V-Speeds Reference for Cessna P210 Pressurized Centurion

Section F Propeller Inspection, Maintenance, and Installation ... .. . 12 -57 CHAPTER 13 Powerplant and Propeller Airworthiness Inspections. 13-1 Section A Airworthiness Inspection Criteria ........ ........ ... ........ . .. ....... 13-2 CHAPTER 14 PowerplantTroubleshooting .......................................... 14-1 Section A Troubleshooti ng Principles .... .. ........ .. .... .......... ... . .. ....... .... 14 -2 Section B Reciprocating Engine Troubleshooti ng .... ... ...... ... ... ... ... .. .. 14-8 Section C Turbine Engine Troubleshooti ng ...... .. .......... . .. .. .. . ... ........ 14 -31 GLOSSERY ............................................ .. ................................................... G-1 ANSWERS ................................................................................................ .. A-1 INDEX ........................................................................................................... 1-1 1/t/i RECIPROCATING ENGINES INTRODUCTION The lack of efficient and practical powerplants has limited aircraft development throughout history . For exan1ple, in 1483 Leonardo daVinci conceived a flying machine he called the aerial screw. However , without a powerplant , the aeria l screw was never developed. In fact , the first patent for a heat engine was taken out in 1791 by John Barber. Un fortunately, Barber's engine was neither efficient nor practical. In 1860, Etienne Lenoir of France built the first practical piston engine. Lenoir's engine , which employed a battery ignition system and used natural gas for fue l, operated industrial machinery such as lathes. The next major breakthrough in piston engine deve lopment came in 1876 when Dr. August Otto developed the four-stroke, five-event cycle. The Otto cycle is still used in most modern reciprocating aircraft engines. DESIGN AND CONSTRUCTION Heat engines convert thermal energy into mechani- cal 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, cre- ating a force that moves a piston and in turn, the pis- ton 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. TYPES OF RECIPROCATING ENGINES Many types of reciprocating engines have been designed for aircraft since the Wright brothers made aviation history using a four-cylinder in -line engine. Reciprocating engines are commonly classified by cylinder arrangement (radial , in-line, V- type, or opposed) and by fuel type (gasoline or diesel). RADIAL ENGINES A radial engine consists of a row, or rows of cylin- ders arranged around a crankcase. The two basic types of radial engines are the rotary-type and the static-type. During World War I, rotary -t ype 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 th e propeller, while the crankshaft remains stationary. Some of the more popular rotary-type engines were the Bentley, the Gnome, and the LeRhone. [Figure 1-1] The large rotating mass of cylinders produced a sig- nificant amount of torque, which made aircraft con- trol difficult. This factor, coupled with complications in carburetion, lubrica t ion, and th e exhaust system, limited the development of the rotary-type radial engine. In the late 1920s, the Wright Aeronautical Corporation, in cooperation with the U.S. Navy , Figure 1- 1 . On rotary - type radial engines, the propeller and cylinders are bolted to the crankcase and rotate around a stationary crankshaft . developed a series of five-, seven -, and nine-cylin - der static -t y pe radial en g ine s. These engines were much more reliable than previous designs. Using these engines, Charles Lindbergh and other aviation pioneers completed l ong 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 28. 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. [Figure 1-2] Single-row radial engines typically have an odd number of cyl inders arranged around a crankcase. A typical configuration consists of five to nine even ly spaced cylinders with all pistons connected to a Reciprocating Engines Figure 1-2. Radial engines helped revolutionize aviation with their high power and dependability. single crankshaft. To increase engine power while maintaining a reasonably-sized frontal area, multi- ple-row radial engines were developed. These engines contain two or more rows of cylinders con- nected 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-4360, which consisted of 28 cylinders arranged in four staggered rows of seven cylinders each. The R-4360 developed a maximum 3,400 horsepower, making it the most 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 in aviation. Development and advancement in turbojet and turboprop engines eclipsed the performance of large multiple-row radial engines. 1-3 IN-UNE ENGINES In-line reciprocating engines generally have an even number of cylinders aligned in a single row parallel with the crankshaft. The pistons are either upright above or inverted below the crankshaft. This engine can be either liquid-cooled or air- cooled. [Figure 1-4] Courtesy of Austro Engine GmbH 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 be enclosed by stream- lined nacelles or cow lings. 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 clear- ance, 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 visibil- ity 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 engine receive relatively little cooling air, in- line engines are typically liquid-cooled or are lim- ited 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 2003, Thielert Aircraft Engines (now Centurion Aircraft Engines) began delivering new, certified kerosene-powered, in-line reciprocating engines for light aircraft. In 2009, Austro Engine certified a similar engine. 1-4 V-TYPE ENGINES In-line engines evolved into V-type engines. 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 horse- power than an in-line engine. Because the cylinder banks share a single crankcase and a single crank- shaft, 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. V-type engines can be either liquid- or air- cooled. V-12 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 experimen- tal racing aircraft. [Figure 1-5] OPPOSED-TYPE ENGINES Opposed-type engines are the most common recip- rocating engines currently used on light aircraft. Opposed engines can be designed to produce as lit- tle as 36 horsepower or as much as 400 horse- power. Opposed engines always have an even number of cylinders, with each cylinder on one side of a crankcase "opposing" a cylinder on the other side. 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, light- weight 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, Figure 1-5. V-type engines provide an excellent combination of weight and power with a small frontal area. Reciprocating Engines Figure 1-6. 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. resulting in less vibration than other engine types. [Figure 1-6] WANKEL ENGINES Although not a reciprocating engine, the Wankel (or rotary) engine deserves mention as an Otto cycle engine with potential for greater use in powered air- craft. 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 mov- ing parts contributes to increased reliability. Early designs had problems associated with sealing the combustion chamber, which affected efficiency and engine life. [Figure 1-7] © MISTRAL Engines, www.mistral-engines.com Figure 1-7. A Wankel engine uses an eccentric shaft to tum a triangular rotor in an oblong combustion chamber. Reciprocating Engines ENGINE COMPONENTS As an aviation maintenance technician, you must be familiar with an engine's components in order to understand its operating principles. Furthermore, your understanding of an engine's basic construc- tion enhances your ability to perform routine main- tenance 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 operating mechanism, crankshaft, and connecting rods are located in the crankcase. [Figure 1-8] For all of the reciprocating engine types discussed, the horizontally opposed and static-type radial INTAKE VALVE / PISTON CRANKSHAFT EXHAUST VALVE / / / /CRANKCASE CONNECTING ROD Figure 1-8. In a basic reciprocating engine, the cylinder forms a chamber where the fuel/air mixture is compressed and burned. The piston compresses the fuel mixture and transmits power to the crankshaft through the connecting rods. The intake valve allows the fuel/air mixture into the cylinder while the exhaust valve lets the exhaust gases out. 1-5 designs represent the majority of reciprocating engines in service today. Because of this, the discus- sion 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. CRANKCASE The crankcase is the core of a reciprocating engine. It contains the engine's internal parts and provides attach points for the cylinders, external acces- sories, and airframe installation. Additionally, the crankcase provides a tight enclosure for the lubri- cating 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 pro- vide the necessary strength and rigidity while reducing weight, most aircraft crankcases are made of cast aluminum alloys. OPPOSED ENGINE CRANKCASES A typical horizontally-opposed engine crankcase consists of two pieces of cast aluminum alloy manu- factured in sand castings or permanent molds. Crankcases manufactured by the permanent mold process, or permamold, as it is called by some man- ufacturers, are denser than those made by sand-cast- ing. 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 sup- porting the crankshaft, these webs add strength and form an integral part of the structure. [Figure 1-9] 1-6 Reciprocating Engines PROPELLER END TRANSVERSE WEB ACCESSORY END Figure 1-9. 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 com- pound. When the crankcase halves are bolted together with appropriate torque, the compound and thread form an effective oil seal without alter- ing 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 the size and type of engine. A typical radial engine crankcase separates into four main sections: nose, power, supercharger, and accessory. [Figure 1-10) 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 alu- minum 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 pro- peller reduction gear assembly. In addition, the nose Reciprocating Engines NOSE SECTION POWER SECTION SUPERCHARGER SECTION '-" ACCESSORY SECTION 1-7 Figure 1-10. The four sections of a radial engine crankcase are nose, power, supercharger, and accessory. 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-piece power section usually consists of a solid piece of aluminum alloy. Multipiece power sec- tions are typically manufactured from aluminum or magnesium and bolted together. The power sec- tion 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 gen- eral, 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 0-ring around the cylinder skirt. This 0-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 com- ponents. A supercharger is an engine device that compresses air for the engine's cylinders, enabling the engine to produce more power. The super- charger 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 pro- vide 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. ENGINE MOUNTING POINTS 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. 1-8 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. CRANKSHAFT The crankshaft receives a linear power pulse from the piston through the connecting rod and changes it to rotary motion to turn the propeller. Because crankshafts must withstand high stress, they are generally forged from a strong alloy such as chromium-nickel molybdenum steel. Some crank- shafts 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. [Figure 1-11] The centerline of a crankshaft runs through the cen- ter of the main bearing journals. These journals sup- port the crankshaft as it rotates. All crankshafts require at least two main journals to support the crankshaft, absorb the operational loads, and trans- mit 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 MAIN JOURNAL \ CRANKPIN MAIN JOURNAL I Figure 1-11. Every crankshaft has main bearing journals, one or more crankpins, and crank cheeks. Reciprocating Engines crankshaft to rotate. Like main journals, crankpins undergo a nitriding process to resist wear and pro- vide a suitable bearing surface. Crankshafts in most aviation engines are usually hollow to reduce weight. This also provides a pas- sage 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 oil from the hollow crankshaft to be sprayed onto the cylinder walls. On opposed engines, the number of crankpins cor- responds with the number of cylinders. The arrange- ment of the crankpins varies with the type of reciprocating engine, but all are designed to posi- tion each piston for smooth power generation as the crankshaft rotates. The relative distance between crankpins on a crankshaft is measured in degrees. [Figure 1-12] Crank cheeks, or crank arms, are required to con- nect crankpins to each other and to the main jour- nal 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 pas- sageways to permit oil to flow from the main jour- nal to the crankpin. CRANKSHAFT BALANCE Excessive engine vibration can cause metal struc- tures to fatigue and fail or wear excessively. An unbalanced crankshaft can cause excessive vibra- tion. To help minimize unwanted vibration, crank- shafts 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 consid- ered in dynamic balance when the centrifugal forces and power pulses are offset with counter- weights. (Crankshafts for smaller engines do not always use counterweights.) A dynamic damper is a counterweight that is fastened to a crankshaft's crank cheek assembly so that it can move back and Reciprocating Engines 1-9 2 3 2,3 Figure 1-12. On a four cylinder engine, crankpins one and four are 180 degrees apart from crankpins two and three. forth in a small arc. Some crankshafts use two or more of these assemblies, each attached to a differ- ent 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 counter- weight and crank cheek. The difference in diameter between the pins and the holes enables the dynamic damper to oscillate. [Figure 1-13] Each time a cylinder fires, force is transmitted to the crankshaft, causing it to flex. This happens hun- dreds of times every minute. Dynamic dampers oscillate, or swing, with every pulse from a firing cylinder to absorb some of this force. [Figure 1-14] figure 1-13. Movable counterweights act as dynamic dampers to reduce the centrifugal and impact vibrations in an aircraft engine. Figure 1-14. 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 oscilla- tion 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. CRANKSHAFT TYPES The type of crankshaft used on a particular engine depends on the number and arrangement of the engine's cylinders. The most common types of crankshafts are single-throw, two-throw, four-throw, and six-throw. The simplest crankshaft is the single- throw, or 360-degree crankshaft, used on single-row radial engines. A single-throw 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. One-piece crankshafts use a connecting rod that splits for 1-10 installation. A two-piece crankshaft uses a crankpin that separates to permit the use of a one-piece con- necting rod. [Figure 1-15] Twin-row radial engines require a two-throw crank- shaft, one throw for each bank of cylinders. The throws on a two-throw crankshaft are typically set 180 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 in- line engines use four-throw crankshafts. On some four-throw crankshafts, two throws are arranged 180 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. [Figure 1-16] CRANK CHEEK ONE-PIECE CRANKSHAFT TWO-PIECE CRANKSHAFT \__ COUNTERWEIGHT COUNTERWEIGHT __/ Figure 1-15. 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. Reciprocating Engines Six-cylinder opposed and in-line engines as well as 12-cylinder V-type engines use six-throw crank- shafts. A typical six-throw crankshaft is forged as one piece and consists of four main bearings and six throws that are 60 degrees apart. [Figure 1-17] Figure 1-16. A typical four-throw crankshaft from a four cylinder, opposed engine is machined from one piece of steel. 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 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 spe- cific 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 move- ment 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. [Figure 1-18] PLAIN BEARINGS Plain bearings are generally used as crankshaft main bearings, cam ring and camshaft bearings, connecting Reciprocating Engines 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 reciprocat- ing engines. Plain bearings are usually made of nonferrous met- als 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. One type of bushing that is used in aviation is the oil impregnated porous Oilite® bushing. With this type of bushing, the heat pro- duced by friction draws the impregnated oil to the bearing surface to provide lubrication during engine operation. BALL BEARINGS 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 hearing races pro- vide 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 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 as thrust bear- 1 11 ings in large radial and gas turbine engines. In appli- cations in which thrust loads are greater in one direction, a larger race is used on the side of the increased load. Most ball bearings that you encounter as a techni- cian are used in accessories such as magnetos, alter- nators, turbochargers, and vacuum pumps. Many of these bearings are prelubricated and sealed to pro- vide trouble-free operation between overhauls. However, if a sealed ball bearing must be serviced, you must use the proper tools to avoid damaging the bearing and its seals. ROLLER BEARINGS Roller bearings are similar in construction to ball bearings except that polished steel rollers are used instead of balls. The rollers provide a greater contact area and a corresponding increase in rolling friction over that of a ball bearing. Roller bearings are avail- able in many styles and sizes, but most aircraft engines either have 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 aircraft engines use straight roller bearings as crankshaft main bearings. Tapered roller bearings, on the other hand, have cone-shaped inner and outer races that enable the bearing to withstand both radial and thrust loads. CONNECTING RODS The connecting rod is the link that transmits the force exerted on the piston to the crankshaft. Most Fi~~re 1-18. The three most common types of bearings in reciprocating engines are plain, roller, and ball. Plain bearings rely on the shdmg movement of one metal against another; both roller and ball bearings use rolling movement. 