Alcock et al. [58] investigated 1972 the effect of wavelength and focal spot size on the breakdown thresholds of Xe, Ne, N 2 , H 2 , CH 4 , air, and D 2 for ignition laser wavelengths of 347.2 and 694.3 nm. The breakdown threshold of hydrogen increased at shorter wavelength while the breakdown threshold of all other gases decreased as the wavelength decreased. Kingdon et al. [59] analyzed the effect of pulse duration and plasma constitution (fine wires and fibers in the focus as a target) on MIE in the year 1978. They found out that for short pulse duration the MIE was independent of plasma constitution, while for longer duration pulses (1 ms) the presence of inhibitors in the plasma could lead to flame extinction. Phuoc et al. [43] showed a decreasing MIE with increasing pressure. Gower [60] measured breakdown thresholds with a KrF laser emitting at 248 nm and Williams et al. [61] studied the breakdown threshold in air with ps pulse duration at 530 nm. Dewhurst [62] measured breakdown thresholds in N 2 and O 2 using 1064, 690 and 530 nm laser beams at very low initial pressures. 21 3 Overview on literature and patents dealing with laser ignition The wavelength dependence of breakdown thresholds in He and Ar was studied by Byron and -2 dependence (longer wavelength => Pert [63] in the year 1979. The argon data showed a Ȝ smaller breakdown threshold) but the helium data were wavelength independent. Syage et al. [64] studied the ignition of hydrogen/air and hydrogen/air/CO 2 mixtures. The study was performed by operating the laser both, in the Q-switched mode to deliver ns laser pulses and in the pulse mode-locked mode to deliver ps laser pulses. In comparison to Syage et al., Lim et al. [65] measured MIE and spark size of CH 4 -air mixtures using the second harmonic of Nd:YAG laser operating either as a Q-switched ns laser (10 ns duration) or as a pulsed mode-locked ps laser (30 ps pulse). They found out that for ps-pulses, higher MIE are needed than in the case of ns-pulses. They explain this fact as follows: for the ps-pulse ignition the time is so short that only multiphoton ionization (MPI) processes are responsible for ionization of the molecules and no, or only a very weak electron avalanche process takes place; hence the efficiency of the ionization process is very low. They showed that the plasma size of ps-pulses is smaller than for ns-pulses of the same energy supporting this hypothesis. MIEs in dependence of fuel/air ratio and different combustion gases have been measured by Beduneau et al. [66] and Phuoc et al. [67] & [43]. Lee et al. [68] showed clearly that with increasing pressure the MIE is decreasing drastically. This conclusion was strengthened by experiments with different fuels. In a very recent paper published 2005, McNeill [69] studied theoretically and experimentally MIE in dependence of the focal length and various other parameters. The main conclusion of this paper is that laser ignition (~ 1 mJ) needs higher MIE than electrical spark ignition (~ 0.3 mJ) for methane-air mixtures. A main reason for that is the fact that the shock wave carries more than 90 % of the ignition energy out of the ignition kernel volume. Unpublished ignition results of methane-air mixtures mainly carried out by Kopecek, showed MPE values for ignition below 0.3 mJ which are equal to or below the MIE for electrical spark ignition. Laser ignition of jet-diffusion flames 3 /s) Laser ignition of jet-diffusion flames with different Reynolds number (RE = 35-103 cm and time-resolved OH emission measurements have been investigated by Phuoc et al. [70]. They showed that the laser radiation can be used to effectively ignite and stabilize the flame under various turbulent flow conditions. Schmieder [71] reported a study where laser-induced sparks were used successfully to either ignite or extinguish a methane jet-diffusion flame. The fact that the laser spark was able to extinguish the flame was attributed to the strong shock wave generated by the sudden deposition of energy, blowing out the flame. Additionally, they observed that the spark could extinguish the flame over larger distances than it could ignite it, and the probability of extinguishing the flame sometimes was higher than the probability of igniting it. Multi-point ignition Phuoc [72] and Morsy et al. [73] showed for the first time the advantages of multi-point ignition resulting in a much shorter combustion time. Morsy et al. [74] & [75] presented the advantages of the interesting idea of multi-point ignition through conical cavities. Further on, they presented an extensive theoretical model of the flame kernel development in the cavity [75]. 