AT501531A1 - METHOD FOR IGNITING A FUEL AIR MIXTURE - Google Patents

Description

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The present invention relates to a method for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine, wherein the ignition of the combustion is timed under introduction of a laser light pulse into the combustion chamber and wherein the laser light pulse is designed such that it in the fuel-air mixture causes the formation of a plasma.

The plasma generated by the laser light pulse acts as a medium, which LaseIicht particularly well absorbed, thus improving the energy absorption by the fuel-air mixture and thus its ignition.

Generic methods have the problem that the application of these methods in internal combustion engines is associated with different disadvantages. For example, the combustion chamber window required for introducing the laser light into the combustion chamber is changed by the action of the laser radiation or by the chemical reactions of the combustion such that its permeability to the laser radiation decreases over time, which reduces the energy of the laser light introduced into the combustion chamber. Furthermore, the action of the laser light causes mechanical disturbances in the combustion chamber window, which in extreme cases can even lead to its destruction.

The object of the invention is to provide a method in which these problems occur, at least to a lesser extent.

This is inventively achieved in that the pulse duration of the laser light pulse is less than 1 nanosecond (ns). Surprisingly, it has been found in measurements by the applicant that for laser pulses with a pulse duration of less than 1 ns, caused by the laser beam damage to the combustion chamber window are smaller than is the case with longer pulses. Furthermore, it has surprisingly been found that a lower Oberflächenbelagsbildung is observed over time, whereby the combustion chamber window retains its permeability with respect to the laser radiation for longer periods. As a result, over time the energy introduced into the combustion chamber by the method according to the invention is greater than is the case in prior art methods.

Particularly pronounced, the advantageous effects occur when the pulse duration of the laser light pulse is less than 0.6 ns, preferably less than 0.5 ns. 57189 36 / fr 2 ··· ♦ ··· < so that it is preferably provided that the pulse duration of the laser light pulse is greater than or equal to 0.1 ns.

In a further variant of the invention, it is provided that the method for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine, wherein the ignition of the combustion is controlled in time under introduction of a Laseriichtpulses into the combustion chamber, in particular according to one of claims 1 to 3, wherein the Laseriichtpuls is designed such that it causes the formation of a plasma in the fuel-air mixture, wherein the laser light pulse is formed such that it has a temporally asymmetric imax

Intensity curve (l (t)), wherein the energy density (JI (t) dt) of the Laseriichtpulses of tStart

Pulse beginning (tstart) until the time (W) of the intensity maximum (lmax) is smaller than the tEnd

Energy density ($ I (t) dt) of the laser light pulse from the time (W) of the maximum intensity f max (lmax) to the end of the pulse (tE "d) · As formula: / max tEnd JI (t) dt < \ L {t) dt. (Start lmax

Intensity I is understood to mean the power P relative to the cross-sectional area A (I = £) of the laser light pulse. The power P is of course equal to the energy E per time t (P = j-). Pulse start tstart or pulse end tEnde is understood as the point in time at which the intensity has risen or fallen to 1 / e2 (e = Euler's number), ie approx. 13%.

This has the advantage that the rising edge which is necessarily steep in the case of such laser light pulses (the intensity maximum must be so large that it comes to plasma formation) results in a particularly rapid plasma formation, which is to be understood quickly with reference to the total pulse duration. As a result, the plasma is available sooner in relation to the total pulse duration and can therefore absorb more energy from the laser light pulse. In other words, the laser light pulse is designed in such a way that the energy introduced into the fuel-air mixture (per area) before the formation of the plasma is smaller than the energy introduced into the fuel-air mixture (per area) after formation of the plasma ,

As long as this criterion is met, the pulse duration can last up to 6 to 8 ns. However, the use of laser light pulses with the short pulse duration specified in claims 1 to 3 can be provided with particular advantage.

The advantage is the greater, the sooner the intensity maximum is reached. It is therefore particularly preferred for the laser light pulse to be designed such that the intensity maximum (lmax) is in the first quarter, preferably in the first sixth of the pulse duration (tEnd-tstart) of the laser light pulse.

The shaping of the laser light pulse can be carried out by measures familiar to the person skilled in the art, for example by a suitably high amplification of the active laser medium (eg by high doping) or by suitable choice of the relevant parameters of the passive-switching medium, such as initial and saturation transmission (T0 or TSat ) or thickness.

The method according to the invention in both variants is also particularly suitable for extremely lean fuel-air mixtures. For example, it can be provided that the air-fuel ratio (λ) is greater than 1.5, preferably greater than 1.8.

Especially with regard to lean mixtures laser light pulses with the specified short pulse duration are particularly well suited due to their steep rising edge. As a result, the plasma formation threshold is reached more rapidly and the plasma arises earlier in relation to the pulse length, whereby more pulse energy of the laser light pulse can be absorbed by the plasma.

In general, however, it can also be provided, after the plasma-generating laser light pulse, to send a further, preferably longer-lasting second laser light pulse into the combustion chamber in order to obtain a sufficiently large energy intake through the fuel-air mixture and thus the ignition of the fuel-air mixture. To ensure mixture.

The method is particularly suitable for gasoline or gasoline powered gasoline engines.

