Fuel volatility and vehicle characteristics have long been recognised as important parameters influencing the exhaust emissions and the driveability of gasoline vehicles. Limits on volatility are specified in a number of world-wide / national fuel specifications and, in addition, many Oil Companies monitor driveability performance to ensure customer satisfaction. However, the relationship between driveability and exhaust emissions is relatively little explored.
A study was carried out to simultaneously measure driveability and exhaust emissions in a fleet of 10 European gasoline vehicles. The vehicles were all equipped with three-way catalysts and single or multi-point fuel injection. The test procedure and driving cycle used were based on the European Cold Weather Driveability test method. Six experimental test fuels of varying volatility were tested in the vehicle fleet, at temperatures of -5°C and+10°C, to provide a range of driveability performance and allow the relationship between driveability and emissions to be investigated.
The main differences in demerits were seen between different vehicles and changes in ambient temperature; fuel volatility and oxygenate content had a relatively minor effect. Analyses of the emissions data showed that a change in the exhaust emissions occurred when driveability malfunctions such as stumble or hesitation were observed. These findings were limited to the data generated at -5°C only as there were too few demerits produced at+10°C. Driveability demerits gave rise to a statistically significant increase in HC and CO2 emissions, an apparent increase in NOx, but no correlation with CO emissions. The data also showed that the control of air/fuel ratio during engine warm up was a factor influencing driveability and emissions performance.