Effects of Combustion Chamber Shapes on Combustion and Emission Characteristics for the N-Octanol Fueled Compression Ignition Engine

The objective of this study is to numerically investigate the effects of the combustion chamber (CC) shapes on the combustion and emission characteristics of the n-octanol fueled compression ignition (CI) engine. At first, to enable the simulation of the combustion process for n-octanol, the skeletal mechanism that consists of 87 species and 422 reactions was obtained by applying directed relation graphs (DRG), directed relation graphs with error propagation (DRGEP), sensitivity analysis, isomer lumping, and peak concentration analysis to the detailed n-octanol reaction mechanism. Then, it was coupled with KIVA4-CHEMKIN to study the effects of the CC shapes for the n-octanol fueled CI engine. In this study, based on the original CC, five different shapes of CCs were designed by changing the parameters of throat radius, depth, and shrinkage angle while keeping the compression ratio constant. Simulations with different CCs were carried out at 1,500 and 2,400 rpm, respectively. The results show that the CCs with different throat radii have important effects on combustion and emission characteristics. As increasing the throat radius, the peak heat release rate (HRR) and the pressure rise rate (PRR) increase at 1,500 rpm; in comparison, both peak HRR and PRR show an opposite trend at 2,400 rpm. Moreover, as widening the throat, the indicated mean effective pressure (IMEP) of 2,400 rpm can be improved significantly due to the extension of combustion duration. For emissions, both narrow and wide throats could generate less NOx under 1,500 and 2,400 rpm. Also, ultra-low soot emissions are realized by fueling n-octanol. As for CO emissions, with the increase of throat radius, it decreases first and then increases at 1,500 rpm; in contrast, a decreasing trend can be observed due to the prolonged combustion duration. Considering the best performance of the engine, NO.5 CC with a radius of 25.75 mm can be selected.