Apparatus for initiating combustion of fuel-air mixtures in an internal combustion engine

Abstract: The invention relates to an apparatus for igniting the combustion of fuel-air mixtures in an internal combustion engine, comprising an electrical circuit including a capacitor (100, 108, 115) for storing a quantity of electrical energy, a discharge device having a pair of spaced electrodes (104, 120) forming a gap (118) across which an electrical discharge channel of alternating electrical current may be established for initiating said combustion using electrical energy stored in said capacitor (100, 108, 113), and a pair of electrical conductors for electrically connecting said capacitor (100, 108, 113) with said electrodes (104, 120) for forming the discharge channel, said capacitor (100, 108, 113) and said conductors being arranged concentrically around a common axis. In this apparatus an insulator body (114) is disposed between said electrodes (104, 120) and said capacitor (100, 108, 113) and between said conductors.

Description: This invention relates to an apparatus for initiating combustion of fuel-air mixtures in an internal combustion engine as defined in the preamble of claim 1. An apparatus of this kind is known from US-A-4,333,125. Furthermore, it is referred to Combustion and Flame 27, published 1976, R. Knystautas and J.H. Lee, On the Effective Energy for Direct Initiation of Gaseous Detonations, pages 221-228. The known apparatus includes a capacitive portion for storing a large quantity of electrical energy therein derived from a power supply, and an electrode portion integral with the capacitive portion which comprises a pair of concentric, rod shaped electrodes for producing a high energy, umbrella shaped plasma discharge, using the inverse pinch technique. Due to the close proximity between the capacitive and electrode portions of the initiating device, rapid energy transfer from the former to the latter creates high magnetic pressures which transform the discharge into a high energy plasma jet which is delivered well into the combustion area. According to this principle, despite the fact that the ignition can be enhanced, the power coupling efficiency from a relatively high impedance ignition source circuit to the very low impedance of an established discharge channel is quite low, resulting in a greater fraction of the available energy being lost through power dissipation in circuit resistance other than the discharge channel itself. Somewhat greater power dissipation in the discharge channel can be achieved by increasing the magnitude of current flow. However, for a given discharge duration, this may be accomplished only at the expense of greater energy input requirements and severe electrode wear. Accordingly, it is the problem to be solved by this invention to create an apparatus for initiating combustion of fuel-air mixtures, which generates a very rapid intense high power electric breakdown. The invention solves this problem by an apparatus having the features of claim 1. Further developments of this apparatus are described in the subclaims. The ignition apparatus of the present invention employs a hard-discharge-ignition (HDI) process which is generated by a very rapid, intense, high-power electrical breakdown which we shall refer to as a "hard" spark discharge. HDI initiation of combustion employs highly effective energy coupling mechanisms which reach high levels of intensity. The term "hard-discharge" as used herein refers to the regime of operation in which the discharge circuit inductance and resistance are sufficiently low that the rate of current flow and rate of energy deposition in the discharge channel during the breakdown phase are largely governed by the resistance of the spark channel itself. This extreme regime of operation is characterized by highly efficient coupling (80-95%) of the initially stored electrical circuit energy, during approximately the first half-period of the discharge current cycle, into the various transient processes associated with gaseous discharge formation and expansion. As a result, hard-discharge operation delivers most of the available pulse energy within the breakdown phase of the discharge (usually within the first few tens of nanoseconds of the discharge), thereby achieving maximum power coupling from the driving circuit to the rapidly dropping effective load impedance of the discharge channel. Using typical discharge circuit energy levels of between 0.05 to 2 joules, and with rates of rise of breakdown current flow on the order of 10¹⁰ to 10¹² amperes per second, the resulting power deposition can approach an order of 10's of megawatts within the time span of a few 10's of nanoseconds. Additionally, the greatly enhanced speed of the overall combustion event significantly reduces the amount of ignition timing advance necessary for MBT (maximum brake torque) operation with a given fuel-air mixture. Depending upon the mixture ratio, engine conditions, and HDI energy and power level, the need for timing advance may be entirely eliminated. Consequently, highly efficient engine operation is provided with significantly reduced ignition timing advance. In the drawings, which form an integral part of the specification and are to be read in conjunction therewith, and in which like reference numerals are employed to designate identical components in the various views: FIGURE 1A is a fragmentary, cross-sectional view of a firing tip geometry which forms a portion of a hard discharge system of the present invention; FIGURE 1B