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Polymeric Engine Bearings for Hybrid and Start 2012-01-1966

Published
Stop Applications 09/24/2012

James W. George
Mahle Engine Systems UK

Ronald Brock
Mahle Industries Inc.

Copyright © 2012 SAE International

doi:10.4271/2012-01-1966

ABSTRACT INTRODUCTION
The challenge of designing internal combustion engine
The implementation of legislations to ban the usage of lead as
components has become more complex and challenging as
an alloying element in internal combustion engines and
the Original Equipment Manufacturers (OEMs) are driven to
incrementally reducing emission output is having a
apply to stricter government legislation. Firstly, the European
fundamental impact on the design and function of half
Directive on End of Life Vehicles has banned the use of
bearings. It has been witnessed that bearing operating
“heavy metals” being used in automotive and light
environments are becoming more aggressive as there is a
commercial vehicle applications. In respect to bearings, this
trend for manufactures to adopt various technologies and
translates into the use of lead as an alloying element. This
strategies such as start stop / hybrid systems, downsizing and
subsection of the Directive was brought into force in 2008
low viscosity oils in an effort to improve fuel consumption
[1]; however for commercial heavy duty applications it is not
and reduce emission output. Engine bearings normally
yet enforced. Although some OEMs are reluctant, there is a
operate under hydrodynamic lubrication, however the
positive trend that they are starting the transition to move
consequence of implementing such technologies, is that the
away from established leaded to lead free materials for their
bearings have to operate under mixed or boundary lubrication
new engine platforms. Commercial vehicles are often
for increased durations without compromising the bearing
expected to operate for a prolonged durability over the life of
system durability. It is considered that the commercial
the application; therefore the robustness of the bearing system
vehicle sector will additionally start to adopt these
is considered paramount. Lead based substrates and overlays
technologies and strategies in an effort to achieve the
have been used for many years as lead was an excellent
specified targets. To attend to this difficult aggressive
bearing material with very good lubricious tribological
operating environment MAHLE have developed a proprietary
properties, so the challenges for bearing suppliers were to
lead free polymer overlay. The foundation of the coating is a
develop lead free materials with comparable and improved
high strength, temperature and chemically resistant polymer
characteristics as alternative solutions.
matrix. An addition of solid lubricant, metallic fillers,
adhesive promoters and specific curing regime generates the
Secondly European legislation has been introduced which
desired properties. Rig and engine test results demonstrate
defines emission standards for all engine applications. A unit
excellent wear and fatigue resistance performance compared
such as a truck or bus has a 2013 Euro IV emissions target for
to conventional uncoated bearing materials. The overlay is
Nitrogen Oxides (0.4 g/kWh), Carbon Monoxide (1.5 g/
suitable to be applied to aluminum and copper based
kWh), Hydrocarbons (0.13 g/kWh) and Particulate Matter
substrates which enables it to be utilized for a wide range of
(0.01 g/kWh) [2], therefore there is an incentive for new
applications. The purpose of this paper is to highlight the
technologies to be adopted to achieve these targets. Start stop,
impact on the bearing operating environment and present a
hybridization, low viscosity oils, downsizing, increasing
lead free polymeric solution to improve the system robustness
power densities, alternative fuels (bio diesel), direct high
and integrity.
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pressure injection systems and complex engine management

maps are all technologies which can be adopted to reduce the
emission output.

Market trends demonstrate that nearly all new passenger car

applications going forward will be manufactured with a start
stop system and it is anticipated that this technology will be
adopted by the commercial and heavy duty vehicle sector. It
would be considered that the use of such systems on buses,
delivery vans, trucks or agricultural vehicles will become a
future trend and due to the duty cycles, there would be
significant opportunities to reduce emissions and improve Figure 1. Schematic to illustrate the impact of the belt
fuel consumption. The purpose of this paper is to highlight load vector on main bearing number 1.
the impact on the bearing operating environment and present
a polymeric solution to improve the system robustness and Half bearings alone attribute a small amount in terms of
integrity. directly reducing CO2; however the optimum design and
material selection can enable and facilitate technologies
The development and implementation of start stop and which have a larger contribution to reducing emission. It is
electric hybrid powertrains systems has resulted in the considered that this technology has had the most influential
bearings having to operate more frequently in mixed or impact on the bearing operating environment for many years.
boundary lubricated conditions. During a start stop cycle the
low engine rotation speed is insufficient to generate a full
hydrodynamic oil film. Furthermore, to avoid delays for the
driver when the engine restarts, typically triggered by load
demand or clutch engagement, the start cycle has to be
quicker and more aggressive than a conventional start. Under
these operating conditions it is unlikely that the oil pressure
within the cylinder block (oil gallery) will fully stabilize,
affecting the robustness of hydrodynamic film generated
before the firing load is subjected to the bearings.

