THE RECRIPROCATING ENGINE

The reciprocating engine is one of the most efficient powerplants used for aircraft power. The combination of the reciprocating engine and propeller is one of the most efficient means of converting the chemical energy of fuel into flying time or distance. Because of the in­herent high efficiency, the reciprocating engine is an important type of aircraft powerplant.

OPERATING CHARACTERISTICS. The function of the typical reciprocating engine in­volves four strokes of the piston to complete one operating cycle. This principal operating cycle is illustrated in figure 2.15 by the varia­tion of pressure and volume within the cylin­der. The first stroke of the operating cycle is the downstroke of the piston with the intake valve open. This stroke draws in a charge of fuel-air mixture along AB of the pressure – volume diagram. The second stroke accom­plishes compression of the fuel-air mixture along line BC. Combustion is initiated by a spark ignition apparatus and combustion takes place in essentially a constant volume. The combustion of the fuel-air mixture liberates heat and causes the rise of pressure along line CD. The power stroke utilizes the increased pressure through the expansion along line DE. Then the exhaust begins by the initial rejection along line EB and is completed by the upstroke along line BA.

The net work produced by the cycle of opera­tion is idealized by the area BCDE on the pressure-volume diagram of figure 2.15. Dur­ing the actual rather than ideal cycle of op­eration, the intake pressure is lower than the exhaust pressure and the negative work repre­sents a pumping loss. The incomplete expan­sion during the power stroke represents a basic loss in the operating cycle because of the re­jection of combustion products along line EB. The area EFB represents a basic loss in the operating cycle because of the rejection of combustion products along line EB. The area EFB represents a certain amount of energy of the exhaust gases, a part of which can be ex­tracted by exhaust turbines as additional shaft power to be coupled to the crankshaft (turbo­compound engine) or to be used in operating a supercharger (turbosupercharger). In addi­tion, the exhaust gas energy may be utilized to augment engine cooling flow (ejector exhaust) and reduce cowl drag.

Since the net work produced during the op­erating cycle is represented by the enclosed area of pressure-volume diagram, the output of the engine is affected by any factor which influences this area. The weight of fuel-air mixture will determine the energy released by combustion and the weight of charge can be altered by altitude, supercharging, etc. Mixture strength, preignition, spark timing, etc., can affect the energy release of a given airflow and alter the work produced during the operating cycle.

The mechanical work accomplished during the power stroke is the result of the gas pres­sure sustained on the piston. The linkage of the piston to a crankshaft by the connecting rod applies torque to the output shaft. During this conversion of pressure energy to mechani­cal energy, certain losses are inevitable because

COMPRESSION COMBUSTION

of friction and the mechanical output is less than the available pressure energy. The power output from the engine will be determined by the magnitude and rate of the power impulses. In order to determine the power output of the reciprocating engine, a brake or load device is attached to the output shaft and the operating characteristics are determined. Hence, the term “brake” horsepower, ВНР, is used to denote the output power of the powerplant, From the physical definition of "power" and the particular unit of “horsepower” (1 h. p.=

33,0 ft.-lbs. per min.), the brake horsepower can be expressed in the following form.

In this relationship, the output power is ap­preciated as some direct variable of torque, Tf and RPM. Of course, the output torque is some function of the combustion gas pressure during the power stroke. Thus, it is helpful to consider the mean effective gas pressure during the power stroke, the “brake mean effective pressure” or ВМЕР. With use of this term, the ВНР can be expressed in the following form.

СВМЕР)(РХЮ

792,000

where

ВНР = brake horsepower BMEP=brake mean effective pressure, psi D=engine displacement, cu. in.

N= engine speed, RPM

The ВМЕР is not actual pressure within the cylinder, but an effective pressure representing the mean gas load acting on the piston during
the power stroke. As such, ВМЕР is a con­venient index for a majority of items of recip­rocating engine output, efficiency, and operat­ing limitations.

The actual power output of any reciprocat­ing engine is a direct function of the combina­tion of engine torque and rotative speed. Thus, output brake horsepower can be related by the combination of ВМЕР and PPM or torque pressure and RPM. No other engine instruments can provide this immediate indi­cation of output power.

If all other factors are constant, the engine power output is directly related to the engine airflow. Evidence of this fact could be appre­ciated from the equation for ВНР in terms of ВМЕР.

