Effects of combustion chamber diameter on the performance and fuel–air mixing of a double swirl combustion system in a diesel engine

A double swirl combustion system (DSCS) in a diesel engine can make significant efficiency improvements and emission reductions. To explore the effects of combustion chamber diameter on the performance at various operations, DSCSs with a diameter of 83, 91, and 98 mm (DSCS83, DSCS91, and DSCS98) were designed and tested in a single-cylinder diesel engine at the maximum torque speed of 1800 rpm under various loads and various excess air coefficients (φ). The experimental results showed that DSCS83 outperformed the other DSCSs, with a 2.1–4.9% decrease in brake specific fuel consumption (BSFC), 12.4–23.1% reduction in soot, and a shortening of the combustion period by 1.6–3.6 °CA. To reveal the mechanism of fuel–air mixing in different DSCSs, simulation models were established with AVL-Fire. The simulation results indicated that the wall-flow-guiding effects and in-cylinder air motion including air entrainment and reversed squish improved as the combustion chamber diameter decreased, which contributed to fast fuel–air mixing in the outer chamber and the clearance, high indicated power and low soot generation. However, when combustion chamber further decreased from 83 to 76 mm, the unutilized air in the inner chamber and the clearance reduced the performance. An analysis of the uniformity index of the equivalence ratio in different parts of DSCS was performed to characterize the fuel–air mixing process. Results found that DSCS83 had a 1.2–4.8% improvements in the whole-chamber uniformity index at 60° CA ATDC at 32% load operating condition. These results could be an essential reference in the optimization and application of DSCSs in diesel engines.