Opportunities for increased energy efficiency from transport technologies are generally based on either improving the physical structure of a vehicle or its mechanical design. 

Improve the physical structure

Better fuel efficiency can be achieved by using technologies for reducing rolling and air resistance. Many technologies exist to improve aerodynamics and wheel/tyre performance. Lightweighting materials save energy—first, by reducing rolling resistance, and second, by enabling the use of more fuel-efficient alternative drivetrains as less power is needed for lighter vehicles.

Reduce rolling-resistance

Tyre rolling-resistance accounts for nearly 13% of energy use by combination trucks. Generally, combination trucks have two sets of wheels and tyres on the drive and trailer axles and do not use tyres that have low rolling-resistance. This presents an opportunity for reducing fuel consumption by switching to low rolling-resistance single-wide tyres. A single-wide tyre and wheel weighs less than two standard tyres and wheels, thereby reducing the overall weight of the vehicle when fitted.

Innovations in the design of tyre treads, rubber compounds and the construction of casings have improved the quality of low rolling-resistance tyres. Trials have indicated a range of fuel savings of 4–13% for heavy vehicles. Using such tyres has the potential in a combination long-haul truck to save more than 2,200 litres of fuel per year.

Use effective wheel/rail lubrication

Trains depend on friction to prevent their wheels from slipping or derailing, especially along curving tracks. Wheel friction contributes consumes a significant proportion of the energy used in rail transport. Using lubricants to reduce friction levels lowers energy use, wear and tear and excessive noise; but it must be done in a way that does not compromise wheel-to-rail contact.

Lubricants can be oil, grease or water. They can be applied automatically from systems installed on the side of the track (wayside) or from onboard lubricating systems.

Wayside grease is the most commonly used type of wheel/rail lubrication; however, onboard lubricating systems are also starting to be used in Australia and around the world.

Use lightweighting materials

The use of lower-density, higher-integrity metals such as aluminium and composites, has enabled the development of more lightweight land, sea and air transportation. Lightweight materials in vehicle design complement the use of alternative drivetrain engine technology.

Lightweighting the mass of vehicles reduces the power needed to accelerate and decelerate, lowers rolling resistance and lowers the power needed for a vehicle to travel at its required speed.

Lightweighting the body and parts of vehicles enables the size of the engine to be reduced accordingly, which leads to further energy efficiency benefits from weight savings. This is especially true for air transportation vehicles where significant fuel savings can be made.

The energy-saving potential from lightweighting for the following types of vehicles is outlined below:

  • cars
  • articulated vehicles
  • trains (passenger carriage)
  • trains (freight)
  • aeroplanes
  • high-speed ferries 

Lightweighting cars

For cars, an estimated 0.15 to 1 L of fuel per 100 km can be saved from a 100 kg weight reduction. Lightweighting the body of the vehicle and its parts allows for the redesign and use of more fuel-efficient alternative drivetrain engines, which further improves fuel efficiency. Combining lightweighting with improved engine design, enables still larger fuel efficiency savings.

Lightweighting articulated vehicles

There is good potential for lightweighting articulated vehicles, such as trucks. Replacement of steel components with aluminium, metal alloys, metal matrix composites and other lightweight parts can lead to improvements in fuel efficiency

The estimates for fuel savings from such initiatives are about 0.03 L per 100 km for a 100 kg weight reduction on a flat highway. Savings can be over 0.1 L per 100 km for a 100 kg weight reduction in urban traffic due to more frequent and rapid acceleration of the vehicle. In addition, compared to a level road, fuel savings are about five times greater on roads with a 2% gradient and almost 10 times higher on roads with a 4% gradient.

Lightweighting trains (passenger carriage)

For subways or other urban train systems, estimated energy savings from weight reduction range from 6.6% to 8.6% per 10% weight reduction. For high-speed passenger trains, estimates for energy savings are about 3.2% per 10% weight reduction, due to their high and steady operating speeds.

Lightweighting trains (freight)

Steel has traditionally been used to make freight carriages for rail freight transport. There is potential to build cars using lighter components such as aluminium, composites or plastics. This would then enable locomotives to pull greater freight loads without exceeding load limits and also to save fuel on empty return runs.

Lightweighting aeroplanes

Lightweighting aeroplanes used on long-haul routes can achieve good return on investment due to the extended flight hours and greater fuel usage involved. A 100 kg weight reduction of an aeroplane can yield fuel efficiency savings of over 100 times higher than for rail and automobiles. Over a lifetime of usage, a 100 kg weight reduction of short-distance aeroplanes will result in energy savings of 10 to 20 TJ, and in the range of between 20 and 30 TJ for long-haul aeroplanes.

Lightweighting high-speed ferries

Significant energy is used by ferries to push against the drag caused by water. This drag increases exponentially against the speed of the ferry. Weight reduction in the manufacture of high-speed ferries has enabled greater fuel efficiency by reducing the submerged surface area of the ferry. Over the lifetime of a ferry, lightweighting results in energy savings in the range of 1,400 GJ for a 100 kg weight reduction.

