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 Ignition Fig. 5: Basic scheme of a laser-induced breakdown process Multiphoton ionization (MPI) process For the electron avalanche process some first free electrons are needed, and two processes in effect can supply them: MPI and seed electrons originating from impurities like aerosols, dust particles or low ionization potential organic vapors. In the MPI process, a gas molecule or atom simultaneously absorbs a number of photons. If the combined absorbed energy is higher than its ionization potential, the gas molecule is ionized. MPI is described by the reaction o e M Ȟ h m M (2) with hQ being the photon energy, m being the number of photons necessary to ionize an atom and M is the symbol for a neutral atom or molecule (see also Fig. 6). 9 Fig. 6: Illustration of a multiphoton ionization process (here: m = 3): E - electron energy, E n - occupied - - electron, hQ - photon energy, E ionization - ionization energy energy level, E ion - ionization energy level, e [20] If the ionization energy of the gaseous medium is E ionization , the number of photons m must be ª t m « ¼ ¬ MPI has been studied by many authors and several reviews of past work are available [15], [21] & [22]. For example, the ionization energy of nitrogen (N 2 ) is about 13 eV [21]. By applying Eq. 3 for Nd:YAG laser radiation (wavelength = 1064 nm), one can calculate the necessity of m = 12 photons having to be simultaneously absorbed by the N 2 molecule to achieve ionization, which is relatively improbable. Most gases have ionization energies larger than 10 eV. This leads to the conclusion that for gases multiphoton ionization is more relevant for shorter wavelengths (< 1 μm) [21] and very low pressures (< 0.01 bar) where collisional effects are negligible as it was stated by Bebb and Gold [23]. Moreover, in a first estimation Kopecek [20] calculated the electrons generated by an Nd:YAG laser with a resulting Gaussian shape, 5 ns pulses at 10 mJ, focused with 50 mm 12 W/cm 2 . Under these focal length at 1 bar and 27°C resulting in an intensity of 4x10 3 electrons can be generated by MPI making it improbable to achieve parameters 5x10 3 electrons eventually could be enough breakdown only by the MPI effect alone. However, 10 to start the impact ionization process described below in more detail, making MPI perhaps an important initial effect for plasma formation. For technical gases, especially for fuel-air mixtures entering a combustion engine, a lot of impurities are present being able to increase the probability for MPI due to the smaller ionization energy of solid particles and fluid droplets as described in the next chapter.
Posted on: Sat, 30 Nov 2013 18:33:05 +0000
Trending Topics
Recently Viewed Topics
© 2015