Large stationary, electricity producing gas engines Large stationary, internal combustion gas engines are widely employed for the generation of electrical and thermal power by using different combustible gases as, for example, natural gas, hydrogen or diverse biogases as fuels. Similar to other Otto-type engines, they are normally ignited by spark plugs of special construction having been optimized for lean mixture operation and high compression ratios (CR) which are the crucial factors for the engine efficiency (Ș) defined as: 1 1 Ș (1) 1 Ȗ CR To increase Ș, the compression ratio and the adiabatic coefficient (Ȗ = c p / c v ) have to be maximized. CR is limited by engine knock which is a serious problem at advanced compression ratios leading to destructive pressure peaks inside the cylinder and probably to a catastrophic damage of the engine. To increase Ȗ, leanest mixtures are necessary. Further on, fuel-lean combustion in natural gas engines is desirable in that it yields lower combustion temperatures which lead to lower NO x emissions. However, there are several aspects of sparkignited lean burn engines that result in ignition and combustion challenges. 1 1 Introduction and goals of this work First, lean mixtures of natural gas and air are relatively difficult to ignite with the required minimum laser pulse energy (MPE) for ignition, increasing asymptotically near both, the rich and lean ignition limits [1]. Second, with increasing CR the resulting increase in in-cylinder pressure at the time of ignition impedes the quality of the electric spark discharge within conventional spark plug based ignition systems. In 1889, Friedrich Paschen published a paper [2] which describes what is known as Paschen’s law. The law essentially states that the breakdown characteristics of a gap at constant temperature are a function of the product of the gas pressure and the spark gap. This is demonstrated in Fig. 1 whose graphs illustrate the theoretically required breakdown voltage for a uniform electric field as a function of pressure for three spark gaps. As it can be seen, the required breakdown voltage increases dramatically with both, pressure and spark gap. But as explained above, the in-cylinder pressures at the instant of ignition of reciprocating engines are increasing requiring higher breakdown voltages which finally ends in high spark plug erosions and dramatically decreased lifetimes because of the exorbitant wear of the electrodes. Further on, the additional required high voltage means increased installation costs. On the other side, the spark gap cannot be scaled down to smallest sizes because the electrode quenching would become too big and, as a conclusion, the spark plug could not ignite the required leanest mixtures. Fig. 1: Paschen’s Law - breakdown voltage in air (tungsten electrode) as a function of pressure for 0.2 mm, 0.4 mm and 0.8 mm spark gaps [2] For all above mentioned problems laser ignition represents a promising alternative, but in this place just some of the advantages for gas engines should be mentioned. The dependence of laser energy demand for ignition on pressure is exactly the opposite of Paschen’s law described above: with higher in-cylinder pressures MPE is decreasing. Further on, because for laser ignition no electrodes are needed, no quenching effects are occurring and hence much leaner mixtures can be ignited. So the combustion temperature is lowered and lowest NO x emissions are the positive consequence. Moreover, in order to reduce the long combustion durations of the lean mixtures and thereby increasing the efficiency of the gas engine, multipoint ignition can be easily applied. This is not the case for conventional spark plug ignition 2 1 Introduction and goals of this work where complex and costly constructions have to be made. For the above mentioned problems and suggested solutions, laser ignition of gas engines possibly can represents a new superior approach and should therefore be investigated in more detail in this work. Direct injection gasoline engines The requirement of a reduction of the fuel consumption of the gasoline Otto engine resulted in the development of the direct injection (DI) of fuel into the engine cylinder. The biggest advantage of such a combustion procedure, in comparison to port fuel injection, is that at part load engine operation the engine can be operated nearly without throttle losses. At port fuel injection the engine load is controlled by the throttle always keeping stoichometric air/fuel equivalence ratios (Ȝ = 1). In contrast to that, in DI engines at part load operation the load is controlled via the air/fuel equivalence ratio (Ȝ >1), but the charge has to be stratified in this case and the rich part of it has to meet the vicinity of the ignition spark. Basically, three different DI schemes are nowadays distinguished as depicted in Fig. 2: Fig. 2: Different direct injection (DI) gasoline schemes: wall-guided, air-guided and spray-guided [3] During the wall-guided DI scheme the rich mixture cloud resulting from the injection nozzle is guided through the piston bowl to the spark plug where it is ignited. It is disadvantageous that the piston partially gets wet from the fuel spray and hence high unburned hydro-carbon (HC) emissions are the result. For the air-guided scheme the rich mixture cloud is guided to the spark plug with the help of a drift which is formed through specially formed inlet valves. This drift