Patent Publication Number: US-6213085-B1

Title: Directed jet spark plug

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to spark plugs of the type that provide torch jet-assisted spark ignition of an air/fuel mixture within a main combustion chamber of an internal combustion engine. In particular, this invention is directed to a torch jet spark plug having a nozzle disposed at an angle to the axis of the plug, which enables flame propagation from the plug to be directed to a specific location within the combustion chamber. 
     BACKGROUND OF THE INVENTION 
     Spark ignition of an air/fuel mixture within a combustion chamber of an internal combustion engine typically involves igniting the air/fuel mixture with an electric spark jumped between an electrode and a ground electrode of a spark plug. An alternative to spark ignition known in the art is torch jet-assisted spark ignition which, as taught by U.S. Pat. Nos. 3,921,605 to Wyczalek, 4,924,829 to Cheng et al., 5,405,280 to Polikarpus et al., and 5,421,300 to Durling et al., offers several advantages over spark ignition approaches. As the name suggests, torch jet-assisted spark ignition utilizes a jet of burning gases that are propelled into the combustion chamber in order to enhance the burning rate within the combustion chamber by providing increased turbulence as well as presenting a larger flame front area. As a result of a faster burning rate, lower cyclic variation in cylinder pressure is achieved, which enables a higher engine efficiency with a higher compression ratio. 
     In a torch jet-assisted spark ignition system, the jet typically emanates from a combustion prechamber within the spark plug, passing through an orifice into the main combustion chamber. The axis of the orifice is parallel and often coaxial with the combustion prechamber. Though an air/fuel mixture can be introduced directly into the prechamber through a separate intake valve or fuel injector, it is generally preferable that the air/fuel mixture originates from the main chamber in order to simplify the construction of the engine and its ignition system. Combustion of the air/fuel mixture within the prechamber can be initiated from within by a separate igniter, or initiated by the flame front within the main chamber. With either approach, combustion typically proceeds relatively simultaneously in both the prechamber and the main chamber. However, because of the small relative volume of the prechamber, a high pressure is developed in the prechamber while the pressure is still relatively low in the main chamber. As a result, a jet of burning gases shoots from the prechamber far into the main chamber, significantly increasing the combustion rate in the main chamber. 
     Engine testing of torch jet spark plugs has verified that torch jet-assisted ignition results in faster burn rates than conventional spark ignition techniques, which produce a fixed flame “kernel” and relies on engine design to achieve suitable flame propagation within the main chamber. Torch jet-assisted ignition also relies on engine design considerations, which include tailoring swirl, turbulence and valve design to control the fuel/air charge for more complete and faster burns. Even with optimal engine design, there are typically regions within a main chamber in which the fuel/air mixture does not burn well, resulting in lower combustion efficiency. Accordingly, further enhancements in combustion efficiency using torch jet-assisted ignition would be desirable, the result of which would provide increased power, reduced emissions and better fuel economy for a given engine design. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a torch jet spark plug for use in a spark ignition system of an internal combustion engine. As with prior art torch jet spark plugs, the spark plug of this invention provides for the ignition of an air/fuel mixture within a combustion prechamber within the plug, and then propels the resulting burning gases through an orifice and into the engine main combustion chamber to increase the burning rate of the air/fuel mixture within the combustion chamber. However, the spark plug of this invention further promotes combustion efficiency by enabling the jet of burning gases to be selectively directed to any desired region within a combustion chamber, such as a region within the chamber that would not otherwise burn well compared to other regions of the chamber. 
     The spark plug of this invention generally includes a body having an interior chamber (“prechamber”) and an orifice in fluidic communication with the chamber for venting the chamber to the exterior of the body. Contrary to prior art torch jet spark plugs, the orifice is oriented in the body so that its axis is not parallel or coaxial with the longitudinal axis of the body, i.e., an angle of greater than zero from the longitudinal axis of the body. The orifice provides the only vent between the chamber and the exterior of the body, and may be disposed at an angle of up to about 30 degrees from the axis of the body. 
