Fuel stratification has been investigated as a means of improving the low-load combustion efficiency in an HCCI engine. Several stratification techniques were examined: different GDI injectors, increased swirl, and changes in injection pressure, to determine which parameters are effective for improving the combustion efficiency while maintaining NOx emissions below U.S. 2010 limits. Performance and emission measurements were obtained in an all-metal engine. Corresponding fuel distribution measurements were made with fuel PLIF imaging in a matching optically accessible engine. The fuel used was iso-octane, which is a good surrogate for gasoline.
For an idle fueling rate (ϕ = 0.12), combustion efficiency was improved substantially, from 64% to 89% at the NOx limit, using delayed fuel injection with a hollow-cone injector at an injection pressure of 120 bar. Relative to this base case, changing to an 8-hole injector provided the single largest improvement, increasing combustion efficiency to 92%. The effects of swirl varied with injector type, but increased injection pressure was beneficial for both injectors. The highest combustion efficiency of 92.5% at the NOx limit was achieved with the 8-hole injector and an injection pressure of 170 bar, with low swirl.
Quantitative fuel-distribution maps derived from the PLIF images showed good agreement with the combustion-efficiency and NOx-emission measurements in the metal engine. The images showed that at the NOx limit, fuel distributions and maximum equivalence ratios (ϕ) are similar for the two injectors, with delayed injection producing a single large fuel pocket. Fuel-mass histograms suggest that the 8-hole injector improved the combustion-efficiency at the NOx limit by reducing the fraction of low-ϕ regions, but a wider field of view is required to fully confirm this. The images also show that increased swirl inhibited the mixing of fuel into the center of the combustion chamber, explaining the slower mixing rates observed in the metal engine. A general finding is that the combustion-efficiency/NOx tradeoff improves when fuel can be injected as late as possible with acceptable levels of NOx. Therefore, techniques that provide even faster mixing have the potential for further improvements.