33
Cummins Inc.
Cummins Inc.
Cummins Inc.
33.1 Introduction
33.2 Power Cylinder Components
Liners/Bores • Rings • Piston • Piston Pin and Connecting
Rod • Friction in Power Cylinders • Tribological Systems •
Ring/liner Interface Tribology • Fuels and Lubricants •
Engine Deposits • Effect of Oil Film Thickness • Effect of
Oil Cleanliness • Examples of Liner and Ring Wear
Mechanisms
33.3 Overhead Components
Cam and Cam Followers • Push Rods, Rocker Levers, and
Crossheads
33.4 Engine Valves
Valve/Seat Wear Mechanisms • Valve Stem/Guide Wear
Mechanisms • Materials Selection Criteria
33.5 Bearings and Bushings
33.6 Turbomachinery
33.7 Fuel System
Fuel Injectors • Fuel Pumps
33.8 Fuels, Lubricants, and Filtration
Diesel Fuel • Diesel Lubricants • Used Oil Analysis •
Filtration: Air System • Filtration: Lubricating System •
Filtration: Fuel System • Filtration: Cooling System
33.9 Future Trends
Diesel engines are based on the principle of compression ignition. Air is introduced alone into the combustion chamber with the opening of the intake valves. The air intake is facilitated by the downward movement of the piston that creates a pressure differential through volume expansion. Turbochargers are often used to force more air into the combustion chamber to increase air density to burn more fuel for the same displacement. The air, once in the system and the intake valve closed, is then compressed to reach high pressure and high temperature. Fuel is injected at this point to initiate combustion. The fuel is ignited by the high temperature induced by compressing air, hence the name compression ignition.
The combustion of the air/fuel mixture creates the expansion that forces the piston to move downward again, producing power output.
The whole diesel engine design is to support the power production through compression ignition.
The key components include the power cylinders that house the combustion process, the cam and valve train system to control the timing of the combustion, the crankshaft and connecting rods to receive power and to provide mechanical movement of the compression/expansion process, the gear train to operate pumps and accessories essential to moving working fluids around, and, finally, all the working
© 2001 by CRC Press LLC

Valve and spring
Liner
Flywheel and housing
Crankcase
Valve cover
Valve pump
Cylinder head
Piston & rings Oil cooler
Connecting rod
Oil filter
Turbocharger
Block
Rocker lever
Push tube
Fuel pump
Oil pump
Electronic controller
Cam followers Camshaft
FIGURE 33.1
Cutaway illustration of a diesel engine.
fluids that provide cooling, lubrication, and power to the engine (Heywood, 1998). All of these components are housed in an engine block with a cylinder head on the top.
Figure 33.1
shows a cutaway view of a typical diesel engine. The engine block starts with the bore that
houses the cylinder liners. Many larger diesel engines have removable liners for longer service life and easy rebuild. The skirt part of the block allows for the camshaft and crankshaft to be inserted. Gears and accessory pulleys are located in the front. A flywheel is attached to the rear that connects to the drive train that transmits power to the transmission. On top of the block is fitted the cylinder head that houses valves and fuel injectors. The oil pan is attached to the bottom of the block.
The power cylinders include the liners, pistons, and piston rings. The liners are fitted inside the bore to guide the movement of the pistons. The movement of the piston inside the liner from the top to the bottom is called a stroke. The displacement of an engine is the volume defined as:
2
Piston rings are used to provide sealing of combustion gas and to control the oil film on the liner wall.
Multiple rings are used for this purpose. The lowest ring is usually called the oil ring to limit the quantity of oil reaching the upper end of the liner to prevent excessive oil accumulation or deposits. The top ring is used to prevent compressed air or combustion gas from leaking past the rings so as to preserve the pressure in the cylinder. It does this by pressing the ring surface against the liner and allowing only an oil film between the liner and the ring. There are usually additional rings between the oil ring and the top ring to facilitate the process. Because the power cylinders are where the heat is generated, the liner is cooled by coolant flowing between the liner and the block. Further cooling is provided by crankcase oils squirted onto the bottoms of the pistons. Most heat, however, is rejected into the exhaust gas.
© 2001 by CRC Press LLC

Because air intake, fueling, and combustion occur in sequence in the power cylinders (i.e., only one cylinder fires at a time), each process must happen in exactly the right time to allow smooth and continuous operation. This is controlled by the design of the cam and the movement of the valve train.
The cam has intake and exhaust lobes for each cylinder. Each lobe is designed with a profile that, when the cam rotates, will actuate the opening and closing of the valves at the right time. Most diesel engines rely on the fuel pump to control the amount and timing of the fuel injection. However, some unit injectors are actuated mechanically by the injector lobes on the cam. The actuation of valve and injector events are delivered by the overhead components, so named because most of them sit on top of the engine head.
The cam lobe rises against a cam follower, which pushes against a push tube. The push tube will raise one end of the rocker lever, forcing the other end of the rocker lever to push down on either injector or valve tips, thus completing the action. The valve springs will force the overhead components to go in the other direction when the cam lobes have moved past their high points. Because the rotation of the cam is tied to the rotation of the crankshaft via gears, and the crankshaft is connected directly to the piston via a connecting rod, the movement of the cam, the crank, the valves, and the pistons are all synchronized.
In addition to synchronizing the movement, the crankshaft also receives the power generated by the combustion. The power is either sent to the transmission for useful work, or used to power accessory pulleys and various pumps through the gear train. These components include the oil pump, the water pump that moves the coolant, the fuel pump, and accessories such as an air conditioner.
Recent diesel engine designs are largely driven by emissions regulations ( Figure 33.2
). Major refinement
in bowl design, air handling, and fuel injection have led to progressive reductions in NO x
and particulate matter. Meanwhile, higher power density, better fuel economy, and extended service intervals are becoming an integral yet conflicting part of modern diesel engines. Currently, exhaust gas recirculation (EGR) and after-treatment devices are considered necessary to achieve the next level of emissions control. All of the above present different challenges to engine tribology. Tribological contacts in each part of the engine are discussed in this chapter.
Simultaneous improvements in engine power density, reliability, and durability over the last five decades are due to advances in design, materials, fuel, and lubricants. The overall function of power cylinders is to convert the gas pressure resulting from the combustion of the air/fuel mixture inside the combustion chamber to the torque applied to the crankshaft. Power cylinder components include liners/bores, piston rings, pistons, piston pins, and connecting rods. In some engine designs, the bores are machined in the cylinder block and are referred to as parent bores and in some engines, liners are fitted into the cylinder
block. A schematic of the ring pack arrangement in a piston is shown in Figure 33.3
. To understand the
tribology of power cylinders, it is important to understand the function of these components.
The functions of liners/bores are to:
1. Confine the combustion gases
2. Transfer the heat from pistons and rings to the coolant
3. Seal the coolant
4. Support piston-side loading
5. Guide the pistons/rings
6. Retain oil for start-ups
Liner/bore materials should have good break-in, wear, and scuff resistance; have a consistent surface finish to limit friction; retain oil in honing grooves for longer engine life; be free from embedded particles; and maintain good heat transfer.
© 2001 by CRC Press LLC

P a r t i c u l a t e g / b h p - h r
0 . 6
0 . 4
0 . 2
0
N O x g / b h p - h r
1 2
8
4
0
1 9 8 6 1 9 8 8 1 9 9 0 1 9 9 2
FIGURE 33.2
Progression of diesel emissions regulations in the U.S.
1 9 9 4 1 9 9 6 1 9 9 8 2 0 0 0
© 2001 by CRC Press LLC