     The torch jet spark plug of this invention is capable of being used as a production plug or adapted for engine design and development. As a production plug, the body includes means for establishing the rotational orientation of the plug in a spark plug well, so that the orifice will be properly oriented to optimize the benefits gained by selectively directing the torch jet into the combustion chamber. In this embodiment, the position of the torch jet spark ignition device is preferably limited to a single orientation within its corresponding well. For design and development purposes, the body is used in conjunction with means that enables the orientation of the body to be selectively varied within a spark plug well, so that combustion conditions can be evaluated with the torch jet directed into different areas of a combustion chamber. In this embodiment, the torch jet spark ignition device is configured to be positively secured in any one of a plurality of orientations in the well. 
     In accordance with the above, the spark plug of this invention can be used to compensate in part for conventional engine design considerations, such as swirl, turbulence and valve design, to control the fuel/air charge for more complete and faster burns. Specifically, the spark plug can be oriented to promote combustion within a region of a combustion chamber in which a fuel/air mixture would not otherwise burn well, resulting in higher combustion efficiency. Simultaneously, jet velocities can be altered by tailoring the chamber and orifice sizes to achieve burn rates and intensities that are compatible with, and possibly augment the effects of, a particular burn direction. Accordingly, this invention enables significant enhancements in combustion efficiency using torch jet-assisted ignition, the result of which is increased power, reduced emissions and better fuel economy for a given engine design. 
     The spark plug of this invention also promotes engine design flexibility by permitting spark plug location to be determined by considerations other than spark location. Specifically, the angled orifice employed by this invention permits the selective “placement” of the torch jet in regions of the combustion chamber other than directly below the spark plug. As a result, spark location within the combustion chamber does not dictate spark plug placement at the expense of other considerations, such as accessibility for service, availability of cooling passages in the cylinder head, and avoidance of engine valves and head bolts. Accordingly, engine packaging and combustion performance can both be improved with the spark plug of this invention. 
     Another significant advantage of this invention is that the plug can be used during engine development and testing to generate combustion data for different flame propagation directions and rates within an engine without necessitating modifications to engine hardware. A particularly notable aspect of this capability is that the plug can assist in efforts to evaluate emission levels and knock-limited power levels, which depend in part on flame propagation and intensity. As a result, use of the plug of this invention during engine cylinder development is able to save time and reduce the costs required to optimize combustion chamber geometry. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows a cross-sectional view along the longitudinal axis of a torch jet spark plug in accordance with this invention; 
     FIG. 2 is a cross-sectional view of a spark plug well configured to receive the spark plug of FIG. 1 in accordance with one embodiment of this invention; 
     FIG. 3 is a cross-sectional view of a shell configured for assembly with the spark plug of FIG. 1 in accordance with another embodiment of this invention; and 
     FIG. 4 schematically shows results of varying the direction of burn within a combustion chamber using the spark plug of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in FIG. 1 is a torch jet spark plug  10  for use in a spark ignition system for an internal combustion engine. In accordance with torch jet-assisted ignition techniques, the torch jet spark plug  10  of this invention serves to increase the burning rate of an air/fuel mixture within a combustion chamber of an internal combustion engine by igniting an air/fuel mixture within a combustion prechamber  12  formed in the insulator body  14  of the spark plug  10 . While those skilled in the art will recognize that the present invention is constructed to be particularly suitable for use in an automotive internal combustion engine, the teachings of the present invention are also applicable to other spark plug configurations, as well as other applications which utilize internal combustion processes for power generation. 
     As with spark plugs typically used with internal combustion engines, the insulator body  14  is preferably formed of a ceramic material, such as alumina (Al 2 O 3 ). One end of the body  14  includes a passage  16  in which an upper terminal  18  is received, by which an electric voltage can be supplied to the spark plug  10 . As seen in FIG. 1, an electric voltage introduced at the upper terminal  18  is conducted to a center electrode  20  through a resistor material  22  disposed in the passage  16  in the insulator body  14 . The center electrode  20  protrudes into the prechamber  12 , which is located in the body  14  opposite the upper terminal  18 . The resistor material  22  is preferably a glass seal resistor material of a type known in the art, which provides electromagnetic interference suppression while also hermetically sealing the passage  16  from the prechamber  12 . 
