Apparatus for igniting the combustion of a fuel-air mixture

Abstract: A pulse forming device for the generation of alternating current pulses which are to be supplied to a gap of an ignitor in an internal combustion engine, comprising an electrical circuit including a capacitor (158) for storing a quantity of electrical energy, and a pair of electrical conductors (160, 164) for electrically connecting said capacitor (158) with said ignitor. The capacitor (122, 158, 144) is inserted and/or distributed along at least a portion of an electrical distribution cable (123, 146, 180).

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 igniting combustion of fuel-air mixtures, which generates a very rapid intense high power electric breakdown. The invention solves this problem by an aparatus 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 10 10 to 10 12 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 1 is a schematic diagram of an equivalent electrical circuit for generating a hard discharge ignition in accordance with the present invention; FIGURE 2 is a longitudinal sectional view of a distribution cable employing a pulse forming network (PFN) having lumped capacitance; FIGURE 3 is a perspective view, parts being broken away in section, of another distribution cable having a pulse forming network employing lumped capacitance; FIGURES 4A and 4B are longitudinal, sectional views of portions of distribution cables employing a pulse forming network having distributed capacitance; FIGURE 5 is a cross-sectional view of a termination connector for use with the distribution cable shown in FIGURE 4A; The rate of admission of energy into the breakdown channel in a spark gap must be maximized in order to achieve high power coupling efficiency and to maximize the intensity of the energy transfer mechanisms which are important in accordance with the present invention for ignition applications. This may be accomplished by using a very low inductance, low impedance, capacitive-discharge or driving circuit represented by the simplified equivalent model shown in FIGURE 1. As used in this description, the term "driving circuit" refers to all of the high voltage discharge circuit components, connecting conductors, and structures other than the breakdown gap and gaseous discharge path itself. Capacitor C represents the total effective discharge circuit capacitance, inductor L o represents the total effective discharge circuit inductance, and resistance R o represents the total effective discharge circuit resistance. The reactive term component of the characteristic impedance of the discharge circuit is expressed as: Z = L o /C C may be a discrete, lumped-element capacitor connected to the spark gap by means of a low inductance lead configuration, or it can be a distributed capacitance in the form of a very low-impedance, low-inductance waveguide structure which acts as a distributed pulse-forming-network (PFN). With operating voltages typically in the range of 20 to 40 kv, the magnitude of the capacitor C will fall within the range of approximately 100 picofarads to about 5 nanofarads. L o includes the inductance of all connecting conductors and the inductance associated with the discrete or distributed capacitive unit and must generally be on the order of a few hundred nanohenries or less. R o includes the resistance of the circuit conductors as well as the effective resistive loss associated with dielectric losses in the capacitive element. In practice, R o should be no more than a few ohms, and preferably should be minimized to the sub-ohm level. In general, this approach toward ignition system operation contrasts with the prior art approach which lays heavy emphasis upon higher impedance, higher inductance, lower capacitance driving circuitry and considerably longer discharge duration at lower intensities. The equivalent lumped circuit model components for the spark gap are indicated by dashed lines in FIGURE 1. Cg is the capacitance of the gap prior to breakdown and is typically on the order of 10 picofarads (10pf). C g is important for storing the charge needed during the very early stages of the breakdown channel formation, but the magnitude of C g is small compared to C and can be neglected once the early breakdown channel has been established. Closing of switch S b represents the onset of the breakdown event in which an ionized current flow path is formed between the spark gap electrodes. The detailed mechanisms involved in this process depend upon the conditions of the gas in the gap and the manner in which the voltage is applied. For purposes of this disclosure, it may be assumed that the establishment of current flow across the gap may be represented by the closing of the switch S b . C g is then effectively shunted by the time-varying channel inductance L g (t) and resistance R g (t). The circuit operation begins after capacitor C is charged to an initial voltage V o . For ignition applications of HDI, maximum performance is obtained using voltages from 20 kV to 40 kV, and a discharge circuit capacitance of 100 picofarads to several nanofarads, values of L/l g on the order of a few hundred nanohenries of discharge circuit inductance (L) per centimeter of discharge gap length (l g ), or less, preferably equal to or less than 80 nanohenries per centimeter, depending on the value of capacitance C and the effective working gap breakdown electric field E o . As a practical matter, reducing the overall circuit inductance to values of L/l g below approximately 10 nH/cm is quite difficult in high voltage discharge circuits where certain minimum physical spacing is required for electrical insulation. In fact, the breakdown channel itself typically has self-inductance on the order of 10nH/cm. In cases where insufficient hardness has been achieved despite the minimization of L/l g to practical limits, the major alternative for increasing hardness are to decrease the capacitance C and/or to effectively increase E o by overvolting the discharge gap. Investigations with hard open air discharges have shown that for values of C less than or approximately equal to 3 nanofarads, an increase in energy caused by increasing the working voltage V o and gap length l g yields a shorter discharge current duration and a longer duration of light output with light output in very hard discharges continuing well beyond the cessation of current flow (afterglow). Reference is now made to FIGURE 2 wherein a discrete capacitance PFN is disclosed. The PFN, generally indicated at 122 is formed in a coaxial cable 123 which connects a power supply (not shown) with a connector (not shown) which is adapted to connect the cable 123 with an ignitor. The PFN 122 comprises an inner conductor 130 surrounded by a sleeve 136 of high dielectric material, such as ceramic. A layer 134 of metalization on the outer surface of the dielectric sleeve 136 is connected with the outer conductor 127 and thus forms a continuous path for the flow of current through the cable 123. The inner conductor 130 is of substantially larger diameter than the central conductor 128 of the cable 123 and is connected at its ends to the central conductor 128 as by welding or the like. A layer of dielectric potting compound 132 surrounds the connection between the central conductor 128 and inner conductor 130. Inner connector 130 in combination with the dielectric sleeve 136 and metalization 134 forms a capacitor which is in close proximity to the ignitor 52. Although the PFN 122 provides a discharge circuit of high impedance and inductance, it possesses the advantage of providing an ignitor which is relatively small and eliminates the problem of deliterious effects on the capacitor by additional heat to which it is subjected if positioned contiguous to the combustion chamber. Another form of discrete capacitance PFN is depicted in Figure 3. The PFN 144 is connected in series with the coaxial power supply cable 146 which connects the power supply (not shown) with a coaxial ignitor 52. The PFN 144 comprises first and second sets of flat plate capacitors 152, 154 which are interleafed and spaced apart using a dielectric material 156 to form a series of capacitor plates. Plates 12 are connected with the outer conductor of cable 146 while capacitor plates 154 are connected with its central conductor 148. A distributed capacitance PFN 158 is depicted in FIGURE 4A, which is formed integral with the distribution cable connecting the ignitor with the high vdltage power supply. The cable including the PFN 158 is substantially flexible but yet does not possess a diameter too large to be used in existing automobile engines. The PFN 158 comprises a stripline geometry in which a plurality of flexible, outer foil conductors 160 are interleafed with a plurality of inner foil conductors 164 and are separated therefrom by a plurality of layers 162 of dielectric material such as a polyamide film. The foil conductors 160, 164 may extend a substantial portion of the length of the entire cable and the sandwiched construction is enclosed by an outer rubber or plastic jacket 166. As shown in FIGURE 5, the stripline configuration may be terminated in a connector 168 which is adapted to. releasably connect the cable with an ignitor. The inner foil conductors 164 are terminated in a single connection which is secured to a center conductor 172 which in turn is connected with a metal contact 174 disposed within a cap 176 which fits over the terminal end of the ignitor. The foil conductors 160 are terminated in a connection with lead lines 170 within the cap 176. Contacts 174 and lead lines 170 respectively interconnect with the electrodes of the ignitor. Another form of distributed capacitance PFN is depicted in FIGURE 4B. The PFN comprises a coaxial cable 123 which is connected to an ignitor (not shown) by a connector 138. The connector 138 includes an outer threaded coupling 142 which is threadably received by a portion of the ignitor, and an inner electrical connecting portion 140 which electrically connects the electrodes of the ignitor with the central conductor 128 and outer conductor 127 of the cable 123. The inner and outer conductors 127 and 128 form the distributed capacitance.

