Internal Combustion Engine Basics
The internal combustion engine is undoubtedly one of the greatest inventions of human civilization. Like so many other inventions, however, the internal combustion engine has caused arguably as many problems as it has solved.
ICEs use either gasoline or diesel fuel to operate, although they can be retrofitted to operate on natural gas as well. Both gasoline and diesel are crude oil derivatives that set the vehicle in motion through, as the name suggests, combustion. The combustion process, like any process involving the burning of a substance, releases various chemicals, most of them harmful.
Carbon monoxide and nitrous oxides are among the biggest culprits for air pollution from vehicles with internal combustion engines, but particulate matter, unburned hydrocarbons, sulfur, and volatile organic compounds are also problematic.
A combination of scientific research, fuel efficiency technological advancements, and regulatory efforts to reduce air pollution and the health risks associated with it has resulted in much cleaner-burning, less polluting ICE fuels.
The group of chemicals called oxygenates have been instrumental in that process.
What are Oxygenates?
Oxygenates are chemical compounds that contain oxygen as one of their components. While any compound that contains oxygen is technically an oxygenate, the term is commonly used for compounds that are used as additives in internal combustion engine vehicles.
Alcohols and ethers
Oxygenate fuel additives are either alcohols, such as ethanol, or ethers, the most popular among them methyl tertiary butyl ether, or MTBE.
An alcohol is any of a number of chemical compounds that contains at least one hydroxyl group (made up from oxygen and hydrogen) and this group is attached to the carbon atom in an alkyl group. Alkyl groups are chains made up of carbon and hydrogen atoms. Chemical specificities aside, alcohols are an important group in organic chemistry because of their versatility: they can be derived from and converted into a wide variety of other compounds. Related: Europe's Largest Economy Is Betting Big On Hydrogen
An ether is a compound that has one oxygen atom attached to two alkyl groups. Like alcohols, ethers are structurally similar to water. The difference between alcohols and ethers is that in alcohols, one of the hydrogen atoms in a water molecule is substituted with an alkyl group, while in ethers, both hydrogen atoms in the water molecule are replaced by alkyl groups.
A History of Oxygenates
In the early days of the automobile industry, cars were noisy, and their engines were not very powerful. Emissions of soot and carbon monoxide, the products of internal combustion engine action, were also abundant. To fix the power and pollution problems, General Motors proposed adding lead to gasoline. This happened in the early 20th century and before long, lead became a standard additive for gasoline.
The problems that lead caused began emerging soon enough. Workers at a Standard Oil facility rose to grim notoriety when they began suffering hallucinations, and several died. Some scientists called for closer scrutiny of the lead additive industry, noting the metal’s history of toxicity. The debate continued for decades as GM funded most of the research into the health hazards of lead. It was only in the 1970s that the lead content in gasoline began to be reduced in the United States, following the passing of the Clean Air Act.
This reduction that would ultimately lead to the elimination of the toxic compound from fuels had to be compensated for by other chemicals to avoid the inevitable drop in engine power.
What Oxygenates Do in the Engine
The main problems with additive-free gasoline include the so-called knocking in the engine and emissions. Knocking refers to the premature explosion of the fuel in the combustion chamber (the cylinder) when mixed with air in the presence of a spark.
First lead and then oxygenates were added to gasoline to prevent this premature explosion. This premature explosion compromised the power of the engine as it interfered with the burning process of gasoline. Oxygenates boost gasoline’s knocking resistance, which is measured as an octane number. The higher the octane number, the smoother the burning of gasoline in the combustion chamber, and the more powerful the car.
The premature explosion also produces more soot because the fuel does not burn completely. It also producers more carbon monoxide. Adding oxygenates to gasoline improves the solubility of oxygen in the fuel, which makes it burn more efficiently—rather, more completely. This, in turn, reduces the emissions of soot and carbon monoxide. What’s more, the oxygen in the additives displaces some components of gasoline, such as benzene and sulphur, which would otherwise be released as polluting exhaust fumes.
This is particularly important in the cold winter months when the low temperatures cause higher carbon monoxide emissions. This is why all American states have strict winter oxygenate fuel programs.
Types of Oxygenates
Oxygenates for gasoline
?he range of oxygenate additives used in gasoline is the widest in both the alcohol and the ether group.
The most popular alcohol oxygenate by far is ethanol. It is more commonly referred to a biofuel that emissions regulation requires to be added to gasoline. Chemically, ethanol is an oxygenate, which is why it merits a place in this guide.
Ethanol is the product of yeast-based fermentation—more specifically, the fermentation of the starch and the sugars in one of several plants: corn, sugar cane, and sugar beets. In the United States, corn is the undisputed leader among these. In other parts of the world, sugar cane and beets are the most popular since fermenting sugar to make alcohol is a quicker process than fermenting starch, which is the case with corn.
In recent years, an alternative fermentation process has emerged for ethanol. That process involves the breakdown of not starch and sugars but cellulose. Called cellulosic ethanol, the product of that process can use any waste feedstock, including grass and tree matter, which makes it a lot more flexible in terms of raw material. The drawback is that the end product comes in much smaller quantities than standard alcohol fermentation. This is why cellulosic ethanol has not yet become a competitor of standard ethanol.
