Energy efficiency in transportation
||This article may require cleanup to meet Wikipedia's quality standards. (March 2008)|
- This page describes energy efficiency in means of transportation. For the environmental impact assessment of a given product or service throughout its lifespan, see life cycle assessment.
- 1 Units of measurement
- 2 Transportation types
- 2.1 Walking
- 2.2 Velomobile
- 2.3 Bicycling
- 2.4 Human power
- 2.5 Automobiles
- 2.6 Aircraft
- 2.7 Ships
- 2.8 Trains
- 2.9 Buses
- 2.10 Rockets
- 2.11 Other
- 3 International transport comparisons
- 4 Caveats
- 5 Footnotes
- 6 See also
- 7 External links
Units of measurement
Efficiency can be expressed in terms of:
- distance per vehicle per unit fuel volume (e.g. km/L or miles per gallon (US or imperial))
- distance per vehicle per unit fuel mass (e.g. km/kg)
- volume of fuel (or total energy) consumed per unit distance per vehicle (e.g. L/100 km or kW·h/100 km),
- volume of fuel (or total energy) consumed per unit distance per passenger (e.g. L/(100 passenger·km))
- volume of fuel (or total energy) consumed per unit distance per unit mass of cargo transported (e.g. L/100 kg·km or MJ/t·km).
A 68 kg (150 lb) person walking at 4 km/h (2.5 mph) requires approximately 210 kilocalories (880 kJ) of food energy per hour, which is equivalent to 4.55 km/MJ. 1 US gallon (3.8 L) of gasoline contains about 114,000 British thermal units (120 MJ) of energy, so this is approximately equivalent to 360 miles per US gallon (0.65 L/100 km).
Velomobiles seem to have the highest energy efficiency in personal transportation. At a speed of 50 km/h (31 mph) the WAW manufacturer claims they need only 0.5 kW·h of food energy per 100 km to transport the passenger, which is around 1/5 (20%) of a normal bicycle, and 1/50 (2%) of an average fossil fuel or electric car. This corresponds to 4700 miles per US gallon (2000 km/L or 0.05 L/100 km). Other sources give a figure of 1/3.4 (29.5%) of the energy efficiency of a normal bicycle.
A relatively light and slow vehicle with low-friction tires and an efficient chain-driven drivetrain, the bicycle is one of the most energy-efficient forms of transport. A 64 kg (140 lb) cyclist riding at 16 km/h (10 mph) requires about half the energy per unit distance of walking: 43 kcal/mi, 27 kcal/km or 3.1 kW·h (11 MJ) per 100 km. This converts to about 732 mpg-US (0.321 L/100 km; 879 mpg-imp). This figure depends on the speed and mass of the rider: greater speeds give higher air drag and heavier riders consume more energy per unit distance.
A motorized bicycle such as the Velosolex allows human power and the assistance of a 49 cm3 (3.0 cu in) engine, giving a range of 160 to 200 mpg-US (1.5–1.2 L/100 km; 190–240 mpg-imp). Electric pedal-assisted bikes run on as little as 1.0 kW·h (3.6 MJ) per 100 km, while maintaining speeds in excess of 30 km/h (19 mph). These best-case figures rely on a human doing 70% of the work, with around 3.6 MJ (1.0 kW·h) per 100 km coming from the motor.
Including the human energy dramatically changes the efficiency of cycles quoted above. As with walking, this would include the increase in food consumption due to caloric efficiency of human muscle and cardiovascular efficiency. However, only the rise in food consumption above the diet of a non-cyclist should be considered.
To be thorough, a comparison must also consider the energy costs of producing, transporting and packaging of fuel (food or fossil fuel), the energy incurred in disposing of exhaust waste, and the energy costs of manufacturing the vehicle. This last can be significant given that walking requires little or no special equipment, while automobiles, for example, require a great deal of energy to produce and have relatively short lifespans. In addition, any comparison of electric vehicles and liquid-fuelled vehicles must include the fuel consumed in the power station to generate the electricity. In the UK for instance the efficiency of the electricity generation and distribution system is around 0.40.
Automobile fuel efficiency is most commonly expressed in terms of the volume of fuel consumed per one hundred kilometres (L/100 km), but in some countries (including the USA, UK and India) it is more commonly expressed in terms of the distance per volume fuel consumed (km/L or miles per US or imperial gallon). This is complicated by the different energy content of fuels such as petrol and diesel. The Oak Ridge National Laboratory (ORNL) states 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 energy. 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.
