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This page describes fuel efficiency in means of transportation. For the environmental impact assessment of a given product or service throughout its lifespan, see life cycle assessment.


The fuel efficiency in transportation ranges from a few megajoules per kilometre for a bicycle to several hundred for a helicopter.

Efficiency can be expressed in terms of consumption per unit distance per vehicle, consumption per unit distance per passenger or consumption per unit distance per unit mass of cargo transported.

Rockets are not necessarily directly comparable since while it takes a very large amount of energy to get into space, once there they can coast with negligible friction thus getting both the highest and lowest fuel efficiency, by far. However, for the maximum landing distance (antipodal) they are not efficient compared with other transportation.

Transportation modes

For freight transport, rail and ship transport are generally much more efficient than trucking, and air freight is much less efficient.)

Walking

  • Walking or running one kilometre requires approximately 330 kJ (70 kcal) of food energy.


Bicycling

As a relatively light vehicle, with low-friction tyres, and an efficient chain-driven drive chain, the bicycle can be an efficient form of transport. Cycling requires about half the energy of walking—around . This figure depends heavily on the speed and mass of the rider: the greater the speeds and heavier the riders the more energy consumed per unit distance.

A motorized bicycle such as the Velosolex affords the rider to cycle under human power or with the assistance of a clean-burning engine which equates to a range of . Electric pedal assisted bikes run on as little as , while maintaining speeds in excess of . These best-case figures rely on a human doing 70% of the work, with around coming from the engine. Including the human energy dramatically changes the quoted efficiency of cycles. This would include the caloric efficiency of human muscle, cardio vascular efficiency, and the energy costs of producing, transporting, packaging and waste disposal of the food itself.

Automobiles

Automobile fuel efficiency is often expressed in volume fuel consumed per one hundred kilometres (i.e., L/100km) but in distance per volume fuel consumed (i.e., miles per gallon) in the US. This is complicated by the different energy content of fuels (compare petrol and diesel). The Oak Ridge National Laboratory (ORNL) state that the energy content of unleaded gasoline is 115,000 BTU per US gallon (32 MJ/L) compared to 130,500 BTU per US gallon (36.4 MJ/L) for diesel.

A second important consideration is the energy costs of producing these fuels. Bio-fuels, electricity and hydrogen, for instance, have significant energy inputs in their production. Because of this, the 50-70% efficiency of hydrogen production has to be combined with the vehicle efficiency to yield net efficiency.

A third consideration to take into account is the occupancy rate of the vehicle. As the number of passengers per vehicle increases the consumption per unit distance per vehicle increases. However this increase is slight compared to the reduction in consumption per unit distance per passenger. We can compare, for instance, the estimated average occupancy rate of about 1.3 passengers per car in the San Francisco Bay Area to the 2006 UK estimated average of 1.58.

Example consumption figures



  • Honda Insight was rated highway.
  • Honda Civic Hybrid- The second most energy efficient automobile in the US, it regularly averages around .
  • Toyota Prius - According to the US EPA's revised estimates, the combined fuel consumption for the 2008 Prius is , making it the most fuel efficient US car of 2008. In the UK, the official fuel consumption figure (combined) for the Prius is .
  • The General Motors EV1 was rated in a test with a charging efficiency of 373 Wh-AC/mile or 23 kWh/100 km.
  • The four passenger GEM NER also uses 169 Wh/mile or 10.4 kWh/100 km, which equates to 2.6 kWh/100 km per person when fully occupied, albeit at only .


Aircraft

A principal determinant of fuel consumption in aircraft is drag, which must be opposed by thrust for the aircraft to progress. Drag increases approximately as the square of lift required for flight, and, as force of lift is directly related to craft weight, to aircraft weight2. Unlike parasitic or form drag, induced drag decreases with the square of velocity, making flight at higher speeds more efficient (see drag).

As induced drag increases as a power function of weight, mass reduction, along with improvements in engine efficiency and reductions in aerodynamic drag, has been a principal source of efficiency gains in craft, with a rule-of-thumb being that a 1% weight reduction corresponds to around a .75% reduction in fuel consumption. Altitude impacts on both air-drag and engine efficiency; the altitude at which aircraft are permitted to fly greatly influences their fuel consumption. Jet-engine cruising efficiency increases at altitude due to the constraint to maintain a combustible fuel mixture: low-pressure air allows reduced fuel injection while maintaining an adequate fuel:air ratio.