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Another advantage of the method according to the invention is that it can be used particularly well in internal combustion engines, in which mixture formation takes place in the combustion chamber.

In a further advantageous embodiment, it is provided that the mixture formation takes place by direct injection of liquid fuel, in particular gasoline, into the combustion chamber. So far, in the direct injection of gasoline the ignition to be made is a problem. The injected gasoline initially has a roughly club shape. There is a steep gradient of mixture ratios (in the core superfat, lean outside) which is both load- and temperature-dependent. The spark plugs commonly used in the prior art are difficult to place at the optimum ignition location, as this location is constantly changing in an unpredictable manner. Furthermore, spark plugs also present mechanical obstacles. Short circuits due to gasoline droplets can also occur. All these problems are avoided in the method according to the invention.

In the method according to the invention, the ignition energy in all variants and exemplary embodiments can be limited to approximately 0.1 to 30 mJ.

The invention further relates to an internal combustion engine which is designed such that it is suitable for operation with a method in one of the aforementioned embodiments. Further advantages and details of the invention will become apparent from the figures and the description of the figures. Show

1 is a schematic representation of an internal combustion engine according to the invention; FIG. 2 is a schematic illustration of a laser light pulse which is embodied according to the first variant of the invention and FIG

Fig. 3 in a schematic representation of a laser light pulse is formed according to the second variant of the invention.

1 shows a cylinder 1 of a (generally several cylinders having) not shown internal combustion engine. In the combustion chamber 2 3 laser light 4 is introduced via a combustion chamber window and focused on the focus volume 5. The laser light is thereby generated in a laser light source 6 (pump laser), which has a pump laser diode 7, a Peltier cooler 8 and a coupling optics for coupling the generated laser light into an optical fiber 10. The optical fiber 10 directs the laser light to the actual ignition laser 11. This has a coupling optics 12, a laser crystal 13 with dichroic coating, an (optional) telescope lens 14 and a

• Passive Q-switch 15. • • · · · · · · · · · · · · · · · · · · ····· The cylinder 1 facing surface of the Q-switch 15 is ground in the form of a Auskoppeloberfläche and suitably coated. Between the ignition laser 11 and the combustion chamber window 3 is still a Aufweiteoptik 16 is provided in order to achieve the lowest possible focus volume 5.

FIG. 2 schematically shows the shape of a laser light pulse according to the invention, the intensity l (t) being plotted on the ordinate and time t on the abscissa. The pulse duration of the illustrated laser pulse is approximately 0.9 ns. It is therefore a short laser pulse within the meaning of claim 1.

In Fig. 3, the pulse duration is about 7 ns. It is therefore a long laser pulse. The laser pulse is designed in such a way that its intensity maximum l.sub.1 lies in the first sixth of the pulse duration tEnd-tstart. The rapid increase in intensity ensures an early plasma formation (based on the total pulse duration).

Innsbruck, 18 February 2005 For the Applicant: The Representatives:

Patent attorneys,

Βιφι / Engelbert Hojinger

Magfoc ~

Dr. Diph-ll

Claims (10)

1. A method for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine, wherein the ignition of the combustion timed under Introducing a laser light pulse is carried out in the combustion chamber and wherein the laser light pulse is designed such that it causes the formation of a plasma in the fuel-air mixture, characterized in that the pulse duration of the laser light pulse is less than 1 nanosecond (ns). 2. The method according to claim 1, characterized in that the pulse duration of the laser light pulse is less than 0.6 ns, preferably less than or equal to 0.5 ns. 3. The method according to claim 1 or 2, characterized in that the pulse duration of the laser light pulse is greater than or equal to 0.1 ns. 4. A method for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine, wherein the ignition of the combustion is timed with the introduction of a laser light pulse into the combustion chamber, in particular according to one of claims 1 to 3, wherein the laser light pulse is designed such that it causes the formation of a plasma in the fuel-air mixture, characterized in that the laser light pulse is formed such that it has a time-inan asymmetrical intensity profile (l (t)), wherein the energy density (jl (t) dt) tStart of the laser light pulse from the beginning of the pulse (tstart) to the instant (W) of the tEnd intensity maximum (lmax) is less than the energy density (J / (f) λ) of the tnan laser light pulse from the time (W) of the intensity maximum (lmax) to the pulse end (^) , 5. The method according to claim 4, characterized in that the laser light pulse is formed such that the intensity maximum (lmax) in the first quarter, preferably in the first sixth of the pulse duration (tEnd - tstart) is the laser light pulse. 6. The method according to any one of claims 1 to 5, characterized in that the air-fuel ratio (λ) is greater than 1.5 - preferably greater than 1.8 - is. 57189 36 / fr »· · i» · · i • · • · > · · I ·· ·· 7. The method according to any one of claims 1 to 6, characterized in that the mixture formation takes place in the combustion chamber (2). 8. The method according to claim 7, characterized in that the fuel is gasoline and the mixture formation by direct injection of liquid fuel, in particular gasoline, in the combustion chamber (2). 9. The method according to any one of claims 1 to 8, characterized in that the internal combustion engine is a gasoline or gasoline-powered gasoline engine. An internal combustion engine adapted to be suitable for operation by a method according to any one of claims 1 to 9. Innsbruck, 18 February 2005 For the Applicant: The Representatives: