Rather than invest large sums of money into the development of clean-sheet airframe designs, in recent years OEMs have elected to redesign, improve and re-engine already successful airframes for the next generation of aircraft. In many of these cases, the engine accounts for a significant proportion of the efficiency savings.
Innovation in the aero engine industry is a constant process to stay one step ahead of the competition. Take the Rolls-Royce Trent 1000 for example. It competes against the GEnx on the Boeing 787 platform. Six years after entering service in 2011, Rolls-Royce developed an improved model, the Trent 1000-TEN, with more than 70 percent of parts either new or changed from the initial models. The Trent 1000-TEN contained technology through research attained from the Advance3 program and its sister engine, the Trent XWB.
There are myriad factors that an engine OEM has to consider when designing and manufacturing an engine. The OEM must ensure the operating and maintenance costs remain low, and reliability is high as these are key considerations for an operator during engine selection. The OEM must also comply with emissions and environmental regulations. These factors are largely tied to the propulsive and thermal efficiencies of an engine. In a nutshell, the goal is to develop an engine with as great a propulsive efficiency — that of the fan system, and thermal efficiency, that of the core — as possible.
Achieving these gains is proving increasingly difficult, particularly with current engine architecture, as much of the improvements are incremental rather than offering a step-change in efficiency. Advanced material development is costly and bypass ratios can only increase so much before size and weight become prohibitive and negate any perceived gain. However, could a demonstrator developed 40 years ago provide the answer?
A new concept was born in the 1970s at the height of the fuel crisis. It was a marriage that brought together the performance of a turbofan and the fuel efficiency of a turboprop. The concept was dubbed the propfan, also known as an open rotor or unducted fan due to its shroudless, contra-rotating fan blades. GE developed perhaps the most notable engine, the GE36. Boeing intended to offer it on the 7J7 while McDonnell Douglas saw it as an option for the MD-94X.
As part of a testing regime, McDonnell Douglas fitted the engine onto the left-side fuselage of a MD-80 aircraft. Astonishingly the open rotor was able to achieve a 30 percent improvement in specific fuel consumption over the JT8D turbojet. With such promising results, GE and McDonnell Douglas flew the aircraft to the 1988 Farnborough Airshow in an attempt to market the engine, but not all was rosy. Noise and vibration was an issue inside the cabin even when mounted at the rear of the fuselage. Airlines never truly bought into the concept, public perception was an issue and, by that time, oil prices had plummeted. Acquisition of the engine would be costly for a minor operating advantage given the economic climate and that turbofans were proving more reliable. The open rotor was dead.
At the same time, sales of the CFM56 took off following issues with the V2500. As a 50 percent shareholder in CFM with Safran, GE didn’t want to impinge on those sales and shelved the project. The success of the CFM56 would continue for the next four decades. Building on that success, CFM developed the LEAP engine to power the next generation of narrowbody aircraft.
Not all was lost from the GE36 programme though. GE leveraged the experience and technology gained from developing composite fan blades for the unducted fan and applied them to the GE90, and later the GEnx. The CFM LEAP engine was able to reap the benefits as it features 18 fourthgeneration 3D-designed carbon fiber composite fan blades. Coupled with a composite fan case, this has enabled CFM to reduce engine weight by around 1,000 pounds, compared to a fully metal fan and case.
Drawing on core technology from the GE90 and GEnx, blisks — bladed disks — are used within the compressor of the LEAP. This reduces the parts count, weight and maintenance costs. Although the LEAP features extra stages compared to the CFM56, which, in theory, should increase maintenance costs, CFM claim that costs and removal intervals should not be much different to its predecessor due to the materials and technology used. Rolls-Royce was able to generate a 15 percent weight saving from the high-pressure compressor (HPC) of the Trent XWB through the use of blisks.
Advancement in materials has seen ceramic matrix composites (CMCs) adopted across hot sections within engine cores. These materials require less cooling, are lightweight and have a higher resistance to temperatures than metals. Both LEAP and the GE9X have components made from CMCs in the High Pressure Turbine (HPT).
A common trend among new engines on the market is the reduction in the number of fan blades. Existing CFM56 models such as the CFM56-7B have 24 fan blades, while CFM56-5B engines have 36. However, with increasingly powerful super computers and the use of 3D air-flow-modelling software, the LEAP fan blades were designed to require just 18 fan blades.
This trend is not restricted to narrowbodies. Large widebody turbofans such as the Trent 7000 feature 20 highly swept fan blades, a reduction from 26 blades on the Trent 700. The next widebody engine to enter the market will be the GE9X in 2020. This will solely power the 777X and features just 16 fan blades compared to the GE90, which had 22. And 3D aerodynamic modelling can be seen throughout the engine. Compressor and turbine blades are modelled with 3D aerodynamics to ensure they minimize losses and inefficiencies and extract the greatest work from the airflow.
To improve thermal efficiency, many OEMs are using lean-burn combustors. GE has developed the twin-annular pre-mixing swirler (TAPS) combustor for the GEnx, a second generation TAPS II combustor for the CFM LEAP and a third-generation model in the GE9X. Pratt & Whitney have the TALON X lean burn combustor fitted to the PW1000G and along with CFM claim the system will reduce NOx emissions by around 50 percent against CAEP/6 (Committee on Aviation Environmental Protection) standards.
