Fuel Quality Testing

Fuels Tested


Gasoline is a complex mixture of relatively volatile hydrocarbons that vary widely in their physical and chemical properties. The fuel quality program inspects and tests all grades of automotive gasoline, gasoline-ethanol blends, E-85 ethanol, aviation gasoline and racing fuels. The fuel properties must be balanced to give satisfactory engine performance over an extremely wide range of operating conditions. The prevailing standards for fuel represent compromises among numerous quality, environmental, and performance requirements. Antiknock (octane) rating, distillation characteristics, vapor pressure, sulfur content, oxidation stability, corrosion protection, and other properties are balanced to provide satisfactory vehicle performance. Additives are often used to provide or enhance specific performance features.

Distillate Fuels

Distillate fuels encompass a broad class of motor fuels, including: No. 1-D and No. 2-D diesel fuel; No. 1 and No. 2 fuel oil; No. 1-K and No. 2-K kerosene; Jet A, Jet A-1 and Jet B aviation fuel.

For further information contact the Fuel Quality Program.

Laboratory Tests Performed…


Alcohol in gasoline is normally added to enhance its octane rating. There are many types of alcohols, but currently only ethanol is added to gasoline.

API Gravity

Producers of petroleum products developed this test so the specific gravity of liquids could be expressed in whole numbers. The API gravity is used along with the distillation temperatures for calculating the cetane index of diesel fuel.

Cetane Index

The cetane index is derived from a formula for estimating the cetane number of diesel fuel.

Cetane Number

The cetane rating is a measure of an important combustion characteristic of a fuel in a compression ignition (diesel) engine. A long ignition delay (low cetane) in a diesel engine will result in rapid pressure rise that can cause undesirable audible knock, high stresses and severe engine vibration. Also, difficult starting in cold weather, misfiring and excessive white smoke often result from too low cetane. The official cetane rating is determined in a special engine by ASTM D 613, Standard Test Method for Cetane Number of Diesel Fuel Oil.

Cloud Point

Cloud point and pour point are measures of winter temperature behavior properties of distillate fuels. Cloud point is the temperature where paraffin first forms in fuel. The pour point indicates the lowest temperature at which the fuel can be pumped.

In practice, cloud point helps to determine the temperature at which paraffin crystals will begin to block fuel filters and lines and cause starting and stalling problems for diesel engines.

For ground based fuels, such as No. 1-D diesel and No. 2-D diesel, the cloud point temperature is defined as a temperature at the tenth percentile minimum monthly temperature for the area. The 10 percentile minimum temperature means that there is only a 10 percent expectation that the minimum daily temperature will be lower than the tenth percentile minimum temperature. The tenth percentile minimum monthly temperatures for Missouri are:

Cloud Point
October 34 °F January 3 °F
November 19 °F February 9 °F
December 7 °F March 18 °F

There are no pour point specifications for diesel fuel; however, pour point temperatures of 8 °F to 15 °F below the cloud points of fuels are most common. In practice, occasionally there are fuels that meet these minimum specifications and cause performance problems in very cold weather. For this reason many marketers "winterize" their No. 2-D diesel by blending kerosene or No.1-D diesel, which typically has a much lower cloud point than No. 2. Some marketers may also add anti-gel additives, which will reduce the pour point and cold temperature filter plugging. The additives normally do not reduce the cloud point temperature. The Low Temperature Flow Test method is designed for predicting the cold behavior properties for the fuels with chemical additives.

Copper Corrosion

The copper corrosion standard ensures that the fuel will not create excessive corrosion in the vehicle fuel system.


The "distillation" standard is one of several tests used to address gasoline’s vaporization characteristics. Gasoline is metered in liquid form, through the fuel injectors (or carburetor), and mixed with air and atomized before entering the cylinders. Therefore, it is very important that a fuel’s tendency to evaporate is controlled to certain standards. A fuel’s ability to vaporize or change from liquid to vapor is referred to as its volatility. In gasoline, the distillation characteristics, along with vapor pressure, define and control starting, warm-up, acceleration, vapor lock, crankcase oil dilution, and, in part, fuel economy and carburetion icing.

The tendency of a fuel to vaporize is also characterized by determining a series of temperatures at which various percentages of the fuel have evaporated (boiling temperatures), as described in ASTM D86, Test Method for Distillation of Petroleum Products. The temperatures at which 10 percent, 50 percent, and 90 percent evaporation occurs are often used to characterize the volatility of gasoline.

The 10 percent evaporated temperature is directly affected by the seasonal blending of the gasoline. This temperature must be low enough to provide easy cold starting, but high enough to minimize the vapor lock and hot weather driveability problems. Most cool weather driveability problems occur from the use of summer season gasoline in the winter months. This is especially true in premium grades, which normally have a high 10 percent evaporated temperature.

