Over-expansion cycle as clean combustion strategy applied to a marine low-speed dual fuel engine

Natural gas marine engines are widely used in the commercial shipping. It is a hot research topic for carbon reduction, energy saving, and emission reduction in shipping. The over-expansion cycle with natural gas high-pressure direct-injection (HPDI) is a promising approach for reducing fuel consumption and emissions in marine engine. Two types of over-expansion cycles in a large-bore (500 mm) low-speed dual-fuel engine were investigated and compared. The variable compression (VC) cycle decreases the effective compression ratio by adjusting the exhaust valve closing (EVC) timing, and the variable expansion (VE) cycle raises the effective expansion ratio by varying the exhaust valve opening (EVO) timing and scavenging ports opening (SPO) timing. A specific sleeve mechanism was designed to precisely control the opening and closing of scavenging ports. The results reveal that the VC cycle exhibits an operable limit with a delay of 40 °CA in EVC, while the VE cycle has a limit with a delay of 60 °CA in EVO. The scavenging effect obtains improvement in the VC cycle, whereas it deteriorates in the VE cycle. Both the two cycles lead to reduced peak pressure and extended combustion duration. Compared to the baseline, the VC cycle and VE cycle can reduce nitrogen oxides (NOx) emissions by a maximum of 51.9% and 70.7%, respectively. However, the VE cycle can even reduce the equivalent indicated specific fuel consumption (EISFC) by 1.14 g/kWh when the EVO is delayed by 5 °CA. Importantly, the VE cycle exhibits a more favorable cost-effectiveness and potential for NOx emission reduction compared to the VC cycle, and it also demonstrates the capability to optimize NOx and EISFC simultaneously. This provides a method to produce cleaner marine engines with less fuel consumption in the future.