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The combustion and emission performance of two high-pressure GDI-type injectors are compared in an automotive HCCI engine during low-load, stratified operation. One of these, an 8-hole injector with 70° spray angle, provides significant reduction in NOX emissions at a given fuel-to-CO2 conversion efficiency (i.e., an improved NOX-CO2 trade-off) compared to the other, a 53° swirl injector. In contrast, attempts to enhance the NOX-CO2 trade-off using alternate charge-stratification strategies such as split injection and high intake velocity are shown to be less successful. The 8-hole and swirl injectors are also compared using the optical techniques of Mie scattering (spray visualization), laser-induced fluorescence imaging (fuel distribution measurement), and direct combustion imaging. The resulting data suggest two possible explanations for the superior performance of the 8-hole injector.

This paper proposes a method for quantitatively predicting emissions from a homogeneous-charge compression-ignition (HCCI) engine using laser-induced fluorescence (LIF) imaging of the in-cylinder fuel distribution. The prediction scheme, which is applied to stratified operation, is based on the premise that local fuel-air packets at a given equivalence ratio burn as if in an isolated homogeneous mixture at the same equivalence ratio. Insofar as the premise holds true, the emissions produced by each packet can be predicted using a look-up table of exhaust emission values measured during homogeneous operation. LIF images of fuel distribution during stratified operation are reduced to probability density functions (PDF) that, together with the look-up tables, allow prediction of engine-out emissions. Despite the simplifications associated with the prediction scheme, predicted values of CO2, CO and HC emissions each agree to within 15% of total fuel carbon for low-load operation.