Article - Issue 53, December 2012
Perfecting the jet engine
Professor Philip Ruffles CBE FREng FRS
The Trent 1000 which entered service with All Nipon Airways in October 2011 delivers the lowest fuel burn, emissions and noise for the Boeing 787 Dreamliner and formed the basis for the Environmentally Friendly Engine copyright Rolls-Royce
Fuel costs and environmental concerns are heavily influencing developments within the civil aerospace industry. The aircraft of the future and the propulsion systems that power them might look radically different in decades to come. Ingenia asked Rolls-Royce, and Professor Philip Ruffles CBE, former director of engineering and technology at the company, about the work being undertaken and where that might lead.
The gas turbine engine has changed dramatically since the father of jet propulsion, Sir Frank Whittle, unveiled his first engine in 1937. Thanks to major advances in materials, design and manufacturing, engines are now much more powerful and efficient. There are, however, plenty of design challenges left to address.
One of the major challenges for aero engine designers today is to find the optimum balance between fuel economy, noise and emissions. To make matters more difficult this balance is constantly changing in response to different requirements from aircraft manufacturers.
To achieve that balance, designers have to make choices. They can drive an engine’s thermodynamic cycle harder, boosting its thermal efficiency and cutting fuel consumption. However, this increases engine maintenance costs and can raise combustion emissions, especially oxides of nitrogen. Alternatively, they can increase propulsive efficiency, principally by increasing fan diameter. But this has to be balanced with increases in weight and drag which in turn increase fuel consumption and consequent CO2emissions. Finally, designers need to design a propulsion system that minimises noise which can increase engine weight and aircraft fuel burn.
Schematic showing the thermal, propulsive and transfer efficiencies of a jet engine today
Making the most of propulsive efficiency is crucial to reducing fuel consumption. A jet engine works by sucking in large amounts of air – see Back to basics. So, one way to boost efficiency is to increase the amount of air being sucked into the engine relative to that going through the engine core. To do this, the engine’s fan blades need to be as large as possible while minimising the weight and drag that increase fuel consumption. There are a number of projects within Rolls-Royce working to increase propulsive efficiency.
At the moment, Rolls-Royce aero engines use super plastically formed/diffusion bonded (SPF/DB) hollow titanium fan blades to deliver the most efficient fan system created to date. With the advances in composite technology, the time is right to produce a new, lighter, composite system. Composite Technology and Applications Limited (CTAL), a joint venture between Rolls-Royce and GKN Aerospace, is developing new ways to manufacture carbon fibre fan blades and fan cases.
Historically, composite blades have been made by hand, but the CTAL team intends to automate production, increase quality and reduce manufacturing time. CTAL is pioneering a technique known as Automated Fibre Placement in which a purpose-built robot lays strips of impregnated carbon fibre tape on to a fan-shaped mould, which is then heated and pressurised to harden the strips. Once baked, the blade’s root and leading edge are machined and coated to protect the surface, edged with titanium to boost strength and, finally, painted with an environmental protective coating.
A joint initiative by GKN and Rolls-Royce, CTAL aims to leapfrog the manuallyintensive production method of producing carbon composites. They have developed an automated process capable of significantly faster production rates. Here, an engineer is supervising the lamination of hundreds of layers of pre-impregnated fibres to produce considerably lighter fan blades © Rolls-Royce
A similar process is being followed to manufacture the fan case, and the process is being further developed so it can be used to make more complex structures. The next generation of Rolls-Royce engine that uses this composite fan system could weigh between 500 and 1,000 Ibs less (depending on the application) than today’s equivalent – a weight saving in the range of 3-6%.
Rolls-Royce is also experimenting with mounting a bladed spinner in front of the main fan blades in order to boost mass flow. Thus additional gas flow can be captured by the fan, without increasing the load on the fan disk. In essence, more flow is squeezed through a fan of a given diameter. The downside to this would be that the additional bladed spinner will increase overall engine noise.
Engineers are studying ways of reducing overall engine noise, for example by using a mini-mixer which mixes some of the fan exit flow with the gases from the hot nozzle. This will reduce noise and improve the transfer efficiency of the engine by optimising the energy between the engine core gas stream and bypass stream.
