Rajan Wilcox
The amount of fuel a plane uses per km depends on a multitude of factors, including the aircraft's empty weight, payload, engine efficiency, flight path, and weather conditions. For example, a Boeing 747 burns approximately 5 gallons of fuel per mile (or 12 litres per kilometre) while carrying 500 people, which equates to 0.01 gallons per person per mile (or 100 miles per gallon per person). On the other hand, the Airbus A380, which is considered more fuel-efficient, burns an average of 4,600 gallons of fuel per hour. The fuel efficiency of aircraft has improved significantly over the years, with jet airliners becoming 70% more fuel-efficient between 1967 and 2007.
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What You'll Learn
Fuel efficiency improvements
The fuel efficiency of an aircraft is the measure of the transport energy efficiency of the aircraft. Fuel efficiency improvements are critical in the aviation sector to reduce CO2 emissions. Here are some ways to improve fuel efficiency:
Engine Efficiency
Improvements in engine efficiency have contributed significantly to the increased fuel efficiency of jet airliners. Between 1967 and 2007, jet airliners became 70% more fuel-efficient, with 40% of that improvement attributed to advancements in engine efficiency. Newer aircraft like the Boeing 787 Dreamliner, Airbus A350, and Bombardier CSeries are 20% more fuel-efficient per passenger kilometre than previous generations.
Airframes
Airframes play a crucial role in fuel efficiency. From 1967 to 2007, improvements in airframes contributed to a 30% increase in the fuel efficiency of jet airliners. Lighter composite material airframes, such as those used in the Boeing 787, help reduce weight and improve fuel efficiency.
Wingtip Devices
Wingtip devices, such as winglets, improve the effective wing aspect ratio, reducing lift-induced drag caused by wingtip vortices. This results in a better lift-to-drag ratio without increasing the wingspan. Airbus has utilized wingtip fences and Sharklet blended-winglets, offering a 3.5% fuel burn reduction on flights over 2,800 km. Boeing 737-800s have also benefited significantly from winglets, with an average efficiency increase of 6.69%.
Aircraft Design
Innovative aircraft designs, such as the Blended Wing Body (BWB) concept, offer improved fuel efficiency by utilizing the entire craft to produce lift, rather than just the wings. This design provides greater range, fuel economy, reliability, and life-cycle savings. NASA is currently testing a scale version of the BWB aircraft, with commercial designs expected by 2035.
Weight Reduction
Reducing the overall weight of an aircraft is crucial to enhancing fuel efficiency. Aircraft designs that relocate engines to the top of the plane body near the tail, such as the "double-bubble" D8 concept, can significantly reduce drag and improve fuel efficiency. Additionally, advancements in materials, such as aluminium metal foam and nanotechnology, hold potential for weight reduction and improved fuel efficiency.
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Jet engine efficiency
The efficiency of a jet engine can be defined as the amount of energy imparted to the plane per unit of energy in the fuel. Jet engines perform in two basic ways, the combined effect of which determines how much waste they produce as a byproduct of burning fuel to do thrust work on an aircraft. The first is an energy conversion as burning fuel speeds up the air passing through, which simultaneously produces waste heat from component losses (thermal efficiency). The second is that part of the power given to the air by the engine is transferred to the aircraft as thrust work, with the remaining part being kinetic energy waste in the wake (propulsive efficiency).
The efficiency of jet engines is calculated using the following formula: airspeed divided by the thrust-specific fuel consumption and the specific energy of the fuel. The overall efficiency of the jet engine is the thermal efficiency multiplied by the propulsive efficiency. The propulsive efficiency of a jet engine is reflected in the work done in the 1800s on ship propellers. The efficiency of this conversion (Froude or propulsive efficiency) is also known as the Froude efficiency.
The waste leaving a jet engine is in the form of a wake that has two constituents: one mechanical, called the residual velocity loss (RVL) due to its kinetic energy, and the other thermodynamic, due to its high temperature. The waste heat in the exhaust of a jet engine can only be reduced at the source by addressing the loss-making processes and entropy generated as the air flows through the engine. For example, a more efficient compressor has lower losses, generates less entropy, and contributes less to the temperature of the exhaust leaving the engine. Another example is the transfer of energy from an engine to air bypassing the engine. In the case of a high-bypass engine, there is a large proportion (~90%) of barely-warm (~60 °F warmer than ambient) thrust-producing air with only a 10% contribution from the much hotter exhaust from the power-producing core engine.
The efficiency of jet engines has been a focus of research and development for many years. Since the 1940s, there have been rapid advances in aero-engine technology. In the 1970s, economic pressure due to rising fuel costs resulted in increased emphasis on efficiency improvements for commercial airliners. Between 1967 and 2007, jet airliners became 70% more fuel-efficient, with 40% of the gains coming from improvements in engine efficiency and 30% from airframes.
Looking to the future, NASA is working on highly efficient hybrid-electric aircraft, with engineers shifting how aircraft engines have traditionally been designed, especially in terms of core size. By shrinking the core, the bypass ratio of the engine is increased, meaning the fuel burn rate is only slightly changed by the addition of a larger inlet fan. Therefore, the engine generates more thrust for roughly the same fuel burn, making it more efficient.
