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Impact of Ethanol Fuel Blends on Automatic Tank Gauging

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Posted / Last update: 14-06-2011
Publication: PetrolPlaza Technology Corner
Issued: May 2011
Author: Frey Ulrich, Hectronic GmbH
  • Figure 1: Ethanol part reducing in E5 fuel phase

By law, 10% ethanol (E10) in gasoline was introduced into Germany since January 2011. This caused a lot of concern as many motorists refuse E10 fuel as they feel unsure if there is an impact on the lifetime of their vehicles. Oil companies and station operators are also not happy as the introduction causes additional costs with no direct return on invest (accommodation of tanks, tanks with low turnover, information needed at price indication, dispensers and nozzles etc). The use of E10 fuel was already established in a number of countries with the USA about to introduce E15 and E25 common in Brazil for a number of years.

How does the addition of ethanol affect equipment such as the tank level measurement system? Ethanol may attack some materials of tank gauges, and systems using only stainless steel and Teflon have a distinct advantage as these materials are compatible with ethanol.

Another problem is the hygroscopic effect of ethanol. Water may ingress to tanks already within the logistic chain or later into storage tanks (of course it should not) and may cause problems in different ways.

Part of the water is absorbed by the fuel phase. Pure gasoline has a water absorptive capacity of up to 120 ppm, E5 fuel may already hold up to 2.500 ppm water (depending on temperature) and E10 even more. Some additives increase the absorption of water. In 100% gasoline nearly all water separates in a phase at the bottom of a tank because of its higher density.

If water is present in a tank with E10 then a smaller part is absorbed in the gasoline phase and the larger part will separate at the bottom as free water phase, until there is a balance of dissolved water in the fuel and free water/ethanol phase at the bottom. As we know from alcoholic drinks, water mixes very well with ethanol. On its way down to the tank bottom water will wash out a considerable part of the ethanol and separate at the bottom as a mixture of water and ethanol. This can easily be demonstrated by taking 1 litre of E10 in a measuring cylinder and pouring 100 millilitres of water into it. Within some seconds there is a clear separated water/ethanol phase at the bottom of more than 130 millilitres. At least 30 millilitres of ethanol is immediately missing from the fuel phase.

The reason is that the hydrogen bond (polar) between water and ethanol is stronger than the non-polar bond between gasoline and ethanol. Scientific report DGMK 645 states for E5 with a 1% separated water phase that 30% - 40% of the ethanol will pass from the fuel phase to the water phase. For a water phase of 5% it is possible that 65 - 78% of the ethanol may be washed out. So water can draw a lot of ethanol out of the fuel phase on its way down to the tank bottom, reducing the Octane number and thus the quality of the fuel considerably.

As an example take a tank with a content of 10.000 litres E10 and ingress of 100 litres of water. Depending on the tank shape, the water level together with the washed out ethanol may get dangerously close to the suction point of the dispenser which is typically 150 mm from tank bottom. While it is still being debated how engine parts react on ethanol or dissolved water in the fuel there is a new problem how to measure the water/ethanol phase in a storage tank. If the water/ethanol phase is not detected in time then the suction pipe from the dispenser may pump from the water/ethanol phase instead of fuel. This will neither amuse the motorist or station operator.

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