is an end view of the firing tip shown in FIGURE 1A; FIGURES 1C-J are views similar to FIGURE 1A but depicting alternate forms of geometry for the firing tip; and FIGURE 2 is a longitudinal sectional view of an ignitor unit employing an integral discrete, lumped capacitance, pulse forming network. Attention is now directed to FIGURE 1 wherein various forms of a discharge tip are depicted. Certain constraints must be placed on the gap between the electrodes at the discharge in order to achieve HDI operation. The predominant factors affecting HDI operation are the value of the inductance of the overall ignitor unit and a gap length sufficient to hold off the voltage level applied to the electrodes. These criteria may be satisfied by numerous discharge tip and gap geometries, providing that inductance and impedance are maintained below a prescribed value. However, it is desirable to provide a geometry and configuration which maximizes the efficiency with which the available circuit energy is coupled into the discharge, and from the discharge to the combustible mixture via light, heat, shock and ion production. Discharge tip geometry also affects longevity of the ignitor in terms of insulator and conductor wear due to the presence of extremely hot plasma and strong shockwave production. Discussed hereinbelow are two preferred forms of discharge tip designs which are highly suitable for achieving HDI operation. One of the tip designs is depicted in FIGURES 1A and 1B and consists of inner and outer coaxial electrodes 80, 76 which are electrically insulated from each other by a cylindrically shaped insulator 82. The outer cylindrical wall of the outer electrode 76 is provided with a thread form 78 which is adapted to be matingly received in an engine block or the like in order to mount the ignitor so that a discharge tip communicates with the combustion chamber. The outer ends of electrodes 76 and 80, as well as the insulator 82, extend along a common plane or flat surface 84. The discharge gap formed by ignitor tip 74 is radial and extends circumferentially around the entire surface 84. Consequently, the electrical field indicated at 85 commences at the outer end of electrode 80 and possesses a radially outward trajectory to all points on the outer electrode 76 along its upper surface 84. The ignitor tip 74 possesses minimum inductance and impedance because of the coaxial geometry of electrodes 76, 80 and the radial nature of the gap. The physical gap length of ignitor tip 74 is given by the difference in conductor radii b-a shown in FIGURE 1B. The gap length will be selected in accordance with the voltage pressure conditions of the particular application and anticipated operating conditions. The wall thickness and nature of the insulator 82 must be selected so as to assure that breakdown between the electrodes 76, 80 does not occur along their lengths. It should be noted that for a coaxial geometry both the inductance and impedance are determined in large part by the natural logarithm of the ratio of conductor radii b/a and that the inductance and impedance may be minimized provided the difference in conductor radii, b/a equals the required thickness of the insulator 82 for internal voltage hold-off. The electric field created by the voltage applied to electrode 76, 80 is shown at 85, with arrows indicating the direction that a positive test charge would move in the field (from positive to negative polarity). The field 85 is non-uniform, moving outwardly away from the surfaces 84, and it is believed that this non-uniformity in addition to the curvature of the lines of the field enhance the resulting discharge. The sharply curving nature of the field 85 changes the characteristic breakdown potential of the gap, accelerates charges moving in the field and tends to push the arc channel outwardly away from the tip due to magnetic forces, particularly where large current densities exist in the discharge. Moreover, the linear flow of current through the central or inner conductor 80 produces a magnetic field which interacts with the fields produced by the discharge to further enhance the discharge. The flat, radial design of ignitor tip 74 tends to produce a discharge which a spatial symmetry and uniformity which maximizes the volume of fuel mixture which is contacted by the discharge. The smooth, unobstructed surface 84 precludes any detrimental effects due to flow conditions within the combustion chamber and exposes larger electrode surface for participation in the discharge, which has a tendency to prolong the life of the electrode. The ignitor tip 74 may be modified in various ways to further enhance its operation. For example, as shown in FIGURE 1C, either or both of the outer ends of the electrodes 76, 80 might be pointed, as at 86, 88 in order to further "peak" the field 85. In other words, the field would tend to emanate from the peaks of the pointed tips 86, 88. In order to avoid possible trenching of the insulator 82 at the surface 84, the outer edge of the insulator 82 may be slightly recessed at 90 as shown in FIGURE 1D. As shown in FIGURE 1E, the discharge gap could be lengthened without increasing wail thickness by extending the insulator 82 outwardly beyond the outer surfaces of electrodes 76, 80; this design would be particularly effective in low pressure combustion environments or where higher breakdown voltage is required. Conversely, as shown in FIGURE 1F, the outer ground electrode 76 might be offset at 96 without comprising the internal hold-off voltage in those cases where