Experimental Study of Cyclic Variations of RCCI Diesel–Methanol Engines at Multiple Simulated Elevations

The cyclic variations of reactivity-controlled compression ignition (RCCI) combustion are studied in this work via experiments conducted in an in-house diesel–methanol dual-fuel (DMDF) engine at multiple simulated elevations. The test engine is maintained at 1800 rpm, with simulated elevations of 10, 700, and 1670 m. Engine load, methanol substitution rate (MSR), and injection parameters are varied to investigate their effects on cyclic variations at multiple elevations. We employ in-cylinder pressure parameters, such as the maximum pressure (P_max) and indicated mean effective pressure (IMEP), to quantify cyclic variations. According to the results, the stability of DMDF combustion in plateau conditions displays a similar trend to that in plain conditions. However, with the increase in elevation, there is a significant increase in the P_max coefficient of variation (COV_Pmax), while that of the IMEP (COV_IMEP) shows an opposite trend. The distribution of crank angle at high elevations corresponding to P_max tends to be more concentrated, and the increase in pilot injections reduces the COV_Pmax in the plateau environment. At all elevations, there is no effect of injection timing on COV_IMEP, while COV_Pmax is more sensitive to injection timing (range from 10 deg CA before top dead center (BTDC) to 3 deg CA BTDC). Compared with other injection parameters, the cyclic variations caused by injection pressure (range from 92 MPa to 112 MPa) are relatively minor.