1-12 connecting rods are made of a durable steel alloy; however, low-horsepower engines sometimes use aluminum. The weight of a connecting rod corre- sponds 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. Engine manufacturers strive to make connecting rods as light as possible, to reduce inertial forces, but still maintain their necessary strength. A typical con- necting rod is forged with a cross-sectional shape resembling an "H" or "I." There are also a few tubu- lar connecting rods. The crankpin end of the con- necting rod connects to the crankshaft and the piston end connects to the piston. The three major types of connecting rod assemblies are plain, mas- ter-and-articulated, and fork-and-blade. PLAIN CONNECTING RODS Plain connecting rods are used in opposed and in- line engines. The piston end of a plain connecting rod is fitted with a bronze bushing to accommodate the piston pin. The bushing is typically pressed into the connecting rod and reamed to a precise dimension to fit the piston pin. The crankpin end is usually fitted with a two-piece bearing, which is held in place by the cap and secured by bolts or by studs and nuts. The bearing inserts are typically steel lined with a nonferrous alloy such as Babbitt, lead, bronze, or copper. Connecting rods are often matched with pistons for balance and crankpins for fit. If a connecting rod is ever removed, it should be replaced in the same cylinder and relative position. Connecting rods and caps might be stamped to identify the correspond- ing cylinder and piston assembly. For example, a number "1" indicates the connecting rod and cap belong with the number 1 cylinder and piston assembly. [Figure 1-19] MASTER-AND-ARTICULATED ROD ASSEMBLY Radial engines use a master-and-articulated rod assembly to connect the pistons to the crankshaft. In this type of assembly, one piston in each row of cylinders is connected to the crankshaft by a master rod. The remaining pistons are connected to the master rod with 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 master rods and 16 articu- lating rods. Master rods are typically manufactured from a steel alloy forging that is machined and heat-treated for maximum strength. Articulated rods are constructed of a forged steel alloy with an I- or H- cross-sectional BRONZE BUSHING Reciprocating Engines SHANK] BEARING SHELLS ==,'JJ.JJ. LINED WITH ~ _ BEARING MATERIAL~8 CRIMP OR PINCH C! ,t_ CONNECTING ROD _;( BOLTS CAP PLAIN ROD Figure 1-19. The two piece bearing shell on a typical plain connecting rod fits tightly in the crankpin end of the con- necting rod. The bearing is held in place by pins or tangs that fit into slots cut into the cap and connecting rod. The piston end of the connecting rod contains a bushing that is pressed into place. profile. Bronze bushings are pressed into the bores in each end of the articulated rods. The master rod serves as the only link between all of the pistons and the crankpin. The piston end of a master rod contains the piston pin bearing. The crankpin end of a master rod contains the crankpin hearing (master rod bearing). A typical crankpin bearing must be able to withstand the radial loads placed on the rod assembly. A set of flange holes is machined around the crankpin end of a master rod to provide an attachment point for the articulated rods. A master rod can be one piece or multiple pieces. As a rule, a one-piece rod is used with a mul- tiple-piece crankshaft, while a multiple-piece (or split-type) master rod is used with a single-piece crankshaft. [Figure 1-20] Each articulated rod is hinged to the master rod by a knuckle pin. Some knuckle pins are 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 holes and articulated rods. In either type of installation, a lock plate on each side retains the knuckle pins and pre- vents lateral movement. [Figure 1-21] As the crankshaft rotates, the crankpin bearing is the only portion of a master rod assembly that travels in a true circle. Because the flange holes on a master rod are arranged around the crankpin, the knuckle pins travel in an elliptical path. [Figure 1-22] Reciprocating Engines Because of the varying angularity, not all pistons move an equal amount in each cylinder for a given number of degrees of crankshaft rotation. To com- pensate for this, the knuckle pin holes in the master rod flange are positioned at varying distances from the center of the crankpin. FORK-AND-BLADE ROD ASSEMBLY The fork-and-blade rod assembly used in V-type engines consists of a fork connecting rod and a blade connecting rod. The forked rod is split at the crankpin end to allow space for the blade rod to fit ONE PIECE MASTER ROD TWO PIECE MASTER ROD Figure 1-20. On a single piece master rod, the master-and- articulated rods are assembled and installed on the crankpin before the crankshaft sections are joined together. On a mul- tiple piece master rod, the crankpin end of the master rod and its bearing are split and installed on the crankpin. The bearing cap is then set in place and bolted to the master rod. 1-13 \_ ARTICULATING ROD Figure 1-21 . Articulated rods are attached to the master rod by knuckle pins. A knuckle pin lock plate retains the pins. Figure 1-22. Knuckle pins rotate in different elliptical paths. Each articulated rod has a varying degree of angularity rela- tive to the center of the crank throw. between the prongs. The fork-and-blade assembly is then fastened to a crankpin with a two-piece bear- ing. [Figure 1-23] PISTONS The piston in a reciprocating engine is a cylindrical plunger that moves up and down within a cylinder assembly. Pistons perform two primary functions; in conjunction with the valves, pistons manage the fuel, air, and exhaust pressures in the cylinder and they transmit the forc-e of combustion through the connecting rod to the crankshaft. 1-14 FORK ROD FORK AND BLADE ROD (V-ENGINES) Figure 1-23. 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. Aircraft engine pistons are typically machined from aluminum alloy or steel forgings. As many as six ring grooves are then machined into a piston's out- side surface to hold a set of piston rings. The por- tion of the piston between the ring grooves is commonly referred to as a ring land. The piston's top surface is called the piston head and is directly exposed to the heat and force of combustion. The piston pin boss is an enlarged area 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 align a piston in a cylinder, the piston base is extended to form the piston skirt. Some pistons have cooling fins cast into the underside of the piston skirt to provide for greater heat transfer to the engine oil. [Figure 1-24] Pistons are sometimes classified according to their head design. The most common types of piston heads are flat, recessed, cupped, and domed. The PISTON PIN BOSS PISTON PIN +PISTON SKIRT Figure 1-24. A typical piston has ring grooves cut into its out- side surface to support piston rings. Cooling fins are some- times cast into the piston interior to dissipate heat. The piston pin boss provides support for the piston pin. Reciprocating Engines three common types of piston skirts are trunk, trunk relieved at piston boss, and slipper. [Figure 1-25] All pistons expand when they heat up. Due to the added mass at the piston boss, more expansion occurs parallel to the piston boss than perpendicu- lar to it. This uneven expansion can cause a piston to take on an oval shape at normal engine operating temperatures, which results in uneven wear of the piston and cylinder. One way to compensate for this is to use a cam-ground piston. A cam-ground piston is machined with a slightly oval shape, such that the diameter of the piston parallel to the piston boss is slightly less than the diameter perpendicular to the piston boss. This compensates for differential expansion and produces a round piston at normal operating temperatures. Furthermore, the oval shape holds the piston centered in the cylinder dur- ing engine warm-up and prevents the piston from moving laterally within a cylinder. [Figure 1-26] PISTON RINGS Piston rings perform three functions. They prevent pressure leakage from the combustion chamber, con- trol oil seepage into the combustion chamber, and transfer heat from the piston to the cylinder walls. Piston rings are spring-loaded and press against the cylinder 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, the ring is machined to the FLAT HEAD CUPPED HEAD FLAT HEAD WITH VALVE RECESSED DOMED HEAD Figure 1-25. The majority of modem aircraft engines use flat- head pistons; however, other designs are still in service. Reciprocating Engines ~4.500" ~ Figure 1-26. 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. desired cross-section, and then split for installation in a piston ring groove. The point where a piston ring is split is called the piston ring gap. The gap can be a simple butt joint with flat faces, an angle joint with angled faces, or a step joint. [Figure 1-27] As an engine reaches operating temperature, piston rings expand. To accommodate expansion, piston rings need a gap. If the gap is too large, the two faces will not come together and provide an ade- quate seal. If the gap is too small, the ring faces will bind against each other and the cylinder wall resulting in scoring damage to the cylinder wall. Ring gaps must be staggered, or offset to create the best seal, which prevents combustion gases from leaking past the rings into the crankcase. This blow-by, as it is often called, results in a loss of power and increased oil consumption. ===:J c= BUTT JOINT ""::::7 ~ ANGLE JOINT Figure 1-27. Of the three types of joints used in piston ring gaps, the butt joint is the most common in aircraft engines. 