22 3 Overview on literature and patents dealing with laser ignition Laser ignition under engine and gas turbine like conditions First laser ignition experiments with a CO 2 laser on an IC engine have been carried out by Dale et al. 1978 [76]. In agreement with other research groups, the laser spark was able to ignite leaner mixtures and the pressure rise time was reduced (shorter ignition delay). In particular, the use of laser ignition increased the peak power by 5 % and 15 %, without exhaust gas recirculation (EGR) and with 16 % EGR, respectively. Hicks et al. [77] studied the ignition probability of a gas turbine using a conventional wallmounted surface discharge igniter (SDI) and a Q-switched Nd:YAG laser. The laser produced pulse energy of about 176 mJ at 532 nm and pulse duration of 10 ns. The conventional wall mounted SDI delivered pulse energy of about 3.1 J and the pulse duration was about 100 ms. They reported that when the laser spark was created close to the conventional wall-mounted location, both methods appeared to produce very similar trends in ignition performance with increasing mass flow at this location. When the laser ignition location was away from the wall, the air-to-fuel ratio for which > 75 % ignition probability could be achieved increased to about 33 %. Thus, the laser ignition could significantly improve the lean ignition limit. Such an improvement was two times higher than the improvement provided by the plasma jet igniter (about 16 %) as reported by Low et al. [78]. In the year 1996, Ishida et al. [79] tested laser ignition applied to a methanol-diesel engine. For this study, the laser beam was focused on a target embedded on the surface of the piston to create a plasma torch. It was found that the laser ignition system had lower performance than a glow plug. A second series of tests was run with a different setup where the laser beam was focused directly on the fuel spray so that a target material was not required. The results showed that with laser energies in the order of 49 mJ it was possible to successfully run the engine. Unfortunately, no comparison was shown between this second laser ignition system and a common glow-plug igniter. Ma et al. [80] analyzed laser ignition of methane-air mixtures in a one cylinder setup and showed the advantages of the shorter overall combustion time and shorter ignition delay. Alger et al. from the Southwest Research Institute (SwRI) published a paper which deals with laser ignition of a one cylinder research engine ignited by a nanosecond Nd:YAG laser [81]. One of their results was the improved combustion process generated through the free choice of positioning of the laser-induced ignition plasma. Laser ignition in a natural gas-fueled engine was studied by McMillian et al. from National Energy Technology Laboratory (NETL) [82] & [83]. They found out that the lean limit could be extended with laser ignition in comparison with conventional spark ignition from Ȝ = 1.87 to 1.95 (Ȝ…air/fuel equivalence ratio). Further on, the ignition delay was 7 % shorter and the knock limit was found to be slightly decreased. In the year 2000, our research group mainly represented by Kopecek et al. [84] first laser ignited one cylinder of a 1 MW gas engine successful. They could expand the lean limit to Ȝ = 2.1 and reached lowest NO x emissions of 0.22 g/kWh. In another publication by Kopecek et al. [85] it was shown for the first time that with laserinduced plasma the start of the combustion in a homogeneous charge compression ignition (HCCI) engine can be controlled. The temperature of the inlet air of the engine was decreased from 215°C to 195°C and as an implication the combustion became unstable. However, when the laser plasma was turned on, the combustion became stable again. This may represent a nice way to control the onset of combustion in the very promising field of HCCI operation. A more detailed explanation of this new combustion concept is given in the Chapter 7 of this PhD thesis. 