Power players: The future of aerospace engines
Alan Peaford looks at the role of engines in reducing emissions produced by the aviation industry, the challenges ahead and the "mindblowing" progress we've already made.
When we talk about the environment and the technology leaps in aviation, we inevitably focus on the airframers. Many years ago, though, I was reminded that an aeroplane is really just a pressurised metal tube strapped to technologically advanced engines. And it is those engines that attention is being drawn to now as the industry is challenged over emissions and noise, and facing stringent demands to reduce both.
It is easy to forget how far the aero-engine business has come over the past 100 years … and indeed over the last decade. It is also easy to gloss over some of the amazing developments that have been achieved and challenges that have been overcome.
The facts are mindblowing – and a little hard to grasp.
Each wide-chord fan blade exerts a centrifugal force of around 70 tonnes, equivalent to the weight of a modern locomotive; each high-pressure turbine blade generates the same amount of power as a Formula 1 car; and the blades in the hot part of the engine have to operate at several hundred degrees hotter than the melting point of the very material they are made from.
Assessing the options
No surprise then a sector that has this track record is being asked to do even more. There are a number of options being considered. Does the answer lie in revolutionary propulsion systems, or will our future travels be powered by the same old jet engine running on renewable fuels?
Mike Whitehead, chief engineer and head of Rolls-Royce Ultrafan programme with its powered gearbox, was able to explain the challenge of the former option in simple terms.
“Our aim is to improve efficiency of the engine by 25% by 2025 relative to a Trent 700,” he said. “There are only two ways you can do that: either improve the propulsive efficiency of the engine – where we need the fan to be as efficient as possible - or the thermal efficiency of the engine, which is down to the turbine.
“So, we are trying to run a big slow fan as slow as possible and pump as much air as we can and have the turbine run hot and fast. Those two things are contradictory you can’t achieve both in a traditional engine. That’s why in the Trent engine, the low-pressure turbine has been getting larger and larger because we are constantly trying to run the fan slower and slower and to do that you need a bigger turbine.”
Whitehead explains: “The Rolls-Royce solution is the powered gearbox which sits between the slow fan and the fast turbine to enable the change in speed between the two. The gearbox transmits power through smallest most compact space we can. Each gear has got two teeth, each the size of a little finger and through that will transmit the same power as the entire grid of a Formula One race.”
Weighing it up
We have already seen massive gear changes affecting efficiency in the latest aircraft with competing engines from Pratt & Whitney (P&W) and CFM.
Pratt & Whitney spent more than 20 years and $1 billion developing its new geared turbofan engines, which use larger fans (more than two metres in diameter on the A320neo) and a gearbox to make the fans rotate more slowly than the internal turbine that drives them, in the way that Whitehead described. Adding the gearbox, however, does make the engines heavier and increases aerodynamic drag.
CFM International – the joint venture between GE and Safran - claims it can achieve many of the same benefits using a conventional turbofan architecture, without the added weight and drag of a gearbox. The Leap engine uses lightweight composite materials, such as carbon fibre fan blades, to achieve energy efficiency gains that the company said is comparable to those of the P&W engine.
The first LEAP-powered commercial flight happened on August 2, 2016 on a Pegasus Airlines flight from Istanbul to Antalya. Since then, more than 130 LEAP-1A and LEAP-1B-powered aircraft have entered service with a total of 24 operators on four continents. Overall, this fleet has logged more than 400,000 flight hours and 200,000 flight cycles.
According to Gaël Méheust, president and CEO of CFM International, the fleet is currently logging 100,000 flight hours per month and on target to achieve the one-million flight hour milestone during the first quarter of 2018.
“The LEAP is providing operators a 15% improvement in fuel consumption and CO2 emissions compared to today’s best CFM engine, along with dramatic reductions in engine noise,” he said.
The real disruptive opportunity, according to many commentators, seems to lie in the area of electric-powered systems.
Embraced (on the surface at least) by the automotive industry which is responding to various governments’ environmental pressures, electric power is being proven in the aviation field with unmanned systems and later this year with the Airbus CityAirbus, an electrically-powered vertical take-off and landing (VTOL) aircraft demonstrator, which is intended to carry four passengers, with a pilot initially and to become self-piloted when regulations allow. The first unmanned flight is scheduled for the end of 2018 with manned flights following next year ahead of a planned entry-into-service in 2023.
This will open new markets and likely increase the number of people switching to air travel for shorter direct routes.
And this is the biggest problem aviation has. It is too popular.
More and more people are flying every year. As countries such as China see a growing middle class with greater spending power, it is no surprise that travel is high on the wish list.
Aviation contributes just over 2 percent of the CO2 emissions from its fossil fuel use (slightly less than the Internet), and as demand for air travel rises, so do emissions. The engine-makers have made those significant reductions already, but as the growth of air travel outstrips any other transport system then the impact by aviation is even greater. The European Union has called for 60% reduction in aviation emissions, a figure that will be impossible with current technologies and the growth in traffic.
Even the greatest optimists accept that long-haul aircraft are unlikely to be fully electric as far as power is concerned. As one senior engineer said: “You can’t get a long enough charger lead.” But there is hope with some alternative systems. Current battery systems provide barely 1% of what kerosene can.
The middle ground: The race is on
Rolls-Royce is taking it seriously for sure. Chief executive Warren East said: “There’s a lot of chatter about hybrid electric flight, not just little aeroplanes but regional aeroplanes. I’m convinced we will see these things happen sooner rather than later.”
He added: “There is a race on. We need to be ready by 2020 because people are talking about entry into service by 2030.”
The British manufacturer has teamed up with Airbus, Siemens and Rolls-Royce to develop a hybrid passenger plane that will use a single electric turbofan along with three conventional jet engines running on aviation fuel.
The three companies aim to build a flying version of the E-Fan X technology demonstrator plane by 2020, based on the existing BAe 146 four-engine regional jet. The hybrid version would generate electric power through a turbine within the aircraft. That power would be used to turn the fan blades of the single electric turbofan engine.
If the system works, a second electric motor could be added. In the US, Washington-based Zunum Aero is working on a 12-seat hybrid-electric commuter jet in partnership with Boeing, jetBlue Technology Ventures, and the Department of Commerce Clean Energy Fund.
Keep it down
Noise is a very complex issue. Former Rolls-Royce head of engineering, Colin Smith, now the chairman of the UK’s Aerospace Growth Partnership and president of the industry body ADS, won a lot of admirers when he told UK parliamentarians that the place to position Heathrow’s third runway is in the centre of London. “With the traffic noise and trains you just wouldn’t hear the airplanes,” he told them.
“That’s one of the problems, he said, “You put aircraft in the countryside and you hear them over the sparrows.”
Smith argues that we have hit the optimum point. “I suspect the latest aeroplanes out of Airbus and Boeing are at a point where it would be daft to try and go any further on noise. There is always a compromise in aerospace – if you make them quieter you would make them slower or less fuel efficient.”
With GE Aviation having successfully test-run its partially 3D-printed Advanced Turboprop engine, which is set to power the Textron Denali single turboprop, it is clear that the propulsion industry is addressing the issues right across the aviation spectrum.
The path to cleaner jets
Although technology is delivering an impressive 1% per annum saving on fuel burn, the 4-5% compound growth of traffic numbers mean it will be an uphill struggle. Efforts to introduce biofuels to power jet engines have so far fallen short of their targets, and I am with those who believe the pathway to cleaner jets will come through advances in engine technology rather than cleaner fuels – unless you know something different.