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The effects of fuel sulfur level on tailpipe emissions from advanced technology vehicles were explored in Phase 1 of a study conducted under the Petroleum Environmental Research Forum. In this study, fuels having three different levels of sulfur were tested according to the Federal Test Procedure (FTP) in low-mileage production vehicles designed primarily to meet California's Transitional Low Emission Vehicle (TLEV) standards. The three fuels consisted of a base gasoline containing 25 ppm by mass of sulfur, with the other two fuels created by doping the base fuel to 300 ppm and 600 ppm sulfur, respectively. Nine TLEVs and one Federal Tier 1 vehicle were tested at least twice on each fuel in an order that was balanced in time and randomized among the vehicles.

For some time, spatial nonuniformities in the unburned mixture within a spark ignition engine have been suspected of contributing to cycle-by-cycle variations in combustion. In this work, an experimental structure was developed that allowed the effects of fuel/air nonuniformity and residual gas/fresh charge nonuniformity to be separately addressed, with special regard given to their impact on the flame development process and combustion rate. Under the conditions of light loads and low speed, it was found that a decrease in fuel-air nonuniformity caused a slight reduction in the variation of the early flame development angle but had no significant impact on the statistics of the main combustion phase or IMEP. Eliminating residual gas nonuniformity by skip firing the engine and adding equivalent residual to the fresh mixture prior to entry to the cylinder had no detectable impact on the flame development process.

In a previous experimental investigation of the effects of residual gas nonuniformity on S.I. engine combustion variability, it was found that eliminating residual gas nonuniformity by skip firing had no detectable impact on the flame development process, but nonetheless caused IMEP fluctuations to drop by about half under very light load conditions. Under further investigation, it has been determined that the observed IMEP fluctuations, particularly for optimally-phased cycles, are controlled by cyclic variations in the amount of fuel burned per cycle. Real-time sampling of the hydrocarbon concentration in the exhaust port has shown that the variation in fuel burned per cycle is not primarily due to variations in combustion completeness, and must therefore be attributed to variations in the amount of fuel trapped within the cylinder prior to combustion.