Wärtsilä plans to reduce the methane slip from its marine low-pressure dual-fuel four-stroke engines to around 1.0g/kWh by 2023, compared with 2.8g/kWh in its 46DF (pictured) at present.
Wärtsilä is confident it will have marine engines operating with a methane slip level of 1.00g/kWh by 2023, equivalent to its best performing stationary engines at present.
Wärtsilä is planning to reduce the methane slip from its marine low-pressure dual-fuel four-stroke engines to around 1.0g/kWh before 2023. The improvement in the methane slip will not be limited to the company’s newest engine platforms, as the engine manufacturer expects to have “retrofittable technologies available to enable similar performance in older engines”.
At present, the modern Wärtsilä 46DF offers methane slip of 2.8g/kWh, while all but one of the engines in Wärtsilä’s portfolio has average methane slip of below 5.5g/kWh.
Like with like comparisons
The Motorship notes that the step change reduction in the company’s methane emissions will alter comparisons between Wärtsilä’s dual-fuel engines and diesel-fuelled engines, which have been the subject of significant scrutiny.
As a result, Wärtsilä noted that all of its gas engines would soon offer a “decisive emissions advantage over diesel”.
Compared with diesel engines, a methane slip of 1g/kWh would cut greenhouse gas emissions on a tank-to-wake basis by 23% over a 100-year timeframe and 14% over 20 years.
Wärtsilä noted that improved control in the emissions produced in the LNG production and supply chain would also reduce methane emissions over the entire life cycle.
Combustion chamber, injection and valve timing
Wärtsilä noted that a combination of different engine design refinements would contribute to the reduction in methane slip.
Combustion in a marine gas engine typically demands high oxygen content and a low temperature to maximise efficiency while producing the lowest NOx emissions. Methane burns more completely at hotter temperatures, some of the gas can pass unburned to the exhaust if it passes through a relatively cooler area of the combustion chamber during combustion.
One of the areas of focus was optimising the design of the combustion chambers to reduce the cooler spots in the combustion chamber, along with crevices where unmixed methane can escape combustion.
Another important parameter is the timing of gas admission and valve overlap duration. The overlap is the time that inlet and exhaust valves are open at the same time. This is often used to allow partial cooling of engine components between the combustion cycles – to reduce NOx formation – but it also improves scavenging as the incoming charge air assists the removal of the remaining exhaust gas in the cylinder. So while this helps with cooling, it also exacerbates methane slip. Working on reducing overlap time, both through the engine control system and the valve train, will minimise methane slip.
The addition of a proportion of hydrogen to the combustion process is another potential improvement to the combustion process that is under consideration. Wärtsilä has extensive experience of combining hydrogen with methane at low concentrations during its hydrogen combustion research. While this would improve combustion and decrease methane slip, the higher combustion pressures and temperatures achieved lead to an increase in NOx formation, challenging IMO Tiers II and III limits.