eBook - A&P Technician Powerplant Textbook

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ISBN-13: 978-0-88487-579-6ISBN-10: 0-88487-579-2

Cover: RAM Aircraft, LP Overhauled TCM TSIO-520-NB engine

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TABLE OF CONTENTS

JEPPESEN INTEGRATED A&P TRAINING SYSTEM

CHAPTER 1 Reciprocating EnginesSection A Design and ConstructionSection B Operating PrinciplesSection C Diesel Engine Technology

CHAPTER 2 Reciprocating Engine Operation, Instruments,Maintenance, and Overhaul

Section A Engine Operation, Instruments, and MaintenanceSection B Engine Removal and Overhaul

CHAPTER 3 Turbine EnginesSection A Design and ConstructionSection B Operating Principles

CHAPTER 4 Turbine Engine Operation, Instruments, Maintenance, andOverhaul

Section A Engine Operation, Instruments, and MaintenanceSection B Engine Removal and Overhaul

CHAPTER 5 Induction SystemsSection A Reciprocating EnginesSection B Turbine Engines

CHAPTER 6 Exhaust SystemsSection A Reciprocating EnginesSection B Turbine Engines

CHAPTER 7 Engine Fuel SystemsSection A Fuel Storage and DeliverySection B Reciprocating Engine Fuel MeteringSection C Turbine Engine Fuel Metering

CHAPTER 8 Electrical, Starting, and Ignition SystemsSection A GeneratorsSection B AlternatorsSection C Motors and Starting SystemsSection D Electrical System Components

Section E Reciprocating Engine Ignition SystemsSection F Turbine Engine Ignition Systems

CHAPTER 9 Engine LubricationSection A Engine Lubricating OilsSection B Reciprocating EnginesSection C Turbine Engines

CHAPTER 10 Cooling SystemsSection A Reciprocating EnginesSection B Turbine Engines

CHAPTER 11 Engine Fire ProtectionSection A Fire Detection SystemsSection B Fire Extinguishing Systems

CHAPTER 12 PropellersSection A Propeller PrinciplesSection B Fixed-Pitch PropellersSection C Adjustable-Pitch PropellersSection D Turboprop PropellersSection E Auxiliary Propeller SystemsSection F Propeller Inspection, Maintenance, and Installation

CHAPTER 13 Powerplant and Propeller Airworthiness InspectionsSection A Airworthiness Inspection Criteria

CHAPTER 14 Powerplant TroubleshootingSection A Troubleshooting PrinciplesSection B Reciprocating Engine TroubleshootingSection C Turbine Engine Troubleshooting

GLOSSARYANSWERS

INTRODUCTIONThe lack of efficient and practical powerplants has limited aircraft developmentthroughout history. For example, in 1483 Leonardo daVinci conceived a flying machinehe called the aerial screw. However, without a powerplant, the aerial screw was neverdeveloped. In fact, the first patent for a heat engine was taken out in 1791 by JohnBarber. Unfortunately, Barbers engine was neither efficient nor practical. In 1860,Etienne Lenoir of France built the first practical piston engine. Lenoirs engine, whichemployed a battery ignition system and used natural gas for fuel, operated industrialmachinery such as lathes. The next major breakthrough in piston engine developmentcame in 1876 when Dr. August Otto developed the four-stroke, five-event cycle. TheOtto cycle is still used in most modern reciprocating aircraft engines.

Heat engines convert thermal energy into mechanical energy. A specific volume of air iscompressed, and then heated through the combustion of a fuel. In a reciprocating engine,the heated air expands, creating a force that moves a piston and in turn, the piston rod,crankshaft, and propeller or rotor. Reciprocating engines derive their name from theback-and-forth (or reciprocating) movement of their pistons. It is the downward motionof pistons, caused by expanding gases, which generates the mechanical energy needed toaccomplish work.

TYPES OF RECIPROCATING ENGINESMany types of reciprocating engines have been designed for aircraft since the Wrightbrothers made aviation history using a four-cylinder in-line engine. Reciprocatingengines are commonly classified by cylinder arrangement (radial, in-line, V-type, oropposed) and by fuel type (gasoline or diesel).

RADIAL ENGINESA radial engine consists of a row, or rows of cylinders arranged around a crankcase.The two basic types of radial engines are the rotary-type and the static-type. DuringWorld War I, rotary-type radial engines were used extensively because of their highpower-to-weight ratio. The cylinders of a rotary-type radial engine are mounted radiallyaround a small crankcase and rotate with the propeller, while the crankshaft remainsstationary. Some of the more popular rotary-type engines were the Bentley, the Gnome,and the LeRhone. [Figure 1-1]

Figure 1-1. On rotary-type radial engines, the propeller and cylinders are bolted to the crankcaseand rotate around a stationary crankshaft.

The large rotating mass of cylinders produced a significant amount of torque, whichmade aircraft control difficult. This factor, coupled with complications in carburetion,lubrication, and the exhaust system, limited the development of the rotary-type radialengine.

In the late 1920s, the Wright Aeronautical Corporation, in cooperation with the U.S.Navy, developed a series of five-, seven-, and nine-cylinder static-type radial engines.These engines were much more reliable than previous designs. Using these engines,Charles Lindbergh and other aviation pioneers completed long distance flights, whichdemonstrated to the world that the airplane was a practical means of transportation.

The most significant difference between the rotary and the static radial engine is thatwith the static engine, the crankcase remains stationary and the crankshaft rotates to turnthe propeller. Static radial engines have as few as three cylinders and as many as 28.The higher horsepower engines proved most useful. Static radial engines also possesseda high power-to-weight ratio and powered many military and civilian transport aircraft.[Figure 1-2]

Figure 1-2. Radial engines helped revolutionize aviation with their high power and dependability.

Single-row radial engines typically have an odd number of cylinders arranged around acrankcase. A typical configuration consists of five to nine evenly spaced cylinders withall pistons connected to a single crankshaft. To increase engine power whilemaintaining a reasonably-sized frontal area, multiple-row radial engines weredeveloped. These engines contain two or more rows of cylinders connected to a singlecrankshaft. The double-row radial engine typically has 14 or 18 cylinders. To improvecooling of a multiple-row radial engine, the rows are staggered to increase the amountof airflow past each cylinder.

The largest, mass-produced, multiple-row radial engine was the Pratt and Whitney R-4360, which consisted of 28 cylinders arranged in four staggered rows of sevencylinders each. The R-4360 developed a maximum 3,400 horsepower, making it themost powerful production radial engine ever used. [Figure 1-3]

Figure 1-3. The Pratt and Whitney R-4360 engine was the largest practical radial engine used inaviation. Development and advancement in turbojet and turboprop engines eclipsed theperformance of large multiple-row radial engines.

IN-LINE ENGINESIn-line reciprocating engines generally have an even number of cylinders aligned in asingle row parallel with the crankshaft. The pistons are either upright above or invertedbelow the crankshaft. This engine can be either liquid-cooled or air-cooled. [Figure 1-4]

Figure 1-4. The Austro Engine company manufactures inline diesel-powered aircraft engines.

In-line engines have a comparatively small frontal area, which enables them to beenclosed by streamlined nacelles or cowlings. Because of this, in-line engines werepopular among early racing aircraft. A benefit of an inverted in-line engine is that thecrankshaft is higher off the ground. The higher crankshaft allowed greater propellerground clearance, permitting the use of shorter landing gear. Historically, in-lineengines were used on tail-wheel aircraft; they enabled manufacturers to use shorter maingear, which increased forward visibility while taxiing.

In-line engines have two primary disadvantages. They have relatively low power-to-weight ratios and, because the rearmost cylinders of an air-cooled in-line enginereceive relatively little cooling air, in-line engines are typically liquid-cooled or arelimited to only four or six cylinders. As a result, most in-line engine designs areconfined to low- and medium-horsepower engines used in light aircraft.

In 2003, Thielert Aircraft Engines (now Centurion Aircraft Engines) began deliveringnew, certified kerosene-powered, in-line reciprocating engines for light aircraft. In2009, Austro Engine certified a similar engine.

V-TYPE ENGINESIn-line engines evolved into V-type engines. Two rows of cylinders, called banks, areoriented 45, 60, or 90 degrees apart from a single crankshaft. Two banks of cylinderstypically produce more horsepower than an in-line engine. Because the cylinder banksshare a single crankcase and a single crankshaft, V-type engines have a reasonablepower-to-weight ratio with a small frontal area. The pistons can be located either abovethe crankshaft or below the crankshaft. Most V-type engines had 8 or 12 cylinders. V-type engines can be either liquid- or air-cooled. V-12 engines developed during WorldWar II achieved some of the highest horsepower ratings of any reciprocating engine.Today, V-type engines are typically found on classic military and experimental racingaircraft. [Figure 1-5]

Figure 1-5. V-type engines provide an excellent combination of weight and power with a smallfrontal area.

OPPOSED-TYPE ENGINESOpposed-type engines are the most common reciprocating engines currently used onlight aircraft. Opposed engines can be designed to produce as little as 36 horsepower oras much as 400 horsepower. Opposed engines always have an even number ofcylinders, with each cylinder on one side of a crankcase opposing a cylinder on theother side. The majority of opposed engines are air-cooled and horizontally mountedwhen installed on fixed-wing aircraft, but they can be mounted vertically in helicopters.