This equation relates that, for a given ВМЕР, the ВНР is determined by the product of en­gine RPM, N, and displacement, D. In a sense, the reciprocating engine could be con­sidered primarily as an air pump with the pump capacity directly affecting the power output. Thus, any engine instruments which relate factors affecting airflow can provide some indirect reflection of engine power. The pres­sure and temperature of the fuel-air mixture decide the density of the mixture entering the cylinder. The carburetor air temperature will provide the temperature of the inlet air at the carburetor. While this carburetor inlet air is not the same temperature as the air in the cylinder inlet manifold, the carburetor inlet temperature provides a stable indication inde­pendent of fuel flow and can be used as a stand­ard of performance. Cylinder inlet manifold temperature is difficult to determine with the same degree of accuracy because of the normal variation of fuel-air mixture strength. The inlet manifold pressure provides an additional indication of the density of airflow entering the combustion chamber. The manifold absolute pressure, MAP, is affected by the carburetor

inlet pressure, throttle position, and super­charger or impeller pressure ratio. Of course, the throttle is the principal control of mani­fold pressure and the throttling action controls the pressure of the fuel-air mixture delivered to the supercharger inlet. The pressure re­ceived by the supercharger is magnified by the supercharger in some proportion depend­ing on impeller speed. Then the high pressure mixture is delivered to the manifold.

Of course, the engine airflow is a function of RPM for two reasons. A higher engine speed increases the pumping rate and the volume flow through the engine. Also, with the engine driven supercharger or impeller, an increase in engine speed increases the supercharger pres­sure ratio. With the exception of near closed throttle position, an increase in engine speed will produce an increase in manifold pressure.

The many variables affecting the character

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subject of reciprocating engine operation. Uniform mixtures of fuel and air will support combustion between fuel-air ratios of approxi­mately 0.04 and 0.20. The chemically correct proportions of air and hydrocarbon fuel would be 15 lbs. of air for each lb. of fuel, or a fuel – air ratio of 0.067. This chemically correct, or “stoichiometric,” fuel-air ratio would provide the proportions of fuel and air to produce maximum release of heat during combustion of a given weight of mixture. If the fuel-air ratio were leaner than stoichiometric, the ex­cess of air and deficiency of fuel would produce lower combustion temperatures and reduced heat release for a given weight of charge. If the fuel-air ratio were richer than stoichio­metric, the excess of fuel and deficiency of air would produce lower combustion temperatures and reduced heat release for a given weight of charge.

The stoichiometric conditions would pro­duce maximum heat release for ideal conditions of combustion and may apply quite closely for the individual cylinders of the low speed re­ciprocating engine. Because of the effects of flame propagation speed, fuel distribution, temperature variation, etc., the maximum power obtained with a fixed airflow occurs at fuel-air ratios of approximately 0.07 to 0.08. The first graph of figure 2.16 shows the varia­tion of output power with fuel-air ratio for a a constant engine airflow, i. e., constant RPM, MAP, and CAT (carburetor air temperature). Combustion can be supported by fuel-air ratios just greater than 0.04 but the energy released is insufficient to overcome pumping losses and engine mechanical friction. Essentially, the same result is obtained for the rich fuel-air ratios just below 0.20. Fuel-air ratios be­tween these limits produce varying amounts of output power and the maximum power output generally occurs at fuel-air ratios of approxi­mately 0.07 to 0.08. Thus, this range of fuel – air ratios which produces maximum power for a given airflow is termed the “best power” range. At some lower range of fuel-air ratios, a maximum of power per fuel-air ratio is ob­tained and this the “best economy” range. The best economy range generally occurs be­tween fuel-air ratios of 0,05 and 0.07. When maximum engine power is required for take­off, fuel-air ratios greater than 0.08 are neces­sary to suppress detonation. Hence, fuel-air ratios of 0.09 to 0.11 are typical during this operation.

The pattern of combustion in the cylinder is best illustrated by the second graph of figure 2.16. The normal combustion process begins by spark ignition toward the end of the com­pression stroke. The electric spark provides the beginning of combustion and a flame front is propagated smoothly through the com­pressed mixture. Such normal combustion is shown by the plot of cylinder pressure versus piston travel. Spark ignition begins a smooth rise of cylinder pressure to some peak value with subsequent expansion through the power stroke. The variation of pressure with piston travel must be controlled to achieve the great­est net work during the cycle of operation.

