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A&P TECHNICIAN POWERPLANT TEXTBOOK PDF

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A&p Technician Powerplant Textbook Pdf

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One type oI bushing that is used in aviation is the oil impregnated porous oilite bushing. With this type oI bushing, the heat produced by Iriction draws the impregnated oil to the bearing surIace to provide lubrication during engine operation.

A ball bearing assembly consists oI grooved inner and outer races, one or more sets oI polished steel balls, and a bearing retainer.

The balls oI a ball bear- ing are held in place and kept evenly spaced by the bearing retainer, while the inner and outer bearing races provide a smooth surIace Ior the balls to roll over.

However, some races have a deep groove that matches the curvature oI the balls to provide more support and allow a bearing to carry high radial loads. Because the balls oI a ball bearing oIIer such a small contact area, ball bearings have the least amount oI rolling Iriction. Because oI their construction, ball bearings are well suited to withstand thrust loads and are, thereIore, used as thrust bearings in large radial and gas tur- bine engines.

In applications where thrust loads are heavier in one direction, a larger race is used on the side oI the increased load. Most oI the ball bearings you will work with as a technician are used in accessories such as magne- tos, alternators, turbochargers, and vacuum pumps.

Many oI these bearings are prelubricated and sealed to provide trouble-Iree operation between over- hauls. However, iI a sealed ball bearing must be removed or replaced, it is important that you use the proper tools to avoid damaging the bearing and its seals. Roller bearings are similar in construction to ball bearings except that polished steel rollers are used instead oI balls.

The rollers provide a greater contact area and a corresponding increase in rolling Iriction over that oI a ball bearing. Roller bearings are avail- able in many styles and sizes, but most aircraIt engine applications use either a straight roller or tapered roller bearing. Straight roller bearings are suitable when the bearing is subjected to radial loads only. For example, most high-power aircraIt engines use straight roller bearings as crankshaIt main bearings.

Tapered roller bearings, on the other hand, have cone-shaped inner and outer races that allow the bearing to withstand both radial and thrust loads. The connecting rod is the link which transmits the Iorce exerted on a piston to a crankshaIt. Most con- necting rods are made oI a durable steel alloy; how- ever, aluminum can be used with low horsepower engines. The lighter a connecting rod is, the less inertia it produces when the rod and piston stop and then accelerate in the opposite direction at the end oI each stroke.

A typical connecting rod is Iorged and has a cross-sectional shape oI either an "H" or an "I. One end oI a connecting rod connects to the crankshaIt and is called the crankpin end, while the other end connects to the piston and is called the piston end. The three types oI connecting rod assemblies you should be Iamiliar with are the plain-type, the master-and-articulated-rod type, and the Iork-and-blade type.

Plain connecting rods are used in opposed and in- line engines. The piston end oI a plain connecting rod is Iitted with a bronze bushing to accommodate the piston pin. The bushing is typically pressed into the connecting rod and then reamed to the dimension required by the piston pin. The crankpin end, on the other hand, is usually Iitted with a two-piece bearing and cap which is held on the end oI the rod by bolts or studs. In this case, the main bearing insert is typically made oI steel that is lined with a nonIerrous alloy such as babbitt, lead, bronze, or copper.

To provide proper Iit and balance, connecting rods are oIten matched with pistons and crankpins. ThereIore, iI a connecting rod is ever removed, it should always be replaced in the same cylinder and in the same relative position. To help do this, con- necting rods and caps are sometimes stamped with numbers to identiIy the cylinder and piston assem- bly with which they should be paired.

For example, Reciprocating Engines Fi gure On a typical pl ai n connecti ng rod, a two pi ece beari ng shell fits tightly i n the crankpi n end of the connect- ing rod. The bearing is held i n place by pi ns or tangs that fit i nto sl ots cut i nto the cap and connecti ng rod. The pi ston end of t he connect i ng rod cont ai ns a bushi ng t hat i s pressed into place.

Figure The master-and-articulated rod assembly is com- monly used in radial engines.

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With this type oI assembly, one piston in each row oI cylinders is con- nected to the crankshaIt by a master rod. The remaining pistons are connected to the master rod by articulated rods.

ThereIore, in a nine cylinder engine there is one master rod and eight articulating rods, while a double row 18 cylinder engine has two master rods and 16 articulating rods. Master rods are typically manuIactured Irom a steel alloy Iorging that is machined and heat-treated Ior maximum strength.

Articulated rods are constructed oI a Iorged steel alloy in either an I- or H- cross-sec- tional proIile. Bronze bushings are pressed into the bores in each end oI the articulated rod to serve as bearings.

The master rod serves as the only connecting link between all the pistons and the crankpin. The small end, or piston end oI a master rod, contains a plain bearing called a piston pin bearing which receives the piston pin. The crankpin end oI a master rod contains the crankpin bearing, sometimes called a master rod bearing. A typical crankpin bearing con- sists oI a plain bearing that is able to withstand the radial loads placed on the rod assembly.

A set oI Figure With a single piece master rod, the master-and-ar ti cul at ed rods are assembl ed and i nst al l ed on t he crankpi n before the crankshaft sections are joined together.

On t he ot her hand, wi t h a mul ti pl e pi ece master rod, t he crankpi n end of the master rod and its bearing are split and i nstal l ed on t he crankpi n. The beari ng cap i s t hen set i n pl ace and bolted to the master rod. Ilange holes are machined around the crankpin end oI a master rod to provide an attachment point Ior the articulated rods. A master rod may be one piece or multiple pieces. As a general rule, a one piece rod is used on multi- ple piece crankshaIts while a multiple piece, or split type master rod is used with single piece crank- shaIts.