FIGURE 33.3
Schematic of ring arrangement in a piston.
TABLE 33.1
Liner Materials Used in Diesel Engines
Low-alloyed gray cast iron Pearlitic matrix with flake-type graphite; hardness 250 BHN
Bainitic-cast gray iron Bainite and tempered martensite; hardness 300 BHN
Boron-alloyed cast iron Lamellar pearlitic material with high percentage of boron carbides; hardness 240 BHN
Liners are typically manufactured from gray cast iron. To improve the wear resistance of liners, alloying elements such as chromium, nickel, molybdenum, copper, vanadium, and phosphorous are added. To improve the wear and scuff resistance of the cast iron liners, various surface modification techniques such as induction hardening or gas-nitriding are also used.
Table 33.1
summarizes typical liner materials
used in diesel engines.
The main functions of both compression and oil rings are to:
1. Seal (to prevent gas and oil leakage between piston and cylinder)
2. Maintain lubricating oil film on the cylinder wall
3. Transfer the heat from the piston to the cylinder wall
Cast iron, ductile iron, and steel are substrate materials used for both compression and oil rings. To improve the durability and performance of the rings, surface treatments that can provide strong adherence to the base material are used. Criteria such as compatibility with liner material, wear and scuff resistance, thermal conductivity, and the ability to survive in dry, partially lubricated, and lubricated sliding conditions, are the basis for choosing surface modification techniques for rings. Ring scuffing can lead to
© 2001 by CRC Press LLC

TABLE 33.2
Properties of Piston Ring Materials
Material
Grey cast iron
Carbidic/malleable iron
Malleable/nodular iron
Sintered irons
Modulus of Elasticity
(GPa)
83–124
140–160
155–165
120
Tensile Strength
(MPa)
230–310
450–580
540–820
250–390
Hardness
(BHN)
210–310
250–320
200–440
130–150
TABLE 33.3
Substrate, Application, and Surface Engineering Methods of Steel Rings
Ring Type Substrate Surface Treatment
Top compression rings
Oil ring
SAE 9254
13% Cr
18% Cr
Low carbon steel
6% Cr
13% Cr
Cr-plated, plasma-sprayed
Nitriding
Nitriding, Cr-plated, plasma-sprayed
Cr-plated
Nitriding
Nitriding high blow-by and catastrophic failure of an engine. The coatings on piston ring faces can be applied on the entire face, or half of the face, or only in the middle. Currently, chrome-plating, gas-nitriding, and plasma-spraying are some techniques that are used for modification of piston ring faces. Also, engine manufacturers are evaluating the high-velocity oxygen fuel (HVOF) process for the application of chrome carbide/nickel chrome with Moly (Cr
3
C
2
-NiCr/Mo) coatings for compression rings (Shuster et al., 1999;
Rastegar and Craft, 1993). The (Cr
3
C
2
-NiCr/Mo) coating has good wear and scuff resistance. Chrome nitride coatings with oxygen (CrNO) deposited by physical vapor deposition (PVD) also have good wear and scuff resistance. These coatings have not been widely used in diesels because of the high cost of the coatings (Rastegar et al., 1997). Shen (1987) explored multilayered PVD coatings for piston rings. Properties of the ferrous-substrate ring materials and low-carbon steels that are currently used in diesels are
summarized in Tables 33.2
and 33.3
(Challen and Baranescu, 1999).
The piston’s roles are to:
1. Transfer force originated from the combustion gas pressure inside the chamber to the piston pin
2. Provide support and guidance to piston rings and pin
3. Dissipate heat energy to the coolant
Materials properties that are important for piston applications are low density, low thermal expansion, and good high-temperature fatigue strength. In addition, manufacturing considerations such as ease of casting, forging and machining, as well as low cost, factor into the materials selection. The original material used for pistons was grey cast iron. Cast iron with spheroidal graphite is commonly used as a piston material due to its good groove resistance, low thermal expansion, and high strength. Despite the difficulties in casting defect free thin sections of cast iron, cast iron pistons were successfully used in some production engines. Oxidation resistance of the piston crown at high temperatures and low wear of the ring grooves is also desirable.
Aluminum-silicon alloys that are widely used for piston materials do not have adequate wear resistance from ring movement at higher operating temperatures and pressures. Therefore, inserts are cast into place to provide a harder and more-resistant surface. To prevent excessive ring or groove side wear, a Niresist ring groove insert (cast iron with high nickel content) is metallurgically bonded to the aluminum piston.
© 2001 by CRC Press LLC

Engine design trends toward high power density have driven the concept of composite pistons. In some engine designs, two-piece or articulated pistons are used, in which the crown is forged steel and the skirt is cast aluminum. In this case, cast-in ring carrier inserts are not necessary.
Squeeze cast-aluminum has also been used for diesel pistons. The mechanical properties of the squeeze cast alloy can be improved by inserting ceramic fibers or metal inserts for reinforcements of grooves, pin bosses, and combustion-bowl rims. For the highest thermal loading, in order to reduce top ring and top ring groove temperatures, a cooling gallery is located in the heat path. Composite aluminum pistons with bolted steel crowns are used in large-bore and slow-speed diesel engines operating at high temperatures and with long durability.
Changes in stress state at the piston combustion bowl can result in thermal fatigue cracks and erosion.
In aluminum pistons, hard anodizing, a process in which the surface is modified to form aluminum oxide, is used to improve the piston crown resistance to thermal cracking.
To help cylinder-kit run-in, soft coatings like graphite, tin, and moly-disulfide are used on piston skirts. Recently developed resin-bonded graphite/molybdenum coatings were reported to have longer life and better wear resistance than sprayed graphite (Challen and Baranescu, 1999).
The piston pin and connecting rod transform the combustion pressure delivered to the piston into torque at the crankshaft, resulting in shaft power. Connecting rods are made of high-quality medium-carbon steel.
Piston pins are made of high-quality, high-carbon steels. The outer surface of the piston pin is surfacehardened, lapped, and polished to a mirror-like finish. Pin and pin bore designs are dictated by piston bore load-carrying capacity. The pin rides in a bushing, which is typically a layered bronze-bearing material.
Figure 33.4a
shows the distribution of the total energy in a typical fired diesel or spark ignition engine
(Richardson, 1999). This figure indicates that 4 to 15% of total energy is lost by mechanical friction.
Richardson cited that 40 to 55% of total mechanical losses are caused by pistons, rings, and piston rods.
This is shown in the Figure 33.4b
. Richardson indicated that 18 to 33% of friction is caused by rods,
28 to 45% by piston rings, and 25 to 47% by pistons. Design for low friction is often compromised by durability and oil consumption requirements.
Based on the function of components, power cylinder components can be divided into eight systems with tribological interfaces: liner/ring; ring and ring groove, liner/piston skirt, piston pin and piston bore, piston pin and connecting rod, piston skirt/piston pin (articulated piston), piston crown/liner
(articulated piston), and oil ring/expander.
Wear can occur in any one of these eight systems. For all practical purposes, engines are rebuilt when oil consumption and blow-by become excessive. In the perspective of power cylinders, oil consumption is most often associated with excessive radial wear of the top compression ring and cylinder liner wear at the top ring reversal.
A properly designed piston skirt has an adequate lubricating film between the skirt and the liner. Lowfriction coatings on the skirt are helpful when the piston skirt breaks through the lubricating film. Under some severe operating conditions, such as when extreme engine thermal transients can alter the clearances, changes in the friction between skirt and the liner can result.
The wear caused by a mechanical contact in a well-designed pin and bore under normal operating conditions is minimal. Embedded particles in the piston pin from the lapping or polishing process can lead to excessive wear in the bore. One abnormal bore failure occurs when an engine experiences hot shutdown. The piston cooling oil ceases to flow, and the piston crown area is hotter than the piston bosses and piston pin. As a result, heat is transferred from the hot crown to the piston bore, which
© 2001 by CRC Press LLC

