Gasoline is the fuel designed for spark-ignition internal combustion engines. There is a “basic recipe” for gasoline, since it must be used in a variety of vehicles with a broad range of engine types and operating conditions. ASTM International, formerly the American Society for Testing and Materials, has developed these specifications.
Conventional gasoline is a mixture of compounds, called hydrocarbons, refined from petroleum crude, plus small amounts of a few additives to improve its stability, control deposit formation in engines, and modify other characteristics. Conventional gasoline also may contain oxygenates, such as methyl tertiary-butyl ether (MTBE) and ethanol to meet octane needs. Gasoline is a sophisticated product with a demanding set of performance expectations that:
- Allow an engine to start easily when cold, warm up rapidly, and run smoothly under all conditions
- Deliver adequate power without engine knocking
- Works in vehicles to provide good fuel economy
- Generate low emissions and enables advanced vehicle emission control systems
- Do not form harmful engine deposits, cause excessive wear, or contaminate or corrode the fuel system
Additives
Each major oil company typically adds its own proprietary additives to the basic recipe for gasoline in order to provide or enhance specific performance features. And, most important, each company provides the technical expertise to back its brand. They also ensure that the additive package is contained in every gallon of gasoline sold under their brand name. Many customers buy branded gasoline because of this consistent quality. Deposit control additives, which are found in all gasoline by law, keep engines clean and make them run more efficiently. Deposits in carburetors or in fuel injectors, for example, can affect the engine’s overall air-fuel ratio as well as an individual cylinder air-fuel ratio, which in turn can affect fuel economy, emissions and driveability.
Octane number is a measurement of fuel’s resistance to engine knock. Engine knock is an abnormal combustion associated with using gasoline with too low of an octane number. Ordinarily, your vehicle will not benefit from using a higher octane than is recommended in the owner’s manual.
Engine Knock
But if your engine knocks or pings at the recommend octane level, you may need a higher octane gasoline to prevent the knock. Knocking may occur under certain conditions. A small percentage of vehicles may knock because of variations in engines of the same model due to manufacturing tolerances, or because of an unusual build-up of engine deposits during the first 15,000 miles of driving. Other factors, such as extremely hot weather, changes in altitude or hard driving conditions (like towing a heavy load) may also cause knocking.
Knock Sensors
Many modern vehicles are equipped with an electronic device that detects and eliminates light knocking before you can hear it. The devices suppress knock by retarding the spark. Those vehicles may experience some deterioration of acceleration performance, without knocking, when operating with lower octane gasolines. In this case, a higher octane number could improve acceleration.
Octane Levels
The most common levels of octane grade are 87 (regular), 89 (mid-grade) and 91-93 (premium). The octane number of the gasolines offered for sale are set by gasoline refiners and marketers based on their perception of the technical and competitive needs in the market. Federal and most state regulations do not regulate minimum octane values, only that the postings on the dispensers accurately reflect the octane number of the gasolines being sold.
Gasoline with a higher heating value (energy content) provides better fuel economy. Traditionally, premium gasoline has had a slightly higher heating value than regular, and, thus, provides slightly better fuel economy, but it is difficult to detect in normal driving. There can be even larger differences in heating value between batches of gasoline from the same refinery, between summer and winter volatility classes, or between brands of gasoline from different refineries because of compositional differences. The differences are small and there is no practical way for the consumer to identify gasoline with a higher-than-average heating value.
Technology Is Always Improving Fuels
The petroleum industry is always on the brink of technological change. Gasoline refined and marketed today is far different from what it was just a few years ago. The properties of gasoline being purchased today are much different than they were in the past. Gasoline has undergone continuing changes to burn cleaner in response to state and federal environmental regulations since the 1970s. Many new blends of gasoline have been developed to comply with regional and state air quality regulations. As a result, there are about 17 different kinds of gasoline that are sold across the country.
The Clean Air Act Amendments of 1990 required two significant types of clean-burning gasoline to reduce carbon monoxide, air toxins and ground-level ozone (smog): federal reformulated gasoline (RFG) and wintertime oxygenated fuel.
RFG is a gasoline blended to reduce vehicle emissions of compounds identified as toxic and compounds that tend to form atmospheric ozone. RFG was introduced for urban areas with special ozone air quality problems. Current federal regulations require RFG to contain oxygen. The most commonly used oxygenates are ethanol and MTBE. However, because of the occurrence of MTBE in groundwater, 17 states have limited the use of MTBE in gasoline.
Wintertime oxygenated gasoline is gasoline to which oxygenates have been added to increase its oxygen content, as required by regulation, to reduce emissions in certain non-attainment areas that do not meet the federal air quality standard for carbon monoxide. Because of improvements in vehicle emissions controls, carbon monoxide air quality has greatly improved, and winter oxygenated fuel sales are now only approximately 3.5 percent of the total gasoline sales in the United States
Conventional gasolines also can contain oxygenates. They are added to help meet octane number specifications and/or to extend the product volume.
Beginning in January of 2004, refiners and importers began to supply lower sulfur gasoline for use in new cleaner Tier 2 vehicles. Gasoline sulfur levels will be phased down by about 90 percent of the levels in 2003, and earlier by the time the three-year phase-in of this program is complete. With the introduction of low-sulfur gasoline, the gap between the emissions benefits of conventional gasoline and cleaner burning gasoline, like RFG, has shrunk considerably.
ASTM
It has been critical to the successful development of gasoline and gasoline-powered vehicles to have consensus among refiners, vehicle manufacturers, and other interested parties on the characteristics of gasoline necessary for satisfactory performance and reliable operation. This consensus is reached in the United States under the auspices of the ASTM International.
The ASTM specification for gasoline, D 4814 – Standard Specification for Automotive Spark-Ignition Engine Fuel, is widely recognized. ASTM International Committee D-2 on Petroleum Products, and Lubricants is responsible for gasoline specifications and test methods. Specifications changes are required to be made in a balanced process, incorporating the positions of the “Producers,” such as the members of API, “Users,” such as the automobile manufacturers and “General Interest” parties, such as consumers or government representatives. Their positions and viewpoints are brought to the D-2 forum by representatives who also are members of ASTM, including:
- Members of the American Petroleum Institute
- Individual refiners
- Petroleum distribution, pipeline, and marketing companies
- Vehicle and engine manufacturers
- Automotive equipment suppliers
- State and federal government officials
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