Passenger airplanes averaged 4.8 L/100 km per passenger (1.4 MJ/passenger-km) (49 passenger-miles per gallon) in 1998. Note that on average 20% of seats are left unoccupied. Jet aircraft efficiencies are improving: Between 1960 and 2000 there was a 55% overall fuel efficiency gain (if one were to exclude the inefficient and limited fleet of the DH Comet 4 and to consider the Boeing 707 as the base case). . Most of the improvements in efficiency were gained in the first decade when jet craft first came into widespread commercial use. Compared to the most advanced turboprop aircraft of the 1950s, the modern aircraft is only marginally more efficient per passenger-mile. Between 1971 and 1998 the fleet-average annual improvement per available seat-kilometrewas estimated at 2.4%. As over 80% of the fully-laden take-off weight of a modern aircraft such as the Airbus A380 is craft and fuel, there remains considerable room for future improvements in efficiency.

  • Airbus state that their A380 consumes fuel at the rate of less than 3 L/100 km per passenger. CNN reports that the fuel consumption figures provided by Airbus for the A380, given as 2.9 L/100 km per passenger, are "slightly misleading", because they assume a passenger count of 555, but do not allow for any luggage or cargo. Typical occupancy figures are unknown at this time. Furthermore, the A380, unlike other airliners, has special dispensation from the FAA to fly higher than .


  • NASA and Boeing are conducting tests on a "blended wing" aircraft. This design allows for greater fuel efficiency since the whole craft produces lift, not just the wings.


  • The Sikorsky S-76C++ twin turbine helicopter gets about at and carries 12 for about 19.8 passenger-miles per gallon (11.9 litres per 100 passenger-kilometres).


  • The Bell 407 single-engine turbine helicopter burns 51 gallons per hour at 120 knots carrying one pilot and six passengers. 2.35 NM per gal for 14.1 passenger-miles per gallon. If the pilot is counted as a passenger, it's 16.4 people-miles per gallon. Increased altitudes can yield better fuel rates. It has operated at 47 gal/hr.


Ships

  • Cunard state that their liner, the RMS Queen Elizabeth 2, travels 49.5 feet per imperial gallon of diesel oil (3.32 m/L or 41.2 ft/US gal), and that it has a passenger capacity of 1777. Thus carrying 1777 passengers we can calculate an effieiency of 16.7 passenger-miles per imperial gallon (16.9 L/100 p·km or 13.9 p·mpg–US).


Trains

UK Freight train average about 1.5-2.0 MPG Loaded. Compared with road transport it is very efficient; if lorries did the same trip they would use 70% more fuel than a freight train. Uk Passenger trains average from 8MPG - 12MPG.

  • Freight: the AAR claims an energy efficiency of over 400 short ton-miles per gallon of diesel fuel in 2004 (0.588 L/100 km per tonne or 235 J/(km·kg))




  • a 1997 ECmarker study on page 74 claims 18.00 kWh/train-km for the TGV Duplex assuming 3 intermediate stops between Parismarker and Lyonmarker. This equates to 64.80 MJ/train-km. With 80% of the 545 seats filled on average this is 0.15 MJ/passenger-km.


  • Actual train consumption depends on gradients, maximum speeds and stopping patterns. Data was produced for the European MEET project (Methodologies for Estimating Air Pollutant Emissions) and illustrates the different consumption patterns over several track sections. The results show the consumption for a German ICE High speed train varied from around . The data also reflects the weight of the train per passenger. For example, the TGV double-deck ‘Duplex’ trains use lightweight materials in order to keep axle loads down and reduce damage to track, this saves considerable energy.


  • A Siemens study of Combino light rail vehicles in service in Basel, Switzerlandmarker over 56 days showed net consumption of 1.53 kWh/vehicle-km, or 5.51 MJ/vehicle-km. Average passenger load was estimated to be 65 people, resulting in average energy efficiency of 0.085 MJ/passenger-km. The Combino in this configuration can carry as many as 180 with standees. 41.6% of the total energy consumed was recovered through regenerative braking.


  • A trial of a Colorado Railcar double-deck DMU hauling two Bombardier Bi-level coaches found fuel consumption to be for , or . The DMU has 92 seats, the coaches typically have 162 seats, for a total of 416 seats. With all seats filled the efficiency would be 468 passenger-miles per US gallon (0.503 L/100 passenger-km; 562 passenger-mpg-imp).


