Most of us tend to take combustion for granted. Everyone knows that certain materials burn, some explosively, while other materials don’t burn. Burning is just a combining of the material with oxygen, right. Pretty simple! Or is it?
Which Materials Burn?
Wood, paper, cloth, dry grass, oil and its products, hair, fur, and if hot enough, bodies; these all burn. You could also add to the list the elements hydrogen and helium. How about the chemical compounds ammonia, butane, methane, ethylene, acetylene, ethane, propane, and silane? In fact, most of the known elements will burn; the conditions have to be right, of course.
If you hold a torch to a hunk of steel, it won’t burn no matter how long you try. Take some steel wool and try the same thing and you’ll see steel burn. The difference should be obvious, but let’s look at it. It’s a bit like trying to light a log with a match and then trying the same thing with sawdust. The sawdust will readily light, but the log won’t, even though they’re made of the same material. The difference, of course, is in the physical structure of the material you are trying to ignite.
The flammable wood particles in sawdust have an enormous amount of surface area relative to their mass, while the log has just the opposite. The structure of sawdust allows the flammable wood particles to be much more effectively exposed to heat, allowing them to reach the temperature necessary for them to combust. Once they do combust, the heat then produced will make the process self-sustaining until all of the fuel is consumed.
Not only do some substances we would not normally consider as ‘dangerously combustible’ actually burn well, but some are capable of explosive combustion. Grain dust, sawdust, and even sugar dust are just three substances that, when suspended in air, can have air/fuel ratios that will support explosive combustion.
Unintended Combustion
On December 23, 1977, 36 people were killed in a grain dust explosion in the New Orleans area, involving silos owned by the Continental Grain Company.
On February 7, 2008, a sugar refinery explosion in Georgia killed 14 people.
From 1996 to 2005, the US experienced an average of 9.7 explosions, 9.4 injuries and 0.8 deaths per year from grain dust explosions. This was substantially down from the mid ‘70’s to mid ‘80’s, where the averages were 21.7 explosions, 44.1 injuries and 14.3 deaths.
The common sources of ignition in these unfortunate instances are static electricity discharge or overheated equipment, such as bearings. Today safety is more of a focus and steps are taken to provide ventilation to prevent enough material from becoming airborne for the mixture to become combustible. In effect, the mixture is kept too lean. Also, sources of static discharge are addressed, and equipment is better monitored for any possible overheating.
DeBruce elevator explosion of June 8, 1988, in the Wichita, Kansas area; pictures from OSHA
Do You Need a Flame?
Obviously, you don’t need a flame to start the combustion process. All that’s needed is heat, and often a flame is a handy source of that heat. Your electric charcoal starter will ignite the charcoal, won’t it? There’s no flame involved there—just heat.
If you had an industrial process oven that would go as high as several hundred degrees (F), you could place a bit of wood in it and see what temperature was needed for it to burn. It might start smoldering at between 300°F and 400°F or so, or it might take a somewhat higher temperature. (Just to be clear, don’t try this. Sure as hell don’t try this at home. If you do, you might be an imbecile.)
If you got your oven up to 600 – 700°F and tossed in your wood block, you might see the block burst into flames straight away. To summarize, we can start the combustion process of fuel and oxygen by supplying enough energy to get it started, with energy in the form of heat. The mention of oxygen was a no-brainer, wasn’t it? We’re talking about a form of oxidation, after all. But what exactly is the part oxygen plays? Doesn’t it just kind of support the process?
I Need My O2!
This begs a question—why is it ‘O2’ and not just ‘O’? It’s simply because an oxygen atom (‘O’) has a strong affinity for another oxygen atom and will naturally bond with it. Why? Well, oxygen has 8 electrons (negative charge) in its two shells. The first is full, with two, and the second would be full with 8, but only has six. Keep in mind that the atom has an equal number of protons in its nucleus, so it has zero net charge. It’s just that our oxygen atom won’t be fully satisfied and fulfilled until it has its outer shell filled, which requires two more electrons.
Luckily, there is usually close at hand another lonely oxygen atom, and these two atoms can share two electrons. Two electrons are therefore in the outer shells of both atoms, which are bonded together as O2, an oxygen molecule. The number of electrons hasn’t changed, and the molecule still has zero net charge.
This shows how two oxygen atoms each share two electrons with their mate.
Hydrogen is similar in this regard, namely its affinity to bond with another hydrogen atom.
The hydrogen atom has just one electron in its one shell. To be full, the shell requires two electrons (this is true for the first electron shell of all elements). Thus, two hydrogen atoms bond, sharing their electrons, and we have a molecule of hydrogen.
What Can I do with Hydrogen?