is also called tumble [3]. In the spray-guided process the fuel is injected through the center of the cylinder head very near to the spark plug. A very exact positioning of the fuel spray and the spark plug has to be assured. If the spark plug penetrates too far into the fuel spray, resulting in a too rich mixture, the spark plug gets coked. But if the spark plug is too far away from the fuel spray a too lean mixture is the result and no ignition can take place. As it can be imagined, the spray-guided scheme is very difficult because at different loads the stream in the cylinder also changes and as a consequence the fuel spray geometry shifts. Hence the position of the spark plug is very sensible. But on the other side, the spray-guided scheme has the highest potential concerning better fuel economy and lowest emissions. In comparison to a port fuel injection engine, an about 15 % better fuel economy can be achieved with a spray-guided DI engine [3]. Further on, the knock border is higher because during the DI process the mixture gets colder as a consequence of the vaporisation heat of the injected fuel. Hence a higher CR with a resulting higher efficiency of the engine is possible. 3 1 Introduction and goals of this work And again, laser ignition offers important advantages in comparison to conventional spark plug ignition for the spray-guided DI Otto engine. For laser ignition no electrodes are needed and so no coking can happen. Employing the right focal length, the ignition plasma can be positioned very precisely. Further on, it is known out of the literature that the laser energy needed for a successful breakdown is about 3 orders of magnitude lower if liquid fuel droplets are in the focal volume [4]. Two different versions for a possible laser ignition system of DI engines are presented in Fig. 3. Furthermore, preliminary tests have shown that the fuel droplets can act as small focusing lenses which are assisting the plasma producing process by placing the plasma automatically to the right position near the fuel spray edge. So, in this thesis laser ignition of DI gasoline engines should be investigated deeply and the possibility of a practical application should be shown. Fig. 3: Setup for laser ignition for the DI Otto engine; comparison of combined and separated focusing optics [5] Laser-triggered homogeneous charge compression ignition engines (HCCI) An HCCI engine combines the advantages of both, diesel and gasoline engine operation: first, the high efficiency of a diesel engine and second the low emissions of a gasoline engine. An HCCI engine has, on the one hand, due to the lower combustion temperatures much lower NO x emissions and, on the other hand lower soot emissions due to a more homogeneous combustion process than in a diesel engine. Compared to spark ignition (SI) engine operation, the high efficiency of an HCCI engine is based on the low throttle losses. In an HCCI engine air and fuel are premixed and as the piston is reaching top dead center (TDC) the mixture auto-ignites at several locations simultaneously. But a serious problem is the control of the onset of the auto-ignition process being influenced by a lot of different parameters from the inlet gas mixture to the shape of the combustion chamber. Several different approaches to overcome this obstacle are still under development, but no reliable solution has been found yet. And again for this problem laser ignition could be a promising future solution. With a laser-induced breakdown the start of the auto-ignition can be “triggered” and so the onset of combustion can be controlled. Different investigations, which have been carried out during this PhD thesis work showed that although the plasma normally would lead to SI, the occurring combustion is a mixture of SI and HCCI so that the advantages of HCCI still are prevailing. 4 1 Introduction and goals of this work Laser-ignited high efficiency dilute gasoline engine (HEDGE) Always stricter regulations regarding the emissions for diesel engines have been and will be implemented in the future. Exhaust gas recirculation (EGR) and exhaust aftertreatment devices will be necessary to control both the NO x and particulate matter (PM) of diesel engines. The use of these devices will increase both initial and maintenance costs and reduce fuel efficiency. As a way out of this increasing cost spiral, the light-duty gasoline technology can be the solution. It has shown that it has the capability to meet the future emission standards. However, light-duty gasoline engines do not have the brake thermal efficiency (BTE) levels of diesel engines, primarily due to pumping losses and the low compression ratios necessary to avoid knock. A key solution to reduce the knock tendency and engine-out NO x levels is high EGR. An EGR rate of about 30 % would produce BTEs which come close to diesel values with BTE reducing after-treatment systems. A new name for this concept was born: HEDGE = high efficiency dilute gasoline engine. But a technology hurdle in this approach is the need for a reliable, high energy ignition system to ensure stable and misfire-free operation across the load range. A high energy ignition system is necessary because the highly diluted mixture with the high percentage of EGR is very difficult to inflame. And here again laser ignition could be a promising alternative to other very expensive high energy ignition systems. 