Initial seed electrons from low ionization potential impurities like aerosol or dust particles As described above, electrons generated through MPI cannot start the electron avalanche process exclusively by themselves. Only at wavelengths far below 1 μm and at very low pressures (< 0.01 bar), MPI can possibly generate enough electrons to start the electron avalanche process. The presence of impurities having low ionization energies can also be expected to contribute significantly to the generation of initial electrons by MPI. 10 2 Principles and advantages of laser ignition For longer wavelengths, MPI cannot furnish any electrons since the high number of photons needed to be absorbed simultaneously by one atom or molecule makes this effect highly unlikely. Experiments conducted in air at a wavelength of 10.6 μm showed breakdown to be a rather sporadic event. It was discovered that the plasma was initiated by aerosols in the focal 7 particles per mm 3 volume [24], [25] & [26]. Under normal conditions, there are more than 10 larger than 0.1 μm in the atmosphere [27]. These particles would heat up under laser irradiation by absorption and could generate electrons by thermionic emission [21]. Experiments were conducted by [28] & [29] showing a steep increase of the breakdown threshold for laser radiation of 10.6 μm wavelength if all particles larger than 0.1 μm where filtered out of the air. Since the conditions of a combustible gas mixture inside the cylinder of a gas engine are everything else than pure, more than enough seeds should be available at any place and time to provide first electrons, whether by MPI or by thermal effects. Electron cascade process After a sufficient number of free electrons having been produced via one or both of the above described effects, the so called electron cascade process takes over. Free electrons gain their energy by absorbing it from an electromagnetic field. This effect is the inverse of bremsstrahlung where high energy electrons emit radiation as they slow down. The highly energetic electrons are losing their energy again by collision with neutral particles. Some electrons will be lost by attachment, but new electrons will be generated by ionizing collisions. Above certain electrical field strength, a few electrons will gain an energy larger than the ionization energy of the medium and thus generate new electrons by impact ionization of the gas. The following equation depicts the reaction process: o M 2e M e (4) Moreover, the electron cascade process is significant at high pressure and longer laser pulse length (nanosecond range) because under these conditions, electron-atom or electron-ion collisions have sufficient time to occur during the laser pulse. According to Morgan [15], a condition determines the effective occurrence if the product of gas pressure and laser pulse -10 bar • s. width is greater than 10 2.3 From the laser spark to combustion Due to the much shorter energy deposition time of several nanoseconds in the case of laser ignition, the effects and processes which lead to ignition and combustion, are quite different to conventional spark plug ignition and therefore of great interest for research. Further on, it is worth to investigate the different timescales from nanoseconds at the plasma phase up to milliseconds in the combustion phase. For this purpose different diagnostic methods can be applied to investigate the different stages of laser ignition in detail. 11 Fig. 7: Scope of timescales of various processes involved in laser-induced ignition: the lengths of the double arrowed lines indicate the duration ranges of the indicated processes. Inserts: (a) typical laser pulse duration; (b) examples for temporal development of spatially resolved OH concentrations in flame kernels; (c) typical pressure rise in the combustion chamber Fig. 7 shows an overview of the processes involved in laser-induced ignition in a constant volume combustion chamber covering several orders of magnitude in time from the nanosecond domain of the laser pulse proper to the duration of the entire combustion lasting several hundreds of milliseconds. The laser energy is deposited in a few nanoseconds leading to shock wave generation. In the first milliseconds an ignition delay can be observed with duration between 5 and 100 ms depending on the mixture. It can last between 100 ms up to 2000 ms again depending on gas composition, initial pressure, pulse energy, plasma size, position of the plasma in the static volume combustion chamber and initial temperature. In an engine, usually turbulences or even the addition of hydrogen [117] are employed to speed up the combustion process while the initiation stays the same as described. Fig. 8 depicts two Schlieren images of a laser-induced breakdown in air. The left picture shows two separated plasma kernels, each leading to an expanding shock wave. The shock wave velocity was measured by [30] using Schlieren visualization, reaching about 5000 m/s in H 2 and about 2000 m/s in air for the time shortly after the detachment from the plasma kernel and decreasing rapidly. It was found out that the peak velocity depends significantly on the incident laser pulse energy. The right picture depicts a multi-exposure image of shock waves in air at 10 bar initial pressure in 500 ns steps after the ignition. 