Over three-quarters of the visible universe is composed of hydrogen, the lightest and simplest element. It’s just one proton (+ charge) in the nucleus orbited by a single electron (- charge). Doesn’t come any simpler. Yes, we can burn it, but nature does something else with it.
Look out the window at that big, hot, bright ball in the sky. Mostly hydrogen. (If it’s night when you do this, it’s our moon, and your mom probably (rightfully) worries about you.)
Just to be clear, the sun doesn’t burn hydrogen. It converts hydrogen into helium in the process of nuclear fusion (‘nu -cu-ler’ if you’re George Bush Jr.).
Your Head is Mostly Empty Space
To help you better appreciate the nature of this, the humblest of elements, consider a scale model of a hydrogen atom. Let’s make the single proton in the nucleus the size of a volleyball. The single orbiting electron will be the size of a really small speck of dust, orbiting the nucleus at a distance of maybe 11 to 13 miles. This simple little atom is 99.99999999999996% empty space! And, yes, this is true of all atoms, including the ones that makeup you (and your head)!
Nuclear Fusion
I have a 1920’s era astronomy book that teaches that the sun is heated by contraction, and that it’s thus only several millions of years old (rather than the 5 billion we know now). Interestingly, the author specifically states that this understanding is so absolutely certain that we no longer have to look for any other explanations! He couldn’t have been more wrong if he tried.
Fusion in the Sun
The reaction that takes place in the core of our sun is called proton-proton fusion. It works like this:
- H nucleus + H nucleus combine. They will likely split, but at times they don’t, and one turns into a neutron, with the release of energy. This proton-neutron is deuterium.
- A third proton collides with the deuterium and forms Helium-3, with energy being released.
- Two of these collide and form Helium-4, plus two extra protons. The resulting mass is 99.3% of the mass we started with, the missing 0.7% having been converted to energy. Energy = mass times speed of light squared, or E = mC2. This 0.7% represents 4.7 million tons of matter per second for our sun.
You might recall that the US space shuttle fleet used liquid oxygen and liquid hydrogen as fuel, the three engines generating some 1.2 million pounds of thrust, or over 37,000,000 horsepower!
A Basic Example
What happens in an example of the burning of hydrogen? This is easier to picture, since we have H2 combining with O2, nothing more. Just hydrogen and oxygen.
2H2 + O2 -> 2H2O + ENERGY
Note that our combustion example does not include any carbon, in the form of a hydrocarbon, so this is a simpler example than burning a substance that does contain carbon (following example uses CH4). The result of this reaction is just water and energy, with no CO or CO2 since there is no carbon!
Joule and Mol
Joule is a unit of energy that’s named after the English physicist. It has many definitions, but perhaps the easiest is that it’s equal to 1 Watt for one second.
The molecular mass of hydrogen is 1gram/mol, so 1 mol of H weighs 1 gram. There are about 28 grams in one ounce. As a reference, a US penny weighs 2.5g.
1mol (‘mole’, 1/28th ounce) of H has 6.02 x 1023 hydrogen atoms. Actually, according to the definition of a mole, one mol of anything is 6.02 x 1023 of whatever you are measuring.
During combustion, the atoms in the materials being burned combine with oxygen in the air to produce different molecules. So much heat is released during these reactions that individual atoms are superheated, and as they leave the combustion zone, they bleed off excess energy in the form of light, both visible and invisible.
Where Does the Light Come From?
In the process of fire, the released energy heats up both the reactants and products. If some of the reactants are solids (such as wood or coal), incomplete oxidation releases solid particles (soot) hot enough to emit light. A flame is the mixture of reacting gases and solids that emits light.
When burning something like hydrogen, there is no carbon. The reason flames such as from burning hydrogen produce any light is that the temperature rise causes an increase in the average velocity of the gas molecules. A small proportion of the gas molecules have enough energy for their collisions to cause electron transitions. Electrons can get bumped up to a higher shell, but they won’t be able to stay there. They will decay by emission of a photon as an electron jumps back down to its normal shell, and it’s the light produced from these decays that we see as the color of the flame. Cool, huh?
Ideal Reaction
Under ideal conditions, where only hydrocarbon and oxygen are present, burning produces only water, carbon dioxide, and energy. This rarely occurs in the real world, as burning in air allows the other non-oxygen gas elements and compounds to take part in the chemical reaction.
Ideal Reaction Equation
CH4 + 2O2 -> 2H2O + CO2 + ENERGY (CH4 is methane)
In the above ideal reaction, the energy gained from the reaction is greater than the energy put into the reaction. Heat is needed to make a hydrocarbon burn. This represents the energy need to break the carbon-carbon and carbon-hydrogen bonds of the hydrocarbon molecule as well as the oxygen-oxygen bond of the oxygen molecule. The typical C-C bond requires 350 kJ/mol to break, the typical C-H bond requires 413 kJ/mol, and the O-O bond requires about 498 kJ/mol.
