As expected, the team detected a relationship between vehicle length and sensitivity to yaw angles, in other words, the longer the vehicle, the greater the increase in drag when subjected to cross winds. Another interesting result of the study was the percentage contribution of friction and pressure drag. As discussed in Section 3. However, as vehicle length increases, the percentage contribution to overall drag from friction drag rises slightly since there is so much more planar surface aligned with the airstream, yet the blunt front face of the vehicle remains unchanged. The study concluded that the percentage contribution of pressure drag on the baseline vehicle was The significance of this is that as vehicle length increase, strategies to reduce friction drag become more effective in reducing fuel consumption.
After studying overall drag, the team analysed the drag at various locations on the combination vehicles, particularly around gaps between trailers. The team concluded that the size of the gap between the lead and trailing trailer played a significant role in the amount of drag experienced by the combination vehicle, particularly at higher yaw angles.
Most operators are not directly concerned with drag, however. To them it is the effect of drag reduction that impacts their operations, via an associated fuel consumption reduction. The following case study describes the effects of drag on LCVs with respect to fuel consumption and thus, fuel costs. A third vehicle is used for comparison: an LCV, consisting of one tractor and two full trailers, connected in a B-train arrangement. It is assumed that engine efficiency and all parasitic drains such as air conditioning and electrical accessories would be relatively similar between the LCV and the conventional vehicles.
Table 5 illustrates the estimated distribution of fuel consumption for the two types of vehicles at five degree yaw wind angles given that a tractor and trailer over an entire year will have a non-zero average wind yaw. LCVs are not a product, per se, therefore there are no relevant claims from manufacturers.
However, user groups such as the Ontario Trucking Association OTA have endorsed the use of LCVs for a variety of reasons, including the potential for fuel savings as a result of aerodynamic reductions. By far the greatest operational concern for an operator wishing to use an LCV remains unloading the front trailer.
Therefore, operators wishing to benefit from the use of LCVs must take this into consideration when planning their delivery and pick up times. These issues can be lessened by the use of special switching yards near depots where pup trailers can be dropped off or by using trailers that have side curtains for loading and un-loading. Although not common, combinations with a tanker trailer in the lead position and a van trailer in the trailing position can also minimize loading and unloading delays. Other concerns involve the perceptions of the driving public as they must negotiate their vehicles around LCVs which are significantly longer than the vehicles they replace, and 15 metres longer than B-trains which most drivers have accepted as the norm on Canadian highways, and even in city centres while delivering fuel.
It is estimated that one LCV would burn approximately 23, fewer litres of fuel when compared to two conventional vehicles, assuming a yearly distance of , km at highway cruising speeds.
However, there are very little data pertaining to North American LCVs experiencing variable wind yaw angles corresponding to a yearly wind average drag. A study such as Cooper and Leuschen [ 6 ] could be conducted whereby a variety of gap fillers, side skirts and boat tails are sequentially added to the LCV in order to determine if the effects of these devices on LCVs is similar to their effect on conventional vehicles.
CMVSS stipulates that all highway tractors must be fitted with side view mirrors, on both sides of the vehicle, with no less than cm 2 50 in 2 of reflective surface. There are many other physical requirements involving reflectivity and mirror curvature, all of which are outside the scope of this document as they do not affect the overall aerodynamics of the mirrors.
However, there is a significant and measurable increase in vehicle aerodynamic drag caused by the addition of the mandatory side view mirrors stipulated in CMVSS The placement in the airstream and the blunt body area of the mirrors remain the principal factors when considering aerodynamic losses. Technological advances have made it possible to replace the rear view mirrors with cameras, thus drastically reducing aerodynamic drag.
However, at the time of writing it is still illegal to remove the side view mirrors on a highway tractor in Canada therefore cameras can, at best, be used as a supplemental vision system. A study [ 7 ] by the Institute for Aerospace Research at the National Research Council NRC-IAR tested the individual contributions to aerodynamic losses from a variety of devices that are, or could be, attached to a highway tractor and trailer combination.
