Section II - Principles of Operation.pdf

8/16/2019 Section II - Principles of Operation.pdf

1/8

Section II. Principles of Operation

1.7. GENERAL

This section covers the principles of turbine engine operation. The three classifications of

turbine engines are turbojet, turboshaft, and ramjet. The term "turbo" means "turbine."Therefore, a turboshaft engine is one which delivers power through a shaft.

1.8. OTTO AND BRAYTON CYCLES

There is an element of similarity to both the reciprocating and jet engines, but thethermodynamic cycle of each is different from the other. The reciprocating engineoperates on the Otto cycle, a constant volume cycle, consisting of four distinct operations.These operations are performed intermittently by a piston reciprocating in an enclosedcylinder. It is important to remember that the piston in a reciprocating engine delivers

power only during one of its four strokes.

The turbine engine operates on the Brayton cycle, a constant pressure cycle containingthe same four basic operations as the Otto cycle, but accomplishing them simultaneouslyand continuously so that an uninterrupted flow of power from the engine results.

1.9. BRAYTON CYCLE OF OPERATION

Ambient air is drawn into the inlet section by the rotating compressor. The compressorforces this incoming air rearward and delivers it to the combustion chamber at a higher pressure than the air had at the inlet. The compressed air is then mixed with fuel that issprayed into the combustion chamber by the fuel nozzles. The fuel and air mixture is thenignited by electrical igniter plugs similar to spark plugs. This ignition system is only inoperation during the starting sequence, and once started, combustion is continuous andself-sustaining as long as the engine is supplied with the proper air-fuel ratio. Only about25 percent of the air is used for combustion. The remaining air is used for internalcooling and pressurizing.

The turbine engines in the Army inventory are of the free-power turbine design, as shownin figure 1.2. In this engine, nearly two-thirds of the energy produced by combustion isextracted by the gas producer turbine to drive the compressor rotor. The power turbineextracts the remaining energy and converts it to shaft horsepower (shp), which is used todrive the output shaft of the engine. The gas then exits the engine through the exhaustsection to the atmosphere. Army helicopters use a divergent duct to eliminate theremaining thrust. The various kinds of exhaust ducting are discussed in detail with theengine using that particular ducting.


2/8

1.10. TURBOJET

The turbojet is the engine in most common use today in high-speed, high-altitude aircraft,not in Army aircraft. With this engine, air is drawn in by a compressor which raisesinternal pressures many times over atmospheric pressure. The compressed air then passesinto a combustion chamber where it is mixed with fuel to be ignited and burned. Burning

the fuel-air mixture expands the gas, which is accelerated out the rear as a high-velocity jet-stream. In the turbine section of the engine, the hot expanded gas rotates a turbinewheel which furnishes power to keep the compressor going. The gas turbine engineoperates on the principle of intake, compression, power, and exhaust, but unlike thereciprocating engine, these events are continuous. Approximately two-thirds of the totalenergy developed within the combustion chamber is absorbed by the turbine wheel tosustain operation of the compressor. The remaining energy is discharged from the rear ofthe engine as a high velocity jet, the reaction to which is thrust or forward movement ofthe engine. The turbojet is shown schematically in figure 1.3.


3/8

Figure 1.3. Axial-Flow Turbojet Engine.

1.11. TURBOPROP ENGINE AND TURBOSHAFT ENGINE

The turboprop engine and turboshaft engines, shown in figures 1.4 and 1.5, are of thesame basic type as the turbojet. Instead of ejecting high-velocity exhaust gases to obtainthrust, as in the turbojet, a turbine rotor converts the energy of the expanding gases torotational shaft power. A propeller or helicopter transmission can be connected to theengine through reduction gearing. This energy may be extracted by the same turbine rotorthat drives the compressor, or it may be a free-power turbine which is independent of thecompressor turbine and only linked to it by the expanding gases.

Figure 1.4. Axial-Flow Turboprop Engine.


4/8

Figure 1.5. Centrifugal-Flow Turbojet Engine.

The free-power turbine is the type used in Army aircraft to harness the energy of thegases and convert this energy to rotational shaft power. This feature of having a free- power turbine enables the power output shaft to turn at a constant speed while the power producing capability of the engine can be varied to accommodate the increased loadsapplied to the power output shaft. Turbine engines may be further divided into threegeneral groups, centrifugal-flow, axial-flow, and axial-centrifugal-flow, depending uponthe type of compressor. Figure 1.4 shows an axial-flow turboprop engine, figure 1.5shows a centrifugal-flow turbojet engine, and figure 1.5a shows an axial-centrifugal-flowcompressor.


5/8

Figure 1.5a. Axial-Centrifugal-Flow Compressor.

