CHAPTER 1 Reciprocating Engines

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RECIPROCATING ENGINES

INTRODUCTION



DESIGN AND CONSTRUCTION

TYPES OF RECIPROCATINGENGINES

RADIAL ENGINES

I, rotary-type radialengines

Figure 1-1. On rotary-type radial engines, the propeller andcylinders are physically bolted to the crankcase and rotatearound the stationary crankshaft.

static-type radial engines.



Reciprocating Engines 1-3

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

single-row radial engine

multiple-rowradial engines

Figure 1-3. The Pratt and Whitney R-4360 engine was thelargest practical radial engine used in aviation. However,

the advent of both the turbojet and turboprop engines hasall but eliminated the usefulness of large multiple-rowradial engines on modern aircraft designs.

double-row radialengine

IN-LINE ENGINES

Figure 1-4. A popular version of the in-line engine consistedof cylinders that were inverted. A typical in-line engine con-sists of four to six cylinders and develops anywhere from 90to 200 horsepower.



1-4 Reciprocating Engines

V-TYPE ENGINES

V-type engine.

inverted V-typeengine.

OPPOSED-TYPE ENGINES

PorscheFlugmotoren (PFM) 3200

power lever.

Figure 1-5. V-type engines provide an excellent combina-tion of weight, power, and small frontal area.

Figure 1-6. A horizontally opposed engine combines a goodpower-to-weight ratio with a relatively small frontal area.These engines power most light aircraft in use today.




ENGINE COMPONENTS

Figure 1-7. In a basic reciprocating engine, the cylinderforms a chamber where the fuel/air mixture is compressedand burned. The piston, on the other hand, compressesthe fuel mixture and transmits the power produced bycombustion to the crankshaft through the connecting

rods. The intake valve allows the fuel/air mixture into thecylinder while the exhaust valve lets the exhaust gases outof the cylinder.

CRANKCASE

OPPOSED ENGINE CRANKCASES

cylinderpad




Figure 1-8. In addition to the transverse webs that support the main bearings, a set of camshaft boss es are typically cast into a

crankcase. These bosses support the camshaft which is part of the valve operating mechanism.

oil galleries

RADIAL ENGINE CRANKCASES



Reciprocating Engines -7

nose section

power section

cylinder pads.

supercharger section

accessory section

Figure 1-9. The four basic sections o f a radial engine crankcase are the nose section, power section, supercharger section, and

accessory section.




ENGINE MOUNTING POINTS

mounting lugs,

CRANKSHAFTS

cranks, throws,

Figure 1-10. All crankshafts consist of a main bearing jour-

nal, one or more crankpins, and several crank cheeks.

main bearing journals, main journals,

Crankpins,

sludge chamber,

考題




Figure 1-11. On a four cylinder engine, the number one and four throws are 180 degrees apart from the number two andthree throws.

crank cheeks, crank arms,

counterweight

dynamic damper

CRANKSHAFT BALANCE

statically balanced

Dynamic balance

Figure 1-12. Movable counterweights serve( 為了要reduce torsional vibrations並不是dynamic vibrations)FAA考題 as dynamic dampers to reduce the centrifugal andimpact vibrations in an aircraft engine..




Figure 1-13. Think of the crankshaft as a pendulum thatswings at its natural frequency once a force is applied. Thegreater the force, the greater the distance the pendulumswings. However, if a second pendulum is suspended fromthe first and a force is applied, the second pendulum beginsto oscillate opposite the applied force. This opposite oscilla-tion dampens the oscillation of the first pendulum leaving itnearly stationary. You can think of a dynamic damper as ashort pendulum hung from a crankshaft that is tuned to thefrequency of the power impulses.

CRANKSHAFT TYPES

single-throw 360degree

Figure 1-14. With a one-piece, single-throw crankshaft, theentire crankshaft is cast as one solid piece. However, with aclamp type two-piece crankshaft, the two pieces are heldtogether by a bolt that passes through the crankpin.

two-throw crank-

shaft,

four-throw crankshafts.




Figure 1-15. A typical four-throw crankshaft used in a hori-zontally opposed engine is machined as one piece withthrows that are 180 degrees apart.

Figure 1-16. With a typical six-throw crankshaft, the throwsare 60 degrees apart. On the six throw crankshaft picturedabove, the crank journals are numbered from the flanged end.If you were to number each throw in 60 increments from theflanged end, the order would be 1,4,5, 2,3,6.

BEARINGS

six-throw crankshafts.

PLAIN BEARINGS

Figure 1-17. Of the three most common types of bearings used in reciprocating engines, the plain bearing relies on the slidingmovement of one metal against another, while both roller and ball bearings have one surface roll over another.




bushings.

BALL BEARINGS

bearing retainer.

bearingraces

ROLLER BEARINGS

Straight roller bearings Tapered roller bearings,

CONNECTING RODS

crankpin end, piston end.

PLAIN CONNECTING ROD




Figure 1-18. On a typical plain connecting rod, a two piece

bearing shell fits tightly i n the crankpin end of the connect-

ing 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 pl ace.

