|
Oil used to be fairly straight forward stuff, but now it seems to take a very long time to work out whether the contents of that five litre can are suitable for your (or, more tricky; the member’s) car.
All engines need a good lubricating oil to prevent moving metal components seizing or bonding with each other. The requirements of modern high speed engines are even more extreme; everything is running at it’s limit so the oil needs to be really durable. Engine oils suffer squashing, shear, intense heat, tricky turbos and all sorts of grot produced by combustion. On top of that they are supposed to last for much longer.
In the 1930s, an engine’s first decoke was done at around 5000 miles, a rebore at 20 000 miles and off to the scrap yard at 60 000.
Now, 200 000 miles doesn’t seem unreasonable for a well maintained engine and the difference in life expectancy can be attributed to a dramatic improvement in oil quality. That, however, does not mean the motoring public have entirely grasped the importance of a good oil.
We need these properties from a good oil:
Good consumption
Friction reduction
Water held in suspension, not sitting in the bottom of the sump
Minimal drag on cold starting
Minimal sludge deposits
Maximum resistance to wear
No metal corrosion
No nasty exhaust emissions
Compatibility between additives
Control of oxidation
Good detergency to keep the engine clean
No deposits to jam up the piston rings
No lacquer deposits
Stable viscosity
GRADING
Early engine oils didn’t bother with any of the issues listed above, but then it was realised that quality needed to improve. Hence the arrival of a grading system.
The first grading system to be introduced was API (American Petroleum Institute). This uses codes from SA to SM. SA was the first oil performance tests, but didn’t consider the requirements listed above. Mercifully, grades SA to SH are now obsolete.
- SA – Obsolete – for older engines
- SB - Obsolete – for older engines
- SC - Obsolete – for 1967 and older engines
- SD - Obsolete – for 1971 and older engines
- SE - Obsolete – for 1979 and older engines
- SF - Obsolete – for 1988 and older engines
- SG - Introduced in 1989, contained much more active dispersant to combat black sludge. Obsolete – for 1993 and older engines
- SH – Introduced in 1993, has same engine tests as SG, but includes phosphorous limit of 0.12 % and control of foam, volatility and shear stability. Obsolete – for 1996 and older engines
- SJ – Introduced 1996 with same engine tests as SG/SH, but phosphorous limit 0.10% together with variation on volatility limits. Current – for 2001 and older automotive engines
- SL - Introduced 2001, all new engine tests, taking into account the latest engine designs meeting current emission standards. Current - for 2004 and older automotive engines
- SM – Introduced in November 2004 with improved oxidation resistance, deposit protection and wear protection. Better low temperature performance over the life of the oil. Current – for all automotive engines presently in use.
Diesel engine oils are different and oils are rated from API CD up to API CI-4. CA, CB and CC are obsolete – the later the final letter the higher the specification.
-
CD - Introduced in 1955. The international standard for turbo diesel engine oils for many years. Only uses the single cylinder test engine.
-
CE - Introduced in 1984 with better control of oil consumption, oil thickening, piston deposits and wear. Uses additional multi cylinder test engines.
-
CF4 - Introduced 1990, further improvements in control of oil consumption and piston deposits and wear. Uses low emission test engine.
-
CF - Introduced 1994, modernised version of CD. Tests revert to single cylinder low emission test engine. Intended for certain indirect injection.
-
CF2 - Introduced 1994. Defines effective control of cylinder deposits and ring face scuffing. Intended for two-stroke diesel engines
-
CG4 - Introduced 1994, a development of CF4 with better control of piston deposits, wear, oxidation stability and holding of soot particles in suspension. uses low sulphur diesel fuel in engine tests.
-
CH4 - Introduced 1998, development of CG4, giving further improvements in control of soot related wear and piston deposits. Uses a morecomprehensive engine test program to include low and high sulphur fuels.
-
CI4 - Introduced 2002, developed to meet 2004 emission standards. May be used where EGR systems are fitted and with fuel containing up to 0.5% sulphur. Can be used where API CD, CE, CF4, CG4 and CH4 are specified.
A little while after API began grading oils, the European ACEA testing scheme was introduced – the standards of which are said to be higher than API.
Again, there are several groups for both petrol and diesel engines where A applies to petrol engines and B applies to diesel engines:
- A1/B1 – Low friction oil
- A2/B2 – Conventional oil for normal use (now obsolete)
- A3/B3 – High performance/long drain interval oils
- A4/B4 – Stable, stay-in-grade intended for high performance petrol and direct injection diesel engines
- A5/B5 - Stable stay-in-grade oil for extended drain intervals in high performance petrol and car and light van diesel engines designed to be capable of using low friction low viscosity oils with a high temperature/high shear rate viscosity.
And then there are the Catalyst Compatibility Oils – C grades:
-
C1 - Stabale stay-in-grade oil intended for use as a catalyst compatible oil in vehicles with a diesel particulate filter (DPF) and Three Way Catalysts (TWC) in high performance car and light van diesel and petrol engines needing low friction, low viscosity, low SAPS (Sulphated Ash, Phosphorous, Sulphur with high HTMS (High Temperature/High Shear rate viscosity. These engines have the lowest SAPS limits and are not suitable for all engines.
