Aircraft technical Basics: TM 1-405, Aircraft Aircraft Engines, 1941: I - Internal-Combustion Engine Principles

SECTION I: INTERNAL-COMBUSTION ENGINE PRINCIPLES

1. General. - a. Rapid progress has been made in the past few years in the development of high-powered aircraft engines; however, insofar as fundamentals are concerned, they have not changed since their conception at the beginning of the twentieth century.

b. The power developed by internal-combustion engines is dependent upon the type of fuel used ; therefore it necessarily follows that the future increases in power obtained from conventional aircraft engines depend upon the development of fuels. However. metallurgy will also take an important part in future development, in that the metals used in an engine of the future must withstand increased stresses.

2. Conversion of heat into mechanical energy.- Internal combustion engines are of a class of prime movers known as "heat engines," that is, they convert heat energy into useful mechanical energy through a process of combustion (fig. 1). A mixture of fuel and air in proper proportion, after it has been compressed to a comparatively high pressure, is burned within the cylinder. The sudden increase in pressure, due to combustion, causes the piston to move against the load and deliver mechanical energy to the engine crankshaft. The fuel must be vaporized or in a gaseous state, when used in an internal combustion engine and must be mixed with the proper proportion of air in order to burn properly. The igniting of the gas is of the utmost importance. The various methods of obtaining and timing the ignition spark and the proper regulation and adjustment of the different parts of the power plant are vital elements in the operation of a conventional aircraft engine.

FIGURE 1.-Method of converting heat energy to mechanical energy in an internal-combustion engine.

3. Engine cycles. - a. In order to operate continuously and deliver power, the engine must go through a routine of operations, each act being performed over and over in the same sequence. Each of these operations is known as an event, and a series of events is known as a cycle, or as a cycle of events. In a gasoline engine. the following events must take place:

(1) Admitting or forcing a charge into the cylinder.

(2) Compressing the charge.

(3) Igniting the charge.

(4) Burning of the charge, developing power on the piston head.

(5) Forcing the burned charge out of the cylinders.

b. Engines are classified by the number of strokes taken to accomplish the above cycle of events, as there are several possible combinations between the events and the number of strokes required for the cycle. Thus, a two-stroke cycle engine completes the five events in two strokes or one revolution of the crankshaft, whereas a four-stroke cycle engine goes through the series in four strokes, or in two revolutions of the crankshaft. Most automotive and aircraft engines constructed at present are of the four-stroke cycle type.

c. A thorough understanding of the four-stroke cycle is of utmost importance in ignition and valve timing as the opening and closing of the valves and the timing of the ignition spark depend entirely upon the time at which the events take place in regard to piston positions.

4. Four-stroke cycle principle. - a. In this type of engine, which is often called the four-cycle engine, the five events take place during four strokes, or two revolutions of the crankshaft ( fig. 2). According to the strokes, the events take place in the following order:

(1) The first stroke is called the intake or admission stroke. The piston moves outward, or toward the crank, and admits a charge of the combustible mixture into the cylinder. During this stroke the intake valves are open.

(2) The second stroke is known as the compression stroke. The piston moves inward or from the crank, compressing the charge. At. the end of the compression stroke the spark occurs and ignites the charge. During this stroke, both intake and exhaust valves are closed.

(3) The third stroke is known as the expansion or power stroke. The hot ignited gases create a high pressure on the piston and again move it outward, or toward the crank. Near the end of the stroke the pressure is much reduced by expansion, the exhaust valve opens, and the burned gas starts to scavenge out of the cylinder to the atmosphere.

(4) The fourth stroke is known as the exhaust, or scavenging stroke. The piston returns inward, or from the crank, and forces out the burned gases left in the cylinder. At the end of the fourth stroke the piston again moves outward admitting another charge of fuel and air mixture, thus starting another similar cycle of events.

Figure 2.-Four-stroke cycle principle.

b. In actual engine operation, a number of varying conditions must be considered in order to obtain the highest engine efficiency. These conditions are governed by the following rules:

(1) The larger the volume of a properly proportioned vaporized fuel and air mixture admitted into an engine cylinder, the more power developed, provided the charge is not compressed above the limiting pressure of the fuel.

(2) The larger the volume of exhaust gases expelled from an engine cylinder. the more power developed.

(a) By opening the intake valve before the piston has reached top center on the exhaust stroke, a larger volume of charge is admitted into the cylinder at the rpm at which the engine operates most of its useful life. This timing results in very poor efficiency at low rpm; however, the sacrifice is well worth the gain. By allowing the intake valve to remain open during the full length of the outward stroke and a certain portion of the inward compression stroke, the volume of charge admitted into the cylinder is still further increased. Therefore, the intake valve actually remains open during the entire intake stroke as well as part of the compression stroke.

