Anantiknock agent (orknock inhibitor[1]) is agasoline additive used to reduceengine knocking and increase the fuel'soctane rating by raising the temperature and pressure at which auto-ignition occurs. The mixture known asgasoline or petrol, when used in highcompression internal combustion engines, has a tendency toknock (also called "pinging" or "pinking") and/or to ignite early before the correctly timed spark occurs (pre-ignition, refer toengine knocking).
Notable early antiknock agents, especiallytetraethyllead, added to gasoline included large amounts oftoxic lead.[2][3] The chemical was responsible for global negative impacts on health, and the phase out of leaded gasoline from the 1970s onward was reported by the United Nations Environmental Programme to be responsible for "$2.4 trillion in annual benefits, 1.2 million fewer premature deaths, higher overall intelligence and 58 million fewer crimes".[2][3] Some other chemicals used as gasoline additives are thought to be less toxic.
In Europe, after the earlier ban of lead antiknock additives, an effective ban of other metal-contained additives (MFA, metallic fuel additives) was established in 2004 with a specific limit formethylcyclopentadienyl manganese tricarbonyl (MMT) of 6 mg/l from 2011, lowered to 2 mg/l in 2014. In recent years no use of MMT has been reported in the annual monitoring reports under the European fuel quality directive. By the end of the 1990s, North American refineries also voluntary stopped the use of MMT in petrol, since less contested antiknock additives and compounds, such asMTBE andethanol, were available.[4]
Early research was led by A. H. Gibson andHarry Ricardo in England andThomas Midgley Jr. and Thomas Boyd in the United States. The discovery thatlead additives modified this behavior led to the widespread adoption of the practice in the 1920s and therefore more powerful higher compression engines. The most popular additive wastetraethyllead. However, with the discovery of the environmental and health damage caused by the lead, attributed toDerek Bryce-Smith andClair Cameron Patterson, and the incompatibility of lead withcatalytic converters found on virtually all US automobiles since 1975, this practice began to wane in the 1980s. Most countries are phasing out leaded fuel although different additives still contain lead compounds. Other additives includearomatic hydrocarbons,ethers andalcohol (usuallyethanol ormethanol).
Typical agents that have been used for their antiknock properties are:
In the U.S., wheretetraethyllead had been blended with gasoline (primarily to boost octane levels) since the early 1920s, standards to phase out leaded gasoline were first implemented in 1973. In 1995, leaded fuel accounted for only 0.6% of total gasoline sales and less than 2,000 tons of lead per year. From January 1, 1996, theClean Air Act banned the sale of leaded fuel for use in on-road vehicles in the United States. Possession and use of leaded gasoline in a regular on-road vehicle now carries a maximum US$10,000 fine in the United States. However, fuel containing lead may continue to be sold for off-road uses, including aircraft, racing cars, farm equipment, and marine engines. The ban on leaded gasoline led to thousands fewer tons of lead being released into the air by automobiles.
Similar bans in other countries have resulted in sharply decreasing levels of lead in people'sbloodstreams.[6][7]
A side effect of the lead additives was protection of thevalve seats from erosion. Manyclassic cars' engines have needed modification to use lead-free fuels since leaded fuels became unavailable. However, "lead substitute" products are also produced and can sometimes[when?] be found at auto parts stores[example needed].[citation needed]
Gasoline, as delivered at the pump, also contains additives to reduce internal engine carbon buildups, improvecombustion, and to allow easier starting in cold climates.
In some parts of South America, Asia, and the Middle East, leaded gasoline is still in use. Leaded gasoline was phased out insub-Saharan Africa, starting 1 January 2006. A growing number of countries have drawn up plans to ban leaded gasoline in the near future.
