Research of ${\mathrm{NO}}_{\mathrm{x}}$ Reduction on a Low-Speed Two-Stroke Marine Heavy-Fuel Oil Engine

Many methods have been developed to reduce emissions from marine heavy-fuel oil (HFO) engines. The effects of exhaust gas recirculation (EGR), Miller cycle, humid air motor (HAM), and start of injection (SOI) were investigated on the combustion and emissions characteristics of a large two-stroke HFO marine engine using computational fluid dynamics (CFD) coupled with chemical reaction mechanisms. A multicomponent-sulfur mechanism was developed to determine the effect of sulfur content on the combustion and emissions characteristics of the HFO marine engine and found that the peak in-cylinder pressure and ${\mathrm{NO}}_{\mathrm{x}}$ emissions decreased by approximately 0.08 MPa and $1.2\mathrm{g}/\mathrm{kW}·\mathrm{h}$, respectively, when sulfur content was reduced from 5% to 0%, which suggests that sulfur content has little effect on in-cylinder pressure and ${\mathrm{NO}}_{\mathrm{x}}$ emissions. A multicomponent surrogate fuel mechanism was then used in CFD simulations to compare different ${\mathrm{NO}}_{\mathrm{x}}$ emissions reductions strategies. In order of effectiveness in reducing ${\mathrm{NO}}_{\mathrm{x}}$ emissions, the strategies were EGR, HAM, SOI, and the Miller cycle. Only EGR reduced ${\mathrm{NO}}_{\mathrm{x}}$ emissions below the regulated limits. Coupling strategies were investigated to balance indicated specific fuel consumption (ISFC) and ${\mathrm{NO}}_{\mathrm{x}}$ emissions. An EGR ratio of 21% coupled with the miller cycle degree of 20 (M20) method reduced ${\mathrm{NO}}_{\mathrm{x}}$ emissions to $3.14\mathrm{g}/\mathrm{kW}·\mathrm{h}$, while the same EGR ratio coupled with HAM level of 2 (HAM2) reduced ${\mathrm{NO}}_{\mathrm{x}}$ emissions to $1.43\mathrm{g}/\mathrm{kW}·\mathrm{h}$. Therefore, coupling EGR with Miller cycle and HAM technology were suggested to reduce ${\mathrm{NO}}_{\mathrm{x}}$ emissions from HFO marine engines.