ParisTech Review

Automobile manufacturers became aware of possible gains in vehicle aerodynamics a long time ago: the German company Rumpler and the American Chrysler Airflow drew their initial inspiration from aircraft wing profiles and, consequently, the automobile sector began benefitting from progress obtained in aeronautics. But the main challenge lay in achieving higher speeds. It was only after the oil crises that energy efficiency began to be taken into account, more so in Europe than in the US and far more as far as private cars are concerned (compared with trucks and lorries).

The rules of the game are changing today: energy efficiency for vehicle fleets is definitely on the agenda. There are two challenges, the first for which is climatic. As EC Commissioner Siim Kallas said in 2013, “cubes are the least aerodynamic shape possible; it is absolutely necessary that we improve on the silhouette of lorry trains running on our motorways. It is an evolution that would make transport greener and safer.” The Commission proposed development of streamlined driver-cabins and aerodynamic deflectors attached at the back of the trailers, which, they estimate, “would generate up to 5 000 euros fuel saving for a standard long-haul truck covering 100 000 km/yr.” This brings us to the second challenge which is economic. Fuel is one of the main cost components for road haulage fleets. In France, for example, and despite having lower taxes than for other grades of engine fuel, diesel fuel is the 2^nd transport cost factor . Moreover, fuel pricing is subject to strong variations that are extremely difficult to forecast.

In the USA, a country notorious for fuel wastage in the past, the policy change took place in a very vigorous manner. Ever since the 1990s and the vehicle grade fuel price hikes, which then impacted the cost of goods delivered by truck, the American DOE (Department of Energy) financed a huge effort into resrecah for vehicle energy efficiency, i.e., where enterprises are concerned, improving ion the quantity of goods delivered per unit of fuel consumed. In this area, the performance level ‘levers’ are numerous: truck frame component and materials, gear-box and tyre quality, load distribution balance, driver behaviour when starting, accelerating and braking, choice of routes…

Performance levers
We must consider three categories of friction to be overcome when a truck begins to move, accelerates and reaches stable road speed: friction in the kinetic power chain (engine, transmission, axles) that accounts for 20% total. Next, friction that relates to overall vehicle + load mass and to kinetic tyre-road rolling friction (25%) and lastly aerodynamic air penetration (55%). Thus, the aerodynamic loading factor represents more than 50% fuel consumption for a freight truck travelling at constant speed. As the American authorities sees matters, aerodynamic improvements are the key priority. They alone could generate 8 to 12% gain in energy efficiency and, according to the DOE subsequently lead to saving some 6 to 9 billion litres of diesel fuel per year across the USA.

The trailer itself, viz., the goods carrying part of the truck that is attached to the tractor (driver cabin and engine assembly) – accounts for more than half of total aerodynamic truck resistance. Likewise, some 10% of this resistance is due to surface friction between the air and the truck surface material (fabric…). If the surface is clean and smooth – and not a badly stretched truck tarpaulin – this will reduce drag. The remainder of the drag factor is due to high and low pressure turbulence create by the vehicle moving through air. There is a high pressure zone in front of the truck, pushing and penetrating the air ahead and a low pressure zone holding up the back of the truck. These two, a high and a low pressure area, account for at least 1/3 total aerodynamic drag. The rest can be attributed to the empty space between the truck-tractor and its trailer and also to air flow road induced turbulence beneath the chassis.

Truck tractor profiles have been studies and improved on for a long time, especially in aerodynamically shaping the cabin roof, but as far as the trailer is concerned, the rear ends are far from correctly shaped and a lot of research work is needed here. The bigger the volume the truck is supposed to carry, the longer the trailer and the more expensive the aerodynamic challenge and solutions; however, as time goes by, the rising price of fuel has made such improvements economically worthwhile. Nevertheless, the tractors and trailers do not necessarily belong to the same operators and there may be a display of reticence on the part of the trailer owners to pay for improvements that will, in the main, prove beneficial to the tractor owners.

Aerodynamic drag is a dissipative loss, which means it cannot be recovered and is one of the most important factors to be borne in mind when trying to reduce fuel consumption and heavy vehicle emissions. Many research laboratories and universities have been invited to explore this area for vehicle improvement. At Georgia Tech, the Georgia Tech Research Institute (GTRI) has developed the concept of active flow control (AFC) fuel-saving technologies with pressurized air techniques combined with conventional aerodynamic streamlining. The method uses pneumatic devices that blow high-pressure air through vents to impact small curved aerodynamic foils attached to the rear of the vehicle. The air jets created in this manner smooth the air flow (i.e., cancel turbulence) on the ‘square’ trailer rear surface. In effect, the prevent the air streams from separating which is the cause for turbulence and low pressure drag on the trailer rear doors, which contributes to a lower truck+trailer drag at normal US highway speeds.

