Effects of n-butanol injection timing on mixture stratification, combustion and emission characteristics of n-butanol/diesel dual direct-injection engine

In this paper, based on the CFD simulations, the effects of injection timing of n-butanol (InjB) on mixture stratification were quantified by a novel Sample Line method. Then InjB's impacts on combustion, emission characteristics and energy and exergy balance of dual fuel direct injection (DFDI) engine fueled n-butanol/diesel were studied in-depth. It was found that, with an earlier InjB, the distribution of n-butanol was more homogenous and the reactivity of mixture was higher, resulting in better combustion process and higher indicated thermal efficiency (ITE). As InjB was retarded, the inhomogeneity of concentration distribution of n-butanol increased. The concentration of n-butanol at the squish region was decreased and more high concentration region of n-butanol appeared at the bottom of combustion chamber pit. In addition, the wall-wetting phenomenon increased, causing more incomplete combustion and lower ITE. Compared with InjB = 150°CA BTDC, when InjB = 300°CA BTDC, the energy loss and exergy loss due to incomplete combustion were reduced by 0.17 kW and 1.88 %, respectively. The ITE enhanced by about 3.5 %, the soot emission was reduced, and the NO emission was slightly increased. Therefore, it was suggested to adopt an earlier InjB under medium loads to achieve high efficiency and clean combustion.