Other
Losses
(47-58%)
(b)
Work
Output
(38-41%)
Pistons,
Rings,
Rod
(40-55%)
Other
(40-60%)
FIGURE 33.4
Distribution of (a) total energy in a fired engine; and (b) total engine mechanical friction. (From
Richardson, E.E. (1999), Review of Power Cylinder Friction for Diesel Engines, ASME, ICE-Vol. 32-3, Paper No. 99-
ICE-196. With permission.) experiences higher temperatures than normal operating conditions. These high temperatures at the piston bore bushing can lead to degradation of bearing material. Another abnormal failure may be caused by pin bending, which results in nonuniform load distribution, with high stresses biased to the inner edge of the bore. In this situation, cracking caused by fatigue is observed on the piston pin boss bushing.
Piston pins and the piston skirts are designed to be the critical load path for the piston thrust load.
In some articulated piston designs, the second land guides the piston movement on the liner. Typically, the contact between piston crown and liner is lubricated. A well-designed system under normal operating conditions shows minimum wear. Potential causes of high piston-liner wear include excessive oil deposits and external contaminants.
Oil consumption is primarily controlled by the tension of the oil ring. Oil consumption is high at low unit pressure, decreases with an increase in unit pressure, and remains constant above a certain unit pressure. Normally, oil rings are designed in the unit pressure region where oil consumption is constant.
In engines that employ oil rings with expanders, the unit pressure of the oil ring may drop either when the expander wears with time or when the spring embeds in the inner diameter of the ring. This results in increased oil consumption. Cr-plating and gas-nitriding are typical surface modification techniques used to extend the life of an oil ring.
The primary factors in determining engine life before major overhauls are ring/liner interface and ring/ring groove wear. Thus, the remainder of the discussion focuses on ring/liner wear. Ring/ring groove wear is covered in Section 33.2.9.
Kodali et al. (1999) reviewed the major factors influencing cylinder liner wear. Most of these factors are also valid for ring wear. The following sections discuss the major variables that influence ring/liner wear, including the aspect of the design, choice of materials, engine operating parameters, fuel sulfur, engine deposits, soot, and lubricant additives.
© 2001 by CRC Press LLC
(a)
Mechanical
Friction
(4-15%)