1.12. ADVANTAGES OF TURBINE ENGINES


6/8

Keeping in mind the basic theory of turbine engines, compare the advantages anddisadvantages of the turbine engine with the piston or reciprocating engine. Theadvantages are covered in the subparagraphs below, and disadvantages are discussed inthe next section.

Power-to-weight ratio. Turbine engines have a higher power-to-weight ratio thanreciprocating engines. An example of this is the T55-L-l11. It weighs approximately 650 pounds and delivers 3, 750 shaft horsepower. The power-to-weight ratio for this engine is5.60 shp per pound, where the average reciprocating engine has a power-to-weight ratioof approximately .67 shp per pound.Less maintenance. Maintenance per hour of operation is especially important in militaryoperations. Turbine engines require less maintenance per flying hour than reciprocatingengines generally do. As an aircraft maintenance officer, this advantage will appeal toyou because of a greater aircraft availability and lower maintenance hour to flying hourratio. The turbine engine also has fewer moving parts than a reciprocating engine; this isalso an advantage over the reciprocating engine.

Less drag. Because of the design, the turbine engine has a smaller frontal area than thereciprocating engine. A reciprocating engine requires a large frontal area which causes agreat deal of drag on the aircraft. Turbine engines are more streamlined in design, causingless drag. Figure 1.6 shows one of the two nacelles that contain reciprocating engines inthe old CH-37 cargo helicopter. Figure 1.7 shows the smaller frontal area of the turbineengines that power the CH-47 Chinook helicopter. Because of this, the engine nacellesare more streamlined in design, causing less drag.

Figure 1.6. Reciprocating Engine Nacelles on CH-37.

Figure 1.7. Turbine Engine Nacelles on CH-47.

Cold weather starting. The turbine engine does not require any oil dilution or preheatingof the engine before starting. Also, once started, the reciprocating engine takes a long


7/8

time to warm up to operating temperatures, whereas the turbine engine starts readily andis up to operating temperature immediately.Low oil consumption. The turbine engine, in general, has a lower rate of oil consumptionthan the reciprocating engine. The turbine engine does not require the oil reservoircapacity to be as large as the reciprocating engine's; because of this, a weight and

economy factor is an additional advantage.

1.13. DISADVANTAGES OF TURBINE ENGINES

Just like everything else, along with the advantages or the good, we have to take thedisadvantages or the bad. This also holds true with the turbine engine. The disadvantagesof the turbine engine are discussed in the following subparagraphs.Foreign object damage. One of the major problems faced by the turbine engine is foreignobject damage (FOD). A turbine engine requires tremendous quantities of air. This air issucked into the engine at extremely high velocities, and it will draw up anything thatcomes near the inlet area. The turbine engines used in Army aircraft are fitted with filters

around the engine inlet to prevent foreign objects from entering the engine and damagingthe compressor vanes. However, even with this precaution, FOD is still a menace toturbine engine operation, as shown in figure 1.8.

Figure 1.8. Compressor Foreign Object Damage.

High temperatures. In the combustion chamber, the temperature is raised to about 3, 500°F. in the hottest part of the flame. Because this temperature is above the melting point ofmost metals, proper cooling and flame dilution must be employed at all times to insurethat the engine is not damaged.Slow acceleration. The acceleration rate of a turbine engine is very slow in comparisonwith that of a reciprocating engine. The pilot must be aware of the time lag in the turbineengine acceleration between the instant when power is requested and when power is

available.High fuel consumption. Turbine engines are very uneconomical when it comes to theamount of fuel they consume. The Lycoming T53 turbine engine, for instance, usesapproximately 1.5 gallons per minute of fuel. Compare it to a reciprocating engine ofapproximately the same horsepower which has a fuel consumption rate of 1 gallon perminute.


8/8

Cost. The initial cost of a turbine engine is very high when compared to the cost of areciprocating engine. For example the T53-L-13B engine costs about $63,000, and thecost of a reciprocating engine of approximately the same horsepower is $20,000.

1.14. SUMMARY

The two turbine engines commonly in use today are the turbojet and turboshaft. Theturbine has surpassed the piston engine in design efficiency. The advantages of the gasturbine are a high power-to-weight ratio, less maintenance, and low oil consumption.Because of the small frontal area, turbines have less aerodynamic drag. Thedisadvantages are foreign object damage to the compressor vanes, high operatingtemperatures, and high fuel consumption. The turbine also has a slower acceleration rate.Because of the high operating rpm, all rotating parts must be in perfect balance. The costto manufacture a turbine is much higher than that of a reciprocating engine. Aircraft

designers have always been limited by the powerplants available for use on aircraft ofnew design. Their constant plea has been for higher power, less weight, and a morecompact design; the turbine engine has been the answer to some, if not all, of their pleas.

Section II - Principles of Operation.pdf

Documents

Transcript of Section II - Principles of Operation.pdf