Basic Engine Oil Theory
The following are taken from these websites:
Petroleum Quality Institute of America
http://www.pqiamerica.com/viscosity.htm
Certas Lubricant Solutions
Castrol
Viscosity
In simplest terms, viscosity is a measure that speaks to a liquid's resistance to flow. Fluids with a high viscosity are thick and flow slowly. Low viscosity fluids are thin and flow quickly. A common example of each is seen when comparing molasses with water. Whereas molasses is thick and flows slowly (high viscosity) when poured, water is "thin" and flows quickly when poured (low viscosity).
The viscosity of lubricants is also the measure of the lubricant's resistance to flow. Low viscosity lubricants are thin and flow easily like water. High viscosity lubricants are more like molasses, they are thick and have high resistance to flow.
The viscosity of an engine oil is significant for three primary reasons. The first is that the higher the viscosity of the lubricants the greater is its ability to provide what is referred to as "boundary lubrication." In practical terms, this means higher viscosity lubricants have a greater ability to maintain a boundary between moving parts. Based on this, one might conclude the higher the viscosity of the engine oil the better. But that’s not necessarily the case, and this leads to the second primary reason viscosity is important in engine oils.
Whereas high viscosity (thick) engine oils help to maintain a barrier between moving parts, they also create drag in the movement of those parts. The industry calls this parasitic load. Rather than burning fuel only to move a vehicle, the higher the viscosity of the lubricant, the more fuel is required to overcome the oils resistance to flow, thus placing additional load on the engine and driving up fuel consumption.
The third reason the viscosity of an engine oil is important is a about its cold flow properties. As the old saying goes, "it's as slow as molasses going uphill on a cold day." Well engine oils are the same, they flow more slowly when it's cold. So, whereas a high viscosity engine oil may be good for boundary lubrication, they can be a real challenge to move when the weather is cold. What this means is that an engine may be slow to get lubricated if the viscosity is too high and the temperature too low. For the car or truck owner this means when they go out to start their vehicle on a cold day, a thinner oil will flow more freely and lubricate more efficiently than a thicker oil.
There are two major goals of oils in engines:
- Viscosity to Coat at High Temperatures. Oil provides a protecting lubricating film at high engine temperatures. Success: Engine oil stays thick enough to coat moving parts. Failure: Engine oil thins and can no longer create a barrier between moving parts.
- Viscosity to Flow at Low Temperatures. Oil must be thin enough at low temperatures to coat and protect cold engines. Success: Engine oil remains fluid enough to move through a cold engine. Failure: Engine oil is too thick to be distributed in engine and parts rub freely.
An engine oil viscosity is mainly measured at three temperatures:
- Kinematic Viscosity measured in centistokes (cSt) at 40°C
- Kinematic Viscosity measured in centistokes (cSt) at 100°C
- Dynamic Viscosity measured in centipoise (cP) at 150°C and called High Temperature High Shear (HTHS) viscosity. It simulates the narrow tolerances and high speeds between moving parts within a hot engine, in particular bearings, camshaft, piston rings and cylinder liners. Shear stress: The force needed to overcome one sliding layer of fluid to another.
Nine ways oils and additives protect engines
- Create Protection. Oils create a thin lubricating film between moving metal surfaces.
- Reduce Buildup. Oils reduce the buildup of sludge and deposits.
- Capture Grime. Oils capture contaminants like dirt, sludge and metal particles before the filter.
- Transfer Heat. Oils transfer heat from the engine to the cooler circulating oil.
- Prevent Corrosion. Oils protect metal surfaces from rust and corrosion.
- Improve Economy. Oils improve fuel economy by reducing friction on moving parts.
- Reduce Foam. Additives help oil flow more freely by reducing the volume of foaming.
- Condition Seals. Oils condition seals in engine assemblies.
- Protect Viscosity. Additives help keep oil at proper viscosity.
Multigrade oil
Years ago, most vehicles used one viscosity grade oil in the summer and a different viscosity grade oil in the winter. But as motor oil technology advanced, additives such as viscosity index improvers allowed for the use of the same grade of oil year-round.
The low temperature viscosity of the oil is a measurement that simulates starting a car on a cold winter day. That value has the letter “W” after the number and has a dash after the W. For example, if the oil is a 5W-30, the 5W part describes the viscosity of the oil at low temperatures. The lower the number, the faster the oil will flow at vehicle start up. The numbers that you can see are 0, 5, 10, 15, 20 and 25.
