Seville STS V8-4.6L VIN 9 (1998)
Fuel: Description and Operation
Fuel Volatility and Octane
FUEL QUALITY
Due to increasing awareness towards vehicle emissions (Evaporative and Exhaust) and their impact on the environment, federal regulations are
limiting certain characteristics of fuel. These limitations are causing driveability problems that are extremely difficult to diagnose. In order to make
a diagnosis, a basic understanding of fuel and its effects on the vehicles fuel system must be gained.
VOLATILITY
Volatility is a fuel's ability to change from a liquid state to a vapor state. Since liquid gasoline will not burn, it must vaporize before entering the
combustion chamber. The rate at which gasoline vaporizes determines the amount of evaporative emissions released from the fuel system, and
therefore has made volatility an environmental concern. The federal government has lowered the maximum allowable volatility, but certain
driveability conditions have resulted.
MEASURING FUEL VOLATILITY
Volatility can be determined through three different tests: the Vapor-Liquid Ratio, the Distillation Curve, and the Reid Vapor Pressure Test
(RVP). The Vapor-Liquid Ratio test determines what temperatures must exist to create a vapor-liquid ratio of 20. The distillation curve is a graph
showing the relationship between temperature and the percentage of fuel evaporated. The fuel components that boil at relatively low temperatures
(below about 32°C [90°F]) are known as 'light ends' and those that boil at about 149°C (300°F) are known as 'heavy ends'. The 'light ends' are
important for cold starting and cold weather driveability. 'Heavy ends' provide engine power and are important for hot weather driveability. It is
the proper mixture of these components that provide proper operation across a wide range of temperatures. However, the distillation curve of a
gasoline usually requires laboratory testing. The Reid Vapor Pressure (RVP) test measures the pressure (psi) vaporized fuel exerts within a sealed
container when heated to 38°C (100°F). Volatility increases proportional to RVP While RVP can easily be measured in the field, it may be
misleading because it is possible for two fuels with the same RVP to have different distillation curves, and therefore, different driveability
characteristics.
As stated, improper volatility can create several driveability problems. Low volatility can cause poor cold starts, slow warm ups, and poor overall
cold weather performance. It may also cause deposits in the crankcase, combustion chambers and spark plugs. Volatility that is too high could
cause high evaporative emissions and purge canister overload, vapor lock, and hot weather driveability conditions. Since volatility is dependent on
temperature, different fuels are used during certain seasons of the year, thus creating problems during sudden temperature changes.
SULFUR CONTENT
The sulfur content in fuel is also regulated to a certain standard. Premium grades of fuel generally have a lower sulfur content than the less
expensive blends. A high sulfur content can promote the formation of acidic compounds that could deteriorate engine oil and increase engine wear.
It could also produce excessive exhaust emissions or a 'rotten egg' smell from the exhaust system.
OCTANE
Octane is a measure of a fuel's ability to resist spark knock. Spark knock occurs in the combustion chamber just after the spark plug fires, when the
air/fuel mixture in the cylinder does not completely burn. The remaining mixture spontaneously combusts due to temperature and pressure. This
secondary explosion causes a vibration that is heard as a knock (ping). Fuel with a high octane number has a greater resistance to spark knock.
OCTANE REQUIREMENTS
This vehicle requires 91 octane ([R+M]/2 method) in order to ensure proper performance of the fuel control system. Using fuel with an octane
rating lower than 91 can create spark knock, which would cause the PCM to retard ignition timing to eliminate the knock. In a case such as this,
poor engine performance and reduced fuel economy could result. Also, in severe knock cases, engine damage may occur.
