Large Jet Engines — Power with Fuel Eficiency

A jet engine burns fuel and generates thrust and how well it does so, is judged by how much fuel it uses to generate it

Issue: 1 / 2019By Anil ChopraPhoto(s): By Snecma
GE90-115B engine generates 127,500 pounds of thrust

Early 2018, General Electric (GE) completed its first flight test of the world’s largest jet engine GE9X being built for the new long-haul Boeing 777X due to take to the skies in 2020. It was tested under the wing of GE Aviation’s Boeing 747 flying test-bed. The engine has approximately the same diameter as the fuselage of a Boeing 737 and houses a 134-inch-diameter front fan. The final engine certification is expected in 2019. The 100,000-pound thrust engine will be the most fuel-efficient engine the company has ever produced and five percent more economical than any twin-aisle engine in the market. On a per-pounds-of-thrust basis, the engine is claimed to be the quietest. GE’s partners in the GE9X project include France’s Safran, Japan’s IHI Corporation and Germany’s MTU Aero Engines. Interestingly, one main engine of the space shuttle produces 375,000 pounds. The world record still belongs to the GE predecessor engine from GE, the GE90-115B, which generated 127,500 pounds of thrust and had record pressure ratio of 23:1. GE90’s fan is an advanced, larger diameter unit made from composite materials and was the first production engine to feature swept rotor blades. It has an in-flight shutdown rate (IFSD) of one per million engine flight-hours. It accumulated more than eight million cycles and 50 million flight hours in 20 years. By the way, GE also built the first US jet engine in 1942.


Air breathing jet engines are essentially combustion engines and include turbojets, turbofans, ramjets and pulse jets. In general, jet engines are combustion engines. Most modern subsonic jet aircraft use complex high-bypass turbofan engines. They give higher speed and greater fuel efficiency. High speed applications (ramjets and scramjets) use the ram effect of the vehicle’s speed instead of a mechanical compressor. Turbofans have an additional fan at the front of the engine, which accelerates air in a duct bypassing the core gas turbine engine and make these more efficient. Turbofans are the dominant engine type for medium and long-range airliners. At high speeds, the large frontal area of turbofans generates more drag and therefore, for supersonic flight, they are not used. The thrust of a typical jetliner engine went from 5,000 pounds in the 1950s to 115,000 pounds in the 1990s. Reliability has gone up from 40 in-flight shutdowns per 100,000 engine flight hours to less than one per 100,000 in the late 1990s. This, combined with greatly decreased fuel consumption, has allowed very long haul flights. Jet engine designs are frequently modified for non-aircraft applications, as industrial turbines and marine power plants.


The engines were assembled at the company’s massive jet engine plant near Peebles, Ohio. The engine also has 3D-printed fuel nozzles and the most extensive use of parts made from lightweight and ultra heat-resistant materials called ceramic matrix composites (CMCs) allowing complex internal shapes that were previously impossible to achieve. CMCs operate in temperatures as high as 2,400 degrees Fahrenheit and allow keep the heat inside the engine while reducing fuel burn and emissions. The hotter the engine can run, the more efficient it is. The engine also uses 16 fourth-generation carbon-fibre fan blades that feed air into an 11-stage high-pressure compressor with a 27:1 pressure ratio, the highest in any engine, which also boosts the engine’s efficiency. GE Aviation, which is the exclusive engine maker for the 777X, has received orders for more than 700 GE9X engines valued at $29 billion from airlines including Emirates, Lufthansa, Etihad Airways, Qatar Airways and Cathay Pacific.


A jet engine burns fuel and generates thrust and how well it does so, is judged by how much fuel it uses to generate it. This is a measure of its efficiency. If air/combustion gases flow more smoothly, the engine will be more efficient and will use less fuel. This is called specific fuel consumption. The thrust-to-weight ratio of jet engines is an important parameter. The lighter the engine, the better the thrust-to-weight ratio and lesser fuel is used to compensate for drag due to the lift needed to carry the engine weight or to accelerate the mass of the engine. Turbofans have a mixed exhaust consisting of the bypass air and the hot combustion product gas from the core engine. The amount of air that bypasses the core engine compared to the amount flowing into the engine determines what is called a turbofan’s bypass ratio (BPR). While a turbojet engine uses all of the engine’s output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan’s cool low-velocity bypass air yields between 30 and 70 per cent of the total thrust produced by a turbofan system. The advent of the turbofan replaced the very distinctive jet noise with another sound known as “buzz saw” noise. Low-bypass engines are preferred for military applications such as fighters due to high thrust-toweight ratio, while high-bypass ratio engines are preferred for civil use for good fuel efficiency and low noise. High-bypass turbofans are usually most efficient when the aircraft is traveling at 800 to 885 kmph, the cruise speed of most large airliners.


