Introduction to Air Breathing Engines
Each engine type will operate only within a certain range of altitudes and Mach numbers (velocities). Similar limitations in velocity and altitude exist for airframes. It is necessary, therefore, to match airframe and propulsion system capabilities. Figure 1 shows the approximate velocity and altitude limits, or corridor of flight, within which airlift vehicles can operate. The corridor is bounded by a lift limit, a temperature limit, and an aerodynamic force limit. The lift limit is determined by the maximum level-flight altitude at a given velocity. The temperature limit is set by the structural thermal limits of the material used in construction of the aircraft. At any given altitude, the maximum velocity attained is temperature-limited by aerodynamic heating effects. At lower altitudes, velocity is limited by aerodynamic force loads rather than by temperature.
The operating regions of all aircraft lie within the flight corridor. The operating region of a particular aircraft within the corridor is determined by aircraft design, but it is a very small portion of the overall corridor. Superimposed on the flight corridor in Fig. 1 are the operational envelopes of various powered aircraft. The operational limits of each propulsion system are determined by limitations of the components of the propulsion system and are shown in Fig. 2.
Figure 1 - Flight limits
Figure 2 - Engine operational limits
The turbojet, turbofan, turboprop, turboshaft, and ramjet engine systems will be discussed in this section. The discussion of these engines will be in the context of providing thrust for aircraft. The listed engines are not all the engine types (reciprocating, rockets, combination types, etc.,) that are used in providing propulsive thrust to aircraft, nor are they used exclusively on aircraft. The thrust of the turbojet and ramjet results from the action of a fluid jet leaving the engine, hence, the name "jet engine" is often applied to these engines. The turbofan, turboprop, and turboshaft engines are adaptations of the turbojet to supply thrust or power through the use of fans, propellers, and shafts.
The "heart" of a gas turbine-type engine is the gas generator. A schematic diagram of a gas generator is shown in Fig. 3. The compressor, combustor, and turbine are the major components of the gas generator which is common to the turbojet, turbofan, turboprop, and turboshaft engines. The purpose of a gas generator is to supply high temperature and pressure gas.
Figure 3 - Gas Generator
By adding an inlet and a nozzle to the gas generator, a turbojet engine can be constructed. A schematic diagram of a simple turbojet is shown in Fig. 4a and a turbojet with afterburner in Fig. 4b. In the analysis of a turbojet engine, the major components are treated as sections. Also shown in Figs. 4a and 4b are the station numbers for each section.
The turbojet was first used as a means of aircraft propulsion by von Ohain (first flight August 27, 1939) and Whittle (first flight May 15, 1941). As development proceeded, the turbojet engine became more efficient and replaced some of the piston engines. A cross-section of the J79 turbojet with afterburner used in the F-4 Phantom II and B-58 Hustler is shown in Fig. 5. The adaptions of the turbojet in the form of turbofan, turboprop, and turboshaft engines came with the need for more thrust at relatively low speeds. Some characteristics of different turbojet, turbofan, turboprop, and turboshaft engines are included in Tables I, II, III, and IV.
Figure 4a - Schematic diagram of a turbojet
Figure 4b - Schematic diagram of a turbojet with afterburner
The thrust of a turbojet is developed by compressing air in the inlet and compressor, mixing the air with fuel and burning in the combustor, and expanding the gas stream through the turbine and nozzle. The expansion of gas through the turbine supplies the power to turn the compressor. The net thrust delivered by the engine is the result of converting internal energy to kinetic energy.
The pressure, temperature, and velocity variations through a J-79 engine are shown in Fig. 6. In the compressor section, the pressure and temperature increase as a result of work being done on the air. The temperature of the gas is further increased by burning in the combustor. In the turbine section, energy is being removed from the gas stream and converted to shaft power to turn the compressor. The energy is removed by an expansion process which results in a decrease of temperature and pressure. In the nozzle, the gas stream is further expanded to produce a high exit kinetic energy. All the sections of the engine must operate in such a way to efficiently produce the greatest amount of thrust for a minimum of weight.
