Effect of low viscosity oils on engine bearings

Low viscosity oils produce less friction and therefore decrease the fuel consumption and combustion gas emissions.

One of the consequences though of lowering oil viscosity is a decrease of the minimum oil film thickness. The shaft surface is getting closer to the bearing surface.
This increases the failure threat level of such factors as surface roughness, shaft grinding irregularities, oil borne foreign particles and geometrical distortions and misalignments.Direct metal-to-metal intermittent contact between the surfaces occurs more frequently.
The wear rate is increased due to the direct metal-to-metal contact.
In this case seizure resistance of the bearing material becomes more important.

The lead based overlay of the conventional tri-metal bearing has a very good seizure resistance, and if the misalignments, shaft grinding defects, etc. are very small, a tri-metal bearing will work perfectly with low viscosity lubricant.
However if the dimension of the geometrical defect is close to the overlay thickness (0.0008”/0.020mm), the overlay is worn fast in some bearing areas and the intermediate layer is exposed.
The intermediate layer has some level of emergency seizure resistance but this level is definitely insufficient for normal bearing operation.

The AlSnSi bearings are much more tolerant to the geometrical defects due to the large thickness of the bearing alloy (~0.01”/0.25mm).
Very good seizure resistance of AlSi alloy is achieved due to the hard silicon particles, which continuously polish the shaft surface and therefore prevent seizure.

Another problem of low viscosity oils is associated with the distribution of oil pressure over the bearing surface.
Oil viscosity does not exert influence on the average pressure however it does affect the value of the pressure peak. The pressure distribution in low viscosity oil is less uniform – the peak pressure is higher therefore the load applied to the bearing material in this particular area is higher.
This may cause fatigue if the fatigue strength of the material is insufficient.
The effect of low viscosity oil on the pressure distribution may be compensated by a decrease of the bearing clearance.

Tin based engine bearing overlays

Tin based overlays contain copper (3-6%) as a hardening component.
The alloys are developed to replace toxic lead based overlays.
Tin based overlays possess excellent corrosion resistance and cavitation resistance.

Overlay multilayer system of Sn6Cu alloy deposited on the bi-layer diffusion barrier Ni-SnNi allows to create a microhardness gradient across the overlay thickness due to varying content of copper (soft top layer and hard bottom layer of the overlay). Such multilayer bearings are capable to withstand alternating loads up to 11600-13000 psi (80-90 MPa).
Hardness of tin-copper overlays is 20-30 HV.

Tin-copper overlays are deposited by the electroplating methods.

Nickel diffusion barrier (nickel dam) is deposited between the intermediate layer and tin containing overlay in order to prevent migration of tin from the overlay into the intermediate material (copper).Diffusion of tin into copper causes formation of brittle Cu-Sn intermetallic compounds (Cu₃Sn, Cu₆Sn₅), which decrease the adhesion strength of the overlay to the intermediate layer. In addition to this decrease of the tin content in the overlay due to migration into intermediate material deteriorates the Corrosion resistance]] of the lead-based overlay alloy.
Thickness of nickel diffusion barrier is about 0.00004”-0.00006” (1-1.5 µm).Nickel electroplating methods (Watts nickel plating solutions or nickel sulfamate solutions) are used for the diffusion barrier deposition.

Engine parameters determining bearing loading

Engine parameters determine the loads applied to the bearings. The load is the resultant force created from the combustion pressure and inertia forces of the moving engine parts.

The load applied to the bearing at any angle of the crankshaft may be calculated and so too can the hydrodynamic bearing parameters (min. oil film thickness, energy loss, oil temperature rise, oil flow, etc.).