Opposed engines have a relatively small, lightweight crankcase that contributes to ahigh power-to-weight ratio. The compact cylinder arrangement provides acomparatively small frontal area, which enables the engine to be enclosed bystreamlined nacelles or cowlings. With opposing cylinders, power impulses tend tocancel each other out, resulting in less vibration than other engine types. [Figure 1-6]

Figure 1-6. A horizontally opposed engine combines a good power-to-weight ratio with a relativelysmall frontal area. This style of engine powers most light aircraft in service today.

WANKEL ENGINESAlthough not a reciprocating engine, the Wankel (or rotary) engine deserves mention asan Otto cycle engine with potential for greater use in powered aircraft. Wankel engineshave a good power-to-weight ratio, and their compact design can be enclosed bystreamlined nacelles or cowlings. Instead of using a crankshaft, connecting rods,pistons, cylinders, and conventional valve train, the Wankel engine uses an eccentricshaft and triangular rotor turning in an oblong combustion chamber. This reduction inmoving parts contributes to increased reliability. Early designs had problems associatedwith sealing the combustion chamber, which affected efficiency and engine life. [Figure1-7]

Figure 1-7. A Wankel engine uses an eccentric shaft to turn a triangular rotor in an oblongcombustion chamber.

ENGINE COMPONENTSAs an aviation maintenance technician, you must be familiar with an enginescomponents in order to understand its operating principles. Furthermore, yourunderstanding of an engines basic construction enhances your ability to perform routinemaintenance operations.

The basic parts of a reciprocating engine include the crankcase, cylinders, pistons,connecting rods, valves, valve-operating mechanism, and crankshaft. The valves,pistons, and spark plugs are located in the cylinder assembly, while the valve operatingmechanism, crankshaft, and connecting rods are located in the crankcase. [Figure 1-8]

Figure 1-8. In a basic reciprocating engine, the cylinder forms a chamber where the fuel/air mixtureis compressed and burned. The piston compresses the fuel mixture and transmits power to thecrankshaft through the connecting rods. The intake valve allows the fuel/air mixture into thecylinder while the exhaust valve lets the exhaust gases out.

For all of the reciprocating engine types discussed, the horizontally opposed and static-type radial designs represent the majority of reciprocating engines in service today.Because of this, the discussion on engine components centers on these types.

The use of diesel fuel in reciprocating engines designed for aircraft is increasing. For adiscussion of components specific to diesel engines, see Chapter 1, Section C.

CRANKCASE

The crankcase is the core of a reciprocating engine. It contains the engines internalparts and provides attach points for the cylinders, external accessories, and airframeinstallation. Additionally, the crankcase provides a tight enclosure for the lubricatingoil. Due to great internal and external forces; crankcases must be extremely rigid andstrong. A crankcase is subjected to dynamic bending moments that change continuouslyin direction and magnitude. For example, combustion exerts tremendous forces to thepistons and the propeller exerts unbalanced centrifugal and inertial forces. To remainfunctional, a crankcase must be capable of absorbing these forces while maintaining itsstructural integrity.

Today, most crankcases consist of at least two pieces; however, some crankcases arecast as one piece, and some consist of up to five pieces. To provide the necessarystrength and rigidity while reducing weight, most aircraft crankcases are made of castaluminum alloys.

OPPOSED ENGINE CRANKCASES

A typical horizontally-opposed engine crankcase consists of two pieces of castaluminum alloy manufactured in sand castings or permanent molds. Crankcasesmanufactured by the permanent mold process, or permamold, as it is called by somemanufacturers, are denser than those made by sand-casting. Greater density permitsmolded crankcases to have relatively thinner walls than similar sand-cast crankcases. Inaddition, molded crankcases tend to better resist cracking due to fatigue. Most opposedcrankcases are approximately cylindrical, with smooth areas machined to serve ascylinder pads. A cylinder pad is the surface on which a cylinder mounts to thecrankcase.

For the crankcase to support a crankshaft, a series of transverse webs are cast directlyinto the crankcase parallel to its longitudinal axis. In addition to supporting thecrankshaft, these webs add strength and form an integral part of the structure. [Figure 1-9]

Figure 1-9. The transverse webs in the crankcase support the main bearings and a set of camshaftbosses support the camshaft.

The crankcase is integral to the lubrication system. Passages are drilled into the casehalves to deliver oil to the moving parts within the crankcase. Additionally, oilpassages are machined into the crankcase to scavenge (collect) oil and return it to themain tank or sump.

Most crankcases split vertically; the halves are aligned and held together with studs,bolts, and nuts. Through-bolts are typically used around the crankshaft bearings andsmaller bolts and nuts are used around the case perimeter. Because the crankcasetypically contains oil, it must be sealed to prevent leakage. To ensure that the seal doesnot affect the tight fit required for the bearings, most crankcase halves are sealed with avery thin coating of a nonhardening gasket compound. In addition, on some engines, afine silk thread extending around the entire case perimeter is embedded in thecompound. When the crankcase halves are bolted together with appropriate torque, the

compound and thread form an effective oil seal without altering the fit of the bearing.

RADIAL ENGINE CRANKCASES

Unlike opposed-engine crankcases, radial-engine crankcases are divided by function.The number of sections can be as few as three or as many as seven, depending on thesize and type of engine. A typical radial engine crankcase separates into four mainsections: nose, power, supercharger, and accessory. [Figure 1-10]

Figure 1-10. The four sections of a radial engine crankcase are nose, power, supercharger, andaccessory.

The nose section is mounted at the front of a radial engine crankcase and bolts directlyto the power section. A typical nose section is made of an aluminum alloy that is cast asone piece with a domed or convex shape. This section typically supports and contains apropeller governor drive shaft, the propeller shaft, a cam ring, and, if required, apropeller reduction gear assembly. In addition, the nose section might have mountingpoints for magnetos or other engine accessories.

The second section of a radial engine crankcase is referred to as the power section andit contains the components that transfer energy from the pistons to the crankshaft. Like anopposed engine crankcase, the power section absorbs stress from the crankshaftassembly and the cylinders. The power section can be one, two, or three pieces. A one-

piece power section usually consists of a solid piece of aluminum alloy. Multipiecepower sections are typically manufactured from aluminum or magnesium and boltedtogether. The power section contains machined bosses that support the crankshaftbearings and add strength.

Cylinders are attached around the perimeter of the power section to machined cylinderpads. In general, studs are installed into threaded holes in the power section to providea means of attaching the cylinders. The inner circumference of a cylinder pad issometimes chamfered or tapered to permit the installation of a large, rubber O-ringaround the cylinder skirt. This O-ring seals the joint between the cylinder and thecylinder pads.

The diffuser or supercharger section is located directly behind the power section andis typically made of cast aluminum alloy or magnesium. This section houses thesupercharger and its related components. A supercharger is an engine device thatcompresses air for the engines cylinders, enabling the engine to produce more power.The supercharger section incorporates attach points to secure the engine assembly to theengine mounts.

The accessory section is usually cast of aluminum alloy or magnesium. On engines withone piece accessory sections, the casting is machined to provide means for mountingaccessories. Two-piece accessory sections consist of an aluminum alloy casting and aseparate magnesium cover plate that provides attach points for the accessories. Possibleaccessories include magnetos, carburetors, pumps, starters, and generators.

The gear train in the accessory section contains both spur- and bevel-type gears to drivevarious engine components and accessories. Spur-type gears drive heavily loadedaccessories or those that would be affected by backlash in the gear train. Bevel-typegears handle lighter loads, but accommodate short drive shafts for various accessories.

ENGINE MOUNTING POINTS

For opposed engines, engine mounting points, sometimes called mounting lugs, can becast as a part of the crankcase or can be a bolt-on addition. Because this mountingarrangement supports the weight of the entire powerplant and propeller, it must bedesigned to accommodate all normal and designed loads in flight and on the ground.

For radial engines, mounting lugs are spaced around the periphery of the superchargersection. As with opposed engines, the mounting lugs on radial engines can be integralwith the casting or bolted on.

CRANKSHAFTThe crankshaft receives a linear power pulse from the piston through the connecting rodand changes it to rotary motion to turn the propeller. Because crankshafts must withstandhigh stress, they are generally forged from a strong alloy such as chromium-nickelmolybdenum steel. Some crankshafts are made from a single forging, while others areformed by joining several components. The number of crankpins varies depending onthe type of engine and the number of cylinders. Regardless of the number of throws orthe number of pieces used in construction, all crankshafts have the same basiccomponents, including main bearing journals, crankpins, and crank cheeks. [Figure 1-11]

Figure 1-11. Every crankshaft has main bearing journals, one or more crankpins, and crank cheeks.

The centerline of a crankshaft runs through the center of the main bearing journals.These journals support the crankshaft as it rotates. All crankshafts require at least twomain journals to support the crankshaft, absorb the operational loads, and transmit stressfrom the crankshaft to the crankcase. To minimize wear, most main bearing journals arehardened through a nitriding process.

A crankshaft has one or more crankpins (also known as throws, crank throws, andconnecting-rod bearing journals) located at specific points along its length. Crankpinsare offset from the main bearing journal to provide attachment points for connectingrods. Because of this offset design, any force applied to a crankpin in a direction otherthan parallel to the crankshaft center line causes the crankshaft to rotate. Like mainjournals, crankpins undergo a nitriding process to resist wear and provide a suitablebearing surface.

