Pratt and Whitney announced, in early June 2013, the successful first flight of the PW1100G Engine: The very engine family (PW1124G,PW1127G, PW1133G) that will be one of two engine options to power the Airbus A320NEO aircraft family (A319NEO, A320NEO, A321NEO). The other engine option is offered by GE-SNECMA’s CFM International’s LEAP engine series.
While CFM’s LEAP engine family is an improvisation of conventional turbofan engines, the Pratt and Whitney PW1000G family, of which the PW1100G series is a member, uses a rarely adopted technique, to promise high fuel burn saving by adopting a bypass ratio hitherto unheard of: 12:1. In this article, we explore, at a high level, the design of the PW1100G family, how it compares with existing A320 engines, the differences, on a high level, of this family with the competitor’s offering: the LEAP 1A, and why the Boeing 737MAX family doesn’t need such a large turbofan engine.
The fuel efficiency of an aircraft is dependent upon three factors: the drag contributed by the airframe, as parasitic drag from the fuselage, wings, horizontal and vertical tail planes, and induced drag from aerodynamic effects such as wing tip vortices; the drag contributed by the engine, by virtue of its shape and size, and effects due to the exhaust gases; and third and most importantly, the efficiency of the engine itself.
From a high level, there are two factors that determine the efficiency of an engine: The thermal efficiency, and the propulsive efficiency. In this article, we disregard mechanical losses in transmissions, gearboxes, and all forms of inter-mechanical conversions. Thermal efficiency deals with how efficiently the engine extracts mechanical work from a unit mass of fuel that is burnt. For all forms of jet engines, including turbojets, turbofans, turboprops, and turboshafts, this is the energy conversion that takes place inside the core of the engine, which includes the compressor, the combustion chamber, and the turbines that extract mechanical energy from the hot, expanding gases.
Propulsive efficiency deals with how effectively the extracted mechanical work is used to generate thrust. In the case of turbojet engines, the mechanical energy generating thrust is the hot expanding gases that exit from the engine at a high speed. For a given size, a “pure” jet engine such as a turbojet can deliver significantly more thrust than other forms of subsonic air breathing propulsions. This is achieved by accelerating a small mass of air to high speeds.
In all other derivatives of a turbojet, such as a turbofan, turboprop, and turboshaft, the propulsive efficiency is determined by the mechanism that extracts energy from the turbines in the engine, and how that is used to move a mass of air. In a turbofan engine, the energy of the hot expanding gases is used to drive a set of turbines, which turn a large ducted fan at the front of the engine. The fan moves a large mass of air, at a good speed. In a turboprop engine, the turbines turn a shaft, which moves to a reduction gearbox that turns slower but with greater torque. The high torque and low speed drives a large propeller, which moves a larger mass of air at a slow speed. In a turboshaft engine, the energy in the shaft is, much like a turboprop engine, passed through a reduction gearbox that drives a large rotor, such as that in a helicopter, which moves a very large mass of air at a slower speed.
In all three derivatives of the “pure” jet engine, the actual jet engine is relegated to the compressor, combustion chamber, and the turbines that drive the compressor. The energy extracted from the hot gases, via an extra set of turbines, drive either the fan, the propeller, or the rotor attached to a shaft, leaving very little energy in the hot gases that leave the turbines. The hot exhaust contributes to little (turbofan), or no thrust (turboshaft, turboprop).
This allows the creation of an engine that marries the best of both worlds: the advantages of a jet engine (such as high reliability, low failure rates, the ability to operate at high altitudes, and the high energy conversion efficiencies), with the benefits of methods that deliver high propulsive efficiency. Watch this video to understand how a jet engine works:
A heavy truck travelling at the same speed as a light car will inflict a greater damage in a collision. A car travelling at a higher speed than an equally lightweight but slower car will inflict a greater damage in a collision. Mass and speed determine momentum: larger of either, or both, will result in a greater momentum. Larger the momentum, more are the forces associated with a collision. Theoretically, the lightweight car mentioned in this example, if travelling fast enough, can inflict as much damage as a slow moving heavy truck.
If the same slow moving heavy truck is brought to a gradual halt, the damage caused is lower. If on the other hand, the truck is brought to a sudden halt, such as hitting a concrete wall, the damage can be far severe. The rate of change of momentum determines the forces associate with, as in this example, a collision.