Figure 6 - Property variations through the General Electric J79 afterburning turbojet engine
The turbofan engine consists of an inlet, fan, gas generator, and nozzle. A schematic diagram of a turbofan is shown in Fig. 7. In the turbofan, a portion of the turbine work is used to supply power to the fan. Generally the turbofan engine is more economical and efficient than the turbojet engine in a limited realm of flight. The thrust specific fuel consumption (TSFC, fuel mass flow rate per unit thrust) is lower for turbofans and indicates a more economical operation. The turbofan also accelerates a large mass of air to a lower velocity than a turbojet for a higher propulsive efficiency. The frontal area of a turbofan is quite large compared to a turbojet and for this reason more drag and weight results. The fan diameter is also limited aerodynamically when compressibility effects occur. Several of the current high bypass turbofan engines used in subsonic aircraft are shown in Figs. 8a through 8f.
Figure 7 - Schematic diagram of a high-bypass-ratio turbofan
Figure 8a - Pratt & Whitney JT9D turbofan
Figure 8b - Pratt & Whitney PW4000 turbofan
Figure 8c - General Electric CF6 turbofan
Figure 8d - Rolls-Royce RB-211-524G/H turbofan
Figure 8e - General Electric GE90 turbofan
Figure 8f - SNECMA CFM56 turbofan
The Afterburning Turbofan Engine
The afterburning turbofan engine is shown in Figure 9a. In this engine, the bypass stream is mixed with the core stream before passing through a common afterburner and exhaust nozzle. Figures 9b and 9c show the Pratt & Whitney F100 turbofan and the General Electric F110 turbofan, respectively. These high performance afterburning turbofan engines are used in both the F15 Eagle and F16 Falcon supersonic fighter aircraft.
Figure 9a - Station numbering for mixed-flow, afterburning turbofan
Figure 9b - Pratt & Whitney F100-PW-229 afterburning turbofan
Figure 9c - General Electric F110-GE-129 afterburning turbofan
A gas generator that drives a propeller is a turboprop engine. The expansion of gas through the turbine supplies the energy required to turn the propeller. A schematic diagram of the turboprop is shown in Fig. 10a. The turboshaft engine is similar to the turboprop except that the power is supplied to a shaft rather than a propeller. The turboshaft engine is used quite extensively for supplying power for helicopters. The turboprop engine may find application in VTOL (vertical takeoff and landing) transporters. The limitations and advantages of the turboprop are those of the propeller. For low speed flight and short field takeoff, the propeller has a performance advantage. At speeds approaching the speed of sound, compressibility effects set in and the propeller loses its aerodynamic efficiency. Due to the rotation of the propeller, the propeller tip will approach the speed of sound before the vehicle approaches the speed of sound. This compressibility effect when approaching the speed of sound limits the design of helicopter rotors and propellers. At high subsonic speeds, the turbofan engine will have a better aerodynamic performance than the turboprop since the turbofan is essentially a "ducted turboprop." Putting a duct or shroud around a propeller increases its aerodynamic performance. Examples of a turboshaft engine are the Pratt & Whitney of Canada PT6 (Fig. 10c) used in many small commuter aircraft, and the Allison T56 (Fig.10b) used to power the C-130 Hercules and the P-3 Orion.
Figure 10a - Schematic diagram of a turboprop
Figure 10b - Allison T56 turboshaft
Figure 10c - Pratt & Whitney of Canada PT6 turboshaft
The ramjet engine consists of an inlet, a combustion zone, and a nozzle. A schematic diagram of a ramjet is shown in Fig. 11. The ramjet does not have the compressor and turbine as the turbojet does. Air enters the inlet where it is compressed and then enters the combustion zone where it is mixed with the fuel and burned. The hot gases are then expelled through the nozzle developing thrust. The operation of the ramjet depends upon the inlet to decelerate the incoming air to raise the pressure in the combustion zone. The pressure rise makes it possible for the ramjet to operate. The higher the velocity of the incoming air, the more the pressure rise. It is for this reason that the ramjet operates best at high supersonic velocities. At subsonic velocities, the ramjet is inefficient and, in order to start the ramjet, air at a relatively high velocity must enter the inlet.