The high temperature viscosity is the number after the dash and is related to the viscosity of the oil as it is moving around your engine after the car has warmed up and is at normal engine temperature of 100°C. In the 5W-30 example, the 30 defines the viscosity of the oil at normal engine temperatures. Again, the lower the number, the lower the viscosity of the oil and the faster the oil will move around the engine. The numbers that you can see are 8, 12, 16, 20, 30, 40, 50, and 60.
Base oil
Base oils are, primarily, hydrocarbons – i.e. a molecule with hydrogen and carbon atoms.
The American Petroleum Institute (API) categorises base oils into five groups: Group I, II, III, IV, and V, based on the Saturates, Sulphur and Viscosity Index.
Automotive oil requires base oil with high levels of saturates. The hydrocarbon molecules are saturated with hydrogen. High saturates levels mean that the molecular bond of the oil is stronger. This increases the resistance to oil oxidation and the reduction in viscosity.
Low sulphur levels are required for automotive oil. Sulphur reacts with oxygen and can cause corrosion and oxidation. It can also damage catalytic converters. Low SAPS engine oil: An engine oil formulated with reduced Sulphated Ash, Phosphorus and Sulphur levels.
Viscosity Index is a scale that measures the oil’s change of viscosity due to temperature.
The higher the Viscosity Index (VI) number, the better, because it represents a smaller change in the oil viscosity as temperatures change. Viscosity is measured at 40 °C and 100 °C.
VI is useful for comparison purposes with an oil analysis report.
Mineral oils are blended from base oils that are primarily hydrocarbons. Paraffinic oil is the most widely used automotive base oil.
Traditional mineral oils tend to be blended from Group II base oils.
Semi synthetic oil tends to be a blend of Group II and Group III base oils. Note, however, that semi synthetic oil is likely to contain less than 30% synthetic oil and sometimes significantly less and can still be called “semi synthetic oil”.
Synthetic oil is blended from either Group III base oil or Group III Gas to Liquid (GTL). Group IV PAO base oil is also used.
Modern automotive oil is blended from Group III or Group III Gas to Liquid (GTL) base oils.
Both types of Group III are classed as fully synthetic in most countries.
Group IV Polyalphaolefin (PAO) is also a fully synthetic engine oil.
Additives
Chemicals are added to an automotive oil to perform specific tasks. Additives deplete within the engine, so regular oil changes are important.
Additives can counteract each other. Even to the degree to which additive gets to the surface first, forms the strongest layer and has the biggest effect.
Lubricant formulas need to balance the types and amounts of each of these additives in order to achieve the desired lubricant properties. For example, too much anti-wear additive is likely to affect the corrosion properties or friction modification performance of the lubricant.
The skill is with the additive and lubricant chemists to balance these challenges and create an automotive oil with all the desired characteristics.
Typical additives in engine oil are:
Viscosity Modifier (VM) | Also known as Viscosity Index Improver (VII) Increases the base oil viscosity to the required SAE viscosity for multigrade oil at 100°C. A high molecular weight polymer additive that is used to thicken the base oil to create the desired viscosity for a multigrade engine oil. At low temperatures, they exist as tightly coiled chains, which have little effect on the oil viscosity. At higher temperatures, the chains become “solvated” and open up into the oil. An example would be an octopus or walking through a crowded room with arms wide open. Automotive engine oil blended from a low viscosity base oil is used to ensure correct viscosity at low temperature. A Viscosity Modifier (VM) is added to ensure the correct viscosity at 100°C. Common VMs: • Olefin Copolymers (OCP) are often used in automotive oils due to their excellent engine performance • Polymethacrylate (PMA) are used in fuel economy engine oils and have excellent low temperature performance (see Additives and Temporary viscosity loss (TVL) entries) |
Anti-wear additives | Helps to combat wear on start-up, which is the boundary lubrication phase |
Corrosion Inhibitor | Stops corrosion of metal components |
Detergent | Keeps the engine clean of engine deposits and helps to neutralise corrosive acids |
Dispersant | Stops soot from forming lumps and blocking the oil galleries |
Oxidation Inhibitor | Reduces oxidation, which causes sludge and viscosity to increase |
Pour Point Depressant (PPD) | Lowers the pour point of the oil. In cold conditions, (PPD) reduces the tendency of the oil to experience wax crystallisation. (PPD) is a polymer similar to the Viscosity Modifier additive.The pour point is the lowest temperature that the oil can be poured under test conditions |
Rust Inhibitor | Similar to corrosion inhibitor |
Anti-foam agent | Used to suppress the oil foaming in engines. It works by breaking up large bubbles into smaller bubbles. Foam can lead to an increased likelihood of oxidation of the oil |
Friction Modifier | Reduces friction in an engine to enhance fuel economy |
Automotive oil – How it is blended from base oil
Base oil provides the start point. Valvoline’s automotive oil, for example, uses high quality Group III base oil. Other methods include Gas to Liquid (GTL) or Group IV Polyalphaolefin (PAO) base oil.