1-15 To form an effective seal, the rings must exert equal pressure around the entire cylinder wall and pro- vide a gas-tight fit against the sides of the ring grooves. New piston rings require some wear-in dur- ing engine operation so that the ring contour matches the cylinder wall. A ring that matches its cylinder is considered to be seated. The two main types of piston rings used in reciprocating engines are compression rings and oil rings. [Figure 1-28] 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 of compression rings used on each piston is determined by the engine manu- facturer. Two or three compression rings on each piston is common. The cross section of a compres- sion ring can be rectangular, wedge shaped, or tapered. Because compression rings receive limited lubrication and are closest to the heat of combus- tion, they are more prone to sticking. 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. Tapered rings have a beveled face to reduce contact area, which reduces friction and hastens ring seating. Wedge-shaped rings also have a beveled face to promote rapid ring seating. Because the profile is wedge shaped, the piston ring grooves must also be beveled. Less material is cut away, so piston ring lands and grooves are stronger. The wedge shape also helps prevent a ring from sticking in a groove. [Figure 1-29] Oil rings control the amount of oil applied to the cylinder walls and prevent oil from entering the combustion chamber. The two types of oil rings rCOMPRESSION RINGS Oil RINGS Figure 1-28. Compression rings are installed in the upper piston ring grooves to help prevent the combustion gases from escaping. Oil rings,, on the other hand, are installed near the middle and bottom of a piston to control the amount of oil applied to the cylinder wall. 1-16 - RECTANGULAR - TAPERED - WEDGE Figure 1-29. Compression rings can have three different ring cross sections. The tapered face presents the narrowest bearing edge to the cylinder wall to reduce friction and accelerate ring seating. that are found on most engines are oil control rings and oil scraper rings. Oil control rings are placed in the piston ring grooves below the compression rings. Pistons can have one or more oil control rings. On some pistons, as many as two rings can be installed in a single ring groove. The primary purpose of oil control rings is to regulate the thick- ness of the oil film on a cylinder wall. An oil con- trol ring returns excess oil to the crankcase through small holes drilled in the piston ring grooves. Additionally, some pistons use ventilated oil con- trol rings with small slots machined around the ring. These slots enable excess oil to return to the engine sump through small holes drilled in the pis- ton ring groove. If excessive oil enters the combustion chamber, it will burn and leave a coating of carbon on the com- bustion chamber walls, piston head, spark plugs, and valves. Carbon buildup on the ring grooves or valve guides can cause parts to stick. Carbon buildup can also cause spark plugs misfiring, cylin- der preignition or detonation, and excessive oil con- sumption. To help prevent this, an oil scraper ring regulates the amount of oil that passes between the piston skirt and the cylinder wall. An oil scraper ring, sometimes called an oil wiper ring, usually has a beveled face and is installed in a ring groove at the bottom of the piston skirt. The ring can be installed with the beveled edge away from the piston head or in the reverse position. If the bevel is 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 upward stroke, the scraper ring retains Reciprocating Engines surplus oil above the ring. On the downward stroke, oil is returned to the crankcase by the oil control rings and piston ring grooves. It is very important that these rings are installed in accordance with the manufacturer's instructions. [Figure 1-30] PISTON PINS A piston pin joins the piston to the connecting rod. Piston pins are tubular, and are machined from a case-hardened, nickel-steel alloy forging. Piston pins are sometimes called wrist pins because the motion of the piston and the connecting rod is simi- lar to a human wrist. Piston pins can be stationary, semifloating, or full- floating. Stationary piston pins are secured to the piston by a setscrew that prevents rotation. Semifloating piston pins are loosely attached to the connecting rod by clamping around a reduced- diameter section of the pin. Full-floating piston pins rotate freely in both the connecting rod and the piston; these pins are used in most modern air- craft engines. A full-floating piston pin must be held in place lat- erally to prevent it from rubbing and scoring the cylinder walls. Three devices that are used to hold a piston pin in place are circlets, spring rings, and metal plugs. A circlet is similar to a snap ring that fits into a groove cut into each end of the piston boss. A spring ring also fits into grooves cut into the ends of a piston boss, but it consists of a single cir- cular spring-steel coil. Both circlets 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. These plugs are inserted into the open ends of the piston pins to pro- vide a good bearing surface against the cylinder walls. Due to the plug's soft aluminum construction CYLINDER WALL / ~PISTON Figure 1-30. 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. However, if the beveled edge faces the cylinder head, the ring scrapes oil toward the crankcase when the piston moves down. Reciprocating Engines and cylinder lubrication, the metal-to-metal contact causes no damage to the cylinder walls. CYLINDERS The cylinder is the combustion chamber where the burning and expansion of gases takes place to pro- duce engine power. Furthermore, a cylinder houses the piston and connecting rod assembly along with the valves and spark plugs. When designing and constructing a cylinder, manufacturers must con- sider several factors. A cylinder must be strong enough to withstand the internal pressures devel- oped during engine operation yet be lightweight to minimize engine weight. Additionally, the materials used in the construction of a cylinder must have good heat-conducting properties for efficient cool- ing. Finally, a cylinder assembly must be relatively simple and cost-effective to manufacture, inspect, and maintain. A typical air-cooled engine cylinder consists of a cylinder head, barrel, mounting flange, skirt, cool- ing fins, and valve assembly. On some of the earliest two- and four-cylinder horizontally opposed engines, the cylinder barrels were cast as part of the crankcase halves. This required the use of remov- able cylinder heads. However, on almost all modern engines, individual cylinders are cast as a compo- nent, separate from the 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 a chilled cylinder barrel. As the head cools, it con- tracts, and as the barrel warms, it expands, resulting in a gas-tight joint. [Figure 1-31] CYLINDER BARRELS The material used to construct a cylinder barrel must be as light as possible, yet have the proper character- istics for operating at high temperatures and pres- sures. 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 steel alloy such as chromium-molybdenum steel or nickel chromium-molybdenum steel. Cylinder barrels are machined from a forged blank, with a skirt that projects into the crankcase 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 engines typically have extended cylinder skirts. The longer skirt helps keep oil from draining into the combus- tion chamber and causing hydraulic lock after an engine has been shut down. The exterior of a cylin- 1-17 Figure 1-31. The cylinder assembly, the piston assembly, connecting rods, crankshaft, and crankcase constitute the power section of a reciprocating engine. der 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. 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 bores are 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 and is designed to compensate for the uneven expansion caused by the higher operating temperatures and larger mass near the cylinder head. With a choke bore cylinder, the greater expansion at the top of the cylinder is compensated for by the taper, resulting in a uniform cylinder diameter at normal operating temperatures. The amount of choke is usually between .003 and .005 inches. [Figure 1-32] The inside wall of a cylinder barrel is continuously subjected to the reciprocating motion of the piston rings. Therefore, in an effort to minimize cylinder barrel wear and increase barrel life, most cylinder walls are hardened. The two most common methods used 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 by infusing the metal with a hardening agent. During the nitriding process, a cylinder barrel is first ground to the 1-18 -----5.245---- -----5.25----"- ,JrCAST ALUMINUM CYLINDER HEAD y-STEEL CYLINDER BARREL Figure 1-32. In most reciprocating engines, the greater mass of the cylinder head retains heat and expands, 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 the diameter at the bottom of the cylinder which helps compensate for the uneven expansion. required size and smoothness and then placed in a special furnace filled with ammonia gas. The fur- nace heats a cylinder barrel to approximately 1,000 degrees Fahrenheit. At this temperature, the ammo- nia gas breaks down into nitrogen and hydrogen. The steel in the cylinder barrel contains a small per- centage of aluminum, which combines with the nitrogen to form a layer of hard, wear-resistant alu- minum nitrides. 