BACKGROUND

Field

This invention relates to the fields of plasma generation, ignitions, and internal combustion (IC) engines. In particular, it relates, but is not limited, to ignition methods and ignition apparatus for use therein; and, specifically, to ignition methods and apparatus for various applications, including but not limited to, high pressure engines. More particularly, some aspects relate to the delivery of discharge current to traveling spark igniters in order to maximize their performance and longevity, especially in internal combustion engines operating at high pressures.

Discussion of Related Art

For a variety of reasons, there is interest today in increasing the pressures in internal combustion engines and similar combustion environments, with a concomitant need for ignition sources capable of operating in these environments. For example, automobile companies and manufacturers of internal combustion engines would like to be able to provide vehicles which have IC engines which operate at much higher pressures than conventional internal combustion engines. To date, however, there has not been an effective and practical ignition system for such engines. Among other concerns are longevity of igniters (spark plugs) and reliability of igniter firing.

The traveling spark igniter (TSI) is a device that has been discussed as a promising spark plug replacement for internal combustion engines, but previously not for high pressure engines. TSIs have, for example, been shown in a number of prior patents including, for example, U.S. Pat. Nos. 6,321,733 and 6,474,321, both assigned to the same assignee as this invention and incorporated by reference in their entireties for their explanations of TSI devices and ignition systems.

Briefly, a TSI-based ignition system provides a large plasma kernel which is propagated along the igniter's electrodes by Lorentz force (along with thermal forces, to lesser degrees) and propelled into a combustion chamber. The Lorentz force acting on the ignition kernel (i.e., plasma) is created by way of the discharge current in the plasma interacting with a magnetic field caused by that same current in the electrodes of the igniter. The magnitude of the Lorentz force is proportional to the square of that current. In engines operating at normal pressures (i.e., a maximum of about 120 psi), traveling spark igniters provide significant advantages over conventional spark plugs due to the large plasma volume they generate, typically some 100-200 times larger than in a conventional spark plug, for comparable discharge energy. Increased efficiency and reduced emissions are attainable.

For higher engine operating pressures, however, the breakdown voltage required for initiating the discharge between the electrodes of the igniter is significantly higher than in engines operating at conventional pressures. This creates problems for TSIs, as for any spark plug. The electrodes in a TSI, as in a conventional spark plug, are maintained in a spaced apart relationship by a member called an isolator, which is formed of an insulating material such as a ceramic. The higher breakdown voltage causes problems for both the isolator and the electrodes.

Along the surface of the isolator running between the electrodes, the breakdown voltage is lower than it is further along the electrodes in a TSI, or in any conventional spark plug with a similar gap between the electrodes. Indeed, this difference in breakdown voltages varies directly with increasing pressure in the combustion chamber. Consequently, although the breakdown voltage along the isolator surface increases with pressure, that increase is less than the increase in the breakdown voltage between the exposed part of the electrodes away from the isolator surface. When breakdown occurs (as a result of which the resistance through the plasma rapidly drops), the current rises rapidly and a very large current is conducted in the forming plasma along the isolator surface, thus giving rise to the Lorentz force acting on the plasma. Such rapidly rising current, though, creates not only a very high temperature plasma, but also a powerful shock wave in the vicinity of the surface of the isolator. The larger the current, the more rapid the plasma expansion and the resulting shock wave. These combined effects can cause deformation and/or breakage of the isolator.

Additionally, the high current produces very rapid erosion of the electrodes in the vicinity of the isolator surface, where they are attacked by the high current, thermal heating and thermionic emission that results therefrom.

Similar problems have been manifest with igniters based on the University of Texas “railplug” design which generates a Lorentz force in a plasma traveling along a high aspect ratio discharge gap (as contrasted with a TSI, which has a low aspect ratio discharge gap).