MASTER-AND-ARTICULATED ROD ASSEMBLY master rod.

articulated rods.

piston pin bearing crankpin bearing, master rod bearing.

Figure 1-19. With 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 the other hand, with a multiple

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.

splittype

knuckle pin.




Figure 1-20. Articulated rods are attached to the master rod

by knuckle pins, that are pressed into holes in the master

rod flanges during assembly. A knuckle pin lock plate is

then installed to retain the pins.

full floating knuckle pins

Figure 1-21. You can see that each knuckle pin rotates in a

different elliptical path. As a result, each articulated rod hasa varying degree of angularity relative to the center of the

crank throw.

Figure 1-22. A fork -and-blade rod assembly used in a V-type

engine consists of a blade connecting rod whos e crankpin

end fits between the prongs of t he fork connecting rod.

FORK-AND-BLADE ROD ASSEMBLY

fork connectingrod blade connecting rod.

PISTONS

Ring grooves ringland. pistonhead

piston pin boss




Figure 1-23. A typical piston has ring grooves cut into its

outside surface to support piston rings. In addition, cooling

fins are sometimes cast into the piston i nterior to help dis-

sipate heat, while the piston pin boss pr ovides support forthe piston pin.

piston pin piston skirt.

Figure 1-24. Most modern airc raft engines use f lat-head pis -tons. However, as an aviation technician, you should be

familiar with all pist on head designs.

Figure 1-25. Several engines now use cam ground pistons

to compensate for the greater expansion parallel to the pis-

ton boss during engine operation. The diameter of a cam

ground piston measures several thousandths of an inchlarger perpendicular to t he piston boss than parallel to the

piston boss.

cam ground piston.

PISTON RINGS




Figure 1-26. Of the three types of joints used in piston ringgaps, the butt joint is the most common in aircraft engines.

piston ring gap.

blow-by,

Figure 1-27. Compression rings are installed in the upperring grooves and help prevent the combustion gases fromescaping by a piston. Oil rings, on the other hand, areinstalled near the middle and bottom of a piston and con-trol the amount of oil applied to the cylinder wall.

Figure 1-28. Of the three different ring cross sections, thetapered face presents the narrowest bearing edge to thecylinder wall to help reduce friction and hasten ring seating.




Oil Oil control rings

venti-lated-type oil control rings

oil scraper ring

oil wiperring,

Figure 1-29. An oil scraper ring installed with its beveled

edge away from the cylinder head forces oil upward along

the cylinder wall w hen the piston moves upward. However,

if the beveled edge is facing the cylinder head, the ring

scrapes oil downward to the crankcase when the piston

moves down.

PISTON PINS

wrist pins

stationarypiston pins Semifloating piston pins, Full-floating

circlet spring ring

piston-pin plug.




CYLINDERS

CYLINDER BARRELS

skirt mounting flange

cooling fins

cylinder bore,

choke borecylinder

Nitriding

Figure 1-30. The cylinder assembly along with the pistonassembly, connecting rods, crankshaft, and crankcase con-stitute the power section of a reciprocating engine.




Figure 1-31. In most reciprocating engines, the greater massof the cylinder head retains heat and expands thereby caus-ing the upper portion of the cylinder to expand more thanthe lower portion. However, with a choke-bored cylinder,the diameter at the top of the cylinder is less than the diam-eter at the bottom of the cylinder which helps compensate

for the uneven expansion.

Chrome plating

electroplating.

chrome channeling.

Figure 1-32. This figure illustrates a reproduction of a pho-

tomicrograph of the tiny cracks that form in chrome platingonce a reverse current is applied. These cracks retain oil andthus aid in lubrication.




CermiCrome NuChrome

CermiNil,

CYLINDER FINISHES

CYLINDER HEADS

Heli-Coil




Figure 1-33. The threaded studs us ed to attach the intake

and exhaust manifolds typically remain threaded into the

cylinder head unless a stud needs to be replaced.

CONTINENTAL FOUR-CYLINDER

ENGINE

CONTINENTAL SIX-CYLINDER

ENGINE

CYLINDER NUMBERING

LYCOMING EIGHT-CYLINDER

ENGINE

Figure 1-34. Since the cylinder numbering varies f rom m an-ufacturer to manufacturer, you should always refer to the

appropriate service information or the numbers indicated

on each cylinder flange to determine how the cylinders on a

specific engine are numbered.

LYCOMING FOUR-CYLINDER

ENGINE

LYCOMING SIX-CYLINDER

ENGINE




Figure 1-35. Looking from the accessory end forward, all sin-gle-row radial engines are numbered consecutively begin-ning at the top cylinder and progressing clockwise. Ontwin-row radials, however, the front row of cylinders are alleven numbered while the rear row of cylinders are odd num-bered.

VALVES

intake valve exhaustvalve.