Energy is released from these reactions when new bonds are formed. The H-O bonds of water release about 464 kJ/mol of energy when formed and the C=O bonds of CO2 release about 800 kJ/mol when formed. The net outcome is the release of energy in the form of heat.
About Heat
Regarding any type of combustion, it’s important to realize that no energy is being ‘created’. Energy can’t be created or destroyed; it can just change forms (1). Combustion is a process in which chemical energy potential is converted into terms with lower energy potentials, with the difference being the released energy. The atoms and molecules involved in the process gain speed (heat) and emit light (photons) at different frequencies. Much of this light is in the visible spectrum, while there are also huge amounts of light in the infrared spectrum, which are light frequencies too low for the eye to see.
You probably already know that visible light represents just a tiny sliver of the spectrum of electromagnetic radiation. When an object radiates heat (it will if it’s above absolute zero), the frequencies of this radiation correspond primarily to infrared. If the object is hot enough, it will also emit in the visible part of the spectrum.
It’s Not Just Infrared
There is often the mistaken belief that infrared is ‘heat energy’, and this misconception comes about naturally, through our human experience. That cup of coffee you just put down was hot, but it’s obviously not glowing. Intuition might tell you that if you were to look at it through an infrared viewer, it would indeed be emitting light in those frequencies. Thus, bolstering our belief that heat = infrared. What we might not realize is that there is a well-defined relationship between the temperature of an object and the frequencies (wavelength) of light that it emits at that temperature. Heat up a poker in a fire and it will get hot enough to burn you well before it glows red.
If you continue to heat it, it changes from red to orange, and then to yellow and white. What might not be obvious is that before we even started our fire, the poker was emitting photons of energy. Put the thing in a freezer, and it will still be emitting photons of energy! In fact, if its temperature is above absolute zero (it will be), it’s emitting photons! If an object has any heat energy (it’s above absolute zero), it is emitting energy in the form of photons. Bottom line—heat we ordinarily encounter is usually in the form of light at infrared frequencies, simply because these objects aren’t hot enough to emit in visible frequencies.
Black Body
This is a representation of a classic black body chart that shows how the radiated energy from objects changes with temperature. As the temperature increases (3000 K – 5000 K) the intensity increases (vertical axis) and the frequencies shift to shorter wavelengths, which correspond to higher frequencies. If you picture the temperature starting at 3000 K (red), you see that most of the radiation is at infrared frequencies. At 4000 K we have strong emissions in the visible part of the spectrum. Increasing further (blue) clearly results in vastly increased energy in the visible part, with the peak frequencies in the orange/yellow. This is precisely why heating an object first produces radiation you can’t see (infrared) and then when it does become visible, the frequencies shift from red to orange to yellow as the temperature increases.
The ‘K’ designation is for ‘Kelvin’. Absolute zero, where all atomic movement has ceased, is 0°K. If you add to this 273.15 you have degrees Celsius. In case you ever wondered, Celsius and centigrade are the same thing. The former is in honor of the Swedish astronomer and the latter is recognizing the 100-degree interval between the water freezing point (0°) and the water boiling point (100°).
Don’t be confused by the super-high temperatures in the diagram above. The same theory (higher the temp the shorter the wavelength) applies to the temperatures we experience daily.
Back to Combustion …
The double bond in O2 is unusually weak, and the formation of the stronger bonds in CO2 and H2O results in the release of heat. The heat of combustion is close to −418 kJ/mol for each mole of O2, of which −306 kJ/mol can be attributed to the weak double bond of O2 and the rest to the stronger bonds in H2O compared to two C–H bonds, and in CO2 compared to two C–C or C–O bonds in the reactants. It is not the organic fuel but rather O2 that is “energy-rich”. The presence of large quantities of O2 in the earth’s atmosphere is due to photosynthesis from H2O by plant life.
It’s important to realize that in no way was matter destroyed or consumed in the process of combustion. It takes 34 lbs. of air to burn 1 lb. of hydrogen. Stated another way, the stoichiometric ratio for hydrogen is 34:1. After combustion is complete, in the case with our 1 lb. of hydrogen, we still have 34 + 1 pound of matter. Nothing was lost; it just attained a state with a lower energy level, reflecting the energy that was produced during the process.
Stoichiometry
Definition: the relationship between the relative quantities of substances taking part in a reaction or forming a compound, typically a ratio of whole integers. It’s pronounced stoikēˈämətrē.
When we speak of stoichiometry for internal combustion engines, we’re referring to the amount of air that has to combine with the fuel to produce complete combustion. This is air, not oxygen, and our air is only 21% oxygen. The remaining portion is overwhelmingly nitrogen, which is largely inert, so we’ll just call it 79% nitrogen.
