Engine Design to Address Environmental Concerns

The engine is the powerhouse of an aircraft. It produces the necessary energy to propel the aircraft, but is also responsible for the high noise levels associated with the platform it powers. In aircraft operations, fuel constitutes the major part of the total cost. The way the cost of Aviation Turbine Fuel (ATF) is escalating, there is a need to look for alternative sources of energy or for designing engines with better efficiency. The growing concerns and consciousness about the environment and global warming, are forcing the air transport sector to revise its norms in respect of emissions and levels of noise generated by the aircraft. As a result, aircraft manufacturers are now becoming sensitive to the requirement of bringing in greener technologies which should be able to reduce polluting emissions, noise levels and enhance efficiency for every unit of ATF consumed.

Alternat ives to Fossil Fuels

The designers of aircraft engines are trying to find ways to utilise renewable sources of energy to fuel their aircraft. For example, engines are now being run on battery power and bio-fuels which are of sustainable nature, but without loss of efficiency. Sustainability means instead of relying on fossil fuels which are a onetime use element and produce noxious or toxic wastes on consumption, new renewable sources of energy be used. Generally speaking, renewable energy includes energy generated by wind, solar and water. Organically produced bio-fuels from algae too are being introduced to make the engines green and reduce their carbon footprints. Such renewable energy will serve the air transport sector by converting their latent and potential energy into kinetic energy. These sources of renewable energy, will, at the same time, be in a position to rejuvenate itself. This means that their supply would be unlimited. So long as the sun is going to be there, it will provide solar power. Of course, water flowing down a slope is going to produce enough hydropower, but it will not be easy to power an aircraft with such a renewable source of energy. Hence the future lies in battery-operated aircraft, use of bio-fuels and designing engines with better efficiency which have low noxious gas emissions and noise levels.

Engine and the Environment

The aviation sector consumes only nine per cent of fuel consumption in the country; but is most committed to reducing its carbon footprint. The National Aeronautics and Space Administration (NASA) of the US has introduced a crucial environmental consciousness programme or mission. By the year 2030, the world is going to witness the reduction in the noise level by 71 decibels below the current Federal Aviation Administration (FAA) standards in respect of noise generated by aircraft. The aim is to ensure that the aircraft of the future are less noisy. Secondly, a 70 per cent reduction in the quantum of nitrogen oxide emissions than what we are seeing today, will mean cleaner air. Thirdly, there will be more than 70 per cent reduction in fuel consumption to reduce Greenhouse gases. This will also reduce the cost of air travel. All this will be possible with improved engine designs, reduced drag and lowering the cost of construction of aircraft as also its fuel-related operating expenditure.

Green aviation efficiency concept primarily is being implemented over four distinct areas. The first is the introduction of green engines followed by airframe modifications to reduce drag. Another plausible method to reduce carbon footprint is by the introduction of Wi-Fi on board to eliminate wires and of course the use of bio fuels.

An engine produces the maximum amount of emissions and noise in an aircraft. Thus, a significant green initiative/development is taking place in this field. An engine is responsible for production of carbon dioxide and nitrogen oxide gases as also noise as it produces thrust and electric power. It is due to this high noise that lot of restrictions on aircraft operations around airports at certain times of the day have been imposed in the Western world. Consequently, airlines have to deploy select category of aircraft which comply with the noise regulations. This leads to increased inventory holding costs and deploying less economical aircraft. It leads to lower efficiency and the users deprived of profits.

Innovation in Engine Design

Engine manufacturers are introducing geared turbine fans on a single shaft to operate at leveraged revolving speeds as per the needed airflow speeds. This enables the adoption of less vigorous turbine fan fabrication techniques and materials. Consequently, it reduces production costs and the all-up weight too. Another factor being considered is that the airflow is maximum and at optimal speeds at the core of the engine. However, the airspeed is higher at the fringes. As a result, a strength differential in the turbine fans can be introduced to reduce the engine manufacturing costs.

An engine is responsible for production of carbon dioxide and nitrogen oxide gases as also noise as it produces thrust and electric power.

Associated with this is the diameter of the engine through which it sucks in air. The larger the diameter; the more effective will be the engine, but higher will be the fuel consumption. If the diameter is small, relatively, less air will be sucked in. As a result, an optimal mix has to be brought in to reduce the power required to run the engine. The large diameter of the engine enables greater efficiency in terms of volumes of air being compressed, but at the same time, this also implies that the manufacturing cost goes up as the strength of material needed to handle such large turbine blades, is also quite large. Moreover, heavier blades generate higher levels of vibration. Hence much stronger mountings will be needed to withstand the higher levels of vibrations.

The air flow towards the core of the engine is slow. So, a volumetric efficiency is achieved by sucking the air along the core which also implies that lighter turbine blades with less intensive manufacturing technology, can be used at the core of the engine turbine. An engine with a smaller diameter will reduce noise and also require lighter turbine blades; but will result in lower levels of efficiency. The manufacturing skills, process and materials for them will be cheaper. So a tradeoff between cheaper, quieter engines versus powerful engines will have to be made.

The engines produce tremendous heat due to combustion. So, there is a need to have a heat removal system through heat sinks. These are bulky and are to be carted along as dead weight in flight. Hence, designers are trying to replace heat sinks by design changes that carry the heat over the shell of the aircraft. As the skin of the aircraft comes in contact with the air, heat dissipation is possible without heat sinks. Another way could be to channelise the engine heat or at least a part of it to the pantry inside the cabin to heat food or beverages. This will avoid the need to off take electricity from the power house of the aircraft.

The introduction of a local limited radius, on board Wi-Fi system to transmit signals/commands between sensors, controls and actuators, will eliminate the need of onboard wires. Such Wi-Fi network will be optimised as per the sensor-transmitter load configuration and the associated spatial spread required. This network will be tailored for each aircraft or sub assembly/part of the aircraft to reduce weight, cost and prevention of redundancy.

Noise reduction is being achieved by designing quieter engines as discussed above. The introduction of chevrons inside the engines reduces the noise level of the aircraft. The different configurations of turbine blades on a single shaft that revolve at individual optimal speeds, also reduce noise levels. Such speed variations and the associated vibrations of individual non similar types of turbine blades or fans, tends to cancel out metallurgical harmonics based resonance in the engine sub-components and prevent cumulative damage and acoustics or noise-based harm. It is something like soldiers being told to break steps as they walk over a bridge to prevent its collapse. In addition the laminar flow of air also reduces sound once it is optimised.

The flow of air over the surface of the aircraft produces drag. NASA along with its partners, is designing aircraft that have their wings towards the rear directly stuck to the fuselage using dependable adhesives and composite materials. This will eliminate the need to have riveting and bolting holes that add to cost, require extra precision in addition to the need of disposing of the debris to the minimum.

It can be seen that the new designs are geared to reduce fuel consumption, introduce renewable energy sources that are as efficient as ATF, reduce noise and emission levels. The specific thrust created per unit of fuel consumed will be dependent on the individual efficiencies introduced by each of the assemblies or facets as discussed above.