Volatility describes how easily the gasoline evaporates (forms a vapor). The definition of volatility assumes that the vapors will remain in the fuel tank or fuel line and will cause a certain pressure based on the temperature of the fuel.
Reid vapor pressure (RVP) is the pressure of the vapor above the fuel when the fuel is at 100 ｡ F (38 ｡ C ). Increased vapor pressure permits the engine to start in cold weather. Gasoline without air will not burn. Gasoline must be vaporized (mixed with air) to burn in an engine.
If winter-blend gasoline (or high-RVP fuel) is used in an engine during warm weather, the following problems may occur:
Hesitation on acceleration
The RVP can be tested using the test kit
Figure 15-1 This gasoline testing kit includes an insulated container where water at 100 degrees F is used to heat a container holding a small sample of gasoline. The reading on the pressure gauge is the Reid vapor pressure (RVP).
The octane rating of gasoline is the measure of its antiknock properties. Engine knock (also called detonation, spark knock, or ping) is a metallic noise an engine makes, usually during acceleration, resulting from abnormal or uncontrolled combustion inside the cylinder.
During periods of spark knock (detonation), the combustion speed increases by up to 10 times to near the speed of sound. The increased combustion speed also causes increased temperatures and pressures, which can damage pistons, gaskets, and cylinder heads.
Figure 15-2 Normal combustion is a smooth, controlled burning of the air-fuel mixture.
Figure 15-3 Detonation is a secondary ignition of the air-fuel mixture. It is also called spark knock or pinging.
The antiknock standard or basis of comparison was the knock-resistant hydrocarbon isooctane, chemically called trimethylpentane (C8H18), also known as 2 2 4 trimethylpentane. If a gasoline tested had the exact same antiknock characteristics as isooctane, it was rated as 100-octane gasoline.
Each uses a model of the special cooperative fuel research (CFR) single-cylinder engine. The research method and the motor method vary as to temperature of air, spark advance, and other parameters. The research method typically results in readings that are six to ten points higher than those of the motor method.
The octane rating posted on pumps in the United States is the average of the two methods and is referred to as (R + M)/2, meaning that, for the fuel used in the previous example, the rating posted on the pumps would be:
Except in high-altitude areas, the grades and octane ratings are as follows:
(Continued) 91 or higher Premium 89 Midgrade (also called Plus) 87 Regular Octane Rating Grades
Figure 15-4 A typical fuel pump showing regular (87 octane), midgrade (89 octane), and premium (92 octane). These ratings can vary with brand as well as in different parts of the country, especially in high altitude areas where the ratings are lower.
When gasoline companies, under federal EPA regulations, removed tetraethyl lead from gasoline, other methods were developed to help maintain the antiknock properties of gasoline. Octane improvers (enhancers) can be grouped into three broad categories:
NOTE: MMT has been proven to be harmful to catalytic converters. However, MMT is currently one of the active ingredients commonly found in octane improvers available to the public and in some gasoline sold in Canada.
Oxygenated fuels contain oxygen in the molecule of the fuel itself. Examples of oxygenated fuels include methanol, ethanol, methyl tertiary butyl ether (MTBE), tertiary-amyl methyl ether (TAME), and ethyl tertiary butyl ether (ETBE).
MTBE does not increase the volatility of the fuel, and is not as sensitive to water as are other alcohols. The maximum allowable volume level, according to the EPA, is 15% but is currently being phased out due to concern with MTBE contamination of drinking water if spilled from storage tanks.
Ethyl alcohol is drinkable alcohol made from grain. Adding 10% ethanol (ethyl alcohol or grain alcohol) increases the (R + M)/2 octane rating by three points. The alcohol added to the base gasoline, however, also raises the volatility of the fuel about 0.5 psi. Most automobile manufacturers permit up to 10% ethanol if driveability problems are not experienced.
Methyl alcohol is made from wood (wood alcohol), natural gas, or coal. Methanol is poisonous if ingested and tends to be more harmful to the materials in the fuel system and tends to separate when combined with gasoline unless used with a cosolvent.
