Author: Bruce Hamilton
Last-modified: 30 March 1996
AKI = Antiknock Index of Gasoline ( (RON+MON)/2 ) CI = Compression Ignition ( Diesel ) Gasoline = Petrol ( Yes, complaints were received :-) ) IC = Internal Combustion MON = Motor Octane Rating Octane = The Octane Rating of the Gasoline RFG = Reformulated Gasoline ( as defined by US Clean Air Act ) RON = Research Octane Rating SI = Spark Ignition (Gasoline)
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The concern about "running out of oil" arises from misunderstanding the significance of a petroleum industry measure called the Reserves/Production ratio (R/P). This monitors the production and exploration interactions. The R/P is based on the concept of "proved" reserves of fossil fuels. Proved reserves are those quantities of fossil fuels that geological and engineering information indicate with reasonable certainty can be recovered in the future from known reservoirs under existing economic and operating conditions. The Reserves/Production ratio is the proved reserves quantity divided by the production in the last year, and the result will be the length of time that those remaining proved reserves would last if production were to continue at the current level . It is important to note the economic and technology component of the definitions, as the price of oil increases ( or new technology becomes available ), marginal fields become "proved reserves". We are unlikely to "run out" of oil, as more fields become economic. Note that investment in exploration is also linked to the R/P ratio, and the world crude oil R/P ratio typically moves between 20-40 years, however specific national incentives to discover oil can extend that range upward.
Concerned people often refer to the " Hubbert curves" that predict fossil fuel discovery rates would peak and decline rapidly. M. King Hubbert calculated in 1982 that the ultimate resource base of the lower 48 states of the USA was 163+-2 billion barrels of oil, and the ultimate production of natural gas to be 24.6+-0.8 trillion cubic metres, with some additional qualifiers. As production and proved resources were 147 billion barrels of oil and 22.5 trillion cubic metres of gas, Hubbert was implying that volumes yet to be developed could only be 16-49 billion barrels of oil and 2.1-4.5 trillion cubic metres. Technology has confounded those predictions [6a]. The US Geological Survey has also just increased their assessment of US ( not just the lower 48 states ),inferred reserves crude oil by 60 billion barrels, and doubled the size of gas reserves to 9.1 trillion cubic metres. When combined with the estimate of undiscovered oil and gas, the totals reach 110 billion barrels of oil and 30 trillion cubic metres of gas .
The current price for Brent Crude is approx. $18/bbl. The world R/P ratio has increased from 27 years (1979) to 43.1 years (1993). The 1994 BP Statistical Review of World Energy provides the following data [6,7].
Crude Oil Proved Reserves R/ Ratio Middle East 89.6 billion tonnes 95.1 year USA 4.0 9.9 years USA - 1995 USGS data 10.9 33.0 years Total World 136.7 43.1 years<> Coal Proved Reserves R/ Ratio USA 240.56 billion tonnes 267 years Total World 1,039.182 236 years<> Natural Gas Proved Reserves R/ Ratio USA 4.7 trillion cubic metres 8.8 years USA - 1995 USGS data 9.1 17.0 years Total World 142.0 64.9 years.<>One billion = 1 x 10^9. One trillion = 1 x 10^12.
If the crude oil price exceeds $30/bbl then alternative fuels may become competitive, and at $50-60/bbl coal-derived liquid fuels are economic, as are many biomass-derived fuels and other energy sources .
Because sulfur in gasoline inhibited the octane-enhancing effect of the alkyl lead, the sulfur content of the thermally-cracked refinery streams for gasolines was restricted. By the 1930s, the petroleum industry had determined that the larger hydrocarbon molecules (kerosine) had major adverse effects on the octane of gasoline, and were developing consistent specifications for desired properties. By the 1940s catalytic cracking was introduced, and gasoline compositions became fairly consistent between brands during the various seasons.
The 1950s saw the start of the increase of the compression ratio, requiring higher octane fuels. Octane ratings, lead levels, and vapour pressure increased, whereas sulfur content and olefins decreased. Some new refining processes ( such as hydrocracking ), specifically designed to provide hydrocarbons components with good lead response and octane, were introduced. Minor improvements were made to gasoline formulations to improve yields and octane until the 1970s - when unleaded fuels were introduced to protect the exhaust catalysts that were also being introduced for environmental reasons. From 1970 until 1990 gasolines were slowly changed as lead was phased out, lead levels plummetted, octanes initially decreased, and then remained 2-5 numbers lower, vapour pressures continued to increase, and sulfur and olefins remained constant, while aromatics increased. In 1990, the US Clean Air Act started forcing major compositional changes on gasoline, resulting in plummeting vapour pressure and increaing oxygenate levels. These changes will continue into the 21st Century, because gasoline is a major pollution source. More comprehensive descriptions of the changes to gasolines this century have been provided by L.M.Gibbs [12,13].
The move to unleaded fuels continues worldwide, however several countries have increased the aromatics content ( up to 50% ) to replace the alkyl lead octane enhancers. These highly aromatic gasolines can result in in damage to elastomers and increased levels of toxic aromatic emissions if used without exhaust catalysts.
The way the hydrogen and carbons hold hands determines which hydrocarbon family they belong to. If they only hold one hand they are called "saturated hydrocarbons" because they can not absorb additional hydrogen. If the carbons hold two hands they are called "unsaturated hydrocarbons" because they can be converted into "saturated hydrocarbons" by the addition of hydrogen to the double bond. Hydrogens are omitted from the following, but if you remember C = 4 hands, H = 1 hand, and O = 2 hands, you can draw the full structures of most HCs.