So, 79% of what we call ‘air’ is nitrogen, and nitrogen generally doesn’t take part in the combustion process. That means that if x amount of nitrogen is involved in the combustion process, x amount comes out of it as nitrogen (usually)!
We’re burning 1lb. of hydrogen. In our 34 lb. of air (table above), how much oxygen is there that will actually take part in the chemical process? Simple! 34 lb. times 21% = 7.14 lb. of O2.
The end product for our hydrogen combustion is 1lb. + 7.14 lbs. = 8.14 lbs. of water (H2O), in either liquid or vapor forms.
How much energy do you think it would take to return this 8.14 lbs. of H2O to 1 lb. of hydrogen and 7.14 lbs. of O2? Assuming the process used was as efficient as the combustion process was, this energy value would be equal to the energy that was liberated by the combustion process.
Oxygen as Limiting Factor
The effect of nitrogen (and other trace elements) in air is that they act as contaminants in the combustion reaction and they act to effectively limit the concentration of oxygen. Rather than 100% oxygen, reactions that occur under atmospheric conditions are subject to only 21% oxygen.
When oxygen is a limiting factor, it is not possible to pair every carbon atom with two oxygen atoms during a combustion reaction. Thus, some carbon atoms end up with only one oxygen atom. This produces carbon monoxide. When a combustion reaction produces CO, it is referred to as incomplete combustion.
Atmospheric combustion would then be more closely modeled by the equation that follows for burning methane (note this is not a balanced equation).
CH4 + 2O2 -> 2H2O + CO2 + CO + ENERGY
Note the inclusion of CO, carbon monoxide, as one of the products of the reaction.
Most of My Fuel is Free!
I have to say, I never fully appreciated this point until the recent era of electric cars came along. Someone pointed out that an electric car carries all of its fuel all of the time while a hydrocarbon-powered car gets most of its ‘fuel’ from the atmosphere, not having to carry it at all!
Using our example above of burning hydrogen, each 1lb. of hydrogen combined with 7.14lb. of O2. 87.7% of the fuel consumed (H2 + O2) is gotten from the atmosphere itself! While we don’t usually speak of oxygen as ‘fuel’, it’s actually more key to the chemical reaction we refer to as ‘burning’ than the particular substance is that’s combining with the O2.
The nature of stoichiometry can be a bit challenging. I’ve seen videos and information on the web that mistook volume for weight. For gasoline, it’s 14.7 POUNDS of air that combines with 1 POUND of fuel. One pound of air occupies 13.33 cubic feet of volume at sea level. Approximately 609 cubic centimeters of gasoline weighs one pound. This is, of course, about 6/10th of a liter. 14.7 pounds of air occupies 13.33 X 14.7 = 195.9 cubic feet.
This image shows the volumes of 14.7lb. of air and 1lb. of gasoline.
This is similar to some images I’ve seen on the Internet, but we’ve crossed it out to indicate that it is incorrect. This equates volumes, not weight. An air/fuel ratio is related to the weight of the components!
Atmospheric Contaminants
Contamination can come from the petroleum refining process or from the atmosphere. The most common atmospheric contaminant, when considering combustion, is nitrogen in the form of N2. When nitrogen is burned at high temperatures, it produces nitrogen oxide and nitrogen dioxide. These two compounds represent about 1% of the output of a common hydrocarbon combustion reaction under atmospheric conditions. We rewrite our equation to reflect this:
CH4 + 2O2 + N2 -> 2H2O + CO2 + CO + NO + NO2 + ENERGY
Both of the nitrogen compounds can participate in the formation of nitric acid (HNO3), which is a component of acid rain. The reactions:
H2O + N2 + NO2 -> HNO2 (HNO2 is Nitrous Acid)
3 HNO2 -> HNO3 + 2 NO + H2O (HNO3 is Nitric Acid)
4 NO + 3 O2 + 2 H2O -> HNO3 (HNO3 is Nitric Acid)
Petroleum Contaminants
Though sulfur is not a major component of the atmosphere, it is often found in petroleum. In fact, sour versus sweet petroleum is determined by the sulfur content where sour petroleum contains more than 0.5% sulfur. Sulfur, when burned during hydrocarbon combustion produces sulfur dioxide, which acts as a precursor to sulfuric acid. Like nitric acid, this contributes to acid rain. We can once again update our hydrocarbon combustion reaction to reflect this new contaminant (reaction not balanced and H2S is not the only sulfur contaminant. Others include COS, CS2, SO2 and more).