The study used a Volvo class 8 highway tractor and a 28 foot van semi-trailer. The coefficient of drag of the tractor and trailer combination without devices was measured and then devices were added back to the combination vehicle, one at a time, to determine the individual contribution to increasing or decreasing aerodynamic drag for the combination vehicle. The drag coefficient C D of the baseline vehicle was measured to be 0. This would result in an estimated Canadian fleet wide savings of approximately ,, litres of fuel every year, or ,, kg of CO2 if the side view mirrors were removed from all , highway tractors.
More fuel could be saved with the removal of mirrors from straight trucks, however, this would be less effective than removal on highway tractors since straight trucks tend to drive at considerably slower speeds than highway tractor. Similar testing was performed on the fender mounted convex mirrors. The results indicated that approximately litres of fuel are burned, per tractor, annually to overcome the drag from fender mirrors on a single tractor.
The projected savings from the removal of mirrors must be tempered by the fact that rear view cameras that could replace mirrors will undoubtedly also project into the airstream and thus create some drag, albeit much less than full CMVSS mirrors. One of the risk factors associated with side view cameras is reliability.
With the exception of vandalism or damage from flying road debris, traditional mirrors have a mean time between failure that far exceeds the practical life of the tractor to which they are mounted. Conversely, rear view cameras and monitors introduce a variety of electronic components, each of which will carry a mean time between failure that is most certainly lower than that of a mirror although the exact amount is not known at this time. It may be necessary for camera equipped vehicles to carry some form of redundancy in the event of camera failure.
This redundancy, however, must be lightweight and stowed in such a manner to not affect the aerodynamic drag of the vehicle when not in use; otherwise the aerodynamic benefits of cameras would be lost. The effect of adding weight to the tractor must be considered for rear facing cameras to ensure that aerodynamic savings are not negated by the addition of weight, above that of conventional mirrors which tend to add very little weight to the tractor.
At a gross weight of, say, 41, kg, the added weight of the camera equipment would, at most, represent a 0. Therefore, the estimated fuel consumption increase would be between 0. Actual specifications of volume and weight of the camera equipment and full scale dynamometer and wind tunnel testing would be required to confirm these estimates. One area of research worth noting is short wave infrared SWIR cameras. These cameras use infrared technology to penetrate through fog and darkness to capture images that would normally be difficult or impossible to see by a human operator.
Combining rear vision cameras with SWIRs would not only reduce aerodynamic drag but would also increase the functionality of the vision system above what is normally provided by conventional mirrors. Similar technologies could be used for drivers of heavy trucks to see other vehicles or obstacles in the road during fog or white-out conditions. Therefore, some consideration could be given to reducing the amount of glass on a CMVSS compliant mirror and thus reducing its area and aerodynamic drag. However, NRC-CSTT measured the reflective surfaces of two in-service Canadian based tractors used for other testing in winter and found that these tractors were equipped with mirrors that were already two and a half to three times larger than what is required under CMVSS regulations, if both the plane and convex sections are considered.
In fact, the plane sections alone of the mirrors were approximately twice the size required in the CMVSS regulation. Therefore, if manufacturers and operators are already accustomed to vehicles equipped with nearly three times the required minimum in order to drive safely, it is not likely that reducing the legal minimum will motivate manufacturers to provide smaller mirrors.
NRC-CSTT interviewed two experienced heavy truck drivers and each of them agreed that rather than alter the size of the mirrors, they would prefer a hybrid side view mirror that included an inboard plane section with an outboard convex section to increase the field of view. This increased field of view would allow them to see smaller and faster vehicles, such as motorcycles, as they pass in the left hand lane, possibly in the left hand portion of the left lane.
Most mirror systems required the driver to look into the plane mirror and then lower their eyes to look into the convex mirror below the plane mirror for the expanded field of view. The hybrid construction would allow them to assess an emergency lane change much faster, without the need to lower their eyes or head. Additionally, one driver indicated that in his opinion , newer drivers could easily be trained to use video camera systems whereas more experienced drivers may find the migration to viewing a video terminal to their right difficult after so many years of looking to their left.