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 0.020 inch. However, the surface hardness gradually decreases with depth until the hardness is the same as the core metal. Because nitriding is neither plating nor coating, it changes a cylinder bore by only two to four ten thou- sandths of an inch. This dimensional change requires a cylinder to be honed to an accurate, micro-smooth finish after the nitriding process is complete. Most manufacturers identify a nitrided cylinder by applying a band of blue paint around the cylinder base, or to certain cooling fins. A disadvantage of nitrided cylinders is that they do not hold oil for extended periods. This increases a cylinder's susceptibility to corrosion. If an engine with nitrided cylinders is out of service for an Reciprocating Engines extended period, the cylinder walls should be coated with sticky preservative oil. Chrome-plating refers to a method of hardening a cylinder by applying a thin coating of chromium to the inside of the cylinder barrels. Chromium is a hard, natural element with a high melting point, high heat conductivity, and a very low coefficient of friction. The process used to chrome-plate a cylin- der is known as electroplating. Chrome-plated cylinders have many advantages over both plain steel and nitrided cylinders. For example, chromed cylinders are less susceptible to rust or corrosion because of chromium's natural corrosion resistance. Therefore, chromed cylinders tend to wear longer. Another benefit of chrome- plating is that after a cylinder wears beyond its usable limits, it can be chrome-plated back to its original size. To identify a cylinder that has been chrome-plated, a band of orange paint is sometimes applied around the cylinder base or to some of the cooling fins. A problem associated with chrome-plating is that, in its natural state, chromium is so smooth 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. The current causes microscopic sur- face cracks to open, forming an interconnected net- work of cracks to retain oil on the cylinder wall. This procedure is often referred to as chrome chan- neling. [Figure 1-33] Engines with chrome-plated cylinders tend to con- sume slightly more oil than engines with nitrided or steel cylinders because the plating channels retain more oil than the piston rings can effectively scav- enge. Furthermore, chrome-plated cylinders are typ- ically 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 neces- sary wear, or seating, of the piston rings during the break-in period. In an effort to overcome the disadvantages of chrome-plated and nitrided cylinders, manufactur- ers have developed some new plating processes. Instead of channeling, one of these processes involves mechanically impregnating silicon carbide particles into a chromed cylinder wall. The silicon carbide provides a somewhat rough finish so it retains lubricating oil, yet is smooth enough to enable effective oil scavenging. Furthermore, the sil- icon carbide provides a surface finish that is more conducive to piston ring seating during the engine break-in period. This plating process is commonly Reciprocating Engines Figure 1-33. Microcracks formed in chrome plating retain oil to aid in cylinder lubrication. This image is an enlarged pho- tomicrograph of the cylinder wall. referred to as either CermiCrome® or Nu-Chrome® plating. This process has been largely discontinued. Another plating process, variously called CermiNil™, Nickel+Carbide™, or Nikasil®, uses nickel with silicon carbide particles as the plating material. Although nickel is not as durable as chromium, it provides for an extremely hard finish while the silicon carbide particles increase the hardness of the material and aid in retaining lubri- cating oil. A unique characteristic of this process is that the silicon carbide particles are infused 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 cylinder barrels to protect the cylinder from corrosion. This special paint would change color when exposed to high tem- peratures, indicating a possible overheat condi- tion that might have damaged the cylinders. Textron-Lycoming cylinders are typically painted with gray enamel that appears burned when exposed to excessive heat. Similarly, Teledyne Continental cylinders are treated with a gold paint that turns pink after an overheat event. CYLINDER HEADS The cylinder head covers the cylinder barrel to form the enclosed chamber for combustion. In addition, cylinder heads contain intake and exhaust valve ports, spark plugs, and valve actuating mechanisms. 1-19 Cylinder heads also transfer heat away from the cylinder barrels. Air-cooled cylinder heads are gen- erally made of forged or die-cast aluminum alloy because it conducts heat well, is lightweight, and is durable. The inner shape of a cylinder head can be flat, semispherical, or peaked. The semispherical type is most widely used because it is stronger and provides for rapid and thorough scavenging of exhaust gases. Cooling fins are cast or machined onto the outside of a cylinder head to transfer heat to the surround- ing air. However, due to the temperature differences across the cylinder head, it is necessary to provide more cooling-fin area on various sections. For exam- ple, because the exhaust valve region is typically the hottest part of the internal surface, that portion of the cylinder head has more fin area. The intake por- tion of the cylinder head typically 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 system redundancy. On older engines, spark plug openings consisting of bronze or steel bushings were shrunk and screwed into the cylinder head. However, most modern engines use stainless steel Heli-Coil® inserts. These inserts can be easily replaced if the threads become damaged. Intake and exhaust ports are machined into each cylinder head to enable the fuel/air mixture to enter the cylinder and the exhaust gases to exit. Gaskets are often used to seal between the cylinder and the intake and exhaust manifolds. A synthetic rubber seal is typically used for attaching the intake mani- fold. Because of the high temperatures associated 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 into the cylinder head. [Figure 1-34] VALVES Engine valves regulate the flow of gases into and out of a cylinder by opening and closing at the appro- priate time during the Otto cycle. Each cylinder has at least one intake valve and one exhaust valve. The intake valve controls the amount of fuel/air mixture that enters through the intake port, and the exhaust valve lets the exhaust gases exit the cylinder through the exhaust port. Some high-powered engines have two intake and two exhaust valves for each cylinder. 1-20 Figure 1-34. Threaded studs for attaching intake and exhaust manifolds typically remain in the cylinder. The valves used in aircraft engine cylinders are sub- ject to high temperatures, corrosion, and extreme operating stresses. Therefore, valves must be designed and constructed for durability. Intake valves operate at lower temperatures than exhaust valves and are typically made of chrome, nickel, or tungsten steel. Because exhaust valves operate under much higher temperatures, they are usually made of materials with greater heat resistance such as Inconel® silicon-chromium or cobalt-chromium alloys. The most common type of valve used in air- craft engines is the poppet valve, which 'pops' open and closed during normal operation. [Figure 1-35] Poppet valves are classified according to their head shape. The four basic designs are flat-headed, semi- tulip, tulip, and mushroom. The flat-head valve is E,STEM ~ /VALVE NECK VALVE FACE ~ Figure 1-35. The basic components of a poppet valve include the valve head, valve face, valve neck, valve stem, and valve tip. Reciprocating Engines typically used only as an intake valve in aircraft engines. The semi-tulip valve has a slightly concave area 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] 1L lllL !FLAT HEAD SEMI-TULIP HEAD TULIP HEAD MUSHROOM HEAD 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 seat are typically ground to an angle of between 30 and 60 degrees to form a tight seal. In some engines, the intake valve face is ground to 30 degrees and the exhaust valve is ground to 45 degrees. The engine manufacturer specifies the exact angle to be ground based on air- flow, efficiency, and sealing ability. Valve faces are often made more durable by welding Stellite®, an alloy of cobalt and chromium, to the valve face. After the Stellite is applied, the face is ground to the correct angle. Stellite resists high temperatures and corrosion and withstands the shock and wear asso- ciated with valve operation. The valve stem keeps the valve head properly aligned as it opens and closes. Most valve stems are surface hardened to resist wear. The tip of a valve stem is also hardened to withstand both wear and hammering. In some cases, a rotator cap is placed over the valve tip to increase service life. A machined groove near the valve stem tip receives a split key, or keeper key, that keeps the valve-spring retaining washers in place and holds the valve in the cylinder head. [Figure 1-3 7] On some radial engines, the valve stems have an additional groove below the split key groove. This second groove is used to hold a safety circlet or spring ring, preventing the valve 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 partially filled with metallic sodium. The sodium melts at approximately 208 degrees Fahrenheit. Due to the up and down motion of the valve, the melted sodium circulates and trans- fers heat from the valve head into the stem where it Reciprocating Engines Figure 1-37. 