5 2 Principles and advantages of laser ignition 2 Principles and advantages of laser ignition The combustion behavior of fuel/air mixtures, especially inside an internal combustion engine, is strongly influenced by the kind of ignition source being applied. There are a lot of similarities between laser and conventional spark plug ignition, like for example the spatially limited ignition volume, the plasma generated by an electrical spark inside the mixture, but also many crucial differences like the different time scales of the energy transfer period (several nanoseconds in the case of the pulsed laser compared to several hundred microseconds for spark plugs) and the influence of quenching surfaces like the electrodes. This chapter should introduce the four basic principles of laser ignition and should then focus on the non-resonant laser breakdown mainly used and investigated in this PhD thesis. Finally, the main advantages of laser ignition in comparison to conventional spark plug ignition should be listed up and discussed. 2.1 Different types of laser ignition According to Ronney [7], there are four inherently different types of laser ignition schemes which can be described as follows: Thermal ignition In this ignition type there is no electrical breakdown of the gas and the ignition energy is transferred via linear absorption effects to increase the kinetic energy of the target molecules. A laser source delivering quasi continuous radiation is employed to excite vibrational, translational or rotational modes of the gas molecules and, as a consequence, heats up the irradiated volume. As a result, molecular bonds are broken and chemical reactions take place. The ignition delay time usually is long. Raffel et al. [8] and Maas et al. [9] studied the laserinduced thermal ignition of O 2 /O 3 and H 2 /O 2 mixtures using a laser pulse from a TEA CO 2 laser at 9.552 μm along the axis of a cylindrical cell. Ignition occurred near the entrance window and, after ignition, the flame moved roughly into the radial and longitudinal directions due to the inhomogeneous absorption of the laser light. The most serious problem is to find a powerful laser emitting at a specific wavelength where molecular absorption takes place. Additionally, the energy can not be deposited locally but is exponentially decreasing over the whole beam path increasing the demand of laser power significantly. Photochemical ignition Highly energetic photons in the UV region are absorbed by gas molecules in this case and cause their dissociation. This process does not involve photoionization, and hence does not lead to a breakdown. The radicals produced by photolysis lead to the usual chain-branching type of chemical reactions if their production rate is greater than their recombination rate. Norrish [10] obtained ignition and combustion of C 2 H 2 /O 2 , CH 4 /O 2 , and C 2 H 4 /O 2 mixtures due to the developing chains initiated by the OH radical. Lavid and Stevens [11] studied the photoignition of premixed H 2 /O 2 and H 2 /air mixtures using laser radiation at 157, 193, and 245 nm wavelengths. In their studies, the dissociation of molecular oxygen was responsible for ignition. Up till now, lasers emitting in the ultraviolet region are either too bulky like excimer lasers or less efficient like frequency-converted solid-state lasers and hence play no role as engine igniters. 6 2 Principles and advantages of laser ignition Resonant breakdown ignition If a bound electron is excited to an upper energy level by resonant absorption of a photon, then further excitation by non-resonant ionization effects (multiphoton ionization or electron cascade growth, both being described later) can lead to ionization of the atom or molecule. Ronney [7] described another kind of resonant breakdown, which involves a non-resonant multiphoton photodissociation of a molecule followed by resonant photoionization of an atom created by the photodissociation process. The free electrons produced by the resonant multiphoton ionization process lead to breakdown of the gaseous media. This ignition type has been demonstrated for O 2 /N 2 O mixtures with a tunable UV laser operating near 225.6 nm [12] and for H 2 molecules near 243.0 nm [13]. Non-resonant breakdown ignition In this type of ignition initiation, the beam of a laser is focused tightly to achieve electrical field strengths in the focal region which exceed the breakdown threshold of the gas. To reach the necessary peak intensities, commonly Q-switched lasers are applied delivering short pulses with pulse durations of several 100 picoseconds to several nanoseconds and pulse energies of several microjoules to several joules. Of the various types of laser energy deposition, this type is the most similar to electric spark discharges, but there are still major differences between the laser and electrical breakdown. For example, the typical breakdown field strength of air at atmospheric pressure for direct current (DC) fields between two long parallel conducting plates is about 30 kV/cm [14], whereas for fields of optical frequency it is 7 MV/cm [15] (corresponding to a focused beam 11 W/cm 2 ). Further on, the breakdown field strength generally increases with intensity of 10 pressure for electric sparks, whereas it decreases for laser-induced sparks [15]. Another important difference between laser and electric sparks is that the presence of even small amounts of aerosols or particles in the atmosphere can reduce the breakdown field strength by orders of magnitude, whereas electric sparks are less sensitive to such effects. Although the needed peak intensities in the focal region are excessively high, they can be easily achieved by Q-switched lasers, which are potentially reliable and robust enough to be employed as the center part of future ignition systems for internal combustion engines. Thus, the aim of this work was to exclusively investigate this kind of laser ignition mode being therefore described in more detail in the following chapter. 