This, he felt, would be particularly difficult in an emergency situation where they react on instinct rather than on process or thought. A cursory review of in-service tractors in Ontario confirmed that drivers are currently accustomed to using mirrors that are nearly three times larger than what is required under CMVSS regulations. Replacing side view mirrors with rear view cameras will most certainly reduce the mean time between failure MTBF of the tractor. NRC-CSTT recommends developing a study to determine the benefits and drawbacks of side view mirror replacement for aspects other than the well known aerodynamic benefits.
If it was determined that side view mirrors could be removed without any negative safety side effects it would be worthwhile to investigate a pilot project to better understand the potential fuel savings under actual revenue driving conditions. The benefits of infrared cameras could also be studied to determine if they could be combined with camera mirrors to enhance the vision of the drivers during inclement weather.
Platooning is the act of coupling two or more vehicles together while they are travelling on a highway. The intent of platooning is to improve safety, traffic flow and efficiency while also improving the efficiency of the individual vehicles within the platoon [ 22 ]. This coupling can be mechanical, electrical, magnetic or electronic. This allows vehicles to travel as a synchronized unit, reducing the distance between vehicles which reduces the lag time as successive vehicles accelerate in the same direction.
It also allows vehicles to brake as one unit, rather than as separate entities, thus reducing the risk of rear end collisions. Although platooning is generally aimed at reducing traffic congestion, one of the greatest side-benefits of platooning is the reduction in aerodynamic drag on all of the vehicles in the platoon, even the lead vehicle. Although aerodynamic drag is a factor that affects all vehicles, the fuel consumption of Class 8 tractor trailer combinations is significantly affected by aerodynamic effects owing to their large size and blunt front faces.
For this reason, the concept of platooning makes sense when applied to very large, blunt face, vehicles. Advanced platooning would allow intelligent vehicles to join or leave the platoon at any time, however, there are many challenges involved with platooning, principally the interaction between the platoon vehicles and non-platoon vehicles as well as strategies to optimize the vehicles spacing.
Although there are many aspects of platooning, the concepts relating to drag reduction of heavy vehicles have been presented in Sections 6. Platooning remains a concept that is being studied, researched and tested in many developed countries, however, there are no examples of platoons that can currently be found on active public roadways. The concept of platooning dates back to a pilot study conducted in by Dr. Robert Fenton at the University of Ohio [ 23 ]. Although computers were used, they filled nearly the entire free space inside the vehicle making the project impractical for everyday use.
This research continued until funding was cut in the early s. However, computing and sensor development and evolution has allowed the concept of platooning to be considered again, as a viable project for the future. Globally, there are several significant projects underway, each attempting to define how a platooning system could be integrated into a smart highway. These projects are as follows:. Although much of the research tended to focus on the effects of platooning on traffic flow, there were also theoretical modules that dealt with the aerodynamic effects. However, the publication [ ] was not clear if that was a fuel consumption savings or a drag savings.
The mission on PATH is to reduce traffic congestion, increase traffic flow and safety and decrease energy consumption. Some of the aerodynamic results that have been demonstrated via the PATH project include:. Safe Road Trains for the Environment SARTRE is a European consortium project aimed at studying the possibility of a multi vehicle type platoon system that could include passenger vehicles, heavy trucks and buses driving on un-altered conventional roads. The main focus of the project is the environment, traffic safety and traffic congestion. The system comprises one lead vehicle, driven by a professional driver who leads the platoon, and a series of autonomous following vehicles, operated by conventional drivers.
The following drivers are then free to engage in activities that would normally be prohibited, such as reading or using a cell phone. One of the principal goals of the project is to determine how a platoon can be successfully integrated with other non-platoon traffic, for such things as lane changes and departing the highway. One of the principal goals of the SARTRE project [ 27 ] is to determine the optimum spacing for vehicles such that aerodynamic gains are maximized while still respecting the needs to maintain a safe distance.