Finally, vehicle energy efficiency calculations would be misleading without factoring the energy cost of producing the vehicle itself. This initial energy cost can of course be depreciated over the life of the vehicle to calculate an average energy efficiency over its effective life span. In other words, vehicles that take a lot of energy to produce and are used for relatively short periods will require a great deal more energy over their effective lifespan than those that do not, and are therefore much less energy efficient than they may otherwise seem. Compare, for example, walking, which requires no special equipment at all, and an automobile, produced in and shipped from another country, and made from parts manufactured around the world from raw materials and minerals mined and processed elsewhere again, and used for a limited number of years.
Example consumption figures
- Solar cars use no fuel, charging the batteries entirely from built-in solar panels, and typically use less than 3 kW·h per 100 miles (67 kJ/km or 1.86 kW·h/100 km).
- The four passenger GEM NER uses 169 W·h/mile or 10.4 kW·h/100 km, which equates to 2.6 kW·h/100 km per person when fully occupied, albeit at only 24 mph (39 km/h).
- The General Motors EV1 was rated in a test with a charging efficiency of 373 Wh-AC/mile or 23 kWh/100 km (approximately equivalent to 2.6 L/100 km for petroleum-fuelled vehicles).
- Chevrolet Volt in full electric mode uses 36 kWh per 100 miles (810 kJ/km), meaning it may be more energy-efficient than walking for 4 or more passengers. Note that the energy at the power station is more than this by approximately 2.5 times.
- The Daihatsu Charade 993cc turbo diesel (1987–1993) won the most fuel efficient vehicle award for going round the United Kingdom consuming an average of 2.82 L/100 km (100 mpg-imp). It was surpassed only recently by the VW Lupo 3 L which consumes about 2.77 L/100 km (102 mpg-imp). Both cars are rare to find on the popular market. The Daihatsu had major problems with rust and structural safety which contributes to its rarity and the quite short production run.
- The Volkswagen Polo 1.4 TDI Bluemotion and the SEAT Ibiza 1.4 TDI Ecomotion, both rated at 3.8 L/100 km (74 mpg-imp; 62 mpg-US) (combined) were the most fuel efficient cars on sale in the UK as of 22 March 2008.[dated info]
- Honda Insight - achieves 48 mpg-US (4.9 L/100 km; 58 mpg-imp) under real-world conditions.
- Honda Civic Hybrid- regularly averages around 45 mpg-US (5.2 L/100 km; 54 mpg-imp).
- Toyota Prius - According to the US EPA's revised estimates, the combined fuel consumption for the 2008 Prius is 46 mpg-US (5.1 L/100 km; 55 mpg-imp), making it the most fuel efficient United States car of 2008. In the UK, the official fuel consumption figure (combined) for the Prius is 4.3 L/100 km (66 mpg-imp; 55 mpg-US).
- 2012 Cadillac CTS-V Wagon 6.2 L Supercharged, 14 mpg-US (17 L/100 km; 17 mpg-imp).
- 2012 Bugatti Veyron, 10 mpg-US (24 L/100 km; 12 mpg-imp).
A principal determinant of energy consumption in aircraft is drag, which must be opposed by thrust for the aircraft to progress.
- Drag is proportional to the lift required for flight, which is equal to the weight of the aircraft. As induced drag increases with weight, mass reduction, with improvements in engine efficiency and reductions in aerodynamic drag, has been a principal source of efficiency gains in aircraft, with a rule-of-thumb being that a 1% weight reduction corresponds to around a 0.75% reduction in fuel consumption.
- Flight altitude affects engine efficiency. Jet-engine efficiency increases at altitude up to the tropopause, the temperature minimum of the atmosphere; at lower temperatures, the Carnot efficiency is higher. Jet engine efficiency is also increased at high speeds, but above about Mach 0.85 the airframe aerodynamic losses increase faster.
- Compressibility effects: beginning at transonic speeds of around Mach 0.85, shockwaves form increasing drag.
- For supersonic flight, it is difficult to achieve a lift to drag ratio greater than 5, and fuel consumption is increased in proportion.
|passenger miles/imperial gallon||17||109|
|passenger miles/US gallon||14||91|
|litres/100 passenger km||16.6||3.1|
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 advanced piston engine airliners of the 1950s, current jet airliners are only marginally more efficient per passenger-mile. Between 1971 and 1998 the fleet-average annual improvement per available seat-kilometre was estimated at 2.4%. Concorde the supersonic transport managed about 17 passenger-miles to the Imperial gallon; similar to a business jet, but much worse than a subsonic turbofan aircraft. Airbus states a fuel rate consumption of their A380 at less than 3 l/100 km per passenger (78 passenger-miles per US gallon).
