Study: Gas becomes stale before water uptake becomes a concern

By Renewable Fuels Association | September 21, 2016

As millions of Americans say goodbye to summer and prepare to store their boats, motorcycles, lawn mowers, leaf blowers, and other equipment for the winter, a new study by the Department of Energy is providing fresh insight into a decades-old debate about the impacts of ethanol-blended gasoline on water uptake and “phase separation” in small and off-road engines.

The study, conducted by DOE’s National Renewable Energy Laboratory, found that the petroleum components of ethanol-blended gasoline become degraded and unfit for use in an engine long before the ethanol portion takes up enough water to cause phase separation in the fuel tank. “Phase separation” occurs when an excessive amount of water is introduced into the fuel tank leading the ethanol and water to mix and sink to the bottom of the tank. In other words, gasoline becomes “stale” and unusable before water uptake by the ethanol component becomes a concern.

“Significant gasoline weathering (evaporation of the most volatile components) can occur over one month of storage in a high-temperature, high-humidity environment, with total mass losses as high as 30-70 percent for certain tanks,” according to the study, which was commissioned by the Renewable Fuels Association. “This means gasoline weathering, which can have a negative effect on fuel quality, generally occurs well in advance of any issues related to phase separation. The fuel vapor pressure may drop to levels where the fuel is not fit for purpose (engine will be difficult or impossible to start) and there may also be gum formation.”

As part of the study, NREL scientists stored gasoline-ethanol blends ranging from E0 (0 percent ethanol) to E85 (83 percent ethanol) in actual lawn mower fuel tanks over several months in a climate-controlled chamber meant to replicate hot, humid environments like Houston and Orlando. The samples were tested at regular intervals for evidence of gasoline weathering and water uptake.

In every case, the hydrocarbon components of the fuel became unfit for use in an engine before water uptake became a concern. Over time, the fuel samples experienced significant loss of volatility, loss of mass, reduced octane rating, increased concentration of sulfur and gum, and other degradations. The study found that ethanol-free gasoline (E0) degraded “to the same degree [as ethanol-blended fuels] during this timeframe…An ethanol-free gasoline stored in the same conditions for the same period of time would likely be problematic despite a lack of phase separation.”

For gasoline-ethanol blends, it often took more than three months for phase separation to occur, meaning the fuel had already weathered to a point it was unusable. “In a small engine fuel tank in a constantly high-temperature, high-humidity environment, it takes three months or longer for E10 and other ethanol blends to take up enough water for phase separation,” the study found.  “This confirms the statement by Mercury Marine that water uptake in E10 blends ‘…does not happen at a level or rate that is relevant.’”

The research also found that an advantage of ethanol blends is that they do in fact hold more water in suspension without phase separation than the hydrocarbon components of gasoline. The scientists found that “…more ethanol improves the fuel’s resistance to phase separation. This is an advantage that can help keep fuel systems ‘dry’ by moving low levels of water out of the system.”

RFA President and CEO Bob Dinneen offered the following comments on the new study:

“Simply put, critics who continue to suggest E10 is a problem for small engines and boat motors are all wet. This research from NREL clearly demonstrates once and for all that ethanol actually helps these engines run more efficiently. It also shows that gasoline goes bad long before the ethanol in the tank could cause any problems due to moisture uptake. This research effectively disproves the half-baked anecdotes and horror stories about E10 and small engines that have been pushed for decades by ill-informed biofuel opponents and snake-oil additive salesmen.

“Every manufacturer of small and off-road engines has approved the use of E10 in their equipment for many years. If owners of this equipment simply follow the manufacturers’ recommendations for fuel, maintenance, and winterization, they won’t have any issues at all. But, as this study shows, letting gasoline sit in your tank for extended periods of time is likely to cause some issues—irrespective of whether the gasoline contains ethanol or not.”

A summary of the NREL study is available here and the full study can be found here.