Since 2010, several systems aimed at reducing rear vehicle drag coefficients have been marketed. The major vehicle equipment assemblers are investing in this field now. In Europe, the pioneer system, of Spanish origin, is called SDR (System Drag Reduction); the system consist of attaching a blade on the top trailing edge of the trailer, enabling an air flow such that the low pressure zone is reduced. JOST, the manufacturer, promises a return on investment in just 6 months on the basis of 150 000 km/yr. Some trailers are fitted with extendible aerodynamic farings, called TrailerTails, a special product of the US company ATDynamics. But there is a legal loophole here, inasmuch as they are allowed even when they extend beyond the legal length of the vehicle trailer, viz., beyond the regulation limit of 13.6m. Future European legislation will make the farings legal as of 2015, but with an extended length limited to 50cm.

The European Commission in Brussels wishes to facilitate the marketing of more aerodynamic vehicles, authorizing the manufacturers to design streamlined, convex cabins and to equip the rear of the vehicle trailers with aerodynamic deflectors. Changes like this “will have a considerable influence on the aero dynamism and fuel consumption, especially in long distance haulage.” According to JOST’s estimates, the savings would amount to approximately 5,000 euros/yr. in fuel costs for a traditional long distance truck travelling 100,000 km/yr.

It is in the interest of the haulage companies to use the largest trucks possible. According to the results of a study commissioned by the CNRC (Canadian National Research Centre), if the operator adds a second trailer to form an extended road load, doubling the payload, there is only a small increase in the drag coefficient (10 to 20%) compared with a single trailer assembly. Such assemblies offer very interesting routes to procuring savings.

Weather conditions are also important. In colder climates, such as we find in Canada, the winter aerodynamic drag can be as much as 20% higher than normal, because the cold air is denser than so-called ambient temperature air. For tractor-trailer assemblies, this drag can lead to a 10% increase in fuel consumption, with respect to a given reference (warmer) temperature.

Again, according to the CNRC research results, the separation gap between the tractor and the trailer also has a significant effect on the vehicle assembly when it exceeds 45cm, increasing 2% for every extra 25cm beyond a 75cm gap. Ongoing research could completely solve this gap issue, with fuel savings of about 6% for a typical tractor-trailer assembly, simply by a lower drag coefficient. At a road speed of 98km/h, fuel saving would amount to approximately 3%. Truck manufacturers are preparing a tractor slider mountings that could be repositioned to reduce tractor-trailer gap at high road speeds.

Several studies have shown that when trucks travel in platoon formation, the aerodynamic drag can be reduced for all vehicles in the convoy, including the lead vehicle. Positioning oneself just behind a moving truck, benefits from the low pressure drag. The lead vehicle makes the effort to penetrate the air ahead and the second vehicle profits from this effort. It is estimated that a vehicle in a convoy can have a fuel savings gain of between 9 and 25%, the exact amount deepening on the separation distance, road speed, position and mass. A Californian company, adopting the name of “Peloton” (platoon), is proposing a combined radar-wifi technology that enables large freight trucks to closely follow the truck ahead and profit from the low-pressure suction without the risk of collision, which is the truck equivalent of ACC (adaptive cruise control) which already equips certain private cars. In order to achieve maximum aerodynamic gain, the inter-truck separation distance must be reduced to 11m. This in fact is very short, given that at 80 km/h a driver will hit the brakes on average 1 s after detecting a danger ahead and in that second delay in response, the vehicle will cover 22 m. The conclusion is that platoon technology must be much more reactive than human reactions.

Why Wal-Mart is leading the field
The corporate search for energy efficiency in truck fleets has tuned into technological marathons for some companies. Strange to say, it is the one company that is among the most criticized for its personnel policies, that wins the day in terms of record fuel savings, viz., the distributor Wal-Mart Stores Inc. For the past ten years or so, Wal-Mart Stores Inc. has become a well-known case study and its competitors are trying to keep up with them.

In 2005, Wal-Mart Stores Inc. set itself the target of doubling the energy efficiency of its trucks (6 500 tractors, 55 000 trailers and 7 000 drivers) to reach 13 miles per gallon (18l/100km) by 2015. Mid-2014, the company claims to be 84% there, i.e., since 2007, in a comparison with reference year 2005, Wal-Mart delivered 850 million tons more parcels, driving some 480 million less km, and this effort is to be again compared with the 1 120 million km travelled by the Wal-mart fleet.

