Martin Weinrotter

Laser Ignition - a New Concept to Use and Increase the Potentials of Gas Engines

Due to market demands aimed at increasing the efficiency and the power density of gas engines, existing ignition systems are rapidly approaching their limits. To avoid this, gas engine manufacturers are seeking new technologies. From the viewpoint of gas engine R&D engineers, ignition of the fuel/air mixture by means of a laser has great potential. Especially the thermodynamic requirements of a high compression ratio and a high power density are fulfilled well by laser ignition. Results of measurements on the test bench confirm the high expectations – with a BMEP of 1.8 MPa it was possible to verify NOx values of a non-optimized system of 30 ppm (70 mg/Nm³ @ 5 % O2) with very high combustion stability. In the meantime, GEJ can look back at 6 years of excellent experience and can see itself as the “technological leader” in the field of laser ignition. Despite this, considerable developmental steps are still necessary to adapt the laser ignition concept fully to desired objectives (especially costs).

Laser Ignition of Methane-Air Mixtures at High Pressures and Diagnostics

Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200°C were ignited in a high-pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (λ). The lean-side ignition limit of methane-air mixtures was found to be λ=2.2. However only λ<2.1 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is λ=1.8. Only with excessive efforts λ=2.0 can be spark ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd: YAG laser at 1064 nm. The engine worked successfully at λ=1.8 for a first test period of 100 hr without any interruption due to window fouling and other disturbances. Lowest values for NO x emission were achieved at λ=2.05 (NO x =0.22 g/KWh). Three parameters obtained from accompanying spectroscopic measurements, namely, water absorbance, flame emission, and the gas inhomogeneity index have proven to be powerful tools to judge laser-induced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition; and the progress of combustion.

Optical Diagnostics of Laser-Induced and Spark Plug-Assisted Hcci Combustion

HCCI (Homogeneous Charge Compression Ignition), laser-assisted HCCI and spark plug-assisted HCCI combustion was studied experimentally in a modified single cylinder truck-size Scania D12 engine equipped with a quartz liner and quartz piston crown for optical access. The aim of this study was to find out how and to what extent the spark, generated to influence or even trigger the onset of ignition, influences the auto-ignition process or whether primarily normal compression-induced ignition remains prevailing. The beam of a Q-switched Nd:YAG laser (5 ns pulse duration, 25 mJ pulse energy) was focused into the centre of the cylinder to generate a plasma. For comparison, a conventional spark plug located centrally in the cylinder head was alternatively used to obtain sparks at a comparable location. No clear difference in the heat releases during combustion between the three different cases of ignition start could be seen for the fuel of 80/20 iso-octane/n-heptane used. However, with optical diagnostic methods, namely PLIF (Planar Laser-Induced Fluorescence), Schlieren photography and chemiluminescence imaging, differences in the combustion process could be evaluated.

Laser ignition of engines - : a realistic option!

Due to the demands of the market to increase efficiencies and power densities of gas engines, existing ignition schemes are gradually reaching their limits. These limitations initially triggered the development of laser ignition as an effective alternative, first only for gas engines and now for a much wider range of internal combustion engines revealing a number of immediate advantages like no electrode erosion or flame kernel quenching. Furthermore and most noteworthy, already the very first engine tests about 5 years ago had resulted in a drastic reduction of NOx emissions. Within this broad range investigation, laser plasmas were generated by ns Nd-laser pulses and characterized by emission and Schlieren diagnostic methods. High-pressure chamber experiments with lean hydrogen-methane-air mixtures were successfully performed and allowed the determination of essential parameters like minimum pulse energies at different ignition pressures and temperatures as well as at variable fuel air compositions. Multipoint ignition was studied for different ignition point locations. In this way, relevant parameters were acquired allowing to estimate future laser ignition systems. Finally, a prototype diode-pumped passively Q-switched Nd:YAG laser was tested successfully at a gasoline engine allowing to monitor the essential operation characteristics. It is expected that laser ignition involving such novel solid-state lasers will allow much lower maintenance efforts.

