Rajan Wilcox
The fuel efficiency of aircraft compared to other modes of transportation is a topic of interest for those concerned about carbon emissions. While take-off is the most fuel-intensive part of a flight, cruising burns the most fuel overall. Fuel efficiency is improved by better aerodynamics, reduced weight, and improved engine brake-specific fuel consumption. The average fuel burn of new aircraft fell by 45% from 1968 to 2014, and in 2018, CO₂ emissions for passenger transport were 747 million tonnes. In comparison, trains are more efficient than planes, with diesel trains being about four times as energy efficient as airplanes. However, the global warming effects of non-CO2 emissions from aircraft are worse than those from trains.
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What You'll Learn
Fuel efficiency and carbon intensity
Planes are currently the most fuel-intensive transport option, with an estimated fuel efficiency of about 43 passenger miles per gallon of jet fuel. This is in contrast to interurban buses, which are the most energy-efficient option, with an estimated 152 passenger miles per gallon gasoline equivalent (MPGge). Interurban rail and cars have intermediate values of 51 and 53 MPGge, respectively.
The fuel efficiency of aircraft has improved over time, with a 45% reduction in average fuel burn for new aircraft from 1968 to 2014. This equates to a compounded annual reduction of 1.3%. However, the total fuel burned and carbon emissions produced by aircraft are still significant. The quantity of fuel burned is influenced by factors such as aircraft capacity, age, and engine type. For example, the Airbus A380 used for the Dubai flight has a higher fuel consumption per hour compared to the newer Airbus A350 used for the Hong Kong flight.
When comparing the fuel efficiency of trains and planes, it is important to consider the type of train. Steam-powered trains that burn coal or fuel oil have a lower fuel efficiency of about 5 passenger-miles per gallon. Electric trains are about twice as efficient as airplanes, while diesel trains are about four times as energy-efficient. It is worth noting that the carbon intensity of planes may be higher due to non-CO2 emissions, such as the formation of clouds from water vapour, which can trap heat and contribute to global warming.
Overall, the fuel efficiency and carbon intensity of transportation modes vary, with planes being the most fuel-intensive option. While improvements in technology have led to reductions in fuel burn, the carbon footprint of different modes should be a key consideration for travellers and the industry.
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Energy efficiency
Firstly, the type of fuel used is important. Trains, especially older models, burn coal or fuel oil, which have a lower energy efficiency than jet fuel. On the other hand, diesel trains are about four times as energy-efficient as airplanes. Electric trains are about twice as efficient as airplanes, but the process of generating electricity requires a lot of fuel, which reduces their overall energy efficiency.
The design and functionality of aircraft also impact their energy efficiency. For example, wingtip devices on planes improve aerodynamics, reducing drag and improving lift, which leads to better fuel efficiency. Additionally, cruising at higher altitudes can reduce drag and maximize endurance and range. However, this is counteracted by the weight penalty of carrying extra fuel on long-haul flights, which can limit the number of available seats.
Comparing different modes of transport is complex, and there are many variables to consider. The number of passengers, distance travelled, speed, and type of fuel all play a role in determining energy efficiency. While planes and trains may be equal in terms of passenger-miles per gallon, other factors, such as non-CO2 emissions, give trains an advantage in terms of their impact on global warming.
In conclusion, when considering energy efficiency, it is important to look beyond simple metrics like miles per gallon. A more comprehensive analysis should consider fuel type, vehicle design, and environmental impact to determine the most energy-efficient mode of transportation.
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Fuel economy
The fuel economy of a vehicle is often measured in miles per gallon (mpg). However, this metric is not suitable for comparing different modes of transport, given the large difference in vehicle size. A better metric is passenger miles per gallon, which is obtained by multiplying mpg by the number of passengers carried.
According to data collected by the Oak Ridge National Laboratory, the average occupancy for passenger vehicles on longer leisure trips (most comparable to plane, bus, and train trips) is 2.2. It is also important to correct for the fact that vehicles use different types of fuels, with buses, planes, and trains typically operated on denser, more carbon-intensive fuels than cars.
In 2016, the average fuel consumption for flights across the Pacific Ocean was 3.23 litres per 100 kilometres per passenger (73 mpg-US). The most fuel-efficient airline on this route was Hainan Airlines, with 2.78 litres per 100 kilometres per passenger (85 mpg-US). Qantas was the least efficient, with 4.55 litres per 100 kilometres per passenger (51.7 mpg-US).
On the same year, Cathay Pacific and Cathay Dragon consumed 37 grams of fuel per revenue passenger kilometre, or 4.63 litres per 100 kilometres (50.8 mpg-US). Aeroflot consumed 2.86 litres per 100 kilometres per seat (82 mpg-US) at an 81.5% load factor.
According to one source, a Boeing 747 uses approximately 1 gallon of fuel (4 litres) every second. Over a 10-hour flight, this amounts to 36,000 gallons (150,000 litres). Another source states that a 747 burns approximately 5 gallons of fuel per mile (12 litres per kilometre).