Crankshafts in most aviation engines are usually hollow to reduce weight. This alsoprovides a passage for lubricating oil and serves as a collection chamber for sludge,dirt, carbon deposits, and other foreign material. Centrifugal force prevents sludge from

circulating in the engine. On some engines, a passage drilled in the crankpin allows oilfrom the hollow crankshaft to be sprayed onto the cylinder walls.

On opposed engines, the number of crankpins corresponds with the number of cylinders.The arrangement of the crankpins varies with the type of reciprocating engine, but allare designed to position each piston for smooth power generation as the crankshaftrotates. The relative distance between crankpins on a crankshaft is measured in degrees.[Figure 1-12]

Figure 1-12. On a four cylinder engine, crankpins one and four are 180 degrees apart fromcrankpins two and three.

Crank cheeks, or crank arms, are required to connect crankpins to each other and to themain journal of the crankshaft. In some designs, the cheeks extend beyond the journal toprovide an attach point for counterweights that help balance the crankshaft. Most crankcheeks have drilled passageways to permit oil to flow from the main journal to thecrankpin.

CRANKSHAFT BALANCE

Excessive engine vibration can cause metal structures to fatigue and fail or wearexcessively. An unbalanced crankshaft can cause excessive vibration. To help minimizeunwanted vibration, crankshafts are balanced statically and dynamically.

A crankshaft is in static balance when the weight of the entire assembly is balancedaround its axis of rotation. To test a crankshaft for static balance, the outside mainjournals are placed on two knife-edge balancing blocks. If the crankshaft tends to favorany one rotational position during the test, it is out of static balance.

After a crankshaft is statically balanced, it must also be dynamically balanced. Acrankshaft is considered in dynamic balance when the centrifugal forces and power

pulses are offset with counterweights. (Crankshafts for smaller engines do not alwaysuse counterweights.) A dynamic damper is a counterweight that is fastened to acrankshafts crank cheek assembly so that it can move back and forth in a small arc.Some crankshafts use two or more of these assemblies, each attached to a differentcrank cheek. The construction of the dynamic damper used in one type of engine consistsof a movable slotted-steel counterweight attached to a crank cheek by two spool-shapedsteel pins that extend through oversized holes in the counterweight and crank cheek. Thedifference in diameter between the pins and the holes enables the dynamic damper tooscillate. [Figure 1-13]

Figure 1-13. Movable counterweights act as dynamic dampers to reduce the centrifugal and impactvibrations in an aircraft engine.

Each time a cylinder fires, force is transmitted to the crankshaft, causing it to flex. Thishappens hundreds of times every minute. Dynamic dampers oscillate, or swing, withevery pulse from a firing cylinder to absorb some of this force. [Figure 1-14]

Figure 1-14. Think of the crankshaft as a pendulum that swings at its natural frequency when aforce is applied. The greater the force, the greater the distance the pendulum swings. However, ifa second pendulum is suspended from the first and a force is applied, the second pendulumbegins to oscillate opposite the applied force. This opposite oscillation dampens the oscillation ofthe first pendulum. You can think of a dynamic damper as a short pendulum hung from a crankshaftthat is tuned to the frequency of power impulses.

CRANKSHAFT TYPESThe type of crankshaft used on a particular engine depends on the number andarrangement of the engines cylinders. The most common types of crankshafts aresingle-throw, two-throw, four-throw, and six-throw. The simplest crankshaft is thesingle-throw, or 360degree crankshaft, used on single-row radial engines. A single-throw crankshaft consists of a single crankpin with two crank cheeks and two mainjournals. A single-throw crankshaft may be constructed out of one or two pieces. One-piece crankshafts use a connecting rod that splits for installation. A two-piececrankshaft uses a crankpin that separates to permit the use of a one-piece connectingrod. [Figure 1-15]

Figure 1-15. A one-piece, single-throw crankshaft is cast as one solid piece. However, a clamptype, two-piece crankshaft is held together by a bolt that passes through the crankpin.

Twin-row radial engines require a two-throw crankshaft, one throw for each bank ofcylinders. The throws on a two-throw crankshaft are typically set 180 degrees apartand can consist of either one or three pieces. Although uncommon, two cylinderopposed engines also use two-throw crankshafts.

Four-cylinder opposed engines and four cylinder inline engines use four-throwcrankshafts. On some four-throw crankshafts, two throws are arranged 180 degreesapart from the other two throws. Depending on the size of the crankshaft and poweroutput of the engine, a four-throw crankshaft has either three or five main bearings.[Figure 1-16]

Figure 1-16. A typical four-throw crankshaft from a four cylinder, opposed engine is machined fromone piece of steel.

Six-cylinder opposed and in-line engines as well as 12-cylinder V-type engines use six-throw crankshafts. A typical six-throw crankshaft is forged as one piece and consistsof four main bearings and six throws that are 60 degrees apart. [Figure 1-17]

Figure 1-17. The crankpins in a typical six-throw crankshaft are 60 degrees apart in the firing order.

BEARINGS

A bearing is any surface that supports and reduces friction between two moving parts.Typical areas where bearings are used in an aircraft engine include the main journals,crankpins, connecting rod ends, and accessory drive shafts. A good bearing must becomposed of material that is strong enough to withstand the pressure imposed on it,while allowing rotation or movement between two parts with a minimum of friction andwear. For a bearing to provide efficient and quiet operation, it must hold two parts in anearly fixed position with very close tolerances. Furthermore, depending on theirspecific application, bearings must be able to withstand radial loads, thrust loads, orboth.

There are two ways in which bearing surfaces move in relation to each other. One is bythe sliding movement of one surface against another, and the second is for one surface toroll over another. Reciprocating engines use bearings that rely on both types ofmovement. Aircraft reciprocating engines typically use include plain bearings, ballbearings, and roller bearings. [Figure 1-18]

Figure 1-18. The three most common types of bearings in reciprocating engines are plain, roller,and ball. Plain bearings rely on the sliding movement of one metal against another; both roller andball bearings use rolling movement.

PLAIN BEARINGS

Plain bearings are generally used as crankshaft main bearings, cam ring and camshaftbearings, connecting rod end bearings, and accessory drive shaft bearings. Thesebearings are typically subject to radial loads only; however, flange-type plain bearingsare often used as axial thrust bearings in opposed reciprocating engines.

Plain bearings are usually made of nonferrous metals such as silver, bronze, Babbitt, tin,or lead. One type of plain bearing consists of thin shells of silver-plated steel; withlead-tin plated over the silver on the inside surface only. Smaller bearings, such asthose used to support various accessory drive shafts, are called bushings. One type ofbushing that is used in aviation is the oil impregnated porous Oilite bushing. With thistype of bushing, the heat produced by friction draws the impregnated oil to the bearingsurface to provide lubrication during engine operation.

BALL BEARINGS

A ball bearing assembly consists of grooved inner and outer races, one or more sets ofpolished steel balls, and a bearing retainer. The balls are held in place and kept evenlyspaced by the bearing retainer, and the inner and outer bearing races provide a smoothsurface for the balls to roll over. However, some races have a deep groove that matchesthe curvature of the balls to provide more support and enable the bearing to carry high

radial loads. Because the balls in a ball bearing assembly provide a small contact area,this type of bearing has the least amount of rolling friction.

Ball bearings are well-suited to withstand thrust loads; because of this, they are used asthrust bearings in large radial and gas turbine engines. In applications in which thrustloads are greater in one direction, a larger race is used on the side of the increasedload.

Most ball bearings that you encounter as a technician are used in accessories such asmagnetos, alternators, turbochargers, and vacuum pumps. Many of these bearings areprelubricated and sealed to provide trouble-free operation between overhauls.However, if a sealed ball bearing must be serviced, you must use the proper tools toavoid damaging the bearing and its seals.

ROLLER BEARINGS

Roller bearings are similar in construction to ball bearings except that polished steelrollers are used instead of balls. The rollers provide a greater contact area and acorresponding increase in rolling friction over that of a ball bearing. Roller bearingsare available in many styles and sizes, but most aircraft engines either have a straightroller or tapered roller bearing. Straight roller bearings are suitable when the bearingis subjected to radial loads only. For example, most high-power aircraft engines usestraight roller bearings as crankshaft main bearings. Tapered roller bearings , on theother hand, have cone-shaped inner and outer races that enable the bearing to withstandboth radial and thrust loads.

CONNECTING RODSThe connecting rod is the link that transmits the force exerted on the piston to thecrankshaft. Most connecting rods are made of a durable steel alloy; however, low-horsepower engines sometimes use aluminum. The weight of a connecting rodcorresponds to the amount of inertia it possesses when the rod and piston stop beforeaccelerating in the opposite direction at the end of each stroke. Engine manufacturersstrive to make connecting rods as light as possible, to reduce inertial forces, but stillmaintain their necessary strength. A typical connecting rod is forged with a cross-sectional shape resembling an H or I. There are also a few tubular connecting rods.The crankpin end of the connecting rod connects to the crankshaft and the piston endconnects to the piston. The three major types of connecting rod assemblies are plain,master-and-articulated, and fork-and-blade.

PLAIN CONNECTING RODS

Plain connecting rods are used in opposed and inline engines. The piston end of a plainconnecting rod is fitted with a bronze bushing to accommodate the piston pin. Thebushing is typically pressed into the connecting rod and reamed to a precise dimensionto fit the piston pin. The crankpin end is usually fitted with a two-piece bearing, whichis held in place by the cap and secured by bolts or by studs and nuts. The bearing insertsare typically steel lined with a nonferrous alloy such as Babbitt, lead, bronze, orcopper.

Connecting rods are often matched with pistons for balance and crankpins for fit. If aconnecting rod is ever removed, it should be replaced in the same cylinder and relativeposition. Connecting rods and caps might be stamped to identify the correspondingcylinder and piston assembly. For example, a number 1 indicates the connecting rodand cap belong with the number 1 cylinder and piston assembly. [Figure 1-19]

Figure 1-19. The two piece bearing shell on a typical plain connecting rod fits tightly in thecrankpin end of the connecting rod. The bearing is held in place by pins or tangs that fit into slotscut into the cap and connecting rod. The piston end of the connecting rod contains a bushing thatis pressed into place.

MASTER-AND-ARTICULATED ROD ASSEMBLY

Radial engines use a master-and-articulated rod assembly to connect the pistons to thecrankshaft. In this type of assembly, one piston in each row of cylinders is connected tothe crankshaft by a master rod. The remaining pistons are connected to the master rodwith articulated rods. For example, a nine-cylinder single-row engine has one master

rod and eight articulating rods and a double-row 18-cylinder engine has two masterrods and 16 articulating rods.

Master rods are typically manufactured from a steel alloy forging that is machined andheat-treated for maximum strength. Articulated rods are constructed of a forged steelalloy with an I- or H- cross-sectional profile. Bronze bushings are pressed into thebores in each end of the articulated rods.

The master rod serves as the only link between all of the pistons and the crankpin. Thepiston end of a master rod contains the piston pin bearing. The crankpin end of a masterrod contains the crankpin bearing (master rod bearing). A typical crankpin bearingmust be able to withstand the radial loads placed on the rod assembly. A set of flangeholes is machined around the crankpin end of a master rod to provide an attachmentpoint for the articulated rods. A master rod can be one piece or multiple pieces. As arule, a one-piece rod is used with a multiple-piece crankshaft, while a multiple-piece(or split-type) master rod is used with a single-piece crankshaft. [Figure 1-20]

Figure 1-20. On a single piece master rod, the master-and-articulated rods are assembled andinstalled on the crankpin before the crankshaft sections are joined together. On a multiple piecemaster rod, the crankpin end of the master rod and its bearing are split and installed on thecrankpin. The bearing cap is then set in place and bolted to the master rod.

Each articulated rod is hinged to the master rod by a knuckle pin. Some knuckle pinsare pressed into the master rod so they do not rotate in the flange holes; other full-floating knuckle pins have a loose fit that enables them to rotate in both the flange holesand articulated rods. In either type of installation, a lock plate on each side retains theknuckle pins and prevents lateral movement. [Figure 1-21]

Figure 1-21. Articulated rods are attached to the master rod by knuckle pins. A knuckle pin lockplate retains the pins.

As the crankshaft rotates, the crankpin bearing is the only portion of a master rodassembly that travels in a true circle. Because the flange holes on a master rod arearranged around the crankpin, the knuckle pins travel in an elliptical path. [Figure 1-22]

Figure 1-22. Knuckle pins rotate in different elliptical paths. Each articulated rod has a varyingdegree of angularity relative to the center of the crank throw.

Because of the varying angularity, not all pistons move an equal amount in each cylinderfor a given number of degrees of crankshaft rotation. To compensate for this, the knucklepin holes in the master rod flange are positioned at varying distances from the center ofthe crankpin.

FORK-AND-BLADE ROD ASSEMBLY

The fork-and-blade rod assembly used in V-type engines consists of a fork connectingrod and a blade connecting rod. The forked rod is split at the crankpin end to allowspace for the blade rod to fit between the prongs. The fork-and-blade assembly is thenfastened to a crankpin with a two-piece bearing. [Figure 1-23]

Figure 1-23. A fork-and-blade rod assembly used in a V-type engine consists of a blade connectingrod whose crankpin end fits between the prongs of the fork connecting rod.

PISTONS

The piston in a reciprocating engine is a cylindrical plunger that moves up and downwithin a cylinder assembly. Pistons perform two primary functions; in conjunction withthe valves, pistons manage the fuel, air, and exhaust pressures in the cylinder and theytransmit the force of combustion through the connecting rod to the crankshaft.

Aircraft engine pistons are typically machined from aluminum alloy or steel forgings.As many as six ring grooves are then machined into a pistons outside surface to hold aset of piston rings. The portion of the piston between the ring grooves is commonlyreferred to as a ring land. The pistons top surface is called the piston head and isdirectly exposed to the heat and force of combustion. The piston pin boss is an enlargedarea inside the piston that provides additional bearing area for the piston pin, which

passes through the piston pin boss to attach the piston to a connecting rod. To help aligna piston in a cylinder, the piston base is extended to form the piston skirt. Some pistonshave cooling fins cast into the underside of the piston skirt to provide for greater heattransfer to the engine oil. [Figure 1-24]

Figure 1-24. A typical piston has ring grooves cut into its outside surface to support piston rings.Cooling fins are sometimes cast into the piston interior to dissipate heat. The piston pin bossprovides support for the piston pin.

Pistons are sometimes classified according to their head design. The most commontypes of piston heads are flat, recessed, cupped, and domed. The three common types ofpiston skirts are trunk, trunk relieved at piston boss, and slipper. [Figure 1-25]

Figure 1-25. The majority of modern aircraft engines use flat-head pistons; however, other designsare still in service.

All pistons expand when they heat up. Due to the added mass at the piston boss, moreexpansion occurs parallel to the piston boss than perpendicular to it. This unevenexpansion can cause a piston to take on an oval shape at normal engine operatingtemperatures, which results in uneven wear of the piston and cylinder. One way tocompensate for this is to use a cam-ground piston. A cam-ground piston is machinedwith a slightly oval shape, such that the diameter of the piston parallel to the piston bossis slightly less than the diameter perpendicular to the piston boss. This compensates fordifferential expansion and produces a round piston at normal operating temperatures.Furthermore, the oval shape holds the piston centered in the cylinder during enginewarm-up and prevents the piston from moving laterally within a cylinder. [Figure 1-26]

Figure 1-26. Cam ground pistons compensate for the greater expansion parallel to the piston bossduring engine operation. The diameter of a cam ground piston measures several thousandths ofan inch larger perpendicular to the piston boss than parallel to the piston boss.

PISTON RINGSPiston rings perform three functions. They prevent pressure leakage from the combustionchamber, control oil seepage into the combustion chamber, and transfer heat from thepiston to the cylinder walls. Piston rings are spring-loaded and press against thecylinder walls; when properly lubricated, they form an effective seal.

Piston rings are usually made of high-grade gray cast iron or chrome-plated, mild steel.The chrome-plated rings can withstand higher temperatures. During manufacture, thering is machined to the desired cross-section, and then split for installation in a pistonring groove. The point where a piston ring is split is called the piston ring gap. The gapcan be a simple butt joint with flat faces, an angle joint with angled faces, or a stepjoint. [Figure 1-27]

Figure 1-27. Of the three types of joints used in piston ring gaps, the butt joint is the most commonin aircraft engines.

As an engine reaches operating temperature, piston rings expand. To accommodateexpansion, piston rings need a gap. If the gap is too large, the two faces will not cometogether and provide an adequate seal. If the gap is too small, the ring faces will bindagainst each other and the cylinder wall resulting in scoring damage to the cylinderwall. Ring gaps must be staggered, or offset to create the best seal, which preventscombustion gases from leaking past the rings into the crankcase. This blow-by, as it isoften called, results in a loss of power and increased oil consumption.

To form an effective seal, the rings must exert equal pressure around the entire cylinderwall and provide a gas-tight fit against the sides of the ring grooves. New piston ringsrequire some wear-in during engine operation so that the ring contour matches thecylinder wall. A ring that matches its cylinder is considered to be seated. The two maintypes of piston rings used in reciprocating engines are compression rings and oil rings.[Figure 1-28]

Figure 1-28. Compression rings are installed in the upper piston ring grooves to help prevent thecombustion gases from escaping. Oil rings, on the other hand, are installed near the middle andbottom of a piston to control the amount of oil applied to the cylinder wall.

The compression rings, located in the ring grooves immediately below the piston head,prevent gas from escaping around the piston during engine operation. The number ofcompression rings used on each piston is determined by the engine manufacturer. Twoor three compression rings on each piston is common. The cross section of acompression ring can be rectangular, wedge shaped, or tapered. Because compressionrings receive limited lubrication and are closest to the heat of combustion, they are moreprone to sticking.

A rectangular compression ring fits flat against a cylinder wall with a large contact areato provide a tight seal. The large contact area requires a relatively long time to seat.Tapered rings have a beveled face to reduce contact area, which reduces friction andhastens ring seating. Wedge-shaped rings also have a beveled face to promote rapidring seating. Because the profile is wedge shaped, the piston ring grooves must also bebeveled. Less material is cut away, so piston ring lands and grooves are stronger. Thewedge shape also helps prevent a ring from sticking in a groove. [Figure 1-29]

Figure 1-29. Compression rings can have three different ring cross sections. The tapered facepresents the narrowest bearing edge to the cylinder wall to reduce friction and accelerate ringseating.

