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Development of a Naturally Aspirated Spark Ignition Direct-Injection Flex-Fuel Engine
Abstract:
Companion empirical and analytical studies were conducted to assess the feasibility and constraints (hardware and combustion perspectives) associated with operating a Spark Ignition Direct-Injection (SIDI) engine on high ethanol and gasoline mixtures ranging from 0 to 85% by volume. Cold start, part and full-load performance aspects were explored.
Analytical experiments were performed to correlate with the empirical data using a commercially available single dimensional engine simulation code. Under WOT operating conditions it was found that the engine's simulated output was overestimated with E85 fuel which was caused by the over prediction of volumetric efficiency. It was necessary to create a sub-routine to accurately model the impingement, vaporization, and heat transfer of fuel on the piston surface. Results could only be correlated after taking fuel impingement into account.
Spray measurements were conducted including white light illumination and phase Doppler interferometery to study the differences in the spray shape, impingement, and the droplet formation regime between gasoline and E85 fuels. Efforts were taken to study the effects of temperature and back pressure on the spray. The results suggested that the spray properties were quite similar between the fuels, that the penetration lengths and droplet sizes of the fuels were comparable for like conditions, however E85 was more resistant to flash boiling effects for typical engine operating conditions.
Selected empirical part-load optimization suggested efficiency improvements of 3-6% were possible over the optimized gasoline baseline. These gains were primarily due to a reduction in heat rejection and increased Exhaust Gas Residual (EGR) tolerance. These efficiency gains mildly offset the inherent energy density deficiency of ethanol based gasoline mixtures.
Full-load empirical data suggested a potential for 13-15% increase in specific output with E85, as compared to a customer intent gasoline baseline fuel. These performance gains were enabled by the anti-knock qualities of the ethanol blends and an increase in volumetric and indicated efficiencies. It was also found that optimized full-load E85 operation was possible with injector hardware that was sized for gasoline (E00) operation. Ultimately, strategies were developed that enabled E85 operation at all operating conditions, as enabled by low smoke operation, improved knock resistance, increased charge cooling, and increased combustion stability.
Cold startability tests with representative Class 3 E85 fuel were conducted at extreme ambient conditions to assess the cold start performance of the SIDI engine. It was found that a Low Pressure Start (LPS) operating strategy enabled engine starts at ambient conditions as cold as -20C, however, High Pressure Stratified Starts (HPSS) were necessary to meet colder startabilty requirements. Ultimately, a HPSS strategy improved startabilty and reduced enrichment requirements for E85.