Fuel efficiency in transportation

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 some hundred kilojoule per kilometre for a bicycle to several megajoule 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.

Transportation modes

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

Walking

• A 140 lb (64 kg) person walking at 3 mi/h (~5 km/h) requires approximately 80 kcal (330 kJ) of food energy per mile (~205 kJ/km).
• Given that 1 gallon (~3.7854 liter) of gasoline contains about 114,000 BTU (120 MJ) of energy, this converts to roughly 360 MPG (0.65 l/100 km).

Bicycling

As a relatively light and slow vehicle, with low-friction tires, and an efficient chain-driven drivetrain, the bicycle can be an efficient form of transport. A 140 lb (64 kg) cyclist riding at 16 km/h requires about half the energy per unit distance of walking: 43kcal/mi or 3.1 kW·h/100 km (0.11 MJ/km; 0.050 kW·h/mi). This figure depends on the speed and mass of the rider: greater speeds give higher air drag and heavier riders also consume more energy per unit distance. This converts to about 732 MPG.

A motorized bicycle such as the Velosolex affords the rider to cycle under human power or with the assistance of a 49 cm³ (3.0 cu in) engine which equates to a range of 160–200 mpg_-US (1.5–1.2 L/100 km; 190–240 mpg_-imp).^{[citation needed]} Electric pedal assisted bikes run on as little as 1.0 kilowatt-hour per 100 kilometres (0.036 MJ/km; 0.016 kW·h/mi),^{[citation needed]} while maintaining speeds in excess of 30 km/h (19 mph).^{[citation needed]} These best-case figures rely on a human doing 70% of the work, with around 3.6 MJ/100 km (55 BTU/mi) coming from the engine.

Human power

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 cardio vascular 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 life-spans. Also 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.

Automobiles

Main article: Fuel economy in automobiles

See also: Fuel economy-maximizing behaviors

Automobile fuel efficiency is often expressed in volume fuel consumed per one hundred kilometres (i.e., L/100 km) 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.

Finally, vehicle energy efficiency calculations would be misleading without factoring the energy or "fuel" cost of producing the vehicle itself. This initial energy cost can of course be "amortized" 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 or fuel 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

• Chevrolet Volt in full electric mode uses 36 kWh per 100 miles (810 kJ/km), meaning it may be more fuel-efficient than walking for 4 or more passenges. Note that the fuel 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 UK doing an average 100MPG. It was only surpassed recently by the VW Lupo 3L which gives about 102MPG. Both cars are surprisingly rare to find on the popular market.
• 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) are the most fuel efficient cars on sale in the UK as of 22 March 2008.

• 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 US car of 2008 according to the EPA. In the UK, the official fuel consumption figure (combined) for the Prius is 4.3 L/100 km (66 mpg_-imp; 55 mpg_-US).
• The General Motors EV1 was rated in a test with a charging efficiency of 373 Wh-AC/mile or 23 kWh/100 km (translates approximately to 2.6L/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 24 mph (39 km/h). All these electric vehicle figures are overoptimistic though, taking into account the coal, oil or natural gas consumed in the power station due to modern high density fossil fuel electricity generating plants 67% thermal efficiency of converting fuel into electricity at best.

Aircraft

A principal determinant of fuel 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. However, 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 five and fuel consumption is increased in proportion.

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 .75% reduction in fuel consumption. Flight altitude affects both parasitic drag and 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.

Concorde fuel efficiency comparison

Aircraft	Concorde	Boeing 747-400
passenger miles/imperial gallon	17	109
passenger miles/US gallon	14	91
litres/passenger 100 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.^{[citation needed]} 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-kilometre was 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 states a fuel rate consumption of their A380 at less than 3 L/100 km per passenger (78 passenger-miles per US gallon).

• 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 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-kilometres).^{[citation needed]}

• 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.^{[citation needed]}

• Concorde the supersonic transport managed about 17 miles to the gallon per passenger; similar to a business jet, but much worse than a subsonic turbofan aircraft.

Related to fuel efficiency is the impact of aviation emissions on climate.

