Laser Ignition of Internal Combustion Engines

Contents

1 Introduction and goals of this work

2 Principles and advantages of laser ignition
2.1 Different types of laser ignition
2.2 Non-resonant laser-induced breakdown
2.2.1 Basic steps of a non-resonant breakdown
2.3 From the laser spark to combustion
2.4 Advantages of laser ignition

3 Overview on literature and patents dealing with laser ignition
3.1 Literature review
3.2 Patent review

4 Experimental investigation of laser ignition in a constant volume combustion chamber
4.1 Basic experimental setup
4.1.1 Employed combustion chambers
4.1.2 Mixture preparation, pressure measurement and experimental procedure
4.2 Extensive comparison of spark plug and laser ignition
4.2.1 Experimental setup
4.2.2 Results and discussion
4.3 Investigation of laser ignition at elevated temperatures
4.3.1 Investigation of the lean limit at elevated temperatures
4.3.2 Investigation of the minimum breakdown energy at elevated temperatures and high pressures
4.4 One-, two- and three-point ignition of hydrogen-air mixtures
4.4.1 Experimental setup
4.4.2 Results and discussion
4.5 Schlieren diagnostics of multi-point laser ignition and spark plug ignition
4.5.1 Experimental setup
4.5.2 Results and discussion
4.6 Resonant initiation of auto-ignition of n-heptane-air mixtures by an Er,Cr:YSGG laser
4.6.1 Experimental setup
4.6.2 Results and discussion

5 The optical focusing element
5.1 Relation of NA and Ethr for different focusing optics
5.2 The aspheric lens window (ALW)
5.3 Theoretical comparison of spheric and aspheric focusing lenses
5.4 Long term tests of different lens window materials at atmospheric conditions
5.4.1 Experimental setup
5.4.2 Results and discussion

6 Different experiments on the IC engine
6.1 Long term tests of different window materials and focusing systems
6.1.1 Experimental setup
6.1.2 Contamination of the combustion window
6.1.3 Comparison of separated and combined spheric focusing optics
6.2 Direct comparison of laser and spark plug ignition
6.2.1 Experimental setup
6.2.2 Results and discussion
6.3 Application of a α-prototype laser to an IC engine
6.3.1 Description and details of the α-prototype laser
6.3.2 First successful 100 h test with laser head from the engine decoupled
6.3.3 First successful test with laser head directly mounted on the cylinder head

7 Laser-triggered HCCI engine
7.1 Fuel: 80% isooctane & 20% n-heptane
7.1.1 Experimental setup
7.1.2 Results and discussion
7.2 Fuel: 100 % natural gas
7.2.1 Experimental setup
7.2.2 Results and discussion

8 Laser ignition of HEDGE engine operation
8.1 Constant volume combustion chamber experiments
8.1.1 Experimental setup
8.1.2 Result and discussion
8.2 Single-cylinder engine experiments
8.2.1 Experimental setup
8.2.2 Results and discussion

9 First design and realization of an own β-prototype laser ignition system
9.1 Basic description of a first β-prototype laser ignition system
9.2 Brief literature review on passively Q-switched, solid-state laser systems
9.3 First experimental setup and used components
9.4 Results and discussion

10 Summary, conclusions and outlook

Nomenclature

References

Appendix A

Appendix B

Collaborations

Scientific achievements

Journal Publications

Conference presentations

Patents

Co-supervision of diploma and baccalaureate theses

Curriculum vitae

1 Introduction and goals of this work

Ignition is defined as the transformation of a combustible material from an unreactive state to a self-propagating state where the ignition source can be removed without the combustion process extinguishing. Most practical combustion devices (for example s park i gnition (SI) engines) require combustion events to be initiated at predetermined locations and times. For more than 100 years, electric sparks mainly generated by spark plugs have been the primary means of accomplishing this task. But because of different reasons like increasing m ean e ffective p ressures (MEP) at stationary gas engines for higher engine efficiencies, the need of ignition of leanest mixtures for lowest NOx emissions and moreover higher efficiencies, low lifetime of the spark plugs and the need of precise location of ignition without electrodes, the electrical spark ignition comes to its physical borders. And here a new ignition system like laser ignition is a promising future solution with all the, in the next chapter described, advantages.

In this PhD thesis different aspects of laser ignition are investigated: from first experiments in combustion chambers to determine the boundary conditions over an extensive characterization of the optical focusing element till detailed engine experiments; different aspects have been studied, various problems could be identified and possible solutions are presented.