Figure Each articulated rod is hinged to the master rod by a knuckle pin. Some knuckle pins are pressed into the Reciprocating Engines Figure Articulated rods are attached to the master rod by knuckle pi ns, that are pressed i nto holes i n the master rod fl anges duri ng assembl y. A knuckl e pi n l ock pl ate i s then i nstall ed to retai n the pi ns. In either type oI installation, a lock plate on each side retains the knuckle pins and prevents lateral movement. Figure Since the Ilange holes on a master rod encircle the center oI the crankpin, the crankpin is the only por- tion oI a master rod assembly that travels in a true cir- cle as the crankshaIt rotates.

The remaining knuckle pins travel in an elliptical path. Figure Figure You can see that each knuckle pi n rotates i n a different elliptical path. As a result, each articulated rod has a varyi ng degree of angul ari ty rel ati ve to the center of the crank throw. A fork-and-blade rod assembly used i n a V-type engi ne consists of a blade connecti ng rod whose crankpi n end fi ts between the prongs of the fork connecti ng rod.

Because oI the varying angularity, all pistons do not move an equal amount in each cylinder Ior a given number oI degrees oI crankshaIt rotation. To com- pensate Ior this, the knuckle pin holes in the master rod Ilange are positioned at varying distances Irom the center oI the crankpin. The Iorked rod is split at the crankpin end to allow space Ior the blade rod to Iit between the prongs.

The Iork-and-blade assembly is then Iastened to a crankpin with a two-piece bearing. Figure The piston in a reciprocating engine is a cylindrical plunger that moves up and down within a cylinder. Pistons perIorm two primary Iunctions; Iirst, they draw Iuel and air into a cylinder, compress the gases, and purge burned exhaust gases Irom the cylinder; second, they transmit the Iorce produced by combustion to the crankshaIt.

Most aircraIt engine pistons are machined Irom alu- minum alloy Iorgings. Ring grooves are then cut into a piston's outside surIace to hold a se t oI piston rings. As many as six ring grooves may be machined around a piston. The portion oI the piston between the ring grooves is commonly reIerred to as a ring land. The piston's top surIace is called the piston head and is directly exposed to the heat oI combus- tion.

The piston pin boss is an enlarged area inside the piston that provides additional bearing area Ior a Reciprocating Engines Fi gure A typical piston has ri ng grooves cut into i ts outside surface to support piston ri ngs. In addition, cooling fi ns are sometimes cast i nto the piston i nterior to help dis- si pate heat, whi le the pi ston pi n boss provi des support for the piston pi n. To help align a p iston in a cylinder, the piston base is extended to Iorm the piston skirt.

On some pistons, cooling Iins are cast into the underside oI the piston to provide Ior greater heat transIer to the engine oil. Figure Pistons are oIten classiIied according to their head design. The most common types oI head designs are the Ilat, recessed, cupped, and domed.

In addition, piston skirts can be the simple trunk type, the trunk type that is relieved at the piston boss, and the slip- per type which is relieved along the piston base to reduce Iriction. Most modern aircraft engi nes use flat-head pis- tons. However, as an avi ati on techni ci an, you shoul d be familiar with all piston head designs.

Fi gure Several engi nes now use cam ground pistons to compensate for the greater expansi on parallel to the pis- ton boss duri ng engi ne operati on. The di ameter of a cam ground pi ston measures several thousandt hs of an i nch larger perpendicul ar to the pi ston boss than parallel to the piston boss. All pistons expand as they heat up. However, due to the added mass at the piston boss, most pistons expand more along the piston boss than perpendic- ular to the piston boss.

This uneven expansion can cause a piston to take on an oblong, or oval shape, at normal engine operating temperatures, resulting in uneven piston and cylinder wear. One way to com- pensate Ior this is with a cam ground piston. A cam ground piston is machined with a slightly oval shape. That is, the diameter oI the piston parallel to the piston boss is slightly less than the diameter per- pendicular to the piston boss. This oval shape com- pensates Ior any diIIerential expansion and produces a round piston at normal operating tem- peratures.

Furthermore, the oval shape holds the piston centered in the cylinder during engine warmup and helps prevent the piston Irom moving laterally within a cylinder. They prevent leakage oI gas pres- sure Irom the combustion chamber, reduce oil seepage into the combustion chamber, and transIer heat Irom the piston to the cylinder walls.

The rings Iit into the piston grooves but spring out to press against the cylinder walls. When properly lubri- cated, piston rings Iorm an eIIective seal. Most piston rings are made oI high-grade gray cast iron. However, in some engines, chrome-plated mild steel piston rings are used because oI their ability to withstand high temperatures.

AIter a ring is made, it is ground to the desired cross-section and then split so it can be slipped over a piston and into a ring Reciprocating Engines Figure Of the three types of joints used in piston ring gaps, the butt joint is the most common in aircraft engines. The point where a piston ring is split is called the piston ring gap.

The gap can be a simple butt joint with Ilat Iaces, an angle joint with angled Iaces, or a step joint. Figure Since the piston rings expand when the engine reaches operating temperature, the ring must have a speciIied clearance between the ring gap Iaces. II the gap is too large, the two Iorces will not close up enough to provide an adequate seal.

On the other hand, an insuIIicient gap will result in the ring Iaces binding against each other and the cylinder wall causing cylinder wall scoring. When installing piston rings, the ring gaps must be staggered, or oIIset, to ensure that they do not align with each other. This helps prevent combustion chamber gases Irom Ilowing past the rings into the crankcase.

