Effects of Pilot Oil Injection Timing on Combustion, Covariance and Knocking of a Natural Gas–Diesel Duel-Fuel Low-Speed Engine

Abstract Dual-fuel engines, which are powered by natural gas while using a small amount of diesel for ignition, have become an attractive option in the marine sector due to their fuel flexibility and relatively good emission characteristics. Altering the fuel injection timing can change the combustion state of natural gas in the cylinder, which in turn affects engine stability and leads to engine knocking. In this study, the effects of different pilot oil injection timings on the combustion stability of a marine low-speed natural gas dual-fuel engine with a pre-combustion chamber are evaluated in terms of the pressure rise, covariance of Pmax and IMEP, combustion phase, and knocking. It is found that the maximum cylinder pressure and pressure rise rate increase with an advance in the pilot oil injection time. After the natural gas enters the combustion chamber, it undergoes a process of mixing with air in the combustion chamber, and earlier pilot oil injection leads to an increase in the ignition delay period and shortens the combustion duration of the engine. Moreover, it is found that earlier pilot oil injection times result in an increase in engine IMEP and Pmax cycle fluctuations, and engine knocking also undergoes an increase when the pilot oil injection time is advanced. Hence, an appropriate pilot oil injection time should be considered in the process of optimising engine performance.