Aero Engines : Green Conscience

In January 1930, Frank Whittle, an Englishman, designed, developed and patented the first gas propelled jet engine. The engine, however, was employed more than a decade later, in May 1941, to propel a jet airplane in England. In 1936 in Germany, Hans von Ohain and Max Hahn patented their own design of a jet engine. In August 1939, this engine powered a Heinkel HE-178 airplane. Although the origin of the jet engine lay in Britain, the first jet powered aircraft flew in Germany.

Till the 1990s, the focus of development of the jet engine was concentrated primarily on achieving higher levels of thrust so as to propel larger commercial aircraft. It is only in this decade that the new mantra, to produce engines that provide for high fuel efficiency without compromising on the thrust, has pervaded the industry. There has also been considerable emphasis on reduction of emissions and noise pollution. (See The Green Engines are Coming, SP’s Airbuz 02/2008). Researchers are now working on engines that would conform to the increasingly stringent norms stipulated for eco-friendly and fuel efficient machines.

Turbo Fans
Propulsive efficiency of modern jet engines (most commercial aircraft are powered by turbo-fan engines) is a product of the bypass airflow, which gives a secondary source of thrust. The ratio of the air flowing outside the core of the engine and the air, which flows through the core of the engine, is termed as Bypass Ratio (Secondary Flow/Primary Flow). During the 1960s, the Bypass Ratio was in the region of 1.5:1. A decade later, the ratio increased to around 5:1. Today, some of the engines have a ratio of around 9:1 and more. However, the drive to increase Bypass Ratio necessitates higher weight and increase in the size of the engine. For example, the designers have to compromise between the diameter of the fan and the ground clearance when mounted below the wings. The Trent 1000 engine has a fan diameter of around 285 cm with a Bypass Ratio of 10:1 as compared to the Trent 700 with a fan diameter of 246 cm and a ratio of 5:1. The Trent 1000 has a Specific Fuel Consumption (SFC) of around 14 per cent higher than that of Trent 700. Similar is the case with the GEnx. This has a 282-cm diameter fan and the SFC is 15 per cent better than that of the GE-CF6, which has a fan diameter of 236 cm and is an engine of the older generation with the same thrust rating. The former has a Bypass Ratio of 9.5:1 as compared to 5:1 of the latter.

According to Rolls-Royce, one of the leading manufacturers of jet engines, the point of diminishing returns in today’s engine technology is a ratio of about 10:1. However, researchers say Bypass Ratio of up to 15:1 could be accomplished by reducing the system weight. Therefore, GE is now focused on lightweight materials. The all new GEnx now has a composite fan case, which reduces weight and improves corrosion control. Manufacturers say this improvement on the engine saves almost 350 lbs of weight as compared to a metal version. Similarly, Pratt & Whitney’s Geared Turbofan (GTF), being developed for the Mitsubishi Regional Jet, produced by Mitsubishi Aircraft Corporation and the C-Series jets forthcoming from Bombardier, is expected to have a Bypass Ratio from 10:1 to as high as 12:1.

The aim, therefore, is to develop turbo-fan engines that weigh less, effectively by reducing the individual component weight with the use of composites. The trend today is to have larger fan sections. This translates into sizeable increase in weight, which is almost three times on account of the increase in size of the fan containment case, enlargement in the engine structure and reinforcements in the airframe structure. This domino effect on the total aircraft weight is required to be catered for without increasing the all-up weight.

CFM International has developed a comprehensive technology update package, marketed under the trade name LEAP 56. CFM began production in 2007 and has already delivered more than 1,000 new engines for new aircraft. The enhanced turbofans feature a more efficient engine core, compressor and turbine, as well as infusion moulded fan blades and a composite fan case. Given these modifications, composite materials now make up roughly 20 per cent, by weight, of each new CFM56 engine. GE says that their costs have reduced with the use of composites instead of metal in the fan components. The main reason for this is the rising prices of the alloys and the long lead time by their suppliers. This has prompted the company and its competitors to explore means of extending the employment of composites to other components of their engines.

Turbine
While the weight of the bigger turbofan is reduced by using composites, there is another problem. Larger diametric fans need to rotate at lower speeds to ensure that the tip speed remains sub-sonic to reduce drag. The speed of the rotation of the fan is linked to the turbine speed. Higher the speed of rotation of the low pressure turbine, better is its efficiency. However, due to the limitations in the rotational speed of the fan, there has necessarily to be a compromise between the rotation speed of the two components of the engine. The most recent innovation is the GTF, designed by Pratt & Whitney. A system of gears is inserted between the fan and the low-pressure turbine which allows running of the fan and the turbine at different speeds. This ensures that with decoupling, the large diametric fan runs at a speed 30 per cent lower than that of a conventional turbofan of similar size. The low-pressure turbine runs almost three times as fast compared to the low-pressure turbines in conventional engines.

As the diameter of the fan increases, the tip-speed is controlled by lower rotations of the fan which results in higher efficiency and lower noise levels. Low pressure turbines are revved up due to the gears and these operate at a more efficient speed. Questions may arise about the gear systems being complex and adding to the weight. Pratt & Whitney says that this is offset by the reduction in the Low-Pressure stages and airfoils. Further, they say that for a given thrust, a bigger fan means the high speed spool can be smaller and reduces weight. For example, a 25,000 lb thrust GTF engine will be almost 10 per cent lighter than a comparable conventional engine because it has fewer stages.

Conventionally, the High Pressure (HP) turbine, which is the first stage of the assembly, and the Low Pressure (LP) turbine, which is in the last stage of assembly, rotate in the same direction. GEnx engines have the high-pressure and low-pressure spools rotate in opposite directions. This concept was first used in the Trent 900 and later, in the Trent 1000. GE says this is beneficial because the airflow which exits the HP turbine is not manipulated before entry into the LP turbine. Besides, fewer vanes to direct the airflow are employed. Computers are used to analyse the airflow and the turbine blades are designed so as to allow the synchronised flow through the blades. This minute tailoring helps to reduce aerodynamic loss and increase efficiency. The company introduced blisks or Bladed Discs for the compressors where the airfoils are machined out of a solid piece of material or have been joined to the discs with friction welding. This increases strength and durability while decreasing the weight of the assembly, as well as, aerodynamic loss. As of now, the GEnx uses blisks in three out of the 10 compressor stages, after weighing the benefits versus the costs. GEnx also uses modified nacelles in the trailing edge to reduce noise by pre-mixing the core air and the by-pass air before they exit the engine.