More radical still, the open rotor concept has the greatest potential to increase bypass ratio and propulsive efficiency. Here, the fan case and nacelle, the outer casing of the engine, are removed from the fan system, leaving the rotors exposed without drag from the surrounding casing, enabling extremely high bypass ratios to be achieved.
A contra-rotating system using two rows of propeller blades would be preferable thus removing the exit swirl of air that occurs in a normal single row propeller. This would eliminate wasted energy, thereby achieving the highest propulsive efficiency.
A key focus of the work Rolls-Royce engineers are undertaking on the open rotor design is around blade pitch control. With airflow on to the fan no longer controlled by a duct, the engine control system must set the blades at the correct angle to the airflow for any given operating condition or stage of the aircraft’s journey. Installing such an engine to the airframe is a challenge and engine designers are working closely with airframers to ensure open rotor engines could be accommodated in new aircraft designs. Future open rotor powered aircraft will also be slightly slower than their conventional turbofan-operated counterparts. Along with noise, this is a key tradeoff that has to be weighed up against the exceptionally high efficiency benefits of this engine design.
A bladed disk, a blisk and a bling
Thermal efficiency is essentially the efficiency at which the gas generator converts chemical energy from fuel into the available thermal energy in the gas stream.
To extract more thermal energy from the fuel and convert it into thrust, the overall pressure ratio (OPR) from the front of the fan to the rear of the compressor must be as high as possible within the constraints of available materials. However, as the OPR increases, so do the operating temperature environment of the core components and the temperature of the air used to cool the turbines.
Rolls-Royce carries out numerous technology programmes to improve engine design and performance. One such is the Environmentally Friendly Engine (EFE) demonstrator which focuses on improving the thermal efficiency of the engine. EFE utilises a three-shaft Trent 1000 engine as used on the Boeing 787 Dreamliner. This has been heavily modified to demonstrate the next generation and beyond of world class engine technologies.
Engineers working on the EFE demonstrator programme have been testing a range of materials, cooling specifications and different methods of film cooling for various blade and nozzle shapes. Additional work on lightweight intercoolers – devices that cool the air between compression stages – allows the engine to pressurise the air more efficiently and reduce the temperature of the cooling air extracted to cool other areas of the engine.
Ongoing studies continue to investigate the ‘statorless’ turbine. The usual set of stationary guide vanes, which direct gas flow into the next stage of rotating rotor blades, would be removed so that the gases exiting the upstream rotor impinge directly on the downstream rotor. This would lead to a large reduction in components and weights, and consequently generating fuel burn benefits.
Lastly, advances in computing power have led to engineers on the project developing new algorithms for electronically monitoring and controlling the engine. Active tip clearance control is a key example. Here, control software has been developed to monitor and alter the gap between the tip of a turbine blade and its casing. The turbine tip seal gap is in the magnitude of the width of a human hair. It minimises airflow losses and keep the blades and casing cool. However, the gap width normally varies throughout flight, as the casing and blade heat up and cool down and as the blade is subjected to centrifugal forces.
With active clearance control, sensors in the EFE continuously monitor the width of this gap and feed data back to the engine’s electronic control unit. Crucially, the control system can use this data to alter the width of the gap, according to engine condition, on a second-by-second basis. Systems such as active tip clearance control are paving the way to an intelligent engine that will ‘morph’ to its operating conditions.
Aircraft of the future
Ultimately, tomorrow’s engines will largely be defined by future aircraft designs. New concepts in development include Boeing’s blended wing body, Lockheed Martin’s box-wing and Northrop Grumman’s flying wing designs. All of the proposed aircraft are designed to halve landing and take-off emissions of nitrogen oxides by reducing the take-off thrust requirements. They also aim to cut fuel consumption by nearly 50%, compared with aircraft flying today, with a large proportion of this benefit being delivered by the engines.
These future aircraft concepts are not only radically different, but all point towards a need for closer integration between the airframe and the engine, even designing the aircraft and the engine as a single, fully integrated entity.
Danish physicist Niels Bohr reportedly said that “prediction is very difficult, especially about the future”. We cannot foresee what the aeroplanes of tomorrow will look like, but all the signs are that they will burn less fuel and emit less carbon dioxide and oxides of nitrogen.
See Professor Philip Ruffles’ talk: Past, present and future – sustaining the traditions of Sir Henry Royce at http://tv.theiet.org/technology/transport/13479.cfm