What are the Solvents Often Found in Antiknock Additives?
The solvents often found in antiknock additives and octane boosters can be classified into three groups. Those three groups of compounds are: i) aromatics, ii) oxygenates, and iii) organo-metallic compounds. These three types of antiknock additives/octane boosters all increase gasoline octane and reduce engine ‘pinging.’ But what about other effects that they might have, such as health effects or effects on the engine that burns them? Also, if you know enough about the solvents often found in antiknock additives, could you mix up your own octane booster? Let’s take a look at each of the groups of octane boosters in turn.
Aromatics (such as Toluene or Xylene)
Aromatics are a group of organic compounds characterized by the presence of a ‘benzene ring’ in their molecule. Benzene is the simplest aromatic organic compound, containing just the ‘benzene ring,’ that is 6 carbon atoms bonded to each other in a ring structure with each of the carbon atoms having a hydrogen atom attached to it. Benzene is naturally present in small amounts in crude oil and its concentration is increased in the refining process. U.S. EPA limits the overall average benzene concentration in refined gasoline to no more than 0.62% because it has been identified as carcinogen, so it is not a candidate to use as an octane booster.
Toluene and xylene are the main aromatic organic solvents often found
in antiknock additives. They are the next two simplest aromatics after benzene. Toluene has one hydrogen ion in the benzene ring replaced with a methyl group (CH3) and xylene has two hydrogen ions replaced with methyl groups. They are both octane boosters, present in crude oil and present at increased concentrations in refined gasoline. Neither has been shown to be carcinogenic, so both are candidates for use as octane boosters, and in fact are two solvents often found in antiknock additives.
Oxygenates (Alcohols and Ethers)
The term oxygenates is appropriate for alcohols and ethers, because alcohols are hydrocarbons with at least one OH ion and ethers are hydrocarbons with at least one oxygen atom bonded to two other atoms. The alcohol that is most widely blended with gasoline is ethanol, but methanol, isopropyl alcohol, and tertiary Butyl alcohol would provide similar octane enhancement. Ethanol addition also improves the emission characteristics of gasoline. Methanol has seen significant use as a fuel for race cars.
The ether that is most widely used as a gasoline additive by refiners is MTBE (Methyl Tertiary Butyl Ether). TAME (Tertiary Amyl Methyl Ether) and ETBE (Ethyl Tertiary Butyl Ether) are other octane enhancing ether additives. The use of MTBE in gasoline is controversial because it is soluble in water and is a threat for contamination of water supplies.
A disadvantage of using alcohols or ethers as gasoline additives in significant quantities is their lower energy content. Alcohol or ether additives increase octane but reduce gas mileage.
Tetraethyl lead was very widely used as an octane enhancement additive for many years, until concern about its environmental effects led to its being banned from use in automotive gasoline in the U.S. Another organo-metallic compound that has seen use as an antiknock additive is MMT (methylcyclopentadienyl manganese tricarbonyl). It was banned from use in the U.S. by the EPA in 1978. Other possible organo-metallic additives are dicyclopentadienyl iron, and nickel carbonyl. In addition to their toxicity problems, the organo-metallic additives in general have caused problems with catalytic exhaust systems, premature spark plug failure, and engine wear.
Summary of Types of Solvents Often Found in Antiknock Additives
Of the three types of antiknock additives discussed above, the organo-metallic compounds are the least useful because of their toxicity and their effects on engines and catalytic exhaust systems. Ethanol and MTBE are widely used as additives in gasoline as it is sold at the pump, because the advantages of their antiknock and emission benefits outweigh their lower energy content. The aromatics, toluene and xylene are the most likely candidates for a good solvent to use as an antiknock additive/octane booster. They are already present in gasoline and no adverse effects due to adding more are apparent.
Recipes for Antiknock Additives
Much of the performance care/DIY octane boost discussion on the web zeros in on toluene as a readily available, not too expensive gasoline octane booster. The following recipe is floating around several places on the internet (for example: https://www.clubwrx.net/forums/engine-modifications/44006-homemade-octane-booster-formulas.html as a homemade octane booster:
- 100 oz of toluene for octane boost
- 25 oz of mineral spirits (cleaning agent)
- 3 oz of transmission fluid (lubricating agent)
Discussion following the above recipe at the same web site points out that the latter two components in the recipe aren’t really needed in modern engines, and that simply using straight toluene as an octane booster additive works fine.