Equivalence Ratio Distributions in a Light-Duty Diesel Engine Operating under Partially Premixed Conditions 2012-01-0692
The performance of Partially Premixed Compression Ignition (PPCI) combustion relies heavily on the proper mixing between the injected fuel and the in-cylinder gas mixture. In fact, the mixture distribution has direct control over the engine-out emissions as well as the rate of heat release during combustion. The current study focuses on investigating the pre-combustion equivalence ratio distribution in a light-duty diesel engine operating at a low-load (3 bar IMEP), highly dilute (10% O₂), slightly boosted (P
ⁿ = 1.5 bar) PPCI condition. A tracer-based planar laser-induced fluorescence (PLIF) technique was used to acquire two-dimensional equivalence ratio measurements in an optically accessible diesel engine that has a production-like combustion chamber geometry including a re-entrant piston bowl. Equivalence ratio distributions are presented at a single vertical plane and three different horizontal planes within the combustion chamber, including two planes within the bowl of the piston. The quantitative measurements provide a detailed picture of the mixture formation process for an early injection PPCI combustion regime. Based on the measured equivalence ratio distributions, the mass of engine-out UHC and CO were approximated and the results confirm that the main source of UHC and CO emissions is from over-mixed, overly lean fuel/air mixtures. The measured equivalence ratio distributions were also combined with previous in-cylinder velocity and CO measurements to describe the mechanism by which these overly lean mixtures are formed and how they are transported to the squish region late in the engine cycle.