(b) The exhaust valve opens when the piston is approximately two-thirds down the power stroke, which not only aids in obtaining better scavenging of burned gases, but results in better cooling of the cylinders. By allowing the exhaust valve to remain open the last one-third portion of the power stroke, the full length of the inward stroke, and approximately one-ninth of the next outward stroke, practically all of the burned gases are expelled from the cylinder.

c. The igniting of the charge is a vital event in the operation of a conventional aircraft engine and must occur at the proper time. The igniting unit is therefore timed to ignite the charge before the piston reaches top center on compression stroke in order to allow sufficient. time for the burning charge to reach its maximum pressure at. the instant, the piston passes over top center. Inasmuch as the rate of burning of the charge is dependent on its degree of compression, which in turn is governed largely by the volume of the charge admitted into the cylinder or by throttle position, it becomes apparent that the time at which the charge is ignited should vary with the throttle in order to obtain maximum efficiencies at all engine speeds. The limited range of rpm at which an aircraft engine operates most of its useful life and the attendant dangers of operating on retarded spark prohibit the use of a variable spark control. Therefore, the ignition unit is timed to ignite the charge at one piston position, which is in advanced position.

5. Two-stroke cycle principle.--a. In the two-stroke cycle engine (fig. 3), some of the events which occur to complete the cycle take place in the crankcase. In contrast to the four-stroke cycle engine, when the fresh charge is drawn or forced directly into the cylinder through the intake valve while the piston is moving outward or toward the crank, the fresh charge in a two-stroke cycle engine is drawn, or forced by a supercharger, directly into the crankcase. while the piston is moving inward or from the crank. Therefore, the crankcase must be sealed airtight. This arrangement obviously eliminates the use of a crankcase splash lubricating system and crank-case breathers, and requires a certain amount of lubricating oil mixed directly with the liquid fuel.

(1) In the first inward movement of the. piston two events are taking place; a fresh charge is being drawn into the crankcase and a fresh charge previously forced into the cylinder, is being compressed in the combustion chamber.

(2) Prior to the piston reaching top center, the compressed charge in the combustion chamber is ignited by the spark plug.

(3) The burning charge forces the piston outward on power stroke and while this event is occurring the fresh charge previously drawn into the crankcase is being compressed.

(4) When the top of the piston reaches a point approximately three-fourths of its total outward travel, the exhaust port or hole is uncovered and the exhaust scavenged out into the atmosphere.

(5) When the top of the piston reaches a point approximately seven-eighths of its total outward travel, the intake. port. or hole (opposite the exhaust port) is uncovered and the compressed fresh charge in the crankcase enters the cylinder. The inrushing charge aids in forcing the exhaust gases out of the cylinder, as shown in figure 3.

(6) The next cycle of events does not take place until the piston has passed bottom center and closed off the intake and exhaust ports on its inward stroke.

b. The foregoing description pertains to the two-port, two-stroke cycle principle; however, the three-port engine operation is very similar, except for an additional intake port designed to improve economy in nonsupercharged engines.

FIGURE 3. Two-stroke cycle principle.

6. Diesel principle.    Diesel engines operate on either the two-stroke or four-stroke cycle principle.

a. In the conventional engine an electric spark is utilized to ignite the mixture of gasoline and air to start combustion, whereas the Diesel engine generates its own heat, to ignite or start combustion by means of highly compressed air.

b. In the Diesel engine, pure air is introduced into the cylinders instead of a mixture of fuel and air as in the conventional engine: this air is compressed into a much smaller space than is possible when using a mixture of fuel and air, which would prematurely detonate if compressed to this extent. The temperature of the air in the cylinder at the end of the compression stroke operating with a compression ratio of approximately 14:1 is about 1.000 F., which is far above the spontaneous ignition temperature of the fuel used. Accordingly, when fuel in a highly atomized condition is injected into the cylinder just before the piston reaches top center on the compression stroke, the fuel burns immediately upon coming in contact with the highly heated air, thus starting combustion. Combustion at this point acts on the piston in the same manner as in the case of the conventional engine.

c. In the place of the metering device which is used on a conventional engine for mixing and metering the gasoline and air in proper proportions, the Diesel engine employs a fuel pump and nozzle that injects the fuel in an atomized condition into the cylinder at the time that combustion is desired. The quantity of fuel injected into the cylinder controls the amount of heat generated. In the conventional engine the fuel and air mixture ratio must be kept reasonably constant and when the fuel supply is reduced for throttling purposes, the air must be correspondingly reduced. In the Diesel engine, the amount of fuel may be varied without varying the amount of air.

d. The fact that the air in a Diesel engine cylinder is compressed and heated to a comparatively high degree permits the use of liquid fuels of comparatively low volatility. Fuels corresponding very nearly to crude petroleum oil therefore are suitable for the Diesel engine.