ENGINE AIRFLOW, LBS. PER HR.

Figure 2.16. Reciprocating Engine Operation

Obviously, spark ignition timing is an impor­tant factor controlling the initial rise of pres­sure in the combustion chamber. The ignition of the fuel mixture must begin at the proper time to allow flame front propagation and the release of heat to build up peak pressure for the power stroke.

The speed of flame front propagation is a major factor affecting the power output of the reciprocating engine since this factor controls the rate of heat release and rate of pressure rise in the combustion chamber. For this reason, dual ignition is necessary for powerplants of high specific power output. Obviously, nor­mal combustion can be accomplished more rapidly with the propagation of two flame fronts rather than one. The two sources of ignition are able to accomplish the combus­tion heat release and pressure rise in a shorter period of time. Fuel-air ratio is another factor affecting the flame propagation speed in the combustion chamber. The maximum flame propagation speed occurs near a fuel-air ratio of 0.08 and, thus, maximum power output for a given airflow will tend to occur at this value rather than the stoichiometric value.

Two aberrations of the combustion process are preignition and detonation. Preignition is simply a premature ignition and flame front propagation due to hot spots in the combustion chamber. Various lead and carbon deposits and feathered edges on metal surfaces can sup­ply a glow ignition spot and begin a flame propagation prior to normal spark ignition. As shown on the graph of figure 2.16, pre­ignition causes a premature rise of pressure during the piston travel. As a result, preignition combustion pressures and tempera­tures will exceed normal combustion values and are very likely to cause engine damage. Be­cause of the premature rise of pressure toward the end of the compression stroke, the net work of the operating cycle is reduced. Preignition is evidenced by a rise in cylinder head tempera­ture and drop in ВМЕР or torque pressure.

Denotation offers the possibility of immedi­ate destruction of the powerplant. The nor­mal combustion process is initiated by the spark and beginning of flame front propaga­tion. As the flame front is propagated, the combustion chamber pressure and temperature begin to rise. Under certain conditions of high combustion pressure and temperature, the mixture ahead of the advancing flame front may suddenly explode with considerable vi­olence and send strong detonation waves through the combustion chamber. The result is depicted by the graph of figure 2.16, where a sharp, explosive increase in pressure takes place with a subsequent reduction of the mean pres­sure during the power stroke. Detonation produces sharp explosive pressure peaks many times greater than normal combustion. Also, the exploding gases radiate considerable heat and cause excessive temperatures for many local tiarts of the engine. The effects of heavy

1 " " O" /

detonation are so severe that structural damage is the immediate result. Rapid rise of cylinder head temperature, rapid drop in ВМЕР, and loud, expensive noises are evidence of detona­tion.

Detonation is not necessarily confined to a period after the beginning of normal flame front propagation. With extremely low grades of fuel, detonation can occur before normal igni­tion. In addition, the high temperatures and pressure caused by preignition will mean that detonation is usually a corollary of preigniticn. Detonation results from a sudden, unstable de­composition of fuel at some critical combina­tion of high temperature and pressure. Thus, detonation is most likely to occur at any op­erating condition which produces high com­bustion pressures and temperatures. Gener­ally, high engine airflow and fuel-air ratios for maximum heat release will produce the critical conditions. High engine airflow is common to high MAP and RPM and the engine is most sensitive to CAT and fuel-air ratio in this region.

The detonation properties of a fuel are de­termined by the basic molecular structure of the fuel and the various additives. The fuel detonation properties are generally specified by the antidetonation or antiknock qualities of an octane rating. Since the antiknock proper­ties of a high quality fuel may depend on the mixture strength, provision must be made in. the rating of fuels. Thus, a fuel grade of 115/145 would relate a lean mixture antiknock rating of 115 and a rich mixture antiknock rating of 145. One of the most common opera­tional causes of detonation is fuel contamina­tion. An extremely small contamination of high octane fuel with jet fuel can cause a serious decrease in the antiknock rating. Also, the contamination of a high grade fuel with the next lower grade will cause a noticeable loss of antiknock quality.