lower voltage or higher compression operation is desired. An alternative approach for lengthening the discharge gap consists of recessing the center electrode 80 from the end of the insulator 82 and outer electrode 76, as shown in FIGURE 1G. A pronounced "jet" action due to the resultant cavity above the center electrode 80 has been noted with ignitors of this type. This jet is not likely due to an expulsion of plasma from the cavity, but rather is caused by reflected shockwaves initially trapped during the channel expansion and/or possibly a stream of heavy ion species originally moving along electric field lines but at a later time following trajectories dictated by their inertia once the field has diminished. To avoid excessive wear on the insulator 82, such insulator could be contoured at 83 as shown in FIGURE 1H to present a tapered surface extending from the end of center electrode 80 radially outward to the outer electrode 76. The geometry shown in FIGURE 1H provides the advantage of a recessed design which reduces insulator wear, but retains the jet or cannon line discharge effect. Extension of the center electrode 80 beyond the end of the outer electrode 76 as shown in FIGURE 1I also provides a means of increasing the discharge gap length. The tapered outer surface 85 of the insulator 82 again reduces wear on the insulator. Such an extension of the center electrode 80 into the combustion chamber assists in coupling and transferring the discharge energy to a fuel charge and is relatively unconfined. As previously mentioned hereinabove, various ignitor tip and discharge gap configurations may be successfully employed to achieve HDI operation and in some cases it may be desirable to employ a linear or longitudinally extending tip gap. One suitable tip design employing a linear gap is shown in FIGURE 1J. The ignitor shown in FIGURE 1J is broadly similar to conventional spark plug designs, with the outer electrode 76 having an L shaped extension 76a which provides an electrode surface axially aligned with the center electrode 80. Although the configuration shown in Figure 1J may be employed with beneficial results in connection with the present invention, it is not the preferred form of ignitor geometry and in any event, it is necessary to minimize inductance and impedance in those components of the ignitor which are directly adjacent to the discharge gap while at the same time allowing sufficient gap length for breakdown at peak voltages. In connection with the linear gap geometry, discharge occurs with virtually no wear upon the insulation due to arc while a desirable cylindrical shockwave is produced which is impeded only in the direction of the extended ground electrode. This exposure of the entire breakdown path lends itself to strong coupling and efficient energy exchange. Multiprong designs can be used in order to increase ignitor life inasmuch as there are additional surface areas between which a discharge can occur. It is important to orient these extra electrodes such that the discharge is not impeded in its growth nor shielded from the fuel charge thus prohibiting or quenching combustion promoting reactions. FIGURE 2 discloses a coaxially configured ignitor 98. The integral PFN-ignitor (PFN = Pulse Forming Network) 98 achieves the lowest possible inductance and therefore provides maximum coupling to the discharge channel. Additionally, a later discussed capacitive portion of the ignitor 98 need not be assigned to have an extended service life since it is removed and replaced periodically when the ignitor tip becomes worn and requires replacement. The ignitor 98 includes a cylindrical outer electrode 100 formed of metal or the like and includes a reduced diameter portion 104 at one end thereof which is connected to the larger diameter portion by a radially extending shoulder 105. The smaller diameter portion 104 is threaded at 107 so as to be threadably received within an engine block or the like. The outer end of the larger diameter portion of the electrode 100 is threaded at 102 so as to threadably connect with a power supply distribution cable. A central, metal electrode 108 is cylindrical in shape and is disposed coaxially within the outer electrode 100. One end of the central electrode 108 includes a reduced diameter extension 120 which is received within a passageway 118 and an insulating sleeve 114 which is secured within the reduced diameter portion 104 of the outer electrode 100. One end of the central electrode 108 is beveled around its entire circumference 109 and a suitable dielectric potting compound 116 is interposed between the end of the insulator 114 and the beveled surface 109 of the central conductor 108. The outer end of the central electrode 108 is defined by a reduced diameter portion or tip 111 which terminates at its outer end in a hemispherical surface 112. The base of the central electrode 108 surrounding the tip 111 is defined by a ring-shaped, radially extending shoulder 110. The outer end of the electrode 100 extends longitudinally approximately the same length as the tip 111 of the central electrode 108. A ring-shaped body 113 formed of a ceramic capacitor compound is disposed between the outer electrode 100 and central electrode 108. Body 113 extends the full length of the outer electrode 100 from the base or shoulder 105. The outer end 106 of body 113 extends beyond the outer longitudinal extremities of tip 111 or electrode 100. The central electrode 108, outer electrode 100 and capacitor compound 113 form the capacitive portion of the PFN.

