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Chemical Octane Project 

 

 

 

 


            The goal of this class project was to research current concerns that involve chemistry and that affect people and the planet.  This specific project is on octane, and how it affects fuel.  Octane is the primary component of most automotive fuels, and this fact shows no sign of changing anytime soon.  It is important to make sure that any given engine uses the correct ratio of octane to other compounds; otherwise, it may cease to function, potentially explosively.  This essay explains what octane is, what it does, and how this information can be used to help automobiles, and any gasoline engine, for that matter, run more safely without wasting money.

 

Text Box: n-heptane
(CH<sub>3</sub>(CH<sub>2</sub>)<sub>5</sub>CH<sub>3</sub>):

H
|
H – C – H
|
H – C – H
|
H – C – H
|
H – C – H
|
H – C – H
|
H – C – H
|
H – C – H
|
H

Text Box: iso-octane (3(CH3)CH2C(2(CH3))H:

        H         H
        |         |
    H – C – H H – C - H
        |         |
    H   |     H   |
    |   |     |   |
H – C – C –- C -– C – H
    |   |     |   |
    H   |     H   |
        |         |
    H – C – H H – C - H
        |         |
        H         H

            Octane is a pure substance; it is not a mixture by the definition used in this Science class.  Its chemical formula is C8H18.  It is a member of the *-ane family of hydrocarbons, where * is a numerical prefix representing the number of carbon atoms in one molecule.  In this case, the prefix is oct-, which means eight.  The number of hydrogen atoms in any substance in this family is equal to 2c + 2, where ‘c’ is the number of carbon atoms.  This is true with octane, as 2(8) + 2 = 16 + 2 = 18.  The reason for this relation is simple: molecules with the standard structural formula of a –ane hydrocarbon is structurally like chains.  This structural formula is indicated by the n- prefix, as in n-heptane (shown below).  The chain is formed by carbon atoms, with each carbon atom bonded with two hydrogen atoms (hence the ‘2c’ in the equation).  Each end of the chain is capped with a hydrogen atom (hence the ‘+ 2’ in the equation).  Forms other than the ‘n-‘, or chain, form do exist, however.  Iso-octane is the form of octane used in gasoline.  It has an irregular formula; it has 5 CH3 groups, connected with an array of carbon and hydrogen atoms.  It is difficult to describe iso-octane’s structure, so a structural formula of it, along with one of n-heptane, the other primary chemical in gasoline, is included on either side of this text.

 

            Energy is obtained from octane by burning it.  When octane burns, it produces a good deal of outward pressure.  The formula that shows how much energy is released by octane, the main component of gasoline, when it is burned, is:

 

2C8H18 (l)+ 25O2(g) ΰ 16CO2(g) + 18H2O(g)      D –7,372kJ

 

Some of this energy becomes heat, causing the gasses that are produced to expand, and much of the rest of the energy goes into kinetic energy, forcing matter out from the location of the explosion.   To understand how this energy is harnessed, it is necessary to have a general understanding of how gasoline engines work.  There are two primary kinds of gasoline engines, the two-stroke and the four-stroke.  Explanations for both will be given in the next two paragraphs.

 

            In a standard two-stroke engine, gasoline in a cylinder in the engine is ignited, and the pressure formed by burning forces a piston in the cylinder to move down.  The piston is connected to the engine shaft, and using an ingenious set of joints, it causes the shaft to rotate.  One stroke is where the piston goes either up or down completely, and it takes two strokes to cause the engine shaft to complete one revolution.  The exhaust is released when the piston is halfway down, and the remaining pressure is used to push the exhaust out of the piston and into the exhaust system.  When the piston is all the way down, the exhaust valve closes and another valve opens.  Air is forced in through this valve.  The valve quickly closes, and the engine’s momentum forces the piston up, thus compressing the air.  When the piston is almost all the way back up, fuel is injected into the cylinder and the spark plug ignites the fuel/air mixture, starting the cycle all over again.  The advantage of this type of engine over a 4-stroke engine is that it only requires two strokes per gasoline explosion, meaning that, theoretically, twice as much power could be gained from the same size engine.  This makes two-stroke engines popular in engines carried by people, such as weed trimmers, as well as some manual lawn mowers.  However, this engine has many disadvantages.  Since the fuel is released from the cylinder before it has finished burning, much of the pressure is not utilized by the engine.  This pressure goes to produce heat and noise, making these engines louder than their counterparts.  Also, because the fuel is released before burning can be completed, there are many unburned products, such as carbon monoxide, released with the exhaust.  This fact is made worse by the lack of pollution control systems on these engines; these systems tend to be large, heavy, and expensive, and these engines are really only used in situations where a small, light, and cheap engine is required.

 

            In a standard four-stroke engine, the process starts off in the same way, by gasoline pushing a piston in an engine cylinder.  The remnants of the reaction, however, are released only when the cylinder has finished its first down stroke.  The exhaust valve stays open throughout the first up stroke, so all the exhaust can be removed, leaving more room for air and gasoline in the piston.  When the engine finishes its first up stroke, the exhaust valve closes and the air valve opens.  The engine’s second down stroke pulls air into the cylinder.  When the stroke is finished, the air valve closes and the second up stroke pressurizes the air.  When this final stroke is nearly finished, gasoline is injected into the cylinder, and the spark plug produces a spark and ignites the gasoline, starting the cycle over again.  Because fuel is burned for a longer period of time in these engines, fewer unburned remnants remain.  The fact that these engines tend to be used on ride-on vehicles means that the presence of pollution control systems like catalytic converters (as well as mufflers to reduce noise) is not a great problem, so these things are generally used.  These engines are more efficient because they use much more of the energy in the fuel than two-stroke engines, although a larger engine with more and/or larger cylinders is required to release the energy at the same rate.  Four-stroke engines are generally used wherever feasable.

