The primary purpose of engine oil is to stop metal from touching metal. When metal surfaces come in contact, they grind together causing friction. That creates heat, stress and wear.
Engine oil is used to reduce friction by creating a slick film between metal parts that lets them glide over one another.
An oil's viscosity - a measure of its resistance to flow, along with the pressure and speed of movement, determines the thickness of the oil film between two moving surfaces. This, in turn, determines the ability of the oil film to keep the surfaces apart, the rate heat is generated by friction and the rate the oil flows between the surfaces to convey the heat away.
Engine oils are commonly referred to as "thick" - having a high viscosity, or "thin" - having a low viscosity.
According to oil manufacturers, an oil should have a viscosity at the operating temperature that is correct for maintaining a fluid film between the engine surfaces.
Changes in an engine oil's viscosity can affect the oil's lubricating (protecting) ability. If too thin or thick, more friction and drag will be created, resulting in premature wear and failure.
ENGINE OIL CLASSIFICATION
Engine oils are classified and rated in order to establish quality levels and appropriate applications for those oils.
For automotive applications in the U.S., engine oils are generally classified by their API (American Petroleum Institute) and SAE (Society of Automotive Engineers) ratings.
Under the API classification system, oils designed for gasoline engines are designed by an "S." Oils appropriate for diesel engines are indicated by a "C."
The SAE rating specifies the viscosity of an oil to flow at certain temperatures. The higher the number, the thicker the oil.
OIL GRADES
There are two common types of SAE classifications: monograde and multigrade.
A monograde oil (also called single grade), such as SAE 30, is designed to be used within a defined temperature range.
Multigrade oils are formulated to give the oil the flexibility to lubricate effectively over a wider temperature range than monograde oils.
By way of example, with an SAE 10W-30 oil, the "W" stands for "winter" and indicates that the oil meets certain viscosity requirements for operation in low temperature.
The number in front of the "W" indicates the low temperature viscosity. This number refers to the viscosity of the oil when the engine is cold, and indicates the oil's performance when the engine is starting up.
The thinner the oil when cold, the quicker it moves around the engine and the faster it protects vital engine components.
The second number, "30," is the high temperature viscosity.
When comparing viscosity numbers, a higher number will not flow as easily as a low number.
An oil displaying the API certification mark and service symbol identify quality motor oils for gasoline- and diesel-powered vehicles that meet performance requirements set by U.S. and international vehicle and engine manufacturers and the lubricant industry.
Some motor oils may also contain a resource conserving or energy conserving rating from the International Lubricant Standardization and Approval Committee (ILSAC) - a joint effort of U.S. and Japanese automobile manufacturers. These designations apply to oils intended for gasoline engines for light duty vehicles.
LIGHTER GRADE OILS
The reasons to switch to using lower viscosity, full synthetic or synthetic blend engine oils continue to build, says Dan Arcy, OEM technical manager for Shell Lubricants, one of the world's leaders in lubricants technology and innovation. "The tried and true SAE 15W-40 conventional engine oils that many use has competition from lower viscosity grade oils that promote better fuel economy and other benefits."
Thicker oils make it easier to maintain oil pressure and reduce the tendency for oil consumption, particularly in worn engines, he notes. Thick oils trade off fuel efficiency to achieve this as the engine has to work harder to move the oil around.
Resource or energy conserving engine oils are lighter viscosity oils. They are intended to maximize energy efficiency for improved fuel economy through the use of additive packages that reduce drag, friction and energy losses.
"A number of diesel engine manufacturers recommend lower viscosity lubricants in their newest engines, and the move to lower viscosity lubricants is reinforced by the recent announcement that one of the focus areas for the next generation of heavy duty diesel engine oils will be fuel economy savings, which lower viscosity oils have demonstrated the ability to provide," says Arcy. "This is particularly important as the first-ever fuel economy regulations for heavy trucks will begin in 2014."
