High compression ratio turbo gasoline engine operation using alcohol enhancement

Gasoline ethanol blends were explored as a strategy to mitigate engine knock, a phenomena in spark ignition engine combustion when a portion of the end gas is compressed to the point of spontaneous auto-ignition. This auto-ignition is dangerous to the operation of an internal combustion engine, as it can severely damage engine components. As engine designers are trying to improve the efficiency of the internal combustion engine, engine knock is a key limiting factor in engine design. Two methods have been used to limit engine knock that will be considered here; retarding the spark timing and addition of additives to reduce the tendency of the fuel mixture to knock. Both have drawbacks. Retarding spark reduces the engine efficiency and additives typically lower the heating value of the fuel, requiring more fuel for a given operating point. To study this problem a turbocharged engine was tested with a variety of combinations of gasoline and ethanol, an additive with very good anti-knock abilities. Pressure was recorded and GT Power simulations were used to determine the temperature within the cylinder. An effective octane number was calculated to measure the ability of the fuel to resist knock. Effective octane numbers varied from 91 for UTG91 to 111 for E25, respectively. Engine simulations were used to extrapolate to points that couldn't be tested in the experimental setup and generate performance maps which could be used to predict how the engine would act inside of a vehicle. It was found that increasing the compression ratio from 9.2 to 13.5 leads to a 7% relative increase in part load efficiency. When applied in a vehicle this leads to a 2-6% increase in miles per gallon of gasoline consumption depending on the drive cycle used. Miles per gallon of ethanol used were significantly higher than gasoline; 141 miles per gallon of ethanol was the lowest mileage over all cycles studied. Thesis Supervisor: John B. Heywood Title: Sun Jae Professor of Mechanical Engineering, Emeritus