With a 5W-30 multigrade oil, the “30” relates to a viscosity of 9.3 to <12.5 cSt at 100°C. To achieve this viscosity, a 4, 6 or 8 cSt base oil is used.
Viscosity Modifiers (VM), known also as Viscosity Index Improvers (VII), are added to “thicken” the oil’s viscosity when it is at 100°C. By using a low viscosity base oil, with thickeners, the cold start and normal operation temperature viscosity standards can be achieved, as set out by the SAE Automotive Lubricant Viscosity Grades: Engine Oils – SAE J300.
Oil chemists develop a balanced set of additives that play specific roles but do not counteract each other.
Film strength
The ability of an oil film to withstand pressure due to load, temperature and speed. A loss of film strength promotes metal to metal contact, creating wear. A shear-stable oil retains its film strength.
An oil can lose its shear stability by the depletion of the Viscosity Modifier (VM).
Lubricating regimes:
There are four lubricating regimes in an engine:
- Boundary >> Start-up conditions
- Mixed >> The phase between start-up and full-flow lubrication
- Hydrodynamic >> Full-flow lubrication
- Elastohydrodynamic >> Full-flow lubrication for roller bearings and camshaft lobes
Stribeck curve
This highlights the relationship between the phases of oil lubrication and wear, film thickness and coefficient of friction.
The four phases of lubrication are boundary, mixed, hydrodynamic and elastohydrodynamic. The diagram below shows that a harmonious lubrication state exists at the juncture of boundary and hydrodynamic lubrication.
The Stribeck curve is named after a German mechanical engineer, Richard Stribeck, who pioneered the four lubrication phases in 1902.
Stribeck curve | ||||
| Boundary | Mixed | Hydrodynamic | Elastohydrodynamic |
Wear | Wear controlled by anti-wear additives | Potential for wear is dropping significantly | Wear is at its lowest | See Hydrodynamic lubrication |
Film thickness | Film thickness is thin | Film thickness is increasing dramatically | Film thickness is at its highest | See Hydrodynamic lubrication |
Co-efficient of friction | Co-efficient of friction is dropping | Co-efficient of friction is low | Co-efficient of friction is increasing slightly | See Hydrodynamic lubrication |
Boundary lubrication
Boundary lubrication is related to metal-to-metal contact. This is especially relevant during start-up or low-speed and subsequent high-load conditions. The oil is not thick enough to overcome the microscopic roughness in the bearings (asperities).
Friction tends to be at its highest during boundary lubrication. 70% of engine wear can occur during start-up.
Elastohydrodynamic lubrication
This phenomenon occurs in roller bearings and camshaft lobes when the curve of the roller and the race are in opposite directions and have a very small contact area. Extremely high pressure is exerted on the bearing by the oil, in the region of 450,000 PSI and there is momentary elasticity or a temporary deformation of the metal bearing. The bearing returns to its normal shape as the rotation continues. The oil film thickness is in the region of 1 micron. The good news is the surface asperities are in the order of 0.4 to 0.8 microns.
Zinc dialkyl dithiophosphates (ZDDP)
An anti-wear additive found in automotive engine oil called Zinc dialkyldithiophosphates (ZDDP).
Limits are set on the amount of ZDDP that can be used in engine oils. ZDDP can coat the precious metals on catalytic converters, rendering them useless.
ZDDP has particular relevance at start-up, when the engine is in the boundary lubrication phase. A classic car oil, for example, Valvoline VR1 20W-50 has 1,400 ppm of ZDDP. A modern, well-formulated 5W-30 fully synthetic Low SAPS C3 oil will contain significantly less ZDDP, around 800 ppm.
This is an extract of an article by Richard Long of the Southern Classics Society, first published in the TVR Magazine:
“Classic car petrol engines (1950 through to 1990) will by now have many years and miles under their pistons, and their care is just as important as modern day units; perhaps even more so. What may be good oil for one type of engine could be an anathema for another. So what do classic car engines need from oil that is suitable for their longevity and protection?
One of the key components is the zinc level in oil and this is defined as "parts per million (ppm)".
