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.
Posted on: Thu, 31 Oct 2013 18:49:15 +0000
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