The same principles apply to thrust generation, though not for destructive purposes. The rate of change of momentum determines thrust. The mass here is the mass of air that the engine ingests and spews out. The velocity is the speed of the exhaust gases, or air. These two determine the momentum. The rate of change of momentum is defined, in the case of an aircraft engine, the time taken to impart the exhaust velocity to the exhaust gases/air. This is the time taken between the engine ingesting a given mass of air, and expelling the air at a higher speed. The mass of air over time is known as mass flow rate.
Additionally, as the speed of the aircraft through the air approaches the speed at which the gases or air are expelled from the engines, the propulsive efficiency starts approaching 100%. If an aircraft engine’s exhaust airstream has a speed of 300kts, then the engine will perform best when the aircraft to which it is attached flies at close to 300kts. Above and below this speed, the propulsive efficiency decreases.
Supposing that an aircraft designed to fly at not more than 300kts through the air, needs a certain amount of thrust, say, 30,000 lbs (136,000N). The thrust can be delivered either by an engine that exhausts gases at 600kt (308 m/s), and has a mass flow rate of 440 kg/s, or by another engine that exhausts gases at 300kt (154m/s), and has a mass flow rate of 881kg /s. The second engine will bode well for the 300kt airplane, but will require a larger mass flow rate.
Ingesting a larger mass of air per second, and keeping the exhaust velocity low, will need a larger larger fan, or a larger propeller.
A large fan has three problems: First, it offers more drag to the oncoming air, which can offset the gains in propulsive efficiency. Second, a larger fan makes the engine heavier, adversely affecting fuel burn. Third the larger the fan gets, the faster the tips of the blade travel through the air, for a given rotational speed. Here is an example.
The fan of an IAE V2500 engine (that power the Airbus A320) spins at a maximum speed of 5,650 RPM. The fan diameter is 63.5 inches (1613mm). The distance covered by the blade tip, in one revolution is 1.613m X π = 5 meters. 5 meters X 5650 RPM = 28,630 m/minute = 477m/s, which is about 1.4 times the speed of sound at sea level. If the PW1100G’s 81 inch diameter fan (2057mm) is spun at the same speed, the blade tips travel at 608m/s, which is almost twice the speed of sound! (This is responsible for the characteristic chainsaw noise that can be heard from an Airbus A320’s engine when taking off at close to full take off power). A large amount of energy will be needed to overcome the drag associated with the high speed of the blades (this is different from the drag that the blades pose to the oncoming air stream). In addition, the airflow becomes more complex.
The Bombardier Q400, for instance, is a slow moving turboprop airplane (in relation to a jetliner) that has a 6 blade propeller, spinning at a 100% speed of 1020 RPM. The diameter of the massive propeller is 13.5ft (4115 mm). The tip speed at 1020RPM is 220 m/s.
In short, a turbofan engine suited for a subsonic airplane must have a larger diameter fan, to produce the same thrust at low exhaust velocities but higher propulsive efficiencies. However, the fan must spin slower, to keep the tips from attaining very high velocities.
The core of the engine, which is the “true” jet engine relegated to the role of a gas generator, is made of many significantly smaller diameter bladed disks. These bladed disks, some of which form the low pressure compressors, and others the high pressure compressors, ingest only a part of the total air sucked in by the large diameter fan. That air is compressed to a significant amount before mixing with fuel and being ignited in the combustion chamber.
For example, on the IAE V2527-A5, 17.24% of the air ingested by the fan enters the compressor. 4, smaller diameter disks with blades serve to initially compress the air ingested by the compressor. These compressors disks run at the same RPM as the fan at the front of the engine, which is a maximum of 5650 RPM.
The air is further compressed by 10 disks spinning at a speed different from those of the fan and low pressure compressor stages. These disks spin at a much higher speed, at a maximum of 14,950 RPM. It is this high rotational speed, that allows the 10 high pressure compressor stages to effectively compress the air to required levels. At the end of the tenth high pressure stage, the air is compressed to 32.8 times that of the ambient air. The small diameter, high RPM, high compressor stages contribute to most of the compression.
Because of the low contribution of the four low pressure compressors (attached to the fan) to the overall compression, the high pressure compressor must incorporate 10 stages. If however, the low pressure compressor could deliver a higher compression, the high pressure section could be reduced in stages. For the low pressure compressor to perform better, the disks must spin at a higher speed. But since the fan is attached to the same shaft that spins the low pressure compressors, and as demonstrated earlier the need for a slow spinning fan, the rotational speed of the low pressure compressors are limited.
Reducing the number of compression stages in an engine reduces weight, decreases system complexity, reduces the overall length of the engine, saves cost, and improves efficiency.