It has been identified that there are two main types of start
stop systems; belt driven and enhanced starter motor units.
The disadvantage of a belt driven unit is that a high load
vector (generated by the belt tension) is positioned at the
timing side of the crankshaft to achieve the required
performance. The consequence of the vector can result in the Figure 2. Stribeck Curve visually illustrating lubrication
translation of load primarily to main bearing number one regimes for bearing running conditions. [3].
(figure 1); however this can additionally affect subsequent
main bearings. The vector not only results in flexing of the
crankshaft, but also impacts the journal orbit. It is the HALF BEARINGS
combination of these parameters which generates a more Connecting rod and crankshaft bearings are components
aggressive boundary or mixed lubricated environment. The within the internal combustion engine. Connecting rod
second unit type is based on standard principles of a bearings are located within the big end and main bearings are
conventional starter unit whereby the drive gear engages the assembled within the engine block with a cap or ladder
flywheel. Irrespective of the method, the frequency of start frame. Irrespective of the configuration, each bearing pair
stop cycles has increased by several orders of magnitude surrounds a crankshaft journal or pin. The function of a
compared to a conventional engine. Figure 2 visually bearing is to support the crankshaft and transmit the firing
illustrates the different lubrication regimes for bearing load whilst maintaining a sufficient clearance. Half bearings
operating conditions. are subjected to cyclic loading and their robust function
requires the generation of a hydrodynamic oil film.
Hydrodynamic lubrication is when the journal is separated
from the bearing by a fluid film (in this case oil) and in
theory, in a hydrodynamic state there should not be any
contact between the surfaces. In basic principles the
formation of oil film is based on the locus delta between the
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bearing and journal which generates a circular oil wedge in material configuration can be adopted in which two different
the clearance space. As a result of shaft rotation and the materials are used in one location. A typical bimetallic
incompressibility of the oil, this generates a high pressure structure is shown in figure 3.
region which separates the journal from the bearing surface to
support the load. Operating on an oil film reduces friction
within the system and the oil flow through the bearings also
helps dissipate heat. Pressurized oil is supplied from the
pump, into the cylinder block via an oil gallery then to the
main bearings. Through drilling configurations within the
crankshaft the pressurized oil is then transported to the
connecting rod bearings.

Typical engine bearings are manufactured in a layered

structure. A steel support backing is the foundation of the
component, which provides sufficient strength to ensure that
an appropriate back contact can be achieved through a press
fit design. Subsequent layers can include an alloy which can
be used as a functional layer or as an alloy substrate with an
additional antifriction layer which provides an optimum
combination of bearing material properties. Figure 3. Typical Bimetal bearing structure
Desired bearing properties include:
For highly loaded (diesel) applications there is a requirement
for a stronger alloy substrate to enable them to withstand the
subjected loads. In this case a copper based alloy is bonded
on the steel support backing. Unfortunately, these alloys do
not have sufficient conformability and compatibility
properties; therefore they are not suitable to be used as
functional layers. Copper based substrates have been used for
many years, however originally they contained lead and
although they were not used as a functional layer, they had
improved conformability and compatibility.

To improve the conformability and compatibility properties a

third soft, thin antifriction layer is applied to the substrate
known as an overlay. Examples of overlays include
aluminum tin Physical Vapor Deposition (PVD) Sputter and
Electroplated tin based layers. The addition of an overlay
provides the product with the correct balance of properties,
however due to the complexity of the layered structures these
are often premium products. A typical trimetallic structure is
shown in figure 4.