Oil rings control the amount of oil applied to the cylinder walls and prevent oil fromentering the combustion chamber. The two types of oil rings that are found on mostengines are oil control rings and oil scraper rings. Oil control rings are placed in thepiston ring grooves below the compression rings. Pistons can have one or more oilcontrol rings. On some pistons, as many as two rings can be installed in a single ringgroove. The primary purpose of oil control rings is to regulate the thickness of the oilfilm on a cylinder wall. An oil control ring returns excess oil to the crankcase throughsmall holes drilled in the piston ring grooves. Additionally, some pistons use ventilatedoil control rings with small slots machined around the ring. These slots enable excessoil to return to the engine sump through small holes drilled in the piston ring groove.

If excessive oil enters the combustion chamber, it will burn and leave a coating ofcarbon on the combustion chamber walls, piston head, spark plugs, and valves. Carbonbuildup on the ring grooves or valve guides can cause parts to stick. Carbon buildup canalso cause spark plugs misfiring, cylinder preignition or detonation, and excessive oilconsumption. To help prevent this, an oil scraper ring regulates the amount of oil thatpasses between the piston skirt and the cylinder wall.

An oil scraper ring, sometimes called an oil wiper ring, usually has a beveled face andis installed in a ring groove at the bottom of the piston skirt. The ring can be installedwith the beveled edge away from the piston head or in the reverse position. If the bevelis installed so that it faces the piston head, the ring pushes oil downward toward the

crankcase. If the bevel is installed to face away from the piston head, on the upwardstroke, the scraper ring retains surplus oil above the ring. On the downward stroke, oilis returned to the crankcase by the oil control rings and piston ring grooves. It is veryimportant that these rings are installed in accordance with the manufacturersinstructions. [Figure 1-30]

Figure 1-30. An oil scraper ring installed with its beveled edge away from the cylinder head forcesoil upward along the cylinder wall when the piston moves upward. However, if the beveled edgefaces the cylinder head, the ring scrapes oil toward the crankcase when the piston moves down.

PISTON PINSA piston pin joins the piston to the connecting rod. Piston pins are tubular, and aremachined from a case-hardened, nickel-steel alloy forging. Piston pins are sometimescalled wrist pins because the motion of the piston and the connecting rod is similar to ahuman wrist.

Piston pins can be stationary, semifloating, or full-floating. Stationary piston pins aresecured to the piston by a setscrew that prevents rotation. Semifloating piston pins areloosely attached to the connecting rod by clamping around a reduced-diameter sectionof the pin. Full-floating piston pins rotate freely in both the connecting rod and thepiston; these pins are used in most modern aircraft engines.

A full-floating piston pin must be held in place laterally to prevent it from rubbing andscoring the cylinder walls. Three devices that are used to hold a piston pin in place arecirclets, spring rings, and metal plugs. A circlet is similar to a snap ring that fits into agroove cut into each end of the piston boss. A spring ring also fits into grooves cut intothe ends of a piston boss, but it consists of a single circular spring-steel coil. Bothcirclets and spring rings are used primarily on earlier piston engines. The current

practice is to install a plug of relatively soft aluminum called a piston-pin plug. Theseplugs are inserted into the open ends of the piston pins to provide a good bearingsurface against the cylinder walls. Due to the plugs soft aluminum construction andcylinder lubrication, the metal-to-metal contact causes no damage to the cylinder walls.

CYLINDERSThe cylinder is the combustion chamber where the burning and expansion of gases takesplace to produce engine power. Furthermore, a cylinder houses the piston andconnecting rod assembly along with the valves and spark plugs. When designing andconstructing a cylinder, manufacturers must consider several factors. A cylinder must bestrong enough to withstand the internal pressures developed during engine operation yetbe lightweight to minimize engine weight. Additionally, the materials used in theconstruction of a cylinder must have good heat-conducting properties for efficientcooling. Finally, a cylinder assembly must be relatively simple and cost-effective tomanufacture, inspect, and maintain.

A typical air-cooled engine cylinder consists of a cylinder head, barrel, mountingflange, skirt, cooling fins, and valve assembly. On some of the earliest two- and four-cylinder horizontally opposed engines, the cylinder barrels were cast as part of thecrankcase halves. This required the use of removable cylinder heads. However, onalmost all modern engines, individual cylinders are cast as a component, separate fromthe crankcase, and the heads are permanently attached during the manufacturing process.To do this, the cylinder head is expanded through heating and then screwed down onto achilled cylinder barrel. As the head cools, it contracts, and as the barrel warms, itexpands, resulting in a gas-tight joint. [Figure 1-31]

Figure 1-31. The cylinder assembly, the piston assembly, connecting rods, crankshaft, andcrankcase constitute the power section of a reciprocating engine.

CYLINDER BARRELS

The material used to construct a cylinder barrel must be as light as possible, yet havethe proper characteristics for operating at high temperatures and pressures. Furthermore,a cylinder barrel must possess good bearing characteristics and high tensile strength.The most commonly used material that meets these requirements is a high-strength steelalloy such as chromium-molybdenum steel or nickel chromium-molybdenum steel.

Cylinder barrels are machined from a forged blank, with a skirt that projects into thecrankcase and a mounting flange that is used to attach the cylinder to the crankcase.The lower cylinders on radial engines and all the cylinders on inverted enginestypically have extended cylinder skirts. The longer skirt helps keep oil from draininginto the combustion chamber and causing hydraulic lock after an engine has been shutdown. The exterior of a cylinder barrel consists of several thin cooling fins that aremachined into the exterior cylinder wall and a set of threads that are cut at the top of thebarrel so that it can be screwed into the cylinder head.

The inside of a cylinder, or cylinder bore, is usually machined smooth to a uniform,initial dimension, and then honed to a final dimension. However, some cylinder boresare machined with a slight taper, so that the diameter of the top of the barrel is slightly

smaller than the diameter at the cylinder skirt. This is called a choke bore cylinder andis designed to compensate for the uneven expansion caused by the higher operatingtemperatures and larger mass near the cylinder head. With a choke bore cylinder, thegreater expansion at the top of the cylinder is compensated for by the taper, resulting ina uniform cylinder diameter at normal operating temperatures. The amount of choke isusually between .003 and .005 inches. [Figure 1-32]

Figure 1-32. In most reciprocating engines, the greater mass of the cylinder head retains heat andexpands, causing the upper portion of the cylinder to expand more than the lower portion.However, with a choke-bored cylinder, the diameter at the top of the cylinder is less than thediameter at the bottom of the cylinder which helps compensate for the uneven expansion.

The inside wall of a cylinder barrel is continuously subjected to the reciprocatingmotion of the piston rings. Therefore, in an effort to minimize cylinder barrel wear andincrease barrel life, most cylinder walls are hardened. The two most common methodsused to provide a hard wearing surface are nitriding and chrome plating.

Nitriding is a form of case hardening that changes the surface strength of steel byinfusing the metal with a hardening agent. During the nitriding process, a cylinder barrelis first ground to the required size and smoothness and then placed in a special furnacefilled with ammonia gas. The furnace heats a cylinder barrel to approximately 1,000

degrees Fahrenheit. At this temperature, the ammonia gas breaks down into nitrogen andhydrogen. The steel in the cylinder barrel contains a small percentage of aluminum,which combines with the nitrogen to form a layer of hard, wear-resistant aluminumnitrides. The depth of a nitrided surface depends on the length of time that the cylinder isexposed to the ammonia gas but a typical thickness is approximately 0.020 inch.However, the surface hardness gradually decreases with depth until the hardness is thesame as the core metal.

Because nitriding is neither plating nor coating, it changes a cylinder bore by only twoto four ten thousandths of an inch. This dimensional change requires a cylinder to behoned to an accurate, micro-smooth finish after the nitriding process is complete. Mostmanufacturers identify a nitrided cylinder by applying a band of blue paint around thecylinder base, or to certain cooling fins.

A disadvantage of nitrided cylinders is that they do not hold oil for extended periods.This increases a cylinders susceptibility to corrosion. If an engine with nitridedcylinders is out of service for an extended period, the cylinder walls should be coatedwith sticky preservative oil.

Chrome-plating refers to a method of hardening a cylinder by applying a thin coating ofchromium to the inside of the cylinder barrels. Chromium is a hard, natural element witha high melting point, high heat conductivity, and a very low coefficient of friction. Theprocess used to chrome-plate a cylinder is known as electroplating.

Chrome-plated cylinders have many advantages over both plain steel and nitridedcylinders. For example, chromed cylinders are less susceptible to rust or corrosionbecause of chromiums natural corrosion resistance. Therefore, chromed cylinders tendto wear longer. Another benefit of chrome-plating is that after a cylinder wears beyondits usable limits, it can be chrome-plated back to its original size. To identify a cylinderthat has been chrome-plated, a band of orange paint is sometimes applied around thecylinder base or to some of the cooling fins.

A problem associated with chrome-plating is that, in its natural state, chromium is sosmooth that it does not retain enough oil to lubricate the piston rings. To overcome this,a reverse current is applied to the cylinder after the chromium has been applied. Thecurrent causes microscopic surface cracks to open, forming an interconnected networkof cracks to retain oil on the cylinder wall. This procedure is often referred to aschrome channeling. [Figure 1-33]

Figure 1-33. Microcracks formed in chrome plating retain oil to aid in cylinder lubrication. Thisimage is an enlarged photo-micrograph of the cylinder wall.

Engines with chrome-plated cylinders tend to consume slightly more oil than engineswith nitrided or steel cylinders because the plating channels retain more oil than thepiston rings can effectively scavenge. Furthermore, chrome-plated cylinders aretypically more difficult to seal, or break in, immediately after an engine is overhauled.This is a result of the oil film on the cylinder wall preventing the necessary wear, orseating, of the piston rings during the break-in period.

In an effort to overcome the disadvantages of chrome-plated and nitrided cylinders,manufacturers have developed some new plating processes. Instead of channeling, oneof these processes involves mechanically impregnating silicon carbide particles into achromed cylinder wall. The silicon carbide provides a somewhat rough finish so itretains lubricating oil, yet is smooth enough to enable effective oil scavenging.Furthermore, the silicon carbide provides a surface finish that is more conducive topiston ring seating during the engine break-in period. This plating process is commonlyreferred to as either CermiCrome or Nu-Chrome plating. This process has beenlargely discontinued.

Another plating process, variously called CermiNil, Nickel+Carbide, or Nikasil,uses nickel with silicon carbide particles as the plating material. Although nickel is notas durable as chromium, it provides for an extremely hard finish while the siliconcarbide particles increase the hardness of the material and aid in retaining lubricatingoil. A unique characteristic of this process is that the silicon carbide particles areinfused throughout the plating, not only on the surface. This tends to improve the wear

properties of a cylinder while maintaining a smooth surface for effective oil scavenging.

CYLINDER FINISHES

In the past, engine manufacturers applied special paints to the exterior of cylinderbarrels to protect the cylinder from corrosion. This special paint would change colorwhen exposed to high temperatures, indicating a possible overheat condition that mighthave damaged the cylinders. Textron-Lycoming cylinders are typically painted withgray enamel that appears burned when exposed to excessive heat. Similarly, TeledyneContinental cylinders are treated with a gold paint that turns pink after an overheatevent.

CYLINDER HEADS

The cylinder head covers the cylinder barrel to form the enclosed chamber forcombustion. In addition, cylinder heads contain intake and exhaust valve ports, sparkplugs, and valve actuating mechanisms. Cylinder heads also transfer heat away from thecylinder barrels. Air-cooled cylinder heads are generally made of forged or die-castaluminum alloy because it conducts heat well, is lightweight, and is durable. The innershape of a cylinder head can be flat, semispherical, or peaked. The semispherical typeis most widely used because it is stronger and provides for rapid and thoroughscavenging of exhaust gases.

Cooling fins are cast or machined onto the outside of a cylinder head to transfer heat tothe surrounding air. However, due to the temperature differences across the cylinderhead, it is necessary to provide more cooling-fin area on various sections. For example,because the exhaust valve region is typically the hottest part of the internal surface, thatportion of the cylinder head has more fin area. The intake portion of the cylinder headtypically has few cooling fins because the fuel/air mixture sufficiently cools this area.

After a cylinder head is cast, spark plug bushings, or inserts, are installed. Typically,each cylinder head has two spark plugs for increased performance and for systemredundancy. On older engines, spark plug openings consisting of bronze or steelbushings were shrunk and screwed into the cylinder head. However, most modernengines use stainless steel Heli-Coil inserts. These inserts can be easily replaced ifthe threads become damaged.

Intake and exhaust ports are machined into each cylinder head to enable the fuel/airmixture to enter the cylinder and the exhaust gases to exit. Gaskets are often used to sealbetween the cylinder and the intake and exhaust manifolds. A synthetic rubber seal is

typically used for attaching the intake manifold. Because of the high temperaturesassociated with exhaust gases, a metal gasket is typically used for the exhaust manifold.Each manifold is held in place by a nut secured to mounting studs or bolts threaded intothe cylinder head. [Figure 1-34]

Figure 1-34. Threaded studs for attaching intake and exhaust manifolds typically remain in thecylinder.

VALVESEngine valves regulate the flow of gases into and out of a cylinder by opening andclosing at the appropriate time during the Otto cycle. Each cylinder has at least oneintake valve and one exhaust valve. The intake valve controls the amount of fuel/airmixture that enters through the intake port, and the exhaust valve lets the exhaust gasesexit the cylinder through the exhaust port. Some high-powered engines have two intakeand two exhaust valves for each cylinder.

The valves used in aircraft engine cylinders are subject to high temperatures, corrosion,and extreme operating stresses. Therefore, valves must be designed and constructed fordurability. Intake valves operate at lower temperatures than exhaust valves and aretypically made of chrome, nickel, or tungsten steel. Because exhaust valves operateunder much higher temperatures, they are usually made of materials with greater heatresistance such as Inconel silicon-chromium or cobalt-chromium alloys. The most

common type of valve used in aircraft engines is the poppet valve, which pops openand closed during normal operation. [Figure 1-35]

Figure 1-35. The basic components of a poppet valve include the valve head, valve face, valveneck, valve stem, and valve tip.

Poppet valves are classified according to their head shape. The four basic designs areflat-headed, semi-tulip, tulip, and mushroom. The flat-head valve is typically usedonly as an intake valve in aircraft engines. The semi-tulip valve has a slightly concavearea on its head while the tulip design has a deep, wide indented area on its head.Mushroom valves have convex heads and are not commonly found on aircraft engines.[Figure 1-36]

Figure 1-36. Aircraft engine valves are classified according to their head profile.

The valve face creates a seal at its respective port. The valve and corresponding seatare typically ground to an angle of between 30 and 60 degrees to form a tight seal. Insome engines, the intake valve face is ground to 30 degrees and the exhaust valve isground to 45 degrees. The engine manufacturer specifies the exact angle to be ground

based on airflow, efficiency, and sealing ability. Valve faces are often made moredurable by welding Stellite, an alloy of cobalt and chromium, to the valve face. Afterthe Stellite is applied, the face is ground to the correct angle. Stellite resists hightemperatures and corrosion and withstands the shock and wear associated with valveoperation.

The valve stem keeps the valve head properly aligned as it opens and closes. Mostvalve stems are surface hardened to resist wear. The tip of a valve stem is alsohardened to withstand both wear and hammering. In some cases, a rotator cap is placedover the valve tip to increase service life. A machined groove near the valve stem tipreceives a split key, or keeper key, that keeps the valve-spring retaining washers inplace and holds the valve in the cylinder head. [Figure 1-37]

Figure 1-37. The groove near the tip of a valve stem allows a split retainer key to hold springtension on a valve as well as keep the valve from falling into the cylinder.

On some radial engines, the valve stems have an additional groove below the split keygroove. This second groove is used to hold a safety circlet or spring ring, preventing thevalve from falling into the cylinder in the event the valve tip breaks off.

To help dissipate heat, some exhaust valve stems are hollowed out and then partiallyfilled with metallic sodium. The sodium melts at approximately 208 degreesFahrenheit. Due to the up and down motion of the valve, the melted sodium circulatesand transfers heat from the valve head into the stem where it is dissipated through thecylinder head. In some cases, sodium-filled valves can reduce valve operatingtemperature by as much as 400 degrees Fahrenheit. [Figure 1-38]

Figure 1-38. Some valves are filled with metallic sodium to reduce their operating temperatures.During operation, the sodium melts and transfers heat to the stem, which conducts it to thecylinder head.

When overhauling an aircraft engine, you must determine whether the old valves aresodium-filled. As a rule, Teledyne Continental engines do not use sodium filled valves,while many Textron-Lycoming engines do. Regardless of the engine, you must followthe manufacturers recommendations and instructions for handling and installing thevalves. Sodium is a dangerous material that burns violently when exposed to air.Because of this, sodium-filled valves should never be cut, broken, or handled in amanner that would expose the sodium to air. In all cases, sodium valves must bedisposed of in an appropriate manner.

VALVE SEATING COMPONENTS

A valves face must seat firmly against the cylinder head. To accomplish this, severalindividual components work together, including valve seats, valve guides, valvesprings, and valve spring retainers. [Figure 1-39]

Figure 1-39. The valve seat insert provides a sealing surface for the valve face while the valveguide supports the valve and keeps it aligned with the seat. Valve springs close the valve and areheld in place by a valve retainer and a split valve key.

A valve seat is a circular ring of hardened metal that provides a uniform sealingsurface for the valve face. A typical valve seat is made of either bronze or steel andmachined to an oversize fit. To install a valve seat, the cylinder head is heated and thevalve seat is chilled and then pressed into the head with a special tool called a mandrel.When the assembly cools, the cylinder head shrinks and firmly retains the valve seat.After it is installed, the valve seat is precisely ground to provide a sealing surface forthe valve face. Typically, the valve seat is ground to the same angle as the valve face.However, there are some instances where a valve face may be ground to an angle that isfrom one-quarter to one full degree shallower than the valve seat. The angulardifference produces an interference fit that helps to ensure a more positive seating.