Small Aircraft

Small aircraft equipped with atmospheric piston engines can't be compared to jet air passenger aircraft. They're low power, low altitude, low speed, low capacity aerial vehicles. Motor-gliders made of composite materials can reach an extremely low fuel consumption for cross-country flights, using thermal air currents and winds. 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 0.6 Gal/100 p.km (2.4 l/100p.km). Old generation World War II aircraft can be up to 50 l/100p.km.^{[citation needed]}

Ships

• Cunard states 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 efficiency of 16.7 passenger-miles per imperial gallon (16.9 L/100 p·km or 13.9 p·mpg_–US).

Trains

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 19–33 kW·h/km (68–120 MJ/km; 31–53 kW·h/mi). The data also reflects the weight of the train per passenger. For example, the TGV double-deck ‘Duplex’ trains use lightweight materials which keep axle loads down and reduce damage to track but also save energy.

Freight

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 (short ton)	energy/mass-distance
Network Rail	UK	Year	87 km/l (tonne)	energy/mass-distance

Passenger

Source	country	year	Train Efficiency	Per passenger km	ref
East Japan Railway Company	Japan	2004	20.6 MJ/car-km	0.35 MJ/passenger-km
EC	EC	1997	18.00 kWh/train-km	64.80 MJ/train-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	year	0.082 kWh/passenger-km for traction	need coversion	no ref
Siemens	Basel, Switzerland	year	1.53 kWh/vehicle-km (5.51 MJ/vehicle-km)	0.085 MJ/passenger-km (0.15 MJ/passenger-km at 80% average load
Amtrak	USA	2005		2,935 BTU/passenger-mile (1.9 MJ/passenger-km)

Braking losses

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 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. Weight is a determinant of braking losses.

Other references

AEA study of road and rail for the United Kingdom Department for Transport: Final report

Buses

• 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 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, 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 the efficiency would be 231 passenger-mpg. At the typical average passenger load of 9 people, the efficiency is only 54 passenger-mpg and could be half of this figure when many stops are made in urban routes.

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, 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 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 a specific energy 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 energy input 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.^{[citation needed]}

If the Space Shuttle were used to transport people or freight from a point to another on the Earth, using the theoretical largest ground distance (antipodal) flight of 20,000 km, energy usage would be about 40 kJ/km/kg of payload.

Other

• 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.

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

Defra 2008 Guidelines to Defra’s GHG Conversion Factors http://www.aef.org.uk/downloads//Grams_CO2_transportmodesUK.pdf

US Passenger transportation

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

Transport mode	Average passengers per vehicle	BTU per passenger-mile	MJ per passenger-kilometre
Vanpool	6.1	1,322	0.867
Efficient Hybrid	1.57	1,659	1.088
Motorcycles	1.2	1,855	1.216
Rail (Intercity Amtrak)	20.5	2,650	1.737
Rail (Transit Light & Heavy)	22.5	2,784	1.825
Rail (Commuter)	31.3	2,996	1.964
Air	96.2	3,261	2.138
Cars	1.59	3,512	2.302
Personal Trucks	1.72	3,944	2.586
Buses (Transit)	8.8	4,235	2.776

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	246
Domestic Waterborne	510	370
Heavy Trucks	3,357	2,426
Air freight (approx)	9,600	6,900

Compare:

• Space Shuttle used to transport freight to the other side of the Earth (see above): 40,000 kJ per tonne kilometre.
• Net energy for lifting: 10,000 kJ per tonne kilometre.

Canadian transportation

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). Here is the link to the 2009 data: [2]

Caveats

Comparing fuel efficiency in transportation is like comparing apples and oranges. 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.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,

• 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 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.

See also

• ACEA agreement
• Alternative propulsion
• Corporate Average Fuel Economy (CAFE)
• Carbon dioxide equivalent and emission standard
• Fuel economy in automobiles
• Fuel efficiency
• Gasoline gallon equivalent
• Gas-guzzler
• Low-energy vehicle

External links

• ECCM Study for rail, road and air journeys between main UK cities [3]
• Flight Emission Calculator
• Transport Energy Consumption Discussion Paper 2004 - Prof. Roger Kemp
• Traction Summary Report 2007- Prof. Roger Kemp
• Transportation Energy Data Book (US)
• Fuel Consumption Ratings