JP2017106406A - Laser ignition device and internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser ignition device for performing ignition at several points under application of simple configuration.SOLUTION: This invention relates to a laser ignition device 301 comprising one light source 201 for injecting a light L'; an optical transmission member 204 for transmitting the light L' injected from the light source 201; optical elements 205a, 205b arranged on an optical path of the light L'; a resonator 206 to which the light L' passed through the optical elements 205a, 205b is inputted; at least one of wavelength conversion elements 207a, 207b for converting a wavelength of the light L injected out of the resonator 206 into a first wavelength and a second wavelength; a condensation optical system 205c for condensing the light L passed through the wavelength conversion elements 207a, 207b; and condensation positions 01, 02 and 03 set by the optical system 205c are placed on an optical axis of the light L and they are formed at [n+1] locations in respect to the number [n] of the wavelength conversion elements 207a,207b.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a laser ignition device and an internal combustion engine including the laser ignition device.

In recent years, a generator engine for a cogeneration system that achieves high efficiency and low NOx has been developed. In order to generate combustion with high efficiency, an ignition device having a high combustion rate and excellent ignitability is required, and a laser ignition device using a laser crystal that oscillates by photoexcitation has attracted attention.
However, if only a single ignition position is ignited, the initial flame kernel cannot grow sufficiently due to a decrease in the growth rate of the flame kernel due to a decrease in the fuel concentration or the inflow of high-concentration fuel. There is a concern that sex cannot be obtained.
In order to solve such a problem, for example, a method of performing multi-point ignition using a plurality of laser light sources and optical systems has been considered (see, for example, Patent Documents 1 and 2).

  However, the configuration using a plurality of light sources and optical systems has a problem that adjustment of each optical system is complicated.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a laser ignition device that performs ignition at a plurality of points using a simple configuration.

In order to solve the above-described problems, a laser ignition device according to the present invention includes a light source that emits light, a light transmission member that transmits light emitted from the light source, and an optical element disposed on an optical path of the light. An element, a resonator to which the light passing through the optical element is input, and at least one wavelength conversion element that converts the wavelength of the light emitted from the resonator into a first wavelength and a second wavelength When,
A condensing optical system for condensing the light transmitted through the wavelength conversion element, and a condensing position by the condensing optical system is on the optical axis of the light, and the number of the wavelength conversion elements n + 1 locations are formed for n.

  According to the laser ignition device of the present invention, ignition at a plurality of points is performed using a simple configuration.

It is a figure showing an example of the whole composition of the internal-combustion engine concerning the embodiment of the present invention. It is a top view which shows an example of a structure of the laser ignition apparatus shown in FIG. FIG. 3 is a plan view showing an example of the configuration of the laser oscillator shown in FIG. 2. It is a top view which shows an example of the focus position in a combustion chamber when igniting using the laser ignition device shown in FIG. It is a figure which shows the comparative example of a laser ignition device. It is a figure which shows the modification of the laser ignition device shown in FIG. It is a figure which shows an example of the adjustment mechanism shown in FIG.

Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows an engine 300 as an internal combustion engine using the laser ignition device 301 according to the first embodiment of the present invention.
The engine 300 includes a combustion chamber 304, a fuel ejection mechanism 302 that ejects a combustible mixture obtained by mixing fuel and air into the combustion chamber 304, and laser ignition that condenses laser light L and ignites the mixture. Device 301.
The engine 300 includes a piston 305 that is pressed downward in FIG. 1 when the air-fuel mixture burns, and an exhaust mechanism 303 for exhausting residual gas after combustion.

The operation of engine 300 will be described.
In the initial state, it is assumed that the piston 305 occupies the fully raised top dead center, and the combustion chamber 304 is in an exhausted state, that is, an exhaust state.
From the exhaust state, first, the fuel ejection mechanism 302 ejects a mixture of fuel and air into the combustion chamber 304 and the piston 305 descends (intake process).
Next, the piston 305 rises and compresses the air-fuel mixture (compression process).
The laser ignition device 301 emits a laser beam L that is a laser beam, and in the vicinity of the condensing position that is the irradiation point of the laser beam L, a part of the air-fuel mixture is turned into plasma and a flame nucleus is generated. As a result, the air-fuel mixture is ignited (ignition process).

