Gasoline Additives

Gasoline Additives

At one time, all a gasoline-producing company to do to produce their product was pump the crude from the ground, run it through the refinery to separate the fractions, dump in a couple of grams of lead per gallon, and deliver the finished product to the service station. Of course, automobiles were much simpler then and what they burned was not very critical. As long as gasoline vaporized easily and did not cause the lower compression engines to knock, everything was fine.

Times have changed. Today, refiners are under constant pressure to ensure that their produce passes a series of rigorous tests for seasonal volatility, minimum octane, existent gum and oxidation stability, as well as to add the correct deicers and detergents to make the product competitive in a price-sensitive marketplace.

Gasoline additives - primarily used in unleaded gasolines - have different properties and a variety of uses.

Anti-Icing or Deicer

Isopropyl alcohol is added seasonally to gasoline as an anti-icing agent to prevent gas line freeze-up in cold climates.

Metal Deactivators and Rust Inhibitors

These additives are used to inhibit reactions between the fuel and the metals in the fuel system that can form abrasive and filter-plugging substances

Gum or Oxidation Inhibitors

Some gasolines contain aromatic amines and phenols to prevent formation of gum and varnish. During storage, harmful gum deposits can form due to the reaction of some gasoline components with each other and with oxygen. Oxidation inhibitors are added to promote gasoline stability. They help control gum, deposit formation, and staleness. Stale gasoline becomes cloudy and smells like paint thinner.

Gum content is influenced by the age of the gasoline and its exposure to oxygen and certain metals such as copper. If gasoline is allowed to evaporate, the residue left can form gum and varnish.

Gasoline must have as little gum residue as possible to prevent gum formation in the intake manifold, carburetor or fuel injectors, and on intake valve stems.


The use of detergent additives in gasoline has been the subject of some public confusion. Detergent additives are designed to do only what their name implies - clean certain critical components inside the engine. They do not affect octane.


By far the most widely used gasoline additive today is ethanol, or grain alcohol. Ethanol's value as an octane enhancer becomes more apparent when considered in the context of the government-mandated phasedown of tetraethyl lead. Blending 10 percent ethanol into gasoline is seen as a comparatively inexpensive octane booster that results in an increase of 2.5 to 3 road octane points.

In addition to octane enhancement, ethanol blending keeps the carburetor or fuel injectors clean due to detergent additive packages found in most of the ethanol marketed. It also inhibits fuel system and injection corrosion due to additive packages and decreases carbon monoxide emissions at the tailpipe due to the higher oxygen content of blended fuel.


Methanol is the lightest and simplest of the alcohols and is also known as wood alcohol. It can be distilled from coal, but most of what is used today is derived from natural gas.

In the early 1980s, a few companies begin selling a blend of 5 percent methanol in gasoline in the Northeast and Midwest. This showed that methanol plug anticorrosion co-solvents could compete in the motor fuel market as an octane enhancer. Today, both General Motors and Ford have working prototypes on the road that can burn any combination of fuel from nearly neat methanol (85 percent methanol with 15 percent gasoline) to straight gasoline. They key is a fuel line sensor that detects methanol concentration and adjusts spark timing and air/fuel mixture accordingly. The federal government is pushing for methanol acceptance through industry incentives and fleet-sponsored programs.

For now, however, many auto maker continues to warn motorists about using a fuel that contains more than 10 percent methanol and co-solvents by volume. Methanol is recognized as being far more corrosive to fuel system components than ethanol, and it is this corrosion that has auto makers concerned.


Methyl tertiary butyl ether (MTBE) has become very popular in the past few years as an octane enhancer and supply extender because of excellent compatibility with gasoline. Current U.S. EPA restrictions on oxygenates limit MTBE in unleaded gasoline to 11 percent of volume. At that level, it increases pump octane (RM/2) by 2.5 points. However, it is usually found in concentrations of 7 to 8 percent of volume.

MTBE has found favor among refiner/suppliers and independent marketers alike because it is not nearly as sensitive to moisture as the other oxygenates, has virtually no effect on fuel volatility, and can be shipped through product pipelines whereas ethanol and methanol blends cannot.

MTBE increases octane while reducing carbon monoxide emissions at the tailpipe and does it at a cost that makes it very attractive to gasoline marketers across the country.

Alcohol-in-Fuel Test

Some gasoline may contain a small quantity of alcohol. The percentage of alcohol mixed with the fuel usually does not exceed 10 percent. An excessive quantity of alcohol in gasoline may result in fuel system corrosion, fuel filter plugging, deterioration of rubber fuel system components, and a lean air/fuel ratio. These fuel system problems may result in driver complaints of a lack of power, acceleration stumbles, engine stalling, and hard or no starts. Some vehicle manufacturers supply test equipment to check the level of alcohol in gasoline. The following alcohol-in-fuel test procedure requires only the use of a calibrated cylinder.

1. Obtain a 100-milliliter (100-ml) cylinder graduated in 1-ml divisions.

2. Fill the cylinder to the 90-ml mark with gasoline.

3. Add 10 ml of water to the cylinder so the cylinder is now filled to the 100-ml mark.

4. Install a stopper in the cylinder and shake it vigorously for 10 to 15 seconds.

5. Carefully loosen the stopper to relieve any pressure.

6. Install the stopper and shake the cylinder vigorously for another 10 to 15 seconds.

7. Carefully loosen the stopper to relieve any pressure.

8. Place the cylinder on a level surface for about 5 minutes to allow the liquids to separate.

9. Any alcohol in the fuel is absorbed by the water and settles to the bottom. If the water content in the bottom of the cylinder exceeds 10 ml, there is alcohol in the fuel. For example, if the water content is not 15 ml, there was 5 percent alcohol in the gasoline.

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