Energy balance characteristics and particulate size–number distributions of an improved double swirl combustion system

As a well-established wall-flow-guided combustion system for direct-injection diesel engines, the double swirl combustion system (DSCS) has been demonstrated to promote the in-cylinder fuel–air mixing and improve the diffusion combustion performance. However, with increasing engine loads, the DSCS still exhibits low efficiency and high soot emissions. To address these limitations, an improved DSCS (I-DSCS) has been proposed. Previous visualization experiment conducted in a constant-volume combustion vessel has verified the I-DSCS wall-flow-guided concept.To further promote the I-DSCS development from conceptual analysis toward practical engineering application, taking the DSCS as a reference, the combustion performance, energy balance characteristics and particulate size–number distributions of the I-DSCS were investigated on a single-cylinder diesel engine. Simulations were performed to reveal the in-cylinder fuel–air mixing, flow field evolutions, and emission formation. Experimental results indicate that, with increased indicated thermal efficiency, reduced accumulation-mode soot particle emissions, reduced cylinder head fire-deck temperature, and reduced cylinder head heat dissipation fraction, the I-DSCS exhibits more favorable combustion performance and energy balance characteristics. The effectiveness of the I-DSCS in improving combustion performance and optimizing energy conversion pathways is verified. However, the I-DSCS also increases NOx emissions. Simulation results indicate that the I-DSCS promotes fuel–air mixing and suppresses soot net formation by generating intense fuel spray swirling motions and inducing coherent large-scale vortices in both the inner and outer chambers. Additionally, the I-DSCS reduces the cylinder head fire-deck temperature by preventing the accumulation of rich fuel–air mixture and high-temperature fluid on the cylinder head bottom face.