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Why does fuel quality deteriorate over time in small engines, such as those found in lawnmowers and weedeaters, when the equipment sits idle for a while?

In order to answer this question adequately, a general understanding of gasoline is required. First, gasoline is not a pure substance, but a mixture of more than 500 hydrocarbons with boiling ranges from ambient temperatures to about 400 degrees F. The general composition of traditional gasoline is about 50 to 70 percent saturates (single-bonded carbon and hydrogen), 10 to 40 percent aromatics (for example, benzene), and 2 to 10 percent olefins (double-bonded hydrocarbons like cyclopentene). Oxygenated fuels add oxygen-containing compounds, such as ethanol and methyl tert-butyl ether, better known as MTBE, in quantities of about 12 to 15 percent.

Saturated hydrocarbons are the major component of gasoline, and lucky for us, they are relatively stable. They can improve octane ratings, depending on branching and the number of carbon atoms, and promote efficient combustion. Unsaturated hydrocarbons (olefins and aromatics) are unstable and tend to burn in air with a smoky flame, but they can improve octane ratings. They tend to be reactive and toxic. Oxygenates provide antiknock value and may also reduce the smog-forming tendencies of exhaust gases.

Because liquid gasoline itself does not burn, but the vapor is what burns, an important item to consider is volatility or the tendency of the gasoline to form vapors. In order to start a cold engine, enough of the low boiling components, or "light ends," must vaporize at cold engine temperatures in order to form a combustible vapor-air mixture. The vapor pressure of fuel must be high enough to facilitate engine starting, yet not so high as to contribute to vapor lock or excess emissions and running losses. The light ends needed for easy starting have the same tendency to vaporize in storage or in the air space within the gas tank of a small engine. Eventually light ends can be evaporated out of the liquid gasoline from the daily temperatures that the storage tank or fuel tank is subjected to in storage. This concentrates the heavier components in the gasoline, thus severely reducing the fuel's performance characteristics.

Water from high humidity in the air also can contribute to fuel degradation. If the water vapor condenses during the cooling cycle, this will contaminate the gasoline with liquid water. The presence of oxygenates in the fuel can cause additional problems because the water will tend to draw out certain oxygenates from the gasoline into a water/oxygenate layer, thereby changing the composition of the fuel.

The big problem that causes the gum or shellac is the reactions of olefins and diolefins, which, under ambient temperatures, slowly combine with oxygen in air, or oxidize. These reactions become faster as the temperature of the gasoline increases. Some of these gums are soluble in gasoline, but will remain as a sticky residue when the gasoline evaporates. Severe oxidation of gasoline may also produce an insoluble gum. This can also be observed in the gasoline containers we keep to "top off" our small engine fuel tank. This fuel may smell sour, which is also an indicator of olefin decomposition. Although things may look bleak, there are solutions. First take a look at the service manual. Most recommend that gasoline be less than 30 days old. In addition, storing the fuel is important. The container must be kept filled to about 95 percent capacity. Also, the container needs to be capped tightly. Finally, containers of gasoline and the fuel tank of the small engine need to be kept out of direct sunlight and in a location where the temperature stays below 80 degrees F. By the way, this is not just a small engine concern - automotive dealers have noted a problem with gum formation in cars left on the lot in the summer with less than three gallons in their tanks.

Additional steps can be taken that will help prevent gum formation in small gasoline engines. First, run the engine until there is no fuel in the tank or fuel delivery system, and store the equipment empty. This is known as dry storage. The only problem is that water can collect in the system due to condensation. Alternatively, a fuel stabilizer or anti-oxidant may be added to the tank and the engine run long enough for the stabilized fuel to make its way through the entire fuel system. However, the best idea is to start with a fresh tank of fuel when you are ready to use the small engine again.

This month's Whizard is Melissa Williams, a research scientist in the Petroleum Products Research Department (Automotive Products and Emissions Research Division, 08).

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April 15, 2014