Abstract: Abstract: It is difficult to control the particle number emission of diesel engine due to the high number concentration of nanometer particles. To solve this problem, a self-made bipolar charged coagulation test-bed was designed in this study to study the relationship between the particle number concentration of diesel engine and charged voltage. The particle number concentration, particle mass density distribution, particle total number concentration, particle mass median diameter of a common rail diesel engine (the operation conditions of diesel engine were 25% load, 50% load, 75% load and 100% load at 1,600 r/min and 2,600 r/min) under different charged voltages（0, 5, 10, 15 and 20 kV） were measured using an engine exhaust particle sizer spectrometer (EEPS-3090). The trap efficiency of diesel particulate filter (DPF) under 0 kV and 10 kV charged voltages at 50% load was measured using particle collection system (AVL SPC 472) and a microgram electronic balance (MX-5). Experimental results showed that bipolar charged coagulation could reduce the total particle number concentration of diesel engine. With the increase of charged voltage, at 50% load, the particle number concentration of nuclear mode particles (5~50 nm) and accumulation mode particles (<93.1 nm) were reduced, and the number concentration of accumulation mode particles (>93.1 nm) increased slightly, and at the same time the peak of particle number concentration dropped significantly. At 100% load, with the increase of charged voltage, the particle number concentration of diesel engine was in a normal bimodal distribution obviously under different charged voltage, the number concentration of decreased nuclear mode particles (5~50nm) and accumulation mode particles (<93.1 nm) reduced sharply, and the number concentration of accumulation mode particles (>93.1 nm) decreased slightly. At 100% load, the particle number concentration decreased mostly in the number concentration peak of small particles. With the increase of charged voltage, the particle total number concentration decreased. At 50% load, compared with 0 kV, the particle total number concentration decreased by 9.28%, 16.23%, 18.17% and 20.56% for 5, 10, 15 and 20 kV, respectively. While at 100% load, compared with 0 kV, the particle total number concentration decreased by 9.71%、16.78%、21.92% and 25.18% for 5, 10, 15 and 20 kV, respectively. At the same charged voltage, the particle total number concentration at 100% load decreased more obviously than 25% load, 50% load and 75% load. With the increase of charged voltage, the peak of particle mass density distribution moved to larger particles. Compared with 0 kV, the peak of particle mass density distribution at 20 kV increased by 8.06%、9.92%、9.51% and 7.28% for 25% load, 50% load, 75% load and 100% load, respectively. The curves of particle mass cumulative distribution moved to larger particles with the increase of charged voltage at 25% load, 50% load, 75% load and 100% load. With the increase of charged voltage from to 0 kV to 20 kV, the range of particle mass median diameter increased from 96~101 nm to 102~110 nm. Compared with 0 kV, the trap efficiency of DPF (diesel particulate filter) at 10 kV increased from 82.1% to 91.1%. In summary, bipolar charged coagulation could increase the trap efficiency of DPF, and reduce the particle number emission and particle mass emission of diesel engine. The results can provide a technical reference for the application of bipolar charged voltage in automobile.