23 3 Overview on literature and patents dealing with laser ignition Jetzinger et al. [86] investigated the performance of laser ignition on a direct injection fuelstratified gasoline engine. They found out that the required laser energy for reliable ignition was independent of the load but increased slightly with increasing engine speed. Particularly, they found a high potential of laser ignition in the case of fuel stratified combustion concepts, since the laser spark could be located directly inside the rich mixture region without the serious problem of soot formation on the electrodes usually leading to misfires and hence to a reduced lifetime of the spark plug. In a very recent publication of 2005, Gupta et al. [87] studied laser ignition of methane-air mixtures in a rapid compression machine (RCM). They used a laser with a bad beam profile 2 < 5) and a short focal length of with f = 13 mm. It was possible to expand the lean limit (M from Ȝ = 1.67 to Ȝ = 2, however requiring pulse energies up to 80 mJ. Once more, the shorter ignition delay and rates of pressure rise have been shown for lean mixtures. ignition. Laser ignition experiments in a combustion chamber Early experiments on laser ignition have been done in constant volume combustion chambers. In these vessels the advantages (or disadvantages) of different ignition systems can be studied in a very detailed way without any interfering effects (turbulence, inhomogeneity, initial temperature not known exactly...) like in internal combustion (IC) engines. The first experiments with laser ignition in a combustion chamber have been carried out by Lee and Knystautas [35] in 1969. They used a Q-switched ruby laser with 1.2 J laser pulse energy and 10 ns pulse duration to investigate the laser spark ignition of stoichiometric propane-air mixtures and acetylene-oxygen mixtures. Diagnostic measurements like Schlieren pictures of the combustion process and shock wave generation have already been performed in this early work. In the year 1974, Hickling and Smith [36] investigated the characteristics of laser-induced sparks for isooctane, cyclo-hexane, n-heptane, n-hexane, clear indolene, and No. 1 diesel fuel. These authors found like Kopecek et al. [39] of our research group at the TU Wien, several years later that the breakdown threshold decreased as the pressure inside the combustion bomb was increased and that the presence of fuel did not affect the energy needed to cause breakdown in the fuel/air mixture. When compared to conventional electric sparks, it was recognized that the laser ignition system was able to ignite much leaner mixtures than the spark ignition system. Moreover, it was noted that the laser had a zero percent misfire rate, as long as the mixture was within the flammability limits, while the spark plug often required multiple firings before the mixture could be successfully ignited. Faster combustion was achieved with laser ignition especially for fuel-rich mixtures in comparison to conventional spark plug ignition. Furuno et al. [37] investigated laser ignition in a combustion bomb filled by a stratified mixture with the rich mixture prepared in the vicinity of the ignition point. They showed the 19 3 Overview on literature and patents dealing with laser ignition potential of lowest NO x emissions from laser-ignited, lean propane-air mixtures. The ideal two-phase mixture was formed with the aid of a soap bubble. Bradley et al. [38] showed by laser ignition experiments of n-heptane-air mixtures in a spherical combustion bomb that the flame propagation speed can be increased in comparison with conventional spark ignition (400 mJ of laser beam energy were used). Our research group represented by Kopecek et al. [39] showed in the basic, fundamental and comprehensive work, the many different advantages of laser ignition on lean methane-air mixtures in a combustion bomb. Approximately at the same time, Gupta et al. [40] investigated laser ignition inside a combustion chamber of constant volume. The main result of this study was that the laser enables ignition of mixtures at pressures being at least 30 % higher than those defining the ignition limits of conventional spark plug ignition. The authors Kopecek et al. [41] investigated in 2004 for the first time the possibility to transport ns-duration, high peak intensity laser pulses via an optical fiber into the engine. They compared different fibers in the paper but leading to the conclusion that only the photonic crystal fiber (PCF) can be considered as a realistic candidate to realize the concept of laser ignition via optical fiber. Flame kernel and laser-induced plasma investigations One of the first flame kernel and laser-induced plasma observation has been done by Santavicca et al. [42] in 1991. They studied the ignition and the flame kernel development of both, laminar and turbulent methane-air flows, at atmospheric pressure for different