This blow-by, as it is oIten called, results in a loss oI power and increased oil consumption. II three piston rings are installed on one piston, it is common practice to stagger the ring gaps degrees Irom each other. In order Ior piston rings to seal against the cylinder wall, the rings must press against the cylinder wall snugly. Furthermore, the rings must exert equal pressure on the entire cylinder wall as well as pro- vide a gas-tight Iit against the sides oI the ring grooves.

The two broad types oI piston rings used in reciprocating engines are compression rings or oil rings. Figure Compression rings prevent gas Irom escaping past the piston during engine operation and are placed in the ring grooves immediately below the piston head.

The number oI compression rings used on each piston is determined by the type oI engine and its design. However, most aircraIt engines typically Figure Heat engines convert thermal energy into mechanical energy. A specific volume of air is compressed, 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 the back-and-forth or reciprocating movement of their pistons.

It is the downward motion of pistons, caused by expanding gases, which generates the mechanical energy needed to accomplish work. Reciprocating engines are commonly classified by cylinder arrangement radial, in-line, V-type, or opposed and by fuel type gasoline or diesel. The two basic types of radial engines are the rotary-type and the static-type. During World War I, rotary-type radial engines were used extensively because of their high power-to-weight ratio.

The cylinders of a rotary-type radial engine are mounted radially around a small crankcase and rotate with the propeller, while the crankshaft remains stationary.

Some of the more popular rotary-type engines were the Bentley, the Gnome, and the LeRhone. Figure On rotary-type radial engines, the propeller and cylinders are bolted to the crankcase and rotate around a stationary crankshaft. The large rotating mass of cylinders produced a significant amount of torque, which made aircraft control difficult. This factor, coupled with complications in carburetion, lubrication, and the exhaust system, limited the development of the rotary-type radial engine.

In the late s, the Wright Aeronautical Corporation, in cooperation with the U. 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, which demonstrated 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 that with the static engine, the crankcase remains stationary and the crankshaft rotates to turn the propeller.

Static radial engines have as few as three cylinders and as many as The higher horsepower engines proved most useful. Static radial engines also possessed a high power-to-weight ratio and powered many military and civilian transport aircraft. Radial engines helped revolutionize aviation with their high power and dependability. Single-row radial engines typically have an odd number of cylinders arranged around a crankcase.

A typical configuration consists of five to nine evenly spaced cylinders with all pistons connected to a single crankshaft. To increase engine power while maintaining a reasonably-sized frontal area, multiple-row radial engines were developed. These engines contain two or more rows of cylinders connected to a single crankshaft. The double-row radial engine typically has 14 or 18 cylinders.

To improve cooling of a multiple-row radial engine, the rows are staggered to increase the amount of airflow past each cylinder. The largest, mass-produced, multiple-row radial engine was the Pratt and Whitney R, which consisted of 28 cylinders arranged in four staggered rows of seven cylinders each. The R developed a maximum 3, horsepower, making it the most powerful production radial engine ever used. The Pratt and Whitney R engine was the largest practical radial engine used in aviation.

Development and advancement in turbojet and turboprop engines eclipsed the performance of large multiple-row radial engines.

The pistons are either upright above or inverted below the crankshaft. This engine can be either liquid-cooled or air-cooled.

In-line engines have a comparatively small frontal area, which enables them to be enclosed by streamlined nacelles or cowlings.

Because of this, in-line engines were popular among early racing aircraft. A benefit of an inverted in-line engine is that the crankshaft is higher off the ground. The higher crankshaft allowed greater propeller ground clearance, permitting the use of shorter landing gear. Historically, in-line engines were used on tail-wheel aircraft; they enabled manufacturers to use shorter main gear, which increased forward visibility while taxiing.

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

In , Thielert Aircraft Engines now Centurion Aircraft Engines began delivering new, certified kerosene-powered, in-line reciprocating engines for light aircraft. In , Austro Engine certified a similar engine. Two rows of cylinders, called banks, are oriented 45, 60, or 90 degrees apart from a single crankshaft.

Two banks of cylinders typically produce more horsepower than an in-line engine. Because the cylinder banks share a single crankcase and a single crankshaft, V-type engines have a reasonable power-to-weight ratio with a small frontal area.

The pistons can be located either above the crankshaft or below the crankshaft. Most V-type engines had 8 or 12 cylinders.

Vtype engines can be either liquid- or air-cooled. V engines developed during World War II achieved some of the highest horsepower ratings of any reciprocating engine. Today, V-type engines are typically found on classic military and experimental racing aircraft. V-type engines provide an excellent combination of weight and power with a small frontal area. Opposed engines can be designed to produce as little as 36 horsepower or as much as horsepower.

The majority of opposed engines are air-cooled and horizontally mounted when installed on fixed-wing aircraft, but they can be mounted vertically in helicopters. Opposed engines have a relatively small, lightweight crankcase that contributes to a high power-to-weight ratio. The compact cylinder arrangement provides a comparatively small frontal area, which enables the engine to be enclosed by streamlined nacelles or cowlings. With opposing cylinders, power impulses tend to cancel each other out, resulting in less vibration than other engine types.

A horizontally opposed engine combines a good power-to-weight ratio with a relatively small frontal area. This style of engine powers most light aircraft in service today. Wankel engines have a good power-to-weight ratio, and their compact design can be enclosed by streamlined nacelles or cowlings.

Instead of using a crankshaft, connecting rods, pistons, cylinders, and conventional valve train, the Wankel engine uses an eccentric shaft and triangular rotor turning in an oblong combustion chamber. This reduction in moving parts contributes to increased reliability. Early designs had problems associated with sealing the combustion chamber, which affected efficiency and engine life.

