems connected to the engine. 
         [0031]    According to an embodiment, the sense resistor is electrically connected to the ground electrode such that, when the spark plug is mounted, the ground electrode electrically connects to the body of the engine through the interposed resistive element. Therefore, the ionization currents flows in the resistive element Rsense on which a voltage drop proportional to the current may be read by a sensing unit. 
         [0032]    The novel sensing scheme is implemented using a modified spark plug architecture. 
         [0033]    An embodiment of a novel spark plug is shown in  FIG. 5 . The ground electrode  6  is not directly in contact with the metal casing  1 , but connects thereto through a resistive layer  8  applied over the outer surface cylindrical open-structured portion adapted to be embedded, alike the central electrode  3 , in a molded or cast insulating material constituting the insulator  2 . The resistive coating  8  may extend as far as the outer end of the metal casing  1  and the ground electrode  6  has an outer end flange, the rim of which emerges out of the cast insulator body  2 , for constituting an accessible sensing terminal  9  outside the combustion chamber. 
         [0034]    As in common spark plugs, the metal casing  1  is in contact with the metallic body of the engine, thus it is grounded upon mounting the spark plug in a cylinder head of the engine body. When a high voltage is applied on the terminal  4  of the first electrode  3 , a spark occurs in the discharge gap  7 , a ionization current flows through the resistive layer  8  and causes a voltage drop between the ground electrode  6  and the grounded metal casing  1 . This voltage drop is substantially proportional to the ionization current and may be read on the accessible end terminal  9  of the ground electrode  6 . 
         [0035]    The ground electrode  6  is at a relatively low voltage and may be connected to an electronic sensing circuit without any problem of electrical isolation. 
         [0036]    Though, in the novel spark plug the ground electrode  6  is not grounded when a ionization current flows, because of the non-null voltage drop on the resistive layer that constitutes the sense voltage, this does not affect the generation of the spark because the first electrode  3  is brought to a voltage greater than 10 kV and the voltage drop on the interposed resistive layer  8  is generally of few Volts. 
         [0037]    Differently from the known scheme of  FIG. 3 , wherein the circuitry does not allow the measurement of ionization current spectral components above few kHz because of the great to value of stray inductances, stray capacitances and of the magnetic losses in the bulky iron core of the transformer, the novel spark plug allows to implement the scheme of  FIG. 4  and thus to easily sense ionization currents even during the ignition phase and the flame-front phase because there is no low-pass filtering on the measured ionization current. Referring to  FIG. 5 , the sense voltage available on the sensing terminal  9  of the ground electrode  6  is proportional to the ionization current and may be used for accurately monitoring the evolution of the combustion of the air/fuel mixture in the combustion chamber. 
         [0038]    In the embodiment of  FIG. 5 , a substantially cylindrical resistive layer  8  covers the ground electrode  6 , substantially along the whole length thereof. 
         [0039]    According to another embodiment, depicted in  FIG. 6 , the cylindrical resistive layer  8  does not extend for the full length of inner part of the ground electrode  6 , but over a relatively short tract thereof, in correspondence of the coupling zone with the metal casing  1 , to which it mechanically and electrically connects. In this case, the remaining portion of the outer cylindrical surface of the foraminous ground electrode  6  becomes embedded in the dielectric material of the molded or cast insulator  2 . 
         [0040]    According to yet another embodiment, not shown in the figures, the resistive layer  8  may be in the form of a strip longitudinally interposed between the opposing cylindrical surfaces of the ground electrode  6  and of the metal casing  1 , the remaining portion of the outer surface of the ground electrode being mechanically coupled to the inner surface of the metal casing  1  through an intervening layer of dielectric material of the molded or cast insulator  2  casing. 
         [0041]    According to yet another embodiment, not shown in the figures, the accessible sensing terminal  9  may be in form of a radially extending lead integral to the metallic ground electrode, protruding out of a molded or cast insulator  2 , with a shape adapted to be easily connected with a spring clip to a sensing unit for reading the voltage thereon. 
         [0042]    The resistive layer  8  may be made of conductive ceramic or cermet material, for example a conductive ceramic material of sub-stoichiometric conductive oxides or mixed oxides and/or containing metallic micro or nano-particles, or a cermet of ceramic particles in a metallic matrix, or of high temperature resistant (&gt;200° C.) molded conductive resins, such as for example polyimide (TPI), polyetherimide (PEI), phenolic resins and mixtures thereof, of suitable resistivity. The electrical resistance of the resistive layer  8  of  FIGS. 5 and 6  should match the electrical constraints of the circuit that reads the sense voltage. 
         [0043]    Preferably but not necessarily, the resistive interlayer  8  has a resistance comprised between 10 and 500 Ω. Preferably, the material with which resistive interlayer  8  is made to should have a relatively small thermal coefficient in order to limit variations of resistance due to fluctuations of the working temperature to less than 10%. 
         [0044]    The resistive element Rsense of the scheme of  FIG. 4 , differently from the embodiments of  FIGS. 5 and 6 , may be realized even with other spark plug structures, for example according to another possible embodiment shown in  FIG. 7 , the resistive element may be in form of a resistive threaded ferrule-adapter  11  for mounting the spark plug into a threaded hole of a cylinder head of the engine and electrically connect the metal casing  1 , which in this case may be commonly shorted to the ground electrode  6  (i.e. without interposition of a resistive layer therebetween, as in the previous embodiments), to the metallic body of the engine. 
         [0045]    The sensing terminal may be constituted by a metal eyelet  10  over which tightens the metal casing  1  driving an outer threaded surface of the metallic ground electrode into the inner thread of the ferrule  11  made of suitably resistive material, as shown in  FIG. 7 . 
         [0046]    As shown in  FIG. 8 , being R the resistance opposed to the flow of current by the resistive ferrule  11 , during combustion phases, a ionization current flows to the metal body of the engine, passing through the resistive ferrule  11 , thus a sense voltage proportional to the ionization current is available on the metal casing  1  and on the metal eyelet  10  (if present). 
         [0047]    According to yet another embodiment, shown in  FIG. 9 , the metal casing  1  is practically isolated from the body of the engine by an insulating or highly resistive threaded ferrule  11 , and has a metal eyelet  10 , that is the casing, electrically connected to the grounded body of the engine by an external sense resistor Rext through which ionization currents may flow. The sense voltage is available on the metal casing  1  and on the metal eyelet  10 . The equivalent circuit is shown in  FIG. 10 . In this case, the insulating ferrule has a great resistance Rtr, thus practically the whole ionization current flows in the external sense resistor Rext. 
         [0048]    If an external sense resistor Rext is used as shown in  FIG. 9 , preferably it may have a resistance comprised between 10 Ω to 500 Ω and the material of the insulating ferrule  11  should have a resistivity such to make the insulating ferrule have an electrical resistance greater than at least 10 kΩ. 
         [0049]    The sense voltage made available with the novel spark plug structures allows to monitor the ionization current during all phases of the combustion and thus potentially to acquire information about the evolution of the combustion process from the ignition of the air/fuel mixture. This may be done for example during a test phase using one of the novel spark plugs and a pressure sensor to sense the pressure in the combustion chamber. The sense voltage to generated by the novel spark plug may be compared with the signal generated by the pressure sensor, with the objective of finding correlations between them. This would allow extrapolating useful information about the combustion process, when the engine is operating, directly from the sense voltage without using any expensive pressure sensor. 
         [0050]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.