Gasoline FAQ


    4.10.2) Vapour Lock Protection Classes

5 classes for vapour lock protection, according to location and/or season. The limit is a maximum Vapour-Liquid ratio of 20 at test temperatures of 41, 47, 51, 56, 60C.

    4.10.3) Antiknock Index (aka (RON+MON)/2, "Pump Octane")

The ( Research Octane Number + Motor Octane Number ) divided by two. Limits are not specified, but changes in engine requirements according season and location are discussed. Fuels with an Antiknock index of 87, 89, 91 ( Unleaded), and 88 ( Leaded ) are listed as typical for the US at sea level, however higher altitudes will specify lower octane numbers.

    4.10.4) Lead Content

Leaded = 1.1 g Pb / L maximum, and Unleaded = 0.013 g Pb / L maximum.

    4.10.5) Copper strip corrosion

Ability to tarnish clean copper, indicating the presence of any corrosive sulfur compounds

    4.10.6) Maximum Sulfur content

Sulfur adversely affects exhaust catalysts and fuel hydrocarbon lead response, and also may be emitted as polluting sulfur oxides. Leaded = 0.15 %mass maximum, and Unleaded = 0.10 %mass maximum. Typical US gasoline levels are 0.03 %mass.

    4.10.7) Maximum Solvent Washed Gum (aka Existent Gum)

Limits the amount of gums present in fuel at the time of testing to 5 mg/100mls. The results do not correlate well with actual engine deposits caused by fuel vaporisation [40].

    4.10.8) Minimum Oxidation Stability

This ensures the fuel remains chemically stable, and does not form additional gums during periods in distribution systems, which can be up to 3-6 months. The sample is heated with oxygen inside a pressure vessel, and the delay until significant oxygen uptake is measured.

    4.10.9) Water Tolerance

Highest temperature that causes phase separation of oxygenated fuels. The limits vary according to location and month. For Alaska - North of 62 latitude, it changes from -41C in Dec-Jan to 9C in July, but remains 10C all year in Hawaii.

Because phosphorus adversely affects exhaust catalysts, the EPA limits phosphorus in all gasolines to 0.0013g P/L.

As well as the above, there are various restrictions introduced by the Clean Air Act and state bodies such as California's Air Resources Board (CARB) that often have more stringent limits for the above properties, as well as additional limits. More detailed descriptions of the complex regulations can be found elsewhere [16,41,42] - I've just included some of the major changes, as some properties are determined by levels of permitted emissions, eg the toxics reduction required for fuel that has the maximim permitted benzene (1.0%), means total aromatics are limited to around 27%. There have been some changes in early 1996 to the implementation timetable, and the following timetable has not yet been changed.

The Clean Air Act also specifies some regions that exceed air quality standards have to use reformulated gasolines (RFGs) all year, starting January 1995. Other regions are required to use oxygenated gasolines for four winter months, beginning November 1992. The RFGs also contain oxygenates. Metropolitan regions with severe ozone air quality problems must use reformulated gasolines in 1995 that;- contain at least 2.0 wt% oxygen, reduce 1990 volatile organic carbon compounds by 15%, and reduce specified toxic emissions by 15% (1995) and 25% (2000). Metropolitan regions that exceeded carbon monoxide limits were required to use gasolines with 2.7 wt% oxygen during winter months, starting in 1992.

The 1990 Clean Air Act (CAA) amendments and CARB Phase 2 (1996) specifications for reformulated gasoline establish the following limits, compared with typical 1990 gasoline. Because of a lack of data, the EPA were unable to define the CAA required parameters, so they instituted a two-stage system. The first stage, the "Simple Model" is an interim stage that run from 1/Jan/1995 to 31/Dec/1997. The second stage, the "Complex Model" has two phases, Phase I (1995-1999) and Phase II (2000+), and there are different limits for EPA Control Region 1 (south) and Control Region 2 (north). Each refiner must have their RFG recertified to the Complex model prior to the 1/Jan/1998 implementation date. The following are some of the criteria for RFG when complying on a per gallon basis, more details are available elsewhere, including the details of the baseline fuel compositions to be used for testing [16,41,42,43].