Figure 1-36. The basic components of a poppet valveinclude the valve head, valve face, valve neck, valve stem,and valve tip.

poppet valve

flat-head valve semi-tulip tulip mushroom

valve seat




Figure 1-37. Aircraft engine valves are classified according

to their head profile.

Stellite,

valve stem valve tip

rotatorcap

split key keeper key

safety circlet spring ring,

metallic sodium.

VALVE SEATING COMPONENTS

Figure 1-39. Some valves are fill ed with m etallic sod ium to

reduce their operating temperatures. During operation, thesodium melts and transfers heat to the valve stem where

the heat i s conduc ted away by the cyli nder head.

Figure 1-38. 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.




valve seat

valve guide

Figure 1-40. The valve seat insert provides a sealing surface

for the valve face while the valve guide supports the valve

and keeps i t aligned wit h the seat. Valve springs close thevalve and are held in place by a valve retainer and a split

valve key.

Valve springs

valve float valve surge.

valve springretainer

VALVE OPERATING MECHANISMS

Figure 1-41. The components in a typical valve operating

mechanism, include a camshaft or cam ring, a tappet or

lifter, a push rod, and a rocker arm.




Figure 1-42. The raised lobe on a camshaft transforms the rotary motion of the camshaft to linear motion.

OPPOSED ENGINES

camshaft. lobes,

valve lifter, tappet, solid lifter

hydrauliclifters.

Figure 1-43. In a typical opposed engine, the camshaft tim-ing gear has twice as many teeth as the gear on the crank-shaft. In this configuration, the camshaft is driven atone-half the crankshaft's rotational speed.




Figure 1-44. A typical hydraulic lifter consists of a push rod socket, a hydraulic plunger and plunger spring, a check valve, a lifter

body, and a cam follower face.

cam follower face

plunger spring hydraulicplunger push rod socket

ball checkvalve oil supply chamber oil pressure chamber.

push rod.

Figure 1-45. A second type of hydraulic lifter uses a

disk-type check valve instead of a ball check valve.




rocker arm

rocker arm bosses

Figure 1-46. One end of this rocker arm is cup-shaped tohold a push rod, while the other end is machined smooth to

push against the tip of a valve stem. When rotated by thepush rod, the rocker arm pivots on its center bushing anddepresses the valve.

Figure 1-47. A rocker arm is supported by a shaft that is sus-pended between a set of rocker arm bosses.

RADIAL ENGINES

cam rings,

camramp cam track.




Figure 1-48. Assume that you want to know how fast thecam turns on a certain nine cylinder engine. If the cam ringhas four lobes and rotates opposite the crankshaft, the camring turns at 1/8 the crankshaft speed.

cam rollers.

tappet tappet guide.

rollers

VALVE CLEARANCE ADJUSTMENT

Valve clearance

Figure 1-49. A radial engine valve operating mechanism per-forms the same functions as the mechanism used in anopposed engine.

cold clearance

hot

running clearance,




zero clearance, zero lash lifters.

floating camring

PROPELLER REDUCTION GEARS

spur gears

Figure 1-50. With a gear reduction system that uses two

externally driven spur gears, the amount of reduction is

determined by the ratio of the gear teeth. For example, if

the 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.




Figure 1-51. With a gear reduction system that uses oneinternal-tooth gear and one external-tooth gear the pro-peller and crankshaft turn in the same direction and aremore closely aligned.

quill shaft

plane-

tary reduction gear. planetary gears.

sun gear ring bell

Figure 1-52. A quill shaft minimizes torsional vibrationbetween the propeller shaft and the crankshaft.

Figure 1-53. 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.



Reciproca ting Engines 1-31

PROPELLER SHAFTS

Tapered propeller shafts

Figure 1-54. As you can see, a tapered propeller shaftchanges in diameter along its length and utilizes a metalkey to keep a propeller from rotating.

splined propellershafts.

Figure 1-55. All splined propeller shafts are identified by an

SAE number. For example, SAE 50 identifies a splined shaftthat meets SAE design specifications for a 50 size shaft. TheSAE number does not refer to the actual number of splines.

Figure 1-56. Before you remove a propeller from a flangedpropeller shaft, it is a good practice to mark the propellerhub and flange. This makes reattaching the propeller eas-ier since you can identify how the propeller should be posi-tioned.

flanged propeller shaft.

ENGINE IDENTIFICATION






OPERATING PRINCIPLES

ENERGY TRANSFORMATION

heat engine

internal combustion

external combustion.

Intake

Compression

Ignition

Exhaust

ENERGY TRANSFORMATIONCYCLES

cycle




Figure 1-57. One stroke is equivalent to the distance a pis-ton head travels between bottom dead center and top deadcenter. In all reciprocating engines, one complete strokeoccurs with each 180 degrees of crankshaft rotation.

four-stroke, Otto Otto, two-stroke cycle.

stroke

top dead center (TDC) bottom dead center (BDC).

FOUR-STROKE CYCLE

Figure 1-58. The four strokes that take place in a four-stroke.Otto cycle include the intake, compression, power, andexhaust.