The difficulty in achieving sizeable improvements from conventional two- or three-spool engines is only getting harder. Increasing the bypass ratio to improve the propulsive efficiency will invariably cause the low pressure turbine (LPT) to grow in size and stage numbers. Large turbine systems tend to be expensive, heavy and increase the cost of maintenance.
Pratt & Whitney has overcome this issue with the PW1000G geared turbofan (GTF). The GTF has a planetary gearbox sitting between the fan and HPC. The reduction gearbox transmits the load from the LPT to the fan allowing the fan to spin three times slower at its optimal speed. This allows the fan to grow in size with little impact to the LPT. In fact, the engine can reduce in length as fewer LPT stages are required, allowing the LPT to spin faster at its optimum speed.
Rolls-Royce has seen the benefit of having a gearbox. As part of the UltraFan research program, they have invested around €65M to build a test bed and facility for their power gearbox. The gearbox is designed to operate on widebody engines and reach up to 100,000 horsepower. To incorporate the gearbox design, Rolls-Royce aims to remove the LPT altogether and have an enhanced intermediate pressure turbine (IPT) to drive the fan via the gearbox. This reduction in weight and improvement in efficiency is claimed to allow UltraFan engines to achieve 25 percent fuel efficiency improvements over first-generation Trent engines.
Little news has emerged from Boeing with regards to their New Midsize Airplane (NMA) program. If it is launched, intense competition will surely follow between many of the major engine OEMs to supply the powerplant. Current estimates place the entry into service in the mid-to-late 2020s with the need for an ultra-high bypass ratio turbofan. This falls in line with the timeframe that RollsRoyce has stated for technology readiness of the UltraFan. CFM International, having also studied an engine with a geared system, and Pratt & Whitney have both stated an interest in the program.
Maintenance, Manufacturing, Monitoring
Material and turbomachinery do not represent the only advancements in the aero engine industry. Maintenance, manufacturing and monitoring have all evolved.
The advent of additive layer manufacturing (ALM), more commonly known as 3D printing, has allowed OEMs the freedom to design and manufacture intricate parts that would otherwise have been impossible with traditional methods. For example, blades with a complex network of cooling passages that aren’t able to be engineered with traditional techniques can be printed. GE, for one, has adopted the 3D-printing capability, having launched its own ALM division, GE Additive, to supply 3D printers, materials and consulting services. The LEAP features 3D-printed fuel nozzles, which are claimed to be 25 percent lighter than its predecessors and five times more durable.
In designing the Advanced Turboprop, GE engineers were able to reduce 855 individual parts to 12 parts. More than a third of the engine is built using ALM parts, which enables a five percent reduction in engine weight. GE also claims the engine will be able to operate for 1,000 hours longer than its competitors, further extending the mean time between overhaul. Fewer parts benefits the operator, in terms of reduced maintenance cost and downtime.
Systems health monitoring has been aided by the explosion of activity in the big data market. During each flight, data is recorded and transmitted live. A 787 can generate anywhere in the region of 500 gigabytes of data every flight. Using analytics, airlines and OEMs can interrogate the data to monitor the health of their fleet and schedule maintenance accordingly. This works particularly well for both the airline and OEM if the airline is enrolled on a flighthour agreement (FHA). Predictive maintenance can reduce downtime, in-service delays or flight cancellations benefitting both parties.
For MROs, the next frontier is to incorporate the use of drones and virtual reality platforms during maintenance. EasyJet has been trialling the use of drones for inspections and damage assessments during the past number of years in a bid to reduce and eliminate technical delays.
Rolls-Royce and GE are both heavily involved in robotics research to assist engine inspections and repairs. With the goal to reduce costs, the OEM can remotely assist a local engineer with assessments and repairs from anywhere in the world rather than fly to the location and incur hefty costs. In the same vein, augmented or virtual reality will allow more qualified engineers to assist local technicians using wearable technology to help with maintenance.
Not only is virtual reality useful for maintenance activities, it can be a powerful tool for training the next generation of maintenance crew. Japan Airlines has recently begun to use virtual reality headsets to train engine mechanics and flight crew.
To the future
As fuel prices continue to climb, the need for highly fuel-efficient engines will continue to grow. Safran, in conjunction with research partners, successfully began ground tests of an open rotor demonstration engine in October 2017. The aim is to develop an engine capable of entering service in 2030 and offering airlines a 30 percent reduction in fuel consumption and carbon dioxide emissions compared to the current CFM56 engines. Coming around full circle, OEMs are once again seeking that step-change in technology to find new ways of generating improvements.
A number of OEMs have invested in and are studying a range of new technologies from embedded propulsion and distributed propulsion to hybrid-electric aircraft. The latter has seen a flurry of activity in recent months. Boeing and JetBlue Airways have backed Zunum Aero, a Seattle-based startup that aims to build a hybrid-electric commuter aircraft. On the European front, Airbus, Rolls-Royce and Siemens have partnered to launch a hybrid-electric demonstrator — the E-Fan X.
There are many exciting technologies already present in the aero engine industry. Those on the horizon are still studies or early concepts, but will they become a reality? Only time will tell.
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