-
C2 - As above, but designed to increase the life of the DPF and TWC and help fuel economy.
-
C3 - As above, but better.
Heavy duty diesel engine oils are classified E2 to E7, designed to help meet the Euro emission standards – but we don’t deal with many of those and I think you’ve suffered enough.
In the ACEA scheme the higher numbers don’t necessarily mean it’s a higher specification oil. New tests for oils are incorporated, but the same rating designation is retained – with a year suffix to show the latest specifications.
API gradings are fixed so the later letters are used as new requirements are adopted.
Four-stroke motorcycles are different again. These are classified as MA or MB. MA oils don’t have the higher levels of friction reducing additives which can cause problems for oil-bath clutches or starters which need friction. Most bikes prefer MA grade oils.
Some oils don’t carry any of the above ratings. These economy oils should be left on the shelf as quality is poor.
VISCOSITY
This is the thickness of an engine oil and the manufacturer of the engine will decide the oil viscosity and the performance grade needed by that engine. Oil viscosity is indicated by a set of SAE numbers such as 0W/40 or 20W/50, 10W/30. The first number tells you how thick the oil is when it’s cold. The second figure gives its thickness when it’s hot. The ‘W’ means ‘Winter’.
If you are still awake we are just getting to a confusing bit – the ‘0’ doesn’t mean it has no viscosity and all oils are thicker when they are cold. The lower ‘W’ numbers make cold starting easier and the higher numbers mean the oil has more ‘body’.
Single Grade Oils
I don’t know anything about kinematic viscosity or centistokes so won’t be explaining how this is measured.
Based on the range of viscosities the oil falls in at that temperature the oil is graded as a SAE number 0, 5, 10, 20, 30, 40, 50, 60 or 70. These numbers are referred to as the weight of an oil. The reference temperature is meant to approximate the operating temperature to which the oil is exposed in an engine.
However, the viscosity of a single grade oil which hasn’t the benefits of additives changes a lot with temperature. As temperature increases the viscosity of the oil decreases in a pretty set pattern. On single grade oils viscosity can be done at cold WINTER (W) temperature as well as checking minimum viscosity at 100ºC to grade an oil as SAE number 0W, 5W, 10W, 15W,20W or 25W.
A single grade oil graded at the hot temperature is expected to test into the corresponding grade at the winter temperature – so a 10 grade oil should match a 10W oil.
If the temperature ranges in a particular engine are not very wide then a single grade oil is ok – but don’t put it in anything more complicated than a lawn mower.
Multigrade
Engines don’t do constant temperature. They have to start from cold and then get very hot when running. Between these extremes of temperature the difference in viscosities in a single-grade oil is too great. So to bring the different viscosities closer together, special polymer additives called viscosity improvers are added to the oil.
These additives make the oil a multigrade. The viscosity still varies with temperature, but the degree of change is reduced. The change with temperature depends on the nature and amount of the additives put in the base oil. The API/SAE designation for multi-grade oils includes two grade numbers – 10W/30 designates a common multigrade oil.
The first number, associated with the W is not rated at any single temperature. The ‘10W’ means this oil can be pumped by an engine when as cold as a single grade 10 weight oil can be pumped. ‘5W’ can be pumped at a lower temperature than 10W. The second number – 30 - denotes the viscosity of this 10W/30 oil at 100ºC operating temperature which corresponds to the viscosity of a single grade 30 oil at the same temperature.
TURBINE OILS
Rotary engines don’t need quite the same things from an oil.
Deposit control and corrosion are not major issues when formulating a turbine oil and the shear stresses are minimal in a rotary engine because turbines are naturally balanced rotating machines. Normal, reciprocating engines are not. Turbine oils favour the ISO VG range 32, 48 and 68 and make extensive use of polyolester and polyalpholefin. They use a Group II base stock because they have to cope with very high temperatures.
In the gas turbine engines used in planes the really high temperatures are not reached until after the engine has shut down – when the heat has migrated from the combuster cans and the compressors into the regions of the engine with lubricated bearings and gearboxes. The gas flow used to run the turbines provides plenty of convective cooling which stops as soon as the engine shuts down, leaving residual heat which makes temperatures within the turbine rise dramatically. I thought you’d find this particularly useful when called out to your next cut-out/non-start on an Airbus.
CHOOSING AN ENGINE OIL
Check the manufacturer’s recommendations in the vehicle handbook – they know best. Pay particular attention to the ACEA or API performance grade and viscosity.
It’s ok to use an oil grade higher than the minimum specified so if the handbook says API SH then you can use a SJ grade without any concerns. Choice also depends on use – for severe use (that’s long, high speed journeys or towing) then a higher grade is a good idea.
An older engine which is starting to drink a bit of oil would benefit from a heavier oil – a 20W/50 instead of a 10W/40.
Modern oils are designed to blacken with use – they hold the soot from combustion in suspension where it does less harm.
