Fuel Efficiency - A drop Saved, a drop Gained

Sometime, in the not too distant a future, the Earth’s supply of fossil oil will run out completely. Long before that it will probably become uneconomical to extract and refine crude. As power-hungry nations begin frantically searching for alternatives to keep transportation systems running and myriad machines humming, the aviation industry will probably be among the hardest hit. Aviation has been heavily oil dependent since the dawn of powered flight. Other industries have managed to reduce their dependence to some extent, but aviation is as oil-addicted as ever.

None of the current alternatives—nuclear power, solar power or hydrogen fuel cells—suit jet airliners yet. Even the most optimistic among the researchers believe it will take years, if not decades, for biofuels to make a significant contribution to satisfying commercial aviation’s insatiable thirst for oil. And biofuels come with problems of their own.

Efficiency is the Watchword
Meanwhile, the rapid increase in airline flights worldwide (barring the current global downturn which is temporary) means that aviation’s carbon footprint is expanding. There are several ways to decrease fuel burn and cut aviation related greenhouse gas (GHG) emissions—improved air traffic management being one of the foremost. Up to a point, weight can be reduced by getting rid of all unnecessary items on board and carrying just the required amount of fuel. A clutch of operational measures, like taxiing with a single engine, selecting the optimum cruise level and airspeed, making use of tail winds, using continuous descent approaches, and so on are also worthwhile. But the important thing is to increase engine fuel efficiency.

Fuel efficiency is a measure of the efficacy of the process that converts chemical energy contained in a particular fuel into kinetic energy or work. In the context of aviation, fuel efficiency refers to the energy efficiency of an aircraft as a ratio of range units per unit of fuel. It is practically synonymous with fuel economy and can be expressed in two different ways. First, the amount of fuel used per unit distance. For instance, litres per 100 km; the lower the fuel figure the more fuel-efficient the engine. Second, the distance travelled per unit volume of fuel burnt expressed as km per litre (km/L). This is a familiar way of describing how economical a vehicle is. The ratio km/L, thus, represents the distance the vehicle can travel on a litre of fuel; hence, the higher the distance figure, the greater the fuel efficiency. It is, however, more accurate to take the weight of the fuel which is independent of temperature rather than volume which varies with temperature.

Fanning the Engine
That brings us to the question: what measures have the aviation industry adopted to improve jet engine fuel efficiency? The first airliners fitted with turbojet engines, over half a century ago, had vastly greater thrust than the piston-engine propeller driven aircraft they replaced. If the jets drank fuel by the tanker-full, it was not a major issue since oil was cheap and plentiful. Still engine manufacturers were untiring in their efforts to improve fuel efficiency, while not compromising on thrust produced, because it made economic sense. Their efforts received a fillip during periods when the price of oil spiked and, for the last couple of decades or so, since green activists began subjecting commercial aviation to increasingly unfriendly scrutiny.

The development of the turbofan engine marked a turning point. In a turbofan, part of the airflow passes through the engine core (which consists of the compressor, combustor and turbine sections) providing oxygen to burn fuel and produce power. However, most of the air bypasses the core, and is accelerated by the fan blades, quite like a propeller does. The latest high-bypass turbofans, such as the Rolls-Royce Trent, push approximately nine times more air around the core than through it. The difference between the two flows is called the bypass ratio (BPR). The turbine can also be smaller, since not all the air passes through it, and this in turn means it needs less fuel. The combination of thrust produced from the fan and the exhaust from the core makes for a more efficient process than earlier designs, resulting in significantly lower specific fuel consumption. For reasons of fuel economy, and also of reduced noise, almost all of today’s jet airliners are powered by high-bypass turbofans. Compared with a 1960s jet, a modern turbofan is some 80 per cent quieter and burns just half as much fuel.

However, there’s a price to be paid: the larger the fan, the greater the size and weight of the engine casing. And though bypass ratios have gradually increased, unless some breakthrough is achieved, there’s only so much fuel efficiency that can be gleaned from changes not already incorporated in today’s commercial aircraft.

Target Tracking
Recently, ICAO’s Group on International Aviation and Climate Change recommended “a global aspirational goal of two per cent annual improvement in fuel efficiency of the international civil aviation in-service fleet”. Considering that the more efficient airlines have been increasing fuel efficiency by around 1 to 2 per cent each year, this should not be too difficult a task, provided the laggards get their act together. Some of the most stringent targets have been set by the EU’s Advisory Council for Aeronautical Research in Europe. By 2020, compared with the baseline year of 2000, it stipulates:
  • 50 per cent reduction in fuel burn and carbon dioxide emissions per passenger km;
  • 80 per cent reduction in nitrous oxide; and
  • 50 per cent reduction in the perceived external noise level.

