Hydrocarbon Permeation in Gasoline Vehicle Fuel Systems Using Isobutanol Blends

In the past decade, a significant market has emerged for automotive fuels produced from renewable sources. Blends containing low concentrations of ethanol have been the readily-available choice for providing renewable content in gasoline fuels.

The simple addition of ethanol to gasoline significantly increases the mixture's vapor pressure, which can promote higher vehicle evaporative emissions. Gasoline specifications and blending practices have been updated to help offset the increase to vapor pressure and evaporative emissions. However, recent studies have shown that even at reduced vapor pressure, ethanol can increase gasoline evaporative emissions by enhancing the permeation of hydrocarbons through the elastomeric materials found in vehicle fuel systems.

Technology is currently in development that will allow for the production of isobutanol from renewable sources. In addition to high energy density, high octane, and good material compatibility, isobutanol has low vapor pressure impact when blended with gasoline and hence, low potential to drive evaporative emissions. However, until recently the impact of isobutanol to permeation emissions had not been determined.

A test program was initiated in 2007 to explore the permeation effects of using isobutanol in gasoline in place of ethanol. The study compared seven isobutanol and ethanol blends at varying concentrations while matching fuel properties such as vapor pressure, aromatics, distillation and oxygen content. Testing was conducted using fuel systems removed from seven vehicles representing three technology groups across model years 1981 through 2006. The program duplicated the Coordinated Research Council (CRC) E-65-3 study protocols which quantified the permeation impacts of MTBE (Methyl Tertiary Butyl Ether) and ethanol. The fuel systems were removed from the vehicles and mounted on custom aluminum frames in positions closely approximating the vehicle layout. The fuel system rigs were then exposed to each fuel until the permeation emissions were stable over a three week average. Permeation emissions were measured in an emissions test SHED (Sealed Housing for Evaporative Determination) at 105°F (40.6°C). After stabilization has been determined, the rigs were placed in a variable temperature SHED (VT-SHED) and tested on the California two-day diurnal (65°F - 105°F, 18.3°C - 40.6°C) to determine the permeation impact of each fuel. The results confirm that isobutanol can be used in gasoline with reduced impact on fuel permeation emissions.