The engine factors determining the load applied to the bearing are as follows:

• Compression ratio. The higher the C.R. the higher the pressure of the gas in the TDC (top dead center) and the higher the bearing load.
• Displacement per one cylinder. Larger displacement means larger amount of fuel-air mixture is intaken into the cylinder in each engine cycle and larger energy is discharged in each cycle.
• Fuel type. Gasoline fuel with higher octane number allows working with higher compression ratio without the risk of detonation. Therefore higher octane fuel produces higher load. Methanol fuel may be used with even higher compression ratio (e.g. 16:1).
• Engine type. Diesel engines especially diesel engines with direct injection produce highest pressure in the combustion cylinder (up to 220 bar) and highest load applied to the bearings.
• Aspiration type. Turbo- and supercharged engines use compressed air in the air-fuel mixture. Compressed air allows to intake larger air amount and larger fuel amount in each engine cycle since air-to-fuel ratio is approximately constant. So the charge of energy is higher than in normally aspirated engines and the load is also higher.
• Bore diameter. The same cylinder pressure produces higher absolute load in the cylinder with higher bore diameter.
• Piston and conrod weights and rotation speed. Inertia forces developed by the moving piston and conrod decrease the resultant force acting on the upper conrod bearing. At the same time they increase the force acting on the lower part. The inertia forces are proportional to the weights of the moving parts and to the squared rotation speed therefore the effect of inertia forces is particularly high at high RPM (6000 – 10000).

Lubrication regimes

Sliding friction is significantly reduced by an addition of a lubricant between the rubbing surfaces.Engine bearings are lubricated by Engine oils constantly supplied in sufficient amounts to the bearings surfaces.

Lubricated friction is characterized by the presence of a thin film of the pressurized lubricant (squeeze film) between the surfaces of the bearing and the journal.
The ratio of the squeeze film (oil film) thickness h to the surface roughness Ra determines the type of the lubrication regime:

• Boundary lubrication (h<Ra).

A constant contact between the friction surfaces at high surface points (microasperities) occurs at boundary lubrication.
This regime is the most undesirable since it is characterized by high coefficient of friction (energy loss), increased wear, possibility of seizure between the bearing and journal materials , non-uniform distribution of the bearing load (localized pressure peaks). Very severe engine bearing failures are caused by boundary lubrication.
Conditions for boundary lubrication are realized mainly at low speed friction (engine start and shutdown) and high loads.Extreme pressure (EP) additives in the lubricant prevent seizure conditions caused by direct metal-to-metal contact between the parts in the boundary lubrication regime.

• Mixed lubrication (h~Ra).

An intermittent contact between the friction surfaces at few high surface points (microasperities) occurs at mixed lubrication.
Mixed lubrication is the intermediate regime between boundary lubrication and hydrodynamic friction.

• Hydrodynamic lubrication (h»Ra).

High rotation speed at relatively low bearing loads results in hydrodynamic friction, which is characterized by stable squeeze film (oil film) between the rubbing surfaces.
No contact between the surfaces occurs in hydrodynamic lubrication.
The squeeze film keeps the the surfaces of the bearing and the shaft apart due to the force called hydrodynamic lift generated by the lubricant squeezed through the convergent gap between the eccentric journal and bearing.
Bearings working under the conditions of hydrodynamic lubrication are called hydrodynamic journal bearings.



The three lubrication regimes are clearly distinguished in the Striebeck curve (see the figure above), which demonstrates the relationship between the coefficient of friction and the bearing parameter η*N/p_av (η - dynamic viscosity of the lubricant, N - rotation speed, p_av - average bearing pressure).

Stability of different lubrication regimes may be explained by means of the Striebeck curve:
Temperature increase due to heat generated by friction causes drop of the lubricant viscosity and the bearing parameter.

According to the Striebeck curve decrease of the bearing parameter in mixed regime causes increase of the coefficient of friction followed by further temperature rise and consequent increase of the coefficient of friction. Thus mixed lubrication is unstable.

Increase of the bearing parameter due to temperature rise (lower viscosity) in hydrodynamic regime of lubrication causes the coefficient of friction to drop with consequent decrease of the temperature. The system corrects itself. Thus hydrodynamic lubrication is stable.

Mechanisms of wear

Wear is the removal of the material from the surface of a solid body as a result of mechanical action of the counterbody.

Wear of Engine bearings is the removal of the bearing material from its surface caused by sliding friction between the bearing and the crankshaft.

Wear may combine effects of various physical and chemical processes proceeding during the friction between two counteracting materials: micro-cutting, micro-ploughing, Plastic deformation, cracking, fracture, welding, melting, chemical interaction.