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. is dissipated through the cylinder head. In some cases, sodium-filled valves can reduce valve operat- ing temperature by as much as 400 degrees Fahrenheit. [Figure 1-38] When overhauling an aircraft engine, you must determine whether the old valves are sodium-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 fol- low the manufacturer's recommendations and instructions for handling and installing the valves. 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 a manner that would expose the sodium to air. In all cases, sodium valves must be disposed of in an appropriate manner. 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 the cylinder head. 1-21 VALVE SEATING COMPONENTS A valve's face must seat firmly against the cylinder head. To accomplish this, several individual compo- nents work together, including valve seats, valve guides, valve springs, and valve spring retainers. [Figure 1-39] A valve seat is a circular ring of hardened metal that provides a uniform sealing surface for the valve face. A typical valve seat is made of either bronze or steel and machined to an oversize fit. To install a valve seat, the cylinder head is heated and the valve seat is chilled and then pressed into the head with a spe- cial 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 pre- cisely ground to provide a sealing surface for the 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 is from one-quarter to one full degree shallower than the valve seat. The angular differ- ence 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 stern and keeps the valve face aligned with the valve seat. Valve guides are made from a variety of materials such as steel, tin-bronze, VALVE SPRING RETAINER LOWER VALVE SPRING RETAINER VALVE GUIDE VALVE SEAT INSERT Figure 1-39. The valve seat insert provides a sealing surface for the valve face while the valve guide supports the vaive and keeps it aligned with the seat. Valve springs close the valve and are held in place by a valve retainer and a split valve key. 1-22 or aluminum-bronze and are installed in the cylin- der head with a shrink fit in the same manner as valve seats. 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 dif- ferent 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. When this occurs, a spring loses its ability to hold a valve closed. By installing two or more springs of dif- fering sizes, one spring is always free to close the valve. An added safety benefit of this arrangement is that two or more springs reduce the possibility of fail- ure due to a spring breaking from excessive tempera- ture or metal fatigue. The valve springs are held in place by a valve spring retainer and a split valve key. A valve spring retainer seat is usually located between the cylinder head and the bottom of the valve springs, while a valve spring retainer is installed on the top of the valve springs. The retainer is fitted with a split valve key that locks the valve spring retainer to the valve stem. VAL VE OPERATING MECHANISMS Reciprocating engines require a valve operating mechanism to open each valve at the correct time, hold it open, and then close it. A typical valve oper- ating mechanism includes an internally driven camshaft or cam ring that pushes against a valve lifter. The valve 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. [Figure 1-40] OPPOSED ENGINES On an opposed engine, valve operation is controlled with a camshaft. A typical camshaft consists of a round shaft with a series of cams, or lobes. These transform the rotational motion of the camshaft to the linear motion needed to actuate a valve. 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. Because cams are continuously mov- ing across another metal surface, lobes are hardened to resist wear. [Figure 1-41] The camshaft is supported by a series of bearing journals that ride in a set of camshaft bosses, which are cast into the crankcase. The force used to rotate a camshaft comes from the crankshaft through a set of gears. The camshaft rotates at one-half of the Reciprocating Engines Figure 1-40. The typical valve operating mechanism includes a camshaft (or cam ring), a tappet (or lifter), a push rod, and a rocker arm. crankshaft speed. In a four-stroke engine, each cylin- der fires once for every two crankshaft rotations. Therefore, each valve should open and close only once for every two rotations of the crankshaft. [Figure 1-42] 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 engines can be solid or hydraulic. A solid lifter is a solid metal cylinder that directly transfers the lifting force from the camshaft to the push rod. The cam follower face of a solid lifter is flat with a polished surface, while the push rod end contains a spherical cavity that houses the push rod. Holes drilled in the lifter enable oil to flow through the lifter to lubricate the push rod. Most opposed engines use hydraulic lifters. Hydraulic lifters use oil pressure to cushion nor- mal impact and remove play within the valve oper- ating mechanism. A typical hydraulic lifter consists of a cam follower face, a lifter body, a hydraulic plunger and spring, a check valve, and a push rod socket. The entire lifter assembly floats in a machined hole in the crankcase and rests on the camshaft. [Figure 1-43] The cam follower face is the smooth, hardened sur- face of the lifter that contacts the lobe. When the fol- lower face is on the back side of a lobe, the hydraulic plunger spring forces the hydraulic plunger out- ward so that the push rod socket presses firmly against the push rod. As the hydraulic plunger moves outward, a ball check valve moves off its seat to let oil flow from the oil supply chamber to the oil Reciprocating Engines 1-23 Figure 1-41 . The raised lobe on a camshaft transforms the rotary motion of the camshaft to linear motion. CRANKSHAFT Figure 1-42. In the typical opposed engine, the camshaft tim- ing gear has twice as many teeth as the crankshaft gear. The camshaft rotates at one-half of the crankshaft's speed. pressure chamber. As the camshaft rotates and the front side of the lobe contacts the follower face, the lifter body and cylinder move outward. This action causes the check valve to seat, trapping oil in the oil pressure chamber. This trapped oil acts as a cushion that dampens the abrupt pressure applied to the push rod. After the valve is lifted off its seat, oil leaks between the plunger and the cylinder to com- pensate for any dimensional changes caused by operation. After the valve closes, the ball check valve is unseated and oil flows from the supply chamber to the pressure chamber in preparation for another cycle. A second type of hydraulic lifter is similar in construction to the lifter just discussed, except that a disk check valve is used instead of a ball check valve. [Figure 1-44] The lifting force of the lobe is transmitted through a lifter and a push rod. A typical push rod is a hollow steel or aluminum-alloy tube with polished ends. One end of the push rod rides in the valve lifter socket while the other end fits into a socket in the rocker arm. Push rods typically have holes drilled in each end to let oil flow from the valve lifter to the valve components in the cylinder head. On most air- craft reciprocating engines, the push rods are enclosed by a thin metal shroud, or tube, that runs from the cylinder head to the crankcase. In many cases, these tubes also provide a return path for the oil that is pumped up to the cylinder head. A rocker arm is a pivoting lever in the cylinder head that changes the lifting movement of the push rod into the downward motion needed to open a valve. A typical rocker arm is made of forged steel and has a cup-shaped socket to hold the push rod end and a polished surface that pushes against the valve tip. [Figure 1-45] The entire rocker arm pivots on a shaft that is sus- pended between two rocker arm bosses cast into the cylinder head. Each rocker arm boss contains a bronze bushing that provides a bearing surface for the shaft. The rocker arm shaft is installed with a light press fit and held in place the rocket-box cover or by covers inserted over the outside of each rocker arm boss. When the push rod pivots the 1-24 Reciprocating Engines PLUNGER SMALL CHECK VALVE 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. L CAM FOLLOWER FACE Figure 1-44. Some hydraulic lifters uses a disk-type check valve