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Engine & fuel engineering FAQ
Engine & fuel engineering
Gasoline (Petrol) composition and properties
Posted: 21 Jun 09
Gasoline or petrol is a fuel, derived from petroleum crude oil, for use in spark-ignited internal combustion engines. Conventional gasoline is mostly a blended mixture of more than 200 different hydrocarbon liquids ranging from those containing 4 carbon atoms to those containing 11 or 12 carbon atoms. It has an initial boiling point at atmospheric pressure of about 35 °C (95 °F) and a final boiling point of about 200 °C (395 °F).[ Gasoline is used primarily as fuel for the internal combustion engines in automotive vehicles as well in some small airplanes.
In Canada and the United States, the word "gasoline" is commonly used and it is often shortened to simply "gas" although it is a liquid rather than a gas. In fact, gasoline-dispensing facilities are referred to as "gas stations".
Most current or former Commonwealth countries use the term "petrol" and their dispensing facilities are referred to as "petrol stations". The term "petrogasoline" is also used sometimes. In some European countries and elsewhere, the term "benzin" (or a variant of that word) is used to denote gasoline.
In aviation, "mogas" (an abbreviation for "motor gasoline") is used to distinguish automotive vehicle fuel from aviation fuel known as "avgas".
GASOLINE PRODUCTION FROM PETROLEUM CRUDE OIL
Gasoline and other end-products are produced from petroleum crude oil in petroleum refineries. For a number of reasons it is very difficult to quantify the amount of gasoline produced by refining a given amount of crude oil:
* There are quite literally hundreds of different crude oil sources worldwide and each crude oil has its own unique mixture of thousands of hydrocarbons and other materials.
* There are also hundreds of crude oil refineries worldwide and each of them is designed to process a specific crude oil or a specific set of crude oils. Furthermore, each refinery has its own unique configuration of petroleum refining processes that produces its own unique set of gasoline blend components. Some crude oils have a higher proportion of hydrocarbons with very high boiling points than other crude oils and therefore require more complex refinery configurations to produce lower boiling point hydrocarbons that are usable in gasolines.
* There are a great many different gasoline specifications that have been mandated by various local, state or national governmental agencies.
* In many geographical areas, the amount of gasoline produced during the summer season (i.e., the season of the greatest demand for automotive gasoline) varies significantly from the amount produced during the winter season.
However, as an average of all the refineries operating in the United States in 2007, refining a barrel of crude oil (i.e., 42 gallons or 159 litres) yielded 19.2 gallons (72.7 litres) of end-product gasoline as shown in the adjacent image. That is a volumetric yield of 45.7 percent. The average refinery yield of gasoline in other countries may be different.
From the viewpoint of performance when used in automotive spark-ignited internal combustion engines, the most important characteristic of a gasoline is its octane rating (discussed later in this article). Paraffinic hydrocarbons (alkanes) wherein all of the carbon atoms are in a straight chain have the lowest octane ratings. Hydrocarbons with more complicated configurations such as aromatics, olefins and branched paraffins have much higher octane ratings. To that end, many of the refining processes used in petroleum refineries are designed to produce hydrocarbons with those more complicated configurations.
Some of the most important refinery process streams that are blended together to obtain the end-product gasolines are:
* Reformate (produced in a catalytic reformer): has a high content of aromatic hydrocarbons and a very low content of olefinic hydrocarbons (alkenes).
* Catalytically cracked gasoline (produced in a fluid catalytic cracker): has a high content of olefinic hydrocarbons and a moderate amount of aromatic hydrocarbons.
* Hydrocrackate (produced in a hydrocracker): has a moderate content of aromatic hydrocarbons.
* Alkylate (produced in an alkylation unit): has a high content of highly branched paraffinic hydrocarbons such as isooctane.
* Isomerate (produced in a catalytic isomerization unit): has a high content of the branched isomers of pentane and hexane.
PROPERTIES THAT DETERMINE THE PERFORMANCE OF GASOLINE
The image below depicts what occurs in one of the combustion cylinders of a gasoline-fuelled, spark-ignited internal combustion engine operating in a 4-stroke cycle. Each cylinder in the engine has a movable piston that can slide upward and downward within the cylinder. Although not shown in the image, the bottom of the piston is connected to a rotating central crankshaft by a so-called connecting rod.