INTAKE STROKE




COMPRESSION STROKE

POWER STROKE

EXHAUST STROKE

VALVE TIMING

valve lead.




valvelag. valve overlap

Figure 1-59. To read this diagram, begin at the inside of thespiral. Notice that the intake valve opens 15 degrees beforetop center on the exhaust stroke while the exhaust valveremains open 10 degrees into the intake stroke. As you fol-low the spiral into the compression stroke, notice that theintake valve closes 60 degrees past bottom center on the

compression stroke. At this point, both valves are closedand ignition takes place at 30 degrees before top center onthe compression stroke. As the cycle proceeds, the exhaustvalve opens 60 degrees before bottom center on the powerstroke and remains open throughout the exhaust stroke.The intake valve, on the other hand, opens 15 degrees priorto top center on the exhaust stroke.

duration,

and




Figure 1-60. The circle divided into even segments repre-

sents the travel of a crankpin whi le the vertical line above

the circl e represents the path of a pisto n. To represent the

connecting rod length, equal length lines are drawn fromeach segment li ne to the pisto n path. From the figure, you

can see that a piston moves m ore per degree of travel when

the piston is near top center than when it is near bottom

center.

FIRING ORDER

Figure 1-61. Notice that the firing pattern and cyli nder num-

bering method varies between engine manufacturers and

engine models.




A

TWO-STROKE CYCLE

Figure 1-62. In a two-stroke engine, the piston controls the flow of gases into and out of the cylinder through the intake andexhaust ports. This eliminates the need for either an intake or exhaust valve and their associated operating mechanisms. This sim-plifies a two-stroke engine's construction and minimizes weight.




WORK-POWER CONSIDERATIONS

foot-pounds.

joule. newton

POWER

WORK

HORSEPOWER




INDICATED HORSEPOWER

indicatedmean effective pressure

P Indicated Mean Effective Pressure, IMEP

FRICTION HORSEPOWER

fric-tion horsepower

BRAKE HORSEPOWER




torque,

Prony brakedynamometer,

2K

Figure 1-63. In the example above, the 3-foot arm of the

prony brake is exerting a force of 200 pounds on the scale.

This results in a torque of 600 foot-pounds.

electric or hydraulicdynamometer.

PISTON DISPLACEMENT

Figure 1-64. With a dynamometer, the power produced byan engine is used to drive an electrical generator or fluid

pump so the power output can be accurately measured.




For example, one cylinder of a four-cylinder aircraftengine has a bore, or diameter, of four inches. Whatis the area of the piston head?

Given:

Bore = 4 inches

Area = 7tr 2

= 3.14X2

2

= 12.56 square inches

Once the area of one piston is known, total pistondisplacement is calculated with the formula:

Total Piston Displacement = A x L x N

Where: : . ..,-..■■-.

A = area of piston head in square inches L =length of the stroke in inches N = numberof cylinders

Using the example presented earlier, determine thetotal displacement if each of the four cylinders has astroke of six inches.

Given:

Area = 12.56 square inchesStroke = 6 inches Numberof cylinders = 4

Piston Displacement = A x L x N; =12.56X6X4

= 301.44 cubic inches

The total engine displacement is 301.44 cubicinches. Since the amount of work done by theexpanding gases is determined in part by the pistonarea and the piston stroke, it should be evident thatincreasing either the cylinder bore or the pistonstroke increases piston displacement.

ENGINE EFFICIENCY

Energy is the capacity for doing work and cannot be

created or destroyed. However, energy can be trans-formed from potential, or stored energy into kineticenergy. Aircraft reciprocating engines transform the

potential, or chemical energy stored in fuel into heatenergy during the combustion process. The heatenergy is then converted to kinetic energy by

mechanical means. Engine design and construction,fuel type, and environmental conditions all play a

part in how efficiently an engine converts a fuel's potential energy. To determine how efficient anengine is, several factors must be examined, includ-ing an engine's thermal, volumetric, and mechani-cal efficiency.

THERMAL EFFICIENCY

An engine's thermal efficiency (TE) is a ratio of theamount of heat energy converted to useful work tothe amount of heat energy contained in the fuel used

to support combustion. In other words, thermal effi-ciency is a measure of the inefficiencies experiencedwhen converting the heat energy in fuel to work. Forexample, consider two engines that produce thesame amount of horsepower, but consume differentamounts of fuel. The engine using less fuel convertsa greater portion of the available energy into usefulwork and, therefore, has a higher thermal efficiency.Thermal efficiency is found by the formula:

, , .,,,. . Horsepowerx 33,000Thermal Efficiency = ----- F x BTTJ x K -------------

Where:

Horsepower = An engine's brake or indicated horsepower 33,000 = Number of foot-pounds ofwork per

minute in one horsepower F = Weight offuel burned per minute BTU = Heat value ofthe fuel burned measured in BTU's

K = Constant representing the number offoot-pounds of work each BTU iscapable of doing in one second.

