Publications

Detailed exhaust speciation measurements were made on an HCCI engine fueled with iso-octane over a range of fueling rates, and over a range of fuel-stratification levels. Fully premixed fueling was used for the fueling sweep. This sweep extended from a fuel/air equivalence ratio (Φ{phonetic}) of 0.28, which is sufficiently high to achieve a combustion efficiency of 96%, down to a below-idle fueling rate of Φ{phonetic} = 0.08, with a combustion efficiency of only 55%. The stratification sweep was conducted at an idle fueling rate, using an 8-hole GDI injector to vary stratification from well-mixed conditions for an early start of injection (SOI) (40°CA) to highly stratified conditions for an SOI well up the compression stroke (325°CA, 35°bTDCcompression). The engine speed was 1200 rpm. At each operating condition, exhaust samples were collected and analyzed by GC-FID for the C1 and C2 hydrocarbon (HC) species and by GC-MS for all other species except formaldehyde and acetaldehyde. These two species were analyzed using high-performance liquid chromatography. In addition, standard emissions-bench exhaust analysis equipment was used to measure total HC, CO, CO2, O2, and NOX simultaneously with the sampling for the detailed-speciation analysis. Good overall agreement was found between the emissions-bench data and total HC from the detailed measurements. Unreacted fuel, iso-octane, was by far the most prevalent HC species at all operating conditions. Numerous other HC and oxygenated HC (OHC) species were found that could be identified as breakdown products of iso-octane. Several smaller HC and OHC species were also identified. At the highest Φ{phonetic}, emissions of all species were low, except iso-octane. As Φ{phonetic} was reduced, emissions of all species increased, but the rate of increase varied substantially for the different species. Analysis showed that these differences were related to the degree of breakdown from the parent fuel and the in-cylinder location where they formed. SOI-sweep results indicated that stratification improves combustion efficiency by reducing the fuel penetration to the crevice and cylinder-wall boundary-layer regions, as well as by creating a locally richer mixture that burns hotter and more completely.

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 NO x-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.

Abstract not provided.