CH4 + 2 O2 + N2 + H2S -> 2 H2O + CO2 + CO + NO+ NO2 + SO2 + ENERGY
CO Carbon monoxide
CO2 Carbon dioxide
CS2 Carbon disulfide
COS Carbonyl sulfide
CH4 Methane
H2S Hydrogen sulfide
HNO2 Nitrous Acid or sodium nitrite
HNO3 Nitric Acid
NO Nitric oxide or nitrogen monoxide
N02 Nitrogen dioxide
SO2 Sulfur dioxide
The reaction that produces sulfuric acid from sulfur dioxide is more complicated than that which governs nitric oxide production.
HO2 Hydroperoxyl
HOSO2 Sulfoxylic acid
H2SO4 Sulfuric acid
OH Hydroxide
SO3 Sulfur trioxide
If you look at the Hydrocarbon Energy Yields table and then at the Air/Fuel Ratios table, it’s easy to come to the conclusion that the substances that can be mixed with more air produce more energy. That makes perfect sense when we remember that it’s the oxygen and its bonds that actually contain the energy that is liberated in combustion.
Molecular Structure
If you look back at the Stoichiometric A/F table, you see that our fuels are composed mostly of carbon and hydrogen, with sometimes a relatively small amount of oxygen. These are hydrocarbons. There are a lot of different hydrocarbons, differentiated primarily by the numbers of hydrogen and oxygen atoms and the manner in which they are linked.
(1) As Albert Einstein conceived, matter and energy have an established relationship and mass can be converted into energy, and vice versa. This does not violate the First Law of Thermodynamics, the Law of the Conservation of Energy.
Gasoline
Gasoline has been an essential part of the lives of most Americans for over 100 years now. Though some of the earliest cars were powered by kerosene, electricity or steam, it wasn’t long before the benefits of gasoline as a fuel became obvious, and engines powered by this fuel began to dominate the market.
The name ‘gasoline’ is thought to have been derived from the British ‘gaseline’, which was a trade name. It’s important to realize that gasoline is comprised on many, many different chemical compounds in various quantities. The substance octane (C8H18) is sometimes used as being representative of gasoline, as it is a constituent of gasoline. The nature of gasoline is such that you can’t define it with a chemical formula (like octane, above), given the fact that it consists of a multitude of different compounds.
Facts About Crude Oil
The refining process begins with crude oil, which is unrefined liquid petroleum. It ranges in color from yellowish to black. Crude oil is composed of thousands of different hydrocarbon compounds, each of which has a different boiling point.
For example, a typical crude oil may begin to boil at 104°F to produce petroleum gas used for heating and making plastics, and finish boiling at greater than 1200°F to produce residuals such as petroleum coke, asphalt and tar.
Crude oil is generally described as sweet or sour according to its sulfur content, and heavy or light according to its API Gravity. The API Gravity index is a relative measure of weight, the lower the number, the heavier the material.
A heavy crude is generally regarded as having a less than 30°API, while a light crude is greater than 30°API. In a bit of bizarreness, the API gravity is the inverse of specific gravity, with water being defined as ’10’. A number greater than 10 is lighter than water, and vice versa.
- Light crude oil has an API gravity higher than 31.1°
- Medium oil has an API gravity between 22.3 and 31.1°
- Heavy crude oil has an API gravity below 22.3°
- Extra heavy oil has an API gravity below 10.0°
What’s With the ‘°’?
API gravity values are dimensionless, but they are often displayed with the degree symbol. Why is this? Honestly, I’m not sure. My guess is that chemistry geeks think it’s funny or there’s some other reason, the explanation of which would bore a normal person to tears.
About API
The American Petroleum Institute is a group that was formed in 1919. Much of the motivation for the formation of the trade group was World War One, which saw the use of gasoline for war purposes increase tremendously.
The U.S. Supreme Court had broken up the Standard Oil monopoly, and the resulting large numbers of smaller companies had no experience in working together. It’s easy to imagine the mayhem that would have resulted if each company had their own standards for motor oils and for gasoline that they produced.
When WWI started, there were less than 400 war planes in the entire world. During the war, over 200,000 aircraft were built! This statistic alone should well illustrate the importance that oil had taken on during this short period. The photo is of a German Fokker DR1 triplane. Despite its overrepresentation in movies about WWI, it was far from the most important German fighter of the war.
In addition to the fantastic increase in numbers of existing types of vehicles during the war, there also emerged vehicles such as this British tank. Interestingly, tanks were designated as ‘male’ if they had canon and ‘female’ if they had machine guns. The steering mechanism seen at the back of this machine would later be discarded. Armor was only about one-half inch thick and could be penetrated by rifles with armor-piercing rounds! Yikes!
Gasoline Refining
All fractions of petroleum find uses, but the greatest demand is for gasoline. One barrel of crude petroleum contains only 30-40% gasoline. To make the process economical viable, more of the petroleum fractions must be converted into gasoline. This may be done by cracking, reforming or isomerization.
- Cracking Large molecules of heavy heating oil and resids (residuals) are broken down.