A Wankel engine uses an eccentric shaft to turn a triangular rotor in an oblong combustion chamber. 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 operating mechanism, crankshaft, and connecting rods are located in the crankcase. The piston compresses the fuel mixture and transmits power to the crankshaft through the connecting rods. For all of the reciprocating engine types discussed, the horizontally opposed and statictype 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 a discussion of components specific to diesel engines, see Chapter 1, Section C. The crankcase is the core of a reciprocating engine. Additionally, the crankcase provides a tight enclosure for the lubricating oil. Due to great internal and external forces; crankcases must be extremely rigid and strong. A crankcase is subjected to dynamic bending moments that change continuously in direction and magnitude.

For example, combustion exerts tremendous forces to the pistons and the propeller exerts unbalanced centrifugal and inertial forces. To remain functional, a crankcase must be capable of absorbing these forces while maintaining its structural integrity.

Today, most crankcases consist of at least two pieces; however, some crankcases are cast as one piece, and some consist of up to five pieces.

To provide the necessary strength and rigidity while reducing weight, most aircraft crankcases are made of cast aluminum alloys. A typical horizontally-opposed engine crankcase consists of two pieces of cast aluminum alloy manufactured in sand castings or permanent molds. Crankcases manufactured by the permanent mold process, or permamold, as it is called by some manufacturers, are denser than those made by sand-casting. Greater density permits molded crankcases to have relatively thinner walls than similar sand-cast crankcases.

In addition, molded crankcases tend to better resist cracking due to fatigue. Most opposed crankcases are approximately cylindrical, with smooth areas machined to serve as cylinder pads. A cylinder pad is the surface on which a cylinder mounts to the crankcase. For the crankcase to support a crankshaft, a series of transverse webs are cast directly into the crankcase parallel to its longitudinal axis.

In addition to supporting the crankshaft, these webs add strength and form an integral part of the structure.

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The transverse webs in the crankcase support the main bearings and a set of camshaft bosses support the camshaft. The crankcase is integral to the lubrication system. Passages are drilled into the case halves to deliver oil to the moving parts within the crankcase. Additionally, oil passages are machined into the crankcase to scavenge collect oil and return it to the main 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 and smaller bolts and nuts are used around the case perimeter. Because the crankcase typically contains oil, it must be sealed to prevent leakage. To ensure that the seal does not affect the tight fit required for the bearings, most crankcase halves are sealed with a very thin coating of a nonhardening gasket compound. In addition, on some engines, a fine silk thread extending around the entire case perimeter is embedded in the compound.

When the crankcase halves are bolted together with appropriate torque, the. 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 the size and type of engine.

A typical radial engine crankcase separates into four main sections: The four sections of a radial engine crankcase are nose, power, supercharger, and accessory. The nose section is mounted at the front of a radial engine crankcase and bolts directly to the power section.

A typical nose section is made of an aluminum alloy that is cast as one piece with a domed or convex shape. This section typically supports and contains a propeller governor drive shaft, the propeller shaft, a cam ring, and, if required, a propeller reduction gear assembly. In addition, the nose section might have mounting points for magnetos or other engine accessories. The second section of a radial engine crankcase is referred to as the power section and it contains the components that transfer energy from the pistons to the crankshaft.

Like an opposed engine crankcase, the power section absorbs stress from the crankshaft assembly and the cylinders. The power section can be one, two, or three pieces. A one-. Multipiece power sections are typically manufactured from aluminum or magnesium and bolted together. The power section contains machined bosses that support the crankshaft bearings and add strength. Cylinders are attached around the perimeter of the power section to machined cylinder pads.

In general, studs are installed into threaded holes in the power section to provide a means of attaching the cylinders. The inner circumference of a cylinder pad is sometimes chamfered or tapered to permit the installation of a large, rubber O-ring around the cylinder skirt. This O-ring seals the joint between the cylinder and the cylinder pads.

The diffuser or supercharger section is located directly behind the power section and is typically made of cast aluminum alloy or magnesium. This section houses the supercharger and its related components. The supercharger section incorporates attach points to secure the engine assembly to the engine mounts.

The accessory section is usually cast of aluminum alloy or magnesium. On engines with one piece accessory sections, the casting is machined to provide means for mounting accessories. Two-piece accessory sections consist of an aluminum alloy casting and a separate magnesium cover plate that provides attach points for the accessories. Possible accessories include magnetos, carburetors, pumps, starters, and generators. The gear train in the accessory section contains both spur- and bevel-type gears to drive various engine components and accessories.

Spur-type gears drive heavily loaded accessories or those that would be affected by backlash in the gear train. Bevel-type gears handle lighter loads, but accommodate short drive shafts for various accessories.

For opposed engines, engine mounting points, sometimes called mounting lugs, can be cast as a part of the crankcase or can be a bolt-on addition. Because this mounting arrangement supports the weight of the entire powerplant and propeller, it must be designed 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 supercharger section. As with opposed engines, the mounting lugs on radial engines can be integral with the casting or bolted on. Because crankshafts must withstand high stress, they are generally forged from a strong alloy such as chromium-nickel molybdenum steel. Some crankshafts are made from a single forging, while others are formed by joining several components. The number of crankpins varies depending on the type of engine and the number of cylinders.

Regardless of the number of throws or the number of pieces used in construction, all crankshafts have the same basic components, including main bearing journals, crankpins, and crank cheeks. 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 two main journals to support the crankshaft, absorb the operational loads, and transmit stress from the crankshaft to the crankcase. To minimize wear, most main bearing journals are hardened through a nitriding process. A crankshaft has one or more crankpins also known as throws, crank throws, and connecting-rod bearing journals located at specific points along its length. Crankpins are offset from the main bearing journal to provide attachment points for connecting rods.