There are some main engine applications where laser ignition can play out its main advantages:

- large stationary, electricity producing gas engines
- d irect i njection (DI) gasoline engines
- laser-triggered h omogeneous c harge c ompression i gnition (HCCI) engines
- laser-ignited h igh e fficiency d ilute g asoline e ngines (HEDGE)

In the following chapters each of the different engine applications will be explained in detail.

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 c ompression r atios (CR) which are the crucial factors for the engine efficiency (η) defined as:

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To increase η, the compression ratio and the adiabatic coefficient (γ = cp / cv) 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 γ, 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 NOx emissions. However, there are several aspects of spark-ignited lean burn engines that result in ignition and combustion challenges.

First, lean mixtures of natural gas and air are relatively difficult to ignite with the required m inimum laser p ulse e nergy (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.

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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 NOx 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, multi-point ignition can be easily applied. This is not the case for conventional spark plug ignition 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 d irect i njection (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 (λ = 1). In contrast to that, in DI engines at part load operation the load is controlled via the air/fuel equivalence ratio (λ >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:

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Fig. 2: Different d irect i njection (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 h ydro- c arbon (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.

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.

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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 NOx emissions and, on the other hand lower soot emissions due to a more homogeneous combustion process than in a diesel engine. Compared to s park i gnition (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 t op d ead c enter (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.

Laser-ignited h igh e fficiency d ilute g asoline e ngine (HEDGE)

Always stricter regulations regarding the emissions for diesel engines have been and will be implemented in the future. E xhaust g as r ecirculation (EGR) and exhaust aftertreatment devices will be necessary to control both the NOx and p articulate m atter (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 b rake t hermal e fficiency (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 NOx 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 = h igh e fficiency d ilute g asoline e ngine.

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.

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.⁶

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 laser-induced thermal ignition of O2/O3 and H2/O2 mixtures using a laser pulse from a TEA CO2 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 C2H2/O2, CH4/O2, and C2H4/O2 mixtures due to the developing chains initiated by the OH radical. Lavid and Stevens[¹¹] studied the photoignition of premixed H2/O2 and H2/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.

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 O2/N2O mixtures with a tunable UV laser operating near 225.6 nm[¹²] and for H2 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 d irect c urrent (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 intensity of 10[11] W/cm[2] ). Further on, the breakdown field strength generally increases with 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 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 m inimum i gnition e nergy 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 laser-induced breakdown is that in the focal point a certain intensity threshold (Ithr) has to be exceeded. This threshold is about 10[12] W/cm[2] which in other words corresponds to a photon flux of about 10[29] photons/cm[2] s[¹⁹]. If the laser pulse exceeds this intensity, plasma is formed and, as it can be easily shown with pulse energies below 1 mJ it is possible to ignite a stoichometric mixture (Ethr). But for ignition of mixtures near the lean limit, which are important for stationary gas engines as explained in the introductory chapter, beside Ithr the plasma also requires certain m inimum p ulse e nergy (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.

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Fig. 4: Principle of laser ignition with indicated intensity threshold (Ithr), minimum breakdown energy (Ethr) and m inimum p ulse e nergy (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.

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 m ulti p hoton i onization (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.

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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

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with hn 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).

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Fig. 6: Illustration of a multiphoton ionization process (here: m = 3): E - electron energy, En - occupied energy level, Eion - ionization energy level, e- - electron, hn - photon energy, Eionization - ionization energy[²⁰]

If the ionization energy of the gaseous medium is Eionization, the number of photons m must be

Abbildung in dieser Leseprobe nicht enthalten (3)

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 (N2) 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 N2 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 focal length at 1 bar and 27°C resulting in an intensity of 4x10[12] W/cm[2]. Under these parameters 5x10[3] electrons can be generated by MPI making it improbable to achieve breakdown only by the MPI effect alone. However, 10[3] electrons eventually could be enough 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.

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 volume[²⁴],[²⁵] &[²⁶]. Under normal conditions, there are more than 10[7] particles per mm[3] 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:

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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 width is greater than 10-[10] bar · s.

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.

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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 H2 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.

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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.

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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 p lanar, l aser- i nduced f luorescence (PLIF); colors refer to relative fluorescence intensities of the OH molecules; laser pulse energy Epulse : 50 mJ; CH4-air mixture; initial pressure : 4 bar; laser spark already ceased[³¹]

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].