7. Compressing the charge.--a. In a conventional internal-combustion engine the mixture of fuel and air must be highly compressed in order to obtain efficient combustion and a reasonable amount of work. If the charge is ignited at atmospheric pressure, the combustion is slow and much heat is radiated and lost. The resulting pressure or power, due to the combustion of the charge, is relatively low. Thus, an engine compressing the charge to 100 pounds per square inch will only develop approximately 1 horse-power for every 4 cubic inches displaced by the piston, while compressing the charge to 140 pounds per square inch will increase the output to a point where 1.8 cubic inches of piston displacement will produce 1 horsepower. Some aircraft engines in use at the present time have compression ratios as high as 7.25 to 1. In general, it may be said that the power and efficiency of an internal-combustion engine increase with the degree of compression. Therefore, an aircraft engine with a compression of 150 pounds per square inch has a much higher output for a given size cylinder than an engine with a compression of only 110 pounds per square inch. Unfortunately. compression pressures are limited by the fuel used and the service for which the engine is intended. Hence, a compression pressure that would be highly desirable from an efficiency point of view would be impossible because of certain conditions. Extremely high compression pressures result in detonation and set up heavy stresses in the structural parts of the engine.

b. The effects of compression, up to certain critical limits, in increasing the actual efficiency and output of an engine are twofold.

(1) The compression produces heat, which aids in the vaporization of the fuel.

(2) At the point of highest compression, the volume is at a minimum and combustion is accomplished more rapidly because of the smaller space through which the flame spreads.

c. The exact compression obtained in any cylinder depends upon a number of factors. The following are the most important:

(1) The ratio of the total volume of the cylinder to the compression space. The compression pressure of the cylinder varies as the ratio of the volume of the clearance space plus the volume swept by the piston to the clearance space. This ratio is known as the compression ratio, and in aircraft engines varies from 5 to 7. For example, in a cylinder having a piston displacement of 100 cubic inches and a combustion chamber space of 20 cubic inches. the compression ratio is    or, as it is commonly written. 6: 1.

(2) The pressure of the charge in the cylinder when the compression process begins. The compression pressure in the cylinder varies as the initial pressure of the charge at the beginning of the compression stroke. The initial pressure is determined by the volume and density of the charge admitted to the cylinder, or volumetric efficiency.

d. In simple terms, volumetric efficiency is the volume of the charge admitted into the engine cylinder compared to piston displacement and is expressed as a percentage. For example, a volume of 95 cubic inches of charge admitted into a cylinder of 100-cubic-inch displacement has a volumetric efficiency of 95 percent,     percent. With the advent of superchargers incorporated or installed on present day aircraft engines, the volumetric efficiency may be increased above 100 percent. because the charge is forced into the cylinder at pressures above atmospheric. Thus, the higher the manifold pressure reading of an aircraft engine in operation, the higher the compression pressure of the charge, resulting in higher power output.

8. Horsepower calculations. - a. A convenient and frequently used method of comparing the output of different engines is to designate the number of cubic inches of piston displacement required per brake horsepower. This is obtained by dividing the total cubic-inch piston displacement by its rated brake horsepower. In many auto-motive racing engines and aircraft engines this figure varies from 1.5 to 4 cubic inches per horsepower, and in some cases is as low as 1 cubic inch, because of the high compression pressures and piston speeds. Much depends on the volumetric efficiency. With the high temperatures and restrictions of the induction system experienced with some cylinders, the volumetric efficiency is seriously affected. The use of two inlet and two exhaust valves per cylinder and supercharging greatly increases the volumetric efficiency, with a corresponding increase in the power per unit of displacement volume.

b. In the inward movement of the piston in an engine cylinder, a specific volume is displaced from its upper extreme to its lower extreme position on the admission or intake stroke. This space or volume displaced by the piston is called "piston displacement," and consists of the product of the area of the cylinder bore and the length of the stroke of the pistons. In case of multiple-cylinder engines, this product is multiplied by the number of cylinders comprising the engine and may be expressed by either of the following formulas:

Displacement = 0.7854 D² LN (all dimensions in inches).

Displacement-3.1416 R² LN (all dimensions in inches).

Where D= Cylinder diameter,

L= Piston stroke,

N= Number of cylinders,

R = Cylinder radius.

