The dual-fuel combustion process, which is offered as a retrofit solution for conventional diesel engines by various manufacturers, represents an option for reducing emissions from internal combustion engines and is already available today. Current dual-fuel engines run on liquefied natural gas (LNG), which is usually of fossil origin. Due to the existing infrastructure and the possibility of producing LNG by means of electrolysis and methanation, LNG can already be produced in a 100% climate-neutral way and thus make a contribution to climate neutrality in the shipping industry.
The adoption of exhaust gas recirculation (EGR) systems in the maritime sector became more significant in 2020 following the enforcement of the sulphur emission cap. By lowering the sulphur content in the fuel, technologies in the exhaust tract are also conceivable without the use of expensive scrubber systems.
Dual-fuel LNG/diesel engines are typically operated in lean-burn mode to reduce the risk of knocking and to comply with the nitrogen oxide limits in accordance with IMO TIER III. However, one disadvantage of the lean-burn combustion process is climate-damaging methane (CH4) slip due to incomplete combustion. To address this issue, an EGR system can be employed to mitigate nitrous oxides (NOx) emissions and prevent engine knocking at richer air-fuel mixtures, as an alternative to increasing the air-fuel equivalence ratio lambda.
In the context of this paper, experimental investigations are conducted on a single cylinder medium speed dual-fuel engine, where EGR rates of up to 35% are examined across various lambda values. The adjustment of lambda is achieved by varying the charge air pressure using an externally driven compressor station for charge air supply. The results of the presented investigations provide insights into the emission reduction potential of a targeted variation in EGR rates and lambda, thus making an important contribution to more environmentally friendly ship propulsion systems.