“Meshing” two speeds
The industry has long been aware of the need for slow spinning fans and fast compressors. By employing the same mechanism that is employed in a turboprop engine, which is a reduction gearbox, the fan essentially gets attached to a third shaft: one that is mechanically linked to the low pressure compressors, but spins at a lower speed. With the addition of a gearbox, the turbofan engine becomes a geared turbofan engine: a multi bladed, shrouded turboprop, in essence. The low pressure compressors run at a high speed, and the gearbox spins the fan at a much lower speed, allowing both sections to run at their optimum speeds.
The Geared turbofan isn’t a new concept. The British Aerospace BAe 146, a regional airliner that first flew in 1981, and produced till 2002, was fitted with four Textron Lycoming ALF 502R-5 geared turbofan engines. The Bombardier Challenger 600 originally were fitted with the ALF 502L geared turbofans. The TFE731, a geared turbofan engine, first ran in 1970, and its variants power popular airplanes such as the Learjet 35, 40,45 and 55, Dassault falcon 900DX, Hawker 800,850XP and 900XP, and a few Cessna Citations.
Pratt and Whitney PW1000 Geared Turbofan Engines
Pratt and Whitney’s PW1000G series of geared turbofan engines are a result of over 15 years of development that started with the work on the PW8000, which was planned as a V2500 and a CFM56 replacement for narrow body airliners such as the Airbus A320 and the Boeing 737. The PW8000 was announced in 1998, almost 11 years before Airbus announced the A320 NEO. A 30,000lb thrust demo geared turbofan engine flew during the conceptual phase for the first time, in 2008, on Pratt and Whitney’s Boeing 747SP Flying Test Bed, and later on an Airbus A340 Flying Test Bed.
The PW1000G “Geared Turbofan Engine” Family will power the Mitsubishi Regional Jet, the Bombardier C Series, the A320NEO family, and the Russian Irkut’s MC-21 series. The family spans across a 15,000lbs to 33,000lbs thrust range, but the architecture remains unchanged throughout.
The PW1000G architecture comprises one fan upstream, followed by a reduction gearbox that allows the low pressure compressors to run faster than the fan, two low pressure compressor stages, 8 high pressure compressor stages, 2 high pressure turbines (to keep the high pressure compressors running) and 3 low pressure turbines that keep the low pressure compressors and the fan running. A listing of the number of bladed disks is referred to as the “stage count”, and in the case of the PW1000G family of engines is:
1-G-3-8-2-3, with the “G” referring to the gear system that links the fan(1 bladed disk) with the low pressure compressor (3 bladed disks).
The reduction gearbox is planetary gear system, with the low pressure shaft driving a sun gear, and five planetary gears enmeshed between the sun gear and a ring gear non-rotating relative to the engine nacelle. A carrier cage, holding the planetary gears, drives the fan.
The PW1000G series have different variant numbers, even for seemingly similar engines. For example, the engines on offer for the A320 and the MC-21 are very similar, the difference lying in the numbering. Pratt and Whitney number their engines as PW-[Generation]-[Customer]-[Thrust Class in thousands of pounds of thrust]-[Specific]. This makes a 27,000lb (12.2kN) engine for the Airbus A320 NEO as PW-1-1-27-G, and the same thrust engine for the MC-21 as PW-1-4-27-G.
The PW1100G series, which will fly on the A320NEO family, feature an 81 inch (2057mm) diameter fan, and an astounding bypass ratio of 12:1, the highest ever. The high pressure compressor is expected to spin at around a maximum of 20,000RPM, and the low pressure compressor is expected to spin at around 10,000RPM. The gear ratio of the gearbox is 3:1, implying the low pressure compressor bladed disks spin at three times the speed of the fan, which is expected at around 3500RPM maximum. This results in optimal performance of both the fan and the low pressure compressor. We’ll see how this engine compares with the existing, highest thrust power plants for the A320: the IAEV2527-A5, and the CFM56-5B4.
At such rotational speeds, the tip of the 81 inch fan is expected to touch close to 370m/s, which is just 1.08 times, or 8% over, the speed of sound. Watch this video to understand more about the PW1100G:
Pratt and Whitney had planned to implement Variable Area Fan Nozzle (VAFN) to ensure optimal efficiencies across the flight spectrum. The VAFN, also known as the fan variable area nozzle (FVAN), attempted to adjust a flap assembly (seen colored in red in the image on the left) that would vary the fan exit area through which the fan air is discharged. This would vary the speed of the exhaust gases, optimising thrust and fuel economy at each flight regime. Pratt and Whitney patented a system, which is the first of its kind for high bypass turbofan engines: a simpler, and inexpensive system of variable area nozzles, as compared to those seen in military jets.
