Spray Combustion of Biodiesel and Diesel in a Constant Volume Combustion Chamber

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Abstract
This paper investigates the combustion of biodiesel and diesel in an optical accessible constant volume chamber that can simulate the conditions in compression ignition engines. The high-pressure and high-temperature environment in the chamber was generated by a controlled premixed combustion with desired content of oxygen left. A common rail and an injector with 160 degree included angle were used to deliver the fuel. Ultra low sulfur diesel (ULSD) and biodiesel from used cook oil were used in this paper. High speed imaging of natural luminosity from the flame was used to study the combustion flame in a time resolved manner. A combination of high speed and intensified imaging of OH chemiluminescence was employed to identify reaction zones. This method has the advantage of capturing the global pictures, thus allowing the studying of jet-to-jet variations, while it does not sacrifice the time-resolved resolution.The effects of different ambient temperature and oxygen concentration on the combustion were studied for ULSD and biodiesel. The results indicate that an early injection of fuel, representing the higher temperature, experiences a prompter auto-ignition, which results in a less content of air entrainment in the early stage of flame development. For ULSD, this produces a higher level of soot, while the effect is less apparent for biodiesel. The soot formation may be more affected by the jet-wall impingement in this case for both ULSD and biodiesel. The 9% oxygen concentration, which simulates the high EGR level, decreases the flame luminosity intensity, which implies the reduction of flame temperature and soot formation. For biodiesel, with the use the high EGR, the soot formation has been dramatically reduced. The experimental results show the possibility to use biodiesel as the alternative of ULSD to suppress soot formation under high EGR level and low temperature combustion mode.

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