Although both the railplug and the TSI generate significant plasma motion at relatively low pressures, when the combustion chamber pressure is increased to a high pressure, the plasma behaves differently and it is this difference in behavior that leads to unsatisfactory results. In a low pressure environment, the force exerted on the plasma by the pressure is relatively small. The plasma moves easily along the electrodes in response to the Lorentz force. As the ignition chamber pressure is increased, however, that pressure provides a force of significant magnitude that resists the Lorentz force and, thus, plasma motion. Consequently, the plasma tends to become more concentrated, and to collapse on itself; instead of having a diffused plasma cloud, a very localized plasma—an arc—is formed between the electrodes below a certain current threshold. This arc, though occupying a much smaller volume than the plasma cloud of the low-pressure case, receives similar energy. As a result, the current density is higher and at the electrodes, where the arc exists, there is a higher localized temperature and more power density at the arc-electrode interfaces. That is, the current density is quite high at those interfaces, producing more localized heating of the electrodes than in the low pressure environment. The localized heating of the electrodes, in turn, produces thermionic emission of electrons and ions. The observed effect is that the arc appears to “attach” itself at relatively fixed locations on the electrodes, producing erosion of the electrodes as the entire discharge energy is deposited at the “attachment point;” this is to be contrasted with the low pressure environment where a lower density, diffused area of plasma contact moves along the electrodes without significantly damaging them.

Concurrently, the plasma, affected by the Lorentz and thermal forces, bows out from the arc attachment points. This causes the magnetic field lines to no longer be orthogonal to the current flow between the electrodes, reducing the magnitude of the Lorentz force produced by a given current. So, in addition to the other problems, there is a loss in motive force applied to the plasma.

Overall, there is a reduction in plasma motion as compared with the lower pressure environments, and dramatically increased electrode wear at the arc attachment points. Accordingly, a variety of needs exist, including needs for plasma generators, in general, needs for improved ignition systems, needs for ignition systems for use in internal combustion engines and needs for an ignition system and method which generates a large ignition kernel and which is usable with high pressure engines, and is commercially practical.

If a traveling spark igniter is to be used in a high pressure combustion environment, a need further exists to overcome the above negative effects on the isolator material and electrodes of the igniter. See U.S. Pat. Nos. 5,704,321, 6,131,542, 6,321,733, 6,474,321, 6,662,793, and 6,553,981, for example, incorporated by reference herein. That is, a need exists for an igniter and ignition system for use in high pressure combustion engines, wherein the isolator and electrodes exhibit substantial lifetimes (preferably comparable to that of conventional spark plugs in low pressure engines) without being destroyed by the discharge process. Desirably, such a traveling spark igniter and ignition system will be usable and useful in internal combustion engines operating not only at high and very high pressures (i.e., several hundred psi), but also at lower, conventional pressures.

SUMMARY

The above and other needs are addressed, and advantages provided, with a new method, and corresponding apparatus, for generating and sustaining a plasma, operating a traveling spark igniter and providing an ignition for internal combustion and other engines, particularly high pressure internal combustion engines. Typically, a high initial breakdown voltage is applied to the igniter to initiate a plasma kernel in a plasma initiation region of the igniter, but preferably at a current lower than that previously employed with TSI ignitions, as the breakdown current need not produce a large Lorentz force. After the breakdown current pulse, various mechanisms may be employed to prolong the plasma while recombination is occurring and to allow the plasma to become easily detached (or detachable) from the initiation region (typically, on or adjacent the surface of an isolator between the igniter electrodes. Before the plasma has a chance to recombine completely, the current is turned on again to provide a short follow-on pulse of energy (preferably at a current substantially less than that of the breakdown pulse). The follow-on current pulse generates a corresponding pulse of Lorentz force to move the plasma away from its previous location, further along the electrodes of the igniter. A number of such follow-on pulses may be provided, with an “off” interval between successive pulses, during which interval one or more mechanisms prolong the plasma and allow only partial recombination of the plasma. This is called “simmering.” Prior to total recombination of the plasma, the next follow-on pulse of current “kicks” the plasma even further along the electrodes; and the final follow-on pulse ejects the plasma from the electrodes. One mechanism for producing simmering is to reduce the current through the igniter to a relatively low (but non-zero) level, called a “simmer current.” Alternatively, if a summer current is not applied, similar effects may be obtained by using any of a number of other techniques for prolonging recombination and preventing “total” recombination of the plasma kernel by the time the next follow-on pulse arrives. For example, the follow-on pulses may be timed and possibly even waveform-shaped to more closely follow each other so that only partial recombination occurs between pulses; or each follow-on pulse may be preceded by a high sub-breakdown voltage; or the plasma may be excited by RF or laser energy. That is, numerous ways are contemplated of preventing total plasma recombination. By “total” in reference to recombination is meant that the plasma effectively has been extinguished and high energy is needed to reignite it.

