Gasoline FAQ

Contents:


  4.6) Why were alkyl lead compounds added?

The efficiency of a spark-ignited gasoline engine can be related to the compression ratio up to at least compression ratio 17:1 [23]. However any "knock" caused by the fuel will rapidly mechanically destroy an engine, and General Motors was having major problems trying to improve engines without inducing knock. The problem was to identify economic additives that could be added to gasoline or kerosine to prevent knock, as it was apparent that engine development was being hindered. The kerosine for home fuels soon became a secondary issue, as the magnitude of the automotive knock problem increased throughout the 1910s, and so more resources were poured into the quest for an effective "antiknock". A higher octane aviation gasoline was required urgently once the US entered WWI, and almost every possible chemical ( including melted butter ) was tested for antiknock ability [24].

Originally, iodine was the best antiknock available, but was not a practical gasoline additive, and was used as the benchmark. In 1919 aniline was found to have superior antiknock ability to iodine, but also was not a practical additive, however aniline became the benchmark antiknock, and various compounds were compared to it. The discovery of tetra ethyl lead, and the scavengers required to remove it from the engine were made by teams lead by Thomas Midgley Jr. in 1922 [9,10,24]. They tried selenium oxychloride which was an excellent antiknock, however it reacted with iron and "dissolved" the engine. Midgley was able to predict that other organometallics would work, and slowly focused on organoleads. They then had to remove the lead, which would otherwise accumulate and coat the engine and exhaust system with lead. They discovered and developed the halogenated lead scavengers that are still used in leaded fuels. The scavengers, ( ethylene dibromide and ethylene dichloride ), function by providing halogen atoms that react with the lead to form volatile lead halide salts that can escape out the exhaust. The quantity of scavengers added to the alkyl lead concentrate is calculated according to the amount of lead present. If sufficient scavenger is added to theoretically react with all the lead present, the amount is called one "theory". Typically, 1.0 to 1.5 theories are used, but aviation gasolines must only use one theory. This ensures there is no excess bromine that could react with the engine.

The alkyl leads rapidly became the most cost-effective method of enhancing octane. The introduction was not universally acclaimed, as the toxicity of TEL soon became apparent, and several eminent public health officials campaigned against the widespread introduction of alkyl leads [25]. Their cause was assisted by some major disasters at TEL manufacturing plants, and although these incidents were mainly attributable to a failure of management and/or staff to follow instructions, they resulted in a protracted dispute in the chemical and public health literature that even involved Midgley [25,26]. We should be careful retrospectively applying judgement to the 1920s, as the increased octane of leaded gasoline provided major gains in engine efficiency and lower gasoline prices.

The development of the alkyl leads ( tetra methyl lead, tetra ethyl lead ) and the toxic halogenated scavengers meant that petroleum refiners could then configure refineries to produce hydrocarbon streams that would increase octane with small quantities of alkyl lead. If you keep adding alkyl lead compounds, the lead response of the gasoline decreases, and so there are economic limits to how much lead should be added.

Up until the late 1960s, alkyl leads were added to gasolines in increasing concentrations to obtain octane. The limit was 1.14g Pb/l, which is well above the diminishing returns part of the lead response curve for most refinery streams, thus it is unlikely that much fuel was ever made at that level. I believe 1.05 was about the maximum, and articles suggest that 1970 100 RON premiums were about 0.7-0.8 g Pb/l and 94 RON regulars 0.6-0.7 g Pb/l, which matches published lead response data [27,28] eg.


For             Catalytic Reformate           Straight Run Naphtha.
Lead g/l                    Research Octane Number
   0                   96                           72
  0.1                  98                           79
  0.2                  99                           83
  0.3                 100                           85
  0.4                 101                           87
  0.5                 101.5                         88
  0.6                 102                           89
  0.7                 102.5                         89.5
  0.8                 102.75                        90<>
The alkyl lead antiknocks work in a different stage of the pre-combustion reaction to oxygenates. In contrast to oxygenates, the alkyl lead interferes with hydrocarbon chain branching in the intermediate temperature range where HO2 is the most important radical species. Lead oxide, either as solid particles, or in the gas phase, reacts with HO2 and removes it from the available radical pool, thereby deactivating the major chain branching reaction sequence that results in undesirable, easily-autoignitable hydrocarbons [21,22].

By the 1960s, the nature the toxicity of the emissions from gasoline-powered engines was becoming of increasing concern and extensive comparisons of the costs and benefits were being performed. By the 1970s, the failure to find durable, lead-tolerant exhaust catalysts would hasten the departure of lead, as the proposed regulated emissions levels could not be economically achieved without exhaust catalysts [29].


  4.7) Why not use other organometallic compounds?