Improve aerodynamics

Aerodynamic drag is created as air resists the movement of a vehicle. The greater the drag, the harder the vehicle engine has to work; so more fuel is consumed. At high speeds, aerodynamic drag can be the highest energy drain on a heavy vehicle, so any improvement in aerodynamic efficiency can lead to fuel savings.

Aerodynamic drag increases exponentially with vehicular speed. In all vehicles, the relationship between the power required to overcome drag is proportional to speed cubed. Therefore, aerodynamic drag has a significant effect on fuel efficiency for long-haul vehicles (trucks, trains and aircraft) which regularly travel at high speeds.

Wind tunnel experiments have led to significant improvements in vehicle aerodynamics over the last few decades. There is potential for further drag reduction through continued refinement of aerodynamic design and materials.

Aerodynamic articulated vehicles

Linfox investigated the benefits of aerodynamic truck and trailer technology. They found that implementing aerodynamic technology in vehicles can reduce fuel consumption by 15%.<

Modern trucks are generally designed to incorporate improved aerodynamics. Vehicles can also be retrofitted with additional aerodynamic equipment. Such modifications include:

  • full roof deflector (5–10% reduction)
  • chassis fairing (1–3% reduction)
  • sloped hood (2% reduction)
  • round corners and aero bumper (2% reduction)
  • air dam, flush headlights (0.5% reduction)
  • slanted windshield, curved windshield and side extenders (1–7% reduction)
  • skirts and under-hood air cleaners (1–4% reduction)
  • concealed exhaust system, recessed door hinges, grab handles, aerodynamic mirrors (1–2% reduction)
  • truck vision systems to replace mirrors (currently are in development) (3–4% reduction).

Further efficiencies can be achieved by better integrating the truck with the trailer.

Aerodynamic trains (passenger)

In passenger trains, assuming the rolling bearings are well designed, rolling friction increases less rapidly with speed than air drag. In most cases, rolling friction of rail passenger vehicles begins to be exceeded by air drag at speeds from 60 to 100 km/h. At speeds above 150 km/h, overcoming air drag is the dominant energy requirement.

Streamlining the already narrow hulls of trains can have a significant impact on reducing air drag.

Aerodynamic trains (freight)

Air resistance accounts for a significant proportion of the energy expenditure of freight trains. Measures to reduce this include:

  • streamlining of train sides and underfloor areas
  • ordering of freight cars to optimise the aerodynamic profile
  • minimising gaps between cars or using air bags to fill gaps
  • covering open top cars or hoppers
  • applying streamlined bogie covers.

Digital simulation is available to assist with determining and/or designing the frequency and size of gaps between wagons, and providing a rating of the overall aerodynamic profile. Simulation tools can also determine the optimum order of containers along the train’s length.

Aerodynamic improvements are greater for intermodal container trains than for unit trains, because aerodynamic drag can be as much as 25% higher.

Optimisation of wagon assignment using digital simulation can deliver reasonable savings in specific cases. Bogie and wheel covers have also been identified as a significant source of aerodynamic drag.

Aerodynamic aeroplanes

The two main sources of aerodynamic drag for aircraft are skin-friction drag and lift-induced drag. These constitute approximately one-half and one-third of the total drag, respectively, for a typical long-range flight at cruise conditions.

Riblets, large eddy break-up devices, hybrid laminar flow technology and innovative wing-tip devices offer the greatest potential for reducing drag. Aircraft aerodynamic performance improvement can also be obtained through trailing edge optimisation, control of the shock boundary layer interaction and control of boundary layer separation.

Improve mechanical design

Energy loss from engines can account for up to 70% of fuel consumed, in the form of waste heat that passes through the exhaust and the cooling systems.

Alternative drivetrain engine technologies can reduce these losses and significantly improve overall energy efficiency. This includes:

  • electric and plug-in drivetrains
  • hybrid and plug-in hybrid drivetrains
  • mechanical electric drive trains.

Reducing energy losses when idling is also beneficial.

Use idle-reduction technologies

An idle-reduction device is generally installed on a vehicle or at a service location. Idle reduction technologies operate on heat, air conditioning and electricity systems.

Use alternative drivetrain technologies

Alternative drivetrains involve a variety of power sources in combination with (or replacement of) internal combustion to provide power to a vehicle. Developments in alternative drivetrains for vehicles have potential to reduce reliance on conventional fuels.

Three key types of drivetrain technologies are:

  • electric and plug-in electric drivetrains 
  • hybrid and plug-in hybrid electric drivetrains
  • mechanical hybrid electric drivetrains
  • Electric and plug-in electric drivetrains

Electric vehicles operating today are either powered by off-board electricity delivered through a conductive contact—usually trams with overhead wires or trains with electrified ‘third rails’—or by electricity from the grid channelled to onboard batteries through plug-in electric drivetrain systems.

An electric drivetrain uses electric motors and controllers instead of an internal combustion engine, with energy stored onboard in suitable electrochemical systems, typically battery packs. The batteries can be charged from a standard electricity outlet. In the future, they may be swapped at specialised exchange stations. Some of the energy normally lost as heat during braking can also be recovered by using the motor as a generator. Fully electric drivetrains are considered most suitable for urban applications as regenerative braking can be maximised in stop-start driving.