In commercial aviation, the major players in the manufacturing of turbofan engines are Pratt & Whitney, General Electric, Rolls-Royce and CFM International (a joint venture of Safran and GE). Aero-engine Corporation of China is a new state owned entrant with 96,000 employees. In 1950, Rolls-Royce Conway became the world’s first production turbofan engine to enter service. In 1968, GE TF39 high bypass turbofan became first engine with greater thrust and much better efficiency. GE Aviation has also set up a cooperative venture with Pratt & Whitney, named Engine Alliance under which the companies have developed an engine for the Airbus A380 named GP7000, based on a 0.72 flow scale of the GE90-110B/115B core. Pratt & Whitney is a subsidiary of United Technologies (UTC). As of 2014, the company reported having 31,500 employees supporting over 11,000 customers in 180 countries around the world and had a revenue of $14.5 billion. Their large commercial engines power more than 25 per cent of the world’s passenger aircraft fleet. Pratt & Whitney’s 11,000 military engines operate in 22 nations and include the F135 for F-35 Lightening II, F119 for F-22 Raptor and F100 family for the F117 among many others. Rolls-Royce engines powers Airbus A380, Airbus A350, Boeing 787 Dreamliner and many regional and corporate jets. They also power fighters such as Tornado, Typhoon and British F-35s. CFM International was formed to build and support the CFM56 series of turbofan engines derived from the two parent companies’ commercial engines GE’s CF6 and Snecma’s M56. The joint venture has delivered 30,700 engines to more than 570 operators and have large orders. GE is the highest ranked engine company with 41st slot in Fortune Global 500 companies.


The aircraft’s maximum flight range is determined by the level of efficiency with which thrust can be applied to overcome the aerodynamic drag. Many devices reduce drag. Lower cruise-speed augments the range, but higher cruise-speed allows more revenue-passenger-miles per day. Jet engine efficiency increases with velocity. Blended-winglets of A320 added 200 kilograms weight, but offered 3.5 per cent fuel burn reduction on flights over 2,800 km. The Boeing Dreamliner 787 was the first major commercial airplane to have composites in most part of airframe to reduce weight and thus reduce fuel consumption. Fuel economy also comes by combining with airline efficiency and passenger load factor. Operational procedures like routing, lesser use of auxiliary power unit (APU), reduced flap approach, reduced thrust reversal on landing, all save fuel. Fuel consumption is measured by passenger-kilometers per litre of fuel or litres/100 km per seat. Typically an ATR 42-500 with 48 seats uses 3.15litre/100km per seat; a Dornier 228 6.22; Airbus A321 Neo 2.19 and Boeing 737 Max 9 2.3. A Boeing 777-200ER on a medium haul flight uses 2.89 litre/100km per seat, but same aircraft on a long haul flight uses 3.08 litres.


Technology is already delivering an impressive one per cent per annum saving on fuel burn. Pratt & Whitney says its new engines will use an internal gearbox to slow down the speed of the fan and could save 20 per cent on fuel consumption. CFM International introduced an advanced engine called the Leap, using lightweight composite materials which could achieve similar improvements without such a radical break from existing technology. Both new engines have been deployed on different versions of Airbus’s A320neo. Efforts to introduce bio-fuels to power jet engines are on. Airbus/Rolls-Royce hybrid electric with gas-turbine engine will allow peak power for takeoff and climb while for descent, the engine is shut down and the electric fans recover. NASA is working on advanced technologies to reduce carbon dioxide emissions from jet engines by more than half. Research is on for plasma jet engines that will use electricity to generate electromagnetic fields instead of fuel by compressing and exciting argon gas into a plasma similar to that inside a fusion reactor. A lot is unfolding.