TECHNICAL FIELD

The present invention relates generally to an internal combustion engine having a sparkplug with an improved firing face.

BACKGROUND OF THE INVENTION The traditional ignition system consists of a sparkplug acting as an electrical feed into a combustion chamber of a gasoline engine, providing concentrated thermal energy release, in the form of a spark, to initiate the combustion. A conventional transformer is used, incorporating electronics to condition and to feed energy from the electrical network in vehicles (Battery, Generator). Typically breaks down voltages are in the range of 30 to 4OkV for spark air gaps of lmm. The required released thermal energy is a result of the magnetic energy which was stored before this event.

Future direct injection combustion systems are targeting stratified fuel injection, promising significant fuel reduction at low speed and partial load. These stratified gasoline mixtures tend to be difficult to ignite with existing ignition systems, mostly because of the limited energy and the very local distribution of the released heat of the spark itself. Furthermore the uncertainty, hence variation of the mixture stoichiometry from cycle to cycle, from wet to rich to lean leading to unacceptable engine and emission conditions. Ignition alternatives are intelligently controlled resonance structures with the help of high frequency electromagnetic fields, which allow emitting heat over larger space, constant over time, and at extended time compared to existing ignition systems. These systems require at minimum a resonance structure, a driving generator and electrodes providing the interface to release the heat. It is well known that with the help of resonance structure high voltages can be generated. The electrode design, commonly called firing face, is of significant importance for the overall performance and has to be optimized in combination with the typical conical cloud like fuel mixtures of a stratified injection strategy inside the combustion chamber. Generally, the electrode configuration of such resonance ignition systems is providing the least interaction with the stratified spray, except of the release of heat under severe conditions. The pressure at which it has to properly ignite the gas mixture may go up to 3MPa at temperatures of approx 400 ⁰ C. It is an objective of the present invention to improve the efficiency of the sparkplug in a simple and cost-effective way, by proposing a spark-plug firing face design for high frequencies in conjunction with various injection strategies, but in particular for stratified fuel injection strategies.

SUMMARY OF THE INVENTION

In order to reach the above mentioned objective, the present invention provides an internal combustion engine having a resonance based ignition system comprising a sparkplug with a center electrode and a ground electrode facing the combustion side of the engine, fuel being injected in the combustion chamber of the engine in the form of a conical stratified spray, characterized in that the ground electrode has a tubular shape coaxial with regards to the center electrode delimiting a coaxial air gap and has an end surface of substantially conical shape, and in that the outer air gap between the end surface of the ground electrode and the mantle line of the conical stratified spray has a substantially conical section. The conical shape of the ground electrode lower end prevents hindering the access of the stratified spray to the lower extremity of the spark plug, in order to place the thermal energy "spark" close by the stratified gasoline spray. According to other features of the present invention:

- the substantially tubular space between the center electrode and the ground electrode is filled with a low loss dielectric material, except at its end portion where the coaxial air gap is delimited;

- the capacitance of the coaxial air gap is at least 500 times smaller relative to the capacitance of the resonating structure of the ignition system;

- the coaxial air gap has a radial dimension included between 0,5 mm and 1,5 mm;

- the conical air gap between the mantle line and the conical coaxial ground electrode varies between 0 mm and 3,5 mm;

- the angle between the mantle line and the end surface of the ground electrode is substantially 30 degrees;

- the ground electrode general axis and the conical stratified spray general axis are substantially coplanar;

- the resonance structure of the ignition system is operated under Industrial Scientific and Medical purposes frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which: - figure 1 is a bloc-diagram showing a resonance based ignition system;

- figure 2 is a bloc-diagram showing the resonating structure of the ignition system of figure 1 ;

- figure 3 is a cross sectional view showing a sparkplug according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown on figure 1 , a resonance based ignition system comprises a resonator 2, a generator 1 and a heat release interface 3. The basic operation is: the generator 1 is feeding its high frequency electromagnetic energy into a resonator 2. The resonator is accumulating this energy and, once the breaks down conditions are met, a plasma is built up at its designed firing face 3. This firing face 3 has to sustain two distinct phases of one single heat release event (spark). The first phase can be characterised by the built up of high voltage at the output of the resonator 2 such that the surrounding gas gets ionised and a second at which the ionised gas is fed continuously by energy from the generator 1.

