Sylvie Willis
The fuel economy of an aircraft is a measure of its transport energy efficiency. Jet aircraft have twice the fuel efficiency of the earliest jet airliners, and jet airliners became 70% more fuel-efficient between 1967 and 2007. Drones, on the other hand, are known to be environmentally friendly alternatives to conventional modes of transportation. Drone flight data reveals that energy per package delivered by drones can be up to 94% lower than conventional transportation modes. Military drones have also achieved better fuel economies than most incumbent military vehicles. While jet aircraft have improved fuel efficiency over time, drones offer a more energy-efficient mode of transportation, particularly for small package deliveries.
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What You'll Learn
- Jet fuel consumption depends on engine type, aircraft weight, and speed
- Jet fuel is kerosene-based, while aviation gasoline is used in small piston-engine planes
- Jet fuel cost and emissions have led to a renewed interest in propfans
- Modern jets are twice as fuel-efficient as the earliest jet airliners
- Jet fuel consumption is influenced by flight conditions and operating modes
Jet fuel consumption depends on engine type, aircraft weight, and speed
The fuel consumption of jet aircraft depends on several factors, including the type of engine, the weight of the aircraft, and its speed. Jet engine efficiency is calculated by dividing airspeed by thrust-specific fuel consumption and the specific energy of the fuel.
Shaft engines, such as piston engines or turboprops, have efficiency that is inversely proportional to their brake-specific fuel consumption. In contrast, jet engines are generally more efficient at higher speeds. For instance, turboprops have an optimal speed of below 460 miles per hour, while jets used by major airlines today typically exceed this speed.
The weight of the aircraft also plays a significant role in fuel consumption. As weight increases, so does the lift-induced drag, which reduces aircraft efficiency. Minimizing weight through the use of lightweight materials and efficient design can help improve fuel efficiency. Additionally, an aircraft's maximum range is determined by how efficiently thrust can overcome aerodynamic drag.
The speed of the aircraft is another critical factor in fuel consumption. At a constant propulsive efficiency, the maximum range speed occurs when the ratio between velocity and drag is minimal. To minimize fuel consumption, an aircraft should cruise close to the maximum altitude where it can maintain sufficient lift.
Other factors that influence fuel consumption include the aircraft's empty weight, carried payload, engine efficiency, flight path, and weather conditions. Overall, jet fuel consumption is a complex interplay of various factors, and improvements in engine and aircraft design, as well as operational practices, can help optimize fuel efficiency.
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Jet fuel is kerosene-based, while aviation gasoline is used in small piston-engine planes
Jet fuel is a kerosene-based derivative, characterised by its higher molecular weight and distinct hydrocarbon makeup. Kerosene is routinely referred to as "fuel oil" in some places. It has a higher energy density than gasoline, making it ideal for the demands of plane turbine engines. Jet fuel also has a higher flash point than gasoline, making it safer to use in air transportation.
Gasoline, on the other hand, is primarily used in spark-ignition or piston engines found in smaller aircraft. It has a lighter hydrocarbon composition, making it suitable for internal combustion engine requirements. Gasoline is easier to ignite than kerosene, and it is also more volatile. This makes it a poor choice for jet engines, as it tends to burn too quickly at the high temperatures jet engines operate at.
Aviation gasoline, or avgas, is a type of gasoline used in piston-engine aircraft, especially smaller general aviation planes. It is a high-octane fuel that is required for older piston engines in sports aircraft and small private aircraft. Avgas is more expensive than jet fuel due to its low production volume, long supply routes, and elaborate quality controls.
While jet fuel is generally used in larger aircraft, some smaller aircraft also use jet fuel. For example, the Bombardier Dash 8 Q400 turboprop is a regional airliner that uses jet fuel due to its improved fuel efficiency.
The type of fuel used in aircraft engines depends on various factors, including engine type, aircraft size, and flight route. The efficiency of an aircraft also depends on factors such as aerodynamics, weight, engine brake-specific fuel consumption, and propulsive efficiency.
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Jet fuel cost and emissions have led to a renewed interest in propfans
Fuel economy in aircraft is a measure of the transport energy efficiency of an aircraft. Fuel efficiency is increased by improving aerodynamics, reducing weight, and improving engine brake-specific fuel consumption and propulsive efficiency or thrust-specific fuel consumption. The average fuel burn of new aircraft fell by 45% from 1968 to 2014, a compounded annual reduction of 1.3%. In 2018, CO₂ emissions from passenger transport totalled 747 million tonnes, giving an average of 88 grams of CO₂ per revenue passenger kilometre.
Rising jet fuel prices and the drive for net zero have led to a renewed interest in improving engine and airframe efficiency to reduce emissions. This has resulted in a renewed interest in propfans, which are aircraft engines that combine features of turbofans and turboprops. Propfans aim to combine the speed capability of turbofans with the fuel efficiency of turboprops, especially at high subsonic speeds. They are sometimes called "ultra-high-bypass (UHB) turbofans".
In the 1980s, General Electric (GE) developed the GE36 UDF propfan, which was intended to replace the CFM56 high-bypass turbofan. The GE36 UDF exhibited a 30% reduction in fuel consumption over the turbo-fan-powered MD-80. However, the project did not come to fruition due to excessive cabin noise and low fuel prices.
