Heinrich Kofler

Laser-induced optical breakdown applied for laser spark ignition

AbstractIn the present article, the experimental investigation of optical breakdown induced by ns/mJ pulses at two wavelengths,1064 nm and 532 nm, in air of atmospheric pressure is reported and discussed. The obtained breakdown thresholds werecompared with theory and are in good agreement. The generated plasmas have been characterized by their amount ofscattered laser light, energy transmission, and change of the transmitted temporal shape. Laser-induced plasmaformation in a gas, in air, also generates an acoustic pressure wave. The acoustic energy is compared to the laser pulseenergy and is found to be linearly dependent. Moreover, the frequency distribution of the characteristic acousticpressure wave was analyzed. The experiments described were accomplished in order to optimize a laser ignition systemwith regard to efficiency and costs. The laser system employed for these investigations is a compact high peak power,passively Q-switched, longitudinally diode-pumped solid-state laser. Such a “laser spark plug” should replaceconventional spark plugs in internal combustion engines because conventional ignition has reached its limits in termsof efficiency and durability. Thereby, a reduction of pollutant emission should also be feasible.Keywords: Acoustic pressure wave; Laser-induced plasma; Optical breakdown; Q-switched; Scattered laser light

Laser cleaning of optical windows in internal combustion engines

Optical access to combustion chambers via windows is desirable for combustion diagnostics as well as for laser ignition. By nature, combustion deposits can form on the inner surface of the light-transmitting window, leading to malfunction. We investigated whether a Nd:YAG ignition laser could cope with combustion-chamber deposits by means of ablation. In a 1.8-kW four-stroke internal combustion engine an optical window was installed to couple in the laser light. Ignition was carried out by a spark plug. Due to inherent high fuel and oil consumption, a deposit layer would form on the substrate within some tens of minutes. Elementary analysis showed carbonaceous as well as inorganic compounds gradually reducing light transmission. With cyclic 5-ns laser shots through the window, the pass-through stayed essentially free of deposits provided the energy fluence was around 10 mJ/mm2. Microanalysis showed evidence of the soundness of the principle. In addition, even single shots with a higher flux were enough to remove a relatively thick layer of deposits at once. Thus an optical window in an internal combustion engine can in principle be kept transmissive by the action of a compact solid-state laser.

Transportation of megawatt millijoule laser pulses via optical fibers

Laser ignition is considered to be one of the most promising future concepts for internal combustion engines. It combines the legally required reduction of pollutant emissions and higher engine efficiencies. The igniting plasma is generated by a focused pulsed laser beam. Having pulse durations of a few nanoseconds, the pulse energy Ep for reliable ignition amounts to the order of 10 mJ. Different methods of laser ignition with an emphasis on fiber-based systems will be discussed and evaluated.

Laser-induced ignition by optical breakdown

This paper is an experimental work of the applied methodical character in which as an attempt to optimize a laser ignition system a systematic study of the best incoupling geometry for the employed Nd:YAG laser was performed. The incoupling geometry comprises the pump fiber and an aspheric collimating lens. In this context, the distance between pump fiber and collimating lens was made continuously variable. The distance between fiber and lens primarily influences the diameter of the pump beam. In this way, it is possible to control the pulse energy as well as the number of pulses generated within a pump cycle. Furthermore, investigations to analyze the focal size dependence of plasma generation were carried out. As a result, it was found that it is possible to reduce optical losses caused by plasma transmission by choosing an optimum focal volume. This experiment was carried out for different pressures and focal volumes.

Laser plasma ignition: status, perspectives, solutions

Laser ignition can yield certain advantages compared to conventional sparkplug ignition. Among other already frequently discussed reasons due to: i) option for sequential or multipoint ignition which can contribute to more reliable ignition in direct injection engines; ii) ignition of leaner mixtures at higher compression being most relevant for gas engines. A satisfying solution to the above mentioned requirements is the longitudinally diode-pumped passively Q-switched Cr4+:YAG/Nd 3+:YAG laser capable of emitting ∼1-ns-pulses of at least 20 mJ . This type of solid-state laser (SSL) confectioned in an engine-compatible form can be called a laser sparkplug. Early versions of this concept comprised a high-power diode pump laser (quasi-cw power <500 W @ ∼500 μs duration) which were placed remote from the engine to avoid detrimental influences of temperature, vibrations, pollution etc. In this case only the SSL is exposed to the elevated temperature in the vicinity of the cylinder walls (<100°C). Recently, technical and cost-oriented considerations allow a change of concept from fiber-based remote pumping via edge emitter arrays to the use of newly developed so-called power VCSELs with two-dimensional stacking. Collimation to form a round pump beam thereby becomes much easier. Their temperature resistance allows lower-cost direct mounting although thereby a wavelength shift is induced. The Q-switched SSL in the sparkplug also faces temperature dependent phenomena like reduction of pulse energy and efficiency, a change of pulse timing and beam profile which will be discussed in the paper.

Laser ignition @ two wavelengths?

The reduction of pollutant emissions and energy consumption represents a central objective in the improvement of internal combustion engines. This ambitious goal can be achieved by a clear optimization of combustion processes as well as ignition mechanisms. Enhanced compression ratios and leaner mixtures allow more efficient engine operation and lower emissions, respectively. Unfortunately, the established electrical spark plug reaches its physical borders and cannot fulfil such requirements. Laser ignition is one of the most promising alternative concepts where the electrical spark plug is replaced by a pulsed laser. The short pulses (∼ ns) are focused into the combustion chamber, and thus a plasma is created igniting the gas mixture. The wavelength of the laser radiation affects the plasma formation and the ignition mechanism in a certain manner being topic of our investigations. Recent investigations on the development of a compact, competitive and reliable ignition laser were based on the fundamental wavelength of Nd:YAG (1064 nm) [1]. Since shorter wavelengths allow smaller focal spots the employment of the SH (second harmonic) wavelength could be advantageous with respect to a lower minimum pulse energy (MPE) required for plasma formation. Moreover, the mixture of two wavelengths (1064 & 532 nm) might be beneficial since the green fraction generates a plasma first and the infrared feeds it up due to better plasma absorption in the infrared regime. Adapting a KTP crystal for SHG (second harmonic generation), the emitted green (532 nm) and infrared (1064 nm) light is compared with respect to plasma characteristics and two-color laser ignition. The experimental setup including all relevant components is shown in Fig. 1.

Development of a High Peak Power Solid-State Laser for Engine Ignition

A compact monolithic Nd:YAG-Cr4+:YAG high peak power, passively Q-switched, longitudinally diode-pumped laser was constructed for laser ignition. The system yielded pulses with energies of 8 mJ and durations of 1ns at 225W pump power.

Laser Ignition of Combustion Engines: Development of an Ignition Laser

Summary form only given. This work focuses on the development of a compact, robust and reliable Nd:YAG solid-state laser. A diagram for the pulse energy in dependence of the reflectivity of the output mirror and the initial transmission of the absorber, as well as the analysis of inject optics and optimum pump duration is established. The highest pulse energies was reached by a reflectivity around 45% and an initial transmission of 40%. Moreover, the pump duration should be in the order of the fluorescence lifetime of the laser active medium, otherwise the laser system becomes more unstable the longer the pump pulse is. Applying this configuration, pulse energies up to 6 mJ and pulse durations under 1.5 ns were reached.