Research on atomization, combustion and emission characteristics of opposed rotary piston engine over various intake pipe deflection angle and injection timing in the full speed range

Opposed rotary piston (ORP) engine demonstrates significant potential in Unmanned aerial vehicle (UAV) due to its compact structure and high power density. Due to the previous research only focused on the single injection optimization factor of straight intake pipe, it is difficult to give full play to the advantages of high power density of ORP engine. Therefore, this study systematically investigates the effect of intake pipe deflection angle (IPDA), fuel injection position, injection angle and injection timing on atomization, combustion and emission characteristics. The results demonstrate that the intake pipe with a larger IPDA exhibits a distinct pressure gradient caused by streamline contraction at the curved pipe, leading to a significant increase in intake flow velocity. Compared to a straight intake pipe, the curved intake pipe with a 60° IPDA exhibits substantially improved flow characteristics, achieving 85.4 % charging efficiency at 5000 r/min and a 14.5 % increase in TKE, while maintaining a flow coefficient of 62.23 % and reducing intake losses. The 60°-position 2 delivers superior fuel atomization, characterized by a SMD of 4.24 μm and the wall oil-film mass of 1.98 mg, while achieving homogeneity mixture (λ = 1.003). Early injection extends the mixing time, enhancing the uniformity of the mixture and improving combustion efficiency. At 1000 and 3000 r/min, a fuel injection timing of −180 °CA yields thermal efficiency of 36.8 and 33.9 %, respectively. Properly delayed injection enhances fuel droplet atomization by leveraging the inertial force of high-velocity airflow. At 5000 r/min, when the injection timing is −150 °CA, the peak in-cylinder pressure increases by 11.3 %, the power output reaches 130.1 kW, and the BSFC is only 173.3 g/kW·h. Moreover, delayed injection reduces wall oil-film mass, when injection timing is delayed from −180 to −165 °CA, the wall oil-film mass decreases from 1.98 to 0.52 mg at 3000 r/min. The optimal injection timing elevates in-cylinder temperature, resulting in a substantial rise in NO_x emission while effectively lowering CO and HC emission.