Establishing Elemental Limit Values for Motor Oils
Limit and warning levels from elemental spectrometric analysis serve as indicators of the amount of foreign particles found in used oil that is still tolerable or, when compared with fresh oil, indicate when the altered lubricant must be changed. Values well above tolerable wear levels can also indicate an acute damage process. However, it is not easy to specify these warning levels. Hardly any engine or equipment manufacturer defines limit levels for used oil. This is because the operating conditions and times are too specific, and the origins of the foreign particles found in the oil are too diverse. Consequently, determining these factors is one of the essential tasks of every oil analysis. After all, the type, quantity and (to a certain extent) the size of the particles provide valuable information about wear, contaminants and the additives in the oil.
When warning and limit levels are used for the diagnosis of a specific oil specimen, the interactions between the values and other criteria should also be taken into account. A variety of factors play a role here, including the engine manufacturer, the engine type, the type of fuel used, the oil volume, the motor oil type, the service life of the motor oil, and any top-up quantities (makeup oil). The operating conditions can also vary markedly from one situation to the next. After all, the engine of a heavy construction machine operates under different conditions than the same engine of a truck traveling long distances on a highway at uniform speed.
However, all of these engines have one thing in common: their motor oil contains a lot of valuable information about the oil itself as well as the state of the engine. For example, the microscopic particles suspended in the oil provide an indication of the amount of wear of the corresponding parts or components. Elements such as sodium, potassium or silicon indicate contamination by road salt, hard water, glycol antifreeze or dust. Comparing the amount of organometallic additive elements (such as calcium, magnesium, phosphorus, zinc, sulfur or boron) in the used oil to fresh oil provides an indication of changes to the oil, such as additive depletion or possibly the mixing of different types of oils.
| Element | Upper Warning Level | Origin |
|---|---|---|
| Iron (Fe) | 80–180 | Cylinder block, cylinder head, timing wheels and timing chains, valves, valve tappets and guides, crankshaft, camshaft, rocker arm shaft, piston pins, roller bearings (with chromium), oil pump; rare in residues of ferrocene, a fuel additive for soot reduction; distinguishing between corrosion and wear based on the PQ index |
| Chromium (Cr) | 4–28 | Piston rings, crankshaft bearings, piston pins, exhaust valves, gaskets, guide bushes, chrome-plated parts and gears; Fe, Al and Cr are usually found in combination with Si in engines because dust causes the most piston (Al), piston ring (Cr) and cylinder (Fe) wear |
| Tin (Sn) | 12–24 | Often together with lead (Babbitt bearings) or copper; running surfaces of connecting-rod bearings, rocker arm shaft and piston pin bearings, solder (consisting of lead and tin) in soldered radiator joints; constituent of some synthetic base oils, additives in fire-resistant fluids |
| Aluminum (Al) | 12–55 | Primarily from pistons, oil pump housings, oil coolers, torque converter parts, turbocharger, guide bushes, plain bearings, cylinder blocks of all-aluminum engines (together with silicon) and dust containing bauxite (aluminum oxide) |
| Nickel (Ni) | 1–3 | Alloy constituent of exhaust valves, valve guides, turbochargers, high-strength gears and turbine blades; instead of being galvanized or chrome-plated, parts such as filter components may be nickel-plated; constituent of heavy oil (together with vanadium) |
| Copper (Cu) | 25–60 | Main constituent of brass and bronze; as wear metal from oil pumps, connecting-rod bearings, piston pin bearings, rocker arm shaft bearings, bronze worm gears, and sintered brake and clutch discs; resulting from the corrosion of oil coolers, piping and seals |
| Lead (Pb) | 10–30 | Usually in combination with tin and/or copper; connecting-rod bearings, nearly all running surfaces of plain bearings and soldered joints in combination with tin |
| Molybdenum (Mo) | 4–20 Up to 500 in fresh oil | Contained in transmission synchronizer rings, piston rings and heat-resistant steels; component of an antioxidant and friction modifier additive package in modern synthetic multigrade oils and gear oils; rarely as MoS2 oil additives |
Inductively coupled plasma (ICP) elemental analysis can be used to determine more than 30 different elements in motor oils. In addition to the presence of the elements, atomic emission spectroscopy (AES) by ICP can be used to determine the concentrations of the elements.
Laboratories routinely determine the following elements and values as part of motor oil testing and list them in the lab report: iron, chromium, tin, aluminum, nickel, copper, lead, calcium, magnesium, boron, zinc, phosphorus, barium, molybdenum, sulfur, silicon, sodium and potassium. In some cases, other elements are also determined, such as silver, vanadium, tungsten or ceramic elements like cerium and beryllium, which are rarely present in motor oils. They are only listed in the lab report if they are actually proven to be present or if the customer specifically requests this. Tables 1-3 show the possible causes for the presence of the elements found in oil, i.e., whether they are related to contaminants, wear or additives.
| Silicon (Si) | 15–30 Up to 15 in fresh oil | Intake air dust, anti-foam additive in motor oil, worn seals containing silicone, residues of silicone greases (also in oil sampling syringes), worn aluminum alloys (aluminum engines) |
|---|---|---|
| Potassium (K) | 2–30 | Additive in aqueous media such as glycol antifreeze or cooling water; mineral salt in road salt or tap water |
| Sodium (Na) | 5–30 Up to 800 in fresh oil | Additive in glycol antifreeze or cooling water; road salt, tap water, wastewater or salty air; additive components in some motor oils as a substitute for calcium or magnesium compounds; thickener in lubricating greases |
| Lithium (Li) | 2–10 | Constituent of multi-purpose greases (thickener); indication of contamination by grease or assembly pastes |
| Antimony (Sb) | 1–3 | Present in some lubricating greases as an EP additive in the form of antimony oxide; in connection with lead or tin in bearing alloys of plain bearings |
| Silver (Ag) | 1–3 | Silver-plated running surfaces of highly loaded plain bearings, such as in locomotive engines; silver solder residues; silver is attacked by additive systems containing zinc |
| Tungsten (W) | 1–2 | Rare in engine construction; alloy constituent for increasing hardness and corrosion resistance |
| Titanium (Ti) | 1–3 | Oil level indicator (float); alloy constituent in springs and valves; from ceramic components; as white titanium oxide in plastics and paints; marker additive in motor oils |
| Vanadium (V) | 1–3 | As a constituent of chrome-vanadium steel alloys in valves and valve springs; like nickel, it is a constituent of petroleum; blow-by product when ship engines are |
| Beryllium (Be) | 1–3 | Cube valves and valve seats; sintered bearings, constituents of sintered ceramic components or in jet engine oils; prohibited in F-1 engines operated with heavy oil fuels |
| Cadmium (Cd) | 1–3 | Components of plain bearings exposed to corrosion; sometimes also deep red pigments in plastics and paints |
| Cobalt (Co) | 1–3 | Possibly from components of turbines or from roller bearing alloys in connection with iron |
| Manganese (Mn) | 1–3 | Alloying element, usually with iron; steel used in valves, roller bearings, gears or shafts; contaminant in manganese mines (with Si); very rarely additives containing manganese |
| Tantalum (Ta) | Only found in oil as a constituent of ceramic components | |
| Cerium (Ce) | Only found in oil as a constituent of ceramic components | |
| Zirconium (Zr) | Only found in oil as a constituent of ceramic components |
Various factors must be taken into account when interpreting a lab report and the values of the elements found in the oil. Naturally, it is not sufficient to simply report the elements and their quantities. In order to assess the measured values, you must know whether the individual elements indicate contamination, wear or changes to the additives. However, these values are also interrelated to a certain extent. The relative proportion of various wear elements provides an indication of the affected machine parts or components, for example. Further, it is important to know how long it has taken for the oil to become enriched with specific wear elements since the last oil change. The operating time of the overall system or the running time of the engine, the oil volume relative to the engine power, and the top-up amounts must also be considered when analyzing or diagnosing warning levels.
In order to reliably assess the values determined for the used oil and their relationship to each other and to other factors, it is necessary to have a suitably large volume of data and analytical expertise. However, additive elements and base oil types can differ considerably depending on the type of oil used, so it is necessary to set suitably broad warning levels. Specific warning levels can only be defined for a specific oil type.
| Calcium (Ca) | 600–5,000 | Oil additive, detergent oil additive; improves cleaning and dispersion capacity as well as heat resistance; occasionally calcium-containing dust from building sites, lubricating grease constituent, or from cooling water or tap water containing calcium |
|---|---|---|
| Magnesium (Mg) | 100–1,500 | Oil additive; improves the corrosion protection, thermal stability and dispersion capacity of motor oils; increases the alkali reserve (BN); alloy constituent of engine blocks; hardening agent in hard tap water or salt water |
| Boron (B) | 10–500 | Improves engine cleanliness as an oil additive; borates are constituents of cooler antifreeze and corrosion protection media |
| Zinc (Zn) | Up to 2,000 in fresh oil | Improves wear protection as an oil additive; zinc-plated components such as filter support cores, threaded fittings, paints containing zinc and vulcanized synthetic materials |
| Phosphorus (P) | 600–2,000 | Oil additive in almost all types of oil; used to improve EP characteristics and reduce wear; has an anti-corrosion and anti-bacterial effect; reduces friction; renders metal surfaces chemically inert |
| Barium (Ba) | 2–20 | Usually not an additive in motor oils; for improving EP characteristics; friction modifier in ATFs; in the form of barium-complex soap; a constituent of greases and assembly pastes |
| Sulfur (S) | 500–6,000 | Constituent of base oils based on mineral oil; for this reason, it is present in almost all oils, but in widely varying amounts; along with phosphorus, sulfur is also a constituent of almost all additive packages for wear and corrosion protection, and is often found in connection with calcium and zinc |
The warning and limit levels listed in Tables 1-3 for wear elements, contaminants and additives are based on a semi-synthetic motor oil (SAE 10W-40) in a modern diesel engine with an oil volume of approximately 25 to 50 liters, using fuel compliant with EN 590 (containing 5 percent fatty acid methyl esters), and with an oil service life of approximately 500 operating hours or a mileage of approximately 47,000 miles.
The basic rule is that warning levels must be set lower for greater oil volume, shorter oil service life, lower engine speed and lighter load conditions.
However, the stated values are distinctly dependent on the oil manufacturer, the correct engine type, the service life of the oil charge, the oil volume and the top-up quantities (if any).