Industry sources, however, believe that current technologies are inadequate to achieve further improvement in noise and fuel burn simultaneously. Hence, jet engine researchers are focusing on next-generation concepts such as open rotor, geared turbofan (GTF) and embedded engines. At the same time, new engines such as the Rolls-Royce Trent 1000 and the General Electric (GE) GEnx already claim improvements in fuel efficiency. The fan is the key since noise level depends on its spin rate. A small fan, rotating at high speed, increases airflow but is noisy. A larger fan can rotate more slowly, reducing noise while improving cycle pressure and fuel burn. But larger fans are heavier, increasing overall weight. Increasing the BPR improves fuel burn and reduces noise till a ratio of about 10, beyond which noise decreases but fuel burn does not. Latest research aims at BPRs of up to 15, which could be accomplished, in part, by using lighter components and reducing system weight.

Meanwhile, GE is concentrating on the aerodynamics of compressor blades and use of lightweight composites. It has developed “blisks” or bladed discs, with airfoils that have been machined out of a solid piece of material or have been joined to the disc with friction welding. Blisks increase strength and durability, while decreasing weight and aerodynamic loss. One drawback is that operators will have to use new techniques to repair or replace damaged airfoils. New in the GEnx is the use of a composite casing, which reduces weight by around 160 kg and improves corrosion control. Recently, CFM International launched the CFM56-7B Evolution engine enhancement programme scheduled to enter service in mid-2011 to coincide with Boeing Next-Generation 737 airframe. CFM also claims the LEAP-X, another advanced turbofan under development, would provide 16 per cent lower fuel consumption than the CFM56-7B. The LEAP-X approach is to use lighter materials and to optimise the aerodynamic performance of the compressor and turbine stages in order to boost fuel efficiency. Its single-stage turbine is one of the highest loaded in existence—the core will run at compressor ratios of roughly 20:1.

Geared Turbofan Vs Open Rotor
Apart from incremental improvements in current engine performance, there are two radical engine architecture concepts being investigated to improve fuel efficiency—the GTF, being developed by Pratt & Whitney, and the open rotor model proposed by CFM International (a GE Aviation/Snecma joint venture) and Rolls-Royce.

GTFs are already under development. On account of the fact that turbines run most efficiently at high speed and fans at low speed, and since both are mounted on the same spool, turbofan engines have to compromise between the two. The introduction of a gearbox, however, allows a turbine to operate at high speed and its fan at lower speed. P&W believes it can obtain a “step change” in both fuel efficiency and noise reduction with this configuration and claims the GTF is the only technical solution that can simultaneously improve both. When the PurePower PW1000G GTF enters service around 2013, it promises to reduce aircraft fuel burn by 12 per cent, as well as noise and emissions by 50 per cent. P&W hopes to continue reducing fuel burn by one per cent a year, for a 20 per cent reduction by 2020. Conventional turbofans will keep getting better too, but P&W declares that the physics of its GTF architecture gives it a six per cent head start the others cannot catch up with. The GTF is also expected to be less susceptible to foreign object damage because the fan runs at lower speed. It will have 40 per cent fewer blades because of the reduction in the number of lowpressure combustor and low-pressure turbine stages.

Meanwhile, some engine manufacturers, among them CFM International and Rolls-Royce, are exploring open rotor designs as a possible way to dramatically increase the fuel efficiency of future airliners. Earlier known as unducted turbofans, the proposed engines bypass the turbine to an even greater extent, with external rotors that look like ungainly propellers. Some designs use two rings of stubby, contra-rotating blades made from composite materials that spin around at the back of the engine. Contra-rotation helps in eliminating swirl that would otherwise reduce engine efficiency. Rolls-Royce believes that when the open rotor enters service around 2020 it would be “the true gamechanger” and could provide a 25 to 30 per cent fuel efficiency gain over present turbofans and even be 10 to 15 per cent more fuel efficient than advanced turbofans that may emerge by then. However, considerable noise is produced when the wake of the front rotor passes through the rear rotor and a method needs to be found to drastically reduce it. Safety concerns also arise over what might happen if a blade should detach, break or shatter in flight.