instead of a ball-type. rocker arm, the rocker arm exerts force against the valve springs to open the valve. [Figure 1-46] Some engines use a newer style rocker arm that is forged out of a single piece of stainless steel and rotates on a pressed-in roller bearing. Additionally, the valve end of the rocker arm is fitted with a roller. This roller helps eliminate side loads when the valve 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 mechanisms as opposed engines, but with some significant differences. For example, in place of a camshaft, a radial engine uses cam rings; the number of rings is the same as the number of cylinder rows. A cam ring is a circular piece of steel with a series of raised lobes on its outer edge. A cam ring for a typical seven-cylinder engine has three or four lobes while a cam ring in a nine-cylinder engine has four or five lobes. The lobes in a radial engine differ from those in an opposed engine in that each lobe is constructed with a cam ramp on each side of the lobe. This ramp LPUSH ROD SOCKET SMOOTH FACE TO DEPRESS VALVE------' Figure 1-45. One end of this rocker arm is cup-shaped to hold a push rod, while the other end is machined smooth to push against the tip of a valve stem. When rotated by the push rod, the rocker arm pivots on its center bushing to depresses a valve. Reciprocating Engines Figure 1-46. A rocker arm is supported by a shaft suspended between a set of rocker arm bosses. reduces the initial shock of an abruptly rising lobe. The smooth area between the lobes is called the cam track. On a single row radial engine a single cam ring with two cam tracks is used. One track operates the intake valve while the second track operates the exhaust valve. In a single-row radial engine, the cam ring is usually located between the propeller reduction gearing and the front end of the power section. In a twin-row radial engine, a second cam for the valves in the rear row is installed between the rear end of the power section and the supercharger section. The cam ring is mounted concentrically with the crankshaft and is driven by the crankshaft through a series of gears. However, unlike a traditional camshaft which rotates at half the speed of a crank- shaft, the rotational speed of a cam ring varies due to size and gearing. To determine the rotation speed of a given cam ring, you must know the num- ber of lobes on the cam ring, the cam ring's direc- tion of rotation relative to the crankshaft, and the number of cylinders on the engine. The direction of cam ring rotation varies on different engines and depends on whether the cam ring has internal or external drive teeth. Externally driven cam rings turn in the same direction as the crankshaft, while internally driven rings turn opposite from crank- shaft rotation. [Figure 1-4 7] If a table is not available, determine cam ring speed by using the formula: Cam Ring Speed = ____l _____ Number of Lobes x 2 In place of a cam follower face, a radial engine uses cam rollers. A cam roller consists of a small wheel 1-25 5 Cylinders 7 Cylinders 9 Cylinders ... II) ... /JI "'Cl ,_ /JI Direction (I) (I) "'Cl Ql Ql (1) (I) "'Cl of .!l J:l (I) .!l .!l (IJ .!l .a (1) EO (I) EO (!) EO (I) Rotation ::; ...I 0.. ::; ...I 0. ::; ...I 0.. Zo en Zo r,J) Zo U) 3 1/6 4 1/8 5 i/10 With Crankshaft 2 1/4 3 1/6 4 1/8 Opposite Crankshaft Figure 1-4 7. This chart identifies cam ring speed for various radial engine configurations. that rolls along the cam track. When the cam roller rides over a lobe on 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 per- forms the same function as an opposed engine push rod. Radial engine rocker arms have adjusting screws and lock screws that enable you to adjust the push rod-to-rocker arm clearance. In addition, many radial engine rocker arms are equipped with rollers on their valve ends to reduce friction, eliminate side loading on the valve stem, and reduce tip deforma- tion. [Figure 1-48] VAL VE CLEARANCE ADJUSTMENT Valve clearance describes the space between the tip of the valve stem and the rocker arm face. For an engine to run properly, the correct valve clear- ance must be maintained. During normal engine operating temperatures, the cylinder assemblies expand and force the cylinder head, along with its valve operating components, further away from the crankcase. 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 valve clearance is not con- trolled, the engine will run poorly, and valve dam- age might result. An engine manufacturer's maintenance manual specifies either a cold or hot valve clearance. As its name implies, a cold clearance is set when the engine is cold. Due to the expansion properties dis- cussed earlier, this clearance is typically less than the hot or running clearance, which is set when the engine is hot. Engines that require valve adjust- ments have adjustment screws and locknuts mounted in their rocker arms at the push rod fitting. Engines that use hydraulic lifters do not require valve adjustments because they automatically maintain a zero running valve clearance during nor- mal operation. For this reason, hydraulic lifters are 1-26 Figure 1-48. The valve operating mechanism for a radial engine performs the same functions as one on an opposed engine. often called zero clearance, or zero lash lifters. However, hydraulic lifters must operate within a specific clearance range when the hydraulic lifter is not filled with oil, or "dry." During cylinder replacement, you must check that the dry-lifter clearance is within specified limits. To perform a dry-lifter clearance check, the lifter body must first have all residual oil removed. The procedure to "bleed down" the lifter varies between engine manufacturers, but usually consists of depressing the check valve to drain all trapped oil. After the oil is drained, check the valve operating mechanism for the proper rocker arm face-to-valve tip clearance using the same procedure employed on engines with solid lifters. Some large radial engines incorporate a floating cam ring that requires a special procedure when Reciprocating Engines adjusting valve clearance. To obtain the correct adjustment, the cam ring must be seated to elimi- nate cam bearing clearance. This usually involves depressing two valves to seat the earn ring, which then enables you to accurately measure a third valve. This procedure is repeated for each cylinder. CYLINDER NUMBERING You will often need to refer to a specific area on an engine or to a specific cylinder. Therefore, you should be familiar with the manufacturer's particu- lar system of cylinder numbering. Regardless of how an engine is mounted in an aircraft, the propeller shaft end is always referred to as the front of an engine, and the accessory end is always the rear of an engine. Furthermore, when referring to either the right or left side of an engine, always assume you are viewing the engine from the rear, or accessory, end. Similarly, crankshaft rotation is always refer- enced from the rear of an engine and is specified as either clockwise or counterclockwise, To identify a specific cylinder, all engine cylinders are numbered. However, opposed engines do not use a standard numbering system. Teledyne Continental Motors and Textron-Lycoming both manufacture four- and six-cylinder horizontally opposed engines, but each company uses a different numbering system. For example, Continental begins its cylinder numbering with the rearward cylinder while Lycoming begins with the forward cylinder, Both companies place the odd numbered cylinders on the right and the even numbered cylinders on the left. [Figure 1-49] In general, single-row radial engine cylinders are numbered consecutively in a clockwise 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 the number one cylinder. The number three cylinder is the next cylinder clockwise from the number two cylinder but is in the rear row. Some radial engines designed in east- ern European countries reverse this pattern. [Figure 1-50] PROPELLER REDUCTION GEARS The amount of power produced by an aircraft recip- rocating engine is determiried by several factors, including the amount of pressure exerted on the pis- tons during each power stroke and the number of Reciprocating Engines CONTINENTAL FOUR-CYLINDER ENGINE LYCOMING FOUR-CYLINDER ENGINE CONTINENTAL SIX-CYLINDER ENGINE LYCOMING SIX-CYLINDER ENGINE LYCOMING EIGHT-CYLINDER ENGINE Figure 1-49. Cylinder numbering varies by manufacturer; always refer to the appropriate service information to deter- mine how the cylinders of a specific engine are numbered. power strokes completed in a given time period. As a rule, the faster an engine turns the more power it produces. However, this rule does not apply to pro- pellers. As a propeller blade tip approaches the speed of sound, it cannot efficiently convert the engine's power into thrust. In other words, a pro- peller needs 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 rated power output while maintaining a slower propeller speed. Reduction gears permit the propeller to turn slower than the 1-27 crankshaft. Reduction gear systems currently installed on aircraft engines use spur gears, plane- tary gears, or a combination of the two. Spur gears have teeth cut straight across their cir- cumference and can be either external or internal. The simplest type of reduction gearing consists of two external tooth spur gears, one small gear on an engine crankshaft and one larger gear on the pro- peller shaft. When configured this way, the amount of reduction is based primarily on the size of the propeller shaft gear. The larger the gear, the slower the propeller turns. However, this reduction system has some disadvantages. For example, when using two external tooth spur gears, the propeller turns opposite the crankshaft. Furthermore, because the SINGLE-ROW RADIAL 6 DOUBLE-ROW ENGINE Figure 1-50. Looking from the accessory end forward, all single-row radial engines are numbered consecutively begin- ning 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. 