The mechanisms of wear:

• Abrasive wear
• Adhesive wear
• Fatigue wear
• Corrosive wear
• Erosive wear

Abrasive wear

 Abrasive wear occurs when a harder material is rubbing against a softer material.

• If there are only two rubbing parts involved in the friction process the wear is called two body wear.

In this case the wear of the softer material is caused by the asperities on the harder surface.

• If the wear is caused by a hard particle (grit) trapped between the rubbing surfaces it is called three body wear. The particle may be either free or partially embedded into one of the mating materials.

In the micro-level abrasive action results in one of the following wear modes:

• Ploughing. The material is shifted to the sides of the wear groove. The material is not removed from the surface.
• Cutting. A chip forms in front of the cutting asperity/grit. The material is removed (lost) from the surface in the volume equal to the volume of the wear track (groove).
• Cracking (brittle fracture). The material cracks in the subsurface regions surrounding the wear groove. The volume of the lost material is higher than the volume of the wear track.

Adhesive wear

 Adhesion wear is a result of micro-junctions caused by welding between the opposing asperities on the rubbing surfaces of the counterbodies. The load applied to the contacting asperities is so high that they deform and adhere to each other forming micro-joints.
The motion of the rubbing counterbodies result in rupture of the micro-joints. The welded asperity ruptures in the non-deformed (non-cold worked) regions.
Thus some of the material is transferred by its counterbody. This effect is called scuffing or galling.
When a considerable areas of the rubbing surfaces are joined during the friction a Seizure resistance (compatibility) seizure of one of the bodies by the counterbody may occur.

The factors decreasing adhesive wear:

• Lower load.
• Harder rubbing materials.
• Contaminated rubbing surfaces.
• Presence of solid lubricants.
• Presence of a lubrication oil.
• Anti-wear additives in oil.

Fatigue wear

 Fatigue wear of a material is caused by a cycling loading during friction. Fatigue occurs if the applied load is higher than the fatigue strength of the material.
Fatigue cracks start at the material surface and spread to the subsurface regions. The cracks may connect to each other resulting in separation and delamination of the material pieces.

One of the types of fatigue wear is fretting wear caused by cycling sliding of two surfaces across each other with a small amplitude (oscillating). The friction force produces alternating compression-tension stresses, which result in surface fatigue.
Fatigue of overlay of an engine bearing may result in the propagation of the cracks up to the intermediate layer and total removal of the overlay.
Corrosive wear

Wear may be accelerated by corrosion (oxidation) of the rubbing surfaces.
Increased temperature and removal of the protecting oxide films from the surface during the friction promote the oxidation process. Friction provides continuous removal of the oxide film followed by continuous formation of new oxide film.
Hard oxide particles removed from the surface and trapped between the sliding/rolling surfaces additionally increase the wear rate by three body abrasive wear mechanism.

Erosive wear

Erosive wear is caused by impingement of particles (solid, liquid or gaseous), which remove fragments of materials from the surface due to momentum effect.

Erosive wear of Engine bearings may be caused by cavitation in the lubrication oil. The cavitation voids (bubbles) may form when the oil exits from the convergent gap between the bearing and journal surfaces. The oil pressure rapidly drop providing conditions for voids formation (the pressure is lower than the oil vapor pressure). The bubbles (voids) then collapse producing a shock wave, which removes particles of the bearing material from the bearing.

Copper based bearing materials

The design of internal combustion engines has a tendency of decreasing the engine dimensions. This tendency leads to more compact bearings and consequently to higher specific alternating loads applied to them.

 Copper based Engine bearing materials provide superior fatigue strength (load capacity).

Copper based bearing alloys commonly contain tin (up to 10%) as a strengthening component and a soft component (lead or bismuth) distributed in a copper-tin matrix as a separate phase in form of small particles.
Soft component imparts to the material anti-friction properties (compatibility, conformability, embedability).
Maximum content of soft phase is about 25%.
Copper based bearing alloys are manufactured by either casting or sintering technology. Sintered materials may be less strong than cast materials of the same composition due to possible voids and pores in the sintered alloys.