The 50 percent evaporated temperature must be low enough to provide good warm-up and cool weather driveability without being so low as to contribute to hot driveability and vapor locking problems. This portion of the gasoline greatly affects fuel economy on short trips.

The 90 percent and end-point evaporation temperatures must be low enough to minimize crankcase and combustion chamber deposits, as well as spark plug fouling and the dilution of engine oil.

If the end-point temperature exceeds the ASTM maximum requirement, it is usually because of the presence of a distillate fuel such as No. 2 diesel. This contamination can be directly attributable to the delivery of diesel prior to the delivery of the gasoline. This problem is avoided if care is taken in handling the product.

Flash Point

Flash point is defined as the temperature to which a fuel must be heated to produce an ignitable vapor-air mixture above the liquid fuel when exposed to an open flame.

Flash point is important primarily from a fuel-handling standpoint. Too low a flash point will cause fuel to be a fire hazard, subject to flashing, and possible continued ignition and explosion. In addition, a low-flash point may indicate contamination by more volatile and explosive fuels, such as gasoline.

A very important reason to maintain the flash point as high as possible is due to the electrostatic hazards in pumping distillate fuels. Never dispense kerosene or diesel fuel into a tank or container that previously contained gasoline or other flammable material.

High Frequency Reciprocating Rig (HFRR)

This test method covers the evaluation of the lubricity of diesel fuels using a high-frequency reciprocating rig (HFRR). Diesel fuel injection equipment has some reliance on lubricating properties of the diesel fuel. Shortened life of engine components, such as diesel fuel injection pumps and injectors, has sometimes been ascribed to lack of lubricity in a diesel fuel.


The Federal Clean Air Act Amendments prohibit the sale of leaded gasoline after December 31, 1995 except for certain aviation, marine, nonroad, and racing applications.


Gasolines are most commonly rated based on their Antiknock Index (AKI), a measure of octane quality. The octane is a measure of gasoline’s ability to resist knock or auto-ignition. The fuel-air mixture in the cylinder of a spark ignition engine will, under certain conditions, auto-ignite. This pre-firing may cause an audible "ping" or knock and may also cause an engine to continue running when turned off. Loss of power and damage to an engine can occur when knocking is severe and prolonged.

The AKI of a motor fuel is the average of the Research Octane Number (RON) and Motor Octane Number (MON) or (R+M)/2. This is also the number displayed on the black and yellow octane decal posted on the gasoline pump. Optimum performance and fuel economy is achieved when the AKI is adequate for the engine in which it is combusted. There is no advantage in using gasoline with a higher AKI than the engine requires to operate knock-free.

The division strictly enforces the Federal Trade Commission’s Fuel Rating Rule (the Automotive Fuel Ratings, Certification and Posting Rule).

Oxidation Stability

Stability standards control a fuel’s tendency to contribute to induction system deposits and filter clogging and also determine the fuel’s storage life.

Saybolt Color

Saybolt color is a physical measure of the clarity of a fuel and is used as an indication of the overall purity of kerosene. It is a useful parameter to assure the freedom from trace contamination with heavier products, which may render the product unsuitable for designated critical applications. Such applications include in-home heating use, where conformance to minimum specifications is essential to ensure the proper burning of the fuel and an adequate margin of safety to the consumer of the product.

Saybolt color is not only useful in determining heavier contaminants but is also helpful in determining the degradation of product over longer periods of storage. Since kerosene has a tendency to degrade faster than most other commercially available petroleum products such as gasoline and diesel fuel, a higher kerosene inventory turnover is desirable.

Some of the most frequent problems noticed where saybolt color is concerned are improper storage, careless handling, and inappropriate dispenser hoses. Frequently, kerosene is stored over the summer months and offered for sale again the following fall. This generally results in discolored sub-quality product. Ideally, inventories should be reduced as much as possible in early spring months then replenished with fresh kerosene in the fall before sale of the product resumes.

Because of the highly refined nature of kerosene and its use in unvented heaters it is imperative that no gasoline or diesel fuel contaminate it through improper handling. This can be avoided by taking precautions to ensure that lines or tanks through which other products have been pumped or stored are thoroughly drained or flushed appropriately. In addition, dispenser hoses manufactured specifically for kerosene is available and should be used at all times.

Silver Corrosion

The silver corrosion test insures that fuel will not create excessive corrosion of silver in the vehicle fuel system.