  • Note that intercity rail in the US reports 3.17 MJ/passenger-km which is several times higher than reported from Japan. Independent transportation researcher David Lawyer attributes this difference to the fact that the losses in electricity generation may not have been taken into account for Japan and that Japanese trains have a larger number of passengers per car.


  • Modern electric trains like the shinkansen use regenerative braking to return current into the catenary while they brake. This method results in significant energy savings, where-as diesel locomotives (in use on unelectrified railway networks) typically dispose of the energy generated by dynamic braking as heat into the ambient air.


  • This Swiss Railroad company SBB-CFF-FFS cites 0.082 kWh per passenger-km for traction.


  • AEA carried out a detailed study of road and rail for the United Kingdom Department for Transport. Final report


  • Amtrak reports 2005 energy use of 2,935 BTU per passenger-mile (1.9 MJ/passenger-km).


  • The Passenger Rail (Urban and Intercity) and Scheduled Intercity and All Charter Bus Industries Technological and Operational Improvements - FINAL REPORT states that "Commuter operations can dissipate more than half of their total traction energy in braking for stops." and that "We estimate hotel power to be 35 percent (but it could possibly be as high as 45 percent) of total energy consumed by commuter railways." Having to accelerate and decelerate a heavy train load of people at every stop is inefficient despite regenerative braking which can recover typically around 20% of the energy wasted in braking.


Buses

  • In July 2005, the average occupancy for buses in the UK was stated to be 9.


  • The fleet of 244 1982 New Flyer trolley buses in local service with BC Transit in Vancouver, Canada, in 1994/95 consumed 35454170 kW·h for 12966285 vehicle-km, or 9.84 MJ/vehicle-km. Exact ridership on trolleybuses is not known, but with all 34 seats filled this would equate to 0.32 MJ/passenger-km. It is quite common to see people standing on Vancouver trolleybuses. Note that this is a local transit service with many stops per kilometre; part of the reason for the efficiency is the use of regenerative braking.


  • A diesel bus commuter service in Santa Barbara, CA, USAmarker found average diesel bus efficiency of (using MCI 102DL3 buses). With all 55 seats filled this equates to 330 passenger-mpg, with 70% filled the efficiency would be 231 passenger-mpg.


Rockets

Unlike other forms of transportation, rockets are commonly designed for putting objects into orbit. Once in sufficiently high orbit, objects have almost negligible air drag, and the orbits decay so slowly that a satellite can be still orbiting decades after launch. For these reasons rocket and space propulsion efficiency is rarely measured in terms of distance per unit of fuel, but in terms of specific impulse which gives how much change in momentum (i.e. impulse) can be obtained from a unit of propellant.

However, to give a concrete example, NASAmarker's space shuttle fires its engines for around 8.5 minutes, consuming 1,000 tonnes of solid propellant (containing 16% aluminium) and an additional 2,000,000 litres of liquid propellant (106,261 kg of liquid hydrogen fuel) to lift the 100,000 kg vehicle (including the 25,000 kg payload) to an altitude of 111 km and an orbital velocity of 30,000 km/h. With an energy density of 31MJ per kg for aluminum and 143 MJ/kg for liquid hydrogen, this means that the vehicle consumes around 5 TJ of solid propellant and 15 TJ of hydrogen fuel.

Once in orbit at 200 km and around 7.8 km/s velocity, the orbiter requires no further fuel. At this altitude and velocity, the vehicle has a kinetic energy of about 3 TJ and a potential energy of roughly 200 GJ. Given the initial energy of 20 TJ, the Space Shuttle is about 16% energy efficient at launching the orbiter and payload just 4% efficiency if the payload alone is considered.

Across a full journey, the space shuttle Atlantis flew approximately 8 million kilometres on the STS-115 mission. Converted to mass of propellant per kilometre, this amounts to 0.125 kg of solid propellant and 0.25 litres of liquid propellant per kilometre. In relation to the theoretical largest ground distance (antipodal) flight of 20,000 km, usage is 50 kg of solid propellant and 100 litres of liquid propellant per kilometre.

Other

  • NASAmarker's Crawler-Transporter is used to move the Shuttle from storage to the launch pad. It uses diesel and has one of the highest fuel consumption rates on record, .