 

            The burning of octane in air is a complex chemical process.  Hydrogen and carbon atoms from within an octane molecule combine with free-floating hydrogen and oxygen atoms (monatomic forms of normally diatomic molecules; they are highly unstable and try to combine with anything possible), as well as HO- and HO2- ions.  These free-floating particles exist in the greatest concentration at the source of the flame, so the burning process primarily takes place there.  However, the intense heat of the engine can cause molecular bonds to disintegrate well away from the flame.  This creates free-floating hydrogen, oxygen, and carbon atoms. Because no bonds exist to resist their combining, these elements combine extremely rapidly into steam and carbon dioxide, and much energy is given off in the process. Straight hydrocarbon chains, such as n-octane and n-heptane, are more susceptible to this problem than geometrically complex structures such as iso-octane, although iso-octane tends to be more expensive to obtain.  If heat builds up in the engine, it is possible for this to happen during the engine’s final engine stroke, just before the engine starts its down stroke.  This causes the pressure to force the piston down too soon, and actually attempts to push the engine backwards.  There is not enough time for it to actually succeed, but it does put a stress on the engine, in a way that it was never intended to be stressed.  It also causes portions of the piston to rattle around inside the cylinder, causing a sound that is referred to as ‘engine knock’.  A severe case of engine knock can destroy an engine in seconds, and given the volume of flammable hydrocarbons that could quickly be exposed to the heat that was in the engine cylinders, it would not be wise to be anywhere near an engine with this problem.  The best way to avoid this problem is to make certain that the correct gasoline is used in a given engine, and to determine what, exactly, is the correct gasoline to use in an engine, it is necessary to understand octane ratings.

 

            Octane ratings are the numbers listed beneath the grades of gas at any gas pump.  In the picture on the proceeding page, the ratings are 87, 89, and 93, for Regular, Super, and Premium, respectively.  The labels, Regular, Super, and Premium, are not as important as the octane ratings.  Octane ratings are obtained through running several tests on a specialized engine.  The engine is a single-cylinder four-stroke engine, with a special mechanism called a ‘knockmeter’ designed to measure the engine knock.  The engine is adjusted so that when the test fuel is used to run it, the knockmeter registers 50 on a scale of 1 to 100.  After this, an assortment of standard reference fuels are tested on the engine.  A fuel that is one octane number beneath the test fuel should register a 60-70 on the knockmeter, and one that is one number above it should register a 30-40.  The reference fuels are defined by the ratio between iso-octane and n-heptane within them.  This same test is run under high and low engine speeds, and the resulting octane numbers are averaged to obtain the octane number displayed at gas stations.  The reference fuels are created by mixing n-heptane with iso-octane; the percent iso-octane in the mixture is the octane rating for those fuels.  Many fuels contain special fuel additives that cause n-heptane to be more resistant to desintegrating in heat, thus making such fuels act like reference fuels with higher octane numbers; this is why the octane percentage is no longer used.  Lead was an example of a chemical that had this effect, and using it was less expensive than obtaining iso-octane; this is part of the reason that it was used in gasoline, despite its toxicity.

            Part of this project was to explain how this information affects consumers.  The vast majority of Americans own at least one gasoline engine, and it is quite common to own two, three, or even more.  The author of this essay has five engines owned by his family; two cars, one old truck, a weed trimmer, and a chainsaw, and there are plans to purchase a generator, probably gasoline-powered, for their house in rural Massachusetts, where blackouts caused by trees falling on power lines are not uncommon.  Other common gasoline-powered appliances include lawn mowers, both ride-on and manually propelled, many boats, and some portable power tools, such as chain saws.  Anyone who owns one of these appliances needs to be wary of purchasing gasoline with too low an octane content, for reasons that have been described in detail within this essay.  However, it may be tempting to purchase higher octane gasolines.  This guarantees that problems associated with having too little octane will not occur, and people might imagine that it would give their engines more power, or improve them in some way.  After all, race car drivers and some airplanes do use very high-octane gasoline, and the stuff does cost more.  However, it is possible to avoid engine problems without wasting money, and race car and airplane engines must be specially designed to take advantage of high-octane fuels; otherwise, they would gain no benefit from them.  The most practical way to go is to choose the octane recommended by the engine manufacturer, and to choose the next value up if that is unavailable.  If your engine starts to experience light knocking regularly when the engine is being taxed heavily (such as going up a steep hill at high altitude at low speeds in a high gear with a fully loaded SUV and a large trailor, and the air conditioning on; less extreme examples can also work), switch to the next higher grade gasoline.  If it starts experiencing knocking regularly, take the engine to a mechanic; it should not do that.  If it starts experiencing heavy, loud knocking at any time, turn off the engine as soon as possible.  If reasonable, bring the engine to a mechanic before starting it again; if not, let it cool down for at least 5-10 minutes (more is better), and try starting it again, putting as little load on the engine as possible (by shifting into a lower gear, for example; driving at a high engine speed in a low gear is much better for the engine than driving at a low engine speed in a high gear; turning off the air conditioner helps a lot, too).

 

            This essay has explained, as stated in its thesis, ‘what octane is, what it does, and how this information can be used to help automobiles, and any gasoline engine, for that matter, run more safely without using a great deal of money’.  Octane was shown to be a hydrocarbon chemical fuel, with the formula C8H18, and with two different structural formulas, represented by the iso- and n- prefixes.      Iso-octane was shown to be the major component of gasoline, providing most of the energy contained therein.  As to how this can be used to make engines run more safely, a great deal of information was shown about how engines work, and how choosing the wrong octane value can cause them not to work.  In addition to enlightening people about what, exactly, they are buying at the gas pump, and what it does when they use it, this information should enable people to take better care of any gasoline engines that they own.