33Diesel Engine Tribology

33.1 Introduction33.2 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 ComponentsCam and Cam Followers • Push Rods, Rocker Levers, and Crossheads

33.4 Engine ValvesValve/Seat Wear Mechanisms • Valve Stem/Guide Wear Mechanisms • Materials Selection Criteria

33.5 Bearings and Bushings33.6 Turbomachinery33.7 Fuel System

Fuel Injectors • Fuel Pumps

33.8 Fuels, Lubricants, and FiltrationDiesel Fuel • Diesel Lubricants • Used Oil Analysis • Filtration: Air System • Filtration: Lubricating System • Filtration: Fuel System • Filtration: Cooling System

33.9 Future Trends

33.1 Introduction

Diesel engines are based on the principle of compression ignition. Air is introduced alone into thecombustion chamber with the opening of the intake valves. The air intake is facilitated by the downwardmovement of the piston that creates a pressure differential through volume expansion. Turbochargersare often used to force more air into the combustion chamber to increase air density to burn more fuelfor the same displacement. The air, once in the system and the intake valve closed, is then compressedto reach high pressure and high temperature. Fuel is injected at this point to initiate combustion. Thefuel 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 downwardagain, 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 valvetrain system to control the timing of the combustion, the crankshaft and connecting rods to receivepower and to provide mechanical movement of the compression/expansion process, the gear train tooperate pumps and accessories essential to moving working fluids around, and, finally, all the working

Malcolm G. NaylorCummins Inc.

Padma KodaliCummins Inc.

Jerry C. WangCummins Inc.

fluids that provide cooling, lubrication, and power to the engine (Heywood, 1998). All of these compo-nents 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 thathouses the cylinder liners. Many larger diesel engines have removable liners for longer service life andeasy rebuild. The skirt part of the block allows for the camshaft and crankshaft to be inserted. Gears andaccessory pulleys are located in the front. A flywheel is attached to the rear that connects to the drivetrain that transmits power to the transmission. On top of the block is fitted the cylinder head that housesvalves 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 boreto guide the movement of the pistons. The movement of the piston inside the liner from the top to thebottom is called a stroke. The displacement of an engine is the volume defined as:

Displacement = π (Bore radius)2 × Stroke

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 quantityof oil reaching the upper end of the liner to prevent excessive oil accumulation or deposits. The top ringis used to prevent compressed air or combustion gas from leaking past the rings so as to preserve thepressure in the cylinder. It does this by pressing the ring surface against the liner and allowing only anoil film between the liner and the ring. There are usually additional rings between the oil ring and thetop ring to facilitate the process. Because the power cylinders are where the heat is generated, the lineris cooled by coolant flowing between the liner and the block. Further cooling is provided by crankcaseoils squirted onto the bottoms of the pistons. Most heat, however, is rejected into the exhaust gas.

FIGURE 33.1 Cutaway illustration of a diesel engine.

CamshaftCam followersElectroniccontroller

Oil pump

Fuel pump

Valve cover

Flywheel andhousing

Liner

Valve and spring

Push tube

Rocker lever

Block

Oil filter

Turbocharger

Oil cooler

Crankcase

Valve pump

Piston &rings

Cylinder head

Connecting rod

Because air intake, fueling, and combustion occur in sequence in the power cylinders (i.e., only onecylinder fires at a time), each process must happen in exactly the right time to allow smooth andcontinuous 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, whenthe cam rotates, will actuate the opening and closing of the valves at the right time. Most diesel enginesrely on the fuel pump to control the amount and timing of the fuel injection. However, some unit injectorsare actuated mechanically by the injector lobes on the cam. The actuation of valve and injector eventsare 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 raiseone end of the rocker lever, forcing the other end of the rocker lever to push down on either injector orvalve tips, thus completing the action. The valve springs will force the overhead components to go in theother direction when the cam lobes have moved past their high points. Because the rotation of the camis tied to the rotation of the crankshaft via gears, and the crankshaft is connected directly to the pistonvia 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 thecombustion. The power is either sent to the transmission for useful work, or used to power accessorypulleys and various pumps through the gear train. These components include the oil pump, the waterpump 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 refinementin bowl design, air handling, and fuel injection have led to progressive reductions in NOx and particulatematter. Meanwhile, higher power density, better fuel economy, and extended service intervals are becom-ing 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. Allof the above present different challenges to engine tribology. Tribological contacts in each part of theengine are discussed in this chapter.

33.2 Power Cylinder Components

Simultaneous improvements in engine power density, reliability, and durability over the last five decadesare due to advances in design, materials, fuel, and lubricants. The overall function of power cylinders isto convert the gas pressure resulting from the combustion of the air/fuel mixture inside the combustionchamber to the torque applied to the crankshaft. Power cylinder components include liners/bores, pistonrings, pistons, piston pins, and connecting rods. In some engine designs, the bores are machined in thecylinder block and are referred to as parent bores and in some engines, liners are fitted into the cylinderblock. A schematic of the ring pack arrangement in a piston is shown in Figure 33.3. To understand thetribology of power cylinders, it is important to understand the function of these components.

33.2.1 Liners/Bores

The functions of liners/bores are to:

1. Confine the combustion gases2. Transfer the heat from pistons and rings to the coolant3. Seal the coolant4. Support piston-side loading5. Guide the pistons/rings6. Retain oil for start-ups

Liner/bore materials should have good break-in, wear, and scuff resistance; have a consistent surfacefinish to limit friction; retain oil in honing grooves for longer engine life; be free from embedded particles;and maintain good heat transfer.

1 9 9 6 1 9 9 8 2 0 0 0

FIGURE 33.2 Progression of diesel emissions regulations in the U.S.

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

0

1 2

8

4

1 9 8 6 1 9 8 8 1 9 9 0 1 9 9 2 1 9 9 4

Liners are typically manufactured from gray cast iron. To improve the wear resistance of liners, alloyingelements such as chromium, nickel, molybdenum, copper, vanadium, and phosphorous are added. Toimprove the wear and scuff resistance of the cast iron liners, various surface modification techniquessuch as induction hardening or gas-nitriding are also used. Table 33.1 summarizes typical liner materialsused in diesel engines.

33.2.2 Rings

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 wall3. 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. Toimprove the durability and performance of the rings, surface treatments that can provide strong adherenceto 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 con-ditions, are the basis for choosing surface modification techniques for rings. Ring scuffing can lead to

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 BHNBainitic-cast gray iron Bainite and tempered martensite; hardness 300 BHNBoron-alloyed cast iron Lamellar pearlitic material with high percentage of boron carbides; hardness 240 BHN

high blow-by and catastrophic failure of an engine. The coatings on piston ring faces can be applied onthe entire face, or half of the face, or only in the middle. Currently, chrome-plating, gas-nitriding, andplasma-spraying are some techniques that are used for modification of piston ring faces. Also, enginemanufacturers are evaluating the high-velocity oxygen fuel (HVOF) process for the application of chromecarbide/nickel chrome with Moly (Cr3C2-NiCr/Mo) coatings for compression rings (Shuster et al., 1999;Rastegar and Craft, 1993). The (Cr3C2-NiCr/Mo) coating has good wear and scuff resistance. Chromenitride coatings with oxygen (CrNO) deposited by physical vapor deposition (PVD) also have good wearand scuff resistance. These coatings have not been widely used in diesels because of the high cost of thecoatings (Rastegar et al., 1997). Shen (1987) explored multilayered PVD coatings for piston rings. Prop-erties of the ferrous-substrate ring materials and low-carbon steels that are currently used in diesels aresummarized in Tables 33.2 and 33.3 (Challen and Baranescu, 1999).

33.2.3 Piston

The piston’s roles are to:

1. Transfer force originated from the combustion gas pressure inside the chamber to the piston pin2. Provide support and guidance to piston rings and pin3. 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 ofcasting, forging and machining, as well as low cost, factor into the materials selection. The originalmaterial used for pistons was grey cast iron. Cast iron with spheroidal graphite is commonly used as apiston material due to its good groove resistance, low thermal expansion, and high strength. Despite thedifficulties in casting defect free thin sections of cast iron, cast iron pistons were successfully used insome production engines. Oxidation resistance of the piston crown at high temperatures and low wearof the ring grooves is also desirable.

Aluminum-silicon alloys that are widely used for piston materials do not have adequate wear resistancefrom ring movement at higher operating temperatures and pressures. Therefore, inserts are cast intoplace to provide a harder and more-resistant surface. To prevent excessive ring or groove side wear, a Ni-resist ring groove insert (cast iron with high nickel content) is metallurgically bonded to the aluminumpiston.

TABLE 33.2 Properties of Piston Ring Materials

MaterialModulus of Elasticity

(GPa)Tensile Strength

(MPa)Hardness(BHN)

Grey cast iron 83–124 230–310 210–310Carbidic/malleable iron 140–160 450–580 250–320Malleable/nodular iron 155–165 540–820 200–440Sintered irons 120 250–390 130–150

TABLE 33.3 Substrate, Application, and Surface Engineering Methods of Steel Rings

Ring Type Substrate Surface Treatment

Top compression rings SAE 9254 Cr-plated, plasma-sprayed13% Cr Nitriding18% Cr Nitriding, Cr-plated, plasma-sprayed

Oil ring Low carbon steel Cr-plated6% Cr Nitriding13% Cr Nitriding

Engine design trends toward high power density have driven the concept of composite pistons. Insome engine designs, two-piece or articulated pistons are used, in which the crown is forged steel andthe 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 squeezecast alloy can be improved by inserting ceramic fibers or metal inserts for reinforcements of grooves, pinbosses, and combustion-bowl rims. For the highest thermal loading, in order to reduce top ring and topring groove temperatures, a cooling gallery is located in the heat path. Composite aluminum pistonswith bolted steel crowns are used in large-bore and slow-speed diesel engines operating at high temper-atures 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 aluminumoxide, 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 pistonskirts. Recently developed resin-bonded graphite/molybdenum coatings were reported to have longer lifeand better wear resistance than sprayed graphite (Challen and Baranescu, 1999).

33.2.4 Piston Pin and Connecting Rod

The piston pin and connecting rod transform the combustion pressure delivered to the piston into torqueat 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 surface-hardened, lapped, and polished to a mirror-like finish. Pin and pin bore designs are dictated by piston boreload-carrying capacity. The pin rides in a bushing, which is typically a layered bronze-bearing material.

33.2.5 Friction in Power Cylinders

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 bydurability and oil consumption requirements.

33.2.6 Tribological Systems

Based on the function of components, power cylinder components can be divided into eight systemswith tribological interfaces: liner/ring; ring and ring groove, liner/piston skirt, piston pin and pistonbore, 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 whenoil consumption and blow-by become excessive. In the perspective of power cylinders, oil consumptionis most often associated with excessive radial wear of the top compression ring and cylinder liner wearat the top ring reversal.

A properly designed piston skirt has an adequate lubricating film between the skirt and the liner. Low-friction coatings on the skirt are helpful when the piston skirt breaks through the lubricating film. Undersome severe operating conditions, such as when extreme engine thermal transients can alter the clear-ances, 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 operatingconditions is minimal. Embedded particles in the piston pin from the lapping or polishing process canlead to excessive wear in the bore. One abnormal bore failure occurs when an engine experiences hotshutdown. The piston cooling oil ceases to flow, and the piston crown area is hotter than the pistonbosses and piston pin. As a result, heat is transferred from the hot crown to the piston bore, which

experiences higher temperatures than normal operating conditions. These high temperatures at the pistonbore bushing can lead to degradation of bearing material. Another abnormal failure may be caused bypin bending, which results in nonuniform load distribution, with high stresses biased to the inner edgeof 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 operatingconditions shows minimum wear. Potential causes of high piston-liner wear include excessive oil depositsand external contaminants.

Oil consumption is primarily controlled by the tension of the oil ring. Oil consumption is high at lowunit pressure, decreases with an increase in unit pressure, and remains constant above a certain unitpressure. 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 whenthe expander wears with time or when the spring embeds in the inner diameter of the ring. This resultsin increased oil consumption. Cr-plating and gas-nitriding are typical surface modification techniquesused to extend the life of an oil ring.

The primary factors in determining engine life before major overhauls are ring/liner interface andring/ring groove wear. Thus, the remainder of the discussion focuses on ring/liner wear. Ring/ring groovewear is covered in Section 33.2.9.

33.2.7 Ring/liner Interface Tribology

Kodali et al. (1999) reviewed the major factors influencing cylinder liner wear. Most of these factors arealso 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, enginedeposits, soot, and lubricant additives.

FIGURE 33.4 Distribution of (a) total energy in a fired engine; and (b) total engine mechanical friction. (FromRichardson, E.E. (1999), Review of Power Cylinder Friction for Diesel Engines, ASME, ICE-Vol. 32-3, Paper No. 99-ICE-196. With permission.)

MechanicalFriction(4-15%)

Work Output

(38-41%)

Other(40-60%)

Pistons,Rings,Rod(40-55%)

OtherLosses(47-58%)

(a)

(b)

33.2.7.1 In-cylinder Mechanical Design

The design of power cylinder components can have a significant effect on system wear. The fundamentalprinciples in designing for low-wear power cylinders are:

• Design the system for hydrodynamic lubrication, which prevents direct contact between slidingsurfaces.

• Choose compatible materials for the sliding surfaces, which results in low wear when contactoccurs between sliding surfaces.

Engine design parameters that modify power cylinder dimensions can influence liner wear. Under-standing the effect of parameters such as distribution of gas pressure, contact force at the ring/linerinterface, 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 remainthe same. The stroke and connecting-rod length affect the piston velocity. This may have some effect onthe hydrodynamic oil films that are developed under the ring face. Excessive bore distortions result inincreased 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 alsocause 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 deadcenter (TDC). Higher speed results in increased wear caused by an increased number of contacts of ringand liner at TDC. Higher rpm also increases the piston speed, which can reduce wear rates by increasingthe 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 boundarylubrication 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. Reducedwidths can decrease this force and potentially reduce wear. The ring face profile influences both the netgas 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 bore-polishing at the top ring reversal area of the liner. Bore polishing results in significant amount of materialloss and loss of oil control.

Land clearances and groove widths affect the flow of gases through the ring pack. This affects theforces 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 surfacesin relative motion. Environmental conditions that affect the contact pressure between two surfaces arediscussed 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

ring can be decreased by proper ring design and can help compensate for increased cylinder pressure. Highestcylinder pressures occur during the compression and expansion strokes. The peak pressure typically occursafter the TDC. Figure 33.5 illustrates the distribution of the gas pressure and the contact force as a functionof 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, differentialexpansion, which may be caused by nonuniform cooling around the circumference and the length of theliner, may lead to bore distortion. The ring/liner wall temperature affects the abrasive wear term indirectlythrough the kinematic viscosity of the lubricant. The liner/ring wear may increase with decrease in theoil viscosity as a result of increasing temperature. Higher sliding temperatures may also aggravate linerwear 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 thestroke, the piston is moving the fastest, resulting in hydrodynamic lubrication. However, at the deadcenters, the piston velocity goes to zero and the hydrodynamic lubrication breaks down. This can resultin ring/liner surface contact that leads to loss of material from both the ring and liner surfaces.

33.2.8 Fuels and Lubricants

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 engineshas been established (McGeehan and Kulkarni, 1987; Dennis et al., 1999; Weiss et al., 1987). Because ofemissions requirements, the trends have been toward decreasing the sulfur content in fuel. Off-highwayapplications are still permitted to use high-sulfur fuels. The SOx products formed as a result of combustioncan 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 determinethe severity of the chemical attack. Material loss as a result of this localized chemical attack is usuallyinsignificant. However, at lower coolant temperatures, acid condensation is promoted, and thus canactivate corrosive wear. Liner wear with 400 ppm sulfur in heavy-duty diesel engines indicated embeddedcorrosion 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 (Wanget al., 1999).

FIGURE 33.5 Variation of contact force and gas pressure with distance below the top dead center.

12000

10000

8000

6000

4000

2000

-2000

-4000

-6000

0

0 20 40 60 80 100 120 140 160

Position (mm)

Gas Pressure

AsperityContact

Force (N)

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 loadedcomponents during operation. The many functions of a modern diesel lubricant are the result of carefulblending of chemical additives and carefully refined base stocks. These additive packages have undergonetremendous improvement over the last 20 years and have played a major role in the improvement inengine life over that period of time.

In a ring/liner interface, the lubricant helps in sealing against the leakage of combustion gases. Deg-radation 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 theviscosity is too high, oil flow may be poor, resulting in poor lubrication conditions that eventually leadto high wear and friction. Buildup of carbonaceous deposits on a piston can increase liner wear anddecrease the ability of the rings to seal the combustion gases. The alkaline detergent additives performa critical, dual role; they neutralize corrosive combustion acids and they help clean deposits that mayform on hot metal surfaces.

The buildup of combustion-derived soot in the lubricant promotes wear. Soot-induced wear mecha-nisms are not well-understood. There are several proposed mechanisms (Kim et al., 1992), such asadsorption of zinc dithiophosphate (ZDTP) by soot, competition between ZDTP and soot for adsorptionsites on contacting surfaces, and the abrasive action of soot. Currently, there is no evidence to illustratethe role of soot on liner wear. The typical size of primary soot particles is reported to range between0.01 to 0.1 µm (Kuo et al., 1998). These particle sizes might only result in polishing wear. When enginesrun 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 wearby soot may result in significant contribution of liner material loss. Similarly, if the soot absorbs theadditives in the oil and thus inhibits formation of the anti-wear film on the contacting surfaces, linerwear may be accelerated. Formation of acids caused by condensation of some combustion products candegrade the oil in localized regions, resulting in chemical attack of the liner.

33.2.9 Engine Deposits

Deposits are known to have an effect on the durability and emissions of an engine. The primary causeof engine deposits is a relatively complex reaction that occurs among the components, fuel, blow-bygases, and the engine lubricant. Most of these deposits derive from the fuel, with some contribution fromthe lubricant. These deposits are accepted to be a mixture of inorganic material (ash) and carbonaceouscombustion 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 ringgroove; and on the under-crown. The deposits formed at different locations of the combustion chamberare chemically different, and probably vary in the mechanical aspect of being soft or hard. Top landdeposits are known to be very hard and abrasive, whereas under-crown deposits, which occur mainlybecause 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 reversalarea of the liner. When these carbon deposits are trapped between the ring and ring groove, side clearancesmay change. Reduced clearances caused by carbon deposits may result in excessive wear, sticking, andpossible scuffing.

33.2.10 Effect of Oil Film ThicknessThe predicted oil film thickness (OFT) under the face of the top compression ring as a function of crankangle for various cases of liner roughness is shown in Figure 33.6a. The OFT is defined as the distancebetween 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 ofthe cycle, the asperities are forced together and the film thickness decreases. In cases of lower surfaceroughness, there are more variations in the predicted OFT. At the mid-stroke, the asperities are small

enough so that it is possible to form a fully hydrodynamic oil film. Near the dead center, part of thepredicted OFT is once again affected by asperity contact between surfaces. The minimum OFT, belowwhich metal-to-metal contact occurs, is known as a lower-limit for fluid film lubrication (LFFL). Thisboundary limit depends on the surface finish, elastic modulus, thermal distortion of sliding surface, andsize 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 OFTfalls below the LFFL value at and near regions of the extreme ends of the stroke. At the mid-strokeposition, the piston speed is highest and thus one can expect a higher OFT than at the top and bottomring reversal area. The OFT for the oil ring is typically smaller than the top compression ring. Oil filmthickness can affect the size of the trapped contaminants under the ring face, whereas the contact pressureat 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 lineris affected by what is deposited by the previous ring. For example, during the down-stroke, the top ring

FIGURE 33.6 (a) Oil film thickness as a function of roughness. (b) Interaction of ring/liner asperities: (i) smoothliner surface, and (ii) rough liner surface.

Film

Th

ickn

ess

(mic

ron

s)

6

5

4

3

2

1

0-360 -270 -180 -90 0 90 180 270 360

Crank Angle (degrees)

TDCFiring

2.0* Roughness

1.0* Roughness

0.5* Roughness

(a)

(b)

sees the oil left behind by the second ring. The second ring sees the oil film left by the oil ring. The size ofthe particles that can be trapped under the ring face depends on its film thickness and the debris that is leftbehind from the previous ring. The bigger particles may get crushed or agglomerate on a ring. The top ringsees 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 insmaller particles passing under the ring face. Especially under high pressure, larger particles may getcrushed under the forces acting on the ring. This, combined with the asperity contact, results in highwear and polishing of the ring and liner surfaces. During times of low pressure in the cycle, and whenthe piston is moving fast, the oil film thickness is larger. Larger particles may pass by the ring face. Becausethe forces are lower, the abrasive wear by debris is less.

33.2.11 Effect of Oil Cleanliness

As a lubricated surface, liner wear occurs primarily through adhesion and abrasion. Adhesive wear iscontrolled by the formation of the oil film around the top ring reversal (TRR) area and by the anti-wearfilm in the vicinity of the TRR, where the film thins to boundary lubrication. Abrasion occurs as third-body wear from particulate contamination in the oil that becomes entrained in the oil film. These particlescan come from both external and internal sources. Externally, they can come from dust-laden air thatpasses by the air filter or from the introduction of replacement oil that has not been handled or transferredin clean conditions. Any time the engine is opened for either scheduled maintenance or unscheduledrepair, there is a possibility of adding external contamination. Internally, particulate contamination cancome from debris left behind by the manufacturing process, such as core sand or machining chips, weardebris, surface fatigue, oil additive precipitation caused by acid neutralization, or massive additive pre-cipitation 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 minimizewear. However, it is not clear to what extent a higher level of filter efficiency, by itself, would extendengine life as measured by oil consumption. Figure 33.7 shows a schematic of a particle-laden oil nearthe TRR as the oil film thins. While this figure shows particles at the ring/liner interface, one should notignore the particle flow by means of lubricant from the back of the ring to the front of the ring. Byhydrodynamic exclusion, larger particles do not pass under the ring face. This should result in finer andfiner 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-micronparticles 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 nolarger than 1 to 2 microns as TRR is approached, and standard filtration becomes much less effective inthat regime. Using surface layer activation, the sensitivity of critical parts to cleanliness for several differentengines has been established (Truhan et al., 1995). Newer engines and lubricants show much moretolerance to dirt in the oil than 15 to 20 years ago. This is reflected in the almost doubling of effectiveengine 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.

system in the first place, because any contamination passed by the air filter does damage going past thering pack, regardless of the efficiency of the oil filter. The wear that occurs on the liner near the TRR isthe wear that determines engine life and is only affected to a small extent by the bulk oil cleanliness. Ringwear is affected to some degree by cleanliness.

33.2.12 Examples of Liner and Ring Wear MechanismsThe major factors that influence ring/liner wear have been identified in previous sections. This sectionprovides an example of liner and top compression ring wear mechanisms that have been identified usingscanning electron microscopy (SEM).

A typical contact load and the measured wear profile are shown in the Figure 33.8. At TRR, the oilfilm thickness is small (Figure 33.7) and the contact load is a maximum. Thus, liner wear at this locationis a maximum.

The liner surface outside the ring travel zone is shown in Figure 33.9a. This is equivalent to a machinedliner surface. The scanning electron micrograph of the TRR interface of the liner after an engine test isshown 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 slidingdirection.

The region below the TRR, taken at higher magnification to look for finer details in the wear mech-anisms, 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 delaminationof 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-bodyparticles. Some of these deposits can be hard and produce a polished surface on the liner even underpoorly lubricated conditions. This loose wear debris can sometimes adhere to the ring face or mechanicallyscrape different regions, and can be transported from one region to another. Adhesive wear is also amechanism between ring/liner interface. The difficulty associated with differentiating between adhesivewear and abrasive wear makes the quantification of the amount of wear caused by adhesive wear impossible.

Combustion products from sulfur-containing fuel include NOx and SOx compounds. These oxides canreact with water vapor to form acids. The condensed acids are rich in sulfurous and sulfuric acids, which

FIGURE 33.8 Contact force at TRR and measured wear profile.

TDO

TRR

Liner wear (microns) Contact force (N)

Dis

tan

ce f

rom

to

p o

f lin

er (

mm

)

Dis

tan

ce f

rom

to

p o

f lin

er (

mm

)

0 5 10 -6000 -4000 -2000 00

5

10

15

20

25

0

5

10

15

20

25

30

40

(a)

(b)

(c)

FIGURE 33.9 (a) Liner surface outside ring travel zone. (b) Region below top ring reversal area. (c) Region showingcorrosive wear.

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 thischemical etch may not be significant.

Figure 33.10 shows the worn face of a Cr-plated top-compression ring after an engine test. This figurereveals a highly polished region and parallel grooves along the sliding direction, the latter an indicationof 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, contactpressure, material compatibility, external dust particles from the air system, the carbon deposits thatform on the piston lands and behind the rings, oil degradation products, and wear debris generatedbecause of the relative motion of the liner and ring. The contributions to loss of material in the liner byeach one of these factors depend on engine operating conditions. These factors also contribute to thesecond ring and oil ring wear.

Among all the tribological systems in the power cylinder system, the ring/liner interface plays asignificant role in determining the effective engine life. One of the ways to meet more stringent emissionsrequirements is to design an engine with cooled exhaust gas recirculation (EGR). If next-generationengines are designed using this approach, wear due to corrosion might be significant during cold cyclesof operation. To retain experience-based knowledge, the choice of materials is currently limited toconventional materials in use. Designing cleaner engines with greater durability remains a challenge toengine manufacturers.

33.3 Overhead Components

Overhead components (Figure 33.11) are parts that link the camshaft and the intake/exhaust valves inthe cylinder head in order to actuate the opening and closing of the valves in accordance with the contourof the cam lobes. This linkage is also used to actuate fuel injection events in engines equipped withelectronically controlled but mechanically actuated fuel systems, although some modern electronic fuelsystems 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 consistof cam followers, push rods, rocker levers, and crossheads. The key tribological contact is between thecam lobes and the cam followers. Additional tribological contacts include the joint between the pushrods and the cam followers, the rods and the rocker levers, the rocker levers and the crossheads, andultimately the contacts with valve and injector tips. Recent emissions controlled engines encounter high

FIGURE 33.10 Worn face of a Cr-plated top compression ring after an engine test.

levels of soot in the oil, therefore exposing overhead components to the risks of high wear — not onlydue to mechanical stress, but also the interference of soot (Kuo et al., 1998).

33.3.1 Cam and Cam Followers

Small- to medium-sized diesel engines usually use flat cam followers, or tappets, similar to small gasolineengines. When the cam rotates, the cam lobes slide across the flat tappet surface, and the contour of thelobe lifts the tappet upward. Once the contact slides past the peak of the lobe, the tappet will come downdue to the spring pressure of the valves (or injectors) to remain in contact with the lobe surface. Tappetsare usually positioned to be off-centered from the contact point. This allows the tappet to rotate whenit is dragged by the friction of the sliding contact. This rotating motion introduces limited rolling toreduce friction, and helps distribute the wear across the tappet surface to prolong component life. Toprovide 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

FIGURE 33.11 Overhead components and their arrangements in a diesel engine.

or chilled ductile iron. Tappets are usually made of various cast or alloyed irons, or steel, depending onthe stress level (Korte et al., 1997). Ceramic tappets using materials such as silicon nitride have also beentested successfully to improve durability at high stress levels (Kitamura et al., 1997). Ceramics have beenparticularly effective in reducing wear when a poor-quality lubricant is used. Superfinishing on cam ortappet surfaces has been used successfully on many occasions, but overly smooth surfaces sometimeslead to early seizure and scuffing. Lubricant is provided by crankcase oil splashed onto the parts due tothe mechanical movement of crankshaft and connecting rods inside the block. Lubrication in this contactis usually in the elasto-hydrodynamic to boundary lubrication regime with a λ ratio below 1. Frictionmodifier and antiwear additives are more important for wear protection than the rheological propertiesof the lubricant.

Medium- to large-sized diesel engines usually use roller followers to accommodate the much higherstress involved. When the cam lobe rotates, it lifts a roller mounted on a roller shaft assembly, which inturn raises the push rod. The cam and roller follower contact now has turned into a rolling contact toreduce friction and improve durability. However, depending on the friction and lubrication characteristicsof the several interfaces involved, the contact often has some sliding motion, or slippage, in addition topure rolling. Many cam and tappet failures are associated with the transition from rolling to sliding. Withlimited slippage, fatigue life is reduced because the high friction of sliding introduces a shear stress thatraises 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 causeddirectly by the lubrication of the cam/follower contact. Many roller followers are mounted on a bronzepin 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/bronzepin interface will cause the roller to “stick” to the pin, then quit rolling (Cusano and Wang, 1993). It isimportant to look at all linkages when troubleshooting a tribology problem.

33.3.2 Push Rods, Rocker Levers, and Crossheads

Push rods are used to transfer the lifting motion initiated by the cam and the cam followers to the rockerlevers. The rocker levers engage on the valve tip or injector link to initiate the valve opening or fuelinjection event. Ideally, the push rod will have only normal loading on both ends as it merely functionsas a link between the follower and the rocker lever. However, both ends of the push rods are usuallyshaped like a ball joint and engage in sockets in the mating parts. This design allows limited sliding andsideward movement of the rod relative to the other two components it connects. As modern enginedesigns call for reduced emissions and higher power density, accurate actuation of valve and injectorevents become critical. Push rods represent a linkage among the overhead components that introduceadditional inaccuracy due to tolerance stack-ups and wear. This linkage is eliminated in overhead camdesigns where the cam and cam followers actuate the valve and injectors either directly or via the rockerlevers.

Many modern diesel engines have more than two valves per cylinder to improve air handling. Rockerlevers sometimes push down on crossheads to actuate two valves simultaneously. This is in contrast todouble overhead cam designs, in which each valve is actuated by individual cam lobes. This lever/cross-head contact has limited sliding in addition to normal loading.

Of all the interfaces between overhead components, sliding is always directly or indirectly involved, andis primarily responsible for wear problems. Oil drillings are usually present to feed oil to the interfacesbetween 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 drillingwithout delay. The presence of contaminants, especially diesel soot, can greatly influence the effectivenessof lubrication to cause wear. Wear modes range from scuffing at the rod tip/bowl interface, to abrasionand scuffing on the crossheads. Another cause of wear is oil delay due to oil thickening or poor cold flowproperties, and loss of wear protection due to oil soot contamination. Hardware improvements can alsoreduce or prevent wear. These may include ceramic inserts, coatings, or enlarged contact areas.

33.4 Engine Valves

The engine valve system (comprising the valve, seat insert, and valve guide) is shown schematically inFigure 33.12.

33.4.1 Valve/Seat Wear Mechanisms

Wear between the valve and seat is thought to occur primarily due to relative motion when the valve isseated, due to cylinder pressure that forces the valve into the seat, causing slight deflections of both valveand seat. Thus, cylinder pressure and rigidity of the valve face and seat are primary variables in deter-mining valve/seat wear. Also, the valve/seat angle controls the contact stress normal to the seat and hencethe tangential stress (friction coefficient times normal stress). Flatter seats (i.e., reduced seat angle θ)reduce the contact stress (which varies as 1/cosθ), but the effect is small, especially when there issubstantial friction between the valve and seat. However, the most important effect of reducing the seatangle 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 orcorrosion due to sulfur compounds or other fuel-derived compounds. Sometimes, the wear surface showssurface waves (“sand dunes,” see Figure 33.13), which occur due to the surface shear stress that causesmaterial to flow down the seating surfaces in addition to being removed as wear debris. Material canalso 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 seattemperatures are lower (e.g., 600°C for exhaust valve seat and 370°C for exhaust seat insert; 370°C forintake valve seat and 270°C for intake seat insert). This is thought to occur because of lack of lubricationon 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, whichfavors adhesive wear. This is also consistent with the observed temperature sensitivity of wear inFigure 33.19 (Section 33.6): some materials show decreasing wear as the temperature is raised from 270to 500°C. Factors affecting valve/seat wear are listed in Table 33.4.

FIGURE 33.12 Engine valve schematic. (From Schaefer, S.K., Larson, J.M., Jenkins, L.F., and Wang, Y. (1997) Evaluationof heavy duty engine valves — material and design, Proc. Int. Symp. Valvetrain Sys. Des. Mater., Dearborn MI, April 14-15.)

Combustion face

Seat

Insert

Guide

Stem

Keeper

Tip

Spring retainer

Spring

Cylinderhead

Fillet

Margin

The end result of valve/seat wear is that the valve recesses into the head (measured as a change in valveprotrusion). 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 seatingload caused by valve/seat wear can also lead to poor heat transfer from the valve to the seat, leading tovalve overheating, which may cause valve torching, excessive valve/seat wear, fatigue failures, or stem-guide galling. Also, in cases of extreme valve wear, chordal fatigue failures of the valve can occur due tothe loss in section.

33.4.2 Valve Stem/Guide Wear Mechanisms

Wear between the valve stem and guide occurs due to the sliding motion of the stem in the guide (two-body abrasion and also adhesive wear under more severe conditions) and also due to three-body abrasionfrom oil deposits. Corrosive wear is also common for exhaust valves (and potentially for intake valvesfor engines equipped with exhaust gas recirculation). Stem/guide wear is sensitive to the rate of lubricantsupply to the interface, which is usually controlled by means of a stem seal, which limits the ingress of

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.”

oil and also the amount blown out by port pressure. Modern engines normally run oil-starved in orderto reduce valve guide oil consumption, which in turn reduces the lubricant contribution to particulateemissions. Factors that affect valve stem/guide wear are listed in Table 33.5.

33.4.3 Materials Selection Criteria

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 mostheavy-duty diesel applications, exhaust valves are two-piece with a high-strength, iron-based austeniticstainless steel head welded to a hardenable martensitic steel stem. The valve seat area can be coated witha variety of hardfacing alloys, including Stellites, Tribaloys, and various nickel-based overlays. Intakevalves are typically made from one-piece martensitic steels such as Silchrome 1, which may be hardfacedat the seat area. Valve stems are often chrome plated or nitrided to improve wear and scuff-resistant forboth 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 inoil 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, nickeland cobalt alloys, and powder metal steels incorporating solid lubricants for improved adhesive wearresistance. PM materials may be copper infiltrated to improve thermal conductivity. Materials selection

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 SO2, SO3, NO, and NO2 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.

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 Exhaust and intake valvesFatigue strength at elevated temperature Exhaust valveHigh temperature yield strength (hot hardness) Exhaust valveTemperature resistance (phase stability and dimensional stability) Exhaust valve and seat insertElevated temperature compressive yield strength Exhaust seatThermal fatigue resistance Exhaust valve and seat insertCorrosion resistance: sulfidation, oxidation, chlorides Exhaust valve and guideCreep resistance Exhaust valve and seat insertIndentation resistance (hot hardness) Exhaust valve and seat insertResistance to adhesive wear (marginal lubrication, high contact stress, low

temperature)Intake valve and seat insert

Resistance to adhesive wear (marginal lubrication, high contact stress, high temperature)

Exhaust valve and seat insert

Resistance to sliding wear (marginal lubrication, low temperature) Intake valve stem and guideResistance to sliding wear (marginal lubrication, high temperature) Exhaust valve steam and guideWeldability to steel Two-piece valvesThermal expansion mismatch Exhaust valve stem to guideResistance to sliding wear (well-lubricated, low temperature, potentially high-soot oil) Stem tip/rocker lever or crosshead

tends to be very application specific. The tribological compatibility of the valve/seat insert system isimportant because some materials couples do not match well.