12 Fig. 8: Left picture: breakdown occurred at two locations simultaneously, therefore two shock waves can be observed; initial pressure : 25 bar, medium : air, temperature : 100°C, laser energy : 50 mJ, time : 8 μs after ignition, image dimensions: 11.6 mm x 9.15 mm. Right picture: multi-exposure image of the shock wave in air at 10 bar in 500 ns steps after ignition. The distance between the first two shock front structures outside of the hot core gas is slightly but visibly larger than between the subsequent exposures [30] During the next microseconds after the plasma has been formed, the fuel-air mixture is heated up starting the chemical reactions necessary for combustion. A flame kernel is generated, which consumes all the fuel in its near vicinity (see Fig. 9). Such flame kernels induced by laser sparks have a characteristic shape consisting of a torus and a front lobe being directed towards the laser source. If the heat generated by the initial chemical reactions can overcome all the losses like conduction, convection, radiation and shock wave development, a self-sustaining flame front will start to propagate away from the kernel consecutively deflagrating the whole volume. Fig. 9: Image of a longitudinal cut of a laser-induced flame kernel 2.2 ms after the laser pulse entering from the right side; measured by planar, laser-induced fluorescence (PLIF); colors refer to relative fluorescence intensities of the OH molecules; laser pulse energy E pulse : 50 mJ; CH 4 -air mixture; initial pressure : 4 bar; laser spark already ceased [31] 13 2 Principles and advantages of laser ignition 2.4 Advantages of laser ignition In this chapter the basic, fundamental advantages in comparison to conventional spark plug ignition should be presented and discussed. Especially for stationary, electricity producing gas engines like depicted in Fig. 10, with high demands on the ignition system, laser ignition can play out all of its main advantages. But also for triggering an HCCI engine or to ignite reliably a DI gasoline engine laser ignition is a promising alternative for the future like mentioned in the introduction chapter. This chapter is partly taken from the PhD thesis of Kopecek [20]. Fig. 10: Large gas engine (GE Jenbacher); max. 3 MW of electrical power; nominal speed: 1500 rpm (revolutions per minute) [32] The following advantages of laser ignition in comparison to conventional spark plug ignition are mainly focused on gas engines: x Ignition of leanest mixtures feasible => lower combustion temperatures => lower NO x emissions x No erosion effects occurring like in the case of spark plugs leading to significantly longer availability of laser ignition systems x Higher load/ignition pressures up to 35 bar applicable => increase in engine efficiency x Choice of arbitrary positioning of the ignition plasma in the cylinder available; advantageously in the center of the combustion chamber, to minimize the path length of the propagating flame front and to increase the engine efficiency especially in the case of very lean mixtures. x Simplified possibility of multipoint ignition to speed up the combustion process for highest engine efficiencies especially for lean mixtures x Precise ignition timing possible for optimal engine performance and maximum efficiency x Shorter ignition delay time x Less space demand in the cylinder head because of the smaller components of a laser oscillator => larger inlet and outlet valve diameters => increase in engine efficiency Some of these advantages are discussed in more detail just below. 14 2 Principles and advantages of laser ignition Ignition of leanest mixtures possible Environmental pollution caused by the emissions of combustion engines became one of the most important topics in engine development over the last decades. Although the chemical reaction equation for the combustion of, for example, methane, promises water and CO 2 as the only output species, in real combustion processes several other reactions take place, additionally producing harmful species, like for example oxides of nitrogen (NO and NO 2 ), unburned hydro-carbons (HC) and carbon monoxides (CO) [33]. Two different techniques were established up till now to reduce these emissions of internal combustion engines. The first is the application of three-way catalysts for