33.2.7.1 In-cylinder Mechanical Design
The design of power cylinder components can have a significant effect on system wear. The fundamental principles in designing for low-wear power cylinders are:
• Design the system for hydrodynamic lubrication, which prevents direct contact between sliding surfaces.
• Choose compatible materials for the sliding surfaces, which results in low wear when contact occurs between sliding surfaces.
Engine design parameters that modify power cylinder dimensions can influence liner wear. Understanding the effect of parameters such as distribution of gas pressure, contact force at the ring/liner interface, the ring pack design and location, and piston type is critical.
33.2.7.2 Bore and Stroke
Bore size has very little effect on ring/liner wear because the net pressures that act on the ring remain the same. The stroke and connecting-rod length affect the piston velocity. This may have some effect on the hydrodynamic oil films that are developed under the ring face. Excessive bore distortions result in increased ring/liner wear.
33.2.7.3 Load and Speed
Engine load is governed by cylinder pressure. The higher or longer the cylinder pressure acts on the rings, the greater the potential for increased wear. Higher loads also tend to increase piston temperatures.
Higher temperatures decrease oil viscosity, reduce oil film thickness, and increase wear; they can also cause distortions that may lead to high wear.
Engine speed is an indication of the number of times that the ring and liner contact near the top dead center (TDC). Higher speed results in increased wear caused by an increased number of contacts of ring and liner at TDC. Higher rpm also increases the piston speed, which can reduce wear rates by increasing the oil film thickness.
33.2.7.4 Liner, Ring, and Piston Design
Liner design is critical in minimizing the bore distortion that can lead to excessive wear. In boundary lubrication conditions, liner surface finish influences liner wear. For example, discontinuous hone marks, and torn or folded metal on the liner surface can cause high liner wear.
During high cylinder pressure, ring width affects the net radial gas force acting on the ring. Reduced widths can decrease this force and potentially reduce wear. The ring face profile influences both the net gas pressure force and the hydrodynamic lubrication of the ring face, and should be optimized for low wear.
Liner wear is also a function of the piston land design. If the top land clearance is not designed properly, carbon can build up. These carbon deposits, when trapped between the piston and liner, result in borepolishing at the top ring reversal area of the liner. Bore polishing results in significant amount of material loss and loss of oil control.
Land clearances and groove widths affect the flow of gases through the ring pack. This affects the forces that are acting on the rings and ultimately the wear. Small clearances may result in excessive wear, sticking, and possible scuffing. Large clearances between ring and ring groove may lead to ring breakage.
33.2.7.5 In-cylinder Physical Environment
The most significant factor that influences abrasive wear in an engine is the contact between two surfaces in relative motion. Environmental conditions that affect the contact pressure between two surfaces are discussed below.
33.2.7.6 Cylinder Pressure
High cylinder pressures force the ring and liner together. If they move relative to each other, then wear occurs.
With proper design, the effects of pressure can be minimized.
For example, the net radial force acting on a
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12000
Force (N)
10000
8000
6000
4000
2000
0
-2000
0
-4000
-6000
20
Gas Pressure
Asperity
Contact
40 60 80 100 120 140 160
Position (mm)
FIGURE 33.5
Variation of contact force and gas pressure with distance below the top dead center.
ring can be decreased by proper ring design and can help compensate for increased cylinder pressure. Highest cylinder pressures occur during the compression and expansion strokes. The peak pressure typically occurs
after the TDC. Figure 33.5
illustrates the distribution of the gas pressure and the contact force as a function
of distance from the TDC. Both gas pressure and contact load are at maximum just below the TDC.
33.2.7.7 Temperature
The temperature at which the ring/liner or liner/piston skirts slide together influences liner wear.
Improper liner cooling may result in higher ring/liner interfacial temperatures. For example, differential expansion, which may be caused by nonuniform cooling around the circumference and the length of the liner, may lead to bore distortion. The ring/liner wall temperature affects the abrasive wear term indirectly through the kinematic viscosity of the lubricant. The liner/ring wear may increase with decrease in the oil viscosity as a result of increasing temperature. Higher sliding temperatures may also aggravate liner wear because of local degradation of the lubricant.
33.2.7.8 Piston Velocity
Lubrication conditions of the ring/liner interface are governed by piston velocity. In the middle of the stroke, the piston is moving the fastest, resulting in hydrodynamic lubrication. However, at the dead centers, the piston velocity goes to zero and the hydrodynamic lubrication breaks down. This can result in ring/liner surface contact that leads to loss of material from both the ring and liner surfaces.
33.2.8.1 Effect of Fuel Sulfur
Correlation between the sulfur content in the diesel fuel and the wear of liners/rings in diesel engines has been established (McGeehan and Kulkarni, 1987; Dennis et al., 1999; Weiss et al., 1987). Because of emissions requirements, the trends have been toward decreasing the sulfur content in fuel. Off-highway applications are still permitted to use high-sulfur fuels. The SO x
products formed as a result of combustion can combine with water moisture to form sulfuric and sulfurous acid, leading to chemical attack.
Concentration of the acid and the length of the time that the acid stays on the liner surface determine the severity of the chemical attack. Material loss as a result of this localized chemical attack is usually insignificant. However, at lower coolant temperatures, acid condensation is promoted, and thus can activate corrosive wear. Liner wear with 400 ppm sulfur in heavy-duty diesel engines indicated embedded corrosion by-products, corrosion pits, and severe abrasion as contributing factors for loss of liner material.
A liner from an engine that ran with 1 ppm sulfur indicated much less abrasion and corrosion (Wang et al., 1999).
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33.2.8.2 Effect of Base Stocks and Additives
One of the most important functions of the lubricant is to control the friction and wear of highly loaded components during operation. The many functions of a modern diesel lubricant are the result of careful blending of chemical additives and carefully refined base stocks. These additive packages have undergone tremendous improvement over the last 20 years and have played a major role in the improvement in engine life over that period of time.
In a ring/liner interface, the lubricant helps in sealing against the leakage of combustion gases. Degradation of that lubricant or formation of deposits in the ring belt area can affect liner wear. For example, increased friction and wear of ring/liner interface are expected if the viscosity is too low. Similarly, if the viscosity is too high, oil flow may be poor, resulting in poor lubrication conditions that eventually lead to high wear and friction. Buildup of carbonaceous deposits on a piston can increase liner wear and decrease the ability of the rings to seal the combustion gases. The alkaline detergent additives perform a critical, dual role; they neutralize corrosive combustion acids and they help clean deposits that may form on hot metal surfaces.
The buildup of combustion-derived soot in the lubricant promotes wear. Soot-induced wear mechanisms are not well-understood. There are several proposed mechanisms (Kim et al., 1992), such as adsorption of zinc dithiophosphate (ZDTP) by soot, competition between ZDTP and soot for adsorption sites on contacting surfaces, and the abrasive action of soot. Currently, there is no evidence to illustrate the role of soot on liner wear. The typical size of primary soot particles is reported to range between
0.01 to 0.1 µm (Kuo et al., 1998). These particle sizes might only result in polishing wear. When engines run for longer periods of time, bore/liner polishing may contribute to significant loss of liner material.
Soot particles greater than 1 µm can result from some engine operating conditions, and the abrasive wear by soot may result in significant contribution of liner material loss. Similarly, if the soot absorbs the additives in the oil and thus inhibits formation of the anti-wear film on the contacting surfaces, liner wear may be accelerated. Formation of acids caused by condensation of some combustion products can degrade the oil in localized regions, resulting in chemical attack of the liner.
Deposits are known to have an effect on the durability and emissions of an engine. The primary cause of engine deposits is a relatively complex reaction that occurs among the components, fuel, blow-by gases, and the engine lubricant. Most of these deposits derive from the fuel, with some contribution from the lubricant. These deposits are accepted to be a mixture of inorganic material (ash) and carbonaceous combustion products (soot), and resinous organic material that serves to bind the mixture together
(Shurvell et al., 1997). Typically, deposits are seen on the crown, top land, and second land; in the ring groove; and on the under-crown. The deposits formed at different locations of the combustion chamber are chemically different, and probably vary in the mechanical aspect of being soft or hard. Top land deposits are known to be very hard and abrasive, whereas under-crown deposits, which occur mainly because of severe operating conditions, do not have serious effects.
Carbon deposits trapped in the piston/liner interface result in bore-polishing at the top ring reversal area of the liner. When these carbon deposits are trapped between the ring and ring groove, side clearances may change. Reduced clearances caused by carbon deposits may result in excessive wear, sticking, and possible scuffing.
The predicted oil film thickness (OFT) under the face of the top compression ring as a function of crank
angle for various cases of liner roughness is shown in Figure 33.6a
. The OFT is defined as the distance between the mean height of the asperities on the surface ( Figure 33.6b
). As the liner roughness increases,
asperities are held apart, resulting in a thicker oil film. However, during the high-pressure portions of the cycle, the asperities are forced together and the film thickness decreases. In cases of lower surface roughness, there are more variations in the predicted OFT. At the mid-stroke, the asperities are small
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(a) 6
5
4
TDC
Firing
2.0* Roughness
3
2
1.0* Roughness
1
0.5* Roughness
0
-360 -270 -180 -90 0 90
Crank Angle (degrees)
180 270 360
(b)
FIGURE 33.6
(a) Oil film thickness as a function of roughness. (b) Interaction of ring/liner asperities: (i) smooth liner surface, and (ii) rough liner surface.
enough so that it is possible to form a fully hydrodynamic oil film. Near the dead center, part of the predicted OFT is once again affected by asperity contact between surfaces. The minimum OFT, below which metal-to-metal contact occurs, is known as a lower-limit for fluid film lubrication (LFFL). This boundary limit depends on the surface finish, elastic modulus, thermal distortion of sliding surface, and size of trapped contaminants found in their clearance space.
Typically, the arithmetic surface roughness of the liner surfaces is no more than 0.8 to 0.9 µm and, thus, the minimum OFT for lubrication can be considered to be the same order of magnitude. The OFT falls below the LFFL value at and near regions of the extreme ends of the stroke. At the mid-stroke position, the piston speed is highest and thus one can expect a higher OFT than at the top and bottom ring reversal area. The OFT for the oil ring is typically smaller than the top compression ring. Oil film thickness can affect the size of the trapped contaminants under the ring face, whereas the contact pressure at the ring/liner interface influences the damage caused by these trapped particles.
Damage on the liner can be understood by a simple assumption that the oil film thickness on the liner is affected by what is deposited by the previous ring. For example, during the down-stroke, the top ring
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sees the oil left behind by the second ring. The second ring sees the oil film left by the oil ring. The size of the particles that can be trapped under the ring face depends on its film thickness and the debris that is left behind from the previous ring. The bigger particles may get crushed or agglomerate on a ring. The top ring sees the particles from the combustion chamber first and is more susceptible to agglomeration of particles.
Near the dead centers and during high pressure, film thickness tends to be smaller. This results in smaller particles passing under the ring face. Especially under high pressure, larger particles may get crushed under the forces acting on the ring. This, combined with the asperity contact, results in high wear and polishing of the ring and liner surfaces. During times of low pressure in the cycle, and when the piston is moving fast, the oil film thickness is larger. Larger particles may pass by the ring face. Because the forces are lower, the abrasive wear by debris is less.
As a lubricated surface, liner wear occurs primarily through adhesion and abrasion. Adhesive wear is controlled by the formation of the oil film around the top ring reversal (TRR) area and by the anti-wear film in the vicinity of the TRR, where the film thins to boundary lubrication. Abrasion occurs as thirdbody wear from particulate contamination in the oil that becomes entrained in the oil film. These particles can come from both external and internal sources. Externally, they can come from dust-laden air that passes by the air filter or from the introduction of replacement oil that has not been handled or transferred in clean conditions. Any time the engine is opened for either scheduled maintenance or unscheduled repair, there is a possibility of adding external contamination. Internally, particulate contamination can come from debris left behind by the manufacturing process, such as core sand or machining chips, wear debris, surface fatigue, oil additive precipitation caused by acid neutralization, or massive additive precipitation caused by the accidental leakage of the coolant into the oil.
The use of air and oil filters is designed to keep the particulate contamination under control to minimize wear. However, it is not clear to what extent a higher level of filter efficiency, by itself, would extend
engine life as measured by oil consumption. Figure 33.7
shows a schematic of a particle-laden oil near
the TRR as the oil film thins. While this figure shows particles at the ring/liner interface, one should not ignore the particle flow by means of lubricant from the back of the ring to the front of the ring. By hydrodynamic exclusion, larger particles do not pass under the ring face. This should result in finer and finer particles being captured by the film as it thins.
For these smaller particles, the efficiency of standard oil filters, as measured by industry standard tests, drops off dramatically. As an example, for a current high-efficiency, single-stage filter, 99% of 30-micron particles and greater can be removed, but the efficiency drops to 85% for particles smaller than 10 microns.
Because oil film thickness is roughly on the order of magnitude as roughness, trapped particles are no larger than 1 to 2 microns as TRR is approached, and standard filtration becomes much less effective in that regime. Using surface layer activation, the sensitivity of critical parts to cleanliness for several different engines has been established (Truhan et al., 1995). Newer engines and lubricants show much more tolerance to dirt in the oil than 15 to 20 years ago. This is reflected in the almost doubling of effective engine life during that period. It is much more effective to keep the external contamination out of the
FIGURE 33.7
Schematic and the observed regions above, at, and below the TRR area.
© 2001 by CRC Press LLC

