Volatility
Gasoline is composed of a variety of ingredients that evaporate at different temperatures. A fuel’s ability to vaporize, or change from a liquid to a vapor, is referred to as VOLATILITY. More volatile components (faster vaporizing) evaporate at lower temperatures. Less volatile components (slower vaporizing) evaporate at higher temperatures.
Gasoline which is not volatile enough results in poor cold start and poor warm-up driveability. These fuels can also contribute to crankcase and combustion chamber deposits as well as spark plug deposits. Gasoline which is too volatile vaporizes too easily and may boil in the fuel line at high operating temperatures. This could cause a decrease in fuel flow to the engine, resulting in symptoms such as VAPOR LOCK, loss of power, rough engine operation, or complete stoppage. Fuel economy could also deteriorate and evaporative emissions could increase.
Reid Vapor Pressure (RVP) is a measure of how easily fuel will vaporize. To test for RVP, a sealed container of fuel is submerged in 100°F water. As the fuel vaporizes, it creates a measurable pressure in the sealed container. The measurement of that pressure is referred to as RVP. RVP ratings of regular unleaded gasoline typically range from eight to as high as fifteen. Lower RVP indicates low volatility, while higher RVP indicates a fuel blended for higher volatility.
Seasonal Blends
The volatility of gasoline is adjusted to compensate for the seasonal changes in temperature. Winter fuels tend to be more volatile (higher RVP) to provide better cold start and warm-up performance. Summer fuels are less volatile (lower RVP) to reduce hot driveability conditions such as vapor lock. Fuel volatility is also adjusted to compensate for high altitude areas. Fuels will boil at lower temperatures in higher altitudes, therefore, they must be less volatile. Adjusting for seasonal and geographic conditions helps to reduce problems, however, they do not eliminate them. Temperatures are unpredictable, particularly in the spring and fall, and may fall well below or above the fuel’s volatility limits.
Oxygenated Fuels
Government and industry are working on a variety of alternative fuels to partially or even completely replace the use of gasoline fuels. Fuels under consideration include Ethanol, compressed gases, and reformulated gasoline.
Oxygenated fuels contain oxygen bearing compounds (ethers or alcohols). Presently, the only oxygenates seeing any significant level of commercial use are Ethanol, and Methyl Tertiary Butyl Ether (MTBE). Since these compounds add oxygen to the air/fuel mixture, they artificially lean the air/fuel mixture, resulting in more complete combustion and lower CO.
While most ingredients in gasoline are made of hydrogen and carbon (hydrocarbons), oxygenates are made of hydrogen, carbon, and oxygen (thus the name oxygenate). Three oxygenates in use are Ethanol, Methyl Tertiary Butyl Ether (MTBE), and Ethyl Tertiary Butyl Ether (ETBE) (Figure 1-13).
In the past, alcohols and ethers were used primarily as octane enhancers. Now, the focus is on emissions reduction. The use of these oxygenates is controlled by the EPA. Their addition to gasoline is regulated to ensure that the finished fuel will not contribute to fuel system failures that could result in increased emissions. Blends exceeding 10% Ethanol, 5% Methanol, or 15% MTBE are not permitted by law.
Types of Oxygenates
ETHANOL is available and widely used in some parts of the country. Like other oxygenates, the use of Ethanol as a gasoline component will improve combustion and reduce CO emission. Although Ethanol and Methanol are both alcohols, they have distinctly different characteristics. Ethanol provides better water tolerance, better fuel system compatibility, and contains less oxygen than Methanol. The oxygen content of a blend containing 10% Ethanol is about 3.5% by weight.
METHANOL is used in combination with heavier cosolvent alcohols as an octane enhancer for addition to gasoline. Since Methanol is 50% oxygen, gasoline containing 5% Methanol would have an oxygen content of 2.5%. This may cause significant changes in the air/fuel ratio.
Methanol blends are more sensitive to water than other alcohol blends and should contain cosolvent alcohols to improve water tolerance. Methanol is considered to be more corrosive than other alcohols. Methanol is bad for your vehicle. It can corrode metal parts in your fuel system and damage plastics and rubber.
MTBE (Methyl Tertiary Butyl Ether) is manufactured by the chemical reaction of Methanol and Isobutylene. The conversion of Methanol to MTBE eliminates the unfavorable characteristics associated with Methanol. Some advantages are that it is not as sensitive to water as are the other alcohols, and does not increase the volatility of most gasoline.
MTBE is used by almost every major oil company in at least some of their gasoline. By law, MTBE can be up to 15%, by volume, however, they are commonly used at levels of 6% to 8% except where higher levels are mandated by law. At maximum permitted levels, MTBE will raise octane by up to 3.0 octane numbers. A 15% by volume MTBE blend contains 2.7% oxygen, by weight, and effectively reduces CO emissions.
end. Copyright 2004 General Motors. Used with my mom's permission.
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