Strategies on Methane Slip Mitigation of Spark-Ignition Natural Gas Engine during Transient Motion 2021-01-5062

The liquefied natural gas (LNG)-fueled ships were provisioned to meet the strict emission legislation in the marine application since 2000. However, the scientific approach of burning the low-emission natural gas in lean combustion uncovered that the engine suffers from high methane slip emission. Serious questions are raised about the quantity of methane slip during marine conditions when the load varies in multiple frequencies and amplitudes. Previous studies by these authors explained how methane slip increases during load oscillation. This paper examined several practical methods to reach stable combustion in transient conditions to reduce the methane slip. Employing Proportional-Integral-Derivative (PID) controllers in a closed loop, implementing open-loop lookup tables, model predictive controller (MPC), and an innovated solenoid method are performed in a high-fidelity medium-speed natural gas spark-ignition (SI) engine model. The transient response of the engine is examined during sea-state oscillating torque. The data from the vessel are used for confirming the implemented models. Moreover, an additional model was formulated to present the sources of methane slip during the transient condition.

The results revealed that all the closed-loop controllers respond similarly during the transient conditions, and changing the position of controllers influences the combustion stability slightly. Moreover, advancing the SI timing to reduce the methane slip for an over-lean mixture is not recommended during the transient marine condition. Actuating the turbocharger in advance with a torque prediction from the wave effectively reduced the methane slip, but the method is not applicable in non-predictable conditions. A rapid response solenoid valve was finally proposed to be installed upstream of the fuel valves to discharge the extra air in a short timescale. This system resulted in a low level of methane slip during the transient condition. Since there is always a trade-off between the emission compounds, this study likewise examined the influence of transient load and the proposed methods on nitrogen oxides (NO_X) formation. A combination of the solenoid method and ignition timing reduced the emission quantity to a value comparable to the steady state.