Some expertsspeculate that leaded petrol was behind a global crime wave in the late 1980s and early 1990s.[8]
To avoid deposits of lead inside the engine, lead scavengers are added to the gasoline together with tetraethyllead. The most common ones are:
As tetraethyllead use declined, industry had to decide how to make up the octane deficit between the principal marketable light fuels produced by their refineries, and the higher octane fuels needed for high-compression gasoline engines in the automobile fleet. Around 70% of the difference was accommodated by more advanced processes at the refinery stage, cracking other hydrocarbon products from the distillation stack to modify them into fuels that would blend gasoline closer the appropriate octane. Most of the rest of the octane deficit required chemical additives not derived from the refinery process. Tetraethyl lead was largely replaced in the US withmethyl tert-butyl ether starting in 1979. MTBE is a toxic water pollutant, and aseries of groundwater contamination scandals starting in the 90's prompted the EPA to begin phasing MTBE out in 2000.[9]
MTBE'swater pollution issues prompted plans for a phaseout, starting in 2000 with an EPA draft proposal, which was addressed several times at the state level in the years to follow, and eventually cemented in place federally with a 9-year phaseout in 2005's Energy Policy Act, with significant proportions of fuel ethanol designated as the replacement antiknock agent for the US automotive fuel system. Congress' attempts to promote ethanol for its geopolitical use as a backstop on any attempts to limit the US' gasoline supply, and also its incentives to reward Iowan corn farmers, whose state political primaries hold a special place in the electoral system, escalated ethanol from an additive to be used as needed, then to a fixed blending proportion of 5%, and then 10%, which is today themost common US fuel blend.[10][11]
Ethanol has several issues as an antiknock additive. It is hydrophilic, pulling water vapor out of moist air, and it also increases the level of free oxygen in the fuel significantly. Both of these cause significant degradation to traditionally constructed engines, posing both residue and corrosion issues in increasing proportion with increasing fractions of ethanol. Whereas age-degraded gasoline may simply polymerize, evaporate, and thus lose its flammability, age-degraded gasoline-ethanol blends can cause severe damage if allowed to sit in an engine. Automotive engines addressed this with the mandated shift over to ethanol-tolerant metals and seals, and with the use of smart electronic fuel injection, which has some flexibility to adjust combustion properties and timing. Automotive engines did not see major issues because of these factors, and because automobiles in active use typically cycle through their gas tank in a matter of weeks. In smallcarburetor engines, like generators and lawnmowers, ethanol damage became the dominant mode of failure.
Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used for many years in Canada and recently in Australia to boost octane ratings. It also allows old cars, designed to use leaded fuel, to run on unleaded fuel without the need for additives to prevent valve stem erosion.
A large Canadian study from 2002 (funded by automakers, who are against its use) concluded that MMT impairs the effectiveness of automobile emission controls and increases pollution from motor vehicles. However, a later study by the Canadian government found that "no Notice of Defect was found to be potentially caused by MMT."[12]
Many studies have been undertaken over time that confirmed the use of MMT is compatible with vehicles and safe for human health and the environment. In particular, a2013 risk assessment on MMT was undertaken by ARCADIS Consulting, following a methodology developed by the European Commission. This risk assessment was verified by an independent panel and found by the EU Commission to be compliant with their methodology. It concluded that "when MMT is used as a fuel additive in petrol, no significant human health or environmental concerns related to exposure to either MMT or its transformation [combustion] products (manganese phosphate, manganese sulphate and manganese tetroxide) were identified even in locations where MMT is approved for use at levels up to 18 mg Mn/L."[13]
As stated by Health Canada in their risk assessment on the widespread use of MMT in Canadian gasoline, "all analyses indicate that the combustion products of MMT in gasoline do not represent an added health risk to the Canadian population"[14]
MMT is manufactured by reduction of bis(methylcyclopentadienyl) manganese using triethylaluminium. The reduction is conducted under an atmosphere of carbon monoxide. MMT is a so-called half-sandwich compound, or more specifically a piano-stool complex (since the three CO ligands are like the legs of a piano stool). The manganese atom in MMT is coordinated with three carbonyl groups as well as to the methylcyclopentadienyl ring. These hydrophobic organic ligands make MMT highly lipophilic, which may increasebioaccumulation. While the structure of MMT suggests lipophilicity and potential to bioaccumulate, comparison of bioconcentration factors (BCF) reported for plant and animal species in comparison to regulatory-based cutoffs (i.e., US EPA and EU REACH) indicates a low bioaccumulative potential of MMT. Figures 2 and 3 of the study (pages 182 & 184) shows the BCF plotted against time and illustrates the potential BCF of MMT. From these figures, the upper curve (A) demonstrates the 9-day MMT BCF plateauing at approximately 400 in plants and 200 in fish, with both values well below the Bioaccumulative / Very Bioaccumulative (B/vB) thresholds of US EPA, EU REACH and Environment & Climate Change Canada.[15]
A variety of related complexes are known, including ferrocene, which is also under consideration as an additive to gasoline.
Ferrocene is theorganometallic compound with the formula Fe(C5H5)2. It is the prototypicalmetallocene, a type oforganometallicchemical compound consisting of twocyclopentadienyl rings bound on opposite sides of a centralmetal atom. Such organometallic compounds are also known assandwich compounds.[16] The rapid growth oforganometallic chemistry is often attributed to the excitement arising from the discovery of ferrocene and its many analogues.
Ferrocene and its numerous derivatives have no large-scale applications, but have many niche uses that exploit their unusual structure (ligand scaffolds, pharmaceutical candidates), robustness (anti-knock formulations, precursors to materials), and redox reactions (reagents and redox standards). Use for global cooling has been proposed.[17]
Ferrocene and its derivatives are antiknock agents added to the petrol used in motor vehicles, and are safer than the now-banned tetraethyllead.[18] Petrol additive solutions containing ferrocene can be added to unleaded petrol to enable its use in vintage cars designed to run on leaded petrol.[19] The iron-containing deposits formed from ferrocene can form a conductive coating on the spark plug surfaces.