Claims: Apparatus for igniting the combustion of a fuel-air mixture in an internal combustion engine consisting of a capacitor (100,108,113) and a discharge device (104,120,114) each of which comprising a central conductor (108,120) surrounded by an outer conductor (100,104) and an insulating material (113,114) between the two conductors (100,108;104,120), said capacitor and discharge device being coaxially arranged such that the two central conductors (108,120) are integrally formed along a common axis thereby forming a single central conductor, and such that the two outer conductors (100,104) are integrally formed with each other, the free ends of the central and outer conductors (120,104) of the discharge device forming the electrodes of a spark gap (118), characterized in that the insulator material (114) of the discharge device fills the space between the discharge device conductors (104,120) and extends axially from the capacitor to a position near the free ends of the discharge device conductors (104,120). The apparatus according to claim 1, characterized in that said capacitor (100,108,113) and said insulator material (114) of the discharge device (104,120,114) overlap each other along their common axis. The apparatus according to claim 2, characterized in that the single central conductor (108,120) has a larger-diameter portion (108) in the capacitor and a reduced-diameter portion (120) in the discharge device, and that a dielectric material (116) is arranged around the central conductor (108,120) in an axial position between the larger-diameter portion (108) and the insulator material (114) of the discharge device. The apparatus according to claim 3, characterized in that said larger-diameter portion (108) has a bevelled end surface (109) facing toward said dielectric material (116). The apparatus according to anyone of the claims 1 to 4, characterized in that said discharge device has an outer diameter reduced with respect to the outer diameter of said capacitor. The apparatus according to anyone of the claims 1 to 5, characterized in that the central and outer conductors (80,76) and the insulating material (82) of said discharge device have outer ends within a common plane (84). The apparatus according to claim 6, characterized in that either one or both of the outer ends (86,88) of said conductors (80,76) are pointed. The apparatus according to anyone of the claims 1 to 5, characterized in that the outer end (90) of the insulating material (82) of the discharge device is reduced with respect to the outer ends of its conductors (76,80). The apparatus according to anyone of the claims 1 to 5, characterized in that the outer ends (92,94) of the conductors (76,80) of the discharge device are recessed with respect to the outer end of its insulating material (82). The apparatus according to claim 7 or 8, characterized in that the outer end of the insulating material (82) of the discharge device presents a tapered surface (83,85) extending from a position adjacent the outer conductor (76) to a position near the outer end of the central conductor (80). The apparatus according to claim 6, characterized in that the outer conductor (76) of the discharge device is offset at its outer end in a radially inward direction and that the insulating material outer end has a correspondingly bevelled peripheral edge.

Inventors: HENSLEY RAYMOND E, PATE ROLAND C, HENSLEY, RAYMOND E., PATE, ROLAND C.
Applicants: HENSLEY PLASMA PLUG, HENSLEY PLASMA PLUG PARTNERSHIP DBA HDI RESEARCH
Application year: 1985
Main classification: F02P9
IPC codes: F02P3, F02P9, H01T13, F02B1