A valve guide is a cylindrical sleeve that provides support to the valve stem and keepsthe valve face aligned with the valve seat. Valve guides are made from a variety ofmaterials such as steel, tin-bronze, or aluminum-bronze and are installed in the cylinderhead with a shrink fit in the same manner as valve seats.

Valve springs are helical-coiled springs that are installed in the cylinder head toprovide the force that holds the valve face firmly against the valve seat. Most aircraftengines use two or more valve springs of different sizes and diameters to prevent aphenomenon called valve float or valve surge. Valve float occurs when a valve springvibrates at its resonant frequency. When this occurs, a spring loses its ability to hold avalve closed. By installing two or more springs of differing sizes, one spring is alwaysfree to close the valve. An added safety benefit of this arrangement is that two or moresprings reduce the possibility of failure due to a spring breaking from excessivetemperature or metal fatigue.

The valve springs are held in place by a valve spring retainer and a split valve key. Avalve spring retainer seat is usually located between the cylinder head and the bottom ofthe valve springs, while a valve spring retainer is installed on the top of the valvesprings. The retainer is fitted with a split valve key that locks the valve spring retainerto the valve stem.

VALVE OPERATING MECHANISMSReciprocating engines require a valve operating mechanism to open each valve at thecorrect time, hold it open, and then close it. A typical valve operating mechanismincludes an internally driven camshaft or cam ring that pushes against a valve lifter. Thevalve lifter, or tappet, transmits the force from the cam to a push rod, which in turn,actuates a rocker arm to overcome the valve spring tension and open the valve. [Figure1-40]

Figure 1-40. The typical valve operating mechanism includes a camshaft (or cam ring), a tappet (orlifter), a push rod, and a rocker arm.

OPPOSED ENGINES

On an opposed engine, valve operation is controlled with a camshaft. A typicalcamshaft consists of a round shaft with a series of cams, or lobes. These transform therotational motion of the camshaft to the linear motion needed to actuate a valve. Theshape of a cam determines the distance that a valve is lifted off its seat and the length oftime that the valve is open. Because cams are continuously moving across another metalsurface, lobes are hardened to resist wear. [Figure 1-41]

Figure 1-41. The raised lobe on a camshaft transforms the rotary motion of the camshaft to linearmotion.

The camshaft is supported by a series of bearing journals that ride in a set of camshaftbosses, which are cast into the crankcase. The force used to rotate a camshaft comesfrom the crankshaft through a set of gears. The camshaft rotates at one-half of thecrankshaft speed. In a four-stroke engine, each cylinder fires once for every twocrankshaft rotations. Therefore, each valve should open and close only once for everytwo rotations of the crankshaft. [Figure 1-42]

Figure 1-42. In the typical opposed engine, the camshaft timing gear has twice as many teeth as thecrankshaft gear. The camshaft rotates at one-half of the crankshafts speed.

As the camshaft rotates, the lobe raises the valve lifter. A valve lifter, or tappet,transmits the lifting force of the cam to the push rod. Valve lifters in opposed enginescan be solid or hydraulic. A solid lifter is a solid metal cylinder that directly transfersthe lifting force from the camshaft to the push rod. The cam follower face of a solidlifter is flat with a polished surface, while the push rod end contains a spherical cavitythat houses the push rod. Holes drilled in the lifter enable oil to flow through the lifter tolubricate the push rod.

Most opposed engines use hydraulic lifters. Hydraulic lifters use oil pressure tocushion normal impact and remove play within the valve operating mechanism. Atypical hydraulic lifter consists of a cam follower face, a lifter body, a hydraulicplunger and spring, a check valve, and a push rod socket. The entire lifter assemblyfloats in a machined hole in the crankcase and rests on the camshaft. [Figure 1-43]

Figure 1-43. A typical hydraulic lifter consists of a push rod socket, a hydraulic plunger and spring,a check valve, a lifter body, and a cam follower face.

The cam follower face is the smooth, hardened surface of the lifter that contacts thelobe. When the follower face is on the back side of a lobe, the hydraulic plunger springforces the hydraulic plunger outward so that the push rod socket presses firmly againstthe push rod. As the hydraulic plunger moves outward, a ball check valve moves off itsseat to let oil flow from the oil supply chamber to the oil pressure chamber. As thecamshaft rotates and the front side of the lobe contacts the follower face, the lifter bodyand cylinder move outward. This action causes the check valve to seat, trapping oil inthe oil pressure chamber. This trapped oil acts as a cushion that dampens the abruptpressure applied to the push rod. After the valve is lifted off its seat, oil leaks betweenthe plunger and the cylinder to compensate for any dimensional changes caused byoperation. After the valve closes, the ball check valve is unseated and oil flows fromthe supply chamber to the pressure chamber in preparation for another cycle. A secondtype of hydraulic lifter is similar in construction to the lifter just discussed, except that adisk check valve is used instead of a ball check valve. [Figure 1-44]

Figure 1-44. Some hydraulic lifters uses a disk-type check valve instead of a ball-type.

The lifting force of the lobe is transmitted through a lifter and a push rod. A typical pushrod is a hollow steel or aluminum-alloy tube with polished ends. One end of the pushrod rides in the valve lifter socket while the other end fits into a socket in the rockerarm. Push rods typically have holes drilled in each end to let oil flow from the valvelifter to the valve components in the cylinder head. On most aircraft reciprocatingengines, the push rods are enclosed by a thin metal shroud, or tube, that runs from thecylinder head to the crankcase. In many cases, these tubes also provide a return path forthe oil that is pumped up to the cylinder head.

A rocker arm is a pivoting lever in the cylinder head that changes the lifting movementof the push rod into the downward motion needed to open a valve. A typical rocker armis made of forged steel and has a cup-shaped socket to hold the push rod end and apolished surface that pushes against the valve tip. [Figure 1-45]

Figure 1-45. One end of this rocker arm is cup-shaped to hold a push rod, while the other end ismachined smooth to push against the tip of a valve stem. When rotated by the push rod, the rockerarm pivots on its center bushing to depresses a valve.

The entire rocker arm pivots on a shaft that is suspended between two rocker armbosses cast into the cylinder head. Each rocker arm boss contains a bronze bushing thatprovides a bearing surface for the shaft. The rocker arm shaft is installed with a lightpress fit and held in place by the rocker-box cover or by covers inserted over theoutside of each rocker arm boss. When the push rod pivots the rocker arm, the rockerarm exerts force against the valve springs to open the valve. [Figure 1-46]

Figure 1-46. A rocker arm is supported by a shaft suspended between a set of rocker arm bosses.

Some engines use a newer style rocker arm that is forged out of a single piece ofstainless steel and rotates on a pressed-in roller bearing. Additionally, the valve end ofthe rocker arm is fitted with a roller. This roller helps eliminate side loads when thevalve is opened, which helps minimize wear on the valve guide and valve stem.

RADIAL ENGINES

Radial engines use some of the same components in their valve operating mechanismsas opposed engines, but with some significant differences. For example, in place of acamshaft, a radial engine uses cam rings; the number of rings is the same as the numberof cylinder rows. A cam ring is a circular piece of steel with a series of raised lobes onits outer edge. A cam ring for a typical seven-cylinder engine has three or four lobeswhile a cam ring in a nine-cylinder engine has four or five lobes. The lobes in a radialengine differ from those in an opposed engine in that each lobe is constructed with acam ramp on each side of the lobe. This ramp reduces the initial shock of an abruptlyrising lobe. The smooth area between the lobes is called the cam track. On a singlerow radial engine a single cam ring with two cam tracks is used. One track operates theintake valve while the second track operates the exhaust valve.

In a single-row radial engine, the cam ring is usually located between the propellerreduction gearing and the front end of the power section. In a twin-row radial engine, asecond cam for the valves in the rear row is installed between the rear end of the powersection and the supercharger section.

The cam ring is mounted concentrically with the crankshaft and is driven by thecrankshaft through a series of gears. However, unlike a traditional camshaft whichrotates at half the speed of a crankshaft, the rotational speed of a cam ring varies due tosize and gearing. To determine the rotation speed of a given cam ring, you must knowthe number of lobes on the cam ring, the cam rings direction of rotation relative to thecrankshaft, and the number of cylinders on the engine. The direction of cam ring rotationvaries on different engines and depends on whether the cam ring has internal or externaldrive teeth. Externally driven cam rings turn in the same direction as the crankshaft,while internally driven rings turn opposite from crankshaft rotation. [Figure 1-47]

Figure 1-47. This chart identifies cam ring speed for various radial engine configurations.

If a table is not available, determine cam ring speed by using the formula:

In place of a cam follower face, a radial engine uses cam rollers. A cam roller consistsof a small wheel that rolls along the cam track. When the cam roller rides over a lobeon the cam ring, the roller pushes against a tappet that is enclosed in a tappet guide.The tappet, in turn, actuates a push rod that performs the same function as an opposedengine push rod. Radial engine rocker arms have adjusting screws and lock screws thatenable you to adjust the push rod-to-rocker arm clearance. In addition, many radialengine rocker arms are equipped with rollers on their valve ends to reduce friction,eliminate side loading on the valve stem, and reduce tip deformation. [Figure 1-48]

Figure 1-48. The valve operating mechanism for a radial engine performs the same functions asone on an opposed engine.

