Experimental investigation into the effects of pilot fuel and intake condition on combustion and emission characteristics of RCCI engine

Due to the excellent physicochemical properties, coal to liquid (CTL) is suitable for use as an alternative fuel in internal combustion engines. It is conducive to promoting efficient and clean utilization of coal. In this study, the experiment of using CTL and conventional diesel as pilot fuel and gasoline as the premixed fuel was carried out on a modified dual-fuel engine. The combustion boundary conditions, including gasoline ratio, intake flow rate, and intake temperature, were modulated at an engine speed of 1400 rpm and under low load. The combustion parameters and emission characteristics were fully discussed. The results show that both gasoline/CTL reactivity controlled compression ignition (RCCI) mode and gasoline/diesel RCCI mode contain low temperature heat release (LTHR) and high temperature heat release (HTHR) phases. Among them, the gasoline/CTL RCCI mode has a higher proportion of LTHR, and the low-temperature combustion occurs later than gasoline/diesel. Besides, the ignition delay (ID) of gasoline/CTL RCCI mode is generally shorter, and CA50 is more forward than that of gasoline/diesel RCCI. Compared with gasoline/diesel RCCI mode, the intake air flow has a more significant effect on the indicated thermal efficiency (ITE) and particle emissions of gasoline/CTL RCCI. In particular, the problem of high accumulation mode particles brought by the CTL can be solved by appropriately increasing the intake flow rate. There is an optimal intake temperature for the ITE of gasoline/CTL RCCI mode, while the ITE of gasoline/CTL RCCI mode remains almost constant as the intake temperature increases. Compared with diesel, when using high cetane number (CN) CTL as a pilot fuel, the combustion is more stable, and the ITE of that at each operating condition is about 3% higher. Under optimum intake conditions, the gasoline/CTL RCCI mode achieves higher thermal efficiency and products lower CO, HC, and particle emissions, but NOx emissions increase slightly.