Thermal efficiency can be calculated using either brake or indicated horsepower. If brake horsepoweris used, the result is brake thermal efficiency (BTE),and if indicated horsepower is used, you get indi-cated thermal efficiency (ITE).

The constant, 33,000, is the number of foot-poundsof work per minute in one horsepower. Therefore,when horsepower is multiplied by 33,000, the out-

put of an engine in foot-pounds per minute results.

Almost all engine performance data relating to fuelconsumption is expressed in terms of gallons per

hour. Therefore, you must be able to convert gallons

your previous study of mathematics and the discus-sion on indicated horsepower (PLANK), the area ofa circle is calculated with the formula:




per hour to pounds per minute. For example, theweight of 100LL aviation gasoline is six pounds pergallon. If a particular engine burns 10 gallons per

hour, you must multiply the gallons consumed perhour by six pounds and divide the product by 60,the number of minutes per hour. The resulting fuel

burn is one pound per minute (10 x 6 -f- 60 = 1).

In the English system of measurement, the relation-ship between heat and work is the British ThermalUnit, or BTU, of heat energy. Each pound of aviationgasoline contains 20,000 BTU's of heat energy,therefore, the number 20,000 is typically used in theformula for determining thermal efficiency.

By multiplying the pounds per minute of fuel an

engine burns by 20,000, you get the total number ofBTU's, or total heat energy that is produced in agiven engine. One BTU is capable of doing 778 foot-

pounds of work. Therefore, when you multiply thetotal number of BTU's by the constant 778, both thetop and bottom of the formula produce a productthat is in foot-pounds.

Based on the information just presented, the for-mula used to calculate thermal efficiency can besimplified to read:

. , rr . . Horsepowerx 33,000Thermal efficiency = ---------------------------

To check your understanding of this formula, deter-mine the brake thermal efficiency of a piston enginethat produces 150 brake horsepower while burning8 gallons of aviation gasoline per hour.

12,448,000

.398 39.8

percent

Most reciprocating engines are between 30 and 40 percent efficient. The remaining heat is lost through

the exhaust gases, the cooling system, and the fric-tion within the engine. In fact, of the total heat pro-duced in a reciprocating engine, 30 to 40 percent is



utilized for power output; 15 to 20 percent is lost incooling; 5 to 10 percent is lost in overcoming fric-tion of moving parts; and 40 to 45 percent is lostthrough the exhaust.

VOLUMETRIC EFFICIENCY

Volumetric efficiency (VE) is the ratio of the volumeof fuel and air an engine takes into its cylinders tothe total piston displacement. For example, if anengine draws in a volume of fuel and air that isexactly equal to the engine's total piston displace-ment, volumetric efficiency would be 100 percent.By the same token, if an engine draws in 288 cubicinches of fuel and air and has a total piston dis-

placement of 320 cubic inches, the volumetric effi-ciency would be 90 percent.

Because the density of the air drawn into an enginevaries with changes in atmospheric conditions, theonly way to accurately calculate volumetric effi-ciency is to correct for nonstandard temperature and

pressure. If you recall from your earlier studies,standard temperature and pressure at sea level is59蚌 (15蚓) and 29.92 inches of mercury (1013.2millibars) respectively. Based on this, the formulafor determining volumetric efficiency is:

The volumetric efficiency of most normally aspi-rated engines is less than 100 percent. The reasonfor this is because bends, surface roughness, andobstructions inside the induction system slow theflow of air which, in turn, reduces the air pressure

within the manifold. On the other hand,tur-bocharged engines compress the air before itenters the cylinders, and often have volumetricefficiencies greater than 100 percent.

Anything that decreases the density, or volume ofair entering a cylinder decreases volumetric effi-ciency. Some of the typical factors that affect volu-metric efficiency of a non-turbocharged engineinclude:

1. Part throttle operation This restricts the volume of air that flows into the cylinders.

2. Long, small diameter, intake pipes As air flowsthrough an induction system, friction slows the airflow, causing a decrease in air density. The amount offriction created is directly proportional to the lengthof the intake pipes and inversely proportional to theircross-sectional area. In other words, long, smalldiameter intake pipes create the most friction whileshort, large diameter intake pipes create less friction.




3. Induction systems with sharp bends Eachtimeintake air turns a corner in an induction system, air

flow slows and less air enters the cylinders.4. High carburetor air temperatures As the tem

perature of the intake air increases, air densitydecreases. A lower air density means less air entersthe cylinders. ■ /■■. > c . .■ u-

5. High cylinder head temperatures As the cylinder heads and corresponding combustion chambersheat up, air density in the cylinders decreases andvolumetric efficiency decreases.

6. Incomplete scavenging If the valve overlap inan engine is incorrect, exhaust gases will displace

some of the incoming fuel/air mixture. When thishappens, less fuel and air is drawn into the cylinders and a lower volumetric efficiency results.

7. Improper valve timing If the intake valve doesnot remain open long enough to allow a completecharge of fuel and air to enter a cylinder, volumetricefficiency drops.