The cycle starts with the piston at the top of the cylinder (i.e., where the piston is farthest away from the crankshaft axis) and the inlet and exhaust valves are closed. Then:
* During the intake stroke, the piston is pulled downward by the rotating crankshaft and the inlet valve opens to admit a mixture of fuel and air.
* During the compression stroke, the inlet valve closes and the piston is pushed upward by rotating crankshaft which compresses the fuel-air mixture.
* During the power stroke, the compressed fuel-air mixture is ignited by a spark from the spark plug. The resulting increase in temperature and pressure of the burning fuel forces the piston down which, in turn, forces the crankshaft to rotate.
* During the exhaust stroke, the outlet valve opens and the rotating crankshaft pushes the piston upward which forces the combustion product gases to be exhausted from the cylinder. That ends the 4-stroke cycle and the cycle then starts again.
In a typical multiple-cylinder engine, the timing of the each cylinder's cycle is such that the crankshaft is kept in continuous rotation.
If the gasoline spontaneously ignites and detonates (i.e., explodes) before it is ignited by the spark plug, it causes an abnormal phenomenon known as knocking, pinging or spark knock. The knocking is quite audible and prolonged knocking will damage an engine.
As briefly mentioned above, the most important performance characteristic of a gasoline is its octane rating, which is a measure of how resistant the gasoline is to knocking. In fact, the octane rating is sometimes referred to as the Anti-knock Index. The octane rating is based upon an arbitrary scale indexed relative to a liquid mixture of iso-octane , which is 2,2,4-trimethylpentane, and n-heptane.
Iso-octane (see above image), with a branched structure and a high resistance to knocking, has arbitrarily been assigned an octane rating of 100. N-heptane (see adjacent image), with a straight-chain structure and poor resistance to knocking has arbitrarily been assigned an octane rating of 0.
The octane rating of a specific gasoline is measured by using it in a single-cylinder test engine with a variable compression ratio and adjusting the ratio to produce a standard knock intensity as recorded by an instrument known as a knockmeter. By comparison to tabulated results from similar testing of various mixtures of iso-octane and n-heptane at the same compression ratio, the octane rating of the gasoline is determined. For example, if the gasoline test results match those of a mixture containing 90 volume % iso-octane and 10 volume % n-heptane, then the octane rating of the gasoline is taken to be 90.
The octane rating is measured at two different operational conditions. The rating measured at the more severe operating conditions is called the Motor Octane Number (MON) and the rating measured at the less severe conditions is called the Research Octane Number (RON). The Motor Octane Number is more representative of the performance of a gasoline when used in an automotive vehicle operated under load. For many gasoline formulations, the MON is about 8 to 10 points lower than the RON.
In the United States and Canada, the octane rating shown on the pumps in gasoline dispensing stations is the average of the gasoline's RON and the MON. That average is sometimes referred to as the Pump Octane Number (PON), the Anti-Knock Index (AKI), the Road Octane Number (RdON) and very often simply as (RON + MON)/2) or (R + M)/2. In Europe and Australia and other countries, the octane rating shown on the pumps is most often the RON.
As a broad generality, the higher is the compression ratio of a spark-ignited internal combustion engine, the higher is the performance level of the engine and the higher is the octane rating required for the gasoline fuel. The design of an engine determines its compression ratio and, therefore, the required gasoline octane. Using a gasoline with an octane rating higher than an engine requires will not improve the engine's performance, it will simply cost more.
The vapor pressure of a gasoline is a measure of its propensity to evaporate (i.e., its volatility) and high vapor pressures result in high evaporative emissions of smog-forming hydrocarbons which are undesirable from the environmental viewpoint. However, from the viewpoint of gasoline performance:
* The gasoline must be volatile enough that engines can start easily at the lowest expected temperature in the geographical area of the gasoline's expected market. For that reason, in most areas, gasoline marketed during the winter season has a higher vapor pressure than gasoline marketed in the summer season.
* Too high a volatility could cause excessive vapor leading to vapor locking in the fuel pump and fuel piping.
Thus, gasoline producers must provide gasolines that make possible the easy starting of engines and avoid vapor locking problems while at the same time complying with the environmental regulatory limitations on hydrocarbon emissions.