- Reforming Molecular structures of low-quality gasoline molecules are changed.
- Isomerization Rearranging the atoms in a molecule so that the product has the same chemical formula but has a different structure.
There are a number of increasing complexities that come into play, depending upon what the desired end product is.
Separation
The first step is the separation of molecules according to their molecular weight, through distillation at atmospheric pressure.
The oil is vaporized by heating it at the bottom of a 60-meter distillation column at temperatures of 350°C to 400°C, causing it to vaporize. The heaviest molecules remain un-vaporized at the bottom, while the lighter ones rise to the top.
The vapors condense into liquids at different heights and temperatures in the column. Liquids can’t reach the top, though vapors do. Lighter liquids will rise higher than heavier ones, and the various densities of liquids are collected in trays, with each tray collecting a different fraction.
The heavier product left over after this process are further processed in another column to recover diesel and fuel oil.
Conversion
At this point the hydrocarbon molecule chains still need to be “cracked” into smaller chains and lighter products.
This conversion process is known as catalytic cracking. It uses a catalyst to speed up the chemical reaction, and takes place at about 500°C. Hydrogen is often added to improve the yield, in a process known as hydrocracking. The products of this process are gas, gasoline and diesel. Naturally, the more complex the conversion process the more expensive it is.
Treating
Crude oils tend to be a mixture of paraffins-naphthenes-aromatics, with paraffins and naphthenes the predominant species. Crude oils can exhibit regional trends in chemical composition, placing them into one of the following groups:
- Paraffinic Paraffinic oils are the preferred option for high-temperature applications and where longer lubricant life is required.
- Napthenic From sweet crude; The unique characteristics of naphthenic mineral oils have often made them good lubricants for locomotive engine oils, refrigerant oils, compressor oils, transformer oils and process oils.
- Aromatic The name “aromatic” refers to the fact that such hydrocarbons are commonly fragrant compounds
Catalytic reforming is the process used to produce high-octane products. This process uses platinum as a catalyst and converts some of the naphthenic hydrocarbons into aromatic hydrocarbons which have a much higher-octane rating. Other chemical reactions, such as alkylation, also improve the octane rating.
Hydrocracking is a catalytic chemical process used in petroleum refineries for converting the high-boiling constituent hydrocarbons in petroleum crude oils to more valuable lower-boiling products such as gasoline, kerosene, jet fuel and diesel oil. The process takes place in a hydrogen-rich atmosphere at elevated temperatures (260 – 425 °C) and pressures (35 – 200 bar).
Gasoline Additives
It should come as no surprise that the use of gasoline additives has increased with time. What might not be so well known is that the use of additives goes way back to the 1920’s.
The first additives to gas were intended to raise the octane of the fuel. In the WWI era, gasoline had octane ratings in the 50 to 60 range. Of course, compression ratios were well under half of what they would climb to in the 1960’s (Model T – 4.5:1).
It should be pointed out that higher octanes can be achieved by additional refining techniques but finding an additive to accomplish this would be more cost effective.
The Magic Additive
Charles F. Kettering had formed Dayton Engineering Laboratories Co. and he embarked on the search for a substance to raise octane. He’s the same man who invented the self-starter and who created the Delco battery. The focus was on compounds of elements that are found at the bottom of the periodic table. A compound of lead and ethylene was already known to science and the compound, tetraethyl lead (TEL), was designated as the substance of choice. (TEL is (CH3CH2)4Pb)
Delco itself was purchased by GM in 1919, and GM patented the use of TEL in motor fuels. They also started referring to the product as “Ethyl”, which conveniently ignored the use of the word “lead”. Anyone who remembers the 1960’s might recall hearing the upper-tier, higher octane offering at a filling station being referred to by this name.
The desired effect of a raised octane was to be able to offer a fuel that would be resistant to causing engine knocking (detonation), even when prevailing temperatures made it more likely. A secondary benefit was that the rising octane numbers allowed automakers to gradually increase the compression ratios of their engines.
Ironically, it was known from the beginning of TEL use in gas that this substance was fatal! TEL manufacturing sites had workers get sick from the exposure and several suffered horrible deaths. This didn’t deter the oil companies, though, and the production and sale of gasoline with TEL, once started, would continue almost to the close of the twentieth century.
There were alternatives to TEL, but they were more expensive to implement. Low cost won out and it was conveniently forgotten that this substance that would be found in every city, town and village in America was a deadly poison.
Octane
We need to address the topic of octane itself. Octane is a standard measure of the performance of an engine fuel and its ability to withstand compression without detonation occurring.
The higher the octane, the harder it is to ignite.
Remember the above statement. Notice that it makes no comment about energy; a higher-octane fuel does not contain more energy potential than a lower octane fuel. There is no correlation between octane and energy content. Zero.