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 fuel efficiency. The mass of an aircraft can be reduced by using light-weight materials such as titanium, carbon fiber and other composite plastics. Expensive materials may be used, if the reduction of mass justifies the price of materials through improved fuel efficiency. The improvements achieved in fuel efficiency by mass reduction, reduces the amount of fuel that needs to be carried. This further reduces the mass of the aircraft and therefore enables further gains in fuel efficiency. For example, the Airbus A380 design includes multiple light-weight materials.
Airbus has showcased wingtip devices (sharklets or winglets) that can achieve 3.5 percent reduction in fuel consumption. There are wingtip devices on the Airbus A380. Further developed Minix winglets have been said to offer 6 percent reduction in fuel consumption. Winglets at the tip of an aircraft wing, can be retrofitted to any airplane, and smooths out the wing-tip vortex, reducing the aircraft's wing drag.
NASA and Boeing are conducting tests on a 500 lb (230 kg) "blended wing" aircraft. This design allows for greater fuel efficiency since the whole craft produces lift, not just the wings. The blended wing body (BWB) concept offers advantages in structural, aerodynamic and operating efficiencies over today's more conventional fuselage-and-wing designs. These features translate into greater range, fuel economy, reliability and life cycle savings, as well as lower manufacturing costs. NASA has created a cruise efficient STOL (CESTOL) concept.
Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) have researched a shark skin imitating paint that would reduce drag through a riblet effect. Aircraft are a major potential application for new technologies such as aluminium metal foam and nanotechnology such as the shark skin imitating paint.
Propfan propulsors are a more fuel efficient technology than jets or turboprops, but turboprops have an optimum speed below about 450 mph (700 km/h). This speed is less than used with jets by major airlines today. However, the decrease in speed reduces drag. With the current [dated info] high price for jet fuel and the emphasis on engine/airframe efficiency to reduce emissions, there is renewed interest in the propfan concept for jetliners that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans. NASA has conducted an Advanced Turboprop Project (ATP), where they researched a variable pitch propfan that produced less noise and achieved high speeds.
Related to fuel efficiency is the impact of aviation emissions on climate.
- Motor-gliders made of composite materials can reach an extremely low fuel consumption for cross-country flights, using thermal air currents and winds.
- At 160 km/h, a diesel powered two-seater Dieselis burns 6 liters of fuel per hour, 1.9 liters per 100 passenger km.
- at 220 km/h, a four-seater 100 hp MCR-4S burns 20 liters of gas per hour, 2.2 liters per 100 passenger km.
- Under continuous motorised flight at 225 km/h, a Pipistrel Sinus burns 11 liters of fuel per flight hour. Carrying 2 people aboard, it operates at 2.4 liters per 100 passenger km.
- Ultralight aircraft Tecnam P92 Echo Classic at cruise speed of 185 km/h burns 17 liters of fuel per flight hour, 4.6 liters per 100 passenger km (2 people). Other modern ultralight aircraft have increased efficiency; Tecnam P2002 Sierra RG at cruise speed of 237 km/h burns 17 liters of fuel per flight hour, 3.6 liters per 100 passenger km (2 people).
- Two-seater and four-seater flying at 250 km/h with old generation engines can burn 25 to 40 liters per flight hour, 3 to 5 liters per 100 passenger km.
- The Sikorsky S-76C++ twin turbine helicopter gets about 1.65 mpg-US (143 L/100 km; 1.98 mpg-imp) at 140 knots (260 km/h; 160 mph) and carries 12 for about 19.8 passenger-miles per gallon (11.9 litres per 100 passenger km).
Cunard states that 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 efficiency of 16.7 passenger miles per imperial gallon (16.9 l/100 p·km or 13.9 p·mpg–US).
Emma Maersk uses a Wärtsilä-Sulzer RTA96-C, which consumes 163 g/kW·h and 13,000 kg/h. If it carries 13,000 containers then 1 kg fuel transports one container for one hour over a distance of 45 km. The ship takes 18 days from Tanjung (Singapore) to Rotterdam (Netherlands), 11 from Tanjung to Suez, and 7 from Suez to Rotterdam, which is roughly 430 hours, and has 80 MW, +30 MW. 18 days at a mean speed of 25 knots (46 km/h) gives a total distance of 10,800 nautical miles (20,000 km).