As the combustion of the air-fuel mixture proceeds, the volume increases in the process of changing the air-fuel mixture into combustion gas and presses the piston 305 downward (combustion step).
After the piston 305 is lowered to the bottom dead center, the piston 305 rises again due to inertia. At the same time, the exhaust mechanism 303 exhausts the residual gas after combustion to the outside of the combustion chamber 304 (exhaust process), and returns to the exhaust state. .

The engine 300 obtains kinetic energy according to the vertical movement of the piston 305 by repeating the above-described intake process to exhaust process.
For example, natural gas or gasoline is used as the fuel.
In the present embodiment, the engine 300 is described as a four-stroke engine using a combustible air-fuel mixture, but may be other internal combustion engines.

As shown in FIG. 2, the laser ignition device 301 includes a surface emitting laser 201 that is a light source that emits excitation light L ′, and an incident-side condensing optical system 203 on which the excitation light L ′ emitted from the surface emitting laser 201 is incident. And have.
The laser ignition device 301 has an optical transmission member 204 that transmits to the laser resonator 206 via the incident side condensing optical system 203.
The laser igniter 301 includes a first lens 205a, a second lens 205b, and a laser resonator that is a resonator that is excited by the excitation light L ′ and emits the laser light L, disposed on the emission surface side of the light transmission member 204. 206.
The laser ignition device 301 includes a first wavelength conversion element 207a and a second wavelength conversion element 207b that are wavelength conversion elements that change the wavelength of the laser light L emitted from the laser resonator 206, and a third lens 205c that is a condensing optical system. And have.
In the following, if there is a particular need, the optical axis direction of the laser beam L is the Z axis, and among the directions perpendicular to the Z axis, the upper direction in FIG. 2 is the X axis, and the direction perpendicular to the X axis and the Z axis is The Y axis is assumed.

The surface-emitting laser 201 is a vertical-cavity surface-emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser).
The surface emitting laser 201 is an excitation light source, and has a plurality of light emitting units arranged within the effective diameter of the incident side condensing optical system 203 (9 mm in this embodiment).
Since the plurality of light emitting units emit light at the same time in this embodiment, the surface emitting laser 201 has a function as a single light source for emitting one excitation light L ′.
Thus, by arranging a plurality of light emitting units within the effective radius, it is possible to increase the light output while operating as a single light emitting source.
Specifically, the surface emitting laser 201 has a function as a light source that emits excitation light L ′ having a wavelength of 808 nm and a light output of about 200 W.

The incident side condensing optical system 203 is an optical system including at least one lens that condenses the excitation light L ′ emitted from the surface emitting laser 201 toward the incident surface 204 a of the transmission member 204.
The light transmission member 204 is an optical fiber disposed so that the core incident surface 204 a is positioned near the focal point of the incident-side condensing optical system 203.
The optical transmission member 204 uses an optical fiber having a core diameter of 1.5 mm and an NA of 0.4.

By providing the optical transmission member 204, the distance d between the surface emitting laser 201 and the laser resonator 206 is increased by the length of the transmission member 204, and the surface emitting laser 201 is moved from the high temperature region or vibration region around the engine 300. keep away.
By placing the surface-emitting laser 201 in a place away from the engine 300 in this manner, it is possible to suppress the occurrence of a malfunction due to heat and the reliability of the laser ignition device 301 is improved.

  The excitation light L ′ that has entered the light transmission member 204 propagates through the core and is emitted from the emission surface 204 b that is the end surface of the light transmission member 204 on the emission side.

The first lens 205a is a collimating lens having a focal length of 8 mm, and transmits the excitation light L ′ emitted from the light transmission member 204 as substantially parallel light.
The second lens 205b is a convex lens having a focal length of 6 mm, and condenses the excitation light L ′ that has passed through the first lens 205a and becomes substantially parallel light to the focal point.

  The first lens 205a and the second lens 205b may be a lens optical system that achieves desired optical conditions by combining a plurality of lenses.