Choices. . .
I remember reading something about marketing that stated you can offer people too many or too few choices. The number three was the minimum. If you offer just two (here – fuel choices), most people will choose the lowest priced item. If you offer three, many people will step up to the mid-priced item. I’ve wondered if that comes into play with most gas stations offering three different levels and prices of gas. Maybe, maybe not.
Octane is measured by either one of two common methods, or, as in the U.S., by a combination of the two methods. The substance heptane has been assigned an octane value of 0, and the substance iso-octane a value of 100. Values above 100 are extrapolated. Basically, heptane and iso-octane are mixed, and this reference mixture is compared to the detonation resistance of the sample fuel. When the mixture has the same detonation resistance, the two octanes are the same. The octane is then taken from the percentage of iso-octane in the reference mixture.
You may have noticed the small print on the octane sticker on the fuel station gas pump. It refers to the measuring method by which the octane number is obtained, which is “(R + M) / 2”. This number is therefore an average of the “R” method and the “M” method. They refer to Research and Motor, respectively. In literature, you will also see Motor Octane Number (MON) and Research octane Number (RON) referred to.
Two Different methods
The Research method uses a small test engine with a variable compression ratio, operating at 600 rpm. The Motor method uses the same type of test engine, but operating at 900 rpm, and with a preheated fuel mixture and variable ignition timing. Octane values obtained with this method are 8 – 12 points lower than those obtained by the Research method.
Though the M octane values will be lower, you cannot use the difference as any sort of an adjustment value! You can’t add a fixed percentage to the M values to get the R values. As previously stated, the octane value on U.S. gas pumps is the average of values gotten from these two methods.
Back in the Day
Back in the muscle car heyday, octane ratings were substantially higher than those commonly offered today. A good part of this difference is that the old 103 octane Sunoco was measured with the Research method, not the average of the R and M that we use today. There is no simple conversion factor between old and new octane rating methods, but it seems that the old 103 octane is equivalent to today’s 94 to 96 octane. So, regardless of the octane ratings change, the gas with the highest-octane ratings then did exceed that of the gas readily available now, provided they’re rated by the same method.
Given what we know, is it safe to assume that two different fuels of the same octane will perform the same? This consideration doesn’t take into account the detergent, anti-corrosion, or other additives. We’re primarily interested in the engine not knocking, as this is the by and large the reason a person chooses a higher-octane number.
You might see some subtle differences between two fuels of the same octane, say, 91 octane. It’s possible that each of the two 91 octane fuels had different RON numbers and MON numbers. Yes, both sets of numbers averaged to 91, but the numbers themselves, before averaging, might have been different. The result might be that your engine operated at the edge of knocking onset with one fuel, while the other fuel was beyond this, into full knocking under some conditions.
Worth the Cost?
You may have heard a conversation where someone filled their tank with a higher-octane fuel, often because they seem to think it’s somehow good for the car to do this once in a while. Invariably, they will make some comment about how the car had more power with the higher-octane gas.
We humans seem to experience what we expect to experience. I’d venture that nine out of ten people who expect a higher-octane fuel to provide more power or better mileage will perceive just that as a result.
There is no shortage of companies very willing and able to use this human trait to their advantage. The automotive market has its share of companies selling products to improve automotive performance that aren’t worth the cardboard they’re packaged in.
Detonation
I have an old automotive textbook from the late ‘80’s that’s a later version of the text we used in auto shop in high school. On the topic of detonation and knocking, it states that this is when the flame front caused by the spark plug collides with another flame front that is caused by the air/fuel mixture self-igniting from the heat and pressure it’s subject to. There was even a lame little drawing to illustrate this, in case anyone had trouble getting their young little brain wrapped around this concept. I distinctly remember thinking “Okay, but how does this make the knock I hear?”. However, with no resources to research the matter, I let it go.
A Bad Explanation
Some years later I happened across an article in one of the car magazines that addressed the issue. It stated that the facts behind this phenomenon were complex, so that the simpler explanation was usually offered without any further elaboration. That vindicated my gut feeling from all those years earlier, but I still had no complete answer for the phenomena.
Engine knock arises from auto-ignition of the end gas ahead of the propagating flame. The figure above presents the pressure trace, pressure oscillation, heat release rate (HRR) and unburned gas temperature (T) of a typical knocking case. The combustion process of the knocking case has two stages: flame propagation induced by spark ignition and end-gas auto-ignition causing pressure oscillation.
The image above clearly shows the excessive peak pressure as well as the pressure oscillations.
What Really Happens
The following “thought experiment” will help you to understand the relationship between detonation and the characteristic knock sound that accompanies it. It is well established that the onset of knock is not audible, and that knock sensors can detect what our ear cannot. Obviously, there are varying degrees of knock, as would seem reasonable.