Pratt and Whitney however dropped the idea, “after the fan blade demonstrated better performance across the flight spectrum.” This indicates a method to further inch toward better propulsive efficiencies, but dropped in the light of the gains expected versus the loss in engine reliability and overall increase in weight associated with the addition of another mechanical system.
The alternate A320NEO engine: the CFM LEAP-1A
CFM hasn’t opted for a geared turbofan approach. Instead, CFM has stuck to conventional engine design: a twin spool engine, with the low pressure rotor connecting the low pressure compressor stages and the fan, making them spin at the same speed, and high pressure compressor stages that run at a different, higher speed.
CFM has endeavoured to target two birds with one stone: increased propulsive efficiency, and increased thermal efficiency. GE has long been working on raising the temperatures that can be withstood by its turbines. By ensuring high compression ratios, high temperatures, and a twin annular pre-swirl combustor contributing to a “lean burn”, the thermal efficiency of the engine is increased. However, the blades of the high pressure turbines, upon which the extremely hot gases impinge, must be able to withstand a higher temperature. CFM uses “advanced cooling” to keep the blades of the turbine cool, while employing a ceramic matrix composite (CMC) shroud to withstand the temperatures. Watch this video to understand more about the LEAP:
CFM’s fan for the Airbus NEO, measured at 78 inches in diameter is bigger than the 68.3 inches on the CFM 56-5B4 on existing A320s. The nearly 10 inch increase in fan diameter translates to a higher propulsive efficiency, but since the fan is attached to the low pressure compressors, it comes at a price. While the sum of the low pressure and high pressure compressor stages remain the same for the LEAP 1A and the CFM 56-5B4, at 13, the number of turbines differ significantly. While the -5B4 has 1 high pressure turbine + 4 low pressure turbines, the LEAP 1A features 2 HP turbines and 7 LP turbines.
Offseting the weight increase contributed by the significantly larger number of bladed disks are the “3D woven carbon fiber composite blades and case”, which are lighter, and promise greater durability.
CFM has been very cautious in the engine design, committing to “Proven Performance”, “Low Risk Execution”, and “Leading Technology”. The advantage that the LEAP will have over the Pure Power is the proven reliability of the conventional engine design, which a geared turbofan engine of such dimensions hasn’t had the chance to demonstrate.
The competition to the A319, A320, and A321 are the Boeing 737-700, 737-800 and 737-900, respectively. However, the thrust requirement for the Boeing 737-900, is 24,200-27,300lbs, which is much lower than the 30,000-33,000lbs thrust range required for the Airbus A321.
While the 737-800 can operate with a thrust range of 24,200-27,300 lbs, the A320CEO operates with a thrust range between 25,000lbs – 27,000lbs, in which case the ranges are similar. The Boeing 737-700 can fly with engines featuring a thrust in the range of 20,600 – 26,300 lbs, and the A319 can fly with 22,000 – 23,500lbs. In all cases, a Boeing 737NG can fly with lower thrust, with the largest gap seen between the Boeing 737-900 and the A321.
The design of a family of engines is determined by the highest thrust requirement. In the case of the PW1100G family, the size of the fan, 81 inches, was determined based on the need for 33,000lbs of thrust from the PW1133G to power the A321NEO. The PW1127G and the PW1124G share the same design and dimensions, but operate at less extreme conditions.
In comparison, the Boeing 737-9 MAX needs a thrust range of 27,000-28,000lbs. Since the 737 family and A320 family have similar cruise speeds, for the same speed of exhaust gases from the respective aircraft engine’s fan, the A321’s engine will need to be larger to move a larger mass of air, as opposed to the Boeing 737-9 MAX’s.
It is for this reason that the LEAP-1A that will power the A320NEO family, has a fan diameter of 78 inches, while the LEAP-1B that will power the Boeing737MAX family has a fan diameter of 69 inches, prompting Boeing to state, “The A320neo pays the economic price for the A321neo’s thrust needs”, with a larger, heavier engine with more turbine stages that offer more dag for the same thrust.
The geared turbofan engine has, theoretically atleast, a reduced reliability in comparison to a standard twin spool turbofan design, because of the inclusion of an extra mechanical stage: a gear system. The engine now houses three shafts, all turning at different speeds. Further, the gear system adds weight.