VALVE CLEARANCE ADJUSTMENTValve clearance describes the space between the tip of the valve stem and the rockerarm face. For an engine to run properly, the correct valve clearance must be maintained.During normal engine operating temperatures, the cylinder assemblies expand and forcethe cylinder head, along with its valve operating components, further away from thecrankcase. However, due to their relatively small mass, the push rods expand less. As a

result, the clearance between the rocker arm and valve stem increases. If this valveclearance is not controlled, the engine will run poorly, and valve damage might result.

An engine manufacturers maintenance manual specifies either a cold or hot valveclearance. As its name implies, a cold clearance is set when the engine is cold. Due tothe expansion properties discussed earlier, this clearance is typically less than the hotor running clearance, which is set when the engine is hot. Engines that require valveadjustments have adjustment screws and locknuts mounted in their rocker arms at thepush rod fitting.

Engines that use hydraulic lifters do not require valve adjustments because theyautomatically maintain a zero running valve clearance during normal operation. For thisreason, hydraulic lifters are often called zero clearance, or zero lash lifters.

However, hydraulic lifters must operate within a specific clearance range when thehydraulic lifter is not filled with oil, or dry. During cylinder replacement, you mustcheck that the dry-lifter clearance is within specified limits.

To perform a dry-lifter clearance check, the lifter body must first have all residual oilremoved. The procedure to bleed down the lifter varies between enginemanufacturers, but usually consists of depressing the check valve to drain all trappedoil. After the oil is drained, check the valve operating mechanism for the proper rockerarm face-to-valve tip clearance using the same procedure employed on engines withsolid lifters.

Some large radial engines incorporate a floating cam ring that requires a specialprocedure when adjusting valve clearance. To obtain the correct adjustment, the camring must be seated to eliminate cam bearing clearance. This usually involvesdepressing two valves to seat the cam ring, which then enables you to accuratelymeasure a third valve. This procedure is repeated for each cylinder.

CYLINDER NUMBERINGYou will often need to refer to a specific area on an engine or to a specific cylinder.Therefore, you should be familiar with the manufacturers particular system of cylindernumbering. Regardless of how an engine is mounted in an aircraft, the propeller shaftend is always referred to as the front of an engine, and the accessory end is always therear of an engine. Furthermore, when referring to either the right or left side of anengine, always assume you are viewing the engine from the rear, or accessory, end.

Similarly, crankshaft rotation is always referenced from the rear of an engine and isspecified as either clockwise or counterclockwise.

To identify a specific cylinder, all engine cylinders are numbered. However, opposedengines do not use a standard numbering system. Teledyne Continental Motors andTextron-Lycoming both manufacture four- and six-cylinder horizontally opposedengines, but each company uses a different numbering system. For example, Continentalbegins its cylinder numbering with the rearward cylinder while Lycoming begins withthe forward cylinder. Both companies place the odd numbered cylinders on the right andthe even numbered cylinders on the left. [Figure 1-49]

Figure 1-49. Cylinder numbering varies by manufacturer; always refer to the appropriate serviceinformation to determine how the cylinders of a specific engine are numbered.

In general, single-row radial engine cylinders are numbered consecutively in aclockwise pattern starting with the top cylinder. The difference with double-row radial

engines is that all of the rear cylinders are odd-numbered and front cylinders are even-numbered. For example, the top cylinder of the rear row is the number one cylinder,while the number two cylinder is the first cylinder in the front row clockwise from thenumber one cylinder. The number three cylinder is the next cylinder clockwise from thenumber two cylinder but is in the rear row. Some radial engines designed in easternEuropean countries reverse this pattern. [Figure 1-50]

Figure 1-50. Looking from the accessory end forward, all single-row radial engines are numberedconsecutively beginning at the top cylinder and progressing clockwise. On twin-row radials,however, the front row of cylinders are all even numbered while the rear row of cylinders are odd

numbered.

PROPELLER REDUCTION GEARSThe amount of power produced by an aircraft reciprocating engine is determined byseveral factors, including the amount of pressure exerted on the pistons during eachpower stroke and the number of power strokes completed in a given time period.

As a rule, the faster an engine turns the more power it produces. However, this ruledoes not apply to propellers. As a propeller blade tip approaches the speed of sound, itcannot efficiently convert the engines power into thrust. In other words, a propellerneeds to be operated at a specific speed to achieve maximum efficiency. Some high-powered engines use a propeller reduction gear system to produce their maximum ratedpower output while maintaining a slower propeller speed. Reduction gears permit thepropeller to turn slower than the crankshaft. Reduction gear systems currently installedon aircraft engines use spur gears, planetary gears, or a combination of the two.

Spur gears have teeth cut straight across their circumference and can be either externalor internal. The simplest type of reduction gearing consists of two external tooth spurgears, one small gear on an engine crankshaft and one larger gear on the propeller shaft.When configured this way, the amount of reduction is based primarily on the size of thepropeller shaft gear. The larger the gear, the slower the propeller turns. However, thisreduction system has some disadvantages. For example, when using two external toothspur gears, the propeller turns opposite the crankshaft. Furthermore, because thepropeller shaft is off-center from the engine crankshaft, the propeller acts as agyroscope applying high torsion loads to the engine case. As a result, the crankcase mustbe built stronger and heavier to withstand these loads. [Figure 1-51]

Figure 1-51. The ratio of the gear teeth in a gear reduction system with two externally-driven spurgears determines the amount of reduction. For example, if a drive gear has 25 teeth and the drivengear has 50 teeth, a ratio of 1:2 exists and the propeller turns at one half the crankshaft speed.

One way to overcome some of the disadvantages of a simple spur gear arrangement is touse an internal-tooth spur gear on the propeller shaft and an external-tooth spur gear onthe crankshaft. In addition to allowing the propeller to turn in the same direction as theengine, this arrangement aligns the propeller shaft more closely with the crankshaft,which eliminates much of the stress placed on the crankcase. [Figure 1-52]

Figure 1-52. A gear reduction system with one internal-tooth gear and one external-tooth gearturns the propeller and crankshaft in the same direction and is more closely aligned than areduction system with two external-tooth gears.

Whenever a reduction gear does not keep the propeller shaft perfectly aligned with the

crankshaft, additional vibration is induced into an engine. To help minimize thisvibration, some engines use a quill shaft between the crankshaft and propeller shaft. Aquill shaft is a hardened steel shaft that is splined on both ends and installed betweentwo gears, or shafts, to absorb torsional vibration. One end of the quill shaft fits into thefront end of the crankshaft, and the opposite end is inserted into the front end of thepropeller drive shaft. With this arrangement, the quill shaft drives the propeller andabsorbs vibration from the gear reduction mechanism. [Figure 1-53]

Figure 1-53. A quill shaft minimizes torsional vibration between a propeller shaft and thecrankshaft.

In a planetary reduction gear system the propeller shaft is attached to a housing thatcontains several small gears called planetary gears. The planetary gears rotatebetween a sun gear and a ring gear (sometimes called a bell gear). The crankshaftdrives either the sun gear or the ring gear depending on the individual installation. Theplanetary gear reduction system keeps the propeller shaft aligned with the crankshaft,transmits power with a minimum of weight and space, and keeps the propellersdirection of rotation the same as the engine. Planetary gears are used on somehorizontally opposed engines as well as radial and turboprop engines. [Figure 1-54]

Figure 1-54. In a planetary gear reduction system, the propeller is attached to the planetary gearspider and the crankshaft turns either the sun gear or the ring gear.

You can determine the reduction rate that a particular gearing arrangement achieves bythis formula:

For example, if there are 72 teeth on the ring gear and 36 teeth on the sun gear, thepropeller turns at a ratio of 1.5 to 1. However, reduction ratios are traditionallyexpressed in whole numbers, so this example is expressed as a 3 to 2 reduction. In otherwords, the crankshaft must turn three revolutions for every two revolutions of thepropeller shaft. Neither the number of teeth on the planetary gears nor the number ofplanetary gears contributes to the computation for gear reduction.

PROPELLER SHAFTSAll aircraft reciprocating engines have a propeller shaft. As an aviation technician, youmust be familiar with the various types of propeller shafts, including tapered, splined,and flanged shafts.

Tapered propeller shafts were used on most of the early, low-powered engines. On atapered propeller shaft, the shaft diameter tapers toward the end of the shaft. To preventa propeller hub from rotating on a tapered shaft, one or more key slots are milled intothe shaft. In addition, the end of the shaft is threaded to receive a propeller retaining nut.[Figure 1-55]

Figure 1-55. A tapered propeller shaft changes in diameter along its length and uses a metal key tokeep a propeller from rotating.

High-powered engines require a stronger method of attaching propellers. Most highpowered radial engines use splined propeller shafts. A spline is a rectangular groovethat is machined into the propeller shaft. Most splined shafts have a master spline that isapproximately twice the size of any other spline. This master spline assures that apropeller is attached to a propeller shaft a specific way so that vibration is kept to aminimum. [Figure 1-56]

Figure 1-56. All splined propeller shafts are identified by an SAE number. For example, SAE 50identifies a splined shaft that meets SAE design specifications for a 50 size shaft. The SAE numberdoes not refer to the number of splines.

Modern horizontally opposed aircraft engines use a flanged propeller shaft. Thecrankshaft