                            1990            Clean Air Act         CARB
                                         Simple    Complex       Phase 2
                                                   I    II    Limit Average 
benzene (max.vol.%)          2           1.00     1.00  1.00   1.00   0.8 
oxygen  (min.mass %)        0.2          2.0      2.0   2.0    1.8     -
        (max.mass %)         -           2.7       -     -     2.2     -
sulfur  (max.mass ppm)     150        no increase  -     -     40     30
aromatics (max.vol.%)       32    toxics reduction -     -     25     22
olefins (max.vol.%)         9.9     no increase    -     -     6.0    4.0
reid vapour pressure (kPa)  60        55.8 (north) -     -    48.3     -
(during VOC Control Period)           49.6 (south)
50% evaporated (max.C)       -            -        -     -    98.9    93
90% evaporated (max.C)     170            -        -     -   148.9   143
VOC Reductions            - Region I    (min.%)  35.1  27.5     -      -
(VOC Control Period only) - Region II   (min.%)  15.6  25.9     -      -
NOx Reductions - VOC Control Period     (min.%)   0     5.5     -      -     
               - Non-VOC Control Period (min.%)   0     0       -      -
Toxics Reductions                       (min.%)  15.0  20.0     -      -
These regulations also specify emissions criteria. eg CAA specifies no increase in nitric oxides (NOx) emissions, reductions in VOC by 15% during the ozone season, and specified toxins by 15% all year. These criteria indirectly establish vapour pressure and composition limits that refiners have to meet. Note that the EPA also can issue CAA Section 211 waivers that allow refiners to choose which oxygenates they use. In 1981, the EPA also decided that fuels with up to 2% weight of oxygen ( from alcohols and ethers (except methanol)) were "substantially similar" to 1974 unleaded gasoline, and thus were not "new" gasoline additives. That level was increased to 2.7 wt% in 1991. Some other oxygenates have also been granted waivers, eg ethanol to 10% volume ( approximately 3.5 wt% ) in 1979 and 1982, and tert-butyl alcohol to 3.5 wt% in 1981. In 1987 and 1988 further waivers were issued for mixture of alcohols representing 3.7% wt of oxygen.

  4.11) What are the effects of the specified fuel properties?

    4.11.1) Volatility

This affects evaporative emissions and driveability, it is the property that must change with location and season. Fuel for mid-summer Arizona would be difficult to use in mid-winter Alaska. The US is divided into zones, according to altitude and seasonal temperatures, and the fuel volatility is adjusted accordingly. Incorrect fuel may result in difficult starting in cold weather, carburetter icing, vapour lock in hot weather, and crankcase oil dilution. Volatility is controlled by distillation and vapour pressure specifications. The higher boiling fractions of the gasoline have significant effects on the emission levels of undesirable hydrocarbons and aldehydes, and a reduction of 40C in the final boiling point will reduce the levels of benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce HC emissions by 20% [44].

    4.11.2) Combustion Characteristics

As gasolines contain mainly hydrocarbons, the only significant variable between different grades is the octane rating of the fuel, as most other properties are similar. Octane is discussed in detail in Section 6. There are only slight differences in combustion temperatures ( most are around 2000C in isobaric adiabatic combustion [45]). Note that the actual temperature in the combustion chamber is also determined by other factors, such as load and engine design. The addition of oxygenates changes the pre-flame reaction pathways, and also reduces the energy content of the fuel. The levels of oxygen in the fuel is regulated according to regional air quality standards.

    4.11.3) Stability

Motor gasolines may be stored up to six months, consequently they must not form gums which may precipitate. Reactions of the unsaturated HCs may produce gums ( these reactions can be catalysed by metals such as copper ), so antioxidants and metal deactivators are added. Existent Gum is used to measure the gum in the fuel at the time tested, whereas the Oxidation Stability measures the time it takes for the gasoline to break down at 100C with 100psi of oxygen. A 240 minute test period has been found to be sufficient for most storage and distribution systems.

    4.11.4) Corrosiveness

Sulfur in the fuel creates corrosion, and when combusted will form corrosive gases that attack the engine, exhaust and environment. Sulfur also adversely affects the alkyl lead octane response, and will adversely affect exhaust catalysts, but monolithic catalysts will recover when the sulfur content of the fuel is reduced, so sulfur is considered an inhibitor, rather than a catalyst poison. The copper strip corrosion test and the sulfur content specification are used to ensure fuel quality. The copper strip test measures active sulfur, whereas the sulfur content reports the total sulfur present.

  4.12) Are brands different?

Yes. The above specifications are intended to ensure minimal quality standards are maintained, however as well as the fuel hydrocarbons, the manufacturers add their own special ingredients to provide additional benefits. A quality gasoline additive package would include:

During the 1980s significant problems with deposits accumulating on intake valve surfaces occurred as new fuel injection systems were introduced. These intake valve deposits (IVD) were different than the injector deposits, in part because the valve can reach 300C. Engine design changes that prevent deposits usually consist of ensuring the valve is flushed with liquid gasoline, and provision of adequate valve rotation. Gasoline factors that cause deposits are the presence of alcohols or olefins [46]. Gasoline manufacturers now routinely use additives that prevent IVD and also maintain the cleanliness of injectors. These usually include a surfactant and light oil to maintain the wetting of important surfaces. Intake valve deposits have also been shown to have significant adverse effects on emissions [47], and deposit deposit control additives will be required to both reduce emissions and provide clean engine operation [48]. A slighty more detailed description of additives is provided in Section 9.1.