The invention is manifested in several ways, or aspects, and example implementations are presented below. Other ways of practicing the invention will become apparent to those skilled in the art. The various aspects may be practiced alone or in any of many combinations, all of which cannot be reasonably enumerated here. It is intended that features of various embodiments be practiced in combinations other than those illustrated, not all features being shown in connection with all embodiments, for brevity. Aspects of the invention include the following, at least:

A method of plasma generation, comprising applying a high voltage to an igniter, said high voltage being of amplitude sufficient to cause breakdown to occur between the electrodes, resulting in a high current electrical discharge in the igniter in an initiation region, and formation of a plasma kernel adjacent said initiation region; and following breakdown, applying to said electrodes a sequence of at least two relatively lower voltage follow-on pulses, whereby the plasma kernel is forced to move toward a free end of said electrodes by said follow-on pulses.

A method of plasma generation, comprising applying a high voltage to an igniter, said high voltage being of amplitude sufficient to cause breakdown to occur between the electrodes, resulting in a high current electrical discharge in the igniter in an initiation region, and formation of a plasma kernel adjacent said initiation region; and following breakdown, applying to said electrodes a sequence of one or more relatively lower voltage follow-on pulses of current sufficiently low as to maintain a diffuse attachment of the current arc to the electrodes, whereby the plasma kernel is forced to, and can, move toward a free end of said electrodes under the influence of said follow-on pulses.

The initiation region may be on or adjacent the surface of an isolator disposed between said electrodes. A current of the follow-on pulses, for an internal combustion engine, may be between about 3 and 450 Amperes. The method may include preventing total kernel recombination of the plasma prior to at least one follow-on pulse. This may be done in various was, including between pulses of the sequence, maintaining a simmer current between the igniter electrodes sufficient to prevent total recombination of the plasma kernel. It also may include, in an interval between follow-on pulses, for at least part of said interval maintaining a voltage across electrodes of the igniter below a breakdown voltage but sufficient to sustain enough current to prevent total recombination before the end of the interval. The igniter may be a traveling spark igniter. Successive pulses in said sequence are separated by intervals of about 2-600 microseconds and preferably about 20-250 microseconds, most preferably 50-100 microseconds. Each of said follow-on pulses may have a maximum amplitude of about 3-450 Amperes. The amplitudes may not be uniform. The follow-on pulses may have a maximum amplitude of about 20-120 Amperes, which may not be uniform. Each of said follow-on current pulses preferably may have an average duration of less than about 200 microseconds, which may not be uniform. The follow-on pulses may have an amplitude of about 10-5000 V and preferably about 20-275 V. The follow-on pulses need not all have the same polarity of voltage and current and the currents of the follow-on pulses need not be constant.

A fuel ignition method, comprising applying a high voltage to an igniter in the presence of a combustible fuel, said high voltage being of amplitude sufficient to cause breakdown to occur between the electrodes of the igniter, resulting in a high current electrical discharge in the igniter in an initiation region, and formation of a plasma kernel adjacent said initiation region; and following breakdown, applying to said electrodes a sequence of two or more relatively lower voltage follow-on pulses, whereby the plasma kernel is forced to move toward a free end of said electrodes by said follow-on pulses. The initiation region may be on or adjacent the surface of an isolator disposed between said electrodes. The igniter may be in an internal combustion engine. A current of the follow-on pulses for a gasoline-fueled internal combustion engine, may be between about 3 and 450 Amperes. Preferably, said method includes preventing total kernel recombination of the plasma prior to a follow-on pulse.