8. Increases in altitude As an aircraft climbs,ambient air pressure drops and airdensitydecreases. As an engine draws the "thin" air into itscylinders, its volumetric efficiency drops.

This problem can be overcome, to a certain degree, byturbocharging an engine. Turbocharging increasesthe induction air pressure above atmospheric pressure which, in turn, increases the density of thefuel/air charge entering the cylinders.

MECHANICAL EFFICIENCY

Mechanical efficiency is the ratio of brake horsepower to indicated horsepower and represents the percentage of power developed in the cylinders thatreaches the propeller shaft. For example, if an

engine develops 160 brake horsepower and 180indicated horsepower, the ratio of brake horsepowerto indicated horsepower is 160:180, which repre-sents a mechanical efficiency of 89 percent. Sinceaircraft engines are mechanically efficient, it is notunusual for ninety percent of the indicated horse-

power to be converted into brake horsepower.

The factor that has the greatest effect on mechanicalefficiency is the friction within the engine itself. Thefriction between moving parts in an engine remainsrelatively consistent throughout an engine's speedrange. Therefore, the mechanical efficiency of an

engine is highest when the engine is running at anrpm that maximum brake horsepower is developed.

FACTORS AFFECTING POWER

According to the general gas law which combinesBoyle's Law and Charles' Law, a definite relation-

ship between gas volume, temperature, and pressureexists. That relationship is the reason why the inter-nal combustion process must be precisely con-

trolled for an engine to produce power efficiently.

MANIFOLD PRESSURE

Changes in manifold air pressure affect the amountof power an engine can produce for a given rpm.Manifold air pressure, or manifold absolute pressure(MAP) readings are monitored by a gauge and pro-vide a means of selecting power settings. Absolute

pressure is the pressure above a complete vacuumindicated in inches of mercury (in. Hg.) or pounds

per square inch absolute (psia). MAP gauges indi-cate absolute pressure of the fuel/air mixture at a

point just outside a cylinder intake port.

Excessive pressures and temperatures shortenengine life by overstressing cylinders, pistons, con-necting rods, bearings, crankshaft journals, andvalves. Continued operation past upper manifoldabsolute pressure limits leads to worn engine parts,decreasing power output and lower efficiency, orworse, engine failure.

DETONATION AND PREIGNITION

A fuel/air mixture burns in a very controlled and predictable way when normal combustion takes

place. Even though the process happens in a frac-tion of a second, the mixture starts burning at the point where it is ignited by the spark plugs, then burns away from the plugs until it is all consumed.The dual spark plug design common in most aircraftreciprocating engines promotes a complete even

burn of the fuel/air charge by providing two ignitionsparks at the same time. The plugs are arrangedacross from one another so that, as the flameadvances from each spark plug, the mixture burnsin a wavelike form toward the center of the cylinder.This type of combustion causes a smooth buildup oftemperature and pressure so that maximum force is

applied to the piston at exactly the right time in the power stroke. [Figure 1-65]

Detonation is the uncontrolled, explosive ignition ofthe fuel/air mixture in the cylinder. Detonationcauses high cylinder temperatures and pressureswhich lead to a rough running engine, overheating,and power loss. If detonation occurs in an engine,damage or even failure of pistons, cylinders, or valvescan happen. The high pressures and temperatures,combined with the high turbulence generated, causea "hammering" or "scrubbing" action on a cylinderand piston that can burn a hole completely through

either of them in seconds. You can detect detonationas a "knock" in the engine. [Figure 1-66]




Figure 1-65. During normal combustion, the fuel/air mixtureburns evenly, producing a steady force similar to the evenpressure of someone pushing down on the piston.

Detonation is caused by several conditions such asusing a fuel grade lower than recommended andallowing the engine to overheat. Wrong ignition tim-

Figure 1-66. When detonation occurs, the fuel/air chargeburns in an explosive fashion causing a rapid increase inpressure that produces a "hammering" action on the piston.

ing, heavy engine load at low rpm, fuel/air mixturetoo lean, and compression ratios of 12:1 or higherare also possible causes of detonation. [Figure 1-67]

Figure 1-67. This chart illustrates the pressure created in a cylinder as it passes through its various strokes. As you can see, whennormal combustion occurs, cylinder pressure builds and dissipates evenly. However, when detonation occurs, cylinder pressurefluctuates dramatically.




Preignition takes place when the fuel/air mixtureignites too soon. It is caused by hot spots in a cylin-der that ignite the fuel/air mixture before the spark

plugs fire. A hot spot can be caused by something assimple as a carbon particle, overheated valve edges,silica deposits on a spark plug, or a red-hot spark

plug electrode. Hot spots are caused by poor enginecooling, dirty intake air filters, or shutting down theengine at high rpm. When the engine continues run-ning after the ignition is turned off, preignition may

be the cause.

Preignition and detonation can occur simultane-ously, and one may cause the other. Sometimes it isdifficult distinguishing between the two, but they

both cause engine roughness and high operatingtemperatures.

COMPRESSION RATIO

All internal combustion engines must compress thefuel/mixture to receive a reasonable amount of workfrom each power stroke. The fuel/air charge in the

cylinder can be compared to a coil spring, in that themore it is compressed, the more work it is poten-tially capable of doing.

An engine's compression ratio is defined as theratio of cylinder volume with the piston at the bot-tom of its stroke to the volume with the piston at thetop of its stroke. For example, if there are 140 cubicinches of space in a cylinder when the piston is at

bottom center and 20 cubic inches of space whenthe piston is at top center, the compression ratio is140 to 20. When this ratio is expressed in fractionform, it becomes 140/20, or 7 to 1, usually repre-sented as 7:1. [Figure 1-68]

To a great extent, an engine's compression ratiodetermines the amount of heat energy that is con-verted into useful work. Specifically, high compres-sion ratios allow the fuel/air mixture to release itsenergy rapidly and produce maximum pressureinside a cylinder just as the piston begins the powerstroke. As a general rule, the higher the compressionratio, the greater an engine's power output.

Figure 1-68. A cylinder's compression ratio compares cylinder volume with a piston at bottom dead center to the cylinder volumewhen the piston is at top dead center. In this example, the compression ratio is 7:1.




Compression ratios can be increased or decreased by altering an engine's design. For example, if thecrankshaft "throw" is lengthened, a piston's stroke

increases which, in turn, increases the compres-sion ratio. By the same token, if you shave a cylin-der head's mating surface, you effectively decreasethe distance between the cylinder head and pistonhead which increases the compression ratio.Another way to increase compression ratios withoutchanging the piston stroke length include installingdomed pistons.

To some degree, the characteristics of availablefuels determine the practical limits of compressionratios that can be used in engine design. For exam-

ple, if the compression rat io of an engine isincreased up to or beyond a fuel's critical pressure,detonation will occur. Because of this, engine man-ufacturers specify the correct grade of fuel to beused based, in part, on an engine's compressionratio. Use of fuel grades lower than recommendedshould be avoided.

tion timing as engine operating conditions change.Aircraft engines, on the other hand, use fixed tim-ing, which is a compromise between the timing

required to give best performance for takeoff and best performance at cruise.

If the ignition event occurs too early, an engineloses power because maximum cylinder pressure

builds too early. In other words, when the fuel/aircharge is ignited early, the force of the expandinggases opposes the engine's rotational inertia

because the piston is still moving upward. On theother hand, late ignition also causes a loss of

power, since cylinder volume is increasing at the

same time the gases are expanding. The result isthat gas pressure on the piston head does not buildto expected levels. Furthermore, late ignition doesnot allow enough time for complete combustion

before the exhaust valve opens. Burning gases thenengulf the valve, increase its temperature, andoften lead to detonation or engine damage due tooverheating.

If an engine is turbocharged, the degree oftur-bocharging limits the engine's compressionratio. Although turbocharging does not change an

engine's compression ratio, it does increasemanifold pressure as well as each cylinder's meaneffective pressure. If you recall from your study ofthe gas laws, as the pressure of a gas increases, thetemperature also increases. As a result,turbocharging raises the temperature of the fuel/airmixture in an engine's cylinders and increases the

possibility of detonation. Therefore, compressionratios in turbocharged engines must be limited toallow for the increased operating temperatures.

IGNITION TIMING

When the ignition event is properly timed, completecombustion and maximum pressure occur just afterthe piston passes top dead center at the beginning ofthe power stroke. To accomplish this, ignition tim-ing is typically set to ignite the fuel/air chargeshortly before a piston reaches top center on thecompression stroke. For example, some smallContinental engines ignite the fuel/air charge

between 25 and 32 degrees before top center on thecompression stroke.

An automobile engine employs a variable timingdevice on the ignition distributor to change the igni-

ENGINE SPEED

The amount of power an aircraft engine produces

is determined by cylinder pressure, piston area,the distance a piston moves on each stroke, andthe number of times this movement occurs in oneminute. Stated in simple terms, the faster anengine runs, the more power it produces.However, there are some limitations to how fast anengine can rotate. Some of these limitationsinclude ignition timing, valve timing, and the iner-tia of rapidly moving pistons. For example, whenintake and exhaust valves move too quickly, theycan "float," or not seat properly. In addition, theinertia of pistons reversing their direction of travelthousands of times per minute can overstresscrankshaft journals and bearings when engine rpmexceeds safe limits.

