Engines that help these big aircraft fly are usually not given much of a credit for the amazing job that they are doing and how far they have come in terms of efficiency and power. The latest design for such an engine is being rolled under the name of ‘PurePower’ by Pratt & Whitney brands and will be installed as an option on the A320neo – the Airbus’ latest product.
It is known as geared turbofan and, as the name suggests, features a gearbox as part of its mechanism. Pratt & Whitney is a division of United Technologies – An American Conglomerate. The firm has spent over 20 years with a total investment of over $10 billion in building these engines. The basic idea being that by connecting an engine’s inlet fan through a gearbox to the compressor and turbine located in the core, fuel economy could be increased while making the engine quieter. The apparent problem in this approach is that the engine size is directly related to the amount of forces that are being applied on the gearbox and the chances of something going wrong increase. This has been the main reluctance in attempting to incorporate gearboxes in 30,000hp engines. Pratt & Whitney remained loyal to the cause because it believes that conventional and ungeared turbofan has reached its limit and if we have to go further, we need to develop geared turbofan.
Let’s talk about how a jet engine works; Newton’s third law of motion. The reaction being achieved is the forward movement that is caused by the action; ejection of fast moving gas from the jet’s back. The gas generates thrust that is proportional to gas’ mass and the speed of ejection. The earlier jet engines made use of thrust that was achieved using the air that entered the engine’s core at the front followed by squeezing using a compressor, mixing with fuel and ignition to create hot gases that sped out of the rear. The mass ejected was small, but the speed of ejection was good enough to keep the aircraft in air.
The turbofan works in a similar fashion with the only difference being that the fan is also turned using the turbine in order to have some of the air move around rather than all of it going through the core. This air is large in quantity and imparts thrust, thus creating a system that is efficient and quieter than earlier jet engines. The bypass ratio is the ratio of air going around the core compared with the air going through it. The latest turbofans have a bypass ratio as high as 9:1. Achieving this ratio meant that the fans and subsequent inlets had to be increased in size and now the size limit has been achieved because the fan tips are travelling at speeds close to the speed of sound and any further increase would cause shock waves and potentially threatening vibrations.
So, how does a gearbox help? It lets the fan turn slowly while allowing the compressor and turbine to move faster. This means, in simpler words that now we can increase the fan size and allow for more volume of air without having to slow down things. This allows PurePower to sport a bypass ratio of 12:1. However, this means that the gearbox has to be incredibly sturdy and reliable. Pratt & Whitney make use of an advanced nickel-based alloy for the components of the box while the fan blades are created from a lightweight alloy of aluminum and lithium. The turbine is created using titanium aluminide – a material created in collaboration with the MTU, a German firm.
The end result being that a pair of PurePower engines fitted under an A320neo’s wings reduces fuel consumption by 15% when compared with a conventional A320. This would save an airline over $1.5 million per year per aircraft.
Let’s see how well these engines perform and if there are any associated problems/risks.