Iron pentacarbonyl, also known as iron carbonyl, is thecompound withformulaFe(CO)5. Understandard conditions Fe(CO)5 is a free-flowing, straw-colored liquid with a pungent odour.
This compound is a common precursor to diverse iron compounds, including many that are useful inorganic synthesis.[20] Fe(CO)5 is prepared by the reaction of fineiron particles withcarbon monoxide. Fe(CO)5 is inexpensively purchased.
Iron pentacarbonyl is one of thehomolepticmetal carbonyls; i.e.metal complexes bonded only toCO ligands. Other examples include octahedralCr(CO)6 andtetrahedralNi(CO)4.
Most metal carbonyls have18 valence electrons, and Fe(CO)5 fits this pattern with 8 valence electrons on Fe and five pairs of electrons provided by the CO ligands. Reflecting its symmetrical structure and charge neutrality, Fe(CO)5 isvolatile; it is one of the most frequently encountered liquid metal complexes.
Fe(CO)5 adopts a trigonal bipyramidal structure with the Fe atom surrounded by five COligands: three inequatorial positions and two axially bound. The Fe-C-O linkages are each linear.
Fe(CO)5 is the archetypalfluxional molecule due to the rapid interchange of the axial and equatorial CO groups via theBerry mechanism on theNMR timescale. Consequently, the13CNMR spectrum exhibits only one signal due to the rapid interchange between nonequivalent CO sites.
InEurope, iron pentacarbonyl was once used as ananti-knock agent inpetrol in place oftetraethyllead. Two more modern alternative fuel additives areferrocene andmethylcyclopentadienyl manganese tricarbonyl. Fe(CO)5 is used in the production of "carbonyl iron", a finely divided form of iron used inmagnetic cores of high-frequency coils for electronics, and for manufacture of the active ingredients of someradar absorbent materials (e.g.iron ball paint). It is famous as a chemical precursor for the synthesis of various iron-basednanoparticles.
Iron pentacarbonyl has been found to be a strong flame speed inhibitor in oxygen based flames.
Toluene is a clear,water-insoluble liquid with a slightly sweet, highly volatile odor reminiscent ofpaint thinners or otherorganic solvents. It is anaromatic hydrocarbon that is widely used as an industrial feedstock and as asolvent. Like other solvents, toluene is also used as aninhalant drug for its intoxicating properties.[21][22]
Toluene and benzene were used asoctane rating boosters for aviation fuel by theRoyal Air Force in theWorld War Two. Tetraethyl lead was manufactured in the USA and was on short supply, so Rolls-Royce engineers built the Rolls-Royce Merlin to work with fuel affed with benzene and toluene. This was called as "aromatic fuel".[23] The Allison V-1710 engine would not run with the RAF fuels as it required tetraethyl lead for lubrication of its valvetrain, but the Packard-built Merlins would. This is why the Merlin-engine P-51 Mustangs had a text "Suitable for Aromatics" on their USAAF type description.[24][25]
Toluene can be used as anoctane booster ingasoline fuels used ininternal combustion engines. Toluene at 86% by volume fueled all the turbo Formula 1 teams in the 1980s, first pioneered by the Honda team. The remaining 14% was a "filler" of n-heptane, to reduce the octane to meet Formula 1 fuel restrictions. Toluene at 100% can be used as a fuel for both two-stroke and four-stroke engines; however, due to the density of the fuel and other factors, the fuel does not vaporize easily unless preheated to 70 degrees Celsius (Honda accomplished this in their Formula 1 cars by routing the fuel lines through the exhaust system to heat the fuel). Toluene also poses similar problems as alcohol fuels, as it eats through standard rubber fuel lines and has no lubricating properties as standard gasoline does, which can break down fuel pumps and cause upper cylinder bore wear.
Toluene has also been used as a coolant for its good heat transfer capabilities in sodium cold traps used in nuclear reactor system loops.
Properties ofxylenes andethylbenzene are nearly identical to toluene, with the latter advertised by a refinery as "component of high performance fuels".
2,2,4-Trimethylpentane, also known asisooctane, is anoctaneisomer which defines the 100 point on theoctane rating scale (the zero point isn-heptane). It is an important component ofgasoline.
Isooctane is produced on a massive scale in thepetroleum industry, usually as a mixture with related hydrocarbons. Thealkylation process alkylatesisobutane withisobutylene using a strong acid catalyst. In the NExOCTANE process,[26] isobutylene isdimerized intoisooctene and then hydrogenated to isooctane.
InWorld War II,xylidine was an important antiknock agent in very high performanceaviation gasolines. Its purpose was to permit high levels of boost pressure in multiple-stage turbochargers, and thus high power at high altitudes, without causing detonation that would destroy the engine. The high pressures brought high temperatures of inlet air, making engines prone to knock. This use and storage stabilization methods were important military secrets.[27][28]
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