Despite the presence of soft components in most copper based bearing alloys their anti-friction properties are poor therefore copper (bronze) bearings are rarely used in engines in monometal (solid) or bi-metal (steel backed) forms.
Typical copper bearings are steel backed and overplated with a thin sliding layer (overlay) of a soft material (tri-metal structure). The overlay improves the anti-friction properties of the copper alloy.
Modern copper based bearing alloys do not contain lead because of its harmful effect on the environment. Lead in lead free alloys may be substituted by harmless bismuth having excellent anti-friction properties comparable with lead.

Load carrying capacity of copper based bearings is determined by the fatigue strength of the overlays:

• Lead based overlays: 7250-10150 psi (50-70 MPa)
• Tin based overlays: 11600-13000 psi (80-90 MPa)
• Sputter overlays: 14500-17400 psi (100-120 MPa)

Characteristics of some copper based engine bearing materials

Composition	Technology	Structure	Hardness, HV	Diffusion barrier	Overlay	Applications
CuPb25Sn1	Cast	Tri-metal	55	Nickel	Pb10Sn2Cu	Low loaded bearings
CuPb23Sn1.5	Cast	Tri-metal	60	Nickel	Al20Sn sputter	Highly loaded conrod bearings
CuPb23Sn1.5	Cast	Tri-metal	60	no or Nickel	Pb10Sn12In	Medium and highly loaded bearings
Cu24Pb243.5Sn	Cast	Multilayer	65	Nickel	Pb10S3Cu + MoS2 in resin	Racing car engine bearings
Cu20Pb3Sn	Cast	Tri-metal	65	Nickel	Pb10Sn3Cu	Medium loaded bearings
Cu15Pb7Sn	Cast	Bi-metal	80	no	no	Small end bushes
Cu15Pb7Sn	Cast	Solid	80	no	no	Thrust washers
Cu10Pb10Sn	Cast	Bi-metal	100	no	no	Highly loaded Small end bushes
Cu10Pb10Sn	Cast	Solid	100	no	no	Highly loaded Small end bushes
Cu8Al	Cast	Solid	145	no	no	Highly loaded bushes, lead free
Cu24Pb1Sn	Sintered	Tri-metal	55	Nickel	Pb10Sn2Cu	Low loaded bearings
Cu5Sn1Ag	Sintered	Tri-metal	80	no	MoS2 in resin	Medium loaded bearings, lead free
Cu22Pb1Sn	Sintered	Multilayer	55	Nickel	NiSn + SnCu	Medium and highly loaded bearings
Cu24Pb3.5Sn	Sintered	Tri-metal	65	Nickel	Pb10Sn3Cu	Medium loaded bearings
Cu20Pb4.5Sn	Sintered	Tri-metal	70	Nickel	Pb10Sn5Cu	Medium and highly loaded bearings
Cu10Pb10Sn	Sintered	Bi-metal	100	no	no	Highly loaded Small end bushes
Cu3Bi10Sn	Sintered	Bi-metal	130	no	no	Highly loaded bushes, lead free

Cast copper-lead alloys

According to the Unified Numbering System (UNS):
Cast Copper-Lead Alloys (Leaded Coppers) are copper alloys, containing 24-51% of lead (Pb) as the major alloying element. Cast Copper-Lead alloys are designated with the numbers from C98000 through C98999.
Cast Copper-Nickel alloys may contain up to 3% of tin (Sn), and up to 5% of silver (Ag) as additional alloying elements.
Cast Copper-Lead alloys possess binary microstructure, consisting of lead particles distributed throughout the copper matrix.
Lead in the Cast Copper-Lead alloys serves as a solid lubricant in sliding friction applications.
Cast Copper-Lead alloys are used for manufacturing sliding bearings, bushings and other sliding friction components.

Chemical compositions of some cast copper-lead alloys

UNS Designation	Pb,%	Sn,%	Cu,%
C98200	21.0-27.0	0.6-2.0	balance
C98400	26.0-33.0	0.5 max.	balance
C98600	30.0-40.0	0.5 max.	60.0-70.0
C98800	37.5-42.5	0.25 max.	56.5-62.5
C98820	40.0-44.0	1.0-5.0	balance
C98840	44.0-58.0	1.0-5.0	balance