Laser ignition of hydrogen/air mixtures in a constant-volume combustion chamber using a pulse-burst Nd:YAG/Cr4+:YAG laser spark plug

Abstract

In this study, characteristics of laser ignition of H2/air mixtures are investigated in a constant-volume combustion chamber using a compact, passively Q-switched Nd:YAG/Cr4+:YAG laser spark plug. Ignition was conducted at a single point, with precise timing achieved through two laser modes of operation: delivering a single laser pulse and operating in pulse-burst mode, emitting trains of up to five laser pulses. Experiments covered a wide range of relative equivalence ratios (λ= 1.5 to 5.0) at 1 bar initial pressure and extended to 9 bar. Under similar pressure conditions, igniting H2/air mixtures with higher λ values (lean mixtures) results in a reduction of maximum pressure and a slower combustion process. Conversely, maintaining the same λ value for H2/air mixtures, while elevating the initial pressure, yields higher maximum pressures, but concurrently decelerates the combustion process. Pulse-burst mode laser ignition expedited combustion, especially for lean mixtures (λ> 4.0), showcasing advantages over single-pulse laser ignition. Additionally, lean ignition limits were determined at different pressures, revealing pressure-dependent differences between single pulse and pulse-burst modes laser ignition. These results provide insights into the features of laser ignition with pulse trains of H2/air mixtures and the use of this ignition technique in hydrogen-powered reciprocating engines.