Optical diagnostic study of coal-to-liquid/butanol blend and dual-fuel combustion of a CI engine

Abstract

The clean alternative fuel represented by coal-to-liquid (CTL) and butanol has attracted the attention of researchers. Previous research by our group has shown that both CTL/butanol blend and dual-fuel combustion can alleviate the problems when burning pure CTL in compression-ignition (CI) engines. To investigate the mechanism of CTL/butanol blend and dual-fuel combustion improving premixed combustion and reducing particulate emissions, an optical diagnostic study was conducted on a self-built dual-fuel optical engine with flexible and adjustable combustion boundary conditions for CTL/butanol blend and dual-fuel combustion. The images of flame development had been acquired and analyzed by means of high-speed photography combined with two-color method. The results show that the blending of butanol into the CTL reduces the reactivity of the blended fuel and leads to a prolonged ignition delay (ID), while the ignition timing of the CTL/butanol dual-fuel combustion is mainly controlled by the CTL with high reactivity, so the ID varies slightly with the butanol addition in the dual-fuel combustion. During the combustion of blend fuel mode, a visible flame is formed near the cylinder wall and the flame in the center of the cylinder is not obvious, whereas during the combustion of dual-fuel mode there are two distinct kinds of flame both around the cylinder wall and in the center of the cylinder. Both butanol blending and butanol pre-spraying can significantly reduce the flame area and flame natural luminosity during combustion, but the effect of butanol blending is more obvious. So that increasing the proportion of butanol is effective in reducing the average in-cylinder temperature at both the CTL/butanol blend and dual-fuel combustion modes. However, compared to dual-fuel combustion, CTL/butanol blend combustion can reduce the in-cylinder temperature gradient more effectively. Since soot is more likely to be generated in the high temperature region, blending butanol into CTL is more effective in reducing soot generation than pre-spraying butanol into the intake.