Answer
Methane Slip refers to the unburned natural gas (methane, CH4) that escapes through the engine exhaust valve into the atmosphere during the combustion process of liquefied natural gas (LNG) fueled marine engines.
Causes
Cylinder Trapping (Dead Volumes): Methane hides in the combustion chamber crevice volumes (the small gaps between the piston crown, cylinder liner, and top piston ring) where the flame front cannot propagate.
Scavenging Air Overlap: In 4-stroke or 2-stroke low-pressure dual-fuel engines, gas is introduced during the air scavenging phase. A small portion mixes directly with air and escapes out the exhaust valve before it closes completely.
Lean-Burn Flame Extinction: If the air-fuel ratio is excessively lean or the local temperature drops, the flame can extinguish near the cold cylinder walls, leaving unburnt methane.
Impact on Environmental Regulations
While LNG burns cleanly with negligible SOx and low NOx, methane is a greenhouse gas with a global warming potential (GWP) over 28 times greater than CO2 over a 100-year timescale. High methane slip can drastically penalize a ship's annual CII (Carbon Intensity Indicator) rating and increase financial liabilities under regional emissions trading systems like the EU ETS.