Bearing system trends would suggest the Maximum Specific

When developing a material and selecting the appropriate Load (MSL), Direct Contact Reaction (DCR) and
product for each engine application, it is essential to ensure Temperatures are increasing to the detriment of the Minimum
there is a correct balance of hard and soft properties. For Oil Film Thickness (MOFT) as a result of increasing power
conventional gasoline applications the specific loads are densities and the effects of downsizing. Historically material
relatively low compared to diesel fuelled units. The selection could be recommended based on Specific Load
consequence of the load enables a bimetallic (two layered) (MPa), however now it has become essential for bearing
bearing composite to be used, consisting of a steel support systems to be analyzed. A comprehensive materials portfolio
layer and an alloy substrate layer. Such a substrate layer includes Aluminum alloys, Electroplated overlays, Thermal
could be an aluminum matrix, with a soft second phase (tin) Spray (High Velocity Oxy Fuelled) and Sputter overlays and
and for a superior product, a hard third phase (silicon historically variants of these materials sufficiently attended to
particles). The maximum specific bearing loads for a the application demands. To support the optimum material
commercial application are often in the operating window of selection for an aggressive operating environment, it was
aluminum bimetallic bearings; alternatively a “cocktail”
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identified that a new innovative low friction material needed considered essential that the operating conditions in modern
be developed with enhanced wear and seizure resistance. engine application required the properties from a polymer
overlay, not just for the initial running in operation, but for
the life of the engine. The development activities reported
here were focused on generating a functional polymeric
overlay. Market trends and application experience
highlighted that although MSL was increasing, there were
additionally applications operating at lower loads which
exhibited accelerated wear. It was ensured that the polymeric
overlay was compatible, to enable it to be applied either to an
aluminum or copper based alloy substrate.

To achieve the desired properties a high performance

polyamide-imide (PAI) base resin was developed which
provided superior mechanical strength at increased
temperatures and furthermore has excellent chemical
resistance.

An important addition to a polymeric overlay is a solid

lubricant to control the coefficient of friction and provide
good compatibility. Due to their laminar structure, solid
lubricants such as Graphite and Molybdenum Disulphide are
Figure 4. Typical Trimetal bearing structure commonly selected. Alternative examples of solid lubricants
include Tungsten Disulphide, Boron Nitride and
New Polymer Coating for Aluminium Polytetraflurethylene (PTFE). PTFE does not have a laminar
structure, however due to its macro molecular makeup this
and Copper Based Bearings enables movement or slip between the molecules generating
Polymers have been used to attend to a variety of different similar comparable structural properties [4]. PTFE is known
automotive functions (mostly dry lubricated environments) in to have good temperature resistance and low surface energy;
which the benefits of applying an addition of solid lubricant furthermore it has a very low dynamic and static coefficient
is that a lower friction coefficient is achieved. Furthermore of friction. Typical PTFE usages within the internal
the solid lubricant assists compatibility and conformability combustion engine can include gaskets, seals and as an oil
which is a desirable characteristic and improves the durability additive. Research revealed the properties and characteristics
and robustness of the product. The operating environment for of PTFE would be the most suitable solid lubricant addition.
these applications are very different compared to the demands
of a hydrodynamic crankshaft bearing, mostly in respect to Interfacial wear of the polymeric overlay was improved by
the high specific loads encountered. the addition of a small amount of metallic particles deposited
parallel to the coated surface. Polymers are often used as
A disadvantage of conventional polymer layers is they insulators in different applications; an insulated bearing could
typically needed to be applied thinly to reduce the possibility result in elevated temperatures, lower oil viscosities and oil
of fatigue as they inherently have a low intrinsic strength. films. Therefore the further advantage of the metallic filler
Moreover spalling of the polymer can occur as a result of was to improve the coefficient of heat transfer through the
contamination within the system and imperfection of the coating. The structure of the coating can be seen on the
mating components. The added benefit of a polymer within a polished micro-section shown in figure 5.
dry lubricated environment is that it would be expected to
transfer some of the material to the mating surface, in a The optimum formulation of PAI resin, fillers and solvent is
hydrodynamic state this is unlikely to occur and therefore applied onto either a premium aluminum alloy or a copper
could result in excess wear of the polymer layer. alloy substrate. An adhesion agent is combined with the PAI
resin to promote adhesion between the overlay and the
There are half bearings with a polymer coating on the market; substrate. Additionally a specific controlled surface
however some of these are sacrificial or running in layers preparation is performed. The combination of these
which are advantageous to improve compatibility and avoid parameters has generated a robust bonding mechanism.
seizure during initial operation. The disadvantage of a Following the polymer application it is finally cured to
sacrificial layer is as it wears this increases the clearance, achieve the desired properties.
which could reduce the oil films, increase noise and generate
excess oil flow which are undesirable characteristics. It was
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Figure 5. Example of an optical image of the new polymeric overlay.