The fuel efficiency of an aircraft depends on its fleet fuel burn, seating density, air cargo and passenger load factor, and operational procedures like maintenance and routing. On average, fuel burn for 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 for 8.5 trillion revenue passenger kilometres, giving an average of 88 grams of CO₂ per revenue passenger kilometre; this represents 28 grams of fuel per kilometre, or a 3.5 litres per 100 kilometres (67 mpg-US) fuel consumption per passenger, on average.
Wingtip devices on aircraft wings can increase fuel efficiency by improving the lift-to-drag ratio. Airbus A321s average a 4.8% improvement in fuel consumption with winglets, while Boeing 737-800s benefit the most from this technology, with a 6.69% increase in efficiency.
To minimize fuel consumption, an aircraft should cruise close to the maximum altitude at which it can generate sufficient lift to maintain its altitude. As the aircraft's weight decreases throughout the flight due to fuel burn, its optimum cruising altitude increases. Flying at optimum altitudes and airspeeds can maximize endurance and range, improving economy.
Trains are also fuel-intensive, especially older steam-powered trains which burned coal or fuel oil and achieved only 5 passenger miles per gallon. Electric trains are about twice as efficient as airplanes, and diesel trains are about four times as efficient.
Overall, planes are currently the most fuel-intensive mode of transport, with an estimated fuel efficiency of about 43 passenger miles per gallon of jet fuel. Interurban buses are the most energy-efficient option, with an estimated 152 passenger miles per gallon of gasoline equivalent. Interurban rail and cars have intermediate values of 51 and 53 miles per gallon of gasoline equivalent, respectively.
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Fuel consumption
The fuel economy of a plane is often measured in miles per gallon (mpg), with newer planes achieving over 100 mpg per person on full flights exceeding 500 miles. However, shorter commuter flights on smaller, older planes may only achieve 30-40 mpg per person. On average, large commercial jets like the Boeing 737-800 have seen a 6.69% increase in efficiency due to wingtip devices, with fuel savings ranging from 4.6% to 10.5%.
When comparing planes to trains, it is important to consider the type of train. Steam-powered trains that burn coal or fuel oil have a fuel economy of around 5 passenger-miles per gallon. Electric trains are about twice as efficient as airplanes, while diesel trains are about four times as efficient. The energy expended by trains and planes can be compared in terms of passenger-miles per BTU of fuel, with trains and planes exhibiting similar energy efficiency in this metric.
The carbon intensity of transport options is also an important consideration. While planes may be more energy-efficient than trains, they contribute more to global warming due to non-CO2 emissions. Condensation trails, or contrails, formed by water vapour at high altitudes, trap heat and contribute to global warming. Therefore, when considering fuel consumption and environmental impact, taking a train may be a more fuel-efficient and environmentally friendly option compared to flying.
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Carbon emissions
The fuel economy of an aircraft is influenced by factors such as aerodynamics, weight, engine brake-specific fuel consumption, propulsive efficiency, and thrust-specific fuel consumption. Flying at optimum altitudes, usually higher, improves fuel economy. Additionally, an aircraft's optimum cruising altitude increases as its weight decreases during the flight due to fuel burn.
According to the International Council on Clean Transportation, planes are currently the most fuel-intensive transport option, with an estimated fuel efficiency of about 43 passenger miles per gallon of jet fuel. In contrast, interurban rail has an estimated fuel efficiency of 51 passenger miles per gallon of gasoline equivalent (MPGge).
When comparing carbon emissions between planes and trains, it is important to consider the type of train. Steam-powered trains that burn coal or fuel oil have lower fuel efficiency, estimated at 5 passenger-miles per gallon. Electric trains are about twice as efficient as airplanes, while diesel trains are about four times as efficient.
It is worth noting that the global warming effects of non-CO2 emissions from aircraft, such as condensation trails, are considered worse than their CO2 emissions. Therefore, while planes may be more energy-efficient than trains, they contribute more to global warming.
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Frequently asked questions
Fuel efficiency is calculated in terms of miles per gallon. While planes and trains are about equal in energy efficiency, planes are currently the most fuel-intensive transport option, with an average of 43 passenger miles per gallon of jet fuel. Trains that run on diesel are about four times as energy-efficient as airplanes, while electric trains are about twice as efficient.
A plane's fuel consumption depends on various factors such as the aircraft's capacity, age, and engine type, as well as altitude, winds, and routing. For instance, a Boeing 747 can burn approximately 36,000 gallons of fuel during a 10-hour flight, while a 737 might burn 30,000 pounds of fuel in under 5 hours. The A380, one of the heaviest aircraft, is known for its high fuel consumption per hour.
Take-off and cruising are the most fuel-intensive parts of a flight. While take-off requires the engines to work the hardest, cruising accounts for a significant portion of fuel burn, especially on longer flights. For instance, on the London Heathrow-Hong Kong route, cruising burned 96% of the total fuel.
The carbon emissions from burning fossil fuels are directly proportional to the quantity of fuel burned. Therefore, longer flights produce more carbon emissions. Additionally, non-CO2 emissions from aircraft, such as the water vapour that forms condensation trails, contribute significantly to global warming.