The fuel metering requirements for an engine are illustrated by the third graph of figure 2.16 which is a plot of fuel-air ratio versus engine airflow. The carburetor must provide specific fuel-air ratios throughout the range of engine airflow to accommodate certain output power. Most modern engines equipped with auto­matic mixture control provide a scheduling of fuel-air ratio for automatic rich or automatic lean operation. The auto-rich scheduling usu­ally provides a fuel-air ratio at or near the maximum heat release value for the middle range of airflows. However, at high airflows a power enrichment must be provided to sup­press detonation. The auto-rich schedule gen­erally will provide an approximate fuel-air ratio of 0.08 which increases to 0.10 or 0.11 at the airflow for takeoff power. In addition, the low airflow and mixture dilution that oc­curs in the idle power range requires enrich­ment for satisfactory operation.

The schedule of fuel-air ratios with an auto­matic lean fuel-air ratio will automatically provide maximum usable economy. If manual leaning procedures are applicable a lower fuel – air ratio may be necessary for maximum possi­ble efficiency. The maximum continuous cruise power is the upper limit of power that can be utilized for this operation. Higher air­flows and higher power without a change in fuel-air ratio will intersect the knee of the detonation envelope.

The primary factor relating the efficiency of operation of the reciprocating engine is the brake specific fuel consumption, BSFC, or simply c.

Brake specific fuel consumption

_ engine fuel flow brake horsepower lbs. per hr.

C_ ВНР

Typical minimum values for c range from 0.4 to 0.6 lbs. per hr. per ВНР and most aircraft powerpiants average 0.5. The turbocompound engine is generally the most efficient because of the power recovery turbines and can ap­proach values of c=0.38 to 0.42, It should be noted that the minimum values of specific fuel consumption will be obtained only within the range of cruise power operation, 30 to 60 per­cent of the maximum power output. Gen­erally, the conditions of minimum specific fuel consumption are achieved with auto-lean or manual lean scheduling of fuel-air ratios and high ВМЕР and low RPM. The low RPM is the usual requirement to minimize friction horsepower and improve output efficiency.

The effect of altitude is to reduce the engine airflow and power output and supercharging is necessary to maintain high power output at high altitude. Since the basic engine is able to process air only by the basic volume displacement, the function of the supercharger is to compress the inlet air and provide a greater weight of air for the engine to process. Of course, shaft power is necessary to operate the engine driven supercharger and a tempera­ture rise occurs through the supercharger com­pression. The effect of various forms of super­charging on altitude performance is illustrated in figure 2.17-

The unsupercharged—or naturally aspi­rated—engine has no means of providing a

EFFECT OF SUPERCHARGING ON ALTITUDE
PERFORMANCE

manifold pressure any greater than the induc­tion system inlet pressure. As altitude is increased with full throttle and a governed RPM, the airflow through the engine is reduced and ВНР decreases. The first forms of supercharging were of relatively low pressure ratio and the added airflow and power could be handled at full throttle within detonation limits. Such a “ground boosted” engine would achieve higher output power at all altitudes but an increase in altitude would produce a decrease in manifold pressure, air­flow, and power output.

More advanced forms of supercharging with higher pressure ratios can produce very large engine airflow. In fact, the typical case of altitude supercharging will produce such high airflow at low altitude operation that full throttle operation cannot be utilized within detonation limits. Figure 2.17 illustrates this case for a typical two-speed engine driven altitude supercharging installation. At sea level, the limiting manifold pressure produces a certain amount of ВНР. Full throttle oper­ation could produce a higher MAP and ВНР if detonation were not the problem. In this case full throttle operation is unavailable because of detonation limits. As altitude is increased with the supercharger or "blower” at low speed, the constant MAP is maintained by opening the throttle and the ВНР increases above the sea level value because of the re­duced exhaust back pressure. Opening the throttle allows the supercharger inlet to re­ceive the same inlet pressure and produce the same MAP. Finally, the increase of altitude will require full throttle to produce the con­stant MAP with low blower and this point is termed the "critical altitude” or "full throttle height.” If altitude is increased beyond the critical altitude, the engine MAP, airflow, and ВНР decrease.

The critical altitude with a particular super­charger installation is specific to a given com­bination of MAP and RPM. Obviously, a lower MAP could be maintained to some higher altitude or a lower engine speed would produce less supercharging and a given MAP would require a greater throttle opening. Generally, the most important critical alti­tudes will be specified for maximum, rated, and maximum cruise power conditions.

