Volatility and vehicle driveability performance of ethanol/gasoline blends: a literature review
The effect of blending ethanol (up to 20% v/v) into gasoline on the volatility of the ethanol/gasoline blend and on the hot and cold weather vehicle driveability performance of these blends has been assessed from published literature. This literature review covers major fuel blending and vehicle driveability studies that have been completed over the past 20 years. Volatility of Ethanol/Gasoline Blends The percentage of an ethanol/gasoline blend that evaporates at 70oC (E70)substantially increases when ethanol is blended into gasoline. The correspondingE100 (the percent of the blend that evaporates at 100oC) also increases but less substantially than the E70 value. The increase in these two volatility parameters with ethanol addition is usually smaller as the volatility of the gasoline increases.Interestingly, “Blending E70” values for ethanol tend to decrease with increasing ethanol content while “Blending E100” values increase. This difference is due to the formation of an azeotrope that affects the volatility behaviour of the ethanol/gasoline blend at different temperatures. Simple predictive models have been developed based on analytical data from the published studies that describe the change in Blending E70 and Blending E100 values with the base gasoline’s E70 and E100 and with ethanol contents in the range of 5 to 20% v/v. Vehicle Driveability Performance of Ethanol/Gasoline Blends Published studies on the impact of ethanol and gasoline volatility on vehicle driveability performance have also been evaluated. These included seven studies on Hot Weather Driveability (HWD) and eleven studies on Cold Weather Driveability (CWD). These studies show that modern vehicles are much less susceptible to HWD performance problems than are older vehicles. Some early model Direct Injection Spark Ignition vehicles tested by CONCAWE/GFC (2003) showed some HWD problems but only on high volatility fuels. Current specification properties appear to be adequate to control HWD but some increases in the E70 maximum limits allowed by the European EN228 gasoline specification may be needed in order to allow ethanol blending into gasoline at 10% v/v and higher. CWD vehicle performance is affected by mid-range gasoline volatility (E100) and is an issue for modern vehicles because it is linked to exhaust emissions performance under cold starting conditions. CWD is degraded by the use of ethanol/gasoline blends at the same volatility level as hydrocarbon-only gasolines. To reduce the impact of ethanol, new Driveability Indices (DI) have been developed and applied in some extensive US studies. These DIs generally include ethanol offset terms in order to control the impact of volatility on CWD performance. Although current E100volatility class limits are fixed in the European EN288 gasoline specification, the published literature indicates that the minimum E100 limits should ideally vary with ambient temperature and should include an ethanol offset term in order to control CWD performance. Based on these results, a European DI including an ethanol offset term should be considered in order to account for the performance of European vehicles under European climatic conditions.