Review of Technologies to Achieve Sustainable (Green) Aviation
In the next few decades, air travel is forecast to experience the most expeditious relative growth among all modes of transportation, especially due to many-fold increase in demand in major developing nations of Asia and Africa. Like any other form of public mass transport that relies on finite planetary resources, aviation cannot be considered sustainable in the very long term. Because of the finite nature of the resources upon which aviation relies, it is more realistic in the medium term to think how best to improve the sustainability of air transport rather than it achieving sustainable development.
Demand for air transport is continually growing and, if this demand is to be met with all the attendant benefits, society must also accept the costs - noise, pollution, climate change, risk, resource use etc. Whilst it is not possible to make aviation sustainable in the very long term, much can be and is being done to improve aviation’s sustainability. Aviation is a global industry necessitating global solutions. Its environmental performance can only be improved if the various players – including airlines, government agencies, air traffic management (ATM) organisations and engine manufacturers, work together in order to develop and implement the best and most efficient, solutions worldwide.
Will we someday be able to fly without the guilt of causing environmental damage? The commercial aviation industry emits 705 million metric tons of carbon dioxide each year. While that represents only about 2 per cent of global carbon emissions, there is evidence that the greenhouse gases in jet fuel have a larger effect on the atmosphere because they are released at high altitude. A handful of firms and regulators hope that the electric revolution in cars will also take to the skies, helping the industry cope with an expected boom in travel and reduce greenhouse gas emissions. "Many people say that we must get rid of air transport because we will never be able to deal with emissions and noise, but this is an outdated approach," said Norwegian Transport Minister Ketil Solvik-Olsen.
In this reflective essay, I will discuss innovative engine technologies. A common definition of innovation “is the creation of better or more effective products, processes, services, technologies, or ideas that are accepted by markets, governments, and society.” An innovator can be an organization that is first to introduce something better than what exists, which often opens up a new area for others to build on. Examples of innovative aircraft engines are the R1340 Wasp, the JT-3, the PT-6, and the JT-9. Pratt & Whitney would be considered an innovator of aircraft engine technologies.
Normally during the journey, the amount of fuel burn varies in inverse proportion to propulsion efficiency and lift-to-drag ratio. Aircraft and engine manufacturers nowadays are developing new engine technologies aimed at enhancing the propulsion efficiency to lessen the fuel burn and also to simultaneously diminish Nitrogen Oxide outflows and noise. The best gains in fuel consume reduction in the past sixty years have come from better engines. The earliest engines were turbojets in which all the air sucked in at the front is compressed, blended with fuel and burned, providing thrust through a jet out the back. A while later, efficient turbofans were designed progressively when it was realized that greater engine productivity could be accomplished by utilizing some of the power of the jet to drive a fan that pushes some of the intake air through ducts around the core.
Turbofan engines use a large-diameter fan to pass air through the engine and have dominated commercial aircraft since the 1960s. Pratt & Whitney spent more than 20 years and $1 billion developing its new geared turbofan engines, which use larger fans (up to 81 inches in diameter on the A320neo) and a gearbox to make the fans rotate more slowly than the internal turbine that drives them, making them more efficient than traditional engines. Adding the gearbox, however, makes the engines heavier and increases aerodynamic drag. CFM International, meanwhile, says it can achieve many of the same benefits using a conventional turbofan architecture, without the added weight and drag of a gearbox.
The Leap engine uses lightweight composite materials, such as carbon fibre fan blades, to achieve energy efficiency gains that the company says are comparable to those of the Pratt & Whitney engine. Other boosts in efficiency have come from better compressors and materials to let the core burn at higher pressure and temperature. As a result, according to International Airport Transport Association (IATA), new aircraft are 70% more eco-friendly then they were forty years ago. In 1998, passenger aircraft averaged 4.8 litres of fuel/100km/passenger; the newest aircraft – Airbus A380 and Boeing B787 use only three litres. The current focus is on making turbofans even more efficient by leaving the fan in the open. Such a ductless “open rotor” design (essentially a high-tech propeller) would make larger fans possible; however one may need to address the noise problem and how to fit such engines on the airframe.
In the short-to-medium-haul market, where most fuel is burned, the open rotor offers an appreciable reduction in fuel burn relative to a turbofan engine of comparable technology, but at the expense of some reduction in cruise Mach number. It is valuable and useful for you to bear in mind that in mid-1980s GE invested outstanding effort in advanced turboprop technology (ATP). The unducted fan (UDF) on a GE36 ultra-high bypass (UHB) engine on MD-81 at Farnborough air show in 1988 created an enormous buzz in the air transportation industry. However, in spite of its potential for 30% savings in fuel consumption over existing turbofan engines with comparable performance at speeds up to Mach 0.8 and altitudes up to 30,000 ft, for a variety of technical and business reasons, the advanced turboprop concept never quite got off the ground.
The answer to getting lower fuel burn is to shift the point of maximum benefit further down on fuel burn trend line by introducing advanced technologies to achieve a paradigm shift. This means the fan bypass ratios and thus efficiency continues increasing, but it also means the fan pressure ratios decreases while the fan diameter increases for the required thrust. The Geared Turbofan (GTF) engine cycle introduced by Pratt & Whitney (P&W) is one of these advanced technologies that enable this paradigm shift. In the GTF architecture, the fan and the core components are separated by a gear system. The gear system allows the fan and the core to operate at different, more efficient speeds. The core can then operate more efficiently and produce a given thrust level at the fan with fewer compressor and turbine stages compared to a direct drive engine, thereby reducing the engine weight and the fuel required to carry that weight around on the aircraft.
In the mid-1980s, NASA in partnership with several U.S. Aerospace companies investigated a radical new propulsion technology for aircraft known as Open Rotors. This propulsion technology held the potential for large increases in efficiency, and therefore significant reductions in fuel burn, compared with conventional turbofans. The predicted performance of Open Rotor propulsion systems showed significant fuel burn benefits even then compared with current high bypass ratio turbofan engines.
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