SYNTHETIC ENGINE OILS
These were initially developed for high temperature gas turbine/jet engines where traditional mineral oils didn’t provide adequate lubrication performance. In the mid 1970s, synthetic motor oils were formulated and commercially used for the first time in automobiles. Improving the efficiency of lubrication reduced the occurrence of oil oxidation and sludge formation. Synthetics also reduced the wear and tear on gears, compared with conventional mineral-oil based lubricants. They are said to offer superior performance and longevity.
Instead of making engine oil with the usual petroleum/mineral based oil the molecules in synthetic oil are made from artificially synthesised polyalpha-olefins. These are specially designed polymers designed to have better motor oil properties.
These are special polymers designed to have better motor oil properties. The polymers [a polymer is a chemical compound with large molecules made of simple molecules of the same kind].
The polymers are made by bonding together alpha-olefin monomers which provide lots of flexible branching groups on the polymer molecule’s backbone.
This side-branching interferes with the ability of the molecules to line up compactly next to each other. This means the flexible molecules can slide past each other more easily and the synthetic oil has good flow-ability, even at low temperatures.
Because the molecules are made artificially they can be made big enough and ‘softer’ to retain good viscosity at higher temperatures, but the side branching interferes with the solidification and so flow is still good at lower temperatures. This means viscosity still decreases as temperature rises, but synthetic oils have a much better viscosity index than the mineral oils. Their special properties allow a wider temperature range and also have a lower pour point [Pour point is the lowest temperature at which oil will flow. This property is crucial for oils that must flow at low temperatures. Generally, when selecting oil, the pour point should be at least 10ºC lower than the lowest expected ambient temperature].
As their viscosity doesn’t change as much with temperature, synthetic oils don’t need the viscosity improvers normally essential in engine oils. These viscosity index improvers are components of oil which are most vulnerable to thermal and mechanical breakdown as the oil ages and wears out. As synthetic oils don’t have any viscosity index improvers they can’t degrade as quickly as traditional motor oils.
However synthetic doesn’t equal perfect - synthetic oils still trap and fill up with soot and other grotty bits so they still need changing regularly, even if it is not as frequently as a conventional mineral oil.
Fully synthetic oils are said to be superior to conventional oils in many respects – offering enhanced engine protection, performance and better flow in cold starts than mineral oils. Synthetic oils are generally used for extended drain periods.
Semi-synthetic oils are a blend of full synthetic oil and conventional mineral based oil – a compromise between fully synthetic and economy.
Beyond synthetic, semi-synthetic and mineral-based oils the differences lie in the additive packages.
TYPES OF OIL
1. Feedstock is separated into distillates and vacuum gas oils
2. Waxy vacuum gas oil molecules flow to the hydrocracker to begin conversion.
3. Hydrogen is introduced to saturate the molecules and remove impurities such as nitrogen, sulphur, oxygen and heavy metals.
4. Hydrocracking, under conditions of extreme temperature and pressure in the presence of a catalyst, converts aromatic molecules into saturated paraffins.
5. The altered stock is much lighter in colour because the contaminants have been removed.
6. Long waxy paraffin molecules are restructured into shorter, branched isoparaffins which resist gelling and improve low temperature pumpability.
7. Hydrogen is introduced again to remove any remaining aromatics and impurities, enhancing the oxidation and thermal stability of the now colourless oil.
Mineral Oil-Based
The base oil is conventional refined crude petroleum
Hydrocrack Oils (HC Oils)
The base product used for manufacturing NT (new-technology). Hydrocrack oil is either paraffin sludge from de-waxed crude petroleum raffinate [raffinate is a peculiar word which just means ‘refined product’] or vacuum gas oil. These are variably referred to in the mineral oil industry as either synthetic or mineral oils. Hydrocrack oils are made by breaking down the mineral oil-based chemical structure into its constituent components via a cracking process.
Synthetic Hydrocarbons (Polyalpha Olefins or PAO)
The basic components of synthetic hydrocarbons are carbon, hydrogen mineral oils and hydrocrack oils. However, the basic product used for making synthetic hydrocarbons is synthesised by means of several processes to achieve a particular molecular structure. Many synthetic oils also contain di-ester oils as well as the PAOs. These are chemically attracted to metal and this provides a microscopically thin lubricant coating. This is particularly useful at times like cold start when there isn’t much oil about and also provides good corrosion protection for metal surfaces which lack continuous lubrication.
Semi/Partially Synthetic Oils
These contain mineral oil, hydrocrack oils (NT) or synthetic hydrocarbons (PAO).
High performance oils cannot meet the requirements without additives. The type and quantity depend on its intended use. Additives protect against wear and corrosion and trap dirt particles in suspension. They also act as oil enhancers and protect the oil against ageing.
NOTE: Synthetic oils are generally regarded as the best – and they are, but they’re not suitable for all engines. Subaru blamed timing chain failures on some of their engines on the insistence of some owners of changing from mineral to synthetic oil. The synthetic oils were found to lack ‘shear’ and the chains broke.
© Vanessa Guyll, June 08 | 