equivalence ratios and compared the results with those obtained using a General Motors high energy electric ignition system. A clearly better performance for the laser ignition system was demonstrated. Phuoc et al. [43] measured the plasma dimensions in dependence of the air/fuel ratio. The average length and radius of a spark with minimum ignition energy (MIE) of 3-4 mJ in a stoichometric methane-air mixture were about 0.8 mm and 0.3 mm, respectively. The flame kernel development initiated by a laser-induced breakdown was also extensively investigated by Phuoc et al. [44]. OH planar laser induced fluorescence (PLIF of hydroxyl radicals) measurements of laserinduced flame kernels in the time range from 100 μs up to 2000 μs after ignition have been performed by Spiglanin et al. [45]. They characterized the toroidal shape and the front lobe of the flame kernel typical for laser ignition. NH PLIF measurements of laser-induced plasma and flame kernel in NH 3 /O 2 were measured by Chen et al. [46]. Beduneau and Ikeda [47] investigated the emission spectra of the laser-induced plasma and consequent flame kernel by a Cassegrain optic system and showed that the maximum emission peaks of the plasma are between 350 nm and 550 nm, 100-200 ns after formation. For the flame kernel the maximum emission peaks are around 670 nm, 2 μs after ignition. Further on, they investigated the plasma dimensions in dependence of the incident energy [48]. Also Horisawa et al. [49] studied the emission spectra of the laser-induced plasma in dependence of time in supersonic air streams. They established a model of the characteristic time scales of the various processes from the ns to the ms range: (I) absorption of an incident -9 -10 -8 s), (II) plasma formation process (10 -8 -10 -7 s), (III) ignition process (10 -6 laser pulse (10 -4 s), and (IV) shock-flow interaction and (V) convective diffusion processes (~ 10 -5 s). This 10 model is in good agreement with the model of the author of this PhD thesis which is explained later. 20 3 Overview on literature and patents dealing with laser ignition Numerical simulations of the flame kernel development initiated by laser ignition have been done by Morsy et al. [50]. They explained in theoretical models the formation of the “front lobe” of the flame kernel, typical for laser ignition which always heads towards the laser beam. The resulting shock wave of the laser-induced plasma was studied by Phuoc [51]. He theoretically calculated and experimentally showed that the shock pressure is proportional to -3 (R…shock wave radius) for spark energies ranging from 15 to 50 mJ. Within the first few R microseconds, the energy loss by the shock-waves was about 51 to 70 %, the radiation energy loss ranged between 22 to 34 %, and the energy remaining in the hot gas was only about 7-8 % of the absorbed energy. Lackner et al. [52], [53] & [54] from our research group at the TU Wien characterized laserinduced ignition of biogas- and methane-air mixtures by the use of absorption spectroscopy to track the generation of water during the ignition process. Further on, they also applied Schlieren photography and OH PLIF to characterize the plasma and expanding flame kernel. Zimmer et al. [55] investigated laser ignition of premixed and preheated methane-air mixtures produced in a low-swirl burner. Most experiments were conducted at temperatures between 127°C and 327°C. They deeply analyzed the increasing spark size with increasing pulse energy. Further on, they showed the increasing flame kernel size with increasing laser pulse energy by OH PLIF measurements. At 312°C, Zimmer et al. achieved minimum pulse energy (MPE) for ignition below 0.5 mJ for a Ȝ of 1.67. Bindhu et al. [56] investigated the flame kernel development from a laser-induced spark in argon. They found out that at increasing gas pressures the plasma can absorb the incident laser energy more effective. This means that the transmitted energy through the focal volume is less and the laser ignition process is more effective. Minimum ignition energy and breakdown threshold measurements First basic measurements of minimum ignition energy (MIE) of methane-air mixtures have been done by Weinberg et al. 1971 [57]. They used a ruby laser with maximum pulse energies of 2 J and 20 ns full width at half maximum (FWHM) pulse duration. In this case the pulse energy was measured by focusing the beam through a small aperture into a totally absorbing spherical calorimeter. They showed for the first time that MIE and the plasma