ELECTRONIC ENGINE CONTROL

ELECTRONIC ENGINE CONTROL

77. As stated in para. 8, some engines utilize a system of electronic control to monitor engine performance and make necessary control inputs to maintain certain engine parameters within predeter- mined limits. The main areas of control are engine shaft speeds and exhaust gas temperature (E.G.T.) which are continuously monitored during engine operation. Some types of electronic control function as a limiter only, that is, should engine shaft speed or E.G.T. approach the limits of safe operation, then an  input is made to the fuel flow regulator (F.F.R.) to reduce the fuel flow thus maintaining shaft speed or E.G.T. at a safe level. Supervisory control systems may contain a limiter function but, basically, by using aircraft generated data, the system enables a more appropriate thrust setting to be selected quickly and accurately by the pilot. The control system then  makes small control adjustments to maintain engine thrust consistent with that pre-set by the pilot, regardless of changing atmospheric conditions. Full authority digital engine control (FAD.E.G.) takes over virtually all of the steady state and transient control intelligence and replaces most of the hydromechani-cal and pneumatic elements of the fuel system. The  fuel system is thus reduced to a pump and control valve, an independent shut-off cock and a minimum of additional features necessary to keep the engine safe in the event of extensive electronic failure.

78. Full authority fuel control (F.A.F.C.) provides full electronic control of the engine fuel system in the sameway as F.A.D.E.C., but has none of the transient control intelligence capability used to control the  compressor airflow system as the existing engine control system is used for these

Principle of a gear pump

20. The most common type of oil distribution device is a simple orifice which directs a metered amount of
oil onto its target. These jet orifices are positioned as close to the target area as possible to overcome the
possibility of the local turbulent environment deflecting the jet of oil. The smallest diameter of a jet
orifice is 0.04 inch which allows a flow of 12 gallons per hour when operating at a pressure of 40 lb. per
sq. in. The use of restrictors upstream can reduce the flow rate if required.

Fig. 8-5 Principle of a gear pump.

21. All engines transfer heat to the oil by friction, churning and windage within a bearing chamber or
gearbox. It is therefore common practice to fit an oil cooler in recirculatory oil systems. The cooling
medium may be fuel or air and, in some instances, both fuel-cooled and air-cooled coolers are used.

22. Some engines which utilize both types of cooler may incorporate an electronic monitoring system

which switches in the air-cooled cooler only when it is necessary. This maintains the ideal oil temperature

and improves the overall thermal efficiency. 

Combustion stability

Fig. 4-11 Combustion stability limits.

28. Combustion stability means smooth burning and the ability of the flame to remain alight over awide operating range.
29. For any particular type of combustion chamber there is both a rich and weak limit to the air/fuel ratio, beyond which the flame is extinguished. An extinction is most likely to occur in flight during a glide or dive with the engine idling, when there is ahigh airflow and only a small fuel flow, i.e. a veryweak mixture strength.
30. The range of air/fuel ratio between the rich and weak limits is reduced with an increase of air velocity, and if the air mass flow is increased beyond a certain value, flame extinction occurs. A typical stability loopis illustrated in fig. 4-11

A vaporizer combustion chamber,

Fig. 4-5 A vaporizer combustion chamber

FUEL SUPPLY
12. Fuel is supplied to the airstream by one of two distinct methods. The most common is the injection of a fine atomized spray into the recirculating airstream through spray nozzles (Part 10). Thesecond method is based on the pre-vaporization ofthe fuel before it enters the combustion zone.
13. In the vaporizing method (fig.4-5) the fuel is sprayed from feed tubes into vaporizing tubes which are positioned inside the flame tube. These tubes turn the fuel through 180 degrees and, as they are heated by combustion, the fuel vaporizes before passing into the flame tube. The primary airflow passes down the vaporizing tubes with the fuel and also through holes in the flame tube entry section which provide ’fans’ of air to sweep the flame rearwards. Cooling and dilution air is metered into the flame tube in a manner similar to the atomizer flame tube.
TYPES OF COMBUSTION CHAMBER 
14. There are three main types of combustion chamber in use for gas turbine engines. These are the multiple chamber, the tubo-annular chamber and the annular chamber.
Multiple combustion chamber 
15. This type of combustion chamber is used on centrifugal compressor engines and the earlier types of axial flow compressor engines. It is a direct development of the early type of Whittle combustion chamber. The major difference is that the Whittle chamber had a reverse flow as illustrated in fig. 4-6 but, as this created a considerable pressure loss, the straight-through multiple chamber was developed by Joseph Lucas Limited.