When gasoline is combusted, any sulfur compounds in the gasoline are converted into gaseous sulfur dioxide emissions which are undesirable from the environmental viewpoint. Some of the sulfur dioxide also combines with the water vapor formed when gasoline combusts and the result is the formation of an acidic, corrosive gas that can damage the engine and its exhaust system. Furthermore, sulfur interferes with the efficiency of the on-board catalytic converters.
Thus, sulfur compounds in gasoline are highly undesirable from either the environmental viewpoint or the engine performance viewpoint. Many countries now mandate that the sulfur content of gasoline be limited to 10 ppm by weight.
Gasoline stored in fuel tanks and other containers will, in time, undergo oxidative degradation and form sticky resins referred to as gums. Such gums can precipitate out of the gasoline and cause fouling of the various components of internal combustion engines which reduces the performance of the engines and also makes it harder to start them. Relatively small amounts of various anti-oxidation additives are included in end-product gasoline to improve the gasoline stability during storage by inhibiting the formation of gums.
Other additives are also provided in end-product gasolines, such as corrosion inhibitors to protect gasoline storage tanks, freezing point depressants to prevent icing, and color dyes for safety or governmental regulatory requirements.
Many gasolines today now contain ethanol which is an alcohol. Gasoline is insoluble in water but ethanol and water are mutually soluble. Thus, end-product gasolines containing ethanol will, at certain temperatures and water concentrations, separate into a gasoline phase and an aqueous ethanol phase.
For example, the graph below shows that phase separation will occur in a gasoline, at temperatures of 5 to 16 °C (40 to 60 °F), containing 10 volume percent ethanol and as little as 0.40 to 0.50 volume percent water.
For the same temperature range, the fraction of water that an ethanol-containing gasoline can contain without phase separation increases with the percentage of ethanol. Thus, gasolines containing more than 10 volume percent ethanol will be less likely to experience phase separation
1. Gasoline FAQ - Part2 of 4, Bruce Hamilton, Industrial Research Ltd. (IRL), a Crown Research Institute of New Zealand.
2. Gary, J.H. and Handwerk, G.E. (2001). Petroleum Refining Technology and Economics, 4th Edition. Marcel Dekker, Inc.. ISBN 0-8247-0482-7.
3. The Relation Between Gasoline Quality, Octane Number and the Environment, Rafat Assi, National Project Manager of Jordan's Second National Communications on Climate Change, presented at Jordan National Workshop on Lead Phase-out, United Nations Environment Programme, July 2008, Amman, Jordan.
4. James Speight (2008). Synthetic Fuels Handbook, 1st Edition. McGraw-Hill, pages 92-93. ISBN 0-07-149023-X.
5. Where Does My Gasoline Come from?, U.S. Department of Energy, Energy Information Administration, April 2008.
6. See the schematic flow diagram in the Petroleum refining processes article.
7. The compression ratio is the ratio of the full volume of a combustion cylinder in an internal combustion engine to its volume of air-fuel mixture when fully compressed at the end of the compression stroke. It is a fundamental specification for most automotive internal combustion engines. Typical compression ratios in range from about 7:1 to about 10:1.
8. Frank Kreith and D. Yogi Goswami (Editors) (2004). CRC Handbook of Mechanical Engineering, 2nd Edition. CRC Press. ISBN 0-8493-0866-6.
9. As per the ASTM test method D2700
10. As per the ASTM test method D2699
11. David S.J. Jones and Peter P. Pujado (Editors) (2006). Handbook of Petroleum Processing, 1st Edition. Springer. ISBN 1-4020-2819-9.
12. John McKetta (Editor) (1992). Petroleum Processing Handbook. CRC Press. ISBN 0-8247-8681-5.
13. CRS Report for Congress "Boutique Fuels" and Reformulated Gasoline: Harmonization of Fuel Standards (May 10, 2006) , Brent D. Yacobucci, Congressional Research Service, Library of Congress
14. Petrol and Diesel, Questions and Answers From website of New Zealand Ministry of Economic Development.
15. E10 and E85 and Other Alternate Fuels Bruce Bauman, American Petroleum Institute (API)
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