The durable functional polymer overlay presented superior Table 1. Block on Ring key test parameters.
wear resistance under mixed, boundary and hydrodynamic
lubrication as well as increased fatigue resistance attributed
by the good adhesion and high strength of the base resin.

Rig Test Evaluation

A simple Block on Ring test was completed to compare the
coefficient of friction and wear of a polymeric coated and
uncoated aluminum alloy sample. The sample (represented by
the block), is applied with a constant load which results in
contact with the rotating steel ring. Within the assembly there Polymeric coated aluminum alloy (represented by the red line
is a reservoir of oil which partially submerses the ring to on the graph) exhibited an initial lower coefficient of friction
provide marginal lubrication. Contact between the sliding (0. 1µ)and then a steady state (0.15 µ)was achieved for the
surfaces generates measurable friction which translates into remainder of the test. Uncoated aluminum alloy, (represented
material loss and wear rate. A Block on Ring schematic is by the blue line on the graph) exhibited an initial higher
illustrated in Figure 6 and the key test parameters are coefficient of friction (0.2µ), indicating an increased wear
presented in Table 1. rate. The data then showed a declining coefficient of friction
for the remainder of the test. It was considered that there was
some surface conditioning of the ring due to the combination
of the initial high coefficient of friction and the silicon
particles within the aluminum alloy. Furthermore it is
plausible that intermittent hydrodynamic effects were
experienced.

Figure 6. Block on ring schematic.

Coefficient of friction was measured and plotted graphically

against time; it was desirable to plot the graph over a 3
minute scale to focus on the initial run in phase. Prolonged
operation under boundary lubrication in an engine would
ultimately result in a scuff seizure event. The results have
been presented in Figure 7. Figure 7. Coefficient of friction against time.
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The wear of the samples was converted into a volumetric as a suitable benchmark to validate the wear resistance of the
mass and plotted in Figure 8. The results clearly show that new polymeric overlay. The results are presented graphically
there was significantly less wear for the aluminum sample in Figure 10.
coated with the polymeric coating.

Figure 10. Accelerated wear test results comparing the

new polymeric overlay against HVOF and a
conventional leaded overlay.
Figure 8. Block on Ring wear evaluation.
The polymeric overlay on the aluminum substrate exhibited
Further wear evaluations were undertaken on representative
comparable wear resistance to HVOF; however when the new
half bearings and the same wear rate trend was observed.
overlay was applied to the copper based alloy superior wear
Tests were performed on a bespoke accelerated wear rig,
resistance was witnessed. The polymeric overlay irrespective
operating at a high specific load under continuous boundary
of the substrate exhibited significantly improved wear
lubrication for set period of time. A schematic of the test is
resistance compared with the leaded product in a boundary
shown in Figure 9 and the test key parameters summarized in
lubricated operating environment.
Table 2.
The polymeric overlay was additionally evaluated using a
bespoke bearing fatigue rig. This test can be used to
determine the fatigue limits of a given material. Load is
applied in determining steps for a set duration then the parts
are inspected visually to quantify whether they have passed
or failed. A schematic of the test is shown in Figure 11 and
the test key parameters summarized in Table 3.

Figure 9. Schematic illustrating the accelerated half

bearing wear rig.

Table 2. Accelerated half bearing wear rig key test

parameters.

A further comparable test was completed using four

materials, HVOF overlay, polymeric coating on an aluminum
and copper based substrate and an established leaded product. Figure 11. Schematic illustrating the half bearing
HVOF (AlSnCu) is one of the most wear resistant bearing fatigue test.
materials within the market place, therefore was considered
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Table 3. Bearing fatigue rig key test parameters. The results conclusively show that the polymeric overlay on a
copper based alloy has an increased fatigue resistance
compared with the HVOF product.

Engine Test Results

It was essential to additionally validate the new polymeric
overlay development in a variety of engines and test
conditions in order to demonstrate the robustness in a variety
of different bearing operating environments. A highly loaded
Several comparable tests were completed during the inline 6 cylinder 6.6 litre application was tested for a 1000
evaluation of fatigue resistance; the results presented within hours durability cycle, initially the bearing material was an
this paper are focused on the polymeric overlay on a copper aluminum alloy and then the test was repeated with the same
based alloy compared with HVOF. The operating load aluminum alloy material coated with the polymeric overlay.
carrying window for HVOF is applicable for high loaded The results have been presented in figure 13.
diesel applications, therefore it was once again considered to
be an appropriate benchmark material for validation. The The uncoated material demonstrated evidence of contact and
results have been presented in Figure 12. polishing, furthermore in one location there was evidence of
fatigue. Polymeric coated bearings exhibited a very good
visual aspect following the completion of the test with no
evidence of fatigue, supporting the addition of the overlay
increases the fatigue resistance of the product.