In order to do this, Wal-Mart used every lever possible: the tractor aerodynamics, the trailer farings, the truck specifications, for example, having lighter load-bearing axles, wide-based tyres, auxiliary power units (APUs) to replace hydraulic power by electricity which, we note, is also a move under way for moving aircraft on airport taxi-ways. Every option was explored. All these innovations, together with rationalized routing have led to huge progress and, indeed, to a new prototype truck, called the Wal-Mart Advanced Vehicle Experience (Wave). The Wave trailer has a convex ‘nose’ – and this feature alone considerably reduces aerodynamic drag – almost completely assembled from carbon fibre parts, thereby reducing weight by practically 75% compared with a steel frame and proving 10 times more resilient. The main innovation lies in roof-top and side design, again using carbon fibre panels each 16m long (a world first). The sections were glued together with special glues [like modern aircraft wings], doing away with now useless rivets. Fed via a micro-turbine, the hybrid tractor-mounted propulsion unit can accept a variety of fuel grades.

In its quest for better energy efficiency, Wal-Mart Stores Inc. also includes its drivers. The company was one of the first in the world to adopt and fit on-board electronic trip recorders. This is a technology that not only allows you to geo-localize the trucks in real-time conditions but also updates their control centre as to truck fuel consumption, and even the exact positioning of the driver’s right-foot (accelerator pedal) choice of gear and a few other decisions he/she makes. In short, this amounts to “total monitoring.” For Wal-Mart management, modifying the drivers’ behavior at the wheel will be the next frontier in improving energy efficiency.

Another way to reduce fuel consumption is to lower the empty-load distances and to optimize loading of the trailers. Wal-Mart Stores Inc. hopes to attain a globally better energy efficiency including its suppliers that leave the warehouses empty after unloading; this phenomenon represents a waste of some 68 million US dollars for year 2012. The suppliers will be invited to re-locate their warehouses so as to reduce the total carbon print of the Group. Routing applications with GPS devices, optimizers, load planning and ‘cubic’ load distribution software have all been designed and made road-ready internally.

Reducing the size of the parcels and increasing “value density” is another strategic axis that has been addressed in a scientific manner. Wal-Mart has been trying throughout2013 to reduce average parcel volume by 5%. Since the parceling of the products changes and unit price drops, in compliance with the historic promise of the company, compression is the name of the game now, not only during transportation but also in warehouse management. Distributors are now coming up with detergents with lower water content. They are also redesigning the product delivery boxes “on demand,” so as to be able to load more items in each truck by limiting the dead-space round the parcels.

Benchmarking
Wal-Mart Stores Inc. is now a case reference and other US companies are closely following the progress as it is recorded. Federal Express (FedEx) has also undertaken far-reaching research in the field of optimization of energy consumption. A company responsible for transporting and delivering 3.6 billion parcels par year in 200 countries and destinations, is in a high fuel consumption business. The first step FedEx took was to have its pilots trained in reducing engine thrust during take-off and climb-to-cruise phases. At its Memphis hub, FedEx freight aircraft take off and land some 500 times a day and currently are engaged in a so-called recategorization of separation standards or RECAT applicable to inflight separation from aircraft ahead. These studies are based on new knowledge about aircraft wake turbulence. The new rules allow for closer distances if the aircraft belong to the same category. FedEx aircraft are almost all in the ‘C’ category and can fly safely at only 2.5 nautical miles (nm) from the aircraft ahead, whereas previously the standard requirement was 4 nm. Taxiing time on the ground is reduced by 3 minutes for each rotation and this leads to fuel savings estimated, for just this airport, at 1.32 million litres/yr.

But, for certain foresight experts, the ongoing debate on energy savings is something for the past, inasmuch as the next generation of truck will be autonomous and GPS guided, in which the driver in the long run will assure surveillance and vehicle control. Mercedes Benz has already experimented an autonomous lorry on German highways, placing an electronic driving system Highway Pilot at the controls. The system camera recognizes traffic lanes, fixed or mobile objects on the autobahn and can identify road traffic signals. Using an automatic data link, safety aid devices and advanced braking systems, the vehicle can maintain a steady 85 km/h speed, a constant separation distance of 60 m to the vehicles preceding, during the test phase, and the on-board radar can order the vehicle to slow down or come to a full stop. The steering is also innovative. Not only can the truck maintain its optimal direction in a given traffic lane, but it can adjust back to this in the case of strong side winds. The potential benefits here are numerous – more fluid traffic flow, lower fuel consumption and accident rate, added value for the human (supervisor) driver – but for the moment, it is not legally permissible to run autonomous vehicles on our roadways. As is the case with the Google Car, we shall have to await a completely novel ecosystem to see this happen one day.

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