Combustion characteristics of a variable compression ratio laser-plasma ignited compressed natural gas engine

Compressed natural gas (CNG) is considered as one of the most promising alternative fuel used in transport sector worldwide with great potential. However it suffers from the problem of high NOx emissions in a stoichiometric engine. Stringent emissions norms and demand for high thermal efficiency can be met by ignition of lean fuel-air mixtures, which do not produce oxides of nitrogen (NOx). However lean combustion is associated with slower flame speed and lower power output. Lean mixtures are very difficult to ignite by conventional electrical spark generated plasma therefore this comes across as a severe limitation, if one tries to develop a lean-burn conventional CNG engine capable of meeting future emission requirements. Ignition system of a spark ignition (SI) engine is responsible for initiating combustion of fuel-air mixture inside the combustion chamber. However, current ignition systems face limitations in catering to requirements of high efficiency, and high power density, which are essential for an environment friendly SI engine. Laser ignition system is an alternative and technically viable ignition system for igniting lean fuel-air mixtures in IC engines and is capable of overcoming most limitations faced by conventional electrical spark ignition systems. In this paper, laser ignition of lean CNG-air mixtures is experimentally investigated at different compression ratios and air-fuel ratios, in order to extract full potential of CNG for transport applications. It is found that lean flammability limit with drivability constraints increased from λ = 1.62 to λ = 1.76 with increased compression ratio from 9.9 to 11.8, which reflects the benefits offered by higher compression ratio application in laser ignition of CNG-air mixtures, which will be vital for developing lean-burn CNG engine.