The present invention concerns an internal combustion engine with a laser light generating device.
Conventional laser light generating devices which are used in the area of laser ignition for internal combustion engines generally have a laser resonator which is so designed that the laser light delivered by the laser light generating device has a Gaussian profile (TEM00-mode structure), that is to say, the intensity distribution falls transversely in an exponential configuration. Furthermore laser light generating devices with an unstable laser resonator are frequently also used in particular in relation to pulsed lasers. That resonator concept also involves a transversely varying intensity distribution over the beam cross-section.
A serious obstacle in regard to use of laser-ignited internal combustion engines, which is suitable for large-scale use, lies in the low level of efficiency with which the laser light is introduced into the plasma volume which is to be heated up for reliable ignition of the fuel-air mixture. Those losses result on the one hand from the transmission losses of the laser radiation which passes through the ignition volume prior to the laser-induced plasma breakdown, and on the other hand from losses which are caused by the laser radiation laterally passing the plasma volume by virtue of an excessively small plasma size or a focus geometry which is laterally excessively far extended.
The object of the invention is to further develop an internal combustion engine of the general kind set forth, in such a way that the level of efficiency with which the laser light is used for ignition is increased.
That object is attained by an internal combustion engine having the features of claim 1.
With an internal combustion engine of that kind it is possible for the minimum energy necessary to produce a plasma core to be introduced into the combustion chamber by laser radiation which has a TEM00-mode structure. That laser radiation has ideal focusing properties.
Furthermore the total energy necessary to produce an ignitable flame core is introduced into the combustion chamber in the form of a higher-energy laser light which is later formed, with a mode structure of higher transverse order. That occurs only when producing a sufficiently large plasma volume so that losses from radiation which passes the plasma beforehand in respect of time or laterally in respect of space are accordingly minimised.
In accordance with this disclosure the term transverse mode structure is used to denote the intensity pattern of an electromagnetic beam in a plane perpendicularly (that is to say transversely) with respect to the direction of propagation of the beam. The mode structures which can be produced by a laser resonator are of the transverse electromagnetic type (TEM).
The structure of the TEM modes is different depending on the respective symmetry of the laser resonator.
The TEM00-mode is the fundamental mode with a Gaussian profile.
Preferably both the laser light with a TEM00-mode structure and also the laser light having a mode structure of higher transverse order are delivered in the form of pulses.
Preferably both the laser light with a TEM00-mode structure and also the laser light having a mode structure of higher transverse order are generated with the same laser light generating device. For that purpose it is necessary to provide that the laser light generating device has a laser resonator which is so designed that it can be operated stably in relation to at least two modes of different transverse order of the laser light which can be delivered.
Preferably a laser resonator of Fabry-Perot type is used in the invention.
The designation ‘transverse’ relates to any direction in right-angled relationship with the optical axis of the laser resonator.
It is known from the literature that, in the case of laser systems with passive quality switching by means of saturatable absorbers a sequential succession in respect of the production of different mode structures can occur (R Wu, T L Chen, J D Myers, M J Myers, C Hardy, Multi-Pulses Behavior in an Erbium Glass laser Q Switched by Cobalt Spinal, AeroSense 2003, SPIE Vol 5086, Orlando, Fla., Apr. 21-25, 2003).
That however involves unplanned effects which occur in a different mode structure and with an unintentional temporal sequence and the marketness of which moreover is not optimised to that for the production and post-heating of a laser-induced ignitable plasma.
For the preferred embodiment of the invention there is proposed a laser light generating device whose configuration of the laser resonator permits variability in respect of the stability condition of the laser resonator, by virtue of a suitable configuration of the optical surfaces. In that respect it is provided that the optical surfaces of the laser resonator are of such a configuration and are arranged relative to each other such that the beam diameter of a light beam introduced into the laser resonator is variable. Such a laser resonator is also known by the term ‘telescope resonator’.
A laser medium and a passive saturatable absorber are arranged in the laser resonator of such a laser light generating device, wherein preferably a respective surface of the laser medium and the absorber are of such a configuration and arrangement that they form a mirror of the laser resonator. In quite general terms it can be provided that the optical surfaces of the laser resonator are formed by the surfaces of the laser medium and the absorber.
It will be appreciated that alternatively it is also possible to provide a separate optical system, in particular separate mirrors.
In that respect the configuration of the surfaces of the active laser medium and/or the passive saturatable absorber in the preferred embodiment has on the one hand the function of determining the geometrical variability of mode production, which is necessary for the stability to be set for the laser resonator.
On the other hand, transverse variability of the passive saturatable absorber is to be guaranteed to the effect that reliable production of a TEM00-mode can take place in a first step. Then, with a delay corresponding to the time development of the plasma core, a higher-energy radiation with a mode structure of higher transverse order can be generated, with the purpose of ensuring increased heating of the plasma to the temperatures necessary for reliable ignition of a fuel-air mixture, by more efficiently coupling in the laser light and avoidance of temporal or spatial passage losses.
