Laser ignition by a dual-pulse approach
Laser ignition has been investigated as a possible replacement to the commonly used capacitive spark plugs due to its non-invasive nature. The typical laser ignition method uses a nanosecond pulse from an Nd:YAG laser to create a high-temperature spark inside the combustion chamber of an engine. This solution is attractive because of the lack of electrodes (which tend to act as heat sink within the combustion chamber), the ability to achieve fine control over ignition timing, flexibility in locating the ignition source (based on focusing optics), and it’s ability to reliably ignite in high-pressure environments (a property of particular interest for large gas-turbines used as generators). However, the single-pulse laser spark solutions that were recently proposed come with some shortcomings: non-resonant laser breakdown is energetically inefficient, it cannot be easily delivered into the combustion chamber (high-power fiber delivery is challenging) and NOx emission have been shown to increase for a given equivalence ratio compared to spark plugs. The solution under investigation is nothing more than an attempt to address some of these problems as the REMPI pulse is very efficient in ionizing the gas molecules. Moreover, by decoupling the photoionization process from the electron avalanche ionization stage using two separate pulses, one gains much more control over the plasma parameters (temperature, electron density and flow-field).
The study published in Scientific Reports (Scientific Reports, Vol. 10, 19916, 2020) presents a novel laser ignition technique for internal combustion engines that uses a dual-pulse approach. The first UV laser pulse is set to resonantly ionize the molecular oxygen present in the air-fuel mixtures via a 2+1 Resonant Multiphoton Ionization (REMPI) scheme centered around 287.6 nm. This pulse generates a weakly preionized plasma (ne~1E16 cm-3, T=500 K) that is subsequently heated up by a secondary non-resonant infra-red pulse (λ=1064 nm) arriving a few nanoseconds (t<10 ns) after the first. The combination of the two pulses yields a hot plasma kernel (ne~8x1017 cm-3, T~8000 K) that is capable of igniting CH4-Air mixtures from stoichiometric down to ϕ=0.55 (mixtures having a low fuel-to-air ratio). Our study starts by first characterizing the laser plasma using laser Rayleigh/Thomson scattering diagnostics. This is followed by ignition demonstration in a high-pressure vessel. Finally, the experimental study is completed by a flame hydrodynamic study using high-speed OH* chemiluminescence imaging. Numerical modeling of the initial stage of plasma kernel development were also conducted using an in-house computational fluid dynamics code.
The most important finding of this study is that this dual-pulse technique uses less energy than the more commonly used single-pulse laser spark technique (factor of ~2.5) while also allowing for leaner combustion than all other ignition techniques previously investigated. The improved energy efficiency could open new possibilities for practical implementation of laser ignition systems for aerospace and energy generation applications. This work was funded by the US Air Force Office of Scientific Research under grant no. FA-9550-18-1-0239.
Dr. Ciprian Dumitrache is a research scientists CSIII at the National Institute for Laser, Plasma and Radiation Physics (INFLPR), conducting research in the area of advanced optical diagnostics for combustion and plasma applications. His main research interests include: plasma-assisted combustion, laser ignition, plasma kinetics, laser spectroscopy using ultra-short pulses, computational fluid dynamics and remote sensing. Ciprian is a former Fulbright scholar (Georgia Institute of Technology) and received his PhD from Colorado State University in 2017. Prior to joining INFLPR, Ciprian was a postdoctoral researcher at Ecole CentraleSupelec where he worked on the development of femtosecond diagnostics for the characterization of nanosecond discharges used in nitridation.