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Lubricating oils perform a number of important functions in the diesel engine:
Engine lubricants consist of a base oil (typically 75 - 83%), viscosity modifier (5 - 8%) and an additive package (12 - 18%) . As the base oil alone cannot provide all of the lubricating oil functions required in modern engines, the additive package has evolved to play an increasingly important role in the oil formulation.
The base oil is composed of a base stock or a blend of a number of base stocks. Base stocks from petroleum feed stocks may be manufactured using a variety of different processes including distillation, solvent refining, hydrogen processing, oligomerization, esterification, and rerefining. Synthesis using the Fischer-Tropsch process can also be used to produce some high quality base stocks from feed stocks such as natural gas (GTL). Bio-synthesis can also be used to produce base stocks from renewable feed stocks such as plant sugar . Base stocks may also be recovered from used oil recycling.
The American Petroleum Institute (API) classifies base stocks for engine lubricants licensed to carry an API classification symbol into several different categories, as outlined in Table 1. In Europe, the Association Technique de L’Industrie Européenne des Lubrifiants (ATIEL) defines base oil groups for use in ACEA oil sequences. The ATIEL Group I to V classifications are identical to those of API (however, between 2003 and 2010, ATIEL included an additional Group VI classification).
|V||-||-||-||-||-||-||not in Groups I to IV|
|* Maximum 90% saturates and/or minimum 0.03% sulfur|
Group I, II and III base stocks are distinguished by the concentrations of saturates and sulfur and by their viscosity index (see below). Group I base stocks are low in saturates and/or high in sulfur. Group II and III are high in saturates and low in sulfur. Group IV base stocks are synthetic oils made up of polyalphaolefins. Finally, Group V base stocks are those that do not fall into Groups I-IV. Group I and Group II base stocks with a viscosity index greater than 110 are sometimes referred to by marketers as Group I+ and Group II+ base stocks respectively. Increased use of Group III base oils has also introduced a similar differentiation for these products. However, the distinction is less clear. Group III+ base oils can be used to refer to base oils with a viscosity index greater than 130-150 depending on the marketer.
Group I base stocks are the lowest quality base stocks. They are produced by physically separating lubricant molecules using solvent refining; a two step process involving the partial removal of aromatics with a solvent and the subsequent removal of wax by precipitation and a different solvent. Group I base stocks can still contain more than 10% aromatics which give these unadditized base stocks poor oxidation resistance and their viscosity a poor temperature response. Special crude oils that contain the desired lubricant base oil molecules must be used so that Group I base stock performance is highly dependent on the crude oil source.
Group II base stocks are manufactured with a variety of hydroprocessing technologies. In retrofit or hybrid Group II plants, a hydrotreating step is added to a Group I plant and allows increased flexibility in crude oil selection over Group I base stocks. In a purpose-built Group II hydrocracking plant, catalytic processes convert non-lubricant molecules into lubricant molecules, giving even greater feedstock flexibility and allowing the use of lower quality/lower cost crude oils. Group II base stock manufacturing can remove a significant amount of the nitrogen and sulfur containing compounds and aromatics. This provides a superior base stock to Group I base stocks. Group II base stocks are more inert and form less oxidation products. Since Group II base stock feed molecules are cracked and re-shaped, product properties are less dependent on the crude oil source.
Group III base stocks are manufactured in much the same way as Group II base stocks but by using higher temperatures or longer residence times in the reactor. This gives them much improved temperature characteristics. Gas-to-liquid (GTL) derived base stocks fall into Group III. Group III+ base stocks can also be bio-synthesized .
Pressure to improve fuel economy and reduce emissions in automotive applications has led to a decrease in usage of Group I base stocks and an increased usage of Group II and III base stocks. The increased availability of these higher quality base stocks has opened up new applications for Group II base stocks beyond that created from the need for higher quality automotive lubricants. For example, switching to lubricants formulated from Group II base stocks for marine trunk piston engines can help reduce maintenance and operating costs .
Group IV base stocks have traditionally been referred to as “synthetic” base stocks. These polyalphaolefins (PAOs) are polymerized from smaller molecules. At the time of their introduction, they were the highest performing base stocks available. As demand grew, manufacturers started using high viscosity index feedstocks to make mineral oils that matched the performance of PAOs. These Group III base stocks matched the performance of PAOs but at a lower cost. In North America, Group III base stocks can also be referred to as “synthetic” . Biosynthesized PAO base stocks have also been developed . Low viscosity PAOs, used in combination with Group III base stocks, offer a tool to achieve low viscosity engine oil formulations for improved fuel economy while maintaining acceptable oil volatility characteristics, Figure 1 .
Group V base stocks include polyalkylene glycols (PAG), alkylated naphthalenes (AN) and esters such polyol esters (pentaerythritol esters and trimethylolpropane esters) and aromatic esters (phthalates and trimellitates). New ones, such as oil-miscible ionic liquids, continue to be developed as well . These synthetic base stocks can have a variety of properties that make them attractive for certain applications: