A RECENT SPATE OF SPECTACULAR JET engine failures reminds us that the development of aircraft has not been so much about aerodynamic advances, but rather, it has been driven by engine development. And the biggest and most fascinating battle amongst the airline manufacturers is not between Airbus and Boeing, but for the next generation of airline engines.

Guy Leitch

Rolls Royce has officially started building the world’s largest aero-engine, the UltraFan, which it says will redefine sustainable air travel for decades to come. The UltraFan will have the world’s largest fan diameter of 140 inches – a London tube train could run through a circle the size of the engine’s fan case. The gearbox can handle more than 50 MW – enough to power 500 cars. Rolls says the engine is the basis for a potential new family of UltraFan engines able to power both narrowbody and widebody aircraft and deliver a 25% fuel efficiency improvement compared with the first generation Trent engine.

Meanwhile the key battle is for the powering of the narrowbodies. The battle to power the Airbus A320 NEO and 737 MAX pits Pratt & Whitney’s PW1100G geared turbofan against CFM’s Leap- 1A & B engine. These new generation engines claim a more modest 17% increase in fuel efficiency over the previous generation engines.

‘what the next generation engine will be..’

This is not a new battle. The ongoing invention of lighter and more powerful engines has been the driving force behind aircraft development in the twentieth century. The crude boxkite aircraft of World War I progressed rapidly as rotary engines gave way to more powerful radials and liquid-cooled V-engines. The ultimate fighter of World War I was the SE-5a which was built around the liquid cooled 150hp V8 Hispano-Suiza engine.

Not much development happened between the wars except that the liquid cooled V8 evolved into the Merlin V12. But as the war progressed, radials proved more powerful, simpler and less vulnerable to battle damage than the beautiful Merlin derivatives, which peaked with the Griffon at 2000 horsepower. The ultimate World War II fighters such as the Sea Fury and Bearcat were powered by 2500 hp radials like the Bristol Centaurus or Pratt & Whitney R-2800. The Germans likewise progressed from the liquid cooled Daimler to the radial BMW engine in their FW-190.

And then the jet engine happened, and instantly the straight-winged piston aircraft such as the Bearcat or B-29 were obsolete, being replaced by graceful jets such as the Sabre and Boeing B47 Stratojet bomber.

E recent spectacular failure of a B777 engine.

Passenger airliners followed the military’s swift progress from piston to jet. After the false start of the Comet 1, the Boeing 707 has set the dominant shape for airliners for sixty five years, since the prototype first flew in 1954. To the untrained eye there is minimal difference between a Boing 707 and an Airbus A340.

Again it is the engines that make all the difference. The slim JT-3 turbojets gave way to early turbofans, such as those on the Boeing 747-200, which was second generation. These were then replaced by high bypass turbofans with massive ducted fans as seen on the Boeing 777 as third generation.

‘performance and cost guarantees the engine makers committed to years ago’

The fourth generation airliners are now here, being the Boeing 787 and Airbus A350. Thanks again to an evolution in engine technology, mainly associated with new heat resistant materials, they are around 20% more efficient per seat than the third generation. Yet the basic airframes are still almost unchanged from when they were launched, hence the A320 NEO and 737 MAX.

Now the technology for the fifth generation airliner has appeared. And it’s here that things get really interesting. Two clearly separate schools of thought have developed as to what the next generation engine will be like. The battle to own the future amongst engine makers is no longer about who has the biggest fans or highest by-pass ratios, but has reduced to two divergent basic philosophies of how to make more efficient engines. The Rolls Ultrafan either cleverly uses the best of both techniques, or is going to fall between the two stools and kill the company.

More heat: General Electric and Snecma, the partners in the CFM International joint venture, have placed their bets on building hotter burning combustion chambers using advanced carbon-fibre and ceramic-matrix composites.

Less heat: Pratt and Whitney focussed on the mechanics of driving the fan by building a gearbox to enable the engine to run cooler, with fewer parts and more conservative technology.

Engines and their failures are now the key technology drivers for next-gen airliners.

