Experimental Study of Ignition and Combustion Characteristics of a Diesel Pilot Spray in a Lean Premixed Methane/Air Charge using a Rapid Compression Expansion Machine 2012-01-0825

The behavior of spray auto-ignition and combustion of a diesel spray in a lean premixed methane/air charge was investigated. A rapid compression expansion machine with a free-floating piston was employed to reach engine-relevant conditions at start of injection of the micro diesel pilot. The methane content in the lean ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber, the ambient equivalence ratio for the methane content ranged from 0.0 (pure air) to 0.65. Two different nozzle tips with three and six orifices were employed. The amount of pilot fuel injected ranged between 0.8 and 1.8 percent of the total energy in the combustion chamber. Filtered OH chemiluminescence images of the combustion were taken with a UV-intensified high-speed camera through the optical access in the piston. Filtered photomultiplier signals of the total emitted light for OH, CH and C₂ radicals as well as the pressure signal were simultaneously recorded and the effect of methane content and nozzle geometry on ignition locations, ignition timing and combustion behavior was analyzed.

It was seen, that higher values of the equivalence ratio promote propagation of the premixed flame as expected, but delay the autoignition of the pilot spray considerably. Furthermore, it was observed that even for very low equivalence ratios, substantial heat release occurs around the spray in the premixed charge, although no flame propagation in the classic sense can be sustained. For the whole range of equivalence ratios, the use of the six-hole injector was seen to considerably assist conversion of the entire premixed charge.