A sailboat, much like a solar car, can locomote without consuming any fuel. However, often energy will be needed for demands other than propulsion, such as cooking, heating or lighting. The fuel efficiency of a single-occupancy boat is highly dependent on the size of its engine, the speed at which it travels, and its displacement. Due to the high viscosity of water, with a single passenger, the equivalent energy efficiency will be lower than in a car, train, or plane.
Trains can be an efficient means of transport for freight and passengers. Efficiency varies significantly with passenger loads, and losses incurred in electricity generation and supply (for electrified systems), and, importantly, end-to-end delivery, where stations are not the originating final destinations of a journey.
Actual consumption depends on gradients, maximum speeds, loading and stopping patterns. Data produced for the European MEET project (Methodologies for Estimating Air Pollutant Emissions) illustrate the different consumption patterns over several track sections. The results show the consumption for a German ICE high-speed train varied from around 19 to 33 kW·h/km (68–119 MJ/km; 31–53 kW·h/mi). The data also reflects the weight of the train per passenger. For example, TGV double-deck Duplex trains use lightweight materials, which keep axle loads down and reduce damage to track and also save energy.
Energy consumption estimates for rail freight vary widely, and many are provided by interested parties. Some are tabulated below.
|Source||Country||Year||Fuel Economy (weight of goods)||Energy Intensity||ref|
|Association of American Railroads||USA||2007||185.363 km/L (1 short ton)||energy/mass-distance|||
|Network Rail||UK||—||87 t·km/L||0.41 MJ/t·km (LHV)|||
|Source||country||year||Train Efficiency||Per passenger-km (in kJ)||reference(s)|
|East Japan Railway Company||Japan||2004||20.6 MJ (5.7 kWh)/car-km||350 kJ/passenger-km|||
|EC||EC||1997||18 kW·h/km (65 MJ/km)||TGV Duplex assuming 3 intermediate stops between Paris and Lyon.|
|Colorado Rail||USA||year||1.125 mpg-US (209.1 L/100 km; 1.351 mpg-imp)||468 passenger-miles/US gallon (0.503 L/100 passenger-km)||Colorado Railcar double-deck DMU hauling two Bombardier Bi-level coaches|
|SBB-CFF-FFS||Switzerland||2011||2300 GWhr/yr||470 kJ/passenger-km|||
|Siemens||Basel, Switzerland||year||1.53 kWh/vehicle-km (5.51 MJ/vehicle-km)||85 kJ/passenger-km (150 kJ/passenger-km at 80% average load)|||
|Amtrak||USA||2009||2,435 BTU/mi (1.60 MJ/km)|||
|Comboios de Portugal||Portugal||2011||8.5 kW·h/km (31 MJ/km; 13.7 kW·h/mi)||77 kJ/passenger-km|||
Considering only the energy spent to move the train, and taking as example the urban area of Lisbon, train seems to be on average 20 times more efficient than automobile for transportation of passengers, if we consider energy spent per passenger-km. Considering an automobile which has a consumptions of around 6 l/100 km (47 mpg-imp; 39 mpg-US) of gasoline, the fact the on average cars in Europe have an occupation ratio of around 1.2 passengers per automobile and that one litre of gasoline amounts for about 8826 Wh, one gets on average 441 Wh (1,590 kJ) per passenger-km. On the other hand, a modern urban train with an average occupation of 20% of total capacity, which has a consumption of about 8.5 kW·h/km (31 MJ/km; 13.7 kW·h/mi), one gets 21.5 Wh per passenger-km, 20 times less than the automobile.
Stopping is a considerable source of inefficiency. Modern electric trains like the Shinkansen (the Bullet Train) use regenerative braking to return current into the catenary while they brake. A Siemens study indicated that regenerative braking might recover 41.6% of the total energy consumed. 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 head-end 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. Weight is a determinant of braking losses.
- In July 2005, the average occupancy for buses in the UK was stated to be 9.
- The fleet of 244 40-foot (12 m) 1982 New Flyer trolley buses in local service with BC Transit in Vancouver, Canada, in 1994/95 used 35,454,170 kWh for 12,966,285 vehicle km, or 9.84 MJ/vehicle km. Exact ridership on trolleybuses is not known, but with all 34 seats filled this equates to 0.32 MJ/passenger km. It is quite common to see people standing on Vancouver trolleybuses. This is a service with many stops per kilometre; part of the reason for the efficiency is the use of regenerative braking.