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Fig. 10: Large gas engine (GE Jenbacher); max. 3 MW of electrical power; nominal speed: 1500 rpm (r evolutions p er m inute)[³²]

The following advantages of laser ignition in comparison to conventional spark plug ignition are mainly focused on gas engines:

- Ignition of leanest mixtures feasible => lower combustion temperatures => lower NOx emissions
- No erosion effects occurring like in the case of spark plugs leading to significantly longer availability of laser ignition systems
- Higher load/ignition pressures up to 35 bar applicable => increase in engine efficiency
- 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.
- Simplified possibility of multipoint ignition to speed up the combustion process for highest engine efficiencies especially for lean mixtures
- Precise ignition timing possible for optimal engine performance and maximum efficiency
- Shorter ignition delay time
- 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.

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 CO2 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 NO2), unburned h ydro- c arbons (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 NOx 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 λ = 1. The mixture ratio can be also characterized (according to English literature) by the symbol φ which is inversely proportional to λ. But because the use of the symbol λ is prevailing in the engine related literature, the air/fuel equivalence ratio will be described by this λ 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 (λ = 1.85 for natural gas using conventional spark plugs[32] ) to reduce NOx emissions, they suffer from the drawback of increased HC emissions.

illustration not visible in this excerpt

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 (λ); 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.

illustration not visible in this excerpt

Fig. 12: Breakdown voltages of the spark plugs of a large gas engine depending on b reak m ean e ffective p ressure (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.

illustration not visible in this excerpt

Fig. 13: Breakdown voltage of the spark plugs of a large gas engine depending on the test duration at two different BMEP levels[32]

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 CO2 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 c ompression r atio (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 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 (c rank a ngle d egree) 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 NOx emissions, it is necessary to keep the combustion temperature as low as possible. Thus, retarded spark timing is sometimes used to control NOx 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.

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 “non- resonant 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 i nternal c ombustion (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 potential of lowest NOx 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 p hotonic c rystal f iber (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 m inimum i gnition e nergy (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 p lanar l aser i nduced f luorescence (PLIF of hydroxyl radicals) measurements of laser-induced 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 NH3/O2 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 laser pulse (10-[9] -10-[8] s), (II) plasma formation process (10-[8] -10-[7] s), (III) ignition process (10-[6] -10-[4] s), and (IV) shock–flow interaction and (V) convective diffusion processes (~ 10-[5] s). This model is in good agreement with the model of the author of this PhD thesis which is explained later.

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 R-[3] (Rshock wave radius) for spark energies ranging from 15 to 50 mJ. Within the first few 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 laser-induced 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 m inimum p ulse e nergy (MPE) for ignition below 0.5 mJ for a λ 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 m inimum i gnition e nergy (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 f ull w idth at h alf m aximum (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, N2, H2, CH4, air, and D2 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 N2 and O2 using 1064, 690 and 530 nm laser beams at very low initial pressures.

The wavelength dependence of breakdown thresholds in He and Ar was studied by Byron and Pert[⁶³] in the year 1979. The argon data showed a λ-[2] dependence (longer wavelength => smaller breakdown threshold) but the helium data were wavelength independent.

Syage et al.[⁶⁴] studied the ignition of hydrogen/air and hydrogen/air/CO2 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 CH4-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 m ulti p hoton i onization (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

Laser ignition of jet-diffusion flames with different Reynolds number (RE = 35-103 cm[3] /s) 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].

Laser ignition under engine and gas turbine like conditions

First laser ignition experiments with a CO2 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 e xhaust g as r ecirculation (EGR) and with 16 % EGR, respectively.

Hicks et al.[⁷⁷] studied the ignition probability of a gas turbine using a conventional wall-mounted s urface d ischarge i gniter (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 S outh w est R esearch I nstitute (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 N ational E nergy T echnology L aboratory (NETL)[⁸²] &[⁸³]. They found out that the lean limit could be extended with laser ignition in comparison with conventional spark ignition from λ = 1.87 to 1.95 (λair/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 λ = 2.1 and reached lowest NOx emissions of 0.22 g/kWh.

In another publication by Kopecek et al.[⁸⁵] it was shown for the first time that with laser-induced plasma the start of the combustion in a h omogeneous c harge c ompression i gnition (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.

Jetzinger et al.[⁸⁶] investigated the performance of laser ignition on a direct injection fuel- stratified 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 r apid c ompression m achine (RCM). They used a laser with a bad beam profile (M[2] < 5) and a short focal length of with f = 13 mm. It was possible to expand the lean limit from λ = 1.67 to λ = 2, however requiring pulse energies up to 80 mJ. Once more, the shorter ignition delay and rates of pressure rise have been shown for lean mixtures.

