Fundamental Volatility/Driveability Characteristics of Oxygenated Gasolines at High Underbonnet Temperatures 831705
Much interest is currently being shown in the use of high octane organic oxygenated compounds as gasoline components, particularly with increasing pressure to lower lead levels. Such usage focuses attention on sensitive areas of vehicle performance like for example, hot weather vehicle driveability/fuel volatility.
To compare the hot fuel handling characteristics of oxygenated fuels with motor gasoline, a series of tests has been carried out on a chassis dynamometer using a variety of European cars and more than 70 different fuels. The fuel compounds assessed included Methanol, Ethanol, Tertiary Butyl Alcohol (TBA) and Methyl Tertiary Butyl Ether (MTBE) blended individually and in combination at up to 25 per cent volume concentration with motor gasoline.
Initial analyses of the dynamometer data showed, under severe operating conditions (eg at and above 35°C ambient air temperature), certain oxygenate types to disadvantage and indicated that existing gasoline volatility criteria were inadequate to predict the driveability performance of oxygenated fuels. However, further laboratory tests to study the fundamental relationship between temperature and fuel vapour pressure characteristics, together with supplementary road driveability trials, have led to the belief that excessive vehicle fuel system temperatures are largely responsible for the unexpected/undesirable behaviour of some oxygenate supplements.
The authors conclude that vehicle fuel system temperatures must not be allowed to rise excessively otherwise there will be a deterioration in the driveability performance of oxygenated fuels and new volatility specification criteria will be required. These could include a determined vapour/liquid ratio measurement or a vapour pressure control at much higher temperature levels than is currently specified for Reid vapour pressure.