TDO
TRR
Liner wear (microns)
0 5 10
0
5
10
15
20
25
-6000
Contact force (N)
-4000 -2000
0
0
5
10
15
30
40
20
25
FIGURE 33.8
Contact force at TRR and measured wear profile.
system in the first place, because any contamination passed by the air filter does damage going past the ring pack, regardless of the efficiency of the oil filter. The wear that occurs on the liner near the TRR is the wear that determines engine life and is only affected to a small extent by the bulk oil cleanliness. Ring wear is affected to some degree by cleanliness.
The major factors that influence ring/liner wear have been identified in previous sections. This section provides an example of liner and top compression ring wear mechanisms that have been identified using scanning electron microscopy (SEM).
A typical contact load and the measured wear profile are shown in the Figure 33.8
. At TRR, the oil
film thickness is small ( Figure 33.7
) and the contact load is a maximum. Thus, liner wear at this location
is a maximum.
The liner surface outside the ring travel zone is shown in Figure 33.9a
. This is equivalent to a machined
liner surface. The scanning electron micrograph of the TRR interface of the liner after an engine test is
shown in Figure 33.7
. This micrograph reveals:
• Typical polished area in the region below the TRR
• Transition from the highly polished area to the region with the parallel grooves along the sliding direction.
The region below the TRR, taken at higher magnification to look for finer details in the wear mechanisms, is shown in
Figure 33.9b
. This micrograph indicates parallel grooves in the sliding direction,
surface cracks along the cell-boundaries, some regions with pits at microscopic level, and delamination of the graphite flakes.
The carbon deposits that form on the piston lands and behind the rings, oil degradation products, soot, and wear debris generated because of the relative motion of the liner and ring can act as third-body particles. Some of these deposits can be hard and produce a polished surface on the liner even under poorly lubricated conditions. This loose wear debris can sometimes adhere to the ring face or mechanically scrape different regions, and can be transported from one region to another. Adhesive wear is also a mechanism between ring/liner interface. The difficulty associated with differentiating between adhesive wear and abrasive wear makes the quantification of the amount of wear caused by adhesive wear impossible.
Combustion products from sulfur-containing fuel include NO x
and SO x compounds. These oxides can react with water vapor to form acids. The condensed acids are rich in sulfurous and sulfuric acids, which
© 2001 by CRC Press LLC

(a)
(b)
(c)
FIGURE 33.9
corrosive wear.
(a) Liner surface outside ring travel zone. (b) Region below top ring reversal area. (c) Region showing
© 2001 by CRC Press LLC

FIGURE 33.10
Worn face of a Cr-plated top compression ring after an engine test.
are aggressive to the liner surfaces chemically. Figure 33.9c
indicates selective attack of the ferrite phase.
This suggests that a chemical etch has taken place on the liner surface. Loss of material because of this chemical etch may not be significant.
Figure 33.10
shows the worn face of a Cr-plated top-compression ring after an engine test. This figure
reveals a highly polished region and parallel grooves along the sliding direction, the latter an indication of abrasive wear.
In summary, the chief mechanism for the loss of material wear in ring/liner systems is abrasive wear.
Factors that govern ring/liner interface tribology include oil film properties and thickness, contact pressure, material compatibility, external dust particles from the air system, the carbon deposits that form on the piston lands and behind the rings, oil degradation products, and wear debris generated because of the relative motion of the liner and ring. The contributions to loss of material in the liner by each one of these factors depend on engine operating conditions. These factors also contribute to the second ring and oil ring wear.
Among all the tribological systems in the power cylinder system, the ring/liner interface plays a significant role in determining the effective engine life. One of the ways to meet more stringent emissions requirements is to design an engine with cooled exhaust gas recirculation (EGR). If next-generation engines are designed using this approach, wear due to corrosion might be significant during cold cycles of operation. To retain experience-based knowledge, the choice of materials is currently limited to conventional materials in use. Designing cleaner engines with greater durability remains a challenge to engine manufacturers.
Overhead components ( Figure 33.11
) are parts that link the camshaft and the intake/exhaust valves in
the cylinder head in order to actuate the opening and closing of the valves in accordance with the contour of the cam lobes. This linkage is also used to actuate fuel injection events in engines equipped with electronically controlled but mechanically actuated fuel systems, although some modern electronic fuel systems now actuate injectors with hydraulic pressure from engine oil (Glassy et al., 1993).
There are many varieties of designs. Overhead components in a typical diesel engine normally consist of cam followers, push rods, rocker levers, and crossheads. The key tribological contact is between the cam lobes and the cam followers. Additional tribological contacts include the joint between the push rods and the cam followers, the rods and the rocker levers, the rocker levers and the crossheads, and ultimately the contacts with valve and injector tips. Recent emissions controlled engines encounter high
© 2001 by CRC Press LLC

FIGURE 33.11
Overhead components and their arrangements in a diesel engine.
levels of soot in the oil, therefore exposing overhead components to the risks of high wear — not only due to mechanical stress, but also the interference of soot (Kuo et al., 1998).
Small- to medium-sized diesel engines usually use flat cam followers, or tappets, similar to small gasoline engines. When the cam rotates, the cam lobes slide across the flat tappet surface, and the contour of the lobe lifts the tappet upward. Once the contact slides past the peak of the lobe, the tappet will come down due to the spring pressure of the valves (or injectors) to remain in contact with the lobe surface. Tappets are usually positioned to be off-centered from the contact point. This allows the tappet to rotate when it is dragged by the friction of the sliding contact. This rotating motion introduces limited rolling to reduce friction, and helps distribute the wear across the tappet surface to prolong component life. To provide optimal performance at this contact, consideration must be given to the selection of materials, the contact stress, surface finish, and the mode of lubrication. Diesel cams are normally made of steel
© 2001 by CRC Press LLC

