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This paper describes the results of a liquefied petroleum gas (LPG) fleet test conducted using para-transit, medium-duty vehicles. The vehicles were part of an active municipal fleet providing daily service on varying operating routes. Over a period of nine months, each vehicle was fueled with a series of butane/propane mixtures. The mixtures tested were HD5 LPG as the baseline fuel, 20 percent butane/80 percent propane, 30 percent butane/70 percent propane, and a final blend of 50 percent butane/50 percent propane by volume. The test vehicles showed improved fuel economy as the butane content increased in the fuel mixture, even without modification to existing LPG fuel systems. The improved fuel performance was consistent with the higher energy content of butane, compared to an equal volume of propane. The vehicles displayed no symptoms of performance or maintenance problems that would be related to operation of the fuel mixtures.

The Coordinating Research Council (CRC) volatility group has issued a new cold-start and warm-up driveability test designed to improve the resolution of volatility effects on MPFI vehicles. Additionally a new demerit weighting procedure is now available to use with the new test. The weighting procedure was developed through an intermediate-temperature cold-start and warm-up driveability program conducted in the fall of 1994. The program also evaluated the effects of MTBE and volatility (driveability index) in late-model vehicles. The new test procedure and demerit calculation ordered fuels by volatility and generally generated higher average demerit levels for MPFI vehicles than the procedure used in previous cold-start programs. As in previous programs, driveability was a nonlinear function of DI and the presence of MTBE was not found to be a statistically significant effect. A volatility index based on percent evaporated was generated.