Fuel Volatility Effects on Driveability of Vehicles Equipped with Current and Advanced Fuel Management Systems

Fuel volatility and cold/hot engine driveability relationships were evaluated in six 1976/1977 model cars representing conventional carburetor and advanced type fuel metering systems. The program objective was to provide guidance for engine modifications to take advantage of fuel benefits or to overcome performance deficiencies. There were large variations among cars in the maximum volatility tolerance relative to vapor lock during summer, hot engine operation, with a fuel-injected and a new design carburetor system tolerating gasoline volatility levels in excess of normal maximum summer levels. Similarly, cold engine start and driveaway performance at low and intermediate ambient conditions varied widely. Fuel-injected cars showed the best performance and least sensitivity to gasoline volatility changes. Performance differences among all cars with a specific fuel were significantly greater than differences resulting from typical variations of fuel volatility for individual cars. This shows that fuel systems can be designed to provide good overall driveability performance and wide tolerance in fuel volatility, permitting greater flexibility to produce unleaded gasoline.