     As depicted in FIG. 1, an inner electrode  24  is disposed on the internal surface  26  of the prechamber  12  surrounding the center electrode  20 , and an outer hollow electrode  30  is located on the wall of an orifice  32  to the prechamber  12 . The inner electrode  24  is in the form of an annular-shaped band that circumscribes the center electrode  20  to form a radial inner spark gap. The hollow electrode  30  is also in the form of an annular-shaped band and is interconnected with the inner electrode  24  by a conductive “stripe”  28  on the surface  26  of the prechamber  12 . As such, the hollow electrode  30  acts as an extension of the inner electrode  24 , and forms one electrode of an outer spark gap, which will be described below. The stripe  28  and the inner and hollow electrodes  24  and  30  are preferably formed by an adherent metal coating on the internal surface  26  of the prechamber  12 , such as in the manner taught by U.S. Pat. No. 5,421,300 to Durling et al. The inner and hollow electrodes  24  and  30  and the stripe  28  can be formed by a metal layer that substantially covers the entire internal surface  26  of the prechamber  12  below the center electrode  20  as taught by U.S. Pat. No. 5,405,280 to Polikarpus et al., such that an electrical capacitor is effectively formed. Various materials and processes can be used to form the electrodes  24  and  30  and stripe  28  in accordance with the teachings of Polikarpus et al. and Durling et al., both of which are incorporated herein by reference. 
     As shown in FIG. 1, the prechamber  12  is elongate and extends along the longitudinal axis of the insulator body  14 . The orifice  32  serves to vent the prechamber  12  to the main combustion chamber of an engine in which the spark plug  10  is installed. The orifice  32  allows for the intake of the air/fuel mixture during the compression stroke of a cylinder in which the plug  10  is installed, as well as the expulsion of combustion gases upon ignition of the air/fuel mixture within the prechamber  12 , which is initiated by the center and inner electrodes  20  and  24 . 
     As shown, the axis of the orifice  32  intersects but is oriented at an angle to the longitudinal axis of the insulator body  14 . While shown as being generally centrally located at the end of the body  14 , the orifice  32  could be radial offset. According to this invention, selective orientation of the plug  10  within a spark plug well, such as the well  34  shown in FIG. 2, can be used to optimize the burn direction and intensity within a combustion chamber  36  in which the plug  10  is installed. In conjunction with the orifice angle, the volume of the prechamber  12  and the area of the orifice  32  can be selected to provide the desired characteristics for a particular engine and effect that is of interest. For a given prechamber volume, a relatively small orifice diameter restricts the exit of gasses from the prechamber  12 , causing higher prechamber pressures and higher velocity jets when the plug  10  is fired, while a relatively large orifice diameter results in lower velocity jets. Excessively small orifices  32  restrict filling of the prechamber  12  during the engine compression stroke, especially at high engine speeds. Larger prechamber volumes produce longer duration jets, but introduce additional surface area to the combustion chamber, which is undesirable from the standpoint of heat loss and emissions. 
     From the above, it can be seen that there is no single preferred orifice angle, orifice diameter and prechamber volume combination for all engines, and persons skilled in the art will recognize that there are potential advantages of various combinations. For illustrative purposes, one such combination which has been found to perform suitably involves the use of a prechamber  12  whose volume is on the order of about 0.2 to about 0.4 cubic centimeters, in combination with a central orifice  32  having a cross-sectional area of about 1.7 to about 3.8 square millimeters and whose axis is disposed about 20 degrees from the longitudinal axis of the prechamber  12 . 
     The well  34  shown in FIG. 2 is configured for production, in the sense that a locating pin  38  is present within the well  34  for dictating the orientation of the plug  10  within the well  34 . For this purpose, the plug  10  is equipped with a suitable surface feature, such as the groove or recess  40  shown in FIG. 1 as being formed in the body  14 . In accordance with the embodiment of FIG. 2, only one orientation of the plug  10  within the well  34  is possible. The plug  10  can then be secured in the well  34  with any suitable means, such as the fitting  42  shown in FIG.  2 . The fitting  42  is threaded to allow tightening until a lower shoulder  44  of the fitting contacts the shoulder  46  of the plug body  14 . A gasket (not shown) formed of a suitable temperature-resistant material, such as copper or soft steel, can be positioned between the fitting  42  and the shoulder  46  of the insulator body  14  to create a gas-tight seal. 