Even sophisticated control systems require some feedback time to brake the trailing vehicle fast enough to prevent a collision, therefore a limit on the minimum distance between vehicles will be required, regardless of aerodynamic effects. One such study, performed by Bonnet and Fritz [ 26 ] studied the aerodynamic effects of having one heavy truck following a similar heavy truck by as little as 5 m.
The principal difference between this study and many others was the focus on fuel consumption rather than drag coefficient. The purpose of the study was to determine the relationship between following distance and potential drag reduction, assuming all other factors such as grade, wind speed, vehicle speed and rolling resistance remained constant.
Infra-red lights mounted on the rear of the lead vehicle were used to send information to the towed vehicle controller TVC which controls the lateral and longitudinal position of the trailing vehicle by accelerating, steering and braking as necessary. However, the driver of the trailing vehicle always maintains the option of overriding the system and driving the vehicle manually if required for emergency or operational reasons.
The vehicles were driven at trailing distances between 6 and 16 metres and the fuel consumption of both vehicles was calculated using a moving average taken every 10 seconds and by calculating the total amount of fuel consumed for each test leg. These two methods were then averaged to obtain an overall fuel consumption difference at various following distances for each of the vehicles compared to when the vehicles were being driven at a typical following distance.
The test team concluded that a trailing distance of 10 m was optimum when considering a variety of speeds as well as the fuel consumption reduction for both the lead and trailing vehicles. Trailing distances of less than 10 m did not produce increased benefits. The test team also investigated the effects of vehicle weight on the potential fuel savings. The lead and trailing test vehicles had masses of Since rolling resistance is greatly affected by vehicle weight, it stands to reason that the relative aerodynamic benefit of platooning decreases as vehicle weight increases as more and more fuel is being used to overcome rolling resistance.
Using extrapolation methods, the team calculated that a lightly loaded trailing vehicle at Platooning remains a largely theoretical study, with no known current practical applications. Therefore it is difficult to itemize all the areas that may be of concern to an operator. Certainly the biggest operational concern relates to how non-platoon vehicles will interact with long platoons of heavy vehicles. Even if long platoons could be arranged in a safe and efficient manner, there will be many challenges regarding how other vehicles will negotiate around the platoons, particularly when trying to exit a highway via the rightmost lane.
Of all the items described in this report, platooning remains the furthest from actual deployment due not only to the large technical challenges, but also to the monumental task of determining how other vehicles would interact with the platoon. Even if technology could allow two or more heavy vehicles to be electronically connected, the logistics of integrating these vehicles into existing traffic flows will prove to be extremely difficult. Further testing and understanding of LCVs would be a more practical approach to multi vehicle aerodynamic reductions until platooning has been perfected in smaller countries in Europe.
Today, there are a large number of Class 8 tractor-trailer drag-reducing devices and technologies both in-use and under development. Many of these have been extensively studied, with the performance benefits well documented in the research press. The section will also describe some of the less well studied, and less commercially-adopted technologies for improving aerodynamic efficiencies of tractor-trailor combinations. In a report by the US National Academy of Sciences that documented current and emerging technologies for fuel reduction of medium- and heavy-duty vehicles [ 2 ], they identified the four following critical areas for aerodynamic improvement of tractor trailers under highway conditions:.
The drag-reduction technologies described in this section can be separated into two major categories; those mounted to the tractor and those mounted to the trailer. As is pointed out by Leuschen and Cooper [ 29 ] and others, there exist as many as three to four times as many trailers in service as there are tractors.
Since trailer manufacturers are typically not also operators, and the cost of gap devices increases overall trailer acquisition costs, there is little motivation on the part of the trailer manufacturers to adopt these devices. The payback period for tractor-mounted devices will be much shorter than that for trailer-mounted devices, which will affect the rate of adoption of such technologies to the transportation industry. As such, the tractor devices and technologies will likely be adopted earlier.
