Oil has the job of allowing the parts in an internal combustion engine to move in relation to other parts with minimal wear. Wear occurs because parts have rough surfaces with tiny imperfections. These take the form of very small raised areas. Dry parts sliding across one another would continually break off the raised areas, resulting in the removal of metal and a rapid process of destruction.
This kind of wear is prevented by lubrication. Lubrication involves something similar to water skiing. Lubricating oil, a relatively thick, almost gooey liquid, is pumped between the moving parts of an engine, forming a liquid layer. As the parts move across one another, the motion causes them to float just slightly apart. It's like water skiing because the parts can actually touch when they stop, as occurs when a piston reaches the top or bottom of its stroke. But, as soon as the moving part is again in motion, the film of lubricant is re-established, and the part floats, much as a water skier moves up onto the surface of the water once the tow boat reaches a certain speed.
What is Oil Viscosity?
Lubricating oil is said to be a viscous liquid, meaning that it is thicker than water or gasoline. Viscosity affects the liquid's ability to cling to metal parts, and to form a thick film that will easily separate parts when they move across one another with a liquid layer that will support great weight or prevent wear even when a part like a piston is forced downward and against the cylinder liner by tremendous pressure. Viscosity can be measured, for example, by simply measuring how fast a liquid will flow through a small hole. Liquids with a higher viscosity will flow more slowly, but will form a thicker film when lubricating parts. Liquids with a lower viscosity will flow more quickly and will generate less friction in the engine, but they will be more limited in their ability to keep the parts separated. One job of a lubrication engineer is to find the ideal viscosity for oil so it will protect the engine’s parts without allowing them to touch and wear excessively, but with minimal viscosity and friction.
How Does Temperature Affect Viscosity?
All lubricating oils get thicker as they get colder, and thin out as they get hot. So, engine oil viscosity becomes a major concern because the engine will typically operate with the oil in the range of 200F under normal operation, but it must be also be possible to crank the engine and start it when it has cooled down to an outside temperature as low as 0F.
If the oil is too thick when it's cold, the engine will not crank fast enough to start, or may not turn over at all. This is a problem with diesels as they ignite the fuel with air heated up due to being compressed into a small volume in the engine's combustion chambers. Faster cranking means much hotter air in the cylinder because the rapid compression keeps the heat in the air from passing into the metal surfaces of the combustion chamber. If the engine cranks too slowly, much of the heat generated as the piston rises will be lost by the time the piston reaches the top of its stroke and the fuel is injected.
As a result, oil that is thick enough to protect the engine at 200F. or more, yet does not get too thick even at 0F, is a very valuable commodity.
Multi-viscosity Oils
Now we get into what are called multi-viscosity oils. Oil is assigned a viscosity number relating to its thickness that in diesels would typically range all the way from the number 5 (very thin) to the number 40 (very thick). Petroleum oils can be distilled from a barrel of crude oil so as to create various levels of thickness or viscosity. The problem is that oil as it comes out of the barrel, even when distilled to a certain thickness, thins out very rapidly as it warms. So, an oil that is right for 200F operation will be too thick for starting at frigid temperatures. If the oil is thin enough at 0F. it will be far too thin at 200F to protect the engine.
Oil that is thick enough to protect the engine at 200F or more, yet does not get too thick even at 0F, is a very valuable commodity.
It takes oil of a viscosity number of at least 30-40 to lubricate a diesel when it's running at normal operating temperature, with the lubricating oil at 200F. On the other hand, if a diesel is to start at frigid temperatures, it must have a viscosity number between 5 and 15.
In order to allow engines to be properly protected at operating temperatures while starting in the winter without pre-heating, multi-viscosity oils were invented. Special chemicals termed "viscosity index improvers" were created in oil company laboratories in order to flatten the viscosity curve of petroleum oils thin enough to allow the engine to start. These exotic chemicals, called polymers, have little effect on cold oil. But, as the oil gets hotter, they keep the oil from getting thinner at the normal rate. With these so-called "VI improvers", a 10-weight oil that allows easy starting can be kept to at least a viscosity number of 30 for hot operation.
10W-30 and 10W-40 Oils
Thus, today truckers are typically offered choices like 10W-30 or 15W-40 multi-viscosity oils. The W refers to "winter" or the viscosity at a temperature of 0F or, say, 5 degrees above 0F. The upper number always tells the buyer how thick the oil will be at the normal operating temperature of 200F or more.
While 15W-40 oils were popular for many years, most engine makers now approve oil with a 10W-30 viscosity for a wide range of operating conditions. Improvements that help the oil maintain its original thickness even after many thousands of miles, and even to lubricate in spite of being slightly thinner due to exotic additives, have made this possible. These changes maximize fuel economy and reduce CO2 emissions.
What Oil Should Truck Owners Use?
Recommended Year Round Oil
The basic recommendation would be for truck owners to use the thinner 10W-30 grade year round if not running under maximum loaded conditions in very hot weather. If hauling heavy or driving under adverse conditions, say in mountains at desert-like temperatures, the 15W-40 oil would probably be ideal, as the oil's actual temperature under hard operation will rise under extreme weather conditions and load even though truck diesels have a large, thermostatically controlled oil cooler.
Recommended Cold Weather Oil
You may also be permitted to use oil of a lower viscosity in very cold weather. A 5W-30 or 5W-40 oil may be approved. Synthetic oils that are manufactured rather than being distilled out of a barrel of petroleum have these special capabilities.
Study your owner's manual and make sure you are using oil of the approved viscosity for the weather and load conditions you experience. If operating where the weather suddenly gets much colder or hotter, it may even make sense to change your oil just to get the ideal viscosity.
CK-4 and FA-4 Oils and Late-Model Engines
There is one more part of the viscosity story. The latest generation of approved heavy-duty diesel engine oils are known as CK-4 and FA-4. CK-4 oils are approved for all engines and will protect every one going far back. FA-4 oils are marked for standard viscosities as they fit broadly into the range for each viscosity number, but they are actually slightly thinner. FA-4 oils have a lower viscosity and reduce friction, but are usable only in the latest engines‚ 2017 and newer. Even if the upper number relating to the viscosity is, say 30, the actual viscosity will be slightly lower. Their viscosity is measured in a different way that is critical for the protection of the tightest fitting parts of the engine‚ the pistons and rings.
FA-4 oils have a lower viscosity and reduce friction, but are usable only in the latest engines 2017 and newer.
Late-model engines have smoother parts. Final machining creates much smoother surfaces that do not require as thick a lubricating film for full protection. Many large fleets and conservative independents are sticking with CK-4 and taking no chances. But, if your engine is fully approved for FA-4 and you use the specified viscosity of that oil, you should not experience any unusual wear or damage, especially if your operating conditions are not extreme. And, the reduced friction will save you money in fuel purchases.
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