Otto cycle (gas engine)

An Otto-cycle gas engine burns a premixed lean charge of gas and air ignited at near-constant volume, the spark-ignition thermodynamic cycle, rather than the diesel diffusion burn. In marine dual-fuel engines the gas is admitted at low pressure and the lean mixture is lit by a small pilot of diesel, so no spark plug is needed. Low-pressure dual-fuel two-strokes such as the WinGD X-DF and the MAN ME-GA, and four-strokes such as the Wartsila and MAN DF gensets, run this way. The Otto cycle gives NOx Tier III compliance in gas mode without after-treatment, but unburned methane that escapes the lean burn produces methane slip, the trade-off against high-pressure diesel-cycle gas engines.