Smoothing HCCI Heat-Release Rates Using Partial Fuel Stratification with Two-Stage Ignition Fuels 2006-01-0629
This work explores the potential of partial fuel stratification to smooth HCCI heat-release rates at high load. A combination of engine experiments and multi-zone chemical-kinetics modeling was used for this. The term “partial” is introduced to emphasize that care is taken to supply fuel to all parts of the in-cylinder charge, which is essential for reaching high power output.
It was found that partial fuel stratification offers good potential to achieve a staged combustion event with reduced pressure-rise rates. Therefore, partial fuel stratification has the potential to increase the high-load limits for HCCI/SCCI operation. However, for the technique to be effective the crank-angle phasing of the “hot” ignition has to be sensitive to the local ϕ. Sufficient sensitivity was observed only for fuel blends that exhibit low-temperature heat release (like diesel fuel). In contrast, for a single-stage ignition fuel (like typical gasoline) the timing of the hot ignition is relatively insensitive to the local ϕ, and this renders partial fuel stratification ineffective for creating a staged combustion event.
The effects of partial fuel stratification were demonstrated experimentally for PRF80 and PRF83 fuels using a hollow-cone DI injector and two fuel-injection pulses. Suitable fuel stratification was created by injecting the main portion of the fuel during the intake stroke, and the remainder during the compression stroke. This smoothed the heat-release and lowered the pressure-rise rate significantly compared to fully-premixed fueling. For naturally aspirated operation with CA50 at 371°CA, partial fuel stratification allowed IMEPg to be increased from 537 to 597 kPa while maintaining acceptable ringing intensity (PRRmax < 8 bar/°CA) and NOx emissions below US 2010 standards. However, partial fuel stratification must be implemented carefully since too much stratification can quickly lead to unacceptable NOx emissions.