Claims: Apparatus for igniting the combustion of a fuel-air mixture in a spark gap of an ignitor in an internal combustion engine, comprising an electrical discharge circuit including a capacitor (122,144,158) for storing a quantity of electrical energy, and a pair of electrical conductors for electrically connecting said capacitor (122,144,158) with said ignitor (52), said capacitor (122, 158, 144) being inserted and/or distributed along at least a portion of an electrical distribution cable (123, 146, 180) for connecting said capacitor (122,144,158) with a high voltage source, characterized in that the capacitor has a capacitance of 100 to 5000 pi-cofarad, whereas the ratio of the inductance of said discharge circuit including the capacitor (122,144,158) to the length of said spark gap is less than approximately 80 nanohenries per centimeter. The apparatus of claim 1, characterized in that said distribution cable is a coaxial cable (123) the inner conductor (130) of which has an increased diameter on a certain length and is surrounded by a sleeve (136) of high dielectric material having an outer metallization layer (134) which is connected with the outer conductor (127) of said coaxial cable (123), the portion of increased diameter being connected at its ends with the inner conductor (128) of small diameter, the connection being surrounded by a layer (132) of dielectric potting compound. The apparatus of claim 1, characterized in that said distribution cable is a coaxial cable (146, 180) whose outer and inner conductor (148) is connected in series with first and second sets of flat interleafed and spaced apart capacitor plates (152, 154), respectively. The apparatus of claim 3, characterized by a stripline capacitor (158) formed integral with said coaxial distribution cable (180) in which a plurality of outer foil conductors (160) forming said first capacitor plates are interleafed with a plurality of inner foil conductors (164) forming said second capacitor plates and are separated therefrom by a plurality of layers (162) of dielectric material. The apparatus of claim 4, characterized in that the stripline capacitor (158) terminates in a connector (168) for the connection with an ignitor, said inner foil conductors (164) terminating in a single connection with a center conductor (172) within a cap (176) of said connector (168), and said outer conductors (160) terminating in a connection with lead lines (170) within said cap (176), said center conductor (172) and said lead lines (170) being connectable with the electrodes of said ignitor. The apparatus of claim 1, characterized in that said capacitor is formed by an inner and an outer conductor (127, 128) of a coaxial distribution cable (123) which terminates in a connector (138) having an electrical connecting portion (140) for connecting the electrodes of the ignitor with the conductors (127, 128) of said coaxial distribution cable (123).

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