As the toxicity of the alkyl lead and the halogenated scavengers became of concern, alternatives were considered. The most famous of these is methylcyclopentadienyl manganese tricarbonyl (MMT), which was used in the USA until banned by the EPA from 27 Oct 1978 [30], but is approved for use in Canada and Australia. Recently the EPA ban was overturned, and MMT can be used up to 0.031gMn/US Gal in all states except California ( where it remains banned ). The EPA has stated it intends to review the whole MMT siuation and , if evidence supports removing MMT, they will revisit banning MMT. Automobile manufacturers believe MMT reduces the effectiveness of the latest emission control systems [31]. Canada also contemplated banning MMT because of the same concerns, as well as achieving fuel supply uniformity with the lower 48 states of the USA [31]. MMT is more expensive than alkyl leads and has been reported to increase unburned hydrocarbon emissions and block exhaust catalysts [32].

Other compounds that enhance octane have been suggested, but usually have significant problems such as toxicity, cost, increased engine wear etc.. Examples include dicyclopentadienyl iron and nickel carbonyl. Germany used iron pentacarbonyl (Fe(CO)5) at levels of 0.5% or less in gasoline during the 1930s. While its cost was low, one of its major drawbacks was that the carbonyl decomposed rapidly when the gasoline was exposed to light. Iron oxide (Fe3O4) also deposited on the spark plug insulator causing short circuits, and the precipitation of iron oxides in the lubricating oil also led to excessive wear rates [33].


  4.8) What do the refining processes do?

Crude oil contains a wide range of hydrocarbons, organometallics and other compounds containing sulfur, nitrogen etc. The HCs contain between 1 and 60 carbon atoms. Gasoline requires hydrocarbons with carbon atoms between 3 and 12, arranged in specific ways to provide the desirable properties. Obviously, a refinery has to either sell the remainder as marketable products, or convert the larger molecules into smaller gasoline molecules.

A refinery will distill crude oil into various fractions and, depending on the desired final products, will further process and blend those fractions. Typical final products could be:- gases for chemical synthesis and fuel (CNG), liquified gases (LPG), butane, aviation and automotive gasolines, aviation and lighting kerosines, diesels, distillate and residual fuel oils, lubricating oil base grades, paraffin oils and waxes. Many of the common processes are intended to increase the yield of blending feedstocks for gasolines.

Typical modern refinery processes for gasoline components include:

The changes to the US Clean Air Act and other legislation ensures that the refineries will continue to modify their processes to produce a less volatile gasoline with fewer toxins and toxic emissions. Options include:

Some other countries are removing the alkyl lead compounds for health reasons, and replacing them with aromatics and oxygenates. If the vehicle fleet does not have exhaust catalysts, the emissions of some toxic aromatic hydrocarbons can increase. If maximum environmental and health gains are to be achieved, the removal of lead from gasoline should be accompanied by the immediate introduction of exhaust catalysts and sophisticated engine management systems,


  4.9) What energy is released when gasoline is burned?

It is important to note that the theoretical energy content of gasoline when burned in air is only related to the hydrogen and carbon contents. The energy is released when the hydrogen and carbon are oxidised (burnt), to form water and carbon dioxide. Octane rating is not fundamentally related to the energy content, and the actual hydrocarbon and oxygenate components used in the gasoline will determine both the energy release and the antiknock rating.

Two important reactions are:

C + O2 = CO2
H + O2 = H2O

The mass or volume of air required to provide sufficient oxygen to achieve this complete combustion is the "stoichiometric" mass or volume of air. Insufficient air = "rich", and excess air = "lean", and the stoichiometric mass of air is related to the carbon:hydrogen ratio of the fuel. The procedures for calculation of stoichiometric air-fuel ratios are fully documented in an SAE standard [35].

Atomic masses used are:- Hydrogen = 1.00794, Carbon = 12.011, Oxygen = 15.994, Nitrogen = 14.0067, and Sulfur = 32.066.


The composition of sea level air ( 1976 data, hence low CO2 value ) is
Gas            Fractional      Molecular Weight         Relative 
Species          Volume            kg/mole                Mass
N2              0.78084             28.0134             21.873983
O2              0.209476            31.9988              6.702981
Ar              0.00934             39.948               0.373114
CO2             0.000314            44.0098              0.013919
Ne              0.00001818          20.179               0.000365
He              0.00000524           4.002602            0.000021
Kr              0.00000114          83.80                0.000092
Xe              0.000000087        131.29                0.000011
CH4             0.000002            16.04276             0.000032
H2              0.0000005            2.01588             0.000001
                                                        ---------
Air                                                     28.964419
For normal heptane C7H16 with a molecular weight = 100.204

C7H16 + 11O2 = 7CO2 + 8H2O

thus 1.000 kg of C7H16 requires 3.513 kg of O2 = 15.179 kg of air.

The chemical stoichiometric combustion of hydrocarbons with oxygen can be written as:

CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O

Often, for simplicity, the remainder of air is assumed to be nitrogen, which can be added to the equation when exhaust compositions are required. As a general rule, maximum power is achieved at slightly rich, whereas maximum fuel economy is achieved at slightly lean.

