Improving the NOX-CO2 Trade-Off of an HCCI Engine Using a Multi-Hole Injector 2007-01-0180

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 NO_X emissions at a given fuel-to-CO₂ conversion efficiency (i.e., an improved NO_X-CO₂ trade-off) compared to the other, a 53° swirl injector. In contrast, attempts to enhance the NO_X-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. First, some fuel leaving the swirl injector at the end of injection burns as a liquid, likely contributing to NO_X emissions. In contrast, no liquid burning is apparent with the 8-hole injector. Second, fuel distributions measured by fluorescence imaging show that probability density function (PDF) statistics of equivalence-ratio distribution are similar for the two injectors, but the 8-hole injector produces smaller and more numerous fuel packets that could lead to lower local peak gas temperatures and therefore lower NO_X emissions.