Gasoline accounts for about 5% of the total energy consumed in this country. It's sold in about 167,000 gas stations throughout the US. Without gasoline, we could not ride our motorcycles. So, we're interested in this stuff. We buy gasoline and put it in our bike. The bike makes power, and we get to ride around and have fun. Simple, right? Well, not exactly.
In the US we consume about 400 million gallons of gasoline a day, corresponding to about 20 million barrels of oil a day. Roughly half of all the oil we use is used as gasoline. On average, a barrel of oil gets refined into about 20 gallons of gasoline, 10 gallons of diesel fuel, 4 gallons of jet fuel. 2 gallons of fuel oil, 2 gallons of liquefied petroleum gas, and 7 gallons of other products such as plastics and motor oils. About half the oil used in the US comes from the western hemisphere, including the US, Canada, and Venezuela. 20% comes from the Persian Gulf, 15% from Africa, and 15% from other places including the North Sea.
World-wide, oil consumption is about 28 billion barrels of oil per year. This number is expected to grow to about 44 billion barrels per year by 2025. Known conventional oil reserves are about 1,000 billion barrels, and it's estimated that there are still 300 to 1,000 billion barrels of oil yet to be discovered. This is enough oil to last well past 2060, perhaps into 2090.
It is estimated that there are another 4,000 to 5,000 billion barrels of oil still in the ground which we currently don't know how to extract economically. Current oil well technology only extracts about 1/3 to 1/2 the underground oil. In addition there are other known oil reserves such as tar oils, oil sands, and bitumen. These reserves are more expensive to tap, however it is estimated there are another 15,000 billion barrels of oil in this form. All told, it's conservative to think that there is enough oil in the world for at least 100 years, and reasonable to project better extraction technology that would keep the world well supplied for 500 or more years. Additionally, at oil prices over $60 / barrel it becomes economic to convert coal into oil, which gets us at least another 100 years of oil supply.
It is popular for various people to claim we will run out of oil soon. In 1977, Jimmy Carter's administration projected a severe energy crisis that would cripple our economy by 1990; in the late 1980s the Club of Rome projected that the world would run completely out of oil between 1997 and 2003. In fact, while there may be environmental issues related to pollution and global warming, it is extremely unlikely that anyone alive in 2005 will see an end to oil supplies in their lifetime. Prices may raise, and politics may dramatically change supply and demand, but there will be oil.
A gallon of gasoline is roughly equivalent to 1.5 kilowatt-hours. A typical American home uses about 900kwh per month, which is about like burning 600 gallons of gasoline to power your home. In the US each year we generate about 2,000 billion kwh by burning coal; about 750 billion kwh of electricity from nuclear power; about 700 billion kwh from burning natural gas and petroleum; and 275 billion kwh from hydro-electric generation in dams. The 400 million gallons of gasoline we burn in our cars each day corresponds to about 220 billion kwh per year, a small fraction of the energy used in this country.
To burn gasoline, you mix it with air, compress it, and ignite it with a spark. Roughly speaking, it takes about 3.5 pounds of oxygen for each pound of gasoline you want to burn. Air is about 23% oxygen, so it takes about 15 pounds of air for each pound of gasoline. A gallon of gasoline weighs about 6.25 pounds, and requires 95 pounds of air to burn. 95 pounds of air is about 11,500 gallons of air. So we see why internal combustion engines are often called air pumps. Five gallons of gas will use up all the oxygen in almost 60,000 gallons of air. This is all the oxygen in a room 20' x 20' x 20', more or less a typical two stall garage.
When you're burning the gasoline, you mix it with air and compress it. The horsepower your engine makes is directly proportional to the compression ratio - the more you can compress the mixture, the more horsepower you get. Unfortunately, at higher compression ratios the fuel / air mixture has the rather annoying habit of burning on its own rather than waiting patiently for the spark plug. When the mixture self-ignites there will be more than one flame front in the cylinder. When these flame fronts collide, the pressure in the cylinder increases very dramatically resulting in a knocking sound and the likelihood of engine damage. If your engine is knocking, it's very important you do something to fix the problem.
To fix engine knocking, you can lower the compression ratio, which will lower the engine's power output. Or, you can use different gasoline which is formulated to burn slower and be harder to ignite. We have a number to tell us how slowly the gasoline ignites - it's called the octane rating. It would more properly be called the anti-knock rating. Higher octane ratings mean slower burning. Slower burning will prevent the knocking, but will also cost us horsepower. In the US it's well known that bigger numbers = more money = better. But with gasoline, lower octane ratings mean less money and often more horsepower. You want to run the lowest octane rating you can run without detonation or knocking. The story of gasoline is the story of finding ways to increase the octane rating, so that car manufacturers can make engines with higher compression ratios and more power.