When evaluating the potential fuel savings of tractor-trailer devices, it is important to understand the context under which any measurements or evaluations have been performed. Results, especially those based on road testing, can be biased depending on the conditions of the vehicle and the environmental under which they were tested. For example, favourable drag-reduction claims based on fuel-economy tests can be biased if a device is tested on a lightly-loaded vehicle in low-wind conditions. This provides difficulty in evaluating various technologies based on separate studies or claims. A systematic and consistent manner in which the devices can be tested would be required to provide a set of recommendation to policy makers, manufacturers, and operators.
Much of the previous research has been conducted on very specific, or a small set of tractor-trailer combinations. Specifically, there has been little study of the possible negative effects that could arise from cab roof fairings and side extensions, when used with certain combinations of trailers. Lifestyle truckers often prefer older boxier-style tractors with many appendages, lights, and no air deflectors. It is also worthwhile to evaluate the effects of some newer technologies, those that may not significantly affect the appearance of the vehicle, on these classic-style tractors.
As noted above, four critical areas are identified for application of drag reduction technologies. In the following, drag-reduction devices and technologies for each of these four areas are described. Initially a general list of concepts was devised based on several references [ 2 ], [ 5 ], [ 6 ], [ 7 ], [ 8 ], [ 14 ], [ 23 ], [ 32 ], [ 33 ], [ 34 ], [ 35 ] that identify technologies and devices that can be evaluated for drag-reduction potential of tractor-trailer combinations.
Further references are provided within specific section below. Tractor streamlining has been a driving factor in tractor development by manufacturers for the last three decades. The fuel crisis of the s promoted the development and subsequent adoption of aero-tractors to the market in the s and through the s. Despite the demand by older-generation drivers for the classic style tractors with square hoods, flat bumpers, and large external appendages such as air filters and exhaust pipes, all manufacturers have aero-tractor models that have been developed with fuel economy in mind.
Current efforts towards incremental drag reduction of tractors are directed at the bumper areas, the underbody, and the gap region between tractor and trailer. The specific details of the calculations are well presented by Leuschen and Cooper. Of note is that the addition of some common devices, such as OEM Mirrors, bug deflectors and fender mirrors negatively impact fuel burn. Table 7 — Tractor Add-ons potential Fuel Savings [ 29 ].
Many other significant areas of possible aerodynamic drag reduction for tractors have been identified or proposed, as described below. Generally, truck manufacturers aerodynamically optimize their standard mirror design. However, as noted in Section 5, these standard mirrors are larger than required by law. Proposed camera systems instead of side mirrors, as described in Section 5, or combinations of both, may require aerodynamic optimization for shape, size and location. The shape of a tractor in its bumper region provides a strong influence on the flow underneath the tractor.
Lowering of the tractor bumper using air dams or spoilers can redirect flow to the sides of the vehicle rather than underneath it, which may provide a reduction in drag by reducing the momentum of fluid directed to the underbody. This indicates that such bumpers may not necessarily reduce fuel consumption, but their proximity to the stagnation region on the front face of a vehicle may make be them aerodynamically neutral.
Many transport companies are choosing heavy-duty bumpers because of the combined effect of very high insurance premiums and the fragility of the lightweight materials used for front-end components. Having a heavy-duty bumper makes it possible to avoid claiming minor collisions but comes with a fuel consumption penalty. Aerodynamic optimization of such bumpers may be possible to limit any fuel-consumption solely to weight increases. The bumper is located in close proximity to the ground, and the relative motion of the ground with respect to the vehicle creates complex aerodynamic flow characteristics that are not intuitive.
Any aerodynamic evaluation of bumper shapes and technologies would require proper accounting of this relative motion. Work done at the NRC in a wind tunnel using a rolling-road system to simulate properly the vehicle-air-ground motions has shown for some lower-vehicle modifications significantly different aerodynamic behavior depending on whether the ground-plane was fixed or moving. Some configurations that provided in a decrease in drag with a fixed floor provided an increase in drag for a moving floor, indicating this strong dependence of lower vehicle aerodynamics on these relative motions.