dimensions are decreasing with increasing pressure. Alcock et al. [58] investigated 1972 the effect of wavelength and focal spot size on the breakdown thresholds of Xe, Ne, N 2 , H 2 , CH 4 , air, and D 2 for ignition laser wavelengths of 347.2 and 694.3 nm. The breakdown threshold of hydrogen increased at shorter wavelength while the breakdown threshold of all other gases decreased as the wavelength decreased. Kingdon et al. [59] analyzed the effect of pulse duration and plasma constitution (fine wires and fibers in the focus as a target) on MIE in the year 1978. They found out that for short pulse duration the MIE was independent of plasma constitution, while for longer duration pulses (1 ms) the presence of inhibitors in the plasma could lead to flame extinction. Phuoc et al. [43] showed a decreasing MIE with increasing pressure. Gower [60] measured breakdown thresholds with a KrF laser emitting at 248 nm and Williams et al. [61] studied the breakdown threshold in air with ps pulse duration at 530 nm. Dewhurst [62] measured breakdown thresholds in N 2 and O 2 using 1064, 690 and 530 nm laser beams at very low initial pressures. 21 3 Overview on literature and patents dealing with laser ignition The wavelength dependence of breakdown thresholds in He and Ar was studied by Byron and -2 dependence (longer wavelength => Pert [63] in the year 1979. The argon data showed a Ȝ smaller breakdown threshold) but the helium data were wavelength independent. Syage et al. [64] studied the ignition of hydrogen/air and hydrogen/air/CO 2 mixtures. The study was performed by operating the laser both, in the Q-switched mode to deliver ns laser pulses and in the pulse mode-locked mode to deliver ps laser pulses. In comparison to Syage et al., Lim et al. [65] measured MIE and spark size of CH 4 -air mixtures using the second harmonic of Nd:YAG laser operating either as a Q-switched ns laser (10 ns duration) or as a pulsed mode-locked ps laser (30 ps pulse). They found out that for ps-pulses, higher MIE are needed than in the case of ns-pulses. They explain this fact as follows: for the ps-pulse ignition the time is so short that only multiphoton ionization (MPI) processes are responsible for ionization of the molecules and no, or only a very weak electron avalanche process takes place; hence the efficiency of the ionization process is very low. They showed that the plasma size of ps-pulses is smaller than for ns-pulses of the same energy supporting this hypothesis. MIEs in dependence of fuel/air ratio and different combustion gases have been measured by Beduneau et al. [66] and Phuoc et al. [67] & [43]. Lee et al. [68] showed clearly that with increasing pressure the MIE is decreasing drastically. This conclusion was strengthened by experiments with different fuels. In a very recent paper published 2005, McNeill [69] studied theoretically and experimentally MIE in dependence of the focal length and various other parameters. The main conclusion of this paper is that laser ignition (~ 1 mJ) needs higher MIE than electrical spark ignition (~ 0.3 mJ) for methane-air mixtures. A main reason for that is the fact that the shock wave carries more than 90 % of the ignition energy out of the ignition kernel volume. Unpublished ignition results of methane-air mixtures mainly carried out by Kopecek, showed MPE values for ignition below 0.3 mJ which are equal to or below the MIE for electrical spark ignition. Laser ignition of jet-diffusion flames 3 /s) Laser ignition of jet-diffusion flames with different Reynolds number (RE = 35-103 cm and time-resolved OH emission measurements have been investigated by Phuoc et al. [70]. They showed that the laser radiation can be used to effectively ignite and stabilize the flame under various turbulent flow conditions. Schmieder [71] reported a study where laser-induced sparks were used successfully to either ignite or extinguish a methane jet-diffusion flame. The fact that the laser spark was able to extinguish the flame was attributed to the strong shock wave generated by the sudden deposition of energy, blowing out the flame. Additionally, they observed that the spark could extinguish the flame over larger distances than it could ignite it, and the probability of extinguishing the flame sometimes was higher than the probability of igniting it. Multi-point ignition Phuoc [72] and Morsy et al. [73] showed for the first time the advantages of multi-point ignition resulting in a much shorter combustion time. Morsy et al. [74] & [75] presented the advantages of the interesting idea of multi-point ignition through conical cavities. Further on, they presented an extensive theoretical model of the flame kernel development in the cavity [75]. 