Fig. 4-6 An early Whittle combustion chamber

An electronically operated bleed valve system

Fig. 3-18 An electronically operated bleed valve system.

Fig. 3-19 Typical types of fan blades.

32. For casing designs the need is for a light but rigid construction enabling blade tip clearances to be accurately maintained ensuring the highest possible efficiency. These needs are achieved by using aluminium at the front of the compression system followed by .alloy steel as compression temperature increases. Whilst for the final stages of the compression system, where temperature requirements possibly exceed the capability of the best steel, nickel based alloys may be required. The use of titanium in .preference to aluminium and steel is now more common; particularly in military engines where its high rigidity to density ratio can result in significant weight reduction. With the development of new manufacturing methods component costs can now be maintained at a more acceptable level in spite of high initial material costs.

A typical rotor blade showing twisted contour

Fig. 3-12 A typical rotor blade showing
twisted contour.

26. The rotor blades are of airfoil section (fig. 3-12) and usually designed to give a pressure gradient along their length to ensure that the air maintains a reasonably uniform axial velocity. The higher pressure towards the tip balances out the centrifugal action of the rotor on the airstream. To obtain these conditions, it is necessary to ’twist’ the blade from root to tip to give the correct angle of incidence at each point. Air flowing through a compressor creates two boundary layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slow air in the boundary layer a localized increase in blade camber both at the blade tip and root has been introduced. The blade extremities appear as ifformed by bending over each corner, hence the term’end-bend’.Stator vanes
27. The stator vanes are again of airfoil section and are secured into the compressor casing or into stator vane retaining rings, which are themselves secured to the casing (fig. 3-13). The vanes are often assembled in segments in the front stages and may be shrouded at their inner ends to minimize the vibrational effect of flow variations on the longer vanes. It is also necessary to lock the stator vanes in such a manner that they will not rotate around the casing.
OPERATING CONDITIONS
28. Each stage of a multi-stage compressor possesses

Methods of securing blades to disc and Rotors of drum and disc construction

Fig. 3-11 Methods of securing blades to disc.

Fig. 3-10 Rotors of drum and disc
construction.

23. The construction of the compressor centres around the rotor assembly and casings. The rotor shaft is supported in ball and roller bearings and coupled to the turbine shaft in a manner that allows for any slight variation of alignment. The cylindrical casing assembly may consist of a number of cylindrical casings with a bolted axial joint between each stage or the casing may be in two halves with a bolted centre line joint. One or other of these construction methods is required in order that the casing can be assembled around the rotor. Rotors

Principles of operation - Pressure and velocity changes through an axial compressor.

Fig. 3-9 Pressure and velocity changes
through an axial compressor.

19. During operation the rotor is turned at high speed by the turbine so that air is continuously induced into the compressor, which is then accelerated by the rotating blades and swept rearwards onto the adjacent row of stator vanes. The pressure rise results from the energy imparted to the air in the rotor which increases the air velocity. The air is then decelerated (diffused) in the following stator passage and the kinetic energy translated into pressure. Stator vanes also serve to correct the deflection given to the air by the rotor blades and to present the air at the correct angle to the next stage of rotor blades. The last row of stator vanes usually act as air straighteners to remove swirl from the air prior to entry into the combustion system at a reasonably uniform axial velocity. Changes in pressure and velocity that occur in the airflow through the compressor are shown diagrammatically in fig. 3-9. The changes are accompanied by a progressive increase in air temperature as thepressure increases.
20. Across each stage the ratio of total pressures of outgoing air

THE AXIAL FLOW COMPRESSOR

Fig. 3-7 Typical axial flow compressors.