The zinc element is actually contained within a compound called Zinkdialkyldithiophosphate (ZDDP] and its inclusion is a critical factor for old style engines. Through past decades the level of ZDDP has been decreasing due to modern catalytic convertors and far more efficient engine design. Modern "cats” cannot deal or survive with the phosphorus that is also contained within ZDDP; as a result modern car manufacturers have progressively required a reduction of this additive in oil. That’s understandable. However, where does that leave a classic car owner in deciding what oil to choose for their classic car in the 21st Century?
It’s an Interesting question and a far from a straight forward topic.
So what does ZDDP bring to the oil party? Firstly we need to look at why it is an important factor. The majority of classic car owners will have tappet followers that are "flat-bottomed", that is to say the bottom surface is flat to the naked eye. These followers have an extremely tough life; probably only second to cylinder head valves. As the camshaft turns, each lobe per revolution makes contact with the followers. The shape of the lobe is designed so that at its peak revolution it will push the follower up which then pushes the tappet rod up and opens the valve in the cylinder head, via the rocker. The valve is seated extremely tight by single or double springs to form a gas tight seal within the cylinder head combustion chamber.
So as you can see the force placed upon the bottom of the cam follower is significant to say the least. This "super-pressure" contact causes friction and as we know friction causes component wear. This is where ZDDP plays its part. Zinc is a polar molecule, so it is attracted to steel surfaces.
Under high heat and extreme load (pressure), the Zinc reacts with the steel surface and creates a phosphate glass film that protects the steel surface by forming a sacrificial layer that covers the peaks and fills in any indents on the steel surface. Basically your flat bottomed follower really does become 99.9% flat and smooth. By forming this protective layer the cam lobes and flat bottomed followers are heavily protected against friction wear; remember friction wear can never be eliminated but much can be done to slow the process down.
So ideally to get the best protection the oil needs a high ZDDP, but modern oils of today do not contain this but rather "other additives“ which oil manufactures keep close to their chest as it’s all about marketing and protecting their "recipe”. That is little or no comfort for the classic car owner who relies on a decent multigrade mineral oil with a generous level of ZDDP contained therein.
Of significance, many oils that say "classic car oil" do not actually contain enough ZDDP or worse still a mere trace. So, what level of this additive does a classic car owner require to feel confident that the oil in their car engine Is not only lubricating but also protecting those parts under extreme pressure. Without a doubt an owner should be looking for an oil that contains a minimum of 1000ppm of ZDDP and to a maximum of 1600ppm. In fact an oil containing in excess of 1500ppm may cause more harm than good; such high levels of ZDDP are specifically manufactured for race engines and It is not an oil that can be purchased off the shelf. Some classic car oils only contain 800ppm and these oils are insufficient to fully protect metallic components. Oils such as Halfords Classic 20W/50, Comma Classic 20W/50 and Castrol XL 20W/50 are below the 1000ppm level and contain only 800 ppm. Sorry to give you that bad news, but the price of those oils may provide a clue.
So what oils provide in excess of 1000ppm? Well, oils such as Penrite, Millers and Morris as these companies specifically manufacture their oil to be suitable for classic cars and deliver a high Zinc content.
What is odd or perhaps not when you look at it, is that companies such as Penrlte and Millers are more than happy to quote their ZDDP levels; whereas to get Castrol or Halfords to divulge their content is another matter entirely. Their corporate stock phrase will be that there are sufficient additives to meet the required standard.
So what is the required standard and again another interesting point; as engine technology has improved over the years the requirement for ZDDP has reduced to the point that it is no longer added in that pure form. Additionally, modern oils are now either semi of fully synthetic with ever decreasing viscosity levels and higher levels of detergent; this is because modern petrol engines are cleaner and their metallic components are under less stress compared to cars from the ’50s through to the ‘90s.
In fact for oil manufacturers to make multigrade mineral oils with a high level or any level of ZDDP costs money and what’s the point In that when classic cars make up a tiny percentage of cars on the road. Additionally, as cars become mechanically more efficient the “make-up" of the oil must correlate with those changes. The internationally recognized index for oils is the API (American Petroleum Index) and this code can be seen on all oils from multigrade to fully synthetic.”
[From the introduction of API SJ in 1996 the API Classifications were more focused on the control of engine oil components that contributed to catalyst poisoning. And wouldn’t you know it, one of the main contributors to catalyst poisoning was phosphorus from ZDDP! So, from API SJ on, there has been a move away from zinc based anti-wear agents, including limiting the phosphorus content of a Passenger Car Motor Oil (PCMO) to below 1000ppm. The zinc level of an engine oil was no longer an absolute indication of that engine oil’s anti-wear performance.