1-28 propeller shaft is off-center from the engine crank- shaft, the propeller acts as a gyroscope applying high torsion loads to the engine case. As a result, the crankcase must be built stronger and heavier to withstand these loads. [Figure 1-51] -+- CRANKSHAFT DRIVE GEAR Figure 1-51. The ratio of the gear teeth in a gear reduction system with two externally-driven spur gears determines the amount of reduction. For example, if a drive gear has 25 teeth and the driven gear 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 to use an internal- tooth spur gear on the propeller shaft and an exter- nal-tooth spur gear on the crankshaft. In addition to allowing the propeller to turn in the same direction as the engine, this arrangement aligns the propeller shaft more closely with the crankshaft, which elimi- nates much of the stress placed on the crankcase. [Figure 1-52] CRANKSHAFT DRIVE GEAR PROPELLER SHAFT DRIVEN GEAR Figure 1-52. A gear reduction system with one internal-tooth gear and one external-tooth gear turns the propeller and crankshaft in the same direction and is more closely aligned than a reduction system with two external-tooth gears. Whenever a reduction gear does not keep the pro- peller shaft perfectly aligned with the crankshaft, Reciprocating Engines additional vibration is induced into an engine. To help minimize this vibration. some engines use a quill shaft between the crankshaft and propeller shaft. A quill shaft is a hardened steel shaft that is splined on both ends and installed between two gears, or shafts, to absorb torsional vibration. One end of the quill shaft fits into the front end of the crankshaft, and the opposite end is inserted into the front end of the propeller drive shaft. With this arrangement, the quill shaft drives the propeller and absorbs vibration from the gear reduction mecha- nism. [Figure 1-53] PROPELLER SHAFT ~OPELLER SHAFT DRIVE GEAR CRANKSHAFT Figure 1-53. A quill shaft mm1m1zes torsional vibration between a propeller shaft and the crankshaft. In a planetary reduction gear system the propeller shaft is attached to a housing that contains several small gears called planetary gears. The planetary gears rotate between a sun gear and a ring gear (sometimes called a bell gear). The crankshaft dri- ves either the sun gear or the ring gear depending on the individual installation. The planetary gear reduction system keeps the propeller shaft aligned with the crankshaft, transmits power with a mini- mum of weight and space, and keeps the propeller's direction of rotation the same as the engine. Planetary gears are used on some horizontally opposed engines as well as radial and turboprop engines. [Figure 1-54] You can determine the reduction rate that a particu- lar gearing arrangement achieves by this formula: G R t . Teeth On Ring Gear + Teeth On Sun Gear ear a 10 =------"'------------ Teeth On Ring Gear Reciprocating Engines RING GEAR ~~=~VEN BY ~NKSHAFT SPIDER SUN GEAR FASTENED TO CRANKCASE PLANETARY GEAR Figure 1-54. In a planetary gear reduction system, the pro- peller is attached to the planetary gear spider and the crank- shaft turns either the sun gear or the ring gear. For example, if there are 72 teeth on the ring gear and 36 teeth on the sun gear, the propeller turns at a ratio of 1.5 to 1. However, reduction ratios are tra- ditionally expressed in whole numbers, so this example is expressed as a 3 to 2 reduction. In other words, the crankshaft must turn three revolutions for every two revolutions of the propeller shaft. Neither the number of teeth on the planetary gears nor the number of planetary gears contributes to the computation for gear reduction. PROPELLER SHAFTS All aircraft reciprocating engines have a propeller shaft. As an aviation technician, you must be famil- iar with the various types of propeller shafts, includ- ing tapered, splined, and flanged shafts. Tapered propeller shafts were used on most of the early, low-powered engines. On a tapered propeller shaft, the shaft diameter tapers toward the end of the shaft. To prevent a propeller hub from rotating on a tapered shaft, one or more key slots are milled into the shaft. In addition, the end of the shaft is threaded to receive a propeller retaining nut. [Figure 1-55] High-powered engines require a stronger method of attaching propellers. Most high powered radial engines use splined propeller shafts. A spline is a rectangular groove that is machined into the pro- peller shaft. Most splined shafts have a master spline that is approximately twice the size of any 1-29 Figure 1-55. A tapered propeller shaft changes in diameter along its length and uses a metal key to keep a propeller from rotating. other spline. This master spline assures that a pro- peller is attached to a propeller shaft a specific way so that vibration is kept to a minimum. [Figure 1-56] Modern horizontally opposed aircraft engines use a flanged propeller shaft. The crankshaft is forged with a flat flange on its end. A propeller is bolted directly to the flange. To provide additional support for the propeller, most flanged propeller shafts incorporate a short shaft forward of the flange and a series of studs around the flange circumference. [Figure 1-57] Figure 1-56. All splined propeller shafts are identified by an SAE number. For example, SAE 50 identifies a splined shaft that meets SAE design specifications for a 50 size shaft. The SAE number does not refer to the number of splines. Figure 1-57. Before' you remove a propeller from a flanged shaft, temporarily mark the propeller hub and flange. This makes it easier to position the propeller when you reattach it. 1-30 ENGINE IDENTIFICATION Almost all reciprocating engines are identified by a series of letters and numbers that indicate the type and size of the engine. For simplicity, most manu- facturers use the same identification system. In most cases, an engine identification code consists of a let- ter or series of letters followed by a number and model designation. The first letters indicate an engine's cylinder arrangement and basic configura- tion. The following list indicates several of the let- ters used as well as their meanings: 0 - Horizontally opposed engine R - Radial engine - In-line engine V - V-type engine T - Turbocharged I - Fuel injected S - Supercharged G - Geared nose section (propeller reduction gearing) L - Left-hand rotation (for multi-engine installations) H - Horizontal mounting (for helicopters) V - Vertical mounting (for helicopters) A - Modified for aerobatics Reciprocating Engines The numbers in an engine identification code indi- cate an engine's piston displacement in cubic inches. For example, an 0-320 indicates a horizon- tally opposed engine with a displacement of 320 cubic inches. Some engine identification codes include a letter designation after the displacement to indicate a model change or modification to a basic engine. Check with the manufacturer's specification sheets to interpret these letters correctly because their meaning differs among manufacturers. Consider an engine with the following identification code: LI0-360-C. This code designates an engine that has left hand rotation, is fuel-injected and hori- zontally opposed, displaces 360 cubic inches, and is a C model. Similarly, a GTSI0-520-F engine is an F- model version of a geared, turbo-supercharged, fuel- injected, horizontally opposed engine that displaces 520 cubic inches. Reciprocating Engines 1-31 SUMMARY CHECKLIST ,I Reciprocating engines operate according to the Otto cycle. ,I The most common type of reciprocating engine used for light aircraft has opposed cylinders . ./ The major parts of a reciprocating engine are the crankcase, cylinders, pistons, connecting rods, valves, valve-operating mechanism, and crankshaft . .I The major sections of a radial engine are nose, power, supercharger, and accessory. ./ The surface finish of cylinder walls is critical for a proper seal between the engine crankcase and cylinder combustion chamber. ,/ A valve face and seat are machined for a tight seal. ,/ A camshaft controls valve operation in an opposed engine. KEY TERMS rotary-type radial engines static-type radial engines single-row radial engines multiple-row radial engines double-row radial engines V-type engines cylinder pad nose section power section supercharger section accessory section main bearing journals crankpins throws crank throws connecting-rod bearing journals sludge counterweight static balance dynamic balance dynamic damper single-throw crankshaft two-throw crankshaft four-throw crankshaft six-throw crankshaft bushings bearing retainer bearing races straight roller bearings tapered roller bearings crankpin end piston end