treatment of the exhaust gas exclusively working at stoichiometric mixture conditions. The second possibility is to run the gas engine with very lean mixtures near to the ignition limit of the specific fuel, where especially NO x emissions are naturally low due to the much lower combustion temperatures (see Fig. 11). A stoichometric mixture is characterized by the air/fuel equivalence ratio Ȝ = 1. The mixture ratio can be also characterized (according to English literature) by the symbol ij which is inversely proportional to Ȝ. But because the use of the symbol Ȝ is prevailing in the engine related literature, the air/fuel equivalence ratio will be described by this Ȝ throughout this work. Unfortunately, the HC emissions for very lean mixtures are increasing due to incomplete and delayed combustion as indicated by Fig. 11. Since advanced gas engines work just below the lean ignition limit (Ȝ = 1.85 for natural gas using conventional spark plugs [32]) to reduce NO x emissions, they suffer from the drawback of increased HC emissions. Fig. 11: Variation of HC, CO and NO concentrations in the exhaust of a conventional spark ignition internal combustion engine for different relative air-fuel equivalent ratios (Ȝ); two different lines in one color mark the maximum/minimum emission value [33] To ignite such lean mixtures by spark plugs, elongated sparks of large volume, obtained by a longer gap distance between the electrodes, are necessary. Unfortunately, the breakdown voltage is increased by increasing this gap distance, resulting in a higher voltage demand for ignition and consecutively in a shorter lifetime of the spark plugs due to enhanced electrode erosion effects (Fig. 12). Also electromagnetic incompatibility can become a serious problem above a certain voltage level. 15 Fig. 12: Breakdown voltages of the spark plugs of a large gas engine depending on break mean effective pressure (BMEP) [32] Although the lean side operation limit can be pushed by such means, the ultimate limit is still influenced by the flame-quenching effects of the electrodes, which is not the case for laser ignition. The ability of igniting leaner mixtures is thus expected by using laser plasma as an ignition source. Longer lifetime of a laser ignition system The lifetime of conventional spark plugs is naturally limited by erosion effects of the electrodes due to interaction of matter with the plasma spark. Usually, electrode erosion is increasing with increasing voltage being applied to achieve breakdown. Additionally, deposits on the electrodes influence the breakdown voltage. Fig. 13 indicates an increase of the required voltage as the test duration advances. As explained in the introduction chapter in Eq. 1, the engine efficiency is rising with increasing CR which is in other words a higher BMEP. Fig. 13: Breakdown voltage of the spark plugs of a large gas engine depending on the test duration at two different BMEP levels [32] 16 2 Principles and advantages of laser ignition Also higher combustion temperatures in the case of pre-chamber ignition reduce the lifetime of the electrodes significantly. Typical lifetimes of about 600 h for a pre-chamber spark plug are normal [32]. Special requirements to the ignition system are given in the case of burning different polluted biological gases containing, for example, silicon compounds which deposit to some extend on the electrodes [32]. Additionally, inert gases like CO 2 strongly worsen the ignitibility of such biogases, thus leading to an increased demand on breakdown voltage and ignition energy. Typical values for the lifetime of spark plugs feasible for gas engines are approximately 2000 h before first maintenance and 6000 h before exchange [32]. Since diode-pumped laser systems are expected to operate over 10000 h, they are promising candidates for a future advanced ignition system since they could reduce maintenance costs. Reliable ignition at advanced ignition pressures Like it was explained in the introduction, the engine efficiency is increasing with the compression ratio (CR) which, as a consequence, means a higher BMEP. But this also goes along with higher pressures at the instant of ignition. As discussed in the introduction chapter 2, the field strength necessary to achieve a breakdown in gases for DC and low frequency electrical fields is approximately linearly proportional to the gas pressure. For laser ignition exactly the opposite case happens: if the ignition pressure goes up the needed laser pulse energy goes down being very advantageous. So this means, in other words, that contrary to spark plug ignition, highest ignition pressures yield most efficient laser ignition. Free positioning of the ignition source and multi-point ignition Especially in the case of very lean mixtures, where the flame speed is particularly low, it is