The production of sequential pulses by the build-up of different modes and the different temporal behavior, resulting therefrom, of the saturatable absorber, aims specifically at first deliberately exciting the production of the TEM00-mode necessary for ideal focusability, in order to ensure plasma formation at the laser focus. Thereafter modes of higher order are to be deliberately excited for production thereof in order to permit further heating of the plasma which has already formed. In that respect the production of modes of higher transverse orders additionally permits more efficient utilisation of the overall volume of the active laser medium.
For optimum utilisation of the laser energy of the following pulses the time spacing (delay), reckoned between the end of the preceding pulse and the beginning of the following pulse, should be between the pulses 100 ns-200 ns (nanoseconds), preferably 30 ns-70 ns. Within that delay the radiation of the subsequent pulses couples efficiently to the existing plasma of the preceding pulse without itself having to reach the high threshold intensity necessary for plasma formation. Therefore even poorly focusable transverse modes of higher order can also contribute to plasma heating. In the case of longer delays of over 200 ns the plasma has cooled down to such an extent that the laser radiation no longer couples and passes through the resulting hot gas volume, without plasma formation. In that case the threshold intensity necessary for plasma formation is even higher than in the normal situation.
The specific production of the TEM00-mode can be achieved in the preferred embodiment (telescope resonator) by the structural measures set forth hereinafter.
By virtue of the provision of the curved surfaces of the laser medium and the saturatable absorber, a beam path is forced to occur in the laser resonator, which is suitable for altering the stability condition of the radiation circulating in the laser resonator. That is achieved by suitable selection of the values for the curvature and the spacing of the optical surfaces which form the telescope. In that respect the stability of the laser resonator is to be so adjusted that the production of radiation in higher transverse modes is not suppressed, but the configuration in principle of the laser resonator in the form of a hemispherically stable resonator allows the production of a TEM00-mode.
In order to achieve the production of the TEM00-mode specifically prior to the production of a mode structure of higher transverse order, modifications can be made to the laser medium (modulation of amplification) and/or the saturatable absorber (modulation of the losses):
In an embodiment the laser medium is such that, by virtue of a variation in the concentration of the laser-active materials, the absorption of the pump radiation produces an excitation energy distribution in such a way that excitation both of the TEM00-fundamental mode and also modes of higher transverse order is guaranteed. Consequently the geometry of the laser resonator is to be so designed that the production both of the TEM00-fundamental mode and also modes of higher transverse order is guaranteed.
In a further embodiment the saturatable absorber is designed in such a way that the initial transmission in the regions which are covered by the TEM00-mode is kept higher than in the regions which are passed through in the production of modes of higher transverse order. The increased initial transmission in those spatial regions can be achieved by virtue of a special design for the saturatable absorber, such as for example by a reduction in the optical path length in the saturatable absorber or by a reduction in the concentration of the doping ions, which are necessary for the saturatable absorber to function, in the form of a gradient profile. That achieves a respective saturation intensity for the absorber, which varies in the transverse direction.
A simplified possible way of altering the effective cross-section in the absorber along the radial co-ordinate—that is to say in the transverse direction—is achieved by fitting into each other saturatable absorbers with differing doping (step profile). Laser modes which are propagated in the outer region of the absorber consequently pass through spatial regions of different saturation intensity and therefore start to oscillate in time-displaced relationship.
In order to adjust the time delay of the delivery of the laser light with a differing mode structure, preferably between the pulses, to a spacing which is necessary for reliable ignition, it may be necessary for the build-up characteristics of the modes of higher transverse order to be specifically controlled in respect of time. If therefore the TEM00-mode should start to oscillate excessively early in comparison with higher modes, the production of the modes of higher transverse order is also to be made possible, by virtue of modulation of the amplification or loss cross-sections, in the direction of easier build-up relative to the TEM00-mode. Depending on the respective pump geometry and excitation energy distribution in the laser medium it may therefore be necessary to increase the loss mechanisms in the saturatable absorber for the TEM00-mode. That is appropriately effected by prolonging the optical path in the saturatable absorber or by a higher level of concentration of the absorber ions at the center with a constant geometry.
A further possible way of definedly exciting the production of time-displaced laser radiation with differing transverse mode structure involves the use of in particular radially or transversely differing levels of reflectivity of the coupling-out mirror. Such mirrors are used in laboratory lasers with what are referred to as unstable resonators. They have a radially varying reflectivity in order to stimulate build-up of the laser along the optical axis. Variants with a different reflectivity variation and on a curvature designed as a stable resonator are suitable in principle for the production of multiple pulses and can be produced more easily in accordance with the state of the art than non-homogeneous doping properties of the crystals.
