Investigation of minimum laser ignition energies of combustible gas mixtures

Minimum ignition energy (MIE) values for electric spark have been calculated and experimentally evaluated long ago, but for laser ignition significantly higher values were found, as explained, due to shorter duration and smaller size of the laser impact region. The feature of electric breakdown ignition is high energy input rate in the discharge channel. For laser ignition, energy input rate depends on gas spectral absorption. So only a small part of incident laser energy may be deposited in laser spark near breakdown threshold. MIE for certain gas mixture contents and pressure is expected to be independent on the way of energy deposition. There is a mess in published data on laser ignition energies, because authors often do not state whether incident or deposited energy is mentioned. To resolve these discrepancies, minimum pulse energy (MPE) term is suggested for laser impact, and is a more practical one. Although MPE is easy to measure, its value depends a lot on experimental conditions, which are not always properly documented, to reduce this effect, we suggest to consider MPE to laser breakdown energy ratio. We have experimentally evaluated MPE for butane (C4H10) based fuel mixture of different equivalence ratios (φ∼0.5-1.5) and pressures (p∼1-4 bar) at the impact of 1064 nm radiation of nanosecond laser. Efficiency of laser ignition was evaluated by MPE to breakdown threshold ratio.