     In the embodiment shown in FIG. 2, a ground terminal  48  is formed by the surrounding metal of the cylinder head. When the plug  10  is installed in the well  34 , the hollow electrode  30  is immediately adjacent and surrounded by the ground terminal  48 , such that the hollow electrode  30  and ground terminal  48  form an outer spark gap that is radially oriented in a manner somewhat similar to the spark gap between the center and inner electrodes  20  and  24 . 
     FIG. 3 depicts a shell  50  for use with the torch jet spark plug  10  of FIG. 1 in accordance with an embodiment of this invention intended for engine development and testing. The insulator body  14  of the plug  10  is installed and secured in the shell  50  with a locknut  56 . When assembled, the upper end of the body  14  extends through a reduced diameter section  60  of the locknut  56 , and a shoulder  62  of the locknut  56  engages the shoulder  46  of the insulator body  14  to secure the body  14  within the shell  50 . A gasket (not shown) of a suitable temperature-resistant material is preferably present between the shell  50  and the insulator body  14  to create a gas-tight seal. External threads  52  and  54  are formed at both ends of the shell  50 . As is conventional, the lower threads  52  are for the purpose of installing the spark plug  10  in a threaded portion of a spark plug well (not shown). The insulator body  14  will project through an opening  58  in the lower end of the shell  50  adjacent the threads  52 . The perimeter of the opening  58  serves as the ground terminal for the hollow electrode  30 , though it is foreseeable that other ground terminal configurations could be used. 
     Once the shell  50  (FIG. 3) is installed in the combustion chamber, the plug  10  (FIG. 1) can be inserted into the shell  50 , rotated to the desired jet direction, and locked into place with a locknut  56  threaded onto the upper set of threads  54 . Importantly, the plug  10  is not restricted by its configuration to any particular angular orientation within the shell  50 . As a result, the locknut  56  can be tightened to secure the plug  10  after the plug  10  has been properly oriented to direct the orifice  32  toward a desired region within the combustion chamber. 
     With either embodiment of this invention, it can be seen that, upon charging the prechamber  12  with a suitable air/fuel mixture from an engine&#39;s main combustion chamber during a compression stroke, an electric voltage supplied to the spark plug  10  via the upper terminal  18  will generate an electric spark at the spark gap between the center and inner electrodes  20  and  24 , which will ignite the air/fuel mixture within the prechamber  12 . Electric current is also then conducted along the metal stripe  28  to the hollow electrode  30 , where a second spark is generated to ignite the air/fuel mixture within the main combustion chamber. Though combustion proceeds relatively simultaneously in both the prechamber  12  and the main chamber, the smaller relative volume of the prechamber  12  results in a high pressure being developed within the prechamber  12  while the pressure within the main combustion chamber is still relatively low. As a result, a jet which initially includes an unburned portion of the prechamber&#39;s air/fuel mixture will be expelled from the prechamber  12 , become ignited by the external flame kernel of the outer spark gap, and then travel far into any predetermined region of the main chamber based on the angular orientation of the orifice  32 , thereby significantly increasing the combustion rate within the main chamber. 
     FIG. 4 represents information gathered from a series of tests using a torch jet spark plug similarly configured to that shown in FIG. 1, which was assembled with a shell similar to that of FIG.  3 . The orifice angle relative to the longitudinal axis of the prechamber  12  was about 20 degrees. The spark plug was indexed through eight different rotational orientations spaced about 45 degrees apart, and the engine run under identical conditions to evaluate what effect orifice orientation would have on the occurrence of engine knocking. As indicated, engine knocking occurred at four of the eight orientations. None of these events could have been predicted with any accuracy. To obtain the same test conditions without the spark plug of this invention, eight different cylinder heads would have to be fabricated at considerable cost and time. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, appropriate materials could be substituted, and the teachings of this invention could be employed in different environments. Accordingly, the scope of the invention is to be limited only by the following claims.