A change of the blower to a high speed will provide greater supercharging but will require more shaft power and incur a greater tempera­ture rise. Thus, the high blower speed can produce an increase in altitude performance within the detonation limitations. The vari­ation of ВНР with altitude for the blower at high speed shows an increase in critical alti­tude and greater ВНР than is obtainable in low blower. Operation below the high blower critical altitude requires some limiting mani­fold pressure to remain within detonation limits. It is apparent that the shift to high blower is not required just past low blower critical altitude but at the point where the transition from low blower, full throttle to high blower, limit MAP will produce greater ВНР. Of course, if the blower speed is increased without reducing the throttle opening, an "overboost” can occur.

Since the exhaust gases have considerable energy, exhaust turbines provide a source of supercharger power. The turbosupercharger (TBS’) allows control of the supercharger speed and output to very high altitudes with a variable discharge exhaust turbine (VDT). The turbosupercharger is capable of providing the engine airflow with increasing altitude by increasing turbine and supercharger speed. Critical altitude for the turbosupercharger is usually defined by the altitude which produces the limiting exhaust turbine speed.

The minimum specific fuel consumption of the supercharged engine is not greatly affected by altitudes less than the critical altitude. At the maximum cruise power condition, specific fuel consumption will decrease slightly with an increase in altitude up to the critical altitude. Above critical altitude, maximum cruise power cannot be maintained but the

specific fuel consumption is not adversely affected as long as auto-lean or manual lean power can be used at the cruise power setting.

One operating characteristic of the recipro­cating engine is distinctly different from that of the turbojet. Wafer vapor in the air will cause a significant reduction in output power of the reciprocating engine but a negligible loss of thrust for the turbojet engine. This basic difference exists because the reciprocating engine operates with a fixed displacement and all air processed is directly associated with the combustion process. If water vapor enters the induction system of the reciprocating engine, the amount of air available for combustion is reduced and, since most carburetors do not distinguish water vapor from air, an enrich­ment of the fuel-air ratio takes place. The maximum power output at takeoff requires fuel-air ratios richer than that for maximum heat release so a further enrichment will take place with subsequent loss of power. The turbojet operates with such great excess of air that the combustion process essentially is unaffected and the reduction of air mass flow is the principal consideration. As an example, extreme conditions which would produce high specific humidity may cause a 3 percent thrust loss for a turbojet but a 12 percent loss of ВНР for a reciprocating engine. Proper accounting of the loss due to humidity is essential in the operation of the reciprocating engine.

OPERATING LIMITATIONS. Recipro­cating engines have achieved a great degree of refinement and development and are one of the most reliable of all types of aircraft power – plants. However, reliable operation of the re­ciprocating engine is obtained only by strict adherence to the specific operating limitations.

The most important operating limitations of the reciprocating engine are those provided to ensure that detonation and preignition do not take place. The pilot must ensure that proper fuel grades are used that limit MAP, ВМЕР, RPM, CAT, etc., are not exceeded. Since

fluid from fouling the plumbing.

When the fuel grades are altered during oper­ation and the engine must be operated on a next lower fuel grade, proper account must be made for the change in the operating limita­tions. This accounting must be made for the maximum power for takeoff and the maximum cruise power since both of these operating con­ditions are near the detonation envelope. In addition, when the higher grade of fuel again becomes available, the higher operating limits cannot be used until it is sure that no contamina­tion exists from the lower grade fuel remaining in the tanks.

Spark plug fouling can provide certain high as well as low limits of operating temperatures. When excessively low operating temperatures are encountered, rapid carbon fouling of the plugs will take place. On the other hand, excessively high operating temperatures will produce plug fouling from lead bromide de­posits from the fuel additives.

Generally, the limited periods of time at various high power settings arc set to mini­mize the accumulation of high rates of wear

Revised January 1965

and fatigue damage. By minimizing the amount of total time spent at high power setting, greater overhaul life of the powerplant can be achieved. This should not imply that the takeoff rating of the engine should not be used. Actually, the use of the full maximum power at takeoff will accumulate less total engine wear than a reduced power setting at the same RPM because of less time required to climb to a given altitude or to accelerate to a given speed.

The most severe rate of wear and fatigue damage occurs at high RPM and low MAP. High RPM produces high centrifugal loads and reciprocating inertia loads. When the large reciprocating inertia loads are not cush­ioned by high compression pressures, critical resultant loads can be produced. Thus, op­erating time at maximum RPM and MAP must be held to a minimum and operation at maxi­mum RPM and low MAP must be avoided.