Diesel Engine Lubricants

Hannu Jääskeläinen, W. Addy Majewski

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Abstract: Diesel engine lubricants are composed of base oil, viscosity modifier and an additive package. To ensure that lubricants provide all of the lubricating oil functions required in modern engines, a number of oil specifications have been developed in the USA, EU and Japan. One of the main drivers in the development of oil formulations for diesel engines with exhaust aftertreatment is the reduction of sulfated ash, phosphorus and sulfur.

Lubricant Formulation

Overview

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%) [1265]. 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.

Base Oil

The base oil is composed of a base stock or a blend of a number of base stocks. Base stocks may be manufactured using a variety of different processes including distillation, solvent refining, hydrogen processing, oligomerization, esterification, and rerefining. Base stocks may also be recovered from used oil recycling. 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).

Table 1
API Base Oil Stock Classification
GroupSaturatesSulfurViscosity IndexOther
minmaxminmaxminmax
I-90%*0.03%*-80120
II90%--0.03%80120
III90%--0.03%120-
IV------polyalphaolefins (PAO)
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 for definition). 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 as Group I+ and Group II+ base stocks respectively.

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 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 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” [464].

Group V base stocks include polyglycols, alkylated naphthalenes 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 [2442]. These synthetic base stocks can have a variety of properties that make them attractive for certain applications:

Up until 2003, ATIEL and API base stock classifications were identical. However, in 2003 ATIEL established an additional Group VI base stocks classification for poly(internal olefins) (PIO) [2452]. This was done to accommodate a request from SASOL, the only manufacturer of PIOs at the time, to allow PIOs to be used as base stocks in ACEA oil sequences. While PIOs were shown to be fully interchangeable with Group IV PAOs, a separate base stock classification was established to retain alignment between ATIEL and API for Group I to V base stock definitions. By 2005, competition from PAO and hydrocracked base stocks made PIO base stocks unprofitable and production was discontinued. By 2010, the ATIEL Group VI base stock classification was withdrawn—bringing the ATIEL and API classifications back into full alignment.

An important property of base stocks is their viscosity and they are often classified according to their viscosity at a given temperature. Lighter Neutral (N) and Solvent Neutral (SN) base stocks are often designated by numbers such as 100N, 150N, 300SN, etc.. The number is the approximate viscosity in Saybolt Universal Seconds (SUS) at 37.8°C (100°F). Heavier Bright Stock (BS) base stocks carry a similar designation (e.g., 150BS) but the number is the approximate viscosity in SUS at 98.9°C (210°F).

Base stocks can also be categorized by their viscosity index (see below) and can carry designations such as Low Viscosity Index (LVI), Medium Viscosity Index (MVI), High Viscosity Index (HVI) or Very High Viscosity Index (VHVI). While no strict range of viscosity indices for these designations exists, LVI base stocks have viscosity index less than about 30, MVI from 30 to 85, HVI from 85 to 115 and VHVI higher than 115.

The base oil used in a lubricant formulation is very important. Among other impacts, it can affect the sulfur level of the final formulation and affect the concentration and type of additives needed to allow the final lubricant to meet its required performance expectations.

Base Oil Interchange. Base oils can have different physical or chemical properties or perform differently in engine testing. In order to accommodate the need for flexibility during engine oil manufacture, base oil interchangeability guidelines have been developed to ensure that the performance of engine oil products is not adversely affected when different base oils are used interchangeably by engine oil blenders.

The API Base Oil Interchangeability Guidelines (BOI) define the minimum testing necessary to ensure that engine oil performance is not adversely affected by substitution of one base oil for another. These Guidelines are subject to modifications based on new data, new or revised test methods, and/or new performance specifications. The current Guidelines must always be used be used [1877]. ATIEL provides similar guidelines for base oil interchange in ACEA oil sequences [2443].

Biolubricants

“Environmentally-friendly” lubricants are considered important for applications were the lubricant can come into contact with water, sensitive environments, food or people. While there is no clear definition of an “environmentally-friendly” lubricant, a class of lubricants that are biodegradable and have low environmental toxicity (bionotox) has emerged. These 'Biolubricants' are often, but not necessarily always derived from vegetable oils.

The use of biolubricants has mainly been limited to applications where the lubricant ends up in the environment (total loss applications) or where leaks may contaminate water or other sensitive environments. This includes two-stroke engine oils, chain saw lubricants and hydraulic oils. However, applications for gasoline and diesel passenger car engine lubricants have also been under development [1888].

Depending on the application demands, biolubricants can be vegetable oil based or synthetic. Vegetable oil based lubricants are typically only suitable for applications where high temperatures are not encountered or where oil stability is not important. Better properties are available from biolubricants that have been synthesized. Synthetic esters are an important class of biolubricants and can be derived from renewable resources such as vegetable oils, solid fats and low grade or waste materials such as tallow. Polyglycols are also used as base stocks in biolubricants.

While pure hydrocarbons can meet the biodegradability and low toxicity requirements of biolubricants, it is often the additive package that determines whether the formulated oil also meets the bionotox requirement. Some options to reduce the need for additives and therefore make it easier to formulate a biolubricant include [1901]:

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