2.2 Non-resonant laser-induced breakdown Soon after the development of the ruby laser, it was observed that by tightly focusing a laser beam, one could cause a breakdown in air [16], [17] & [18]. A bright spot would be visible at the focus similar in appearance to a discharge between two electrodes connected to a high DC voltage. As many experiments have shown, such laser sparks are able to ignite combustible gas mixtures like DC sparks do, too. The criteria for successful plasma formation beside the emission of light in the visible region by the plasma are the attenuation of the transmitted laser beam and the level of ionization in the focal region. Lewis and von Elbe [1] described ignition of deflagrations in premixed gases as follows: “If a subcritical quantity of energy in the form of heat and/or radicals (chemically active atoms or molecules) is deposited in a combustible mixture, the resulting flame kernel decays rapidly because heat and radicals are conducted away from the surface of the kernel and dissociated away from the surface of the kernel and dissociated species recombine faster than they are 7 2 Principles and advantages of laser ignition regenerated by chemical reaction in the volume of the kernel. The kernel extinguishes after consuming a small quantity of reactant. In the other hand, if the ignition energy exceeds a certain threshold (called the minimum ignition energy MIE) at the time when the peak temperature decays to the adiabatic flame temperature, the temperature gradient in the kernel is sufficiently shallow that heat is generated in the kernel faster than it is lost due to conduction to the unburned mixture.” This basic description was originally meant for electrically produced sparks but the same fundamental process could be seen for laser-ignited combustible gas mixtures. Fig. 4 depicts schematically the basic setup and important thresholds for successful laser ignition. The basic components which are needed for successful laser ignition are a short laser pulse in the lower nanosecond or picosecond regime which is focused with the help of for example, a single lens setup into a focal point. The first condition for a successful ignition by a laserinduced breakdown is that in the focal point a certain intensity threshold (I thr ) has to be 12 W/cm 2 which in other words corresponds to a photon exceeded. This threshold is about 10 29 photons/cm 2 s [19]. If the laser pulse exceeds this intensity, plasma is flux of about 10 formed and, as it can be easily shown with pulse energies below 1 mJ it is possible to ignite a stoichometric mixture (E thr ). But for ignition of mixtures near the lean limit, which are important for stationary gas engines as explained in the introductory chapter, beside I thr the plasma also requires certain minimum pulse energy (MPE) to support a self-propagating flame kernel followed by flame propagation. For lean methane/air mixtures this energy is about 5-10 mJ for a nanosecond laser pulse. Fig. 4: Principle of laser ignition with indicated intensity threshold (I thr ), minimum breakdown energy (E thr ) and minimum pulse energy (MPE) for successful ignition MPE has to be discerned from MIE; whereas MIE is exactly the energy necessary to yield ignition inside the combustion vessel, MPE is the total pulse energy needed to generate ignition. All other parameters held constant, MPE is greater than MIE because a significant part of the laser pulse is transmitted through the plasma. It also includes losses such as reflections at the window of the combustion vessel. Since MIE is prone to experimental errors and uncertainties, MPE is a much more significant parameter than MIE. 8 2 Principles and advantages of laser ignition 2.2.1 Basic steps of a non-resonant breakdown In this chapter the basic steps from the first free electrons to laser-induced plasma should be coarsely discussed. The different steps are graphically depicted in Fig. 5. The process begins with multiphoton ionization (MPI) of a few gas molecules which release electrons that readily absorb more photons via the inverse bremsstrahlung process to increase their kinetic energy. The presence of impurities, such as aerosol particles or low ionization potential organic vapors, can also significantly facilitate the generation of the initial electrons. The electrons liberated by this process collide with other molecules and ionize them, leading to an electron avalanche, and finally to a breakdown of the gas. For very short pulse duration (few picoseconds) the multiphoton processes alone must provide breakdown, since there is insufficient time for electron-molecule collision to occur. In the next sub-chapters the different steps will be discussed more deeply. 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. 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.
Posted on: Mon, 28 Oct 2013 17:36:10 +0000
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