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Hot-weather driveability performance is influenced by vehicle design, ambient temperature and fuel volatility. For individual markets, the gasoline producer can control this aspect of performance by adjusting the fuel volatility level to meet the requirements of the car population under the seasonal ambient temperature conditions. The volatility expression that has been successfully used for several years to control hot-weather iriveability is (RVP + 0.7E70). A standard CBC test procedure is used by a volatility data-sharing group of European oil companies to characterise the performance of modern car models available in European markets. The data thus produced are used to determine the hot-weather driveability performance of European car populations. The data reveal that the performance of these car populations has improved significantly over recent years, although some modern car models with high under-bonnet temperatures show some deterioration.

Evaporative emission levels have been determined in a CONCAWE* programme for a range of ten uncontrolled European vehicles using a modified SHED test procedure as developed by the CEC*. Three extra vehicles were tested which were equipped with evaporative and exhaust emission control systems, but of the same make and model as three of the uncontrolled test cars. The vehicles were tested using several warm-up cycles and on a range of fuels whose volatility parameters were independently varied, including oxygenate blends. Exhaust emissions were determined and a few measurements of true diurnal emissions carried out. Vehicle fuel system design had the greatest effect on evaporative emissions which varied between 4 to 16 g/test on a typical European summer fuel. Gasoline volatility had a significant but smaller effect and RVP was shown to be the dominant fuel parameter. At the same volatility, oxygenate blends gave similar or lower emissions than hydrocarbon fuels.