Dilution limits of n-butane/air mixtures under conditions relevant to HCCI combustion

The role of a spark discharge in extending the operating limits of homogeneous change compression ignition (HCCI) combustion has been investigated using engine experiments and computational flame modeling. The flammability limits of ultra-dilute n-butane/air mixtures are calculated over ranges of temperature, pressure, and dilution levels relevant to HCCI operation. The results suggest that with the elevated temperatures required to achieve HCCI combustion the in-cylinder charge is capable of supporting a propagating flame over most of the HCCI operating regime. However, under light-load and idle conditions the dilution levels are too large and the spark has no effect on HCCI combustion. Thus, some other mechanism must be found to control combustion phasing under these conditions. Since the true eigenvalue for the flame propagation calculation is the mass burning rate and not the flame speed, these results demonstrate that using an arbitrary flame speed cut-off criteria for determining the dilution limit significantly underestimates the actual flammability ranges.