Effect of Ambient Temperature on Flame Lift-off and Soot Formation of Biodiesel Sprays 2010-01-0606
Pure diesel and biodiesel were tested inside a constant-volume combustion chamber which simulates the in-cylinder conditions similar to a diesel engine and is more flexible to change the engine operation boundary conditions. The ambient temperature effect on flame lift-off length for both fuels was first investigated with fixed injection pressure, duration, ambient density, and ambient oxygen concentration. This was determined from time-averaged OH chemiluminescence imaging technique. Then, the impacts of the observed lift-off length variations on oxygen ratio upstream of the lift-off location and the soot formation process were also studied.
A Forward Illumination Light Extinction (FILE) soot measurement technique was adopted to study the soot formation process. The FILE technique with the capability of two-dimensional time-resolved quantitative soot measurement provides the much-needed information to investigate the soot formation mechanism.
The results show that as the ambient temperature increases, the lift-off length decreases for both fuel sprays. The reduction of the lift-off length from 800 K to 900 K ambient temperature is much larger than the reduction observed at higher temperatures. Biodiesel has a longer lift-off length than diesel at all ambient temperatures. Similar trends are seen with the oxygen ratio as those of the lift-off length. A significant decrease of soot emission for biodiesel is observed compared to diesel under all ambient temperatures. The soot formation duration increases with increasing ambient temperature. Shorter soot formation duration was also detected for biodiesel despite its earlier start of combustion.