Because of this offset design, any force applied to a crankpin in a direction other than parallel to the crankshaft center line causes the crankshaft to rotate. Like main journals, crankpins undergo a nitriding process to resist wear and provide a suitable bearing surface. Crankshafts in most aviation engines are usually hollow to reduce weight. This also provides 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. On some engines, a passage drilled in the crankpin allows oil from 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 all are designed to position each piston for smooth power generation as the crankshaft rotates. The relative distance between crankpins on a crankshaft is measured in degrees. On a four cylinder engine, crankpins one and four are degrees apart from crankpins two and three.

Crank cheeks, or crank arms, are required to connect crankpins to each other and to the main journal of the crankshaft. In some designs, the cheeks extend beyond the journal to provide an attach point for counterweights that help balance the crankshaft. Most crank cheeks have drilled passageways to permit oil to flow from the main journal to the crankpin. Excessive engine vibration can cause metal structures to fatigue and fail or wear excessively. An unbalanced crankshaft can cause excessive vibration.

To help minimize unwanted vibration, crankshafts are balanced statically and dynamically. A crankshaft is in static balance when the weight of the entire assembly is balanced around its axis of rotation.

To test a crankshaft for static balance, the outside main journals are placed on two knife-edge balancing blocks. If the crankshaft tends to favor any one rotational position during the test, it is out of static balance. After a crankshaft is statically balanced, it must also be dynamically balanced. A crankshaft is considered in dynamic balance when the centrifugal forces and power. Crankshafts for smaller engines do not always us e counterweights. Some crankshafts use two or more of these assemblies, each attached to a different crank cheek.

The construction of the dynamic damper used in one type of engine consists of a movable slotted-steel counterweight attached to a crank cheek by two spool-shaped steel pins that extend through oversized holes in the counterweight and crank cheek.

The difference in diameter between the pins and the holes enables the dynamic damper to oscillate. Movable counterweights act as dynamic dampers to reduce the centrifugal and impact vibrations in an aircraft engine.

Each time a cylinder fires, force is transmitted to the crankshaft, causing it to flex. This happens hundreds of times every minute. Dynamic dampers oscillate, or swing, with every pulse from a firing cylinder to absorb some of this force. Think of the crankshaft as a pendulum that swings at its natural frequency when a force is applied. The greater the force, the greater the distance the pendulum swings. However, if a second pendulum is suspended from the first and a force is applied, the second pendulum begins to oscillate opposite the applied force.

This opposite oscillation dampens the oscillation of the first pendulum. You can think of a dynamic damper as a short pendulum hung from a crankshaft that is tuned to the frequency of power impulses. The most common types of crankshafts are single-throw, two-throw, four-throw, and six-throw. The simplest crankshaft is the single-throw, or —degree crankshaft, used on single-row radial engines.

A singlethrow crankshaft consists of a single crankpin with two crank cheeks and two main journals.

A single-throw crankshaft may be constructed out of one or two pieces. Onepiece crankshafts use a connecting rod that splits for installation. A two-piece crankshaft uses a crankpin that separates to permit the use of a one-piece connecting rod. A one-piece, single-throw crankshaft is cast as one solid piece. However, a clamp type, 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 of cylinders. The throws on a two-throw crankshaft are typically set degrees apart and can consist of either one or three pieces.

Although uncommon, two cylinder opposed engines also use two-throw crankshafts. Four-cylinder opposed engines and four cylinder inline engines use four-throw crankshafts. On some four-throw crankshafts, two throws are arranged degrees apart from the other two throws. Depending on the size of the crankshaft and power output of the engine, a four-throw crankshaft has either three or five main bearings.

A typical four-throw crankshaft from a four cylinder, opposed engine is machined from one piece of steel. Six-cylinder opposed and in-line engines as well as cylinder V-type engines use sixthrow crankshafts. A typical six-throw crankshaft is forged as one piece and consists of four main bearings and six throws that are 60 degrees apart. The crankpins in a typical six-throw crankshaft are 60 degrees apart in the firing order.

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 be composed 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 and wear. For a bearing to provide efficient and quiet operation, it must hold two parts in a nearly fixed position with very close tolerances.

Furthermore, depending on their specific application, bearings must be able to withstand radial loads, thrust loads, or both. There are two ways in which bearing surfaces move in relation to each other. One is by the sliding movement of one surface against another, and the second is for one surface to roll over another. Reciprocating engines use bearings that rely on both types of movement.

Aircraft reciprocating engines typically use include plain bearings, ball bearings, and roller bearings. 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 and ball bearings use rolling movement. Plain bearings are generally used as crankshaft main bearings, cam ring and camshaft bearings, connecting rod end bearings, and accessory drive shaft bearings.

These bearings are typically subject to radial loads only; however, flange-type plain bearings are 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; with lead-tin plated over the silver on the inside surface only.

Smaller bearings, such as those used to support various accessory drive shafts, are called bushings. With this type of bushing, the heat produced by friction draws the impregnated oil to the bearing surface to provide lubrication during engine operation. A ball bearing assembly consists of grooved inner and outer races, one or more sets of polished steel balls, and a bearing retainer.

The balls are held in place and kept evenly spaced by the bearing retainer, and the inner and outer bearing races provide a smooth surface for the balls to roll over. However, some races have a deep groove that matches the curvature of the balls to provide more support and enable the bearing to carry high. Most ball bearings that you encounter as a technician are used in accessories such as magnetos.

In applications in which thrust loads are greater in one direction. Engine manufacturers strive to make connecting rods as light as possible. The crankpin end of the connecting rod connects to the crankshaft and the piston end connects to the piston.