Prototype laser ignition systems

This last sub-chapter of the literature review is aimed to introduce different research groups in the world which are developing ignition laser prototypes which should be cheap, reliable and capable to be employed on an IC engine. McMillian et al. from NETL[⁸⁸] &[⁸⁹] published a paper on a prototype laser ignition system with maximum output energy around 6 mJ at 10 ns. They used a passively Q-switched, transversal diode-pumped laser system at pump powers between 250 up to 300 W.

The group around Kroupa from C arinthian T ech R esearch (CTR) developed a transversal diode-pumped, passively Q-switched laser system with pulse energy of about 15 mJ at 8 ns. It should be noticed however, that this system is pumped by a very high power of 1.2 kW and hence the crystal has to be cooled in a very complex way involving three different cooling circles.

The company BOSCH with the researchers Ridderbusch and Herden[⁹⁰] are currently developing a longitudinally diode-pumped, fiber-coupled and passively Q-switched laser in an oscillator amplifier arrangement with maximum pulse energies of about 10 mJ at around 5 ns which is to the knowledge of the author the highest value for this type of laser system.

3.2 Patent review

The patents discussed in the following deal with different aspects of laser ignition. This list does not claim to be complete. It should just give an insight into the patent situation on the market which is connected to the topic of this PhD thesis.

Different patents about laser ignition

The US patent by Gupta et al.[145] shows a concept where the laser beam is produced in one master laser and is then distributed through a rotating mirror system to the different engine cylinders which are each supplied with a laser plug. Further on, with a fiber detecting system the invention can detect misfires.

Winkelhofer et al.[146] designed a passively Q-switched, radially diode-pumped, water-cooled solid-state laser specially designed for laser ignition applied to a cylinder of an engine. It can be directly mounted in a standard spark plug hole and delivers about 20 mJ in about 7 ns. It is the only patent containing a description how to build a laser capable for the ignition of an engine.

Patent No. DE 101 45 944 A1[147] describes an invention where in a standard spark plug an optical waveguide is integrated which is capable to guide the highly intensive laser pulse for ignition. The end of the fiber is specially designed so that it can focus the laser beam by itself. Hubert and Keesmann[148], Vowles[149] and Mourad[150] are presenting older patents which are describing very basic but fundamental ideas about laser ignition systems.

Schick and Lindstedt[151] report in their invention on a laser ignition system based on a gas laser of the year 1986. The gas laser used for the system is not specified. Feichtinger et al. of AVL List GmbH[152] suggest a high energy transporting fiber which is mounted in the gasket between the cylinder head and cylinder. The fiber is molten at the end so that it can focus the laser light.

Endo[153] shows a laser ignition unit which consists of semiconductor laser capable to produce high energy ignition pulses. Finally, Tsutsomu from Nissan Motor Co LTD[154] presents an adjustable optical focusing unit of a laser ignition system for engine operation. The flame propagation is detected by eight optical fibers and the focal point is dynamically adjusted to optimize the combustion process.

One of the best patent (or THE best), in the opinion of the author of this PhD thesis, is presented by Herden et al., employee of the company Robert BOSCH GmbH[155]. This patent describes exactly the same laser system design which was and still is under investigation in the research group of the author of this PhD thesis and which is most promising. The laser system design exists mainly out of a longitudinally, diode-pumped, fiber-coupled and passively Q-switched solid-state laser and is described in detail, theoretical and experimental in chapter 9. In this patent, also the need of a hot combustion window for minimized contamination is identified which is described later in chapter 6.1.2. In cooperation with BOSCH, the company SLS with the employee Ozygus[156] presents an invention for a cheap beam shaping optics of the diode pump radiation for efficient and most important, cheap coupling into the fiber. This can be an important patent to reduce the high costs of the pump power supply.

Patents dealing with the combustion window needed for laser ignition

Another very important part of a future laser ignition system is the combustion window or l ens w indow (LW) through which the highly intensive laser pulse is guided into the combustion chamber. In the following the different ideas on how to keep the window clean from combustion products are presented.

Patent No. JP 9250438[157] contains an idea in which the combustion window is situated in a cavity in the cylinder head. After ignition, during the combustion process pressurized air is blown into the cavity, maintaining the cavity clean from combustion products and keeping the window clean. Shohei et al.[158] suggests a high pressure fuel spray for cleaning the window before each ignition attempt.

Direct injection, fuel spray laser ignition patents

Ufermann[159] presents a high pressure fuel injector with two integrated optical waveguides which transport and focus the highly intensive ignition pulse. The focal points are placed at the edge of the spray for optimum air/fuel ratios and ignition characteristics. Further on, it is explained that the two beams can be focused at one point so that the first short, low energy pulse produces plasma and the second long, high energy pulse is delivering the real ignition energy. DeFreitas et al.[160] explains a laser ignition system which ignites an atomized fuel spray of a combustor with at least one nozzle with a nanosecond, focused laser beam. An ignition system for gas turbines is presented in ref.[161] from Few & Lewis. Here again, in a very detailed way the ignition of fuel droplets is explained.

Laser target ignition patents

A number of patents are dealing with highly intensive laser pulses focused on a target and producing the ignition plasma there. Advantage: lower breakdown energies because of the lower ionization potential of a solid target; disadvantage: need of a target => quenching, limited choice for free positioning. Noriyasu and Hirokazu of Mitsubishi Heavy Ind. LTD[162] intend to separate the target fluid spray (for example water) to be introduced in the combustion chamber and absorbing the ignition laser pulse. In this way the ignition plasma in the target fuel is generated and this consecutively ignites the air-fuel mixture.

Mukainakano et al.[163] from the spark plug producing company Nippondenso Co., LTD introduce an invention in which particles with a high light absorption factor are introduced in the combustion chamber and are capable to absorb enough laser light to produce a torch, igniting the mixture. Different versions of two-point ignitions and deflection of the laser pulse through the combustion chamber via mirrors are presented. A solid-state target mounted in the engine cylinder on which the laser beam is focused and ignition plasma is produced is described in ref.[164] by Junichiro. A fuel spray is guided towards the target for ignition. Junichiro[165] moreover suggests in another patent, to focus the igniting laser beam on the piston surface for ignition.

Resonant laser ignition patents

Schick[166] describes a resonant ignition system for lean burn engines. In this invention a layer of many laser beams is created so that the energy is absorbed by the air-fuel mixture and ignition occurs. In the year 1980 Giacchetti[167] presented a resonant laser ignition system for internal combustion engines. In the combustion chamber different mirrors are embedded and consecutively the laser beam is distributed throughout the air/fuel mixture for best ignition properties. Moreover in the year 1980, Kroy[168] introduced in a very interesting invention the idea of resonant laser ignition with a ruby laser. He also explains the advantage of a high n umerical a perture (NA) for good focussing properties. And finally, Few & Lewis published a patent dealing with resonant ignition of fuel droplets in a gas turbine[169]. The laser source is an ultraviolet flashlamp or an excimer laser.

Two- and multi-point ignition patents

A two-point ignition system for engine operation is described by Toshibumi of the year 1987[170]. The laser beam is divided by a beam splitter into two parts and focussed by two lenses situated in the cylinder head to ignite the combustible mixture. Patent No. JP 9303244[171] uses three beam splitter to divide the incident beam into three parts which are focussed through one lens to three neighbouring focal spots, igniting the mixture.

The invention published by Lenz et al.[172] uses an electrical switchable mirror which guides the beam to two lenses with different focal lengths. So the ignition point can be adjusted in association with the engine operation point. Further on, the laser beam can be split into two beams to achieve two focal points with different distances to the cylinder head.

Sequential laser ignition patents

The idea of a sequential laser ignition is more than 20 years old. A patent of the year 1983 of Nishida et al.[173] of the company Nippondenso Co. LTD published this idea first. In principle a first, very short, low energy but highly intensive laser pulse is generating a breakdown in the combustible mixture. After some nanoseconds a second, longer pulse but with higher energy and lower peak intensity increases the energy of the plasma and ensures the ignition of the combustion mixture. With the first high intense pulse which has a sharp temporally rising pulse edge, the plasma is formed very fast and the losses through transmission are reduced. The second longer pulse with higher energy increases the energy of the plasma to ensure the ignition of very lean mixtures and the transmission losses are minimized. Early et al. published two patents about sequential laser ignition[174] &[175]. He explained in detail different laser system options how to get this special temporal laser pulse shapes.

4 Experimental investigation of laser ignition in a constant volume combustion chamber

These experiments, carried out in several constant volume combustion chambers represent a systematic continuation of the diploma and PhD thesis of Kopecek whose results are published in numerous papers and are concluded in his PhD thesis with the title “Laser Ignition of Gas Engines”[20]. In a constant volume combustion chamber (“bomb”) the main characteristics of laser ignition can be investigated much more easily and with less effort compared to a real engine. The main drawback of such experiments is of course that in the bomb laminar conditions predominate and turbulent conditions like in a cylinder of a real engine never can be investigated. Nevertheless, the various results indicate the direction in which the research work should be focused and important cognitions for first prototype laser ignitions systems on the real engines could be obtained. In this chapter, a systematic comparison of laser ignition and conventional spark plug ignition especially at the lean limit was carried out. Further on, high temperature ignitions up to 400°C have been carried out. Schlieren pictures of the early stages of laser ignition experiments have been taken. The correlations of NA, initial temperature and pressure of different optical focussing optics have been investigated. And finally, the resonant start of auto-ignitions of n-heptane-air mixtures with an Er,Cr:YSGG emitting at 2.77 µm, absorbed by small amounts of introduced water have been studied.