The laser resonator 206 is a Q-switched laser resonator that emits the laser light L when the excitation light L ′ is incident and the internal laser medium 206a and the saturable absorber 206b serve as a Q-switched laser oscillator.
As shown in FIG. 3, the laser resonator 206 is formed at the laser medium 206a, the saturable absorber 206b, the first dielectric multilayer film 206c1 formed at the −Z direction end, and the + Z direction end. And a second dielectric multilayer film 206c2.
The laser resonator 206 is a composite crystal integrally formed by joining a laser medium 206a and a saturable absorber 206b, and a laser medium 206a is arranged on the incident side, and a saturable absorber 206b is arranged on the emission side. Has been.
The incident side of the laser medium 206a, that is, the end on the −Z direction side in FIG. 3 and the end on the + Z direction side of the saturable absorber 206b are optically polished, and further the first dielectric multilayer film 206c1 and the second dielectric multilayer film 206c1. A dielectric multilayer film 206c2 is formed.
With this configuration, both end portions in the ± Z direction of the laser resonator 206 have a function as mirror surfaces that reflect the laser light L excited inside in a facing manner.

The laser medium 206a is an Nd: YAG crystal doped with 1.1% Nd.
The saturable absorber 206b is a Cr: YAG crystal, and the initial transmittance is about 30%.
The first dielectric multilayer film 206c1 is a coating that exhibits high transparency with respect to the wavelength 808 nm of the excitation light L ′ and high reflectivity with respect to the wavelength 1064 nm of the laser light L emitted from the laser medium 206a.
The second dielectric multilayer film 206c2 is a coating that exhibits a reflectance of 30 to 80% with respect to the laser light L having a wavelength of 1064 nm.
As described above, by forming different dielectric multilayer films on both ends in the ± Z directions, the laser resonator 206 more efficiently opposes and reflects the laser light L excited inside.

The excitation light L ′ enters the laser medium 206a and is excited to create an inverted state.
The saturable absorber 206b has a function as a passive Q switch. That is, it acts as an absorber when the light amount of the laser light L is less than a predetermined value, and transmits the laser light L when it exceeds the predetermined value.
With this configuration, the laser light L is resonated and amplified by the excitation light L ′ incident in the laser resonator 206. Note that the resonator length of the laser resonator 206 in this embodiment is 5 to 10 mm.

The surface emitting laser 201 operates under a driving condition with a pulse width of about 0.1 ms to 10 ms, and the intensity of the laser light L emitted from the laser resonator 206 is about several mJ to several tens of mJ.
Further, the number of operations of the Q switch, that is, the number of pulses is determined according to the light output as the intensity of the excitation light L ′ of the surface emitting laser 201. In the present embodiment, it is desirable that the number of pulses is designed to be several to tens of times.
When the light intensity of the laser light L becomes sufficiently strong according to the prescribed number of pulses, the laser light L passes through the saturable absorber 206b and is emitted in the + Z direction.
Furthermore, it is desirable that the laser light L oscillated from the laser resonator 206 is linearly polarized light in order to suppress a decrease in light amount at the time of wavelength conversion of a wavelength conversion element described later.

The first wavelength conversion element 207a and the second wavelength conversion element 207b change the wavelength of the laser light L emitted from the laser resonator 206, as shown in FIG.
The first wavelength conversion element 207a is an LBO crystal, and when a laser beam L having a wavelength of 1064 nm is incident, a fundamental light L ₁ having a fundamental wavelength of 1064 nm and a second harmonic L ₂ having a wavelength of 532 nm are emitted.

That is, the first wavelength conversion element 207a is a wavelength conversion part as well as transmits the laser beam L of the first wavelength serving wavelength 1064nm as a basic light _{L 1} as the second harmonic wave _{L 2} of the second wavelength serving wavelength 532nm To do.
Similarly, the second wavelength conversion element 207b is LBO crystal, as well as transmits the fundamental light _{L 1} having a wavelength of 1064nm as a basic light _{L 1,} the third harmonic of wavelength 355nm and the second part of the harmonic _{L 2} It is emitted as L _3.

With this configuration, the laser beam L, a first wavelength conversion element 207a, the second wavelength conversion element 207b, a basic light _{L 1} having a wavelength of 1064 nm, a second harmonic wave _{L 2} having a wavelength of 532 nm, the wavelength 355nm 3 and harmonic L _3, is wavelength-converted into a state including.
Also, the basic light _{L 1,} and the second harmonic wave _{L 2,} a third harmonic _{L 3,} the intensity ratio of each light of, for example, Ya thickness of the first wavelength conversion element 207a and the second wavelength conversion element 207b It may be adjusted by optical characteristics such as absorptance. Alternatively, the amount of light may be controlled using a band pass filter or the like.

