Additives can make a major difference in petrol performance, as all the ads will tell you. For nearly 40 years, every litre of petrol sold at the pump has contained up to a dozen different additives ranging from a few drops for minimally-treated fuels to about six millilitres for the premium grades. These additives are designed to help the product meet petrol specifications as well as the individual requirements of petroleum companies which differentiate their products. Besides the various additives added to protect pipelines and tankers, the final package of additives defines how the petrol will perform.

Since 1985 (the year automakers began the wholesale switch from carburettors to fuel-injection), industry attention has focused heavily on petrol engine cleanliness, first for the control of fuel injector deposits and later, for the prevention of intake valve deposits. Thus, the need for additives keeps increasing.

Merely distributing a petrol which meets the US ASTM standards (generally accepted worldwide) is not enough. These standards are developed by representatives of the automobile and petroleum industries, regulatory agencies, and consultants, and are generally updated at least once a year. The standards are approved through the consensus process, a safe and thorough, but time-consuming, procedure. Changes must be supported by comprehensive technical data which requires time to develop.

The standards set limits for a minimum acceptable quality. They do not include requirements for corrosion protection (other than copper) or engine cleanliness. However, as virtually all cars have fairly sophisticated engine designs today, the companies which develop fuels are expected to continuously offer products of higher and higher quality.

Some petrol additives go in at the refinery. The first additives to be blended into petrol are usually oxidation inhibitors. These were introduced in the mid-1930s and are added to certain components at the process unit to inhibit oxidation and gum formation.

In most large refineries, component streams are mixed to make petrol in an in-line blender. This is where they get other additives. Dyes may also be added to differentiate types of fuel and earlier, the laws in many countries stipulated that leaded petrol must contain dye to indicate the presence of lead compounds.
Some premium fuels get their improved octane ratings from oxygenated materials like methyl tertiary butyl ether (MTBE). These oxygenates are really components, not additives, and added in volume percent.
If the fuel is shipped to the market along pipelines, the petrol gets another dose of additives to inhibit iron corrosion. This practice began in 1948 and not only saves the pipes from corrosion but also protects the petrol from rust contamination.
 
It is at the end of the production chain, before the petrol actually goes to the service stations, that the differences between one brand of petrol and another start to occur. This is when additives specifically selected by a company are introduced. Some additives may be proprietary and the formulation (the 'recipe’) is a trade secret.

New fuel formulations and the use of more advanced additives go through extensive lab testing

Control and prevention of deposits is the main issue in fuel development (pictures from Chevron)

Since 1954, many petroleum companies have added amines or amine carboxylates as detergents to prevent deposit formation in carburettor throttle bodies. This was in response to carburettor "gumming" which was found to be prevalent then. Polybutene amines were introduced in 1970 to help control exhaust emissions caused by deposits forming in critical engine areas. They are still in wide use today. Since 1980, companies have used polyether amines which are specially for unleaded petrol. These deposit control additives not only clean up carburettors and fuel injectors (and other areas of the fuel delivery system) but also keep them clean.
The difference between the performance of detergents and deposit control additives is best illustrated by imagining them to work as soap and water. The additive ("soap") breaks up the deposits and the petrol ("water") washes them away. But not all types of additives will themselves be washed away; while cleaning efficiently, higher temperatures may cause them to build up their own scum on intake valves and actually leave them dirtier than they would be if the detergents were not there.

Deposit control additives disperse these accumulated solids far better than simple detergents. They break caked carbon into tiny particles that wash away easily through the injectors and intake valves, and into the combustion chamber where they burn with the air-fuel mixture. All leading brands of petrol now contain some deposit control additives as well as carburettor/injector detergents. Dosages of the former increased from 1988 onwards as many automobile manufacturers expressed concern about intake valve deposits.

In the past, when cars had large, 'tolerant’ engines and no emission control systems, carburettor detergents were good enough. However, as fuel injection systems became more widely used and more complicated emission control systems had to be incorporated, the mechanisms became more delicate with finer tolerances. A tiny deposit could clog an injector nozzle, preventing efficient fuel delivery.

Detergent levels that cleaned carburettors were thus not effective for injectors, and the higher levels which were effective actually caused more deposit accumulation on intake valves. This resulted in a minor crisis in 1985 as many news cars coughed and spluttered, particularly high-per models like BMWs. The German manufacturer was so concerned about the problem that it established a vehicle test procedure to determine satisfactory intake valve cleanliness. This procedure, which undergoes periodic upgrades, is among those which petroleum companies strive to pass.

The trend these days is to use higher dosages of deposit control additives and less carburettor detergents. The choice of additive types and dosages varies from company to company according to marketing requirements because the additives are not cheap. So it is a myth that petrol is the same, regardless of brand.

Modern engines must be kept clean to provide optimum performance and additives in fuel help to achieve this

The leading companies always carry out extensive tests on the use of a new additive in their petrol. Although such data gives  a good indication of performance, the most reliable measure is a field test. This is even more effective that increasing test severity as it will often reveal possible side effects... valves getting scorched or sticking, compatibility, etc. - under "real world conditions" which will vary from country to country.

For the motorist who often wonders if mixing 'third-party' additives with the petrol will be beneficial, the petroleum companies warn of the possibility of incompatibility. The extra additives may diminish or even destroy the performance of the petrol's own additives and cause other undesirable chemical reactions. Additionally, there is also the possibility that these additives, while giving 'enhanced performance’ may leave behind extra deposits after combustion.

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