Many of these bearings are prelubricated and sealed to provide trouble-free operation between overhauls. Straight roller bearings are suitable when the bearing is subjected to radial loads only. The three major types of connecting rod assemblies are plain. The weight of a connecting rod corresponds to the amount of inertia it possesses when the rod and piston stop before accelerating in the opposite direction at the end of each stroke. Tapered roller bearings.

Ball bearings are well-suited to withstand thrust loads. The rollers provide a greater contact area and a corresponding increase in rolling friction over that of a ball bearing. Because the balls in a ball bearing assembly provide a small contact area. For example. Roller bearings are available in many styles and sizes. Most connecting rods are made of a durable steel alloy. In this type of assembly. The piston end of the connecting rod contains a bushing that is pressed into place.

The bearing is held in place by pins or tangs that fit into slots cut into the cap and connecting rod. Connecting rods and caps might be stamped to identify the corresponding cylinder and piston assembly. The piston end of a plain connecting rod is fitted with a bronze bushing to accommodate the piston pin. If a connecting rod is ever removed. The bushing is typically pressed into the connecting rod and reamed to a precise dimension to fit the piston pin.

The two piece bearing shell on a typical plain connecting rod fits tightly in the crankpin end of the connecting rod. Connecting rods are often matched with pistons for balance and crankpins for fit. The remaining pistons are connected to the master rod with articulated rods. The bearing inserts are typically steel lined with a nonferrous alloy such as Babbitt.

The crankpin end is usually fitted with a two-piece bearing. The master rod serves as the only link between all of the pistons and the crankpin. As a rule. A typical crankpin bearing must be able to withstand the radial loads placed on the rod assembly. Articulated rods are constructed of a forged steel alloy with an I.

Bronze bushings are pressed into the bores in each end of the articulated rods. A master rod can be one piece or multiple pieces. The crankpin end of a master rod contains the crankpin bearing master rod bearing. A set of flange holes is machined around the crankpin end of a master rod to provide an attachment point for the articulated rods.

The piston end of a master rod contains the piston pin bearing. Master rods are typically manufactured from a steel alloy forging that is machined and heat-treated for maximum strength. On a multiple piece master rod. 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.

On a single piece master rod. Some knuckle pins are pressed into the master rod so they do not rotate in the flange holes. In either type of installation. Each articulated rod has a varying degree of angularity relative to the center of the crank throw. A knuckle pin lock plate retains the pins. Because the flange holes on a master rod are arranged around the crankpin. Articulated rods are attached to the master rod by knuckle pins. As the crankshaft rotates. Knuckle pins rotate in different elliptical paths.

The piston pin boss is an enlarged area inside the piston that provides additional bearing area for the piston pin. A fork-and-blade rod assembly used in a V-type engine consists of a blade connecting rod whose crankpin end fits between the prongs of the fork connecting rod. To compensate for this. The fork-and-blade assembly is then fastened to a crankpin with a two-piece bearing.

Because of the varying angularity. Pistons perform two primary functions. Aircraft engine pistons are typically machined from aluminum alloy or steel forgings. The portion of the piston between the ring grooves is commonly referred to as a ring land. The forked rod is split at the crankpin end to allow space for the blade rod to fit between the prongs. The piston pin boss provides support for the piston pin.

The three common types of piston skirts are trunk. Some pistons have cooling fins cast into the underside of the piston skirt to provide for greater heat transfer to the engine oil. To help align a piston in a cylinder. A typical piston has ring grooves cut into its outside surface to support piston rings. The most common types of piston heads are flat. Pistons are sometimes classified according to their head design.

Cooling fins are sometimes cast into the piston interior to dissipate heat. One way to compensate for this is to use a cam-ground piston. This uneven expansion can cause a piston to take on an oval shape at normal engine operating temperatures. Due to the added mass at the piston boss. The majority of modern aircraft engines use flat-head pistons.

All pistons expand when they heat up. A cam-ground piston is machined with a slightly oval shape. This compensates for differential expansion and produces a round piston at normal operating temperatures.

The gap can be a simple butt joint with flat faces. Cam ground pistons compensate for the greater expansion parallel to the piston boss during engine operation. The diameter of a cam ground piston measures several thousandths of an inch larger perpendicular to the piston boss than parallel to the piston boss. During manufacture. The point where a piston ring is split is called the piston ring gap.

The chrome-plated rings can withstand higher temperatures. Piston rings are usually made of high-grade gray cast iron or chrome-plated. Piston rings are spring-loaded and press against the cylinder walls. They prevent pressure leakage from the combustion chamber. New piston rings require some wear-in during engine operation so that the ring contour matches the cylinder wall.

To form an effective seal. If the gap is too small. The two main types of piston rings used in reciprocating engines are compression rings and oil rings. A ring that matches its cylinder is considered to be seated. Of the three types of joints used in piston ring gaps. As an engine reaches operating temperature. To accommodate expansion.

This blow-by. If the gap is too large. Ring gaps must be staggered. Compression rings are installed in the upper piston ring grooves to help prevent the combustion gases from escaping. Less material is cut away. The cross section of a compression ring can be rectangular. Because compression rings receive limited lubrication and are closest to the heat of combustion. Because the profile is wedge shaped.

Wedge-shaped rings also have a beveled face to promote rapid ring seating. The wedge shape also helps prevent a ring from sticking in a groove. Tapered rings have a beveled face to reduce contact area. Oil rings. The compression rings. A rectangular compression ring fits flat against a cylinder wall with a large contact area to provide a tight seal. The large contact area requires a relatively long time to seat. Two or three compression rings on each piston is common. The number of compression rings used on each piston is determined by the engine manufacturer.

These slots enable excess oil to return to the engine sump through small holes drilled in the piston ring groove. The primary purpose of oil control rings is to regulate the thickness of the oil film on a cylinder wall. An oil scraper ring. The tapered face presents the narrowest bearing edge to the cylinder wall to reduce friction and accelerate ring seating. The two types of oil rings that are found on most engines are oil control rings and oil scraper rings.

On some pistons. Carbon buildup on the ring grooves or valve guides can cause parts to stick. To help prevent this. Pistons can have one or more oil control rings. The ring can be installed with the beveled edge away from the piston head or in the reverse position.

Oil rings control the amount of oil applied to the cylinder walls and prevent oil from entering the combustion chamber. An oil control ring returns excess oil to the crankcase through small holes drilled in the piston ring grooves. If the bevel is installed so that it faces the piston head.

Oil control rings are placed in the piston ring grooves below the compression rings. Carbon buildup can also cause spark plugs misfiring. If excessive oil enters the combustion chamber. Compression rings can have three different ring cross sections. On the downward stroke. Semifloating piston pins are loosely attached to the connecting rod by clamping around a reduced-diameter section of the pin. A spring ring also fits into grooves cut into the ends of a piston boss.

A full-floating piston pin must be held in place laterally to prevent it from rubbing and scoring the cylinder walls. If the bevel is installed to face away from the piston head. Stationary piston pins are secured to the piston by a setscrew that prevents rotation. Full-floating piston pins rotate freely in both the connecting rod and the piston. Piston pins are sometimes called wrist pins because the motion of the piston and the connecting rod is similar to a human wrist.

Three devices that are used to hold a piston pin in place are circlets. Piston pins can be stationary. A circlet is similar to a snap ring that fits into a groove cut into each end of the piston boss. An oil scraper ring installed with its beveled edge away from the cylinder head forces oil upward along the cylinder wall when the piston moves upward.

The current. Piston pins are tubular. Both circlets and spring rings are used primarily on earlier piston engines. A cylinder must be strong enough to withstand the internal pressures developed during engine operation yet be lightweight to minimize engine weight. When designing and constructing a cylinder. This required the use of removable cylinder heads. A typical air-cooled engine cylinder consists of a cylinder head. As the head cools. On some of the earliest two. These plugs are inserted into the open ends of the piston pins to provide a good bearing surface against the cylinder walls.

To do this. The inside of a cylinder. The most commonly used material that meets these requirements is a high-strength steel alloy such as chromium-molybdenum steel or nickel chromium-molybdenum steel. The longer skirt helps keep oil from draining into the combustion chamber and causing hydraulic lock after an engine has been shut down.

The cylinder assembly. The lower cylinders on radial engines and all the cylinders on inverted engines typically have extended cylinder skirts. Cylinder barrels are machined from a forged blank. The exterior of a cylinder barrel consists of several thin cooling fins that are machined into the exterior cylinder wall and a set of threads that are cut at the top of the barrel so that it can be screwed into the cylinder head.

This is called a choke bore cylinder and is designed to compensate for the uneven expansion caused by the higher operating temperatures and larger mass near the cylinder head. During the nitriding process. The furnace heats a cylinder barrel to approximately 1. Nitriding is a form of case hardening that changes the surface strength of steel by infusing the metal with a hardening agent.

The inside wall of a cylinder barrel is continuously subjected to the reciprocating motion of the piston rings.

The amount of choke is usually between. The two most common methods used to provide a hard wearing surface are nitriding and chrome plating. With a choke bore cylinder. In most reciprocating engines. Chrome-plated cylinders have many advantages over both plain steel and nitrided cylinders. The depth of a nitrided surface depends on the length of time that the cylinder is exposed to the ammonia gas but a typical thickness is approximately 0.

The current causes microscopic surface cracks to open. Most manufacturers identify a nitrided cylinder by applying a band of blue paint around the cylinder base. To overcome this. At this temperature. Chromium is a hard. The process used to chrome-plate a cylinder is known as electroplating. Chrome-plating refers to a method of hardening a cylinder by applying a thin coating of chromium to the inside of the cylinder barrels.

A disadvantage of nitrided cylinders is that they do not hold oil for extended periods. Because nitriding is neither plating nor coating. This dimensional change requires a cylinder to be honed to an accurate. Another benefit of chrome-plating is that after a cylinder wears beyond its usable limits. This procedure is often referred to as chrome channeling.

The steel in the cylinder barrel contains a small percentage of aluminum. If an engine with nitrided cylinders is out of service for an extended period. A problem associated with chrome-plating is that. To identify a cylinder that has been chrome-plated. Although nickel is not as durable as chromium. In an effort to overcome the disadvantages of chrome-plated and nitrided cylinders. Instead of channeling. This is a result of the oil film on the cylinder wall preventing the necessary wear.

This tends to improve the wear. A unique characteristic of this process is that the silicon carbide particles are infused throughout the plating. Microcracks formed in chrome plating retain oil to aid in cylinder lubrication. This image is an enlarged photo-micrograph of the cylinder wall.

Engines with chrome-plated cylinders tend to consume slightly more oil than engines with nitrided or steel cylinders because the plating channels retain more oil than the piston rings can effectively scavenge. The silicon carbide provides a somewhat rough finish so it retains lubricating oil.

This process has been largely discontinued. Another plating process. A synthetic rubber seal is. Textron-Lycoming cylinders are typically painted with gray enamel that appears burned when exposed to excessive heat.

