Boosted HCCI - Controlling Pressure-Rise Rates for Performance Improvements using Partial Fuel Stratification with Conventional Gasoline

This study investigates the potential of partial fuel stratification for reducing the knocking propensity of intake-boosted HCCI engines operating on conventional gasoline. Although intake boosting can substantially increase the high-load capability of HCCI, these engines would be more production-viable if the knock/stability load limit could be extended to allow higher loads at a given boost and/or to provide even higher thermal efficiencies. A technique termed partial fuel stratification (PFS) has recently been shown to greatly reduce the combustion-induced pressure-rise rate (PRR), and therefore the knocking propensity of naturally aspirated HCCI, when the engine is fueled with a φ-sensitive, two-stage-ignition fuel. The current work explores the potential of applying PFS to boosted HCCI operation using conventional gasoline, which does not typically show two-stage ignition. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) at 1200 rpm. The engine was equipped with a compression-ratio 14 piston, and combustion phasing was controlled by EGR addition.

PFS is produced by premixing the majority of the fuel and then directly injecting the remainder (up to about 20%) in the latter part of the compression stroke. For PFS to be effective, the fuel's autoignition chemistry must vary with the local equivalence ratio (φ) to produce a staged combustion event. Accordingly, tests were conducted to determine the φ-sensitivity of gasoline. They show that at naturally aspirated conditions (Pin = 1 bar), gasoline is not φ-sensitive, and PFS is not effective for reducing the PRR. However, with sufficient intake boost (e.g., Pin = 2 bar), gasoline is found to become highly φ-sensitive, and PFS very effectively reduces the PRR. Varying the amount of PFS, by adjusting either the timing or amount of DI fuel, allows control of the PRR reduction. Applying PFS to high loads at Pin = 2 bar substantially shifts the knock/stability limit and increases the maximum IMEPg from 11.7 (premixed) to 13.0 bar (PFS). Maximum load improvements with PFS are also seen for other intake pressures ranging from 1.6 to 2.4 bar. Finally, because it allows more advanced combustion phasing without knock, PFS is also effective for increasing the thermal efficiency of boosted HCCI over a range of loads for each Pin, yielding typical fuel economy improvements of 2 - 2.5%. Overall, PFS has a strong potential for improving gasoline-fueled boosted HCCI operation.