or chilled ductile iron. Tappets are usually made of various cast or alloyed irons, or steel, depending on the stress level (Korte et al., 1997). Ceramic tappets using materials such as silicon nitride have also been tested successfully to improve durability at high stress levels (Kitamura et al., 1997). Ceramics have been particularly effective in reducing wear when a poor-quality lubricant is used. Superfinishing on cam or tappet surfaces has been used successfully on many occasions, but overly smooth surfaces sometimes lead to early seizure and scuffing. Lubricant is provided by crankcase oil splashed onto the parts due to the mechanical movement of crankshaft and connecting rods inside the block. Lubrication in this contact is usually in the elasto-hydrodynamic to boundary lubrication regime with a
λ
ratio below 1. Friction modifier and antiwear additives are more important for wear protection than the rheological properties of the lubricant.
Medium- to large-sized diesel engines usually use roller followers to accommodate the much higher stress involved. When the cam lobe rotates, it lifts a roller mounted on a roller shaft assembly, which in turn raises the push rod. The cam and roller follower contact now has turned into a rolling contact to reduce friction and improve durability. However, depending on the friction and lubrication characteristics of the several interfaces involved, the contact often has some sliding motion, or slippage, in addition to pure rolling. Many cam and tappet failures are associated with the transition from rolling to sliding. With limited slippage, fatigue life is reduced because the high friction of sliding introduces a shear stress that raises the maximum stress level closer to the surface, causing early spalling or pitting. With more slippage, immediate seizure or scuffing may result due to localized welding. Sometimes, the slippage is not caused directly by the lubrication of the cam/follower contact. Many roller followers are mounted on a bronze pin with the soft bronze material acting as a bearing material to better accommodate the steel roller.
Lubricant is usually fed through oil drillings to this interface. Wear or corrosion of the steel roller/bronze pin interface will cause the roller to “stick” to the pin, then quit rolling (Cusano and Wang, 1993). It is important to look at all linkages when troubleshooting a tribology problem.
Push rods are used to transfer the lifting motion initiated by the cam and the cam followers to the rocker levers. The rocker levers engage on the valve tip or injector link to initiate the valve opening or fuel injection event. Ideally, the push rod will have only normal loading on both ends as it merely functions as a link between the follower and the rocker lever. However, both ends of the push rods are usually shaped like a ball joint and engage in sockets in the mating parts. This design allows limited sliding and sideward movement of the rod relative to the other two components it connects. As modern engine designs call for reduced emissions and higher power density, accurate actuation of valve and injector events become critical. Push rods represent a linkage among the overhead components that introduce additional inaccuracy due to tolerance stack-ups and wear. This linkage is eliminated in overhead cam designs where the cam and cam followers actuate the valve and injectors either directly or via the rocker levers.
Many modern diesel engines have more than two valves per cylinder to improve air handling. Rocker levers sometimes push down on crossheads to actuate two valves simultaneously. This is in contrast to double overhead cam designs, in which each valve is actuated by individual cam lobes. This lever/crosshead contact has limited sliding in addition to normal loading.
Of all the interfaces between overhead components, sliding is always directly or indirectly involved, and is primarily responsible for wear problems. Oil drillings are usually present to feed oil to the interfaces between components to provide boundary lubrication under all these sliding contact conditions. However, rheological properties like cold flow performance are still relevant in terms of pumping oil to the drilling without delay. The presence of contaminants, especially diesel soot, can greatly influence the effectiveness of lubrication to cause wear. Wear modes range from scuffing at the rod tip/bowl interface, to abrasion and scuffing on the crossheads. Another cause of wear is oil delay due to oil thickening or poor cold flow properties, and loss of wear protection due to oil soot contamination. Hardware improvements can also reduce or prevent wear. These may include ceramic inserts, coatings, or enlarged contact areas.
© 2001 by CRC Press LLC

Spring retainer
Tip
Keeper
Stem
Guide
Spring
Cylinder head
Fillet
Insert
Margin
Seat
Combustion face
FIGURE 33.12
Engine valve schematic. (From Schaefer, S.K., Larson, J.M., Jenkins, L.F., and Wang, Y. (1997) Evaluation of heavy duty engine valves — material and design, Proc. Int. Symp. Valvetrain Sys. Des. Mater., Dearborn MI, April 14-15.)
The engine valve system (comprising the valve, seat insert, and valve guide) is shown schematically in
Figure 33.12
.
Wear between the valve and seat is thought to occur primarily due to relative motion when the valve is seated, due to cylinder pressure that forces the valve into the seat, causing slight deflections of both valve and seat. Thus, cylinder pressure and rigidity of the valve face and seat are primary variables in determining valve/seat wear. Also, the valve/seat angle controls the contact stress normal to the seat and hence the tangential stress (friction coefficient times normal stress). Flatter seats (i.e., reduced seat angle θ reduce the contact stress (which varies as 1/cos θ
)
), but the effect is small, especially when there is substantial friction between the valve and seat. However, the most important effect of reducing the seat angle is to reduce lateral valve/seat displacements, which in turn reduce wear.
The primary mechanism of valve/seat wear is adhesive wear, often combined with oxidative wear or corrosion due to sulfur compounds or other fuel-derived compounds. Sometimes, the wear surface shows surface waves (“sand dunes,” see
Figure 33.13
), which occur due to the surface shear stress that causes
material to flow down the seating surfaces in addition to being removed as wear debris. Material can also transfer from the valve to the seat insert, or vice versa.
In diesel engines, especially in constant-speed applications (typically industrial or power generation), intake valve recession is usually more severe than exhaust valve recession, although the intake seat temperatures are lower (e.g., 600°C for exhaust valve seat and 370°C for exhaust seat insert; 370°C for intake valve seat and 270°C for intake seat insert). This is thought to occur because of lack of lubrication on the intake side (especially when a dry stem seal is used), high boost pressure blowing off any oil films, and the lack of oil/fuel deposits or beneficial oxide layers leading to clean metal-to-metal contact, which favors adhesive wear. This is also consistent with the observed temperature sensitivity of wear in
Figure 33.19
(Section 33.6): some materials show decreasing wear as the temperature is raised from 270
to 500°C. Factors affecting valve/seat wear are listed in Table 33.4
.
© 2001 by CRC Press LLC

FIGURE 33.13
Wear of (a) intake valve seat insert, and (b) intake valve showing dark imaging oxide/transfer layers
(left hand image) on the peaks of the “sand dunes.”
The end result of valve/seat wear is that the valve recesses into the head (measured as a change in valve protrusion). Valve recession tends to offset the increase in valve train lash due to wear of the cam follower, pushtubes, adjusting screws, rocker levers, crossheads, and valve tips. If excessive valve recession occurs, the valve-train lash can become zero, leading to higher contact stresses on the cam nose. Loss of seating load caused by valve/seat wear can also lead to poor heat transfer from the valve to the seat, leading to valve overheating, which may cause valve torching, excessive valve/seat wear, fatigue failures, or stemguide galling. Also, in cases of extreme valve wear, chordal fatigue failures of the valve can occur due to the loss in section.
Wear between the valve stem and guide occurs due to the sliding motion of the stem in the guide (twobody abrasion and also adhesive wear under more severe conditions) and also due to three-body abrasion from oil deposits. Corrosive wear is also common for exhaust valves (and potentially for intake valves for engines equipped with exhaust gas recirculation). Stem/guide wear is sensitive to the rate of lubricant supply to the interface, which is usually controlled by means of a stem seal, which limits the ingress of
© 2001 by CRC Press LLC