22 3 Overview on literature and patents dealing with laser ignition Laser ignition under engine and gas turbine like conditions First laser ignition experiments with a CO 2 laser on an IC engine have been carried out by Dale et al. 1978 [76]. In agreement with other research groups, the laser spark was able to ignite leaner mixtures and the pressure rise time was reduced (shorter ignition delay). In particular, the use of laser ignition increased the peak power by 5 % and 15 %, without exhaust gas recirculation (EGR) and with 16 % EGR, respectively. Hicks et al. [77] studied the ignition probability of a gas turbine using a conventional wallmounted surface discharge igniter (SDI) and a Q-switched Nd:YAG laser. The laser produced pulse energy of about 176 mJ at 532 nm and pulse duration of 10 ns. The conventional wall mounted SDI delivered pulse energy of about 3.1 J and the pulse duration was about 100 ms. They reported that when the laser spark was created close to the conventional wall-mounted location, both methods appeared to produce very similar trends in ignition performance with increasing mass flow at this location. When the laser ignition location was away from the wall, the air-to-fuel ratio for which > 75 % ignition probability could be achieved increased to about 33 %. Thus, the laser ignition could significantly improve the lean ignition limit. Such an improvement was two times higher than the improvement provided by the plasma jet igniter (about 16 %) as reported by Low et al. [78]. In the year 1996, Ishida et al. [79] tested laser ignition applied to a methanol-diesel engine. For this study, the laser beam was focused on a target embedded on the surface of the piston to create a plasma torch. It was found that the laser ignition system had lower performance than a glow plug. A second series of tests was run with a different setup where the laser beam was focused directly on the fuel spray so that a target material was not required. The results showed that with laser energies in the order of 49 mJ it was possible to successfully run the engine. Unfortunately, no comparison was shown between this second laser ignition system and a common glow-plug igniter. Ma et al. [80] analyzed laser ignition of methane-air mixtures in a one cylinder setup and showed the advantages of the shorter overall combustion time and shorter ignition delay. Alger et al. from the Southwest Research Institute (SwRI) published a paper which deals with laser ignition of a one cylinder research engine ignited by a nanosecond Nd:YAG laser [81]. One of their results was the improved combustion process generated through the free choice of positioning of the laser-induced ignition plasma. Laser ignition in a natural gas-fueled engine was studied by McMillian et al. from National Energy Technology Laboratory (NETL) [82] & [83]. They found out that the lean limit could be extended with laser ignition in comparison with conventional spark ignition from Ȝ = 1.87 to 1.95 (Ȝ…air/fuel equivalence ratio). Further on, the ignition delay was 7 % shorter and the knock limit was found to be slightly decreased. In the year 2000, our research group mainly represented by Kopecek et al. [84] first laser ignited one cylinder of a 1 MW gas engine successful. They could expand the lean limit to Ȝ = 2.1 and reached lowest NO x emissions of 0.22 g/kWh. In another publication by Kopecek et al. [85] it was shown for the first time that with laserinduced plasma the start of the combustion in a homogeneous charge compression ignition (HCCI) engine can be controlled. The temperature of the inlet air of the engine was decreased from 215°C to 195°C and as an implication the combustion became unstable. However, when the laser plasma was turned on, the combustion became stable again. This may represent a nice way to control the onset of combustion in the very promising field of HCCI operation. A more detailed explanation of this new combustion concept is given in the Chapter 7 of this PhD thesis. 