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 . 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 .
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 .
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:
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 .
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 28.964419For 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 :
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 :
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 42.5Obviously, 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).
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:-