TABLE 33.4
Factors Affecting Valve/Seat Wear
1.
Valve/seat deflections during firing. The magnitude of the deflection depends on cylinder pressure and valve, seat, and head design (particularly seat angle).
2.
Valve and seat temperature ( see text).
3.
Thermal and mechanical distortion of the seat insert (and possibly the valve) due to the temperature gradient from exhaust to intake and due to the effects of cylinder pressure and clamping stresses (e.g., head bolts, injector clamping). Swirl increases temperature gradients across intake valve head, which increases head distortion. This effect leads to deviations from circularity and nonuniform loading. In extreme cases, this may lead to loss of sealing, which can lead to valve “torching” or “guttering,” in which the high thermal flux around a leak results in gross local overheating of the valve and seat, resulting in oxidation/thermal fatigue and the complete loss of a piece of valve material.
4.
Impact wear caused by high seating velocities.
5.
Abrasion by oil/fuel deposits or by protruding carbides in the counterface. Surface deposits may also reduce the heat flow from the valve, causing high valve temperatures.
6.
Insufficient lubrication due to high boost pressure (intake) blowing off the oil film or use of a non-optimum valve stem seal which does not allow sufficient oil to flow into the guide. (Too much oil, on the other hand, can lead to torching due to buildup of oil deposits on the seat.)
7.
Excessive valve rotation, especially if a non-optimum valve rotator is used. Valve rotators normally reduce valve wear by evening out circumferential temperature variations and thermal distortions and removing oil deposits that reduce heat transfer from the valve. However, excessive rotational speeds may cause increased wear, partly by the increased sliding action and partly by removing beneficial surface deposits and oxides.
8.
Corrosive wear from fuel-derived species. Corrosion may occur due to condensation of exhaust gas constituents such as SO
2
, SO
3
, NO, and NO
2
at low temperatures (e.g., <150°C) and by hot corrosion at high temperatures (e.g.,
>730°C, 1350°F).
9.
Excessive temperatures, leading to structural or dimensional changes in the valve and seat, softening of the seating surfaces, and increased oxidation. High temperatures may be caused by engine operating conditions (e.g., overfueling, retarded timing) and may also be due to weak contact between the valve and seat or too narrow seat contact or improper seating of the seat insert in the cylinder head, reducing the heat flux from the valve to the seat.
Seat deposits may also contribute to poor heat flow and overheating.
10.
Valve/seat misalignment, leading to uneven contact stresses and poor sealing. Guide/seat concentricity is important and is influenced by the machining processes. Ideally, the guide bores and seating surfaces are machined in place in the cylinder head at the same time.
11.
Permanent elongation of the valve due to “cupping” of the valve head by creep. This leads to a decrease in lash, which can lead to torching of the valves due to insufficient seating force.
oil and also the amount blown out by port pressure. Modern engines normally run oil-starved in order to reduce valve guide oil consumption, which in turn reduces the lubricant contribution to particulate
emissions. Factors that affect valve stem/guide wear are listed in Table 33.5
.
As shown in Table 33.6
, materials selection criteria include more than tribological properties. The evo-
lution of valve materials to meet these demands has been reviewed by Schaefer et al. (1997). For most heavy-duty diesel applications, exhaust valves are two-piece with a high-strength, iron-based austenitic stainless steel head welded to a hardenable martensitic steel stem. The valve seat area can be coated with a variety of hardfacing alloys, including Stellites, Tribaloys, and various nickel-based overlays. Intake valves are typically made from one-piece martensitic steels such as Silchrome 1, which may be hardfaced at the seat area. Valve stems are often chrome plated or nitrided to improve wear and scuff-resistant for both exhaust and intake valves.
Valve guides are typically made from either pearlitic grey cast iron or iron-based powder metal (PM) materials, which often include machinability aids and solid lubricants, as well as porosity which aids in
oil retention. PM guide materials provide superior scuffing and wear resistance ( Figure 33.14
).
A wide variety of valve/seat inserts is available (Rodrigues, 1997), including cast irons, steels, nickel and cobalt alloys, and powder metal steels incorporating solid lubricants for improved adhesive wear resistance. PM materials may be copper infiltrated to improve thermal conductivity. Materials selection
© 2001 by CRC Press LLC

TABLE 33.5
Factors Affecting Stem/Guide Wear
1.
Oil supply, which is primarily controlled by the valve stem seal design or (if no seal is used) the stem/guide clearance.
Other factors include boost pressure (intake valves), duty cycle (constant-speed applications tend to blow oil out of the guides, while varying speeds and loads can periodically bring in a fresh oil supply).
2.
Inadequate oil retention on the guide bore. Often, the guide bore is knurled for improved oil retention. Powder metal guides have inherent oil retention capabilities due to the porosity of the material. Some powder metal guides incorporate a solid lubricant for improved wear and reduced friction.
3.
Cocking and side loading of the valve stem due to actuation forces from the rocker lever or crosshead. This effect is increased by the use of non-guided crossheads.
4.
Excessive temperatures if the guide protrudes into the port.
5.
Displacements caused by inadequate mechanical support of the guide in the head.
6.
Valve lift, since the wear rate should be proportional to the sliding distance.
7.
Stem/guide clearance. Too tight a clearance can cause scuffing as the stem expands thermally relative to the guide
(because the stem is normally hotter than the guide and also because the stem material may have a higher CTE than the guide). Too open a clearance can allow cocking of the stem in the guide, leading to excessive wear at the guide top and bottom ends (180° apart).
8.
Stem/guide wear should increase with cylinder pressure because the side loading/misalignment forces scale with cylinder pressure.
9.
Errors in as-machined roundness, straightness, and taper of the guide and/or stem will lead to heavy contact between the stem and guide in local areas.
10.
Thermal distortion of the stem and guide due to temperature gradients in the head.
11.
Mechanical distortion of the guide due to improper press fit in the head.
12.
Side loading and cocking caused by poor concentricity of the valve seat and the guide.
13.
Uneven seat wear, leading to side loading and cocking of the valve in the guide.
14.
Excessive stem and guide temperature, leading to reduced stem/guide clearance, corrosive wear, excessive oil deposits, loss of oil film, etc.
15.
Excessive valve rotation, especially if a non-optimum valve rotator is used.
16.
Abrasive wear of intake stems and guides caused by corrosion of the intake system, for example due to a leaking charge air cooler, use of EGR, inadequate air filtration, or improper installation of the air inlet where it can ingest excessive road dust or water.
17.
Corrosive wear of exhaust stems and guides caused by high fuel sulfur, other fuel impurities, or unstable oil additives.
This may be linked with excessive temperatures or may be caused by condensation of acidic gases at low temperatures
(e.g., during engine idling).
TABLE 33.6
Materials Selection Criteria for Valve System Components
Requirement Component
Ability to forge and machine
Fatigue strength at elevated temperature
High temperature yield strength (hot hardness)
Temperature resistance (phase stability and dimensional stability)
Elevated temperature compressive yield strength
Thermal fatigue resistance
Corrosion resistance: sulfidation, oxidation, chlorides
Creep resistance
Exhaust and intake valves
Exhaust valve
Exhaust valve
Exhaust valve and seat insert
Exhaust seat
Exhaust valve and seat insert
Exhaust valve and guide
Exhaust valve and seat insert
Indentation resistance (hot hardness)
Resistance to adhesive wear (marginal lubrication, high contact stress, low temperature)
Resistance to adhesive wear (marginal lubrication, high contact stress, high
Exhaust valve and seat insert
Intake valve and seat insert
Exhaust valve and seat insert temperature)
Resistance to sliding wear (marginal lubrication, low temperature)
Resistance to sliding wear (marginal lubrication, high temperature)
Weldability to steel
Thermal expansion mismatch
Intake valve stem and guide
Exhaust valve steam and guide
Two-piece valves
Exhaust valve stem to guide
Resistance to sliding wear (well-lubricated, low temperature, potentially high-soot oil) Stem tip/rocker lever or crosshead
© 2001 by CRC Press LLC