Gasoline contains over 500 hydrocarbons that may have between 3 to 12 carbons, and gasoline used to have a boiling range from 30C to 220C at atmospheric pressure. The boiling range is narrowing as the initial boiling point is increasing, and the final boiling point is decreasing, both changes are for environmental reasons. Detailed descriptions of structures can be found in any chemical or petroleum text discussing gasolines .
normal heptane C-C-C-C-C-C-C C7H16
C C (aka 2,2,4-trimethylpentane) | | C-C-C-C-C C8H18 | C
C / \ C C | | C6H12 C C \ / C
C | C5H10 2-methyl-2-butene C-C=C-C
_ Acetylene C=C C2H2
C C // \ // \ C C C-C C Benzene | || Toluene | || C C C C \\ / \\ / C C<> C6H6 C7H8
C C // \ / \\ C C C Naphthalene | || | C10H8 C C C \\ / \ // C C
Ethanol C-C-O-H C2H5OH C | Methyl tertiary butyl ether C-C-O-C C4H9OCH3 (aka tertiary butyl methyl ether) | CThey can be produced from fossil fuels eg methanol (MeOH), methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). MTBE is produced by reacting methanol ( from natural gas ) with isobutylene in the liquid phase over an acidic ion-exchange resin catalyst at 100C. The isobutylene was initially from refinery catalytic crackers or petrochemical olefin plants, but these days larger plants produce it from butanes. MTBE production has increased at the rate of 10 to 20% per year, and the spot market price in June 1993 was around $270/tonne . The "ether" starting fluids for vehicles are usually diethyl ether (liquid) or dimethyl ether (aerosol). Note that " petroleum ethers " are volatile alkane hydrocarbon fractions, they are not a Cx-O-Cy compound.
Oxygenates are added to gasolines to reduce the reactivity of emissions, but they are only effective if the hydrocarbon fractions are carefully modified to utilise the octane and volatility properties of the oxygenates. If the hydrocarbon fraction is not correctly modified, oxygenates can increase the undesirable smog-forming and toxic emissions. Oxygenates do not necessarily reduce all exhaust toxins, nor are they intended to.
Oxygenates have significantly different physical properties to hydrocarbons, and the levels that can be added to gasolines are controlled by the 1977 Clean Air Act amendments in the US, with the laws prohibiting the increase or introduction of a fuel or fuel additive that is not substantially similar to any fuel or fuel additive used to certify 1975 or subsequent years vehicles. Waivers can granted if the product does not cause or contribute to emission device failures, and if the EPA does not specifically decline the application after 180 days, it is taken as granted. In 1978 the EPA granted 10% by volume of ethanol a waiver, and have subsequently issued waivers for <10 vol% ethanol (1982), 7 vol% tertiary butyl alcohol (1979), 5.5 vol% 1:1 MeOH/TBA (1979), 3.5 mass% oxygen derived from 1:1 MeOH/TBA = ~9.5 vol% of the alcohols (1981), 3.7 mass% oxygen derived from methanol and cosolvents = 5 vol% max MeOH and 2.5 vol% min cosolvent - with some cosolvents requiring additional corrosion inhibitor (1985,1988), 7.0 vol% MTBE (1979), and 15.0 vol% MTBE (1988). Only the ethanol waiver was exempted from the requirement to still meet ASTM volatility requirements 
In 1981 the EPA ruled that fuels could contain aliphatic alcohols ( except MeOH ) and/or ethers at concentrations until the oxygen content is 2.0 mass%. It also permitted 5.5 vol% of 1:1 MeOH/TBA. In 1991 the maximum oxygen content was increased to 2.7 mass%. To ensure sufficient gasoline base was available for ethanol blending, the EPA also ruled that gasoline containing up to 2 vol% of MTBE could subsequently be blended with 10 vol% of ethanol .
Initially, the oxygenates were added to hydrocarbon fractions that were slightly-modified unleaded gasoline fractions, and these were known as "oxygenated" gasolines. In 1995, the hydrocarbon fraction was significantly modified, and these gasolines are called "reformulated gasolines" ( RFGs ), and there are differing specifications for California ( Phase 2 ) and Federal ( simple model ) RFGs, however both require oxygenates to provide Octane. The California RFG requires the hydrocarbon composition of the RFG to be significantly more modified than the existing oxygenated gasolines to reduce unsaturates, volatility, benzene, and the reactivity of emissions. Federal regulations only reduce vapour pressure and benzene directly, however aromatics are also reduced to meet emissions criteria .
Oxygenates that are added to gasoline function in two ways. Firstly they have high blending octane, and so can replace high octane aromatics in the fuel. These aromatics are responsible for disproportionate amounts of CO and HC exhaust emissions. This is called the "aromatic substitution effect". Oxygenates also cause engines without sophisticated engine management systems to move to the lean side of stoichiometry, thus reducing emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can reduce HC by 10%) , and other researchers have observed similar reductions also occur when oxygenates are added to reformulated gasolines on older and newer vehicles, but have also shown that NOx levels may increase, as also may some regulated toxins [18,19,20].
However, on vehicles with engine management systems, the fuel volume will be increased to bring the stoichiometry back to the preferred optimum setting. Oxygen in the fuel can not contribute energy, consequently the fuel has less energy content. For the same efficiency and power output, more fuel has to be burnt, and the slight improvements in combustion efficiency that oxygenates provide on some engines usually do not completely compensate for the oxygen.
There are huge number of chemical mechanisms involved in the pre-flame reactions of gasoline combustion. Although both alkyl leads and oxygenates are effective at suppressing knock, the chemical modes through which they act are entirely different. MTBE works by retarding the progress of the low temperature or cool-flame reactions, consuming radical species, particularly OH radicals and producing isobutene. The isobutene in turn consumes additional OH radicals and produces unreactive, resonantly stabilised radicals such as allyl and methyl allyl, as well as stable species such as allene, which resist further oxidation [21,22].