Gasoline is made up of a lot of different chemicals, with many different boiling points, vapor pressures, and ignition temperatures. High vapor pressures are good for evaporating the gasoline droplets quickly and mixing it thoroughly with the air, but high vapor pressures also generally mean lower combustion temperatures which cause knocking, and also get government pollution dweebs all excited about air pollution caused by gasoline evaporating out of your gas tank.
Octane rating means comparing the gasoline to a mixture of heptane and iso-octane. If the anti-knock properties of the gasoline are exactly equal to the heptane, the octane rating is 0. If the anti-knock properties are exactly equal to pure iso-octane, the octane number is 100. If the anti-knock properties of the gasoline match a mixture of 13% heptane and 87% iso-octane, then the octane number is 87. This rating system was invented in 1927 by Graham Edgar. The limitations of this test have resulted in several other proposed octane rating systems. We won't be concerned with these details, because even if we completely understood all the issues, we're still limited to buying whatever the gas companies want to sell us. Today there are two tests, the "research octane number" which comes from basic chemistry, and the "motor octane number" which comes from running the fuel in a specific test motor. The octane rating of the fuel you buy is the average, hence the pump label (RON + MON) / 2.
To raise the octane rating of the gasoline you can refine it more carefully and use more of the chemicals in oil that burn slowly. For example, in many countries they are increasing the arene content of the gasoline to 50% or more to raise octane ratings. In the US it is believed that arenes don't burn as completely and cause smog, so the arene content of US gasoline is below 40% and being lowered to below 20%.
Alternatively, you can add chemicals to the gasoline to slow the burning. For decades the most popular octane boosting additive was tetra-ethyl lead, so we said we had leaded gas in the US. We were burning gasoline that contained lead, resulting in over 1,000 tons of lead going into our atmosphere every day. What did 1,000 tons of lead per day do to our food supplies? What about young children, who can suffer brain damage from eating too much lead? No one cared about this in the slightest until it was found that the lead was destroying the catalytic converters that reduced smog. This was serious, and lead was quickly phased out of our gasoline.
After lead was phased out, the refineries were forced to add more and more arenes to the gasoline to prevent knocking. However, the arenes don't burn as completely as the normal alkane molecules, so there was then pressure to find a different way to raise octane and prevent knock. The next chemical family to be tried was oxygenates.
Oxygenates are basically hydrocarbon molecules that have been partially pre-burned. These pre-burned molecules slow combustion, preventing detonation and raising the octane rating. The most popular oxygenate molecules are MTBE and ethanol. After a couple years it was "discovered" that MTBE was cleaning up our air but polluting our water, so it was phased out. At roughly the same time it was discovered that mid-west farmers were growing corn faster than humans and cows could eat it, lowering the price of corn and lowering the profits of the large farming combines. A few political donations later it was "discovered" that ethanol made from corn could also raise octane ratings, prevent engine knocking, and increase the donations and votes for certain political candidates. Today in the US most gasoline sold is about 5% ethanol, and by federal edict that percentage will be increased in the coming years. The legal requirement to post on the gas pump that the product is "gasohol" or part alcohol has been removed, so we mostly have no idea what we're buying.
Modern fuel-injected engines have engine management systems that can adjust to burn almost anything. However, if the octane rating is too low, the engine management system will have to richen the mixture and retard the timing to prevent destructive knock, resulting in lowered gas mileage. As you increase the octane rating of the fuel, you will hit a point where the engine management system can run the engine at optimum levels. At this point you have best gas mileage and best power. If you continue to increase the octane rating of the fuel, the engine management system does nothing, so all you are doing now is burning more expensive fuel for the same result. So, buy the lowest octane number that you can and still get your best gas mileage.
In the winter, or when it's very humid, you can often lower the required octane number. In very dry heat, for example in Texas, New Mexico, Arizona, S.California, you will most likely need to raise the octane level a bit to get best gas mileage.
It has been suggested that adding a small amount of acetone to your gas tank can add 10% - 35% to your fuel mileage. This works because acetone is a surfactant for gasoline - acetone reduces the surface tension of gasoline rather dramatically, allowing the droplets to evaporate more quickly and burn more completely. This means better fuel mileage and less pollution. In fact many mechanics for years have used acetone to improve an old engine's pollution performance and get the engine to pass a smog test.
Acetone is a rather strong solvent, so there is some concern that it can damage the fuel lines over time, This is usually not a problem in cars with OEM fuel lines, as the OEM fuel lines are made to handle solvents like this. Gasoline in the US is about 25% aromatics, meaning Benzene, Toluene, and Xylene. These are pretty strong solvents too.
Acetone binds strongly to water, so if you get water in your fuel tank this can be a big problem. Although acetone dissolves nicely in gasoline, acetone and water do not. The little acetone-water droplets can clog your fuel injectors and cause problems.
