Radical problems demand radical solutions – and sometimes, incremental change is not enough.
This is the thinking behind a three-year European project which ends in August this year and aims to boost the efficiency of aero engines by considering new, innovative concepts. The project, called Ultimate – short for Ultra Low-emission Technology Innovations for Mid-century Aircraft Turbine Engines – aligns with the European Union’s long-term goal to reduce emissions.
“The EU is targeting a 75% reduction in carbon dioxide emissions – per passenger kilometre – between 2000 and 2050,” says Tomas Grönstedt, coordinator of the project and professor in turbomachinery at Chalmers University in Sweden.
He says that previous programmes, such as Clean Sky, have helped to make such a challenging target achievable but that a final push to the summit is now required.
“To reach the 75% reduction target, we estimate that the last 18% will have to come from radical technology [such as that] developed within our project,” he adds.
The best modern aero engines only convert 40% of energy stored in the fuel to do useful work, he notes. The Ultimate project identifies the major loss sources in a state-of-the-art turbofan – then aims to overcome them by applying a range of radical engine concepts and designs, which “perfectly match the main fuel burn and emission objectives of the project and can be considered real breakthroughs”.
The project partners are looking to combine an ultra-efficient engine, with revolutionary core, installed on an advanced tube and wing aircraft. In addition to curbing carbon dioxide, they must also meet stringent noise and nitrous oxide emission targets.
“We will mature engine concepts that today only exist as ideas, by combining technologies in an unprecedented way,” states Grönstedt. “For instance, a composite cycle engine (CCE) that combines conventional aero-engine combustion technology with piston engine solutions.”
The CCE combines a gas turbine and piston engine into a single engine concept. Rather than one combustion chamber – followed by a High Pressure Turbine – the CCE has two banks of V-10 motors that drive the High Pressure Compressor (HPC). A high-speed turbine drives the low-pressure system.
The high peak pressure (300 bar at take-off) and temperature within the piston cylinders increase overall thermal efficiency. An intercooler between the Intermediate Pressure Compressor (IPC) and HPC reduces propulsion system weight by 18.5%, to make the intercooled CCE the best performing option.
The CCE reduces design mission fuel burn by 52% – compared to a year 2000 engine – and outperforms a reference Geared Turbofan engine of the year 2050 by 12.5% in terms of fuel burn.
Radical concepts
This is only one of the concepts that the project has addressed. Some of the others include: an inter-cooled core with pulsed detonation combustion (PDC); an ultra-thin adaptive inlet concept; secondary fluid recuperation (SFR); and three engine concepts developed at Cranfield University in the UK – an open rotor with nutating-disc topping cycle, and two different designs of turbofan.
Together, the concepts try to address the ‘big three’ sources of loss: combustor loss; core exhaust loss; and bypass flow loss. Exhaust air from the engine, for example, is 500-700°C hotter than ambient temperature.
“If this wasted heat can be recycled, then major improvements can be expected,” Grönstedt explains.
For instance, secondary fluid recuperation (SFR) relies on two heat exchangers inside the engine’s core: the first (which is ‘cold’) is installed before the combustor, while the second (‘hot’) comes downstream of the low pressure turbine – inside the hot-gas exhaust nozzle. They are linked by a separate closed circuit that is filled with a secondary working fluid such as liquid metal. This flows independently of the core engine air and gas flow. Heat from the hot exhaust gas is then used to preheat the cold air before the combustor resulting in a significant reduction in fuel consumption and emissions.
Pre-cooled cores
At the same time, researchers at Chalmers University have investigated the concept of pre-cooled cores with pulsed detonation combustion (PDC).
The engine core is developed to power a counter-rotating open-rotor propulsor with a novel Boxprop design in the front rotor. Pre-cooling the core flow – before detonation combustion – improves the volumetric efficiency, boosts combustion pressure ratios, reduces the risk of pre-ignition and reduces the need for engine cooling.
“This will result in a quieter propulsion unit delivering significant improvements in performance and fuel burn,” researchers say.
This engine concept is optimised for a typical short-hop European flights. A second concept – combining intercooling with PDC in turbofan engines – is optimised for long-haul flights.
Triple concept
Researchers at Cranfield University in the UK have developed three separate engine concepts – two of which involve turbofans.
One is a turbofan with a closed-circuit bottoming cycle. Bottoming cycles can boost engine efficiency by extracting energy from the exhaust heat. Open-circuit versions use this heat to raise the temperature of compressed air, then expand this hot air through a turbine – generating more power. Closed-circuit systems can use different working fluids, but must cool them before compressing and recirculating them.
Cranfield has looked at adding a supercritical carbon dioxide bottoming cycle to a turbofan engine. This gives more compact and efficient bottoming cycles than air or steam turbines, but the weight and drag of an air-cooled pre-cooler partly offsets this performance benefit.
A variant on this is a turbofan with an open-circuit air bottoming cycle and a ‘nutating disc’ topping cycle. The aim is to take advantage of synergies between the systems to boost power, reduce weight and increase thermal efficiency.
A third concept also uses a nutating disc topping cycle – but in this case, applies it to an open rotor.
Cranfield is currently developing a mock-up of its nutating-disc topping cycle for display at the forthcoming Farnborough Airshow. Several other Ultimate partners will present their own technologies at the event.
The Ultimate goal
Because the Ultimate project is not quite complete, the organisers have not yet released details of which technologies have proved to be most promising. However, Grönstedt says that they now know “a lot more” than they did at the start.
“We’ve confirmed that many of our initial assumptions were feasible – but have also discovered new challenges,” he concludes. “We now know which configurations look most promising, and want to continue their maturation – and hope to bring one or two of the most promising propulsion concepts into new projects.”
Industry is often criticised for having a short-term approach, but here is an example of extreme long-term thinking – as researchers look to develop technologies that they hope will be introduced onto aircraft engines in 2050.