The use of alcohol can result in the clogging of fuel filters with dirt and other debris cleaned from the fuel tank, pump, and lines.
Alcohol raises the volatility of fuel about 0.5 psi; this can cause hot-weather driveability problems.
Alcohol reduces the heat content of the resulting fuel mixture (it has about one-half of the energy content of gasoline): 60,000 to 75,000 British thermal units (BTUs) per gallon for alcohol versus about 130,000 BTUs per gallon for gasoline.
Alcohol absorbs water and then separates from the gasoline, especially as temperature drops. Separated alcohol and water on the bottom of the tank can cause hard starting during cold weather. Alcohol does not vaporize easily at low temperatures.
Figure 15-5 This fuel tank indicates that the gasoline is blended with 10% ethanol (ethyl alcohol).
Figure 15-6 This container holds pure gasoline and water. Notice how the water separates and sinks to the bottom.
Add 2 mL of water to the graduated cylinder by counting the number of drops from an eyedropper. (Before performing the test, the eyedropper must be calibrated to determine how many drops equal 2.0 mL.)
Put the stopper in the cylinder and shake vigorously for 1 minute. Relieve built-up pressure by occasionally removing the stopper. Alcohol dissolves in water and will drop to the bottom of the cylinder.
If the combustion process is complete, all gasoline or HCs will be completely combined with all the available oxygen. This total combination of all components of the fuel is called stoichiometric air-fuel ratio.
As the altitude increases, atmospheric pressure drops. The air is less dense because a pound of air takes more volume. The octane rating of fuel does not need to be as high because the engine cannot intake as much air. This process will reduce the combustion (compression) pressures inside the engine.
Vehicle engines produced after 1971 for sale in the United States were required to operate on unleaded fuel. Most engine manufacturers started induction hardening of valve seats to help prevent valve recession.
Reformulated gasoline (RFG) is manufactured by refiners to help reduce emissions. The gasoline refiners reformulate gasoline by using additives that contain at least 2% oxygen by weight and reducing the additive benzenes to a maximum of 1% by value.
Two other major changes done at the refineries are as follows:
Reduce light compounds. Refineries eliminate butane, pentane, and propane, which have a low boiling point and evaporate easily. These unburned hydrocarbons are released into the atmosphere during refueling and through the fuel tank vent system, contributing to smog formation.
Reduce heavy compounds. Refineries eliminate heavy compounds with high boiling points such as aromatics and olefins. The purpose of this reduction is to reduce the amount of unburned hydrocarbons that enter the catalytic converter. Increased use of oxygen-containing MTBE helps the combustion process to reduce emissions.
Keep the fuel tank above one-quarter full, especially during seasons in which the temperature rises and falls by more than 20 ｡ F between daytime highs and nighttime lows. This helps to reduce condensed moisture in the fuel tank and could prevent gas line freeze-up in cold weather.
Do not purchase fuel with a higher octane rating than is necessary.
Do not purchase fuel from a retail outlet when a tanker truck is filling the underground tanks. During the refilling procedure, dirt, rust, and water may be stirred up in the underground tanks. This undesirable material may be pumped into your vehicle's fuel tank.
Do not overfill the gas tank. After the nozzle clicks off, add just enough fuel to round up to the next dime. Adding additional gasoline will cause the excess to be drawn into the charcoal canister. This can lead to engine flooding and excessive exhaust emissions.
Aircraft are always filled with fuel from a tanker truck after a ground strap is attached to both the plane and the truck and then attached to ground.
Figure 15-8 Many gasoline service stations have signs posted warning customers to place plastic fuel containers on the ground while filling. If placed in a trunk or pickup truck bed equipped with a plastic liner, static electricity could build up during fueling and discharge from the container to the metal nozzle creating a spark and possible explosion. Some service stations have warning signs stating not to use cell phones while fueling to help avoid the possibility of an accidental spark creating a fire hazard.