Laser Ignition, Optics and Contamination of Optics in an I.C. Engine

Due to the progresses in exhaust emission after-treatment systems and in the development of new combustion processes, the S.I. engine has been booming in the past few years. But the efficiency will have to be improved in the future. Because of its thermodynamic benefits, the S.I. direct injection engine of the second generation — so called air guided system — shows the highest potential for gasoline engines to reduce fuel consumption. However, there are restrictions when using conventional spark ignition system. They concern the optimum position of ignition initialization and spark-plug wear, the latter being caused by inhomogeneous mixture distribution. The laser-induced ignition enables a flexible choice of the ignition location and a wear resistant initialization of the combustion process. The most crucial component here is the optics (the combustion-chamber window), through which the laser beam passes into the combustion chamber. In this paper, laser-induced ignition is discussed and its potential compared to a conventional ignition system is presented. In addition, several optic configurations are presented as well as tests regarding the minimum required laser energy and the optic contamination and self-cleaning effect of the optics. At the Institute of Internal Combustion Engines at the Vienna University of Technology the optic contamination and self-cleaning effect, which is crucial for a long-term operation, was tested on a two-cylinder research engine.Copyright

GE Jenbacher’s Update on Laser Ignited Engines

The focus in research year 05 was on the optimization of optical coupling and minimization of laser energy especially in connection with very lean combustion and with high exhaust gas recirculation rates for low NOx emissions. The direct comparison of laser ignition with conventional spark ignitions, without any measures implemented in favor of laser ignition (high compression ratio, high turbulence ratio), consistently shows advantages in the case of laser ignition. With extension of the Lambda window, in the case of a spark ignition engine with a 2.4 1 piston displacement it is possible to shift the engine 0.3 units in the direction of “lean combustion” (possible reduction of NOx level less than 30% of the state of the art); EGR compatibility is increased by about 15% to a recirculation rate of about 40%. With regard to EGR compatibility, in coordination with SWRI (HEDGE Program) similar tests on determination of potential were carried out as well. In this case too no essential measures were implemented in favor of the exploitation of the potential of laser ignition; however, a minor increase of the compression ratio already allows recognition of the theoretically possible and expected potentials. Regarding stoichiometric conditions, from the viewpoint of the researchers working jointly on the project it is possible to reduce the energy to less than 1 mJ. Conversely, in the event of the utilization of lean-burn combustion, appreciably more energy must be provided. Additionally, measures regarding combustion control in the area of the extended lean-burn limit must also be carried out. Only then is it possible to ensure optimal values for burning durations and the variation coefficient. Initial results in this regard will also be presented.Copyright

Laser-induced ignition characteristics of methane- and hydrogen-air mixtures at high pressures

A Nd:YAG laser was employed to ignite methane- and hydrogen-air mixtures to investigate relevant parameters of laser ignition. The lean side ignition limit of methane was found to be at air/fuel-equivalence ratios (λ) of 2.4 applying a laser pulse energy of 50 mJ. It has to be mentioned, however, that above λ = 2.2 only slowest combustions causing weak pressure rises could be observed. Successful ignitions of hydrogen-air mixtures were achieved up to λ = 8 but it was not possible to examine the lean side limit due to weakest pressure rises far below detection limits for λ >8. Despite much lower values of minimum ignition energy for reported hydrogen-air mixtures in the literature, the minimum laser pulse energies examined for ignition are of the same magnitude as for ignition of rich methane-air mixtures lying around 5 mJ. Minimum pulse energy needed for ignition was decreasing with increasing pressure for hydrogen-air mixtures showing the same trend as in case of methane. The ignition delay time for hydrogen at λ = 2.0 could be observed as ~7 ms being 40 times shorter compared to methane at the same air/fuel ratio. Unfavorable transmission losses of laser energy were observed for methane/air mixtures below λ = 2.1 demanding optimized focusing optics and temporal pulse shaping for future laser ignition systems.