However, countering both effects seem to be the reduced number of engine stages, which have many benefits: decreased part count, increased reliability, and reduced weight. How the two effects: increased mechanical complexity and decreased stages counter each other’s effects is to be seen.
It seems unlikely, with the larger fan and the gear system, that the GTF will be lighter than existing IAE or CFM engines that power the A320 with the highest available thrust. Interestingly, the maximum thrust ratings for the PW1127G, as published by Pratt and Whitney, remain unchanged from what the IAE and CFMs offer for the A320: 27,000 lbs of thrust.
What changes, however, is the amount of drag that the new engine offers, due to its larger size. If there seems to be no significant weight difference between the existing engine options (2,500 kg) and the PW1100G series, there may be a performance penalty, marked by slower cruise speeds for the same thrust setting, possibly slower max cruise speeds, slower green dot (best L/D speed), slightly shallower climbs, and possibly degraded single-engine performance. The single engine performance difference is expected to be the most prominent, with a possibly larger yaw, and a definitely reduced climb gradient, consequently lowering obstacle clearances.
With a gear ratio of 3:1, the equivalent moment of inertia of the fan, as seen by the low pressure spool, is only 1/9th of the actual moment of inertia. The radius of the fan is 27% more than the IAEV2527-A5’s fan, making the volume of the fan approximately 2 times that of the IAE’s, implying very crudely that the mass is double that of the IAE’s, assuming the same material is used to make the blades. With double the mass and 27% greater radius, the moment of inertia of the PW1100G’s fan is about 3.3 times that of the IAE’s. This makes the equivalent moment of inertia of the fan, as seen by the low pressure spool, just 36% of the IAE’s, despite the larger mass and radius. Considering that the PW1100G’s low pressure spool has only 3 compressors and 3 turbines, as opposed to larger and heavier 4 compressors and 5 turbines on the IAEV2527-A5, the overall equivalent moment of inertia of the low pressure spool of the PW1100G’s is atleast around 25% that of the IAEV25257’s. With the PW1100G’s low pressure spool estimated to spin at twice the angular speed of the IAEV2527’s, and the overall moment of inertia around 25%, the spool up time may be reduced to around 50% of that in an IAEV2527-A5. This allows thrust to be made available faster, increasing safety margins through enhanced engine response times.
However, with the increased aerodynamic drag, the V1 will be lower, and the take off run is expected to be longer than A320s which feature high thrust IAE and CFM engines, with the Sharklets.
In all, while the PW1100G series engine is expected to contribute to around 11.5% of the proclaimed 15% fuel burn savings on the A320NEO, the A320 may not fare as well as the high thrust sharklet equipped A320CEO in high altitude, terrain challenged operations. However, neighbourhood noise is expected to be lower. The slow, large fan produces lesser noise, making the airplane quieter for both passengers and communities around airports.
The PW1000G series of engines, and especially the PW1100G family, are expected to be disruptive implementations, by employing the largest bypass ratio in the history of turbofan engines, and adopting a geared turbofan engine design of scales hitherto unmatched, promising double digit fuel burn savings. The slow speed of the fans, contribute to low noise, promising an enhanced passenger experienced, and reduced flight-related fatigue.
However, as with any new system, the reliability of such a huge geared turbofan engine isn’t known, casting initial doubts on dispatch reliability for airlines. Further, the PW1100G series focuses primarily on propulsive efficiency, forcing the engine to take on a large fan diameter of 81 inches, which will offer more drag than the competing LEAP 1A engine, which features a fan of diameter 3 inches smaller. This once sided effort towards better fuel savings increases drag, and may cost the Airbus A320’s takeoff, climb, and cruise performance, especially at areas that have short runways, and/or challenged by terrain. Although the spool up time of the GTF engine is expected to be lower, allowing the airplane to respond faster to a terrain alert, a penalty on climb performance is expected to exist, reducing, to an unknown extent, safety margins related to obstacle clearance.
The CFM LEAP 1A, on the other hand, with the reduced drag footprint, increased thermal efficiency, and optimised propulsive efficiency (although probably not as optimised as the PW1100G’s), may lead to similar fuel burn savings, with a lesser penalty on performance. However, the spool up time may be considerably longer than the PW1100G’s.
Either engine option will affect the 320’s performance, and may not be able to match upto the climb performance, safety and statistical reliability offered by today’s sharklet equipped A320 with either the IAEV2527-A5, or the CFM 56-5B4.