necessary to reduce the path length covered by the flame front, resulting in a shorter overall combustion time and thus in an increased engine efficiency. Unburned HCs are then reduced because the mixture can burn completely. Therefore a single-point ignition source would be best located at the center of the combustion chamber. Unfortunately, this is practically impossible in the case of large gas engines by using a conventional spark plug, because the solid body of the spark plug would significantly distort the propagating flame front. This is quite the opposite in the case of laser ignition, where the spark stays completely free of any electrodes or other solid parts and can be principally positioned everywhere inside the chamber. For further reduction of the combustion duration in the case of very lean mixtures, sometimes more than one ignition source is applied per cylinder, being hard to implement by using conventional spark plugs due to the lack of space on the cylinder head of a typical gas engine. Such multi-point ignition arrangements can be more easily performed by laser ignition since specific optical components are available to split an incoming laser beam into several parts and focus them at pre-determined locations inside the chamber. Thus it is thinkable to use just one optical window for multi-point laser ignition (see also chapter 4.4). Another interesting application of a laser ignition system is the reliable ignition of stratified DI engines, where a locally restricted mixture of rich consistence is ignited by the spark, leading to full combustion of the rest of the lean mixture. The optimum position of the ignition source for these concepts should be near to the center of the rich mixture region. However, up to now, spark plugs have to be placed at the border of these bulbs to avoid misfiring and a significant reduction of the electrode lifetime due to settlings and erosion effects. The laser does not suffer from these problems, and hence is becoming an interesting 17 2 Principles and advantages of laser ignition candidate as the ignition source for such fuel stratified engines as explained in the introduction. Proper spark timing Spark timing is one major parameter to control engine efficiency and emissions. For maximum engine performance, the instant when 50 % of the fuel is burned should be around 8 CAD (crank angle degree) after top dead center, which determines optimum ignition timing [33]. Additionally, a certain delay time between spark generation and the onset of combustion is always present and has to be taken into account. Spark timing affects peak cylinder pressure and therefore peak unburned and burned gas temperatures. Retarding spark timing from the optimum reduces these variables. For minimum NO x emissions, it is necessary to keep the combustion temperature as low as possible. Thus, retarded spark timing is sometimes used to control NO x emissions and to avoid knock, although at the expense of efficiency. The exhaust temperature is affected by spark timing, too. Retarded timing increases, from the point of optimum engine performance, the exhaust temperature. Both, engine efficiency and heat loss to the combustion chamber walls are decreased [33]. All these crucial effects make it absolutely necessary to apply an ignition system which can cope with the rigid requirement of a well-timed ignition of the combustible mixture. A typical value for the required accuracy of ignition timing is 0.5 CAD resulting in 55 μs at a speed of 1500 rpm. 18 3 Overview on literature and patents dealing with laser ignition 3 Overview on literature and patents dealing with laser ignition 3.1 Literature review This chapter describes references dealing only with laser ignition introduced by “nonresonant breakdown” as explained in the excellent basic publication by Ronney [7]. The other three basic mechanisms described there how laser ignition can be realized (i.e. thermal ignition, resonant breakdown and photochemical ignition) are not taken into account because the basic idea of this PhD thesis in general is not bases on them (except chapter 4.6). This review should give a deep insight in the published work which has been done up to now, but does not claim to be complete. Reviews Phuoc [19] and Bradley et al. [34] are presenting extensive reviews about laser-induced breakdown ignition. They both present detailed descriptions of the different ignition mechanisms and spark / flame evolution processes accompanied by explicit theoretical models. These publications represent nice and detailed introductions into the different processes associated with laser 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. 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.
Posted on: Sat, 30 Nov 2013 18:33:43 +0000
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