The power and torque requirements of heavy trucks suggest that the technology is most appropriate for light commercial vehicles in an urban setting, although some trials have shown electric freight vehicles up to 10 tonnes can be successful. Despite a high capital cost, fully electric vehicles can achieve major fuel savings. For example, Adelaide City Council found a 50% decrease in fuel costs associated with the Tindo electric bus compared with a diesel bus.

Hybrid and plug-in hybrid electric drivetrains

The primary benefits of using hybrids are lower fuel consumption and reduced emissions. A hybrid drivetrain employs a combination of two or more power sources in the same vehicle. The most commonly used arrangement has an electric motor coupled with an internal combustion engine. Most hybrid systems also incorporate a regenerative braking feature which captures some of the energy normally wasted during braking. This is well suited to urban transport conditions where frequent stop-starts maximise the benefits.

Hybrid engines save energy in at least six ways, by:

  • shutting down when the vehicle is stopped
  • using the motor for braking and harnessing the electricity generated to recharge the battery
  • using the motor to boost power, allowing engine downsizing and improving engine efficiency
  • switching to the electric motor at low load, eliminating engine operation during its lowest efficiency mode
  • allowing a more efficient cycle than the standard Otto cycle (in some hybrids)
  • shifting power-steering and other accessories to more efficient electric operation.

Isuzu reports that in hybrid engine trials, fuel consumption savings of over 20% were achieved on inner city runs, and almost 10% on a mixed urban run. TNT has reported real-world fuel and emissions reductions of 14% in its fleet. Hybrid technologies are also used in two- and three-wheel vehicles, which operate in urban regions where stop-start driving is frequent. Honda developed a 50cc hybrid scooter prototype, which, compared to similar scooters, uses 30% less fuel with a 30% reduction in GHG.

Even higher levels of GHG emission reductions are possible through the use of plug-in hybrid vehicle technologies which enable road-based transport fleets to be powered by renewable energy. Plug-in Electric Hybrid Vehicles (PHEVs) are a combination of hybrid electric and battery electric. PHEVs get some of their power from the electricity grid. Plug-in hybrid technology will be useful in the long-term for a range of light-to-medium duty vehicles, including urban buses and delivery vans.

Mechanical electric drivetrains

A mechanical hybrid drivetrain is similar to the more common hybrid electric system in that it can recover braking energy, provide supplementary power for better acceleration and reduce fuel consumption. In a mechanical hybrid system, hydraulic or pneumatic accumulators are used to store energy rather than the batteries typical of electrical hybrids. Alternatively, clutches can be used to transfer energy to a flywheel which can store energy in a rotating mass.

The main determinant of suitability within the Australian market appears to be whether sufficient energy can be recovered from braking to offset the disadvantage of carrying the system’s additional weight. The most suitable applications are for vehicles that involve frequent stop-start driving, such as delivery vans, buses and waste-disposal trucks operating in urban vicinities.

Fuel and emissions reductions from implemented mechanical hybrid systems in light commercial vehicles are in the range of 25–70%. For example, in 2006, United Postal Service began a trial with the US Environmental Protection Agency using a hydraulic-hybrid delivery truck. The trial achieved fuel savings of up to 70%. Capital costs have been cited as being approximately 15% higher than for a conventional vehicle, with a payback period of 3–4 years.

Maintenance and service requirements and the service life of the component systems also need to be taken into consideration when evaluating whole-of-life costs.

Use fuel cells

Fuel cell electric vehicles generate electricity onboard from a hydrogen source using a fuel cell rather than batteries. The hydrogen source can be either hydrogen gas or a hydrogen carrier, such as methanol or natural gas. In the latter case, the vehicle uses a reformer to release the hydrogen from the carrier. The fuel cell then chemically combines hydrogen and oxygen to produce water and generate electricity, with the excess electricity being stored in a battery.

The vehicle’s maximum power requirement could be met with a large fuel cell. However, a cheaper option is the combination of a small fuel cell that meets the vehicle’s cruising power requirement and a high-power device (such as a high-power-density battery, ultracapacitor, or flywheel) to provide a boost for acceleration.

Light-duty vehicles could use two types of fuel cells—proton-exchange membrane (PEM), and solid-oxide. PEM fuel cells are closest to commercialisation, but their current manufacturing costs are too high to be commercially viable. Although hydrogen fuel cells do not produce carbon dioxide, the process of reforming fossil fuels does produce carbon dioxide. In addition, current onboard hydrogen storage technologies are either too expensive and/or too heavy and large.

Few cities have a suitable hydrogen distribution network. The costs of developing such a network are considerable, and there is the potential for losses and leaks during refuelling.

Buses may be the easiest transport market for fuel cells to enter because refuelling is concentrated at bus depots. Projects in many countries have demonstrated the use of fuel cell buses. Fuel cell buses are more competitive with an additional cost of about US$100,000 over the conventional buses.