Predicting NOX Emissions from HCCI Engines Using LIF Imaging

Our previous work applied LIF measurements of in-cylinder fuel distribution to predict CO₂, CO, and HC emissions from an HCCI engine under low-load stratified-charge conditions. The prediction method is based on the premise that local fuel-air packets at a given equivalence ratio (characterized using LIF imaging) burn as if in a homogeneous charge at the same equivalence ratio. Thus, emissions measured during homogeneous operation provide an emission-versus- equivalence-ratio look-up table for predicting stratified-charge emissions.

The present paper extends the technique to predict engine-out NO_X emissions. Because of operating-range limitations, NO_X look-up data for homogeneous operation cannot adequately be determined by experiment. Instead, a CHEMKIN-based model provides this look-up table data instead. The model is validated against pressure traces measured at low equivalence ratios where experiments are possible, and then extended to higher equivalence ratios, to determine NO_X emissions at these conditions. The NO_X look-up table is then combined with LIF fuel-distribution measurements to predict NO_X emissions during stratified-charge operation. The predicted NO_X trends compare well with corresponding measurements. This paper goes on to postulate in-cylinder fuel distributions that can provide both high efficiency and low NO_X emissions.