International transport comparisons

UK Public transport

Rail and bus are generally required to serve 'off peak' and rural services, which by their nature have lower loads than city bus routes and inter city train lines. Moreover, due to their 'walk on' ticketing it is much harder to match daily demand and passenger numbers. As a consequence, the overall load factor on UK railways is 35% or 90 people per train :

Conversely, Air services work on point-to-point networks between large population centres and are 'pre-book' in nature. Using Yield management overall loads can be raised to around 70-90%. However, recently intercity train operators have been using similar techniques, with loads reaching typically 71% overall for TGV services in France and a similar figure for the UK's Virgin trains services.

For emissions, the electricity generating source needs to be taken into account. Up to date figures for the UK can be found here:

http://www.atoc-comms.org/admin/userfiles/Energy20%&%20Emissions%20Statement%20-%20web%20version.pdf

(Bad link)

http://www.aef.org.uk/downloads//Grams_CO2_transportmodesUK.pdf

US Passenger transportation

The US Transportation Energy Data Book states the following figures forPassenger transportation in 2006:

Transport mode Average passengers
per vehicle
BTU per passenger-mile MJ per passenger-kilometre
Vanpool 6.1
Efficient Hybrid 1.57
Motorcycles 1.2
Rail (Commuter) 31.3
Rail (Transit Light & Heavy) 22.5
Rail (Intercity Amtrak) 20.5
Cars 1.57
Air 96.2
Buses (Transit) 8.8
Personal Trucks 1.72


US Freight transportation

The US Transportation Energy book states the following figures for Freight transportation in 2004:
Transportation mode Fuel consumption
BTU per short ton mile kJ per tonne kilometre

Class 1 Railroads 341
Domestic Waterborne 510
Heavy Trucks 3,357
Air freight (aprox) 9,600


Caveats




Comparing fuel efficiency in transportation is a bit like comparing apples and oranges in some ways. Here are a few things to consider. Traction energy Metrics produced by the UK Rail and Safety Standards Board is also a useful review of the problem of comparison http://www.rssb.co.uk/pdf/reports/research/T618_traction-energy-metrics_final.pdf

  • There is a distinction between vehicle MPGe and passenger MPGe. Most of these entries cite passenger MPGe even if not explicitly stated. It is important not to compare energy figures that relate to unsimilar journeys. An airline jet cannot be used for an urban commute so when comparing aircraft with cars the car figures must take this into account.


  • There is currently no agreed upon method of comparing electric vehicle efficiency to heat engine (fossil fuel) vehicle efficiency. However, current typical emissions and thermal energy consumption can be compared. Vehicle speed is also an important parameter, and a peer-reviewed evaluation which convolves these criteria may be found at http://www.bentham-open.org/pages/content.php?TOEFJ/2008/00000001/00000001/11TOEFJ.PDF


  • If the issue is rapid investment in new electric mass transit it is important to use emissions associated with the most polluting fuel because increased demand for electricity increases the use of the most polluting fuel used in generation for the immediate future.


  • Systems that re-use vehicles like trains and buses can't be directly compared to vehicles that get parked at their destination. They use energy to return (less full) for more passengers and must sometimes run on schedules and routes with little patronage. These factors greatly affect overall system efficiencies. The energy costs of accumulating load need to be included. In the case of most mass transit distributing and accumulating load over many stops means that passenger kilometres are inherently a small proportion of vehicle kilometres see Transport Energy Metrics, Lessons from the west Coast Main line Modernisation and figures for London Underground in transport statistics for Great Britain 2003. Lessons from the west coast mainline modernisation suggest that long passenger rail should operate at less than 40% capacity utilisation and for London underground the figure is probably less than 15%.


  • Most cars run at less than full capacity, with the usual average load being between 1 and 2. Cars are also subject to inefficiencies because of congestion and the need to negotiate road junctions. The impact of transport road building to reduce congestion should always be considered as should the improving efficiency of cars see http://www.hm-treasury.gov.uk/media/9/5/pbr_csr07_king840.pdf,


  • Vehicles are not isolated systems. They usually form a part of larger systems whos design inherently determines energy consumption. Judging the value of transport systems by comparing the performance of their vehicles alone can be misleading. For instance, metro systems may have a poor energy efficiency per passenger kilometre, but their high throughput and low physical footprint makes the existence of high urban population densities viable. Total energy consumption per capita declines sharply as population density increases, since journeys become shorter.
  • See also Logistics and Transport Focus (the Journal of the Charter Institute of Transport)vol 9 number10 through volume 10 number 6 for a series of articles debating the general issues of fuel efficiency in transportation in the context of impact on climate change.


Footnotes

See also



External links




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