- A commuter service in Santa Barbara, California, USA, found average diesel bus efficiency of 6.0 mpg-US (39 L/100 km; 7.2 mpg-imp) (using MCI 102DL3 buses). With all 55 seats filled this equates to 330 passenger mpg; with 70% filled, 231 passenger mpg. At an average load of 9 people the efficiency would be[weasel words] 54 passenger mpg and could be[weasel words] half of this figure when many[weasel words] stops are made in urban[weasel words] routes.
- In 2011 the fleet of 752 buses in the city of Lisbon had an average speed of 14.4 km/h and an average occupancy of 20.1 passengers per vehicle.
||This section possibly contains original research. (May 2015)|
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.
NASA's space shuttle fires its engines for around 8.5 minutes, consuming 1,000 tons of solid propellant (containing 16% aluminium) and an additional 2,000,000 litres of liquid propellant (106,261 kg of liquid hydrogen) 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 a specific energy of 31MJ/kg for aluminum and 143 MJ/kg for liquid hydrogen, the vehicle consumes around 5 TJ of solid propellant and 15 TJ of hydrogen fuel.
- NASA'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, 150 US gallons per mile (350 l/km; 120 imp gal/mi).
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.
US Passenger transportation
The US Transportation Energy Data Book states the following figures for passenger transportation in 2009:
|Transport mode||Average passengers
|BTU per passenger-mile||MJ per passenger-kilometre|
|Rail (Intercity Amtrak)||20.9||2,435||1.596|
|Rail (Transit Light & Heavy)||24.5||2,516||1.649|
US Freight transportation
|Transportation mode||Fuel consumption|
|BTU per short ton mile||kJ per tonne kilometre|
|Class 1 Railroads||289||209|
|Air freight (approx)||9,600||6,900|
From 1960 to 2010 the efficiency of air freight has increased 75%, mostly due to more efficient jet engines.
1 US gal (3.785 l, 0.833 imp gal) of fuel can move a ton of cargo 857 kilometres (533 mi) by barge, or 337 km (209 mi) by rail, or 98 km (61 mi) by truck.
- Space Shuttle used to transport freight to the other side of the Earth (see above): 40 MJ per tonne kilometre.
- Net energy for lifting: 10 MJ per tonne kilometre.
Natural Resources Canada's Office of Energy Efficiency publishes annual statistics regarding the efficiency of the entire Canadian fleet. For researchers, these fuel consumption estimates are more realistic than the fuel consumption ratings of new vehicles, as they represent the real world driving conditions, including extreme weather and traffic. The annual report is called Energy Efficiency Trends Analysis. There are dozens of tables illustrating trends in energy consumption expressed in energy per passenger km (passengers) or energy per tonne km (freight).
|This section does not cite any references or sources. (March 2008)|
Comparing fuel efficiency in transportation has several challenges:
- All transportation methods require a different infrastructure. The building, repairing and renovation of this must also be considered, and heavy usage of infrastructure will require earlier maintenance, which will also be energy consuming. Road transport requires an extensive network of well-maintained paved roads and trains require railroads and stations along the whole length of the track. Meanwhile, ships and aircraft only require a more limited amount of harbours or airports, without any en route infrastructure except for the occasional navigation beacon.
- Some types of transportation can take shorter routes, thereby (partly) offsetting an increase in energy consumption per unit of distance travelled. For example, a trip from the UK to Italy would take a boat an enormous detour via the Strait of Gibraltar, while cars need to travel much less distance, a train can cross through the channel tunnel, and aircraft can (theoretically; in reality it's going from good to better) take by the shortest route or, in case of favourable winds, a more efficient route.
- 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 dissimilar 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 method of comparing electric vehicle efficiency to heat engine (fossil fuel) vehicle efficiency, but 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.org/open/toefj/articles/V001/11TOEFJ.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 polluting fuel used in generation for the immediate future, as well as low emissions fuels in the case of some countries.
- 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,[dead link]
- Vehicles are not isolated systems. They usually form a part of larger systems whose 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 number 10 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.
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- Defra 2008 Guidelines to Defra’s GHG Conversion Factors
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- Energy return wheel (ERW)
- Fuel economy in automobiles
- Fuel efficiency
- Gasoline gallon equivalent
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- ECCM Study for rail, road and air journeys between main UK cities 
- Flight Emission Calculator
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