Employing the first formula, the piston displacement of a 12-cylinder engine having a 5-inch bore and 7-inch stroke would be computed as follows :

0.7854 x 5² x 7" x 12 = 1.649.34 cubic inches

c. Low weight per horsepower is an essential requirement. for aircraft engines; therefore, it is quite natural that weight per horse-power should be a basis used for comparing engines. The term "weight per horsepower" may be applied to an engine with four different meanings.

(1) Dry weight of complete engine with carburetors, air stack, magnetos, and pumps.

(2) Weight with attached accessories such as magnetos. carburetors, and pumps, and with coolant in the cylinder jackets, connecting pipes, and coolant pump.

FIGURE: 4.--Recorded indicator card pressures.

(3) Weight with attached accessories such as radiators. tanks, batteries, and instruments, or, in other words, the entire power plant.

(4) Weight including all accessories and supplies. This basis is the most important and is dependent upon the length of required flight and fuel and oil consumption of the engine.

d. In comparing engines it may be found that an engine light in weight may prove to be low in horsepower or an engine high in horse-power may be heavy; therefore, it is essential that, in addition to weight per horsepower, the horsepower per cubic inch of piston displacement be known.

(1) Horsepower per cubic inch of piston displacement is dependent on the brake mean effective pressure (b. m. e. p.). or the average constant pressure which may be substituted to produce the same work as the actual varying pressures. Brake mean effective pressure is derived from the brake horsepower measured by the use of a Prony brake, a calibrated club, or dynamometer. The indicated mean effective pressure of an engine may be obtained by recording the cylinder pressures by means of an indicator card apparatus (fig. 4). and is always higher than brake mean effective pressure, because the latter includes the power necessary to overcome the friction of the various moving parts.

(2) The difference, then, between indicated horsepower (I. H. P.) and brake horsepower (B. H. P.) is designated as friction horsepower (F. H. P.) which can only be obtained from actual brake test. By knowing any two of these three horsepowers. the third may be obtained by the use of the following formula :

B. H. P. = I. H. P. - F. H. P.

(3) Friction horsepower ranges from 4 to 10 percent of the total power developed in the engine cylinders. It determines the mechanical efficiency of the engine which is expressed as a percentage and varies in aircraft engines from 90 to 96 percent.

e. The horsepower of an engine varies with its speed: therefore, without changing the weight of an engine, the horsepower may be increased by increasing the speed up to a certain limit. Consider next the effect of engine speed upon engine weight. It is evident that, in general, the higher the speed, the lower will be the engine weight per horsepower. The effect of engine speed upon engine reliability and engine life is important. At higher speeds, the reliability and the life of an engine are lessened.

f. When the B. H. P. is obtained by brake test, the b. m. e. p. may be calculated by the use of the following formula:

b. m. e. p. = B. H. P.  x  

Where L= Stroke in feet,

A= Bore in square inches.

N= Power strokes per minute.

In a four-stroke cycle engine. N is one-half the engine rpm multi-plied by the number of cylinders. The number 33.000 remains constant (33,000- -foot pounds per minute=1 H. P.).

g. Energy is neither created nor destroyed. The actual efficiency of an engine is the ratio of the power developed to the energy furnished in the form of heat from fuel-charge combustion. This ratio is necessarily low because of several factors. To prevent the metals used in the construction of the cylinders from melting, a large amount of heat must be dissipated through cooling, conduction, radiation, and exhaust. As previously stated, a small amount is utilized in overcoming friction of moving parts, etc. Considering the total heat supplied as 100 percent at full throttle and full load, the various losses may be divided as follows:

Heat loss through exhaust   30 %

Heat loss through cooling, conduction, and radiation    30 %

Heat loss through friction (including pumping losses and determined by mechanical deficiency)    6%

Total heat loss 66 %

If the total heat loss is 66 percent, then the energy available for power is only 34 percent. Although the percentage of heat losses has been reduced in the past few years, particularly in reducing friction, no further appreciable reductions may be expected from present-day conventional engines. Increased power per cubic inch of piston displacement has been obtained by increasing the total amount of heat supplied by an improvement in fuels, which permits higher volumetric efficiencies and higher compression ratios.

h.. In summing up the fundamental power factors which determine. the actual efficiency of an engine (excluding engine speed), it is noted that the compression ration, the volumetric efficiency, and the kind of fuel and mixture strength greatly affect the heat of combustion. If only 34 percent of this heat at full throttle, full load operation. is utilized as work in rotating the propeller, it is of interest to note that an aircraft engine which consumes 100 gallons of gasoline per hour utilizes only 34 gallons in actual work.