Gasoline FAQ


[Document Version: 1.09] [Last Updated: Mar_30_1996]

Chapter 1) Introduction, Intent, Acknowledgements, and Abbreviations

  1.1) About the Author

FAQ: Automotive Gasoline

Author: Bruce Hamilton
Version: 1.09
Last-modified: 30 March 1996
Archive-name: autos/gasoline-faq/part1

  1.2) Introduction and Intent

The intent of this FAQ is to provide some basic information on gasolines and other fuels for spark ignition engines used in automobiles. The toxicity and environmental reasons for recent and planned future changes to gasoline are discussed, along with recent and proposed changes in composition of gasoline. This FAQ is intended to help readers choose the most appropriate fuel for vehicles, assist with the diagnosis of fuel-related problems, and to understand the significance of most gasoline properties listed in fuel specifications. I make no apologies for the fairly heavy emphasis on chemistry; it is the only sensible way to describe the oxidation of hydrocarbon fuels to produce energy, water, and carbon dioxide.

  1.3) Acknowledgements

Thanks go to all the posters in and who spend valuable time responding to questions. I would also like to acknowledge the considerable effort of L.M.Gibbs of Chevron, who has twice spent his valuable time courteously detailing errors and providing references for his corrections. All remaining errors and omissions are mine.

  1.4) Abbreviations

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)

Chapter 2) Table of Contents

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Chapter 3) What Advantage will I gain from reading this FAQ?

This FAQ is intended to provide a fairly technical description of what gasoline contains, how it is specified, and how the properties affect the performance of your vehicle. The regulations governing gasoline have changed, and are continuing to change. These changes have made much of the traditional lore about gasoline obsolete. Motorists may wish to understand a little more about gasoline to ensure they obtain the best value, and the most appropriate fuel for their vehicle. There is no point in prematurely destroying your second most expensive purchase by using unsuitable fuel, just as there is no point in wasting hard-earned money on higher octane fuel that your automobile can not utilize. Note that this FAQ does not discuss the relative advantages of specific brands of gasolines, it is only intended to discuss the generic properties of gasolines.

Chapter 4) What is Gasoline?

  4.1) Where does crude oil come from?

The generally-accepted origin of crude oil is from plant life up to 3 billion years ago, but predominantly from 100 to 600 million years ago [1]. "Dead vegetarian dino dinner" is more correct than "dead dinos". The molecular structure of the hydrocarbons and other compounds present in fossil fuels can be linked to the leaf waxes and other plant molecules of marine and terrestrial plants believed to exist during that era. There are various biogenic marker chemicals ( such as isoprenoids from terpenes, porphyrins and aromatics from natural pigments, pristane and phytane from the hydrolysis of chlorophyll, and normal alkanes from waxes ), whose size and shape can not be explained by known geological processes [2]. The presence of optical activity and the carbon isotopic ratios also indicate a biological origin [3]. There is another hypothesis that suggests crude oil is derived from methane from the earth's interior. The current main proponent of this abiotic theory is Thomas Gold, however abiotic and extraterrestrial origins for fossil fuels were also considered at the turn of the century, and were discarded then. A large amount of additional evidence for the biological origin of crude oil has accumulated since then.

  4.2) When will we run out of crude oil?

It has been estimated that the planet contains over 6.4 x 10^15 tonnes of organic carbon that is cycled through two major cycles, but only about 18% of that contributes to petroleum production. The primary cycle ( turnover of 2.7-3.0 x 10^12 tonnes of organic carbon ) has a half-life of days to decades, whereas the large secondary cycle ( turnover 6.4 x 10^15 tonnes of organic carbon ) has a half-life of several million years [4]. Much of this organic carbon is too dilute or inaccessible for current technology to recover, however the estimates represent centuries to millenia of fossil fuels, even with continued consumption at current or increased rates [5].

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 [6]. 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 [7].

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.
One barrel of Arabian Light crude oil = 0.158987 m3 and 0.136 tonnes.

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 [8].

  4.3) What is the history of gasoline?

In the late 19th Century the most suitable fuels for the automobile were coal tar distillates and the lighter fractions from the distillation of crude oil. During the early 20th Century the oil companies were producing gasoline as a simple distillate from petroleum, but the automotive engines were rapidly being improved and required a more suitable fuel. During the 1910s, laws prohibited the storage of gasolines on residential properties, so Charles F. Kettering ( yes - he of ignition system fame ) modified an IC engine to run on kerosine. However the kerosine-fuelled engine would "knock" and crack the cylinder head and pistons. He assigned Thomas Midgley Jr. to confirm that the cause was from the kerosine droplets vaporising on combustion as they presumed. Midgley demonstrated that the knock was caused by a rapid rise in pressure after ignition, not during preignition as believed [9]. This then lead to the long search for antiknock agents, culminating in tetra ethyl lead [10]. Typical mid-1920s gasolines were 40 - 60 Octane [11].

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.

  4.4) What are the hydrocarbons in gasoline?

Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and carbon, both of which are fuel molecules that can be burnt ( oxidised ) to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is not complete, carbon monoxide ( CO ) may be formed. As CO can be burnt to produce CO2, it is also a fuel.

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 [14].

    4.4.1) Saturated hydrocarbons ( aka paraffins, alkanes )

    4.4.2) Unsaturated Hydrocarbons

  4.5) What are oxygenates?

Oxygenates are just preused hydrocarbons :-). They contain oxygen, which can not provide energy, but their structure provides a reasonable antiknock value, thus they are good substitutes for aromatics, and they may also reduce the smog-forming tendencies of the exhaust gases [15]. Most oxygenates used in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain 1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and MTBE was first used in commercial gasolines in Italy in 1973, and was first used in the US by ARCO in 1979. The relative advantages of aromatics and oxygenates as environmentally-friendly and low toxicity octane-enhancers are still being researched.

Ethanol                              C-C-O-H      C2H5OH

Methyl tertiary butyl ether          C-C-O-C      C4H9OCH3
(aka tertiary butyl methyl ether)      |
They 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 [15]. 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 [16]

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 [16].

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 [16].

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%) [17], 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].

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