Combustion Control Strategies for Dual-Fuel Marine Engines Operated with Fluctuating LNG Qualities

The world of shipping is at a turning point. Alongside methanol, ammonia and other PtL (Power to Liquid) fuels, liquefied natural gas (LNG) offers one way of achieving climate-friendly ship operation. Although currently still derived from fossil sources, LNG combines the properties of already having a well-established land-based infrastructure, of enabling a 100 % climate-neutral supply via electrolysis and methanization, and of its ability for any high proportion of climate-neutral LNG to be used as a drop-in fuel during the transformation process in the next decades; proven by the first bunkering of the container vessel “ElbBlue” with 20 tons of SNG (Synthetic Natural Gas) in 2021 [1]. Up to now, LNG fueled marine engines have predominantly been operated in fixed operation areas. Therefore, they can bunker stable gas qualities at specific ports and can be optimized for a specific gas quality. This has mostly been done by means of adapted engine control maps and hardware adjustments such as the engine’s compression ratio. In the case of worldwide operation with frequently changing bunker locations, this approach would lead to disadvantages in efficiency, emissions and reliability. This study characterizes the combustion behavior of a large, medium-speed dual-fuel engine on the basis of test bench measurements while varying the gas composition to mimic the strongly fluctuating gas qualities found worldwide. Engine control unit measures will be presented which enable the engine to operate reliably in a methane number (MN) range from 105 down to 65 without any hardware adjustments and without loss of performance. Adaptation of the pilot injection strategy, especially with regard to injection pressure, quantity and timing shows great potential for reducing engine knock. It is recommended to ignite some fuel gases with higher pilot quantities. Furthermore, pilot injections should then be set relatively early before top dead center (TDC) in order to achieve a stable center of combustion (CoC).