Besides ethanol, other alcohol oxygenates used in gasoline are methanol—the simplest alcohol—and two propylene derivatives, n-butanol and tert-butanol. Another common alcohol oxygenate for gasoline is isopropyl alcohol, also produced from propylene.
The most popular ether oxygenate until relatively recently was methyl tertiary butyl ether (MTBE). As the name suggests, MTBE is made from methanol, like another ether oxygenate, tertiary amyl methyl ether (TAME). The other popular ethers for gasoline—ethyl tertiary butyl ether (ETBE) and tertiary amyl ethyl ether (TAEE) are made from ethanol.
Ethers can be made from either petrochemical feedstock or renewable matter. They are considered superior as oxygenates to alcohols, capable of achieving higher octane numbers, which is why, at first, MTBE was used precisely as an octane booster, to replace lead.
Yet ethers also lowered harmful emissions of carbon monoxide and partially burned hydrocarbons, which made them a good fit as oxygenates as well. The amount of MTBE used in gasoline kept growing as refiners preferred it to alternative additives because of its economical nature and strong performance as an octane booster and emissions reducer. However, it emerged that while MTBE reduced CO2 and unburned hydrocarbon emissions, it often increased the emissions of nitrogen oxide: also a toxic substance.
The MTBE controversy
There have been a number of studies that have found traces of MTBE in groundwater—some of it a source of drinking water. Further research into the compound has suggested it is safe to use as oxygenate in gasoline and that more data is needed to determine the actual health risks that could be associated with human consumption of MTBE-contaminated water.
In the late 1990s, however, U.S. refiners began phasing out MTBE production and to date, MTBE is produced only in relatively small amounts in the country. As a replacement for MTBE in gasoline, refiners are using ethanol, which is not toxic.
Oxygenates for Diesel
Because of the difference in operation between a gasoline engine and a diesel engine (spark ignition in gasoline engines versus air compression in diesel engines), there are also differences in the oxygenates that can be used for each fuel. While some alcohols and ethers are compatible with diesel as oxygenates, the most compatible compounds for diesel fuel are fatty acid esters.
Fatty acid esters are chemical compounds that are made by combining fatty acids with alcohols. The most well known of this group is biodiesel, which is made from recycled cooking oil, animal fats, and a variety of other oils. The production process involves the removal of glycerine from the fats and oils, which leaves compounds officially known as methyl esters. Related: Why Oil Traders Are Stockpiling Crude
Yet esters tend to have a lower cetane number: the time interval between the start of auto-ignition of the fuel in the chamber and the start of pressure increase during ignition. Heavier ethers and alcohols feature a higher cetane number and could therefore also make good oxygenates for diesel engines.
The general rule with diesel oxygenates is that the longer the alkyl chain in the oxygenating additive, the better it is. The reason for this is that heavier alcohols and ethers are denser than those with lower molecular weight, have a higher boiling point and higher viscosity, and they are less volatile and flammable.
This is also why ethanol is not a good oxygenate fit for diesel fuel, and neither is methanol. Both basic alcohols have low molecular weight, which makes it harder to maintain the state of emulsion that fuel and oxygenate need to be in. What’s more, ethanol compromises some properties of diesel fuel, which constitutes a safety hazard: it lowers the flash point of the fuel and it is also flammable within a wider range of temperatures than other oxygenates.
Unlike with gasoline engines, the biggest emission problem with diesel engines is nitrous oxide and particulate matter. Oxygenates that add just 1 percent oxygen to the diesel fuel can achieve particulate matter reductions of up to 67 percent. If the oxygen content of the fuel is high enough, emissions of particulate matter could be entirely eliminated.
In 1990, after Congress passed a set of amendments to the Clean Air Act, a program for tweaking the contents of gasoline so they are less harmful began. The reformulated gasoline program targeted the most polluted parts of the country and involved almost a third of gasoline supply. The reformulation aimed to reduce the content of volatile organic compounds released as a result of the burning of the fuel, nitrous oxides, and other polluting elements.
To date, the quality of conventional gasoline with added oxygenates is virtually undistinguishable from that of reformulated gasoline largely because of other emissions regulation passed over the last 30 years that has tightened requirements for gasoline quality. Fuel efficiency progress has also helped the convergence in quality between conventional gasoline and the reformulated variety.
Current Oxygenate Statistics
Today, more than 31 million barrels of fuel oxygenates are produced monthly in the United States. Most of this is ethanol. Its average monthly production for 2019 hovered around 30 million barrels. Production of methyl tertiary butyl ether, or MTBE, averaged more than 1.7 million barrels monthly in 2019. Biodiesel production hovered around 150 million gallons monthly in 2019.
Ethanol is by far the most popular and environmentally trendy fuel oxygenate. There is production capacity totalling 1.1 million barrels per day in the country and this has been growing. However, in 2019 this growth began to slow down as the market approached the point of saturation.
Ethanol is being blended with gasoline at a low percentage because for all its advantages such as cleaner burning and lower emissions, the biofuel also causes engine corrosion. However, there is a blend that contains predominantly ethanol: E85, which contains 85 percent ethanol and 15 percent gasoline. With fuel and energy efficiency advancements it is not impossible that in the future oxygenates will no longer be needed because vehicles will run on either electricity or renewable fuel such as ethanol.
By Tom Kool for Oilprice.com
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