23 3 Overview on literature and patents dealing with laser ignition Jetzinger et al. [86] investigated the performance of laser ignition on a direct injection fuelstratified gasoline engine. They found out that the required laser energy for reliable ignition was independent of the load but increased slightly with increasing engine speed. Particularly, they found a high potential of laser ignition in the case of fuel stratified combustion concepts, since the laser spark could be located directly inside the rich mixture region without the serious problem of soot formation on the electrodes usually leading to misfires and hence to a reduced lifetime of the spark plug. In a very recent publication of 2005, Gupta et al. [87] studied laser ignition of methane-air mixtures in a rapid compression machine (RCM). They used a laser with a bad beam profile 2 < 5) and a short focal length of with f = 13 mm. It was possible to expand the lean limit (M from Ȝ = 1.67 to Ȝ = 2, however requiring pulse energies up to 80 mJ. Once more, the shorter ignition delay and rates of pressure rise have been shown for lean mixtures. Prototype laser ignition systems This last sub-chapter of the literature review is aimed to introduce different research groups in the world which are developing ignition laser prototypes which should be cheap, reliable and capable to be employed on an IC engine. McMillian et al. from NETL [88] & [89] published a paper on a prototype laser ignition system with maximum output energy around 6 mJ at 10 ns. They used a passively Q-switched, transversal diode-pumped laser system at pump powers between 250 up to 300 W. Prototype laser ignition systems This last sub-chapter of the literature review is aimed to introduce different research groups in the world which are developing ignition laser prototypes which should be cheap, reliable and capable to be employed on an IC engine. McMillian et al. from NETL [88] & [89] published a paper on a prototype laser ignition system with maximum output energy around 6 mJ at 10 ns. They used a passively Q-switched, transversal diode-pumped laser system at pump powers between 250 up to 300 W. The group around Kroupa from Carinthian Tech Research (CTR) developed a transversal diode-pumped, passively Q-switched laser system with pulse energy of about 15 mJ at 8 ns. It should be noticed however, that this system is pumped by a very high power of 1.2 kW and hence the crystal has to be cooled in a very complex way involving three different cooling circles. The company BOSCH with the researchers Ridderbusch and Herden [90] are currently developing a longitudinally diode-pumped, fiber-coupled and passively Q-switched laser in an oscillator amplifier arrangement with maximum pulse energies of about 10 mJ at around 5 ns which is to the knowledge of the author the highest value for this type of laser system. 3.2 Patent review The patents discussed in the following deal with different aspects of laser ignition. This list does not claim to be complete. It should just give an insight into the patent situation on the market which is connected to the topic of this PhD thesis. Different patents about laser ignition The US patent by Gupta et al. [145] shows a concept where the laser beam is produced in one master laser and is then distributed through a rotating mirror system to the different engine cylinders which are each supplied with a laser plug. Further on, with a fiber detecting system the invention can detect misfires. Winkelhofer et al. [146] designed a passively Q-switched, radially diode-pumped, watercooled solid-state laser specially designed for laser ignition applied to a cylinder of an engine. It can be directly mounted in a standard spark plug hole and delivers about 20 mJ in about 7 ns. It is the only patent containing a description how to build a laser capable for the ignition of an engine. 24 3 Overview on literature and patents dealing with laser ignition Patent No. DE 101 45 944 A1 [147] describes an invention where in a standard spark plug an optical waveguide is integrated which is capable to guide the highly intensive laser pulse for ignition. The end of the fiber is specially designed so that it can focus the laser beam by itself. Hubert and Keesmann [148], Vowles [149] and Mourad [150] are presenting older patents which are describing very basic but fundamental ideas about laser ignition systems. Schick and Lindstedt [151] report in their invention on a laser ignition system based on a gas laser of the year 1986. The gas laser used for the system is not specified. Feichtinger et al. of AVL List GmbH [152] suggest a high energy transporting fiber which is mounted in the gasket between the cylinder head and cylinder. The fiber is molten at the end so that it can focus the laser light.
Posted on: Thu, 31 Oct 2013 18:49:49 +0000
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