Additives in lubricating oil
In addition to the viscosity index improvers, motor oil manufacturers often include other additives such as detergents and dispersants to help keep the engine clean by minimizing sludge buildup, corrosion inhibitors, and alkaline additives to neutralize acidic oxidation products of the oil. Most commercial oils have a minimal amount of zinc dialkyldithiophosphate as an anti-wear additive to protect contacting metal surfaces with zinc and other compounds in case of metal to metal contact. The quantity of zinc dialkyldithiophosphate is limited to minimize adverse effect on catalytic converters. Another aspect for aftertreatment devices is the deposition of oil ash, which increases the exhaust back pressure and reduces over time the fuel economy. The so-called "chemical box" limits today the concentrations of sulfur, ash and phosphorus (SAP).
There are other additives available commercially which can be added to the oil by the user for purported additional benefit.
Gasoline and Diesel Additives
Legislation is now restricting the use of organo-metallic compounds for improving the octane rating of gasoline. Consequently, they are not covered here, but a discussion of their use, the other additives that must be used in association with them, and the consequences of their withdrawal are discussed in Stone (1999). The most significant additives are detergents and antioxidants, but corrosion inhibitors, metal deactivators, biocides, anti-static additives demulsifiers, dyes and markers, and anti-icing additives also are used. These are discussed in detail by Owen and Coley (1995).
Antioxidants are needed in gasoline to inhibit the formation of gum, which usually is associated with the unsaturated hydrocarbons in fuel. Formation of gum can interfere with the operation of fuel injectors.
Detergents are added to reduce the deposits in fuel injectors, the inlet manifold, and the combustion chamber. Surfactants inhibit the formation of deposits in the injectors and the inlet manifold, but a different mechanism is needed to combat valve and port deposits because these deposits are associated with higher temperatures. High-boiling point, thermally stable, oily materials such as polybutene are used, and these appear to dissolve the deposits. 49 Diesel additives to improve the cetane number will be discussed first, followed by additives to lower the cold filter plugging point temperature, then additives that are used with low sulfur fuels, and finally other additives.
The most widely used ignition-improving additive currently is 2-ethyl hexyl nitrate (2EHN), because of its good response in a wide range of fuels and comparatively low cost (Thompson et al., 1997). Adding 1000 ppm of 2EHN will increase the cetane rating by approximately 5 units. In some parts of the world, legislation limits the nitrogen content of diesel fuels, because although the mass of nitrogen is negligible to that available from the air, fuel-bound nitrogen contributes disproportionately to nitric oxide formation. Under these circumstances, peroxides can be used, such as ditertiary butyl peroxide (Nandi and Jacobs, 1995).
Diesel fuel contains molecules with approximately 12 to 22 carbon atoms, and many of the higher molar mass components (e.g., cetane, C16H34) would be solid at room temperature if they were not mixed with other hydrocarbons. Thus, when diesel fuel is cooled, a point will be reached at which the higher molar mass components will start to solidify and form a waxy precipitate. As little as 2% wax out of the solution can be enough to gel the remaining 98%. This will affect the pouring properties and (more seriously at a slightly higher temperature) block the filter in the fuel-injection system. These and other related low-temperature issues are discussed comprehensively by Owen and Coley (1995), who point out that as much as 20% of the diesel fuel can consist of higher molar mass alkanes. It would be undesirable to remove these alkanes because they have higher cetane ratings than many of the other components. Instead, use is made of anti-waxing additives that modify the shape of the wax crystals.
Wax crystals tend to form as thin "plates" that can overlap and interlock. Anti-waxing additives do not prevent wax formation. They work by modifying the wax crystal shape to a dendritic (needle-like) form, and this reduces the tendency for the wax crystals to interlock. The crystals are still collected on the outside of the filter, but they do not block the passage of the liquid fuel. The anti-waxing additives in commercial use are copolymers of ethylene and vinyl acetate, or other alkene-ester copolymers. The performance of these additives varies with different fuels, and the improvement decreases as the dosage rate is increased. It is possible for 200 ppm of additive to reduce the cold filter plugging point (CFPP) temperature by approximately 10 K.
Additives can be used with low-sulfur diesel fuels to compensate for their lower lubricity, lower electrical conductivity, and reduced stability. To restore the lubricity of a low-sulfur fuel to that of a fuel with 0.2% sulfur by mass, then a dosage on the order of 100 mg/L is needed. Care is required in the selection of the additive, if it is not to interact unfavorably with other additives (Batt et al., 1996).
Electrical conductivity usually is not subject to legislation, but if fuels have a very low conductivity, then there is the risk of a static electrical charge being built up. If a road tanker, previously filled with gasoline, is being filled with diesel, then there is the possibility of a flammable mixture being formed. The conductivity of untreated low-sulfur diesel fuels can be less than 5 pS/m (Merchant et al., 1997). Conductivities greater than 100 pS/m can be obtained by adding a few parts per million of a chromium-based static dispersant additive. Low-sulfur fuels and fuels that have been hydro-treated to reduce the aromatic content also are prone to the formation of hydroperoxides. These are known to degrade neoprene and nitrile rubbers, but this can be prevented by using antioxidants such as phenylenediamines (suitable only in low-sulfur fuels) or hindered phenols (Owen and Coley, 1995).
Other additives used in diesel fuels are detergents, anti-ices, biocides, and anti-foamants.
Detergents (e.g., amines and amides) are used to inhibit the formation of combustion deposits. Most significant are deposits around the injector nozzles, which interfere with the spray formation. Deposits then can lead to poor air-fuel mixing and particulate emissions. A typical dosage level is 100-200 ppm.
Anti-ices (e.g., alcohols or glycols) have a high affinity for water and are soluble in diesel fuel. Water is present through contamination and as a consequence of humid air above the fuel in vented tanks being cooled below its dewpoint temperature. If ice formed, it could block both fuel pipes and filters. Biocides act against anaerobic bacteria that can form growths at the wateddiesel interface in storage tanks. These are capable of blocking fuel filters.
Anti-foamants (1 0-20 ppm silicone-based compounds) facilitate the rapid and complete filling of vehicle fuel tanks.