THE AXIAL FLOW COMPRESSOR
13. An axial flow compressor (fig. 3-7 and fig. 3-8 consists of one or more rotor assemblies that carry blades of airfoil section. These assemblies are mounted between bearings in the casings which incorporate the stator vanes. The compressor is a multi-stage unit as the amount of pressure increase by each stage is small; a stage consists of a row of rotating blades followed by a row of stator vanes. Where several stages of compression operate in series on one shaft it becomes necessary to vary the stator vane angle to enable the compressor to operate effectively at speeds below the design condition. As the pressure ratio is increased the incorporation of variable stator vanes ensures that the airflow is directed onto the succeeding stage of rotor blades at an acceptable angle, ref. para. 30, Airflow Control.

Fig. 3-8 Typical triple spool compressor.

14. From the front to the rear of the compressor, i.e. from the low to the high pressure end, there is a

Construction - Typical impellers for centrifugal compressors

Construction
10. The construction of the compressor centres around the impeller, diffuser and air intake system. The impeller shaft rotates in ball and roller bearings and is either common to the turbine shaft or split in the centre and connected by a coupling, which is usually designed for ease of detachment. Impellers
11. The impeller consists of a .forged, disc with integral, radially disposed vanes on one or both sides (fig. 3-5) forming convergent passages in conjunction with the compressor casing. The vanes may be swept back, but for ease of manufacture straight radial vanes are usually employed. To ease the air from axial flow in the entry duct on to the rotating impeller, the vanes in the centre of the impeller are curved in the direction of rotation. The curved sections may be integral with the radial vanes or formed separately for easier and more accurate manufacture.Diffusers
12. The diffuser assembly may be an integral part of the compressor casing or a separately attached assembly. In each instance it consists of a number of vanes formed tangential to the impeller. The vane passages are divergent to convert the kinetic energy into pressure energy and the inner edges of the vanes are in line with the direction of the resultant airflow from the impeller (fig. 3-6). The clearance between the impeller and the diffuser is an important factor, as too small a clearance will set up aerodynamic buffeting impulses that could be transferred to the impeller and create an unsteady airflow and vibration.

Fig. 3-6 Airflow at entry to diffuser.

 

 

Fig. 3-5 Typical impellers for centrifugal
compressors.

THE CENTRIFUGAL FLOW COMPRESSOR

THE CENTRIFUGAL FLOW COMPRESSOR

Fig. 3-3 Pressure and velocity changes
through a centrifugal compressor.

5. Centrifugal flow compressors have a single or double-sided impeller and occasionally a two-stage, single sided impeller is used, as on the Rolls-Royce Dart. The impeller is supported in a casing that also contains a ring of diffuser vanes. If a double-entry impeller is used, the airflow to the _rear side is reversed in direction and a plenum chamber is required. Principles of operation
6. The impeller is rotated at high speed by the turbine and air is continuously induced into the centre of the impeller. Centrifugal action causes it to flow radially outwards along the vanes to the impeller tip, thus accelerating the air and also causing a rise in pressure to occur. The engine intake duct may contain vanes that provide an initial swirl to the air entering the compressor.

Fig. 3-4 Impeller working clearance and
air leakage.

Compressors - INTRODUCTION

INTRODUCTION
1. In the gas turbine engine, compression of the air before expansion through the turbine is effected by one of two basic types of compressor, one giving centrifugal flow and the other axial flow. Both types are driven by the engine turbine and are usually coupled direct to the turbine shaft.

Fig. 3-1 A typical centrifugal flow compressor.

2. The centrifugal flow compressor (fig. 3-1) is a single or two stage unit employing an impeller to accelerate the air and a diffuser to produce the required pressure rise. The axial flow compressor (fig. 3-7 and fig. 3-8) is a multi-stage unit employing alternate .rows of rotating (rotor) blades and stationary (stator) vanes, to accelerate and diffuse the air until the required pressure rise is obtained. In some cases, particularly on small engines, an axial compressor is used to boost the inlet pressure to the centrifugal.

Fig. 3-2 Specific fuel consumption and
pressure ratio.

3. With regard to the advantages and disadvantages of the two types, the centrifugal compressor is usually more robust than the axial compressor and is also easier to develop and manufacture. The axial compressor however consumes far more air than a centrifugal