33.5 Bearings and Bushings

Table 33.7 characterizes some of the journal bearing systems found in diesel engines. Bearings andbushings are required to support the load applied to rotating or oscillating shaft/components withminimum 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 toconform to imperfect shaft alignment or distortion and to embed debris, but they also need to be hardenough 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 POFPindicates the potential for overlay or lining fatigue. Durability estimates are often made by comparing

FIGURE 33.14 Results of a split test run with cast iron and powder metal exhaust valve guides. The engine was runat rated for 5 minutes with the coolant level below the head gasket and the water pump disconnected (the steamtemperature from the head was measured at 180°C). This procedure was repeated three times. One exhaust valve wasstuck at the end of the third cycle. Three of the cast iron guides had galled, with no galling of the powder metal guides.

TABLE 33.7 Characterization of Bearing Systems for Heavy-Duty Diesel Engines

Location Motion Unit Load Speed

Piston pin bushing Oscillating High LowConnecting rod (small end) bushing Oscillating High LowConnecting rod (large end) bearing Rotating High MediumCrankshaft bearing Rotating High MediumCamshaft bushing Rotating Low MediumCam roller pin Rotating Medium MediumRocker lever bushing Oscillating Medium LowTurbocharger bushing Rotating Low HighFuel pump bushings (several locations) Rotating Low High

Castiron

Powdermetal

Stuck Gall Gall

calculated MOFT and POFP values for new designs with those for established engines with knowndurability (“comparative engineering”).

Bearing materials selection depends on many design- and application-specific factors. For most highlyloaded bearings used in heavy-duty diesel engines, fatigue requirements dictate the use of steel-backeddesigns and high-strength leaded bronze lining materials. Steel-backed aluminum bearings are used wherefatigue requirements are less stringent. These aluminum bimetal materials are used without overlaysbecause of the difficulty of electroplating onto aluminum. In practice, this, along with fatigue require-ments, tends to further limit the use of aluminum bearings in heavy-duty engines, although they findwidespread 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, harderoverlays may be selected for improved wear and fatigue resistance. Sputtered aluminum-tin overlaysprovide the ultimate in wear and fatigue resistance and can be applied to both bronze and aluminumlinings, but at relatively high cost. Electroplated lead-tin-copper and lead-indium overlays are morecommonly used. For the Pb-Sn-Cu system, the higher the copper level, the greater the hardness/wearresistance/fatigue resistance. Recently, electroplated overlays incorporating ceramic hard-phase particleshave been introduced, providing performance and cost levels intermediate between conventional elec-troplating and sputtering. An alternative bearing design (termed “Rillenlager”) is to form inlaid bandsof overlay material in the lining surface. This design provides improved wear resistance (provided by theexposed lining material), while still maintaining some degree of conformability and embeddability.Figure 33.16 shows wear comparisons between standard electroplated, “Rillenlager,” and sputtered Al-Snoverlays.

TABLE 33.8 Bearing Material Properties

Property Steel BackLeaded

Bronze LiningAluminum/Tin

LiningLead-Tin or

Lead-Indium Overlay

Sputtered Aluminum-Tin

Overlay

Fatigue strength High High Medium Low HighConformability Low Medium Medium High MediumEmbeddability Low Medium Medium High MediumCorrosion resistance High Medium High Medium HighFriction reduction Low Medium Medium High MediumWear resistance High Medium Medium Low HighSeizure resistance Low Medium Medium High MediumHeat resistance High Medium High Low High

FIGURE 33.15 Steel-backed “trimetal” bearing design.

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.

33.6 Turbomachinery

Although the majority of engine components are oil-lubricated, many turbocharger components arerequired to function at high temperatures with no (intentional) lubrication. Although loading is generallylight for such components, durability expectations are the same as for the rest of the engine (e.g., amillion miles for a heavy-duty diesel engine).

An important example of this type of unlubricated, high-temperature system is the turbochargerwastegate mechanism (Figure 33.17), which typically comprises a flap valve with a valve shaft rotatingin a bushing. Maximum operating temperatures range up to approximately 550°C for diesels and 650°Cfor natural gas engines. Wear occurs due to both intended actuator motion and also that induced byexhaust pressure pulses acting on the valve. The vibration amplitude depends on many factors, includingthe stiffness of the actuator mechanism, but is often in excess of 100 microns in amplitude, which cangive rise to substantial wear because the frequency of the excitation pulses is extremely high. The effectof 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 theorder of 10 to 50 microns) is exceeded (Figure 33.18). Temperature has at least three competing effectson wear and scuffing: (1) softening of the materials, (2) formation of protective oxide layers/reactionproducts, and (3) formation of undesirable (soft, nonprotective and/or loosely attached) oxides/reactionproducts. The overall effect depends on material and temperature range (and also other factors, such ascontact 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 responseis optimal because the mechanism is required to function at all temperatures from ambient (on enginestart-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 andthe crank pin (which are external to the turbocharger), leading to loss of control function and consequentdeterioration 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).

FIGURE 33.16 Wear of sputtered Al-Sn, “Rillenlager,” and electroplated overlays. (From Miba Gleitlager AG,Technical Information, Laakirche, Austria.)

0

4

8

12

16

20

100 200 300 400running period (%)

over

lay

Rillenlager

Ni

trimetal bearing

sputter bearingw

ear

[µm

]