Experimental investigation combined with steady-state and transient-state tests on soot characteristics of hydrogenated catalytic biodiesel/n-butanol blends

The utilization of biodiesel and biobutanol offers a promising approach to achieve energy conservation and emission reduction in combustion engines. This study investigates the soot characteristics of hydrogenated catalytic biodiesel mixed with n-butanol (20% by volume) under steady-state and transient-state conditions using diffused background-illumination extinction imaging and two-color method, respectively. Both conditions in constant volume combustion chamber (steady-state) and optical engine (transient-state) exhibit similar soot stages, namely Formation Dominating and Oxidation Dominating. Between the above Dominating stages, the soot development under steady-state conditions is further segmented into Initial Oxidation and Secondary Oxidation, while a Stalemate stage manifests under transient-state conditions owing to wall impingement and strong turbulence. Under steady-state conditions, formation and oxidation processes are mainly controlled by the soot peak of spray head. By decreasing ambient temperature and increasing injection pressure, soot accumulation is notably diminished by 30.27% and 45.72% during the Formation Dominating stage, achieved through the deceleration of precursor reactions and enhancement of air-fuel mixing characteristics within the flame lift-off zone, respectively. Additionally, higher injection pressure notably amplifies the efficacy of the Initial Oxidation, primarily through the reduction of soot residence time. Under transient-state conditions, the Formation Dominating stage for SOI-25 and SOI-20 occur away from the early combustion, resulting in a reduction of 10.26% and 68.36% in soot compared to SOI-30. The Stalemate stage is characterized by a competition between the oxidation of high-temperature flames above 2000 K and the formation of low-temperature flames below 1800 K. A portion of soot originating from the high-temperature flames becomes integrated into the low-temperature flames after cooling, subsequently emerging as the primary hindrance to the ensuing oxidation process.