Another limitation on an engine's maximum rota-tional speed is propeller tip speed. In order to effi-ciently produce thrust, the tip speed of a propeller

blade must not exceed the speed of sound. If yourecall, the further from the propeller hub a point is,the faster that point moves through the air.Therefore, engines that operate at high rpm's musteither be fitted with short propeller blades or some

form of propeller reduction gearing. Reduction gearsallow an engine to turn at higher speeds to produce




Figure 1-69. As this figure illustrates, a propelleris speed is

highest at the blade tips. If you recall from Section A, when

propeller tip s peeds exceed the speed of s ound, propellerefficiency drops.

more power while the propeller rotates at a slower,more efficient speed. [Figure 1-69]

SPECIFIC FUEL CONSUMPTION

An engine's specific fuel consumption is the number of pounds of fuel burned per hour to produceone horsepower. For example, if an engine burns 12gallons per hour while producing 180 brake horse-

power, the brake specific fuel consumption is .4

pounds per horsepower hour. Most modern aircraftreciprocating engines have a brake specific fuel con-sumption (BSFC) that is between .4 and .5 pounds

per horsepower hour. While not actually a measureof power, specific fuel consumption is useful forcomparing engine efficiencies.

An engine's specific fuel consumption varies withseveral factors including: engine speed, enginedesign, volumetric efficiency, and friction losses. The

best specific fuel consumption for most enginesoccurs at a cruise power setting when producingapproximately 75 percent power. The amount anengine's specific fuel consumption varies withengine rpm can be illustrated in a chart. [Figure 1-70]

Figure 1-70. Aircraft engines operate most efficiently

around 2,400 rpm. Below 2,400 rpm an engine is not devel-

oping as much power as i t is capable of for the amount of

fuel it is using. On the other hand, above 2,400 rpm, friction

horsepower increases, causing an overall drop in brake

horsepower. As shown by the lower curve, a typical engine

requires about 0.51 pounds of fuel per hour for each horse-

power it produces at 2,400 rpm.

ALTITUDE

As an aircraft climbs, ambient air pressure dropsand air density decreases. Density altitude is a termthat describes the density of the air at a given alti-tude corrected for nonstandard pressure and tem-

perature. Any time an aircraft engine operates at adensity altitude that is higher than sea level, less airis drawn into the engine for combustion. Whenever

less air is available for combustion, engine poweroutput decreases.

Even though the actual, or true altitude at a locationdoes not change, density altitude can change con-stantly. For example, on a hot day as the air heatsand pressure drops, air becomes less dense, causingthe density altitude to increase. One way to over-come the problems associated with high densityaltitudes is by turbocharging an engine.Turbocharging increases the induction air pressureabove atmospheric pressure which, in turn,

increases the density of the fuel/air charge enteringthe cylinders.

FUEL/AIR RATIO

Gasoline and other liquid fuels must be convertedfrom their liquid state to a vapor before they will

burn. In addition, the ratio of fuel vapor to oxygenin the air must be chemically correct for completecombustion. A stoichiometric mixture is a perfectly

balanced fuel/air mixture of 15 parts of air to 1 partof fuel, by weight. A fuel/air mixture that is leanerthan 15:1 has less fuel in the fuel/air mixture, whilea rich mixture has more fuel. Combustible fuel/airratios range from 8:1 to 18:1.




Mixture controls allow adjustment of the fuel/airratio from idle cut-off to full rich conditions. Leaningraises engine operating temperatures while enrich-

ing provides a cooling effect. Leaning becomes nec-essary as altitude increases, because air densitydrops, causing the fuel/air ratio to gradually becomericher. Best power mixture develops maximum

power at a particular rpm and is typically used dur-ing takeoff. Best economy mixture provides the bestspecific fuel consumption which results in an air-craft's maximum range and optimum fuel economy.

DISTRIBUTION OF POWER

When considering the amount of power that is avail-able in aviation gasoline compared to the amount of

power that is actually delivered to the propellershaft, you can easily see that an aircraft engine is afairly inefficient machine. For example, a typical sixcylinder engine develops 200 brake horsepowerwhen burning 14 gallons of fuel per hour. However,the burning fuel releases enough heat energy to pro-duce 667 horsepower. When you examine the distri-

bution of power you will see that 200 of the 667horsepower is delivered to the propeller whileapproximately 33 horsepower is used to turn theengine and compress the air in the cylinders. Inaddition, an equivalent of about 434 horsepower is

lost to the air through the cooling and exhaust sys-tems. The power loss continues when power isdelivered to the propeller, because in order to pro-

pel an aircraft through the air, the torque produced

by an engine must be converted into thrust. Since a propeller converts only about 90% of the torque itreceives into thrust, the actual thrust horsepower

delivered by the propeller is 180 horsepower. Thrusthorsepower represents the actual horsepower devel-oped by the thrust of the propeller. [Figure 1-71]

POWER CURVES

Most engine manufacturers produce a set of powercurves for each engine they build. These chartsshow the power developed for each rpm as well asindicate the specific fuel consumption at each

power setting. [Figure 1-72]

Figure 1-72. This figure illustrates a typical power curve fora four-cylinder aircraft engine. The upper curve shows themaximum amount of power produced at full throttle on adynamometer. The diagonal power curve represents theamount of horsepower produced with less than full throttle.The two bottom curves represent the specific fuel con-sumptions for full throttle operations and propeller loadconditions. To use this chart, let's assume you have anengine operating at a cruise power setting of 2,400 rpm(item 1) At this power setting the specific fuel consump-

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CHAPTER 1 Reciprocating Engines