The fundamental light L ₁ , the second harmonic L ₂ , and the third harmonic L ₃ are all a series of lights having the same optical axis and synchronized pulse timing and the like.

A fundamental light _{L 1,} and the second harmonic wave _{L 2,} a third harmonic _{L 3,} is the third lens 205c serving focusing optical system, it is focused towards the interior of the combustion chamber 304.
Here, the third lens 205c is a convex lens having a focal length of 11 mm and an Abbe number of 50.

At least one of the first wavelength conversion element 207a and the second wavelength conversion element 207b has the laser beam L converged most on the + Z direction side of the laser oscillator 206 in order to improve the wavelength conversion efficiency. It is desirable to arrange in the vicinity of the point.
Note that “near” in the present specification indicates that the optical design is substantially matched within a range where the optical design is not changed.

  By the way, the lens is generally known as chromatic aberration, and since the refractive index of the lens is different for each wavelength, the characteristic that the focal point shifts depending on the wavelength is known.

Utilizing such characteristics, as shown in FIG. 5 as a comparative example, using the laser beams L _a and L _b having two wavelengths from two different light sources 501a and 501b, the ignition positions O _a and O _b are used. A configuration in which the combustion chamber 304 is ignited by using a laser igniter 500 having a plurality of _b is considered.
Note that, in this comparative example, the same reference numerals are given to the same configurations as those of the first embodiment, and description thereof will be omitted as appropriate.

The light source 501a is, for example, a YAG laser emitting device with a wavelength of 1064 nm, and the light source 501b is, for example, a ruby laser emitting device with a wavelength of 600 nm.
However, simply mounting two systems of laser light sources in this way inevitably increases the cost and complexity of the structure because an additional laser light source is required to secure a larger number of ignition positions. .
In addition, in order to condense laser beams from two different light sources on the same optical axis, there is a great limitation in optical design.

Furthermore, in order to control the ignition timing of the engine 300, the laser igniter 500 having such two systems of light sources 501a and 501b controls the two systems of laser beams of the light source 501a and the light source 501b in synchronization. There is a need to.
For example, in an engine that rotates at 3000 rpm, if the crank rotation angle is different by 20 °, the characteristics at the time of ignition change greatly. Therefore, the difference in ignition timing between the ignition positions O _a and O _b is several tens of times. It is necessary to control in a few hundred μs.
As described above, since a control mechanism for performing laser light oscillation control on the order of μs is also required, there is a problem that the cost is increased and the structure is inevitably complicated.

Therefore the third lens 205c of the present embodiment, by utilizing such a chromatic aberration, a basic light L _1, and the second harmonic wave L _2, varying the third harmonic L _3, the condensing position of each light .
Specifically, the focusing position _{O 1} of the fundamental beam L1 at the focusing position _{O 2} of the second harmonic _{L 2,} resulting in displacement Z-axis direction of about 0.3 mm, the focusing position _{O 1} second 3 focusing position _{O 3} and the deviation of approximately 0.8mm at the harmonic _{L 3} is generated in the Z-axis direction.

In this embodiment, the third lens 205c and two wavelength conversion elements 207a disposed between the laser resonator 206, by using 207b, 3 pieces of current collection point _{O 1,} O 2, _{O 3} Focus to the light position.
That is, the third lens 205c forms n + 1 condensing positions with respect to the number n of wavelength conversion elements.
With this configuration, since a plurality of condensing positions can be easily created on the same optical axis, the laser ignition device 301 performs ignition at a plurality of points using a simple configuration, and stability during ignition is improved.

In the present embodiment, a surface emitting laser 201 that is a VCSEL laser is provided as a light source of the excitation light L ′.
With this configuration, the temperature dependency of each wavelength of the fundamental light L ₁ , the second harmonic L ₂ , and the third harmonic L ₃ due to the thermal load is reduced. It is difficult to be affected by heat, and stability during ignition is improved.

