Pre-mixed combustion

Pre-mixed combustion burns a charge in which fuel and air are already blended before ignition, so a flame front propagates through a roughly uniform mixture, the Otto-cycle mode. In marine low-pressure dual-fuel two-strokes such as the WinGD X-DF and the MAN ME-GA, gas is admitted into the scavenge or cylinder at low pressure and mixed with air during compression, then a pilot oil injection ignites the lean mixture. Pre-mixed lean burn keeps peak temperatures and NOx low enough for Tier III without after-treatment, but the lean flame can leave unburned methane in crevices, the source of methane slip. It contrasts with the diffusion combustion of high-pressure gas injection on the ME-GI.