API SH Zinc: 1300 ppm max, Phosphorus: 1200 ppm max
API SL/SJ Zinc: 1100 ppm max, Phosphorus: 1000 ppm max
API SM Zinc: 870 ppm max, Phosphorus: 800 ppm max]
Do Older Engines Need Oil With ZDDP Additives? (From Amsoil blog)
What is high-zinc motor oil?
Zinc dialkyldithiophosphate (ZDDP) is the most common zinc-based additive and is used primarily as an anti-wear agent to prevent premature engine wear. It also provides corrosion and oxidation protection.
However, because the zinc and phosphorus found in ZDDP can negatively affect catalytic converters, it has been phased out of motor oil formulations in recent years.
Reducing ZDDP has drawbacks, as classic-car owners have found. Older vehicles with flat-tappet camshafts and, in particular, engines that include high-tension valve springs or other modifications that create high contact pressures, can suffer premature wear due to reduced ZDDP levels.
For best protection, engine builders and gearheads typically use high-zinc and high-phosphorus motor oil to offer extra protection for flat-tappet cams, lifters and other components during break-in.
AMSOIL Break-In Oil, for example, contains 2,200 ppm zinc and 2,000 ppm phosphorus.
How do ZDDP additives work?
ZDDP anti-wear additives are heat-activated, meaning they provide wear protection in areas of increased friction.
As temperatures rise and surfaces come closer together, ZDDP decomposes and the resulting chemistry protects critical metal surfaces.
When parts move during operation, any sliding or rolling motion takes place on top of or within the ZDDP anti-wear film, which reduces metal-to-metal contact.”
 |
2ZZS-GE Flat Rocker Arm Pad for High Speed (6200+ rpm) |
“This is especially important in engines with flat-tappet camshafts or engines modified to create more horsepower. High-tension valve springs, often used in racing applications, also increase the potential for cam wear and require additional ZDDP.
Flat-tappet cams vs. roller cams
The design of flat-tappet cams makes them especially vulnerable to wear. As the name indicates, the tappet – or lifter – is flat. During operation, the cam-lobe slides rapidly over the tappet, increasing friction and temperatures.
A thin oil film is the only barrier that prevents the lifter and cam lobe from welding together.
If the oil film fails or provides insufficient wear protection, the two components can eventually wear the flat-tappet cam and affect valve operation.
Engine power and efficiency can decline if the flat-tappet cam cannot lift the valves enough to adequately charge the chamber for ignition or release exhaust fumes. Because most V-8 engines of the muscle-car era came standard with flat-tappet cams, the problem is especially prevalent to classic-car and hot rod owners.
Roller cams, on the other hand, are differentiated by rolling contact rather than sliding contact. Although costlier, roller cams are common in most modern vehicles and can be retrofitted into classic-car and hot-rod engines.
The role of piston rings
Even though Hugh didn’t ask about it specifically, I should also mention the importance of seating the piston rings during break-in.
Although a new or freshly honed cylinder appears smooth to the naked eye, it actually contains microscopic peaks and valleys. If the valleys are too deep, they collect excess oil, which burns during combustion and leads to oil consumption.
The sharp peaks, meanwhile, provide insufficient area to allow the rings to seat tightly. That means highly pressurized combustion gases can blow past the rings and into the crankcase, reducing horsepower and contaminating the oil.
Find out why championship engine builder Jesse Prather requires his customers to use AMSOIL Break-In Oil.
Breaking in the engine wears the cylinder-wall asperities, providing increased surface area for the rings to seat tightly. The result is maximum compression (i.e. power) and minimum oil consumption.
Do you always need an oil with ZDDP additives?
Do you need to use high-zinc motor oil in an older engine after break-in, which is typically about 500 miles?
Yes, if you want to maintain horsepower and promote longevity.
Even after the cam has seasoned, it’s still exposed to tremendous heat and pressure, especially in a heavily modified or racing engine. The constant barrage of pressure can rupture the oil film responsible for preventing wear, leading to metal-to-metal contact.
After break-in, we recommend using an oil with at least 1,000 ppm ZDDP in a flat-tappet engine.
That way, you’re providing your expensive engine with maximum protection against wear and horsepower loss.”
Interesting articles about ZDDP:
Zinc Additive for Oil—How Much ZDDP is Enough
Low Speed Pre Ignition (LSPI)
Premature combustion of the air/fuel mixture in Gasoline Direct Injection engines (GDI).
The phenomenon occurs most commonly at low RPM and high engine loads. Extremely high cylinder pressures can be experienced, which can lead to serious engine damage.
High concentrations of calcium in the detergent additive of the oil has shown to increase the frequency of LSPI.
No comments:
Post a Comment