Moreover, in this embodiment, it has the laser resonator 206 which is a Q switch laser resonator.
With this configuration, even if the excitation light L ′ is a pulse laser, the laser light L is emitted according to the passive Q switch, so that the output of the laser light L is stabilized and the stability at the time of ignition is improved.

In this embodiment, the laser resonator 206 includes a composite crystal in which a laser medium 206a that is a YAG crystal doped with Nd and a saturable absorber 206b that is a YAG crystal doped with Cr are joined. It is used.
With this configuration, since the interface between the saturable absorber and the laser medium is not separated, reflection or peeling at the interface does not occur even when a thermal load is applied, as with a single crystal. Therefore, it is advantageous in terms of optical characteristics and mechanical strength.
Therefore, the stability of the laser resonator is improved, and the stability of the laser ignition device 301 during ignition is improved.

As shown in FIG. 6 as a modified example, the laser ignition device 301 includes a position of the laser resonator 206, the first wavelength conversion element 207a, and the second wavelength conversion element 207b, or between the optical members of each other. An adjustment mechanism 208 for adjusting the interval z is provided.
As shown in FIG. 7, the adjusting mechanism 208 is an annular spacer in which a hollow opening 208a is formed in a portion through which the laser light L passes.
The adjustment mechanism 208 is not limited to such a configuration as long as it adjusts the interval z between the optical members.
With this configuration, the positional relationship among the laser resonator 206, the first wavelength conversion element 207a, and the second wavelength conversion element 207b is adjusted to improve the efficiency with which the laser light L is output, and the laser ignition device 301 is ignited. Improves time stability.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments, and the present invention described in the claims is not specifically limited by the above description. Various modifications and changes are possible within the scope of the above.

For example, in the above embodiment, the internal combustion engine may be a rotary engine, a gas turbine engine, or a jet engine.
In addition to the internal combustion engine, it may be used in a cogeneration system that uses exhaust heat to extract power, heat, and cold to increase energy efficiency.
In addition to the laser ignition device, a laser emission device that supplies laser light to a laser processing machine, a laser peening device, a terahertz generator, or the like may be used.

  The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

201 Light source 204 Light transmission member 205a, 205b Optical element (first lens, second lens)
206 Resonators 207a and 207b Wavelength conversion element 205c Condensing optical system 301 Laser ignition devices O ₁ , O ₂ and O ₃ Condensing positions L Light (laser light)
L 'light (excitation light)
Light transmitted through the L ₁ , L ₂ , and L ₃ wavelength conversion elements (fundamental light, second harmonic, and third harmonic)
Z optical axis

Japanese Patent No. 5630765 Japanese Patent No. 4772600

Claims (9)

One light source that emits light;
An optical transmission member for transmitting light emitted from the light source;
An optical element disposed on the optical path of the light;
A resonator to which the light that has passed through the optical element is input;
At least one wavelength conversion element that converts the wavelength of the light emitted from the resonator into a first wavelength and a second wavelength;
A condensing optical system for condensing the light transmitted through the wavelength conversion element;
Have
A laser igniter in which a condensing position by the condensing optical system is formed on the optical axis of the light, and n + 1 places with respect to the number n of the wavelength conversion elements.   The laser ignition device according to claim 1, further comprising an adjustment mechanism for adjusting a position of the resonator. The laser ignition device according to claim 2, wherein
The laser igniter characterized in that the adjustment mechanism adjusts an interval between the resonator and the wavelength conversion element.   The laser ignition device according to any one of claims 1 to 3, wherein the light source is a surface emitting laser.   The laser ignition device according to claim 1, wherein the resonator is a Q-switched laser resonator.   6. The laser ignition device according to claim 5, wherein the resonator is a composite crystal in which a laser medium and a saturable absorber are joined.   The laser ignition device according to claim 6, wherein the laser medium is a YAG crystal doped with Nd, and the saturable absorber is a YAG crystal doped with Cr. In the laser ignition device according to claim 2 or 3,
The laser igniter characterized in that the adjustment mechanism is a spacer having an opening in a portion through which the light is transmitted. In an internal combustion engine that generates combustion gas by burning fuel,
An internal combustion engine comprising the ignition device according to any one of claims 1 to 8 for igniting the fuel.

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