4.1 Basic experimental setup

The different experimental setups for the specific experiments are explained in the corresponding chapters. In this chapter, only the two combustion chambers which have been used and basic gas mixture preparation, measurements techniques and the experimental procedure will be explained.

4.1.1 Employed combustion chambers

For the following experiments 2 different types of combustion chambers have been used: combustion chamber 1 (constructed by four students of the HTL Steyr, Austria) has a volume of 0.9 dm[3] with an inner diameter and length of 70 mm and 220 mm, respectively, as depicted in Fig. 14. It can withstand max. 250 bar and can be heated up to 200°C to avoid water condensation at the windows. The chamber is equipped with 4 sapphire windows of 13 mm aperture and 5 mm thickness to allow ignition and optical diagnostics of the plasma and flame kernel development perpendicular to the ignition setup. Sapphire (Al2O3) is a crystalline material of high mechanical and chemical resistance and has only 7 % reflection per surface for all wavelengths between 300 nm and 3 µm.

In order to investigate the characteristics of laser ignition at higher temperatures and to be able to do auto-ignition experiments, a new chamber was constructed by Tesch[115] which could be heated up to 400°C and a max. pressure of 200 bar. Combustion chamber 2 has a volume of 0.6 dm[3] and an inner diameter and length of 70 mm and 160 mm, respectively, as depicted in Fig. 15. The chamber is equipped with two sapphire windows of 13 mm aperture and another two windows with 22 mm aperture for better optical diagnostic capability.

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Fig. 14: Combustion chamber 1: pressure resistance of 250 bar; can be heated up to 200°C; equipped with four sapphire windows of 13 mm diameter and 5 mm thickness; 0.9 dm[3] volume; diameter and length: 70 mm x 220 mm

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Fig. 15: Combustion chamber 2: pressure resistance of 200 bar; can be heated up to 400°C; equipped with two 13 mm diameter and two 22 mm diameter sapphire windows; 0.6 dm[3] volume; diameter & length: 70 mm x 160 mm

4.1.2 Mixture preparation, pressure measurement and experimental procedure

Fig. 16 shows the gas filling setup, chamber heating technique and how the pressure history was recorded. This experimental setup basically was the same for both combustion chambers used. The gaseous fuels methane, hydrogen and biogas with purities of < 99.9 % have been used in order to yield data relevant for practical applications. As the oxidizer water free air has been employed. To achiev­e the intended ratio of the gaseous mixture compo­nents according to the partial pressure method (Dalton), it was necessary to measure the partial pressures of fuel and air by using a high resolution pressure meter. This method was confirmed by accompanying measurements of the methane content in the combustion chamber before ignition with gas chro­matography. Beside gaseous fuels also isooctane and n-heptane have been used. For the liquid fuels isooctane and n-heptane the calculated amount of fuel was injected with an injection nozzle into the chamber and proximate it was filled up with air.

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Fig. 16: Basic experimental setup for combustion bomb experiments: 1 - combustion chamber; 2 - eight heating rods mounted inside boreholes of the cylinder wall; 3 - four sapphire windows for the igniting laser beam and optical diagnostics; 4 - pressure transducer; 5 - temperature controller for heating rods; 6, 9, 10, 15 - ball valves; 7 - exhaust gas analysis (O2, CO, CO2); 8 - vacuum pump; 11, 16 - pressure meter to measure partial pressure of the fuel and initial pressure of the mixture respectively; 12, 17 - back flow protection valves; 13, 18 - needle valves to achieve precisely filled mixtures; 14, 19 - pressure controller to reduce and stabilize high output pressure of the gas bottles

Before each ignition event, the combustion chamber was first flushed with air and evacuated thereafter to remove all residual products of the previous combustion. To reliably ensure a homogeneous mixture, the fuel being the species of lower partial pressure was filled in first. In this way, homogeneity could be easily achieved by the turbulences of the following incoming stream of air. Additionally, three minutes of waiting time were applied before each ignition attempt in order to achieve stabilized mixture conditions. After successful ignition and combustion it was possible to measure O2, CO and CO2 in the exhaust gas with an exhaust gas analysis device (Madur Electronics GA-20).