These days, most gasoline in the US contains 5% to 10% ethanol. Alcohol also tends to bind to acetone, and although this does not cause problems, it does reduce the ability of acetone to lower the surface tension. So it's not at all clear that adding acetone to a gasohol mix will improve your gas mileage by much.
Generally, you would add about 2-3 oz (60-90 ml) of acetone to 10 gallons (40 liters) of gasoline. You can buy acetone at any hardware store for about $6 / quart or $13 / gallon, which works out to about 10 cents per ounce, so for $0.30 worth of acetone in a tank you can improve your mileage on your $30 worth of gas dramatically.
Gasoline prices vary considerably over the US. The cheapest gas is in the deep south, because that's where a lot of oil wells and oil refineries are. As you move further from the deep south into the mid west, rocky mountains, and New England, prices raise due to transportation costs. California and the west coast are a special case. CARB, the California Air Resources Board, has special requirements for California gasoline. There are only a few refineries that can make these special blends, so the price is typically considerably higher than the rest of the country. When (not if) one of these refineries has problems, the price goes even higher. Roughly, gasoline prices are made up of crude oil prices (50%), refining (20%), state and federal taxes (20%), and distribution and marketing (10%).
At the time of this writing (8/05) gas prices are quite high, but we continue to read that they are not that high in historical terms. What does this mean? When you adjust historic prices of gasoline in dollars per gallon for the inflation rate, you find that gasoline was quite inexpensive in the 1990s, but was more expensive before and after that decade.
|Connecticut||32.5||50.9||Delaware||23.0||41.4||Dist of Col||20.0||38.4|
|New Jersey||14.5||32.9||New Mexico||18.0||36.4||New York||39.6||58.0|
|North Carolina||24.9||43.3||North Dakota||21.0||39.4||Ohio||26.0||44.4|
|Rhode Island||31.0||49.4||South Carolina||16.8||35.2||South Dakota||24.0||42.4|
Under President Bush, ethanol has become an important and growing part of the gas consumed in the US. The requirement for ethanol enhanced gasoline to be labeled "gasohol" has been lifted. At the time of this writing (9/05) the US uses about 10 million barrels of gasoline a day, which is enhanced with about 250,000 barrels of ethanol a day. However, recently enacted laws require a growing fraction of US gasoline to include ethanol. In 2004 about 11% of the US corn crop went to ethanol production. In 2005 that number is expected to be more like 14.5%, and to continue to grow to at least 18% in the next couple of years.
Ethanol is economically competitive with gasoline when oil prices are above $40 per barrel, a price level which is easily met by the current $65 per barrel. This price condition can be expected to continue to be met for the indefinite future unless there is a world wide economic recession which dramatically cuts oil demand. In my opinion, it is very difficult to forecast oil selling for under $40 / barrel in the current world economic conditions.
Refineries produce raw gasoline, which is then loaded into tanker trucks. The additive packages promoted by the various brands of gas are added into the truck. So, you may always buy Chevron gas, for example, but this only means you're getting the Chevron additive package - the gas may have been produced at any companies' refinery. The various gas companies cooperate with each other in getting gas to the various stations. So, that Chevron truck may have filled up at any refinery you can imagine, then poured the Chevron additive package into the truck tank just before leaving the refinery.
It used to be that the "Premium" grades of gasoline had better additive packages than "Regular." However, for many years now the EPA has required all gasoline sold in the US to have detergents to keep your engine clean. There is little reason to buy premium gas just to get a better additive package.
We're all familiar with smog, that brown-tan-orange stuff in the air that looks so very unappealing, except right at sunset when it's quite romantic. Smog is caused by chemical reactions in the air. These chemical reactions involve sunlight and automotive exhaust.
An unfortunate side effect of combustion is that some of the nitrogen in the air is burned into nitric oxide and nitrogen dioxide, which are collectively called NOx. NOx + sunlight + unburned hydrocarbons = colorful smog chemicals. Interestingly, these colorful chemicals have not been proven to be particularly harmful to people. On the other hand, diesel trucks put out "particulates," little tiny balls of carbon, much like you would get if you pulverized the graphite in a pencil. These particulates are directly linked to asthma and lung cancer. So, since we can see the smog but it's the particulates which are actually most harmful, what do the politicians do? We car drivers all have catalytic converters to help re-tune the motors to eliminate NOx, meanwhile there are no pollution standards for diesel particulates. You can see the particulates in the worst cases as clouds of black smoke coming out of the diesel exhaust. The worst offenders? Public transportation busses and school busses, owned by the governments.
