June 28, 2009
Gasoline is not a single substance. It is a complex mixture of components which vary widely in their physical and chemical properties. There is no such thing as pure gasoline. Gasoline must cover a wide range of operating conditions, such as variations in fuel systems, engine temperatures, fuel pumps and fuel pressure. It must also cover a variety of climates, altitudes, and driving patterns. The properties of gasoline must be balanced to give satisfactory engine performance over an extremely wide range of circumstances. In some respects, the prevailing quality standards represent compromises, so that all the numerous performance requirements may be satisfied.
Auto manufacturers have, for many years, used materials that are compatible with oxygenated fuels. However, with the widespread use of oxygenated fuels and reformulated gasoline, certain myths have resurfaced, so they warrant mention here. In earlier versions of this manual this topic was covered in greater detail, including photographs from various tests and applicable service bulletins.
The information presented was segmented into two categories, metals and elastomers.
Most metal components in automobile fuel systems will corrode or rust in the presence of water, air or acidic compounds. The gasoline distribution system usually contains water, and additional moisture may collect in the automobile tank from condensation. Gasoline may also contain traces of sulfur and organic acids. Gasoline has always been recognized as potentially corrosive. Pipelines which distribute gasoline routinely require that corrosion inhibitors be contained in gasoline to protect their plain steel pipe. Therefore, corrosion inhibitors have been routinely added to gasoline for many years.
Alcohols are more soluble in water than MTBE. The addition of ethanol will increase a gasoline’s ability to hold water. Therefore, an ethanol enhanced gasoline may have a slightly higher moisture content than non-blended gasoline. Several tests have been reported on ethanol enhanced gasolines. Vehicle fuel tanks and fuel system components from autos operated for extended periods on these blends were removed, cut open, and examined. These tests have generally concluded that ethanol does not increase corrosion in normal, everyday operation.
Auto manufacturers have indicated they do not have major concerns about metal corrosion, provided that all fuels contain effective corrosion inhibitors at the proper treatment levels. Responsible ethanol producers recognize that not all commercial gasolines are adequately treated for blending, and have, for some time, included a corrosion inhibitor in their ethanol.
Elastomer compatibility is more difficult to generalize. A number of gasoline ingredients can have an effect on elastomer swelling and deterioration. For instance, aromatics, such as benzene, toluene, and xylene, have been shown to have detrimental effects on some fuel system elastomers. Gasolines sold today have a higher level of aromatics than those sold in the 1970s.
The addition of alcohols or ethers to gasoline can also cause swelling in fuel system elastomers. Swelling can be severe with methanol, but relatively insignificant with other alcohols. Ten volume percent ethanol contributes less swelling than the amount of additional aromatics needed to obtain the same increase in octane number. The combination of ethanol or MTBE with high aromatic levels may cause greater swelling than either product by itself.
Automobile and parts manufacturers have been responsive to the changes occurring in today’s gasoline. Materials problems are less likely to occur with newer vehicles because of the upgrading of fuel system materials that has occurred since the introduction of higher aromatic unleaded gasolines and the addition of alcohols and ethers. All major automobile manufacturers have indicated that their late model vehicles are equipped with fuel system components upgraded for use with these fuels.
While all auto manufacturers warrant the use of 10 percent ethanol blends or gasoline containing MTBE. Fuel systems in 1975 to 1980 model years were
upgraded, but not to the same extent as later models. Pre- 1975 models may have fuel system components that are sensitive to high aromatic gasolines, alcohols and ethers. Specific documentation of the effect fuel components have on older fuel system parts is often lacking.
Technicians who find themselves replacing parts on pre-1980 vehicles should specify that replacement parts be resistant to such fuel components. These products include Viton® (EGR valves, fuel inlet needle tips) and fluoro elastomers (fuel lines, evaporative control lines, etc.)
It is interesting to note that many of the aromatic components of gasoline such as benzene, toluene, and xylene do not fare very well on chemical compatibility charts with common elastomers used in modern day engines. However, many of the same elastomer components show a good to excellent rating in the presence of ethanol.
I have provided a link to a chemical compatibility chart so you too can see how ethanol and other components of aromatic gasolines fares in the presence of elastomers.
I have also provided a link to Changes in Gasoline III, The Auto Technicians Gasoline Quality Guide. Changes in Gasoline III is the latest in the ongoing series of Changes in Gasoline manuals. The first manual, entitled Changes in Gasoline & the Automobile Service Technician, was originally published in 1987. Over a four year period it was periodically updated to focus on fuel related areas of greatest interest to automobile service technicians. The first version of the manual achieved a circulation of 345,000 copies.
This is by far the most comprehensive guide that I have even seen concerning common components of gasoline and it effects on use in new and older model vehicles.
There is also a great deal of information on use in small engines including outboard motors. This manual is a must read for the automobile and small engine techicians.
Chemical compatibility chart
Changes in Gasoline IV