10.4 Introduction: Air-Breathing Aircraft Engine Types

Note:

The burner is also known as the combustion chamber (CC) and the nozzle as the exhaust duct.

Overall engine efficiency improves if the higher energy of the exhaust gas of a straight-through turbojet is extracted through an additional turbine, which can drive a fan in front of a compressor (i.e., for a turbofan engine) or a propeller (i.e., for a turboprop engine).

10.4.2 Turbofan: Bypass Engine

The energy extraction through the additional turbine lowers the rejected energy at the exhaust, resulting in a lower exhaust velocity, pressure, and temperature. The additional turbine drives a fan in front of the compressor. The large amount of airmass flowing through the fan provides thrust. Part of the intake airmass flow through the fan is diverted (i.e., bypassed as the cold secondary flow) around the engine core and does not burn. The primary flow flows through the combustion chamber and is known also as the core flow or hot flow. Figure 10.5 is a schematic diagram of a turbofan engine (the top of the figure is a bare PW 4000). The lower exhaust velocity reduces engine noise. At the design point (i.e., LRC), the lower exhaust pressure permits the nozzle exit area to be sized to make the exit pressure equal to the ambient pressure (i.e., in a perfectly expanded nozzle). This is unlike simple turbojets, which can have a higher exit pressure.

Readers should note that the component-station-numbering system follows the same pattern as for the simple straight-through turbojet. The combustion chamber in the middle maintains the same numbers (i.e., 2–3). The only difference is the fan exit, which has the subscript f. The intermediate stages of the compressor and the turbine are primed.

Typically, the BPR (see Equation 10.2) for commercial jet-aircraft turbofans (i.e., high-subsonic flight speeds of less than Mach 0.98) is around 4 to 7. Recently, turbofans for the newer Boeing787, Airbus350 and Bombardier Cseries have reached BPR of 8 to 12. For military aircraft applications (i.e., supersonic flight speeds of up to Mach 2.5), the BPR is around 1 to 3. A lower BPR keeps the fan diameter smaller and, hence, lowers the frontal drag. Multispool drive shafts offer better efficiency and response characteristics, mostly with two concentric shafts. The shaft driving the low-pressure (LP) section runs inside the hollow shaft of the highpressure (HP) section (see Figure 10.5). Three shaft turbofans have been designed, but most of the current designs use a twin spool. The recent advent of a geared turbofan is indicative of better fuel efficiencies.

Figure 10.7. Schematic diagram of a turboprop engine

The simple straight-through turbojets have a relatively small frontal area resulting in low drag and excess air in the exhaust flow. If additional fuel can be burned in the exhaust nozzle beyond the turbine exit plane, additional thrust can be generated to propel an aircraft at a considerably higher speed and acceleration, thereby also possibly improving propulsive efficiency. However, the reason for using AB arises from the mission demand, such as at takeoff with a high payload and acceleration to engage or disengage during combat and evasion maneuvers. Mission demand overrides the fact that there is a high level of energy rejection in the high exhaust velocity. Fuel economy degrades with AB – it takes 80 to 120% more fuel burn to gain a 30 to 50% increase in thrust. Currently, most supersonic aircraft engines have some BPR when AB is done in the cooler mixed flow past the turbine section of the primary flow.

10.4.4 Turboprop Engine

Lower-speed aircraft can use propellers for thrust generation. Therefore, instead of driving a smaller encased fan (i.e., turbofan), a large propeller (i.e., turboprop) can be driven by a gas turbine engine to improve efficiency because the exhaust energy can be further extracted to a very low exhaust velocity (i.e., nearly zero nozzle thrust). Some residual jet thrust is left at the nozzle exit plane when it needs to be added to the propeller thrust. The nozzle thrust is converted to HP and, together with the SHP generated, it becomes the equivalent SHP (ESHP).

However, a large propeller diameter limits rotational speed due to both aerodynamic (i.e., transonic blade tips) and structural (i.e., centrifugal force) considerations. Heavy reduction gears are required to reduce the propeller rpm to a desirable level. Propeller efficiency decreases when aircraft are operating at flight speeds above Mach 0.5. For shorter-range flights, a turboprop’s slower speed does not become time-critical to the users, yet it offers better fuel economy. Figure 10.7 is a schematic diagram of a typical turboprop engine. Modern turboprops have up to eight blades (see Figure 10.31), which allow a reduction of the diameter size and operate at a relatively higher rpm and aircraft speed.

10.4.5 Piston Engine

Most aircraft piston engines are the reciprocating type (i.e., positive displacement, intermittent combustion): The smaller ones have an air-cooled, two-stroke cycle; the

Figure 10.8. Aircraft piston engine with installation components

larger ones (typically, more than 200 HP) have a liquid-cooled, four-stroke cycle. There are a few rotary-type positive-displacement engines (e.g., Wankel) – attractive in principle but they have sealing problems. Cost-wise, rotary-type positivedisplacement engines are not yet popular; cost will decrease with increased production. Figure 10.8 shows an aircraft piston engine with installation components.

To improve high-altitude performance (with low air density), supercharging is used. Figure 10.8 shows a vane-supercharging type for precompression. Also, AVGAS differs slightly from MOGAS. Recently, some engines for the homebuilt category have been allowed to use MOGAS. Recently for small aircraft application, diesel fuel-powered piston engines have appeared in the market.

Piston engines are the oldest type used for powering aircraft. Over the life cycle of an aircraft, gas turbines are more cost effective for engine sizes of more than 500 HP. Currently, general-aviation aircraft are the main users of piston engines. Small recreational aircraft invariably are powered by piston engines.

10.5 Simplified Representation of the Gas Turbine Cycle

Figure 10.9a depicts a standard schematic diagram representing a simple straightthrough turbojet engine, as shown in Figure 10.4, with appropriate station numbers. The thermodynamic cycle associated with gas turbines is known as the Joule cycle (also known as the Brayton cycle). Figure 10.9b is the corresponding temperature– entropy diagram of an ideal Joule cycle in which compression and expansion take place isentropically.

Real engine processes are not isentropic and losses are involved associated with increased entropy. Figure 10.10 is a comparison of real and ideal cycles.