The resonance structure is operated beneficially under ISM (Industrial Scientific and Medical purposes) radio bands, which were originally reserved internationally for the use of RF electromagnetic fields for industrial, scientific and medical purposes other than communications. Beneficial but not exclusive are the following ISM frequencies: 13,56MHz, 27,12MHz, 40,68MHz. As a consequence the interface 3 is operated with electromagnetic waves of the ISM frequency. The electrodes of this interface are exposed to the stratified gas mixture. The electrodes represent an additional capacitive firing interface (4) for the resonating structure 5, 6, 7, using lumped electrical elements, as shown in figure 2. The elements 4, 5, 6, 7 are described as well by a capacitive loaded lossy quarter wavelength transmission line with a termination of an electrical open circuit condition. This electrical open-circuit termination is practically built out of the mechanical geometries of the electrode configuration, called firing face, with the surrounding gas condition as dielectric filler material. Both mechanical geometries and the dielectric filler are representing a capacitance 4 at which the plasma built up takes place.

The additional capacitor 4 is variable in its value due to the fact that its dielectric permittivity is changing with the permittivity of the surrounding gas mixture. Generally dielectric permittivity is split into two fractions, one which describes the ability to store electromagnetic energy and a second fraction which describes the ability to conduct it and therefore creating thermal energy losses. A few different mechanisms are explaining this behavior of dielectric materials. In this case we are observing two effects affecting the resonance structure, while a stratified gas mixture approaches the electromagnetic field of the electrode configuration. One fraction of permittivity of the gas mixture increases the capacitance, hence lowering the resonance frequency of the structure (see equation 1). Another fraction of the permittivity is creating an electric dissipative loss path through the affected gas mixture which is in reach of the electrodes.

Equation 1 : the resonance frequency is: J

ILC

An electrode design is superior, if the dynamic change of its capacitance, due to a permittivity changes derived from gas mixtures, has its least impact with regards to the resultant resonance shift. Therefore the capacitance of the electrode design has to be significantly inferior to the capacitance of the entire resonance. An electrode design is superior, if most dielectric loss can be directed into the gas, creating thermal losses (means heat generation) to support the desired ignition function.

A new electrode configuration according to the invention is shown in figure 3. It shows a fraction of the electrical feed through and the electrode configuration relative to the stratified spray. The electrical feed through consists of a conductive center electrode 8 and a coaxial ground electrode 9. The lower end surface 15 of the ground electrode 9 is of conical shape, forming an annular conical outer surface. The coaxial tubular space 14 between both elements 8, 9 is filled preferably with low loss dielectric materials such as a ceramic insulator as in standard spark plugs. It represents a fraction of the capacitance 7.

The air gaps 10, 11 consist of two elements. The first element, an inner air gap 10, is a small cavity in the lower portion of the spark plug and represents a capacitance 4 with coaxial geometry. The second element is an outer air gap 11 which results out of the relative position of the stratified mixture 12, here in form of a cone in relation to the ground electrode 9. The outer air gap 11 has a conical, or truncated, shape.

The arrows Fl, F2 on figure 3 represent the mantle line of a conical shaped fuel spray 12 as typically used in stratified injection regimes. The spray 12 is coming out of the nozzle of an injector. This spray 12 is very wet at its mantle line; inside its cone is air with little gas. The spray 12 has a very high velocity at the very beginning of its injection, due to the fuel pressure, and gets slower after some millimeters of travel inside the combustion chamber.

The air gap thickness 13 between the mantle line of the conical stratified spray 12 and the conical coaxial end surface 15 of the ground electrode 9 is increasing outwardly from the spark plug axis along the end surface 15 between two values, preferably between 0mm and 3,5mm. Preferably, there is an angle al of approximately 30 degrees between the mantle line and the conical coaxial end surface 15 of the ground electrode 9, as shown on the figure. According to a preferred embodiment, the ground electrode 9 general axis and the conical stratified spray 12 general axis are substantially coplanar. According to alternative embodiments, the arrangement of said axes could be different from coplanar provided that the mantle line of the conical stratified spray 12 and the conical coaxial end surface 15 are close by. Preferably, the capacitance of the coaxial air gap 10 is at least 500 times smaller relative to the capacitance of the resonating structure 7.

Advantageously, the coaxial air gap 10 has the following dimensions: 0.5mm<d< 1.5mm where "d" is the radial dimension and an axial depth included between lmm and 2mm. The conical, or truncated shape, air gap 11 between the mantle line and the conical coaxial end surface 15 of the ground electrode 9 has a minimal air gap thickness 13 of Omm and a maximal air gap thickness 13 of 3,5mm. The initial plasma formation occurs inside the coaxial air gap 10. It provides a protected niche partially isolated from the turbulences originated by the dynamic gas flow inside the combustion chamber. Because the applied high frequency electromagnetic fields are preferably conducting thru media with high dielectric loss, any fuel droplet and fuel gaseous constituents will be additionally heated which are in reach to these electromagnetic field.

This plasma initiation niche allows stabilizing the system behavior in regards of dynamic change of the resonance effect and in addition to take benefit of typical high frequency break down effects that are reducing the break down voltages.