Ignition Quality Effects on Lift-Off Stabilization of Synthetic Fuels

The ignition and flame stabilization characteristics of two synthetic fuels, having significantly different cetane numbers, are investigated in a constant volume combustion vessel over a range of ambient conditions representative of a compression ignition engine operating at variable loads. The synthetic fuel with a cetane number of 63 (S-1) is characterized by ignition delays that are only moderately longer than n-dodecane (cetane number of 87) over a range of ambient conditions. By comparison, the synthetic fuel with a cetane number of 17 (S-2) requires temperatures approximately 300 K higher to achieve the same ignition delays. The much different ignition characteristics and operating temperature range present a scenario where the lift-off stabilization may be substantially different. At temperatures below 1000 K, the S-2 fuel undergoes a long transient stabilization phase during which the lift-off location moves as much as 15 mm upstream (i.e., toward the injector orifice) after the ignition of the first flame kernel. This behavior is much different than S-1, n-dodecane, or with conventional diesel, in which past research shows that the lift-off location stabilizes very close to the ignition location shortly after the premixed burn. The longer ignition delays for S-2 frequently result in fuel-lean mixtures at the ignition location where the spray becomes over-mixed (i.e., too fuel-lean) and the high-temperature ignition event is noticeably less robust (i.e., smaller and less intense ignition kernels) as observed by high-speed chemiluminescence imaging. High-speed chemiluminescence imaging and pressure measurements show strong evidence of cool-flame (i.e., first-stage or low-temperature) reactions prior to high-temperature ignition for S-1 while they are less evident for S-2.