When a rising piston tries to further compress an air/fuel mixture that’s already under extremely high pressure (and resistant to further compression), it’s something like trying to compress a solid.
Most solids will compress, although the forces required are far above those needed to compress any gas. The mechanical forces and stresses on all of the hardware components are going to spike, particularly those of the piston, rings, piston pin, connecting rod, and bearings.
An Example
Think of trying to drive something into place, such as a wheel bearing race. As your hammer impacts the driver, you may sense no real motion of the race you’re driving into place. However, when it’s finally fully seated and you make an additional blow, it’s very obvious from the sound (and feel) that the race is seated, isn’t it? The hammer and driver kind of bounce back and the sound it makes is higher pitched.
As the race is being driven into place, the energy imparted by the hammer blows is doing the work of moving the race and overcoming the substantial friction between the race and the rotor. Both the race and the rotor will gain energy in the form of heat. The extra blow that you provide, after the race is in place, has nothing to move to readily absorb the energy of the hammer blow and turn this energy into heat.
This energy has to go somewhere, right? It’s going to cause the entire mechanical “system”, that consists of the hammer, the driver, the (already seated) race, and the rotor to absorb the energy, which they can only do by physically moving. The entire system will briefly oscillate. In fact, if you had a vibration sensor mounted to the rotor and a control system to monitor it, you could capture the waveform of the rotor oscillating. It would look like a pulse, followed by positive and negative pulses of ever diminishing amplitude.
Energy Has to Go Somewhere
This is essentially what’s happening when an engine detonates and knocks. The cylinder pressure, which the rising piston is working against, is already excessively high due to the two flame fronts consuming the available air/fuel mixture, creating an enormous amount of heat. As contrasted with the normal case where the spark plug is the only source of ignition, the pressure is abnormally high due to combustion having started early by the mixture self-igniting from the pressure and temperature.
As the piston tries to further compress this gas, the energy of the piston, connecting rod, crankshaft, etc. that would ordinarily be absorbed by the gas (and returned on the power stroke) is forced to go elsewhere. Some of it goes into causing parts of the mechanical assembly to move in the form of an oscillation.
Detonation Damage
Severe detonation can damage connecting rods, bearings, pistons and rings. Below are some of the results of severe detonation.
Octane needs of an Engine
It’s not hard to grasp the concept of higher compression engines requiring, in general, higher-octane fuels. Of course, there are many other factors that come into play.
The early ‘70’s mandate that all new cars be required to run on unleaded fuel caused compression ratios to plummet, generally into the 8:1 range. This was the situation until the mid ‘80’s, when performance engine compression ratios were once again in the 9:1 range. A handful of years later saw the new Chevrolet LT1 with ratios in the mid 10:1 range.
Now, as we approach the mid 2020’s, compression ratios of performance engines are as high as the ranges of 11:1 and 12:1. Ratios like these were considered high in the muscle car era!
You might wonder how an engine with a 12:1 compression ratio can operate safely on 93 (or so) octane gas. Engines today have aluminum alloy heads with sophisticated combustion chamber and port designs, along with fuel injection (port and/or direct) that allow operation at compression ratios that would have been considered as high, even back in the day.
Different Needs
Each engine type will have different octane requirements. In fact, even two engines of the exact same type might have subtle performance differences when running on the same fuel. This could be accounted for by differences in compression ratio caused by carbon deposits in the combustion chamber, camshaft differences, carburetion differences or timing differences. An engine running hot, as from a cooling problem, will exacerbate any tendency to detonate.
Be aware that detonation and preignition are two different phenomena. Preignition is ignition of the air/fuel mixture due to a localized source of heat that’s intense enough to cause ignition. A common cause if preignition is a carbon deposit that has attained enough heat for a portion of it to glow. An engine running on (keeps trying to run when shut off) is a type of preignition.
Methyl Tertiary-Butyl Ether (MTBE)
It was decided in the 1980’s that additional changes needed to be made to some motor fuels. This was reflected in the Clean Air Act Amendments (CAAA) of 1990. Certain metro areas were identified as having excessive levels of ground-level ozone. The response was to make reformulated gasoline (RFG) available. These fuels have an increased oxygen content to allow more complete combustion, thus lowering the production of “ozone precursors”. Two of these precursors are oxides of nitrogen (NOx) and volatile organic compounds (VOCs).
When producing RFG, petroleum refiners were not required to use any particular oxygenate. A product known as methyl tertiary-butyl ether (MTBE) was favored, but refiners also used ethanol, especially where there was a ready supply of it.
MTBE was found to contaminate water resources, so its use was phased out in the U.S. RFG is still sold in the US, but the oxygenate is ethanol, not MTBE. I know it’s hard to imagine the EPA putting in place measures that actually made things worse, but there you have it.