Gasoline is a mix of a lot of different chemicals, which all have quite different burning properties. These chemicals are all variants on a basic theme: a string of carbon atoms with hydrogen atoms stuck on. Carbon atoms have four available chemical bonds. In a string of carbon atoms, one of these bonds connects to the carbon on the left, and another to the carbon on the right, leaving two bonds unused.
Our favorite chemicals in gasoline are the alkanes. These have names which tell us how many carbon atoms are in the string. The chemicals are Hexane with 6 carbons, Heptane with 7 carbon atoms, Octane with 8 carbons, Nonane with 9 carbons, and Decane with 10 carbons. Normally, the atoms are in a straight line as:
normal heptane C-C-C-C-C-C-C C7H16
In heptane, there are seven carbon atoms. Each carbon atom in the middle of the string has two available chemical bonds. These are attached to hydrogen atoms. The two carbon atoms at the ends of the string have three available bonds, which are each attached to hydrogen atoms. All told there are two hydrogen atoms for each carbon atom plus two additional hydrogen atoms to cap the ends. So heptane, with seven carbon atoms, has 16 hydrogen atoms = 7 * 2 + 2.
If every available carbon bond has a hydrogen atom, we call the molecule "saturated." If there are any double bonds between the carbons, then the molecule is unsaturated. This is exactly the same meaning as for saturated fats in food.
Life isn't always that simple, however. The carbon atoms can be put together in something other than a string. For example, there is octane which is a simple string, and there are variants called "iso-octane." In iso-octane there are eight carbon atoms, but they are not in a straight line. Cyclo-octane has the carbons in a ring, and is therefore missing two hydrogens because the two extra carbon bonds on the ends are used to make the ring.
C C C-C-C C=C C=C C | | / \ / \ / \ | C-C-C-C-C-C-C-C C-C-C-C-C C C C C C C-C C-C=C-C | \ / \\ // \\ // Octane C C-C-C C-C C-C 2-methyl- iso-octane Cyclo-octane Benzene Toluene 2-butene
In addition, there are alkenes which have carbon double bonds (aka olefins) and arenes (aka aromatics) which are carbon rings with double bonds. Alkenes and arenes have very high octane ratings and are very common in some gasoline outside the US. A common alkene is 2-methyl-2-butene. Two common arenes are Benzene and Toluene. In some countries arenes make up as much as 50% of the gasoline you buy; however in the US this is being reduced to below 20%. Alkenes are also the base chemicals for some synthetic oils.
Oxygenates are more or less gasoline molecules that have an oxygen atom substituted for one of the carbon atoms. There are two very popular oxygenates, MTBE and ethanol.
C C-C-O-H | Ethanol C-C-O-C | C Methyl Tertiary Butyl Ether
These molecules increase octane ratings, allowing the refiners to reduce arene content. It is sometimes claimed that the oxygenates also reduce smog-forming chemicals in the exhaust, but this is not proven. In order to have a positive effect, the hydrocarbon fractions in the gasoline must be very carefully controlled. If the hydrocarbon fractions are off by just a bit, the oxygenates can actually make the smog producing components in the exhaust worse. However, there are a lot of corn farmers in the US, so now there are laws requiring most of the gasoline sold in the US to have about 6% alcohol content.
There are a huge number of regulations in the US about underground gas tanks - the fear is that gasoline may leak out of the tanks and pollute our ground water. In fact, I believe this fear is completely silly. Gasoline has been sitting around in various places in Alaska, Texas, Saudi Arabia, and under the North Sea for hundreds of millions of years and it hasn't gone anywhere. If gasoline liked to move through the ground, it would have dispersed long ago. On the other hand, MTBE just loves to move through the ground and get into ground water, where it takes hundreds of years to deteriorate. There are now many cities whose historic ground water wells are unusable due to MTBE pollution. The idea that we would put our ground water at risk for a questionable improvement in air quality, and make this change virtually overnight to the tune of millions of gallons of chemicals per day is something only a government could think up.
These chemicals come about because of fears of gasoline evaporating. So, with the People's Republic of California leading the way, in the US gasoline formulation regulations are changing to reduce the volatility of gasoline. Reducing the volatility decreases the octane rating, so oxygenates are used to get the octane rating back to where it needs to be. In other words, we've done near permanent damage to substantial amounts of ground water due to the unproven fear that gasoline spilled by consumers refueling and evaporating out of gas tanks is forming a lot of smog.
Oxygen molecules do not have energy content for a gasoline engine, so oxygenated gasoline has less energy per gallon. Modern fuel-injected motors compensate for this by increasing the flow of gasoline, thereby reducing your gas mileage. So, over the fear of evaporative emissions we're paying more at the pump, burning more oil, getting worse gas mileage and producing less horsepower. The good news: huge industrial farming combines in Nebraska and Iowa are making lots of money on corn.
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