A further split test was performed on another highly loaded

inline 6 cylinder, 6.7 litre application for a 250 hour
durability test. Upper connecting rod bearings 1, 2, 3 and 5
were the uncoated aluminum alloy and bearings 4 and 6 were
the same aluminum alloy coated with the polymeric overlay.
The results have been presented in figure 14.

Following this aggressive test, the polymeric bearings

Figure 12. Fatigue test comparison evaluation results. exhibited a good visual aspect with a low level of contact in

Figure 13. Diesel 6.6L connecting rod bearings following ∼ 1000 hour durability test.
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Figure 14. Diesel 6.7L connecting rod bearings following ∼ 250 hour durability test.

Figure 15. 1,4L Turbo charged diesel passenger car application upper connecting rod bearings following a 450 hour full power
test.

the loaded region witnessed. This result supports the SUMMARY/CONCLUSIONS

robustness and wear resistance in difficult operating
environments. Innovative durable polymer overlays suitable to attend to the
bearing operating demands of a modern application have
Additionally it was desired to validate the polymeric coating been developed. Rig and engine test results confirm excellent
on a copper based alloy in a high speed diesel application. wear and fatigue resistance particularly in mixed or boundary
The bearing ratio and the calculated sliding speeds were lubricated environments. The overlays demonstrated
comparable with commercial vehicle applications; excellent functional performance for highly loaded
furthermore, the aggressiveness of the test was magnified by connecting rod and crankshaft main bearings and furthermore
the high bearings MSL, low oil film thicknesses and the operating on steel and cast iron running surfaces. A number
usage of a cast iron crankshaft. A good visual aspect was of engine tests have been completed for passenger car
witnessed after test with no evidence of fatigue or scuffing in applications with start stop systems, however further testing
this difficult operating environment. The results have been is required for commercial vehicles.
presented in figure 15.
To attend to aggressive operating environment it is
considered that wear resistance is essential. Leaded materials
are still considered to be the “benchmark” within the
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industry, however due to focused R&D on lead free materials

the leaded technology has stood still; while the bearing
DEFINITIONS/ABBREVIATIONS
system has continually changed and become more aggressive. HVOF - High Velocity Oxy Fuelled
The improved wear resistance between the new polymeric
CO2 - Carbon Dioxide
and the leaded overlay, demonstrates the advanced
developments within the field of lead free materials and in OEMs - Original Equipment Manufacturers
particular polymeric overlays. Developments of such PVD - Physical Vapour Deposition
materials should provide the OEM's with the confidence that MSL - Maximum Specific Load (Maximum actual load
robust lead free materials are not only available, but can (firing + inertia) / projected bearing area))
withstand the adoption of new technologies in an effort to
DCR - Direct Contact Reaction (wear indicator based on film
improve fuel consumption and reduce CO2 emissions.
thickness and surface finish)
MOFT - Minimum Oil Film Thickness
REFERENCES
MPa - Megapascals
1. Contact Directive 2000/53/EC of the European Parliament
and of the council of 18 September 2000 on end-of life °C - Degrees Celsius
vehicles. N - Newton's
2. http://www.dieselnet.com/standards/eu/hd.php - accessed RPM - Revs per Minute
on 20/03/2012 PAI - Polyamide-imide
3. Tomanik, E. and Ferrarese, A., Low Friction Ring Pack PTFE - Polytetraflurethylene
for Gasoline Engines, ASME ICEF Fall 2006,
ICEF2006-1566, 2006.
4. www.tribology-abc.com/abc/solidlub.htm 25/03/2012

CONTACT INFORMATION
Mr. Ronald Brock

Address:

MAHLE Industries, Incorporated

23030 Haggerty Road
Farmington Hills
Michigan 48335 USA

Contact Details:

Phone: +1 (248) 596-3187

Fax: +1 (248) 596-8976
ronald.brock@us.mahle.com

ACKNOWLEDGMENTS
The authors would like to thank the Projects Leaders Jon
Forder and Cedric Fortune of MAHLE Engine Systems UK
Ltd for their work during the development of this product.

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