Effect of focal size on the laser ignition of compressed natural gas–air mixture

• Laser ignition of CNG–air mixture was done in CVCC and an engine.
• Strong impact of focal length on minimum ignition energy was seen.
• Combustion duration at λ depends on energy density and focal spot size.
• Minimum ignition energy decreases for lower M2 at all λ.
• Engine performance improved with shorter focal length lens.

Laser ignition of compressed natural gas–air mixtures was investigated in a constant volume combustion chamber (CVCC) as well as in a single cylinder engine. Laser ignition has several potential advantages over conventional spark ignition system. Laser ignition relies on the fact that optical breakdown (plasma generation) in gases occurs at high intensities of ≈1011 W/cm2. Such high intensities can be achieved by focusing a pulsed laser beam to small focal sizes. The focal spot size depends on several parameters such as laser wavelength, beam diameter at the converging lens, beam quality and focal length. In this investigation, the focal length of the converging lens and the beam quality were varied and the corresponding effects on minimum ignition energy as well as pressure rise were recorded. The flame kernel was visualized and correlated with the rate of pressure rise inside the combustion chamber. This investigation will be helpful in the optimization of laser and optics parameters in laser ignition. It was found that beam quality factor and focal length of focusing lens have a strong impact on the minimum ignition energy required for combustion. Combustion duration depends on the energy density at the focal spot and size of the flame kernel.