When involving homogeneously doped laser crystals and saturatable absorbers of uniform thickness as well as coupling-out mirrors which are coated evenly with a constant reflectivity it is also possible to achieve multiple pulse production by the non-homogeneous distribution of the pump light. Passively quality-switched lasers with stable resonators have a tendency just to produce the TEM00-mode or simultaneous build-up of a plurality of transverse modes. In order to guarantee the production of time-displaced modes of higher order, the pump light distribution can be non-homogeneous in such a way that, by virtue of the use of suitable optical elements in the beam path of the pump laser, only the energy necessary to build up the TEM00-mode is coupled in along the optical axis, but an increased proportion of the pump energy is distributed into the volume of higher transverse modes. In principle that additional optical system also allows controlled distribution of the pump energy and affords a possible way of controlling the time spacing of the pulses. Alternatively non-homogeneous light distribution could also be afforded by a plurality of pump light guide fibers or pump light sources which light to varying degrees. It will be appreciated that a combination of various ones of the above-indicated measures can also be used for producing the laser light with a transverse mode structure which changes in respect of time.
A great advantage of the variant of the laser light generating device in which the laser light is delivered pulse-wise is that firstly the efficiency of utilisation of the laser energy is markedly increased by multiple pulse production (a short, well-focusable pulse for laser generation is followed by a second pulse in order to increase the energy content of the laser or at least to maintain it over a longer period of time) and secondly by virtue of the spatially different propagation or form of the laser modes the plasma is to be markedly enlarged in its volume, which is of advantage in particular in terms of the ignition of lean mixtures.
In addition a laser of that kind can have a positive influence on the unwanted effect of deposits at the combustion chamber window used as the energy density at the window is distributed to two or more pulses.
By way of example laser light which in time sequence has a TEM00-mode structure and a TEMp=0, l=8-mode structure could be introduced into the combustion chamber of the internal combustion engine. Light with a TEMp=0, l=8-mode structure has approximately the structure of a hollow cylinder, wherein there are tangentially a plurality of zero locations.
Protection is also claimed for a method of igniting a fuel-air mixture in the combustion chamber of an internal combustion engine, in particular according to one of claims 1 through 11, by the laser light delivered by a laser light generating device, wherein the transverse mode structure of the laser light is varied in respect of time.
By way of example with a method of that kind it can be provided that firstly to produce a plasma in the fuel-air mixture laser light with a TEM00-mode structure is introduced into the combustion chamber and then to post-heat the plasma laser light with a mode structure of higher transverse order is introduced into the combustion chamber.
Further advantages and details of the invention will be apparent from the Figures hereinafter and the related specific description in which:
a and 1b show two different embodiments of the laser resonator which is to be used in an internal combustion engine according to the invention,
a-d show diagrammatic views of a saturatable absorber, the operative cross-section of the absorber in a transverse direction, the intensity of the delivered laser light in dependence on time and the spatial structure of the delivered laser light in dependence on time,
a-d show diagrammatic views of a saturatable absorber, the operative cross-section of the absorber in a transverse direction, the intensity of the delivered laser light in dependence on time and the spatial structure of the delivered laser light in dependence on time, for a further embodiment of an absorber,
a shows a variant in which the reflectivity of the surface coating of the output mirror is varied in such a way that both pump light distribution and also laser threshold permit multiple pulse production,
b shows a view relating to the varying reflectivity of the surface coating of
a and 6b show embodiments of an internal combustion engine according to the invention.
a shows an embodiment of a laser light generating device 1 according to the invention including a laser resonator 2 of the length L and a coupling-in optical system 3 for the radiation of a pump laser (not shown in
In
The laser light generating device 1 shown in
As an alternative to the measure of a homogeneously doped passive absorber 5 as adopted in
In
In
a shows that different reflectivity in respect of the coating 27 of the coupling-out mirror 7 of the laser light generating device 1 can be so selected that different laser thresholds occur in spatially different regions of the resonator.
b shows how, depending on the respective distribution of the intensity of the pump light, a reduction (broken line) or an increase (solid line) in the degree of reflection (Ref) can be necessary at the edge (−r, +r) with respect to the optical axis (o) in order to permit time-displaced build-up of the laser. The radially varying reflectivity (Ref) of the coupling-out mirror 7 can naturally also be produced in some other way, apart from a coating 27.
a shows an internal combustion engine 15 with a laser light generating device 1 according to the invention. The laser light 16 delivered by the laser light generating device 1 is introduced into the combustion chamber 21 of a cylinder 22 by way of a light guide 17, an enlargement optical system formed by the lenses 18 and 19 and a combustion chamber window 20. In that arrangement the combustion chamber window 20 is of such a configuration that the laser light 10 is focused on the focus volume 23 in the combustion chamber 21.
The variant shown in
Number | Date | Country | Kind |
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A 1559/2005 | Sep 2005 | AT | national |