Cylinder heads also transfer heat away from the cylinder barrels. Cooling fins are cast or machined onto the outside of a cylinder head to transfer heat to the surrounding air. The inner shape of a cylinder head can be flat.

This special paint would change color when exposed to high temperatures. These inserts can be easily replaced if the threads become damaged. Teledyne Continental cylinders are treated with a gold paint that turns pink after an overheat event. Air-cooled cylinder heads are generally made of forged or die-cast aluminum alloy because it conducts heat well.

In addition. Gaskets are often used to seal between the cylinder and the intake and exhaust manifolds. The semispherical type is most widely used because it is stronger and provides for rapid and thorough scavenging of exhaust gases. After a cylinder head is cast. On older engines. Intake valves operate at lower temperatures than exhaust valves and are typically made of chrome.

VALVES Engine valves regulate the flow of gases into and out of a cylinder by opening and closing at the appropriate time during the Otto cycle. Some high-powered engines have two intake and two exhaust valves for each cylinder.

Threaded studs for attaching intake and exhaust manifolds typically remain in the cylinder. Because of the high temperatures associated with exhaust gases. The valves used in aircraft engine cylinders are subject to high temperatures. Each manifold is held in place by a nut secured to mounting studs or bolts threaded into the cylinder head.

The most. Because exhaust valves operate under much higher temperatures. Each cylinder has at least one intake valve and one exhaust valve. The flat-head valve is typically used only as an intake valve in aircraft engines. In some engines. The semi-tulip valve has a slightly concave area on its head while the tulip design has a deep.

The valve and corresponding seat are typically ground to an angle of between 30 and 60 degrees to form a tight seal. Mushroom valves have convex heads and are not commonly found on aircraft engines. The engine manufacturer specifies the exact angle to be ground. Aircraft engine valves are classified according to their head profile.

Poppet valves are classified according to their head shape.

Aircraft Handbooks & Manuals

The valve face creates a seal at its respective port. The basic components of a poppet valve include the valve head. The four basic designs are flat-headed.

On some radial engines. The valve stem keeps the valve head properly aligned as it opens and closes. A machined groove near the valve stem tip receives a split key.

Due to the up and down motion of the valve. This second groove is used to hold a safety circlet or spring ring. To help dissipate heat. Stellite resists high temperatures and corrosion and withstands the shock and wear associated with valve operation. Most valve stems are surface hardened to resist wear. After the Stellite is applied.

In some cases. The groove near the tip of a valve stem allows a split retainer key to hold spring tension on a valve as well as keep the valve from falling into the cylinder. The tip of a valve stem is also hardened to withstand both wear and hammering.

The sodium melts at approximately degrees Fahrenheit. Sodium is a dangerous material that burns violently when exposed to air. When overhauling an aircraft engine. Regardless of the engine. Some valves are filled with metallic sodium to reduce their operating temperatures. To accomplish this. During operation. Teledyne Continental engines do not use sodium filled valves. In all cases. Because of this.

To install a valve seat. When the assembly cools. The valve seat insert provides a sealing surface for the valve face while the valve guide supports the valve and keeps it aligned with the seat.

The angular difference 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 keeps the valve face aligned with the valve seat.

Valve guides are made from a variety of materials such as steel. A valve seat is a circular ring of hardened metal that provides a uniform sealing surface for the valve face.

After it is installed. A typical valve seat is made of either bronze or steel and machined to an oversize fit. Valve springs close the valve and are held in place by a valve retainer and a split valve key. Valve springs are helical-coiled springs that are installed in the cylinder head to provide the force that holds the valve face firmly against the valve seat. Most aircraft engines use two or more valve springs of different sizes and diameters to prevent a phenomenon called valve float or valve surge.

Valve float occurs when a valve spring vibrates at its resonant frequency. The valve springs are held in place by a valve spring retainer and a split valve key. An added safety benefit of this arrangement is that two or more springs reduce the possibility of failure due to a spring breaking from excessive temperature or metal fatigue.

The valve lifter. The retainer is fitted with a split valve key that locks the valve spring retainer to the valve stem. A valve spring retainer seat is usually located between the cylinder head and the bottom of the valve springs.

A typical valve operating mechanism includes an internally driven camshaft or cam ring that pushes against a valve lifter. When this occurs.

By installing two or more springs of differing sizes. A typical camshaft consists of a round shaft with a series of cams.

The raised lobe on a camshaft transforms the rotary motion of the camshaft to linear motion. Because cams are continuously moving across another metal surface. The camshaft is supported by a series of bearing journals that ride in a set of camshaft bosses.

The camshaft rotates at one-half of the crankshaft speed. In a four-stroke engine. The force used to rotate a camshaft comes from the crankshaft through a set of gears. These transform the rotational motion of the camshaft to the linear motion needed to actuate a valve. The typical valve operating mechanism includes a camshaft or cam ring.

The shape of a cam determines the distance that a valve is lifted off its seat and the length of time that the valve is open. Valve lifters in opposed engines can be solid or hydraulic. Holes drilled in the lifter enable oil to flow through the lifter to lubricate the push rod. Hydraulic lifters use oil pressure to cushion normal impact and remove play within the valve operating mechanism. Most opposed engines use hydraulic lifters. A solid lifter is a solid metal cylinder that directly transfers the lifting force from the camshaft to the push rod.The use of diesel fuel in reciprocating engines designed for aircraft is increasing.

To help prevent this. Supercharged G. As its name implies. A double-row radial engine may be thought of as two single-row radial engines sharing a common crankshaft. To determine the rotation speed of a given cam ring. The valves used in aircraft engine cylinders are subject to high temperatures. The number of crankshaft degrees that a valve opens before the piston reaches dead center is called valve lead.

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