Texaco demonstrated that a well-formulated package could improve fuel economy, reduce NOx emissions, and restore engine performance because, as well as the traditional liquid-phase deposit removal, some additives can work in the vapour phase to remove existing engine deposits without adversely affecting performance ( as happens when water is poured into a running engine to remove carbon deposits :-) )[49]. Chevron have also published data on the effectiveness of their additives [50], and successfully litigated to get Texoco to modify some of their claims [51]. Most suppliers of quality gasolines will formulate similar additives into their products, and cheaper product lines are less like to have such additives added. As different brands of gasoline use different additives and oxygenates, it is probable that important fuel parameters, such as octane distribution, are slightly different, even though the pump octane ratings are the same.

So, if you know your car is well-tuned, and in good condition, but the driveability is pathetic on the correct octane, try another brand. Remember that the composition will change with the season, so if you lose driveability, try yet another brand. As various Clean Air Act changes are introduced over the next few years, gasoline will continue to change.

  4.13) What is a typical composition?

There seems to be a perception that all gasolines of one octane grade are chemically similar, and thus general rules can be promulgated about "energy content ", "flame speed", "combustion temperature" etc. etc.. Nothing is further from the truth. The behaviour of manufactured gasolines in octane rating engines can be predicted, using previous octane ratings of special blends intended to determine how a particular refinery stream responds to an octane-enhancing additive. Refiners can design and reconfigure refineries to efficiently produce a wide range of gasolines feedstocks, depending on market and regulatory requirements. There is a worldwide trend to move to unleaded gasolines, followed by the introduction of exhaust catalysts and sophisticated engine management systems.

It is important to note that "oxygenated gasolines" have a hydrocarbon fraction that is not too different to traditional gasolines, but that the hydrocarbon fraction of "reformulated gasolines" ( which also contain oxygenates ) are significantly different to traditional gasolines.

The last 10 years of various compositional changes to gasolines for environmental and health reasons have resulted in fuels that do not follow historical rules, and the regulations mapped out for the next decade also ensure the composition will remain in a state of flux. The reformulated gasoline specifications, especially the 1/Jan/1998 Complex model, will probably introduce major reductions in the distillation range, as well as changing the various limits on composition and emissions.

I'm not going to list all 500+ HCs in gasolines, but the following are representative of the various classes typically present in a gasoline. The numbers after each chemical are:- Research Blending Octane : Motor Blending Octane : Boiling Point (C): Density (g/ml @ 15C) : Minimum Autoignition Temperature (C). It is important to realise that the Blending Octanes are derived from a 20% mix of the HC with a 60:40 iC8:nC7 ( 60 Octane Number ) base fuel, and the extrapolation of this 20% to 100%. These numbers result from API Project 45, and are readily available. As modern refinery streams have higher base octanes, these Blending Octanes are higher than those typically used in modern refineries. For example, modern Blending Octane ratings can be much lower ( toluene = 111RON and 94MON, 2-methyl-2-butene = 113RON and 81MON ), but detailed compilations are difficult to obtain.

The technique for obtaining Blending Octanes is different from rating the pure fuel, which often requires adjustment of the test engine conditions outside the acceptable limits of the rating methods. Generally, the actual octanes of the pure fuel are similar for the alkanes, but are up to 30 octane numbers lower than the API Project 45 Blending Octanes for the aromatics and olefins [52].

A traditional composition I have dreamed up would be like the following, whereas newer oxygenated fuels reduce the aromatics and olefins, narrow the boiling range, and add oxygenates up to about 12-15% to provide the octane. The amount of aromatics in super unleaded fuels will vary greatly from country to country, depending on the configuration of the oil refineries and the use of oxygenates as octane enhancers. The US is reducing the levels of aromatics to 25% or lower for environmental and human health reasons.

Some countries are increasing the level of aromatics to 50% or higher in super unleaded grades, usually to avoid refinery reconfiguration costs or the introduction of oxygenates as they phase out the toxic lead octane enhancers. An upper limit is usually placed on the amount of benzene permitted, as it is known human carcinogen.

