5. Combustion Instability at Lean Limit

In recent years gas engine manufacturers have faced stringent emission regulations on oxides of nitrogen (NOx) and unburned hydrocarbons (UHC) [1, 2]. Operating internal combustion engines at ultra-lean conditions can reduce NO_x emissions and also improve thermal efficiency [3, 4]. An approach that can potentially solve the challenge of igniting ultra-lean mixtures is to use a reacting/reacted hot turbulent jet to ignite the ultra-lean mixture instead of a conventional electric spark [5–10]. The hot turbulent jet is produced by burning a small amount of stoichiometric or near-stoichiometric fuel/air mixture in a small volume separated from the main combustion chamber called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushes combustion products into the main combustion chamber in the form of a hot reacting/reacted turbulent jet, which then ignites the ultra-lean mixture in the main combustion chamber. Compared to conventional spark ignition, the hot turbulent jet has a much larger surface area containing numerous ignition kernels over which ignition can occur. Hot jet ignition has the potential to enable the combustion system to operate near the fuel’s lean flammability limit, leading to ultralow emissions.