In a wind tunnel simulation, the moving floor represents most accurately the on-road case. A potential exists for accessories such as hub-caps and mud-flaps to provide some aerodynamic benefit to reducing fuel consumption of vehicles. Leuschen and Cooper [ 29 ] note a small decrease in the drag of a tractor trailer with smooth hub caps, based on wind-tunnel tests with stationary wheels.
The decrease in drag was within the experimental uncertainty of the tests, however with rotating wheels there may be an advantage to covering the wheels. If a net benefit can be gained by using hub caps or wheel covers, it will also be necessary to evaluate their effect on brake cooling to ensure they do not restrict air flow to the brakes.
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Mud flaps inhibit the lifting of soil, rocks and mud into the air by blocking the motion of air in the direct downwind vicinity of the wheels. This provides restriction to the flow and therefore introduces drag to the overall vehicle. Some concepts for slotted mud-flaps have been proposed [ 14 ] that block the motion of solid particles while allowing some air to pass through. Depending on where ice accretes, an increase in drag may be observed for tractor trailers in icy conditions.
Additionally, ice buildup on a tractor-trailer also leads to weight increases which lead to higher fuel consumption. Finally, there is an increased risk to other road users should large chunks of attached ice and snow dislodge at highway speeds. One proposal to alleviate the build-up of ice and snow on tractor-trailer and van bodies is the adoption of superhydrophobic coatings.
Superhydrophobic surfaces, such as the leaves of the lotus plant, are highly hydrophobic which makes them extremely difficult to wet. There have been significant advances in the development of these superhydrophobic coatings, whose applicability to reducing snow and ice accumulation could be investigated. Additionally, the water droplet interaction at the surface of a treated surface could be investigated to see if there are benefits in the reduction of road spray. It is possible, though as yet unstudied, that superhydrophobic surfaces may cause water shedding that reduces the effects of road spray on other road users.
If this angle is large, contact forces between the droplet and the leaf are small, resulting in low droplet adhesion. A large pressure on the surface facing the flow, coupled with a negative pressure on the surfaces facing away from the flow result in significant aerodynamic drag. One proposed technique to minimize this source of drag is called base bleeding. The result is that the differential pressure between the two surfaces is reduced, and consequently drag is reduced. Two methods for achieving this pressure equalization have been studied. These methods comprise so-called active base bleeding and passive base bleeding.
In active base bleeding, a mechanical source compressor is used to generate volumes of air which are introduced into the low pressure regions. These techniques are claimed to be aerodynamically effective under research conditions wind tunnel testing and computational analysis but in practice, can be very difficult to implement. These systems add weight, electrical drag on the engines i.
As well, active base bleeding techniques would most likely suffer similar reliability issues due to sleet, snow and dirt accumulation. Passive base bleeding involves the careful manipulation and ducting of naturally occurring high pressure air to areas of low pressure. Though the electrical system load drag is eliminated, there are still weight and space penalties and the systems need to be carefully tuned in order to operate most effectively.
This fine tuning is often speed dependent, and reliant on maintaining exacting geometries. In practice, this might be difficult to maintain during normal trucking operations on roads where snow, sleet and debris are often present, resulting in the possible obstruction of the base bleeding structures.
Operationally, active systems will be much more difficult to maintain, especially those that require air exchange through thin slots and holes such as those studies by ATDynamics. Given the expected reliability issues of active and passive base bleeding systems when operating under normal weather conditions, further study in this area is not recommended. Engine cooling is accomplished by airflow through the engine compartment, either driven by fans at low driving speeds or by ram effects at higher speeds.
As the cooling air passes through the engine compartment after passing through the radiator, condenser and other auxiliary components, it is then reintroduced into the main airstream. The manner in which the cooling air flow exhausts from the engine compartment may have an effect on the drag characteristics, particularly for underbody effects. For exhausting below the vehicle, the relative motions between the vehicle, the road, and the main airflow will be important, and one could conceive of scenarios to create either a drag reduction or a drag penalty.
Many aerodynamic treatments exist that have been proposed for the tractor-trailer industry. Some, like side skirts, were adopted long ago for tractors and are beginning to be