Bruno Schneider

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

The behaviour 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 auto-ignition 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.

Integration of a Cool-Flame Heat Release Rate Model into a 3-Stage Ignition Model for HCCI Applications and Different Fuels

The heat release of the low temperature reactions (LTR or cool-flame ) under Homogeneous Charge Compression Ignition ( HCCI ) combustion has been quantified for five candidate fuels in an optically accessible Rapid Compression Expansion Machine (RCEM). Two technical fuels (Naphthas) and three primary reference fuels (PRF), (n-heptane, PRF25 and PRF50) were examined. The Cetane Numbers (CN) of the fuels range from 35 to 56. Variation of the operating parameters has been performed, in regard to initial charge temperature of 383, 408, and 433K, exhaust gas recirculation (EGR) rate of 0%, 25%, and 50%, and equivalence ratio of 0.29, 0.38, 0.4, 0.53, 0.57, and 0.8. Pressure indication measurements, OH-chemiluminescence imaging, and passive spectroscopy were simultaneously implemented. In our previous work, an empirical, three-stage, Arrhenius-type ignition delay model , parameterized on shock tube data, was found to be applicable also in a transient, engine-relevant environment. The pressure rise due to cool-flame heat release , which is crucial for the induction of main ignition , was included in the experimental pressure traces that have been used. To fully predict the ignition delay in HCCI -engine applications however, the cool-flame heat release characteristics need to be known in advance. In this work, the cool-flame heat release characteristics have been investigated with regard to operating parameters. A simplified, cool-flame heat release model is proposed, that is mathematically independent from the three-stage ignition delay model . It provides the cool-flame heat release profile that is used to reconstruct a pressure/temperature trace including the effect of the cool-flame . The reconstructed trace is the input to the three-stage model , and thus both cool and hot-flame ignition delays can be predicted. The overall performance of the combined three-stage/ cool-flame heat release model was assessed. Very good agreement was observed between the experimental ignition delay and the combined cool-flame /three-stage ignition model computations.