Figure 11 - Schematic diagram of a ramjet
The combustion process in an ordinary ramjet takes place at low subsonic velocities. At high supersonic flight velocities, a very large pressure rise is developed that is more than sufficient to support operation of the ramjet. Also, if the inlet has to decelerate a high supersonic velocity air stream to a subsonic velocity, large pressure losses can result. The deceleration process also produces a temperature rise and, at some limiting flight speed, the temperature will approach the limit set by the wall materials and cooling methods. Thus when the temperature increase due to deceleration reaches the limit, it may not be possible to burn fuel in the air stream.
In the past few years, research and development have been done on a ramjet that has the combustion process taking place at supersonic velocities. By using a supersonic combustion process, the temperature rise and pressure loss due to deceleration in the inlet can be reduced. This ramjet with supersonic combustion is known as the SCRAMJET (supersonic combustion ramjet). Figure 12a shows the schematic of a scramjet engine similar to that proposed for the National AeroSpace Plane (NASP) research vehicle, the X-30 shown in Fig. 12b. Further development of the scramjet for other applications (e.g., the Orient Express) will continue if this research and development produces a scramjet engine with sufficient performance gains. It must be remembered that since it takes a relative velocity to start the ramjet or scramjet, another engine system is required to accelerate aircraft like the X-30 to ramjet velocities.
Figure 12a - Schematic diagram of a scramjet
Figure 12b - Conceptual drawing of the X-30
Two of the Pratt & Whitney J58 Turbojet engines (see Fig. 13a) are used to power the Lockheed SR71 Blackbird (see Fig. 13b). This was the fastest aircraft (Mach 3+) when retired in 1989. The J58 operates as an afterburning turbojet engine until it reaches high Mach when the six large tubes (Fig. 13a) bypass flow to the afterburner. When these tubes are in use, the compressor, burner, and turbine of the turbojet are essentially bypassed and the engine operates as a ramjet with the afterburner acting as the ramjet's burner.
Figure 13a - Pratt & Whitney J58 turbojet
Figure 13b - Lockheed SR-71 Blackbird
Aircraft Engine Performance Parameters
The two most useful performance parameters for airbreathing engines in aircraft propulsion are the thrust of the engine (F) and the trust specific fuel consumption (S). Values of thrust (F) and fuel consumption (S) for various jet engines at sea level static conditions are listed in Tables I, II, III, and IV. The predicted variations of uninstalled engine thrust (F) and uninstalled thrust specific fuel consumption (S) with Mach number and altitude for an advanced fighter engine are plotted in Figs. 14a through 14d. Note that the thrust (F) decreases with altitude and the fuel consumption (S) also decreases with altitude until 36k ft (the start of the isothermal layer of the atmosphere). Also note that the fuel consumption increases with Mach number and the thrust varies considerably with Mach number. The predicted partial throttle performance of the advanced fighter engine is shown at three flight conditions in Fig. 14e.
Figure 14a Uninstalled Thrust at Maximum Power
Figure 14b Uninstalled Thrust Specific Fuel Consumption at Maximum Power
Figure 14c Uninstalled Thrust at Military Power
Figure 14d Uninstalled Thrust Specific Fuel Consumption at Military Power
Figure 14e Uninstalled Partial Power Performance
The takeoff thrust of the JT9D high bypass turbofan engine is given in Figure 15a versus Mach number and ambient air temperature for two versions. Note the rapid fall-off of thrust with rising Mach number that is characteristic of this engine cycle and the constant thrust at a Mach number for temperatures of 86F and below (this is often referred to as a flat rating). The partial throttle performance of both engine versions is given in Figure 15b for two combinations of altitude and Mach number.
Figure 15a JT9D-70/-70A Takeoff Thrust at Sea Level
Figure 15b JT9D-70/-70A Cruise Performance