Preventing total recombination may include, between pulses of the sequence, comprises maintaining a current (termed a simmer current) through the plasma kernel sufficient to prevent total recombination of the plasma kernel. Preventing total recombination of the plasma kernel also may include, in an interval between follow-on pulses, for at least part of said interval maintaining a voltage across electrodes of the igniter below a breakdown voltage but sufficient to sustain enough current through the plasma to prevent total recombination before the end of the interval.

Follow-on pulses need not all have the same polarity of voltage and current, which need not be constant.

The igniter may be in an internal combustion engine in which there is a relatively high pressure at the time of ignition.

The methods may further include, after a follow-on pulse, re-triggering or re-striking the plasma kernel at a time an ionization level of the plasma kernel has fallen below a desired level, with a current and at a relatively low voltage sufficient to cause the plasma kernel to grow before total recombination occurs, followed by a next follow-on pulse.

The methods also may include simmering the plasma kernel between at least some follow-on pulse pairs.

An ignition circuit for powering an igniter in an internal combustion engine, comprising means for providing a high voltage capable causing an electrical breakdown discharge, at a high current, between electrodes of an igniter, in an initiation region between said electrodes, when said igniter is disposed in a fuel-air mixture of an engine, whereby a plasma kernel is formed in said region by said discharge; and means for providing a sequence of one or more relatively lower voltage and lower current pulses having voltage and current amplitude and timing sufficient to force the plasma kernel to move toward a free end of said electrodes by said lower voltage, lower current pulses. The means for providing a high voltage capable of causing electrical breakdown discharge may include a high voltage, low inductance ignition coil having a primary winding and a secondary winding, the secondary winding having a lead for connection to one electrode of an igniter, and a circuit for triggering a signal in the primary winding to induce a high voltage pulse in the secondary winding. The means for providing a sequence of relatively low voltage pulses may comprise a relatively low voltage source and, for each said pulse, a capacitor charged by the relatively low voltage source and a pulse transformer having a secondary winding connected to said lead and a primary winding through which the capacitor is discharged in response to a trigger signal, inducing said pulse in said lead. The ignition circuit may further include means for providing to the igniter, in an interval between the breakdown discharge and a first follow-on pulse a simmer current sufficient to prevent total recombination of the plasma kernel in said interval. It also may include means for providing to the igniter, in an interval between each successive pair of follow-on pulses a simmer current sufficient to prevent total recombination of the plasma kernel in said interval. The ignition coil preferably includes a saturable core on which the primary and secondary windings are formed and the core substantially saturates when said electrical breakdown occurs, whereby the secondary winding thereafter has substantially reduced inductance.

An ignition circuit for powering an igniter in an internal combustion engine, comprising a high voltage pulse generator which generates on an output for connection to an igniter a pulse whose maximum voltage, when delivered to the igniter, is capable causing a breakdown discharge and consequent high current between electrodes of the igniter, in an initiation region between the electrodes, when said igniter is disposed in a fuel-air mixture, whereby a plasma kernel is formed adjacent said surface by said discharge; and a lower voltage pulse generator which generates on the output a sequence of one or more relatively lower voltage and lower current follow-on pulses having voltage and current amplitude and timing sufficient to force the plasma kernel to move toward a free end of said electrodes by said lower voltage, lower current pulses. There may also be included a simmer current source which supplies on the output line, in an interval between the breakdown discharge and a first follow-on pulse a simmer current sufficient to prevent total recombination of the plasma kernel in said interval. As well, there may be a voltage source which maintains between follow-on pulses, for at least a portion of an interval between said follow-on pulses, a voltage on the igniter electrodes below a breakdown voltage but sufficient to prevent total recombination of the plasma kernel during said interval.

An ignition circuit substantially as shown and described in the drawing figures, particularly any ofFIGS. 8-10.

The ignition circuit also may include means operable after a follow-on pulse, for re-triggering or re-striking the plasma kernel at a time an ionization level of the plasma kernel has fallen below a desired level, with a current and at a relatively low voltage sufficient to cause the plasma kernel to grow before total recombination occurs, followed by a next follow-on pulse.