In 2021, CFM International, a joint venture between GE and Safran Aircraft Engines, announced the Revolutionary Innovation for Sustainable Engines (RISE) development program to produce a single-stage, gear-driven propfan. The engine is expected to produce 20,000–35,000 lbf of thrust, with a 20% increase in fuel efficiency. The company claimed that its motivation was the global emphasis on reducing emissions.
Propfans have the potential to significantly improve fuel efficiency and reduce emissions, but they have been criticized for creating excessive noise both inside the cabin and on the ground. However, CFM has assured that the RISE engine is being thoroughly tested to meet rigorous noise emission standards.
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Modern jets are twice as fuel-efficient as the earliest jet airliners
The earliest jet engines were hybrid designs that aimed to compress air with an external power source, mix it with fuel, and burn it for jet thrust. However, these early attempts were unsuccessful, and engineers soon realized that propeller efficiency was limited by the speed of sound. This challenge spurred the development of the gas turbine engine, which became the most common form of jet engine. Modern jet aircraft have come a long way since these early experiments, boasting twice the fuel efficiency of their pioneering predecessors.
Over time, the focus on engine efficiency has intensified, with modern jet engines designed to have multiple sets of turbines rotating at optimal speeds. This "advanced technology engine" concept has led to triple-spool engines, allowing different blades to revolve at varying speeds. Additionally, the advent of high-bypass turbofan jet engines revolutionized fuel efficiency, matching the performance of the best piston and propeller engines.
The efficiency gains in jet aircraft are evident when comparing fuel burn rates. From 1968 to 2014, the average fuel burn of new aircraft decreased by 45%, translating to a compounded annual reduction of 1.3%. Jet airliners, in particular, witnessed a remarkable improvement, achieving a 70% increase in fuel efficiency between 1967 and 2007. This progress is attributed to both engine efficiency enhancements (40%) and airframe improvements (30%).
While modern jet aircraft have made significant strides in fuel efficiency, the aviation industry continues to explore innovative designs and technologies to further reduce fuel consumption and emissions. For instance, Airbus has patented aircraft designs featuring twin rear-mounted counter-rotating propfans, aiming to bridge the efficiency gap between turboprops and high-bypass turbofans. These efforts are driven by the growing emphasis on sustainability and the rising costs associated with jet fuel.
In summary, modern jets have achieved remarkable progress in fuel efficiency, doubling the efficiency of their earliest counterparts. This advancement is the result of continuous innovation in engine technology, aircraft design, and operational procedures. As the aviation industry navigates the challenges of sustainability and rising fuel prices, further improvements in fuel efficiency are expected, making air travel more environmentally friendly and economically viable.
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Jet fuel consumption is influenced by flight conditions and operating modes
Fuel consumption in jet aircraft is influenced by a multitude of factors, including flight conditions, operating modes, and aircraft design.
Altitude significantly affects fuel efficiency. Higher altitudes offer reduced air resistance, resulting in lower fuel consumption. Specifically, flying at 15,000 meters can reduce fuel consumption by up to 30% compared to a low-altitude flight at around 3,000 meters. This is due to reduced air density and increased engine efficiency at higher altitudes. However, during rapid ascents to high altitudes, fuel consumption temporarily increases, only to be offset by lower consumption when cruising.
Operating modes, such as high-intensity maneuvers like tight turns and rapid acceleration, considerably increase fuel usage. Combat and training missions, which involve frequent speed and altitude changes, result in irregular fuel burn rates. Similarly, in-flight refueling operations require specific aircraft positioning, leading to increased fuel consumption during these maneuvers.
Aircraft design also plays a role in fuel efficiency. Modern jet aircraft, for example, have twice the fuel efficiency of the earliest jet airliners due to advancements in aerodynamics, weight reduction, and improved engine technology. Additionally, wingtip devices like winglets and blended-winglets can offer fuel burn reduction, with some Airbus models showing improvements of up to 10.75%.
Other factors influencing fuel consumption include the aircraft's empty weight, payload, engine efficiency, flight path, and weather conditions. Optimizing these variables through effective fuel planning and management is crucial for maximizing operational efficiency, minimizing costs, and ensuring mission success.
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Frequently asked questions
The amount of fuel used by jets per flight depends on several factors, including the type of jet, the distance travelled, the weight of the jet, and the number of passengers or cargo on board. On average, large commercial jets like the Boeing 737-800 have benefited the most from winglets, which can reduce fuel burn by up to 10.5%.
Drones typically use battery power rather than fuel, and their flight time depends on the weight of the drone and the efficiency of the battery. For example, the DJI FlyCart 30 has a hover endurance of 8 minutes when fully loaded, while the Jetson One can fly for around 20 minutes. Small, next-generation electric drones can achieve well over 1,000 mpg-equivalent, while military drones like the Reaper and Predator drones have achieved 3mpg and 8mpg respectively.
Jets and drones have different fuel efficiency depending on their design and usage. Modern jet aircraft have twice the fuel efficiency of the earliest jet airliners, and jet airliners became 70% more fuel-efficient between 1967 and 2007. Drones that are used for package delivery can have up to 94% lower energy consumption per package compared to conventional modes of transportation.