15% n-paraffins                       RON   MON    BP      d     AIT  
        n-butane                      113 : 114 :  -0.5:  gas  : 370
        n-pentane                      62 :  66 :  35  : 0.626 : 260
        n-hexane                       19 :  22 :  69  : 0.659 : 225
        n-heptane (0:0 by definition)   0 :   0 :  98  : 0.684 : 225
        n-octane                      -18 : -16 : 126  : 0.703 : 220
     ( you would not want to have the following alkanes in gasoline, 
       so you would never blend kerosine with gasoline )
        n-decane                      -41 : -38 : 174  : 0.730 : 210
        n-dodecane                    -88 : -90 : 216  : 0.750 : 204
        n-tetradecane                 -90 : -99 : 253  : 0.763 : 200
30%  iso-paraffins  
        2-methylpropane               122 : 120 : -12  :  gas  : 460
        2-methylbutane                100 : 104 :  28  : 0.620 : 420
        2-methylpentane                82 :  78 :  62  : 0.653 : 306
        3-methylpentane                86 :  80 :  64  : 0.664 :  -
        2-methylhexane                 40 :  42 :  90  : 0.679 : 
        3-methylhexane                 56 :  57 :  91  : 0.687 :
        2,2-dimethylpentane            89 :  93 :  79  : 0.674 :
        2,2,3-trimethylbutane         112 : 112 :  81  : 0.690 : 420
        2,2,4-trimethylpentane        100 : 100 :  98  : 0.692 : 415
          ( 100:100 by definition )
12% cycloparaffins 
        cyclopentane                  141 : 141 :  50  : 0.751 : 380
        methylcyclopentane            107 :  99 :  72  : 0.749 : 
        cyclohexane                   110 :  97 :  81  : 0.779 : 245
        methylcyclohexane             104 :  84 : 101  : 0.770 : 250
35% aromatics        
        benzene                        98 :  91 :  80  : 0.874 : 560
        toluene                       124 : 112 : 111  : 0.867 : 480
        ethyl benzene                 124 : 107 : 136  : 0.867 : 430
        meta-xylene                   162 : 124 : 138  : 0.868 : 463
        para-xylene                   155 : 126 : 138  : 0.866 : 530
        ortho-xylene                  126 : 102 : 144  : 0.870 : 530
        3-ethyltoluene                162 : 138 : 158  : 0.865 : 
        1,3,5-trimethylbenzene        170 : 136 : 163  : 0.864 : 
        1,2,4-trimethylbenzene        148 : 124 : 168  : 0.889 :
8% olefins               
        2-pentene                     154 : 138 :  37  : 0.649 :
        2-methylbutene-2              176 : 140 :  36  : 0.662 :
        2-methylpentene-2             159 : 148 :  67  : 0.690 :
        cyclopentene                  171 : 126 :  44  : 0.774 :
    ( the following olefins are not present in significant amounts
      in gasoline, but have some of the highest blending octanes )   
        1-methylcyclopentene          184 : 146 :  75  : 0.780 :
        1,3 cyclopentadiene           218 : 149 :  42  : 0.805 :
        dicyclopentadiene             229 : 167 : 170  : 1.071 :
Oxygenates Published octane values vary a lot because the rating conditions are significantly different to standard conditions, for example the API Project 45 numbers used above for the hydrocarbons, reported in 1957, gave MTBE blending RON as 148 and MON as 146, however that was partly based on the lead response, whereas today we use MTBE in place of lead.

        methanol                      133 : 105 :  65  : 0.796 : 385
        ethanol                       129 : 102 :  78  : 0.794 : 365
        iso propyl alcohol            118 :  98 :  82  : 0.790 : 399
        methyl tertiary butyl ether   116 : 103 :  55  : 0.745 : 
        ethyl tertiary butyl ether    118 : 102 :  72  : 0.745 :
        tertiary amyl methyl ether    111 :  98 :  86  : 0.776 : 
There are some other properties of oxygenates that have to be considered when they are going to be used as fuels, particularly their ability to form very volatile azeotropes that cause the fuel's vapour pressure to increase, the chemical nature of the emissions, and their tendency to separate into a separate water-oxygenate phase when water is present. The reformulated gasolines address these problems more successfully than the original oxygenated gasolines.

Before you rush out to make a highly aromatic or olefinic gasoline to produce a high octane fuel, remember they have other adverse properties, eg the aromatics attack elastomers, may generate smoke, and result in increased emissions of toxic benzene. The olefins are unstable ( besides smelling foul ) and form gums. The art of correctly formulating a gasoline that does not cause engines to knock apart, does not cause vapour lock in summer - but is easy to start in winter, does not form gums and deposits, burns cleanly without soot or residues, and does not dissolve or poison the car catalyst or owner, is based on knowledge of the gasoline composition.

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