Abstract: Reactivity controlled compression ignition (RCCI) has great potential for improving thermal efficiency and reducing pollutant emissions. RCCI can also control the combustion phase and suppress the maximum pressure rise rate. The RCCI is characterized by the in-cylinder reactivity stratification, which is formed by directly injecting the high reactivity fuel into the low-active homogeneous gas mixture in the cylinder. Based on a heavy-duty six-cylinder turbocharged diesel engine, in this paper, the influence of different start of injection (SOI) timings of diesel fuel on the low-load combustion and emission characteristics of ethanol/diesel RCCI was studied and compared with the conventional combustion mode (CCM). During the experimental study, the rotation speed of engine was 1500 r/min, the total cycle energy was 1 281 J, the ethanol energy premixed ratio was 40%, the SOI timing was -6, -15, -24, -40 °CA ATDC, respectively, and brake mean effective pressure (BMEP) was 0.098, 0.106, 0.109, 0.093 MPa, respectively. The results showed that，in terms of combustion characteristics, with the advance of SOI timing, the heat release rate of the cold flame stage of RCCI was advanced, the heat release rate increased, and the duration increased. During the stage of high temperature heat release, with the advance of SOI timing, the instantaneous heat release rate of RCCI increased first and then decreased，the in-cylinder average temperature peak gradually increased, and the combustion duration gradually increased, the combustion center of gravity (CA50) was advanced first and then postponed, and the maximum cumulative heat release gradually increased. Compared with the CCM, RCCI had a longer ignition delay, slower combustion speed, longer combustion duration, and lower combustion temperature. With the advance of SOI, the difference of combustion characteristics between CCM and RCCI gradually increased. In terms of emission characteristics, as SOI advances, the NOx emission at RCCI first increased and then decreased, HC gradually decreased, and CO first decreased at a small rate and then increased substantially. Compared with CCM, less NOx was emitted from RCCI combustion mode, and with the advance of SOI, the difference gradually increased. In the earlier SOI, the HC emission of RCCI was smaller, and when the SOI was retarded, the HC emission of RCCI was deteriorated. CO emission from RCCI combustion mode was more than that from CCM, and as SOI advances, the difference gradually decreased. With the advance of SOI, the number of the RCCI particulate matter emission was first decreased and then increased. Compared with CCM, there was a smaller amount of particulate matter from the RCCI combustion mode, especially nuclear mode particulates. With the advance of SOI, the particle surface area concentration of RCCI gradually decreased. Compared with CCM, when the SOI range was from -6 to -24°CA ATDC, the surface area concentration of particulate matter emitted by RCCI was relatively larger, and the difference gradually decreased with the advance of SOI. When the SOI range was from -24 to -40° CA ATDC, the surface area concentration of RCCI particulate matters emitted was smaller compared to CCM, and the difference gradually increased with the advance of SOI. The mass concentration of large particulate matters determined the mass concentration of total particulate matter emission. With the advance of SOI, the mass concentration of particulate matter emission from RCCI combustion mode gradually decreased. The proportion of mass concentration of accumulation mode (>13.3-273.8 nm) particulate matter was increased from 15.88% to 66.36% in the total particulate matter emission. The proportion of mass concentration of coarse mode (>273.8-1 000 nm) particulate matter was decreased from 84.09% to 32.87% in the total particulate matter emission. The total mass of RCCI particulate matter was larger than CCM, and the difference gradually decreased with the advance of SOI.