Using LNG as a ship fuel is a step in preparing engines for use of low-carbon fuels on the way to carbon-neutrality. The escape of unburnt methane raises questions on the climate benefits of LNG that are being addressed by engine makers like MAN Energy Solutions
Without countermeasures, there are several routes by which unburnt methane can escape from both two- and four-stroke gasburning engines[ds_preview] via the engine exhaust as well as the crankcase ventilation, a phenomenon called »methane slip«. LNG is typically 85 % to 95 % CH4, a greenhouse gas considerably more potent than CO2. As MAN lays out in a recent Whitepaper entitled »Managing methane slip«, the phenomenon is most prevalent on gas-burning engines operating according to the Otto combustion process, where gas fuel and air are mixed homogenously prior to ignition and combustion. It affects engines in which the premixed fuel and air are ignited by a spark-plug (spark-ignited (SI) gas engines) as well as dual-fuel (DF) engines where a liquid fuel »pilot« initiates ignition of the air-gas mixture.
MAN’s ME-GI two-stroke dual-fuel engines are already achieving very low levels of unburnt methane emission due to operation on the Diesel combustion principle. This is explained by the fact that in the ME-GI two-stroke DF engines, the gaseous fuel is injected into the compressed charge air around top dead centre and only slightly after the liquid fuel pilot, when the pilot has already ignited. This ensures complete combustion with maximised heat release. Since the gaseous fuel only enters the cylinder after the exhaust valve has closed and ignites immediately, there is no opportunity for methane to escape during cylinder scavenging. In addition, virtually no unburnt methane is trapped in crevice volumes, such as the »top land« between the piston and cylinder liner. Methane slip levels lie in a range from 0.2 – 0.3 g/kWh over the ME-GI engines’ load range.
The countermeasures devised and under investigation at MAN’s technical department for four-stroke engines involve all the major aspects of internal engine design: control of engine operation; combustion processes and their control; basic engine architecture. MAN’s four-stroke SI gas and DF engines operate on the Otto principle, where gaseous fuel is pre-mixed with air before ignition. This mixture is compressed and ignited by a spark-plug or liquid fuel pilot injection and is thus in the cylinder for all of the induction and compression strokes and part of the power stroke. As four-stroke engines rely much more for gas exchange on inlet and exhaust valves, there are increased opportunities for the gas to evade combustion.
An important line of attack centres the reduction in the overlap of inlet and exhaust valve openings and the timing of gas admission. Valve overlap creates an unavoidable period during the inlet stroke of a four-stroke engine when both inlet and exhaust valves are open in order to »scavenge« the cylinder. Minimising the overlap reduces the time in which air/gas mixture can reach the exhaust port, while control of the timing and duration of gas admission from electrically-controlled valves in the inlet ports limits the time that gas can enter the cylinder during exhaust valve opening. In terms of engine architecture, a successful approach is the reduction of »crevice volumes« in the combustion chamber, i.e. areas where pockets of unburnt gas can be trapped and not be reached by the flame.