Sulfur is an element that is normally found in varying amounts in all petroleum products. As a result of combustion, sulfur compounds of an acidic or corrosive nature are produced and contribute to combustion chamber and valve deposits, exceptional wear on the engine, and increased atmosphere pollution. Through recent EPA regulations, the sulfur limit for on-highway diesel fuels has been reduced from 0.50 percent to 0.05 percent by weight. High sulfur fuels are still available. However, they are not for highway use and must be colored red. Also, the Internal Revenue Service requires certain fuels to be dyed for tax purposes. Therefore, highway diesel should not contain any dyes.

Total Oxygen

Total oxygen is generally new concept to petroleum distributors, retailers, and consumers. However, with the growth of reformulated fuels this could be an even greater issue in the future. Oxygenated compounds are not normally present in gasolines, but are introduced intentionally in the form of alcohols or ethers to improve octane or reduce the polluting effects upon combustion. A fuel’s total oxygen content is defined as the weight percent of chemically bonded oxygen in alcohols and ethers.

In the Midwestern grain-producing states, ethanol-blended fuels are common. Many base fuels available to blenders may also contain oxygenates added to the fuel in the form of an ether at the refinery level. Without adequate care, ethanol may be added to a base fuel already containing an oxygenate. The ethanol addition may tip the balance to the point that the performance/emission results are unfavorable and exceed specified limits for oxygenates.

Fuel ethanol blended to 10 percent volume meets the 3.7 percent total oxygen specification for motor fuels and in some cases the Environmental Protection Agency has allowed up to an additional 2.0 percent volume Methyl tertiary Butyl Ether (MTBE) at 4.1 percent total oxygen. A total oxygen content in excess of this limit will result in rapidly progressive deterioration of performance and may result in some small increase in evaporative emissions for conventional vehicles. Oxygenates which are not blended to their proper proportion in a fuel may leave that fuel substandard on its octane rating. Missouri marketers of ethanol-blended fuels are educated in blending with the proper base gasoline. Therefore, the maximum total oxygen content is rarely exceeded.

Vapor Pressure

Vapor Pressure is a physical measure of gasoline volatility. High vapor pressures and a low distillation temperature for 10 percent evaporated both help cold starting. Under hot-operating conditions, high vapor pressure also contributes to vapor lock and increases vapor formation in fuel tanks and carburetors. The amount of vapors formed in fuel tanks and carburetors, which must be contained by evaporative loss control system, are related to the vapor pressure and distillation temperatures. In order to reduce overloading the vehicle’s control systems, the Environmental Protection Agency has placed maximum limits on gasoline’s vapor pressure during the ozone season (May 1 - September 15). Also, the Missouri Department of Natural Resources has placed even lower vapor pressure requirements for ozone non-attainment areas of St. Louis and Kansas City during the period of June 1 - September 15. Hydrocarbon emissions are precursors to the formation of ozone (smog).

To assure that fuels have the proper volatility characteristics, refineries adjust gasoline seasonally. During the cold, winter months, fuel will be more volatile. This helps good "cold" start and warm-up performance. During the hot summer months, the fuel tends to be less volatile. This will help to minimize the incidence of vapor lock and hot driveability problems.

Since there are six seasonal classes of gasoline, only a sufficient quantity of fuel that can be consumed in a short period of time should be sold to a consumer/user.


Viscosity is an important characteristic of diesel. Fuels outside the required range may cause power loss or improper atomization of the fuel in a diesel engine.

Water and Sediment

Water and sediment in fuel is our program’s most frequent consumer complaint. As most are aware, water makes a very undesirable fuel and sediment has a tendency to clog filters, carburetors and injectors. ASTM requires gasoline to be visually free of undissolved water, sediment, and suspended matter: it shall be clear and bright at ambient temperatures or 70 °F, whichever is higher.

Usually, the largest source of contamination is at the service station. Following is a list of commonly occurring problems that contributes to water in fuel:

Water Tolerance

The term water tolerance is used to indicate the ability of a gasoline alcohol blend to dissolve water without phase separation. Blending gasoline with ethanol will dissolve only a very small amount of water under normal conditions before experiencing phase separation. When blends are exposed to a greater amount of water than they can dissolve or severe temperature reductions, they separate into an alcohol-rich aqueous phase and an alcohol-poor hydrocarbon phase. Since the aqueous phase can be highly corrosive to many metals and the engine cannot operate on it, such separation is very undesirable. The problem of phase separation can usually be avoided if the fuels are sufficiently water-free initially and care is taken during distribution to prevent contact with water.

If separation should occur the alcohol deficient gasoline will be left at a reduced octane level. This separation of water/alcohol and fuel may occur in storage or in the tank of the consumer’s vehicle, with associated problems.

For further information contact the Fuel Quality Program.