Cast iron
Stuck Gall Gall
Powder metal
FIGURE 33.14
Results of a split test run with cast iron and powder metal exhaust valve guides. The engine was run at rated for 5 minutes with the coolant level below the head gasket and the water pump disconnected (the steam temperature from the head was measured at 180°C). This procedure was repeated three times. One exhaust valve was stuck at the end of the third cycle. Three of the cast iron guides had galled, with no galling of the powder metal guides.
tends to be very application specific. The tribological compatibility of the valve/seat insert system is important because some materials couples do not match well.
Table 33.7
characterizes some of the journal bearing systems found in diesel engines. Bearings and
bushings are required to support the load applied to rotating or oscillating shaft/components with minimum friction and wear. Bearing materials selection is complicated by a number of requirements,
which are often contradictory ( Table 33.8
). For example, bearings need to be compliant enough to
conform to imperfect shaft alignment or distortion and to embed debris, but they also need to be hard enough to resist wear and fatigue. To overcome these conflicting requirements, a steel-backed “tri-metal”
design is often used for connecting rod, crankshaft, and other bearings ( Figure 33.15
).
Bearing design involves trade-offs between friction reduction (for best fuel economy) and durability.
Durability is addressed by calculating the minimum oil film thickness (MOFT) and peak oil film pressure
(POFP) at various design points. Low MOFT indicates the potential for wear and seizure, and high POFP indicates the potential for overlay or lining fatigue. Durability estimates are often made by comparing
TABLE 33.7
Characterization of Bearing Systems for Heavy-Duty Diesel Engines
Location
Piston pin bushing
Connecting rod (small end) bushing
Connecting rod (large end) bearing
Crankshaft bearing
Camshaft bushing
Cam roller pin
Rocker lever bushing
Turbocharger bushing
Fuel pump bushings (several locations)
Motion
Oscillating
Oscillating
Rotating
Rotating
Rotating
Rotating
Oscillating
Rotating
Rotating
Unit Load
High
High
High
High
Low
Medium
Medium
Low
Low
Speed
Low
Low
Medium
Medium
Medium
Medium
Low
High
High
© 2001 by CRC Press LLC

TABLE 33.8
Bearing Material Properties
Property
Fatigue strength
Conformability
Embeddability
Corrosion resistance
Friction reduction
Wear resistance
Seizure resistance
Heat resistance
Steel Back
High
Low
Low
High
Low
High
Low
High
Leaded
Bronze Lining
High
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Aluminum/Tin
Lining
Medium
Medium
Medium
High
Medium
Medium
Medium
High
Lead-Tin or
Lead-Indium Overlay
Low
High
High
Medium
High
Low
High
Low
Sputtered
Aluminum-Tin
Overlay
High
Medium
Medium
High
Medium
High
Medium
High
FIGURE 33.15
Steel-backed “trimetal” bearing design.
calculated MOFT and POFP values for new designs with those for established engines with known durability (“comparative engineering”).
Bearing materials selection depends on many design- and application-specific factors. For most highly loaded bearings used in heavy-duty diesel engines, fatigue requirements dictate the use of steel-backed designs and high-strength leaded bronze lining materials. Steel-backed aluminum bearings are used where fatigue requirements are less stringent. These aluminum bimetal materials are used without overlays because of the difficulty of electroplating onto aluminum. In practice, this, along with fatigue requirements, tends to further limit the use of aluminum bearings in heavy-duty engines, although they find widespread use in light-duty applications.
Overlay selection depends on the level of conformability/embeddability required for the application.
In engines with good shaft tolerances, clean manufacturing processes, and good filtration systems, harder overlays may be selected for improved wear and fatigue resistance. Sputtered aluminum-tin overlays provide the ultimate in wear and fatigue resistance and can be applied to both bronze and aluminum linings, but at relatively high cost. Electroplated lead-tin-copper and lead-indium overlays are more commonly used. For the Pb-Sn-Cu system, the higher the copper level, the greater the hardness/wear resistance/fatigue resistance. Recently, electroplated overlays incorporating ceramic hard-phase particles have been introduced, providing performance and cost levels intermediate between conventional electroplating and sputtering. An alternative bearing design (termed “Rillenlager”) is to form inlaid bands of overlay material in the lining surface. This design provides improved wear resistance (provided by the exposed lining material), while still maintaining some degree of conformability and embeddability.
Figure 33.16
shows wear comparisons between standard electroplated, “Rillenlager,” and sputtered Al-Sn
overlays.
© 2001 by CRC Press LLC

0 100 200 300 running period (%)
400 sputter bearing
4
8
Rillenlager
12
16 trimetal bearing
20
FIGURE 33.16
Wear of sputtered Al-Sn, “Rillenlager,” and electroplated overlays. (From Miba Gleitlager AG,
Technical Information, Laakirche, Austria.)
A variety of proprietary bench tests is available for materials development and selection. Typically, such tests are used for comparing materials rather than to establish design limits.
Although the majority of engine components are oil-lubricated, many turbocharger components are required to function at high temperatures with no (intentional) lubrication. Although loading is generally light for such components, durability expectations are the same as for the rest of the engine (e.g., a million miles for a heavy-duty diesel engine).
An important example of this type of unlubricated, high-temperature system is the turbocharger wastegate mechanism ( Figure 33.17
), which typically comprises a flap valve with a valve shaft rotating in a bushing. Maximum operating temperatures range up to approximately 550°C for diesels and 650°C for natural gas engines. Wear occurs due to both intended actuator motion and also that induced by exhaust pressure pulses acting on the valve. The vibration amplitude depends on many factors, including the stiffness of the actuator mechanism, but is often in excess of 100 microns in amplitude, which can give rise to substantial wear because the frequency of the excitation pulses is extremely high. The effect of vibration amplitude is often greater than the effect of high temperature. For example, Waterhouse
(1992) shows that wear rates increase by several orders of magnitude once a critical amplitude (of the order of 10 to 50 microns) is exceeded ( Figure 33.18
). Temperature has at least three competing effects on wear and scuffing: (1) softening of the materials, (2) formation of protective oxide layers/reaction products, and (3) formation of undesirable (soft, nonprotective and/or loosely attached) oxides/reaction products. The overall effect depends on material and temperature range (and also other factors, such as contact pressure). The effect of temperature on wear rates for various materials is shown in Figure 33.19
.
Wear rates can increase or decrease with increasing temperature. In general, a flat temperature response is optimal because the mechanism is required to function at all temperatures from ambient (on engine start-up) to maximum full-throttle conditions.
Typical failure modes are high wear, galling, and material transfer on the shaft and bushing
( Figure 33.20
), which can lead to sticking of the mechanism. Wear also occurs on the actuator rod and the crank pin (which are external to the turbocharger), leading to loss of control function and consequent deterioration of engine emissions performance. Corrosion can also be a problem for internal components
(due to high-temperature oxidation/sulfidation and condensation of exhaust acids at low temperatures) and external components (due to road salt).
© 2001 by CRC Press LLC