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Fuel costs
The fuel consumption of an aircraft is typically measured in litres per 100 kilometres per passenger or litres per 100 passenger kilometres. Modern passenger jets have a fuel economy of about 100 mpg per seat. The fuel efficiency of an aircraft can also be measured by fuel consumption per hour.
The amount of fuel used by an aircraft per kilometre varies depending on the type and size of the aircraft and the length of the flight. For example, the Airbus A380, one of the largest jet airliners in the world, burns an average of 4,600 gallons (11,400 litres) of fuel per hour. On the other hand, the newer Airbus A350 consumes around 38 pounds of fuel per nautical mile, which is approximately 17,000 gallons of fuel for a flight between New York and London of just over 3,000 nautical miles.
The fuel efficiency of aircraft has improved over time. Jet airliners became 70% more fuel-efficient between 1967 and 2007, with a 45% reduction in average fuel burn from 1968 to 2014. Newer aircraft, such as the Boeing 787 Dreamliner, Airbus A350, and Bombardier CSeries, are 20% more fuel-efficient per passenger kilometre than previous-generation aircraft.
The use of wingtip devices, such as winglets and sharklets, has also contributed to improving fuel efficiency. These devices increase the effective wing aspect ratio, reducing drag and improving the lift-to-drag ratio, resulting in fuel savings of up to 10.5%.
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Carbon emissions
The type of aircraft and its fuel efficiency play a crucial role in reducing carbon emissions. Jet airliners have made significant strides in fuel efficiency, improving by 70% between 1967 and 2007, with a 45% reduction in average fuel burn from 1968 to 2014. Newer aircraft like the Boeing 787 Dreamliner, Airbus A350, and Bombardier CSeries are 20% more fuel-efficient per passenger kilometre than previous generations. Additionally, modern twin jets are significantly more efficient than quadjets.
Wingtip devices, such as winglets and sharklets, have been effective in improving fuel efficiency by reducing lift-induced drag and improving the lift-to-drag ratio. These devices can lead to fuel savings of up to 10.5%, with the Airbus A321s achieving an average improvement of 4.8% in fuel consumption.
While the aviation industry has been working to improve fuel efficiency, sustainable aviation fuel (SAF) still accounts for a very small percentage of global jet fuel usage. The high cost of SAF has been a barrier to its wider adoption, but rising fuel prices and environmental concerns are driving the search for more sustainable alternatives.
The number of passengers on a flight also impacts carbon emissions per person. A full flight is more fuel-efficient than a half-empty one, and a plane with more seats, such as the A380, can be more efficient per passenger. However, low-cost airlines tend to have better fuel efficiency due to their higher load factors.
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Fuel-efficient aircraft
Fuel efficiency in aircraft is a measure of the transport energy efficiency of aircraft. The average fuel burn per kilometre of new aircraft fell by 45% from 1968 to 2014, and in 2018, the average fuel burn was 28 grams of fuel per kilometre.
There are several ways to improve the fuel efficiency of aircraft. Firstly, aircraft efficiency can be improved by maximising the lift-to-drag ratio. This can be achieved by minimising parasitic drag and lift-generated induced drag, which are the two components of aerodynamic drag. Parasitic drag can be reduced by minimising the frontal area of the aircraft and streamlining its shape for a lower drag coefficient, while induced drag can be reduced by decreasing the size and weight of the airframe and increasing the wing aspect ratio.
Another way to improve fuel efficiency is by reducing the weight of the aircraft. A lighter airframe generates lower drag, and weight reduction can be achieved through the use of lightweight materials and construction methods. Additionally, new technology can be employed to reduce engine fuel consumption, such as higher pressure and bypass ratios, geared turbofans, open rotors, and hybrid or fully electric propulsion.
Some specific examples of fuel-efficient aircraft include the Aerion AS2, which has since been discontinued, and the Airbus A320, which features Sharklet blended-winglets that offer a 3.5% fuel burn reduction on flights over 2,800 kilometres. The Boeing 737-800 also benefits significantly from the use of winglets, with an average increase in efficiency of 6.69%.
In terms of manufacturers, Boeing tends to come out on top in terms of fuel efficiency, with their 787 Dreamliner being 20% more fuel-efficient per passenger kilometre than previous-generation aircraft. Airbus is a close second, with their A350 and A380 models also offering impressive fuel economy.
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Frequently asked questions
Fuel consumption depends on the type of plane, its age, the number of passengers, the distance travelled, and other factors. On average, a plane uses 3 to 4 litres of fuel per passenger per 100 kilometres. For example, a Boeing 747 uses approximately 1 gallon (about 4 litres) of fuel every second, which equates to 5 gallons of fuel per mile (12 litres of fuel per kilometre).
Long-haul flights tend to be more fuel-efficient per passenger kilometre than short-haul flights. This is because shorter flights have a higher proportion of fuel burn during taxiing, take-off, climb, and approach, which are more significant on shorter flights.
Modern twin-engine aircraft like the Airbus A350 and its Rolls-Royce Trent XWB engines are considered some of the most fuel-efficient widebody planes on the market. It consumes around 38 lb of fuel per nautical mile, which equates to approximately 17,000 gallons of fuel for a flight between New York and London.