The combustion process was characterized by its pres­sure history measured by a piezoelectric pressure trans­ducer (Kistler). The sen­sor signal had to be amplified in a charge amplifier and was recorded in a digital storage oscilloscope connected to a personal computer. In both chambers, in the walls eight heating rods were mounted to be able to investigate ignition and combustion under elevated temperatures and further on to avoid water condensation on the sapphire windows.

4.2 Extensive comparison of spark plug and laser ignition

As explained in the introduction chapter, for lowest NOx emissions it is very important to drive the engine at the leanest operation point possible. One main advantage of laser ignition is that it is capable to ignite leaner mixtures than conventional spark plug ignition. This should be demonstrated in detail by an extensive comparison of laser and spark plug ignitions in the constant volume combustion chamber. To have statistically confirmed results, more than 1800 ignition attempts have been carried out. This work is published in the SAE paper No. 2005-01-0248 and was presented on the SAE conference in Detroit in the year 2005[113].

4.2.1 Experimental setup

The experimental setup with emphasis on the optical scheme of the igniting beam is depicted in Fig. 17. The beam of a Q-switched Nd:YAG laser (Litron NANO-T-200-20) was focused into combustion chamber 1 (see Fig. 14 and description). The laser pulse duration was about 5 ns (FWHM), the original beam had a diameter of 2.5 mm (1 / e[2] ), and the beam quality described by the M[2] factor was about 1. A part of the laser beam (about 4 %) was used to sample the energy of each pulse by using a pyroelectric detector (Spectro­Las PEM21) and a power meter (LEM2020). For a re­liable comparison with the spark plug experiments, the laser pulse energy was held constant at 25 mJ per pulse which is far above the minimum pulse energy normally needed for a plasma breakthrough achieved with this optical setup and initial conditions. After the beam sampler the laser beam was guided by 3 mirrors (reflectivity ~ 99.5 %) to an expanding and collimating lens which expanded the beam diameter from 2.5 to 8.2 mm to yield a higher n umerical a perture (NA). In these experiments the NA was 0.041. A lens with focal length of f = 100 mm focu­sed the laser beam to cal­culated intensities of about 10[12] W/cm[2]. The distance of the laser-induced plasma to the chamber wall was 60 mm.

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Fig. 17: Setup for the experimental comparison of laser and spark plug ignition in the combustion bomb: 1 - Nd:YAG ignition laser; 2 - λ/2 waveplate; 3 - beam sampler (4 %); 4 - pyroelectric detector (SpectroLas PEM21); 5 - power meter (LEM2020); 6 - high reflection mirrors (99.5 %); 7 - expanding lens (f = -50 mm); 8 - collimating lens (f = 200 mm); 9 - focusing lens (f = 100 mm); 10 - sapphire window; 11 - combustion bomb 1; 12 - spark plug (GE Jenbacher P7); 13 - ignition system (Woodward)

For the spark ignition experiments a spark plug from the company GE Jenbacher (type P7) with an electrode dis­tance of 0.35 mm and a maximum electrode voltage of 30 kV was used. The spark had a typical duration of 400 - 500 µsec. With this spark plug it is possible to reliably run a Jenbacher gas engine with a maximum BMEP of 19 bar at a leanest limit of λ = 1.77. The energy amount for one spark plug discharge was around 180 mJ with delivering an energy of about 20-25 mJ at the electrodes. Four peripheral electrodes are located around the center one. An illustration of the spark plug can be found in Fig. 18. Such a commercially available gas engine spark plug was deliberately chosen for this com­parison to achieve more practically relevant results. For a correct compari­son of the two different ignition systems, the electrically generated spark had the same distance of 60 mm to the chamber wall like the laser-induced plasma (see Fig. 17).

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Fig. 18: Spark plug type P7 for gas engines of GE Jenbacher; electrodes are located peripherally around the center one; electrode distance: 0.35 mm; electrode voltage: 30 kV; typical spark duration: 400-500 µsec.

4.2.2 Results and discussion

Fig. 19 depicts a comparison of the ignition probabilities when using the two different ignition systems. For the sake of a statistically significant result, more than 1800 ignition attempts were carried out. The figure is a 2D-diagram where on the abscissa the air/fuel equivalence ratio l and on the ordinate the initial chamber pressure is plotted. The ignition probability is illustrated by different colours. Every crossing of the black grid lines depicts at least 4 experiments. In the more interesting lean zone more experiments were made. Between these crossings an extrapolation of the ignition probability was made. The ignition probability Pign was defined as

Abbildung in dieser Leseprobe nicht enthalten (5)

where Zign is the number of successful ignitions and Ztot the total number of ignition attempts. Ztot was kept vari­able to minimize experimental efforts but still obtaining reliable results. A completely satisfying performance of the ignition source is indicated by values for Pign close to 100 %. Pign = 0 % indicates that the mixture cannot be ignited under the current conditions.

