Cyclopentanol is a promising alternative biofuel for automobiles. In this study, an optical engine was employed to investigate the effects of cyclopentanol blending ratios on combustion characteristics. Cyclopentanol was blended with diesel by 10% and 20% volume, denoted as CP10 and CP20, respectively. Comparison tests between blends and diesel (denoted as D100) were performed under three injection pressures (100, 120, and 140 MPa) and four injection timings (-6 degrees,-9 degrees,-12 degrees and-15 degrees crank angle after top dead center (degrees CA ATDC)). The experimental results indicated that the addition of cyclopentanol prolongs the ignition delay, shortens the combustion duration and improves the indicated thermal efficiency. As the injection pressure increases or the injection timing advances, the peak combustion pressures (PCPs) and peak heat release rates (PHRRs) increase. Compared to D100, the PCPs and PHRRs of cyclopentanol/diesel fuel blends are lower and decrease with the increase of cyclopentanol ratio. Blending cyclopentanol in diesel is effective in reducing soot emission, and the reduction effect weakens as the injection pressure increases or the injection timing advances. CP10 is superior to CP20 according to the comprehensive evaluation of combustion and emission, especially at the injection timing of -12 degrees CA ATDC under the injection pressure of 140 MPa. (c) 2020 Elsevier Ltd. All rights reserved.