The energy content of the gasoline is measured by burning all the fuel inside a bomb calorimeter and measuring the temperature increase. The energy available depends on what happens to the water produced from the combustion of the hydrogen. If the water remains as a gas, then it cannot release the heat of vaporisation, thus producing the Nett Calorific Value. If the water were condensed back to the original fuel temperature, then Gross Calorific Value of the fuel, which will be larger, is obtained.

The calorific values are fairly constant for families of HCs, which is not surprising, given their fairly consistent carbon:hydrogen ratios. For liquid ( l ) or gaseous ( g ) fuel converted to gaseous products - except for the 2-methylbutene-2, where only gaseous is reported. * = Blending Octane Number as reported by API Project 45 using 60 octane base fuel, and the numbers in brackets are Blending Octane Numbers currently used for modern fuels. Typical Heats of Combustion are [36]:


Fuel     State  Heat of Combustion      Research        Motor
                    MJ/kg                Octane         Octane	
n-heptane  l        44.592                  0              0
           g        44.955
i-octane   l        44.374                100            100
           g        44.682
toluene    l        40.554                124* (111)     112*  (94)
           g        40.967                
2-methylbutene-2    44.720                176* (113)     141*  (81)
Because all the data is available, the calorific value of fuels can be estimated quite accurately from hydrocarbon fuel properties such as the density, sulfur content, and aniline point ( which indicates the aromatics content ).

It should be noted that because oxygenates contain oxygen that can not provide energy, they will have significantly lower energy contents. They are added to provide octane, not energy. For an engine that can be optimised for oxygenates, more fuel is required to obtain the same power, but they can burn slightly more efficiently, thus the power ratio is not identical to the energy content ratio. They also require more energy to vaporise.


            Energy Content   Heat of Vaporisation   Oxygen Content    
              Nett MJ/kg          MJ/kg                   wt%
Methanol        19.95             1.154                  49.9
Ethanol         26.68             0.913                  34.7
MTBE            35.18             0.322                  18.2
ETBE            36.29             0.310                  15.7
TAME            36.28             0.323                  15.7
Gasoline       42 - 44            0.297                   0.0<>
Typical values for commercial fuels in megajoules/kilogram are [37]:


                                Gross        Nett      
Hydrogen                        141.9       120.0
Carbon to Carbon monoxide        10.2          -
Carbon to Carbon dioxide         32.8          -
Sulfur to sulfur dioxide          9.16         -
Natural Gas                      53.1         48.0
Liquified petroleum gas          49.8         46.1
Aviation gasoline                46.0         44.0
Automotive gasoline              45.8         43.8
Kerosine                         46.3         43.3
Diesel                           45.3         42.5
Obviously, for automobiles, the nett calorific value is appropriate, as the water is emitted as vapour. The engine can not utilise the additional energy available when the steam is condensed back to water. The calorific value is the maximum energy that can be obtained from the fuel by combustion, but the reality of modern SI engines is that thermal efficiencies of only 20-40% may be obtained, this limit being due to engineering and material constraints that prevent optimum thermal conditions being used. CI engines can achieve higher thermal efficiencies, usually over a wider operating range as well. Note that combustion efficiencies are high, it is the thermal efficiency of the engine is low due to losses. For a water-cooled SI engine with 25% useful work at the crankshaft, the losses may consist of 35% (coolant), 33% (exhaust), and 12% (surroundings).


  4.10) What are the gasoline specifications?

Gasolines are usually defined by government regulation, where properties and test methods are clearly defined. In the US, several government and state bodies can specify gasoline properties, and they may choose to use or modify consensus minimum quality standards, such as American Society for Testing Materials (ASTM). The US gasoline specifications and test methods are listed in several readily available publications, including the Society of Automotive Engineers (SAE) [38], and the Annual Book of ASTM Standards [39].

The 1995 ASTM edition includes:

D4814-94d Specification for Automotive Spark-Ignition Engine Fuel. This specification lists various properties that all fuels have to comply with, and may be updated throughout the year. Typical properties are:-


    4.10.1) Vapour Pressure and Distillation Classes.

6 different classes according to location and/or season. As gasoline is distilled, the temperatures at which various fractions are evaporated are calculated. Specifications define the temperatures at which various percentages of the fuel are evaporated. Distillation limits include maximum temperatures that 10% is evaporated (50-70C), 50% is evaporated (110-121C), 90% is evaporated (185-190C), and the final boiling point (225C). A minimum temperature for 50% evaporated (77C), and a maximum amount of Residue (2%) after distillation. Vapour pressure limits for each class ( 54, 62, 69, 79, 93, 103 kPa ) are also specified. Note that the EPA has issued a waiver that does not require gasoline with 9-10% ethanol to meet the required specifications between 1st May - 15 September.


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