So far Pratt seems to be ahead with four engine derivatives already certificated. CFM has responded by claiming its Leap-1A has up to 3% lower specific fuel consumption (SFC) than the PW1100G. CFM’s claims are based on results from the latest tests of its key technologies in the Leap-1. These include 3-D woven composite fan blades, compressor variable bleed valves, and an uncooled ceramic matrix composite (CMC) turbine shroud.

Pratt was quick to dismiss CFM’s claims, but was stung into defending the technology in its engine and detailing its plans to introduce new advances over time to continue driving down fuel burn. Pratt’s President David Hess said that there is no way CFM can achieve the advantages in fuel burn and maintenance cost it is claiming “unless they defy the laws of physics.”

Both CFM and Pratt have achieved a fuel burn reduction of at least 15% from the engines now powering the A320. CFM says 50% of the Leap- 1A’s fuel savings come from improved propulsive efficiency, with increased fan size and bypass ratio, and 50% from improved thermal efficiency, including higher temperatures.

Surprisingly, Pratt says only a third of the PW1100G’s fuel savings come from the gearbox, which enables a larger, slower-turning fan to be driven by a faster, more efficient low-pressure (LP) turbine. “The other two thirds is from the rest of the engine,” says Bob Saia, vice president next-generation engine family. “Everyone thinks it’s only the gear, but the GTF family also has a new core with an ‘industry-best’ overall pressure ratio (OPR) of almost 50:1.

A faster LP turbine speed also helps when it comes to temperatures, which drives component and maintenance costs. The PW1100G runs hotter than current A320 engines, but “significantly cooler” than the Leap-1A, says Saia. “You can either move more air slower or add more temperature, and it’s cheaper to make power with air than temperature,” he says. “We are not running exotic gas-path temperatures; we use the fundamental physics of speed.”

These differences may seem academic when both rivals promise similar fuel savings on the same platform. The airlines’ demand determines engine prices and maintenance-cost agreements ensure the manufacturers carry the risks of their technology choices. But they could become more significant as Pratt looks to expand its GTF applications to large commercial aircraft. On paper at least, the conservative temperatures and technologies in the PW1100G give Pratt plenty of room to grow.

Pratt’s Hess says his company has a technology road map to achieve 20-30% fuel savings over today’s engines by the mid-2020s. The plan covers the complete engine from advanced aerodynamics and lightweight rotors for the fan, through higher gearbox ratios to increase the bypass ratio, cores with OPRs beyond 60, active combustor control, and new materials and cooling schemes in the turbine. The road map is aimed at averaging a 1% per year reduction in SFC for new applications and capturing half of that improvement for re-insertion into existing engines. Notably absent from Pratt’s road map are the ceramic-matrix composites which CFM is introducing.

‘Rolls Royce’s Ultrafan is the biggest ever – and will make or break Rolls’

Although they are behind Pratt, CFM’s high-tech approach using CMC blades is rapidly gathering momentum for the move to widebody engines. The Ultrafan’s competitor is Pratt’s huge GE9X which is already in the final stages of testing. Firmly established in the 15,000-33,000-lb thrust range for single-aisle aircraft, Pratt is now eyeing the 70,000-100,000-lb thrust requirements for the twin-aisle market dominated by GE and Rolls- Royce.

The battle has not been easy. Pratt battled to overcome a condition known as ‘rotor bow’ which impacts virtually all gas turbines to some degree and, in the case of the PW1100G, led Pratt to impose a more conservative start time to ensure the blade tips of the compressor rotors do not rub against the walls on start up. This set back Airbus A320 NEO delivery by months and angered early operators.

The battle is long running. Boeing and Airbus and the airlines who will fly the engines are demanding ever-greater reliability at entry into service, forcing engine manufacturers to select technologies earlier and spend more time and money maturing them, even before beginning development of a specific engine.

The bets each company placed on philosophies and advanced material technologies a decade or more ago are now face the ultimate test: that of the airlines who demand that the engines live up to the performance, maintenance and operating cost guarantees that the engine makers committed to years ago. So far all three are struggling to mature their new engines.


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