The first and most important result to be seen in Fig. 19 is the leaner ignition limit for laser ignition, especially in the low initial pressure region. For example, for an initial pressure of 15 bar the lean limit for laser ignition is about l = 2.0 in comparison to the one for a spark plug of about l = 1.65. Only in the high initial pressure region (35-40 bar) about the same lean limit of laser and spark plug ignition could be observed.

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Fig. 19: Comparison of ignition probability of spark plug and laser-ignited methane-air mixtures; initial chamber pressure versus air/fuel equivalence ratio; initial temperature: 200°C

Further on, the transition from 100 % to 0 % ignition probability (lean limit) for the spark plug is not well defined in comparison to the laser ignition expe­riments. This can be explained by the exactly defined quantity of ignition energy deposited in the ignition plas­ma. By the spark plug, sometimes two and occasionally even three sparks were generated. The lean limit of the spark plug experiments shows a strong dependence on the initial pressure which can be explained in the lower breakdown voltage at lower pressures leading to reduced energy feed into the spark. Less energy in the spark cor­responds to a smaller spark which again cannot ignite ultra-lean mixtures. On the other side, at high initial pres­sures (30-40 bar), high breakdown voltages had to be used, consequently resulting in a spark probability below 100 % and high electrode erosion which is very disadvantageous for the application in stationary, electricity producing gas engines where long lifetimes are indispensable.

Fig. 20 illustrates a comparison of the ignition delay times of spark plug and laser ignition at an initial pressure of 30 bar and an air/fuel equivalence ratio of l = 1.73. In these measurements, the ignition delay was defined as the time interval between the appearance of the laser pulse (discharge of the spark coil) and the instant when 5 % of the peak pressure was reached. A clearly shorter ignition delay can be found and also the standard deviation of laser ignition is smaller.

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Fig. 20: Ignition delay of laser and spark plug ignition; initial pressure: 30 bar; l: 1.73; initial temperature: 200°C

Fig. 21 shows the ignition delay at a fixed l = 1.73 depending on the initial pressure. Again, like in Fig. 20, the slightly shorter ignition delay of the laser ignition can be seen. But the most important result in this diagram is the smaller standard deviation of the ignition delay for laser ignition which can be explained in the well defined deposition of energy in the laser plasma. Such a reduced variance is advantageous for the ignition timing and con­tributes to a reduction of the cycle-to-cycle variations of engine combustion.

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Fig. 21: Ignition delay and standard deviation of the ignition delay versus initial pressure of laser and spark plug ignition; complete combustions only; l: 1.73; initial temperature: 200°C

To conclude, laser ignition shows a significantly higher ignition performance compared to conventional spark plug ignition for initial pressures up to 30 bars. Further on, the ignition delay is shorter and due to the always exactly identical quantity of energy deposition in the ignition plasma the ignition delay is much more precisely defined.

4.3 Investigation of laser ignition at elevated temperatures

To provide more understanding of laser ignition, also for higher initial temperatures than 200°C provided by the combustion chamber 1, a new combustion chamber which can be heated up to maximum temperatures of 400°C was constructed (combustion chamber 2, see details in chapter 4.1.1 and the master-thesis of Tesch[115] ). Higher initial temperatures are also interesting because they are nearer to engine like conditions.

4.3.1 Investigation of the lean limit at elevated temperatures

The following results have been published at the European Combustion Meeting in Louvain-la-Neuve, Belgium 2005 and are included in the corresponding Conference Proceedings[114].

The experimental setup is basically the same like explained in Chapter 4.2.1, Fig. 17. As ignition laser again the Litron NANO-T-200-20 was used like in the previous chapter. Only the NA was increased to 0.16 because of the slightly different construction of the combustion chamber 2. The beam was expanded to 16 mm and for the focusing setup a plano-convex focusing lens (f = 100 mm) and a meniscus lens (f = 100 mm) for the minimization of the spherical aberrations with a final focusing length of 50 mm was used (see Fig. 22). With this special setup the laser beam had a focal waist of about 21 µm width, again yielding calculated intensities of about 10[12] W/cm[2]. In this way the minimum laser pulse energy necessary for ignition (MPE) was minimized to very small values.

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Fig. 22: Difference of the focusing properties of a single spheric plano-convex lens and a meniscus focusing system [Thorlabs]

[...]

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