Antiknock agents are compounds which include the antidetonants proper, scavengers and other substances which improve the product performance.
Lead compounds has been used as some of the most effective antiknock agents for over seventy years. The most known of these is tetraethyllead, a transparent, colorless highly toxic dense liquid (1.6524 g/cm3). Tetraethyllead dissolves well in gasoline, ethyl, acetone and some other solvents. It boils and decomposes at around 200°С. The vapor in small concentrations have a sweet scent, as concentration grows, the smell becomes rather unpleasant.
Another well known lead antiknock agent is tetramethyllead. It is also a liquid with unpleasant aroma, boiling at 110°С. It’s density is 1.995 g/cm3. Due to the relatively low boiling temperature, which constitutes about 50% of gasoline boiling temperature, this substance distributes more evenly in gasoline fractions and the cylinders of the engine. Tetramethyllead is more thermally stable than tetraethyllead: at 744°С tetraethyl lead decomposes to 65% in 5.6 ms, while tetramethyllead only decomposes to 8%. This difference ensures better efficiency of tetramethyllead as compared to tetraethyllead in high pressure ratio internal combustion engines and when used in highly aromatic gasolines.
A common drawback of both compounds is the extremely high toxicity of the agents and burn products, with severe impact on the environment and negative influence on exhaust gas aftertreatment devices. For these reasons the use of tetraethyllead and tetramethyllead is being decreased and intensive research into other efficient antiknock agents is underway.
The research of the antiknock agents progresses in two directions: organometallic and organic compounds. Among the organometallic compounds, apart from tetraethyllead and tetramethyllead, the most efficient are manganese and ferrous compounds.
Some of the compounds which have already been studied and tested and used at various times as antoknock agents, are Methylcyclopentadienyl manganese tricarbonyl (MMT), Cyclopentadienyl manganese tricarbonyl (CMT), Dicyclopentadienyl iron and its alkyl derivatives, iron pentacarbonyl etc. In terms of antiknock efficiency, manganese compounds are analogous and iron compounds are only somewhat inferior to lead.
CMT is a volatile crystalline substance of yellow color (melting point 77°С). It is stable in the air, well soluble in organic solvents and is insoluble in water.
MMT is a clear low viscosity liquid of light amber color with a grassy smell, boiling point of 233°С, density of 1.388 g/cm3 and freezing point of 1.5°С, well soluble in gasoline and practically insoluble in water.
Ferrocene is a solid orange crystalline substance, with melting temperature 173°С, subliming temperature 100°С and decomposition temperature 474°С.
Iron pentacarbonyl is a straw color liquid with boiling temperature of 102.5°С and freezing temperature -2°С, insoluble in water.
Ferrocenyldimethylcarbinol is a fine crystal powder with melting temperature 59.5°С.
Organometallic anti-knock agents cause sedimentation of metals on the walls of the cumbustion chamber and on spark plug electrodes. This sediment leads to increased cylinder and piston ring wear, as wells spark plug problems.
Therefore organometallic anti-knock additives are used in combination with scavengers: materials which convert churly metal oxides into volatile compounds.
Common scavengers are alkyl halides: bromo-ethane (boiling temperature 34.4°С), dibromoethane (131.7°С), dichloroethane (83.3°С), naphthalene monochloride (25°С). Some phosphorus and sulfur compounds are used with manganese and iron anti-knock agents. However, the scavengers for these agents are now efficient enough yet, which limits their widespread use.
Due to extraordinary toxicity of lead anti-knock agents, newly found significant disdvantages and high cost of manganese and iron anti-knock additives, the research for an organic antiknock material, which does not contain metal, is ongoing.
Such anti-knock agents are first of all the organic amines: methylaniline, xylidine, extralyne (a mix of 7% aniline, 88% methylaniline and 5% xylidine). The latter was added to blue aviation gasoline in late 40s in the amount of 2% by volume.
When adding aromatic amines to a blend of primary standards (70% isooctane and 30% n-heptane) in the amount of 2% vol, the octane number increase by 5 – 7 points (MM) and 8 – 9 point (IM).
When adding these antiknock agents to gasoline with octane number 86 (IM) in the amount of 2% by volume, the octane number increases by 4 – 5 (MM and by 5 – 6 (IM) points, and by 7 – 8 and 9 – 11 points respectively, when the amount is 5%.