Less energy is consumed when starting a cold engine that is lubricated by a synthetic SAE 5W-40 compared to than is consumed with a conventional SAE 15W-40, he says.
"Synthetic engine oils are formed by a variety of chemical processes that modify the oil molecules to a structure that will better resist higher engine heat and offer better cold temperature performance," explains Arcy. "The synthetic oil will have a lower viscosity and thicken less at cold temperatures, which allows it to flow quickly under cold climate conditions.
"Synthetic engine oil, like mineral oils, helps keep the engine clean through improved sludge, deposit and varnish protection, and helps provide overall engine wear under extreme operating conditions.
"Synthetic engine oils typically have more stable viscosity control and provide better protection when the engine is running under high temperature and high stress conditions, such as high speeds and heavy towing."
Just like with conventional engine oil, as the engine heats up, the viscosity of a synthetic engine oil will change to adapt to the higher operating temperatures, he notes. The SAE 5W-40 synthetic oil will change less than conventional oil as heat increases and will offer improved high temperature viscosity protection.
Concept lubricant achieves significant fuel economy benefit
Late last year, a collaborative effort between research teams in England at Shell Lubricants and automotive design company Gordon Murray Design (GMD) led to the development of an innovative concept engine lubricant capable of achieving a 6.5 percent¹ improvement in fuel efficiency.
This is "a step change compared to the improvements of around 2.5 percent achieved in typical fuel economy lubricant development programs," said Selda Gunsel, Shell Lubricants' vice president of lubricants and B2B products technology.
Using GMD's new T.25 city car as the test bed, Shell engineers were able to work beyond current industry specifications to formulate an ultra-low viscosity 0W-10 engine oil for even greater efficiency.
The T.25 is an ultra-lightweight, compact car, about half the size of the average car. It was designed to have world-leading efficiency and ultra-low emissions, according to the company, and is optimized for performance, cost, safety, usability, recyclability and ease of assembly.
NEW STANDARDS
Changes in legislation and new emission standards are putting pressure on vehicle manufacturers to improve fuel efficiency and reduce emissions.
While the development of increasingly fuel efficient engines is ongoing and has made significant progress in recent years, lubrication is one area that can provide an extra boost to help meet those targets.
The co-engineering approach between the Shell and GMD development teams used sophisticated mathematical modeling techniques to define the optimum lubricant for the T.25 engine design, helping to achieve the lowest possible engine friction, Shell's Gunsel said.
"Blending low viscosity oil to improve fuel efficiency is actually relatively simple," she noted. "The challenge comes when you look to balance it with engine protection and acceptable oil drain intervals.
Although in the concept stage, the 0W-10 engine oil is "represents a major advancement in lubricant technology."
"Of course, engine oil is just one part of the fuel efficiency story," continued Gunsel. "But when we take into account the pressure and incentives for vehicle manufacturers to reduce CO2 emissions, the contribution from lubricants can become very significant.
"This is due to the fact that with less engine friction comes less fuel use and, ultimately, less CO2 is emitted."
DESIGN PROCESSES
"We have challenged every aspect of car design to create the T.25 and the environmentally positive iStream design process, and the lubricant is no exception," said Professor Gordon Murray, GMD's CEO and technical director.
The iStream "is a complete re-think on high-volume materials and the manufacturing process, and will lead to a significant reduction in CO2 emissions over the lifecycle of the vehicles produced using it, compared with conventional ones," he explained.
Using the new Shell 0W-10, the T.25 achieved 96 mpg in the Royal Automobile Club Future Car Challenge, winning the award for the most economic small, passenger internal combustion engine vehicle.
The annual event is designed to demonstrate, promote and challenge the development of new-energy and related technology for tomorrow's highways.
¹The 6.5 percent figure was derived from testing on an urban cycle (designed to mimic the conditions of city driving). Combined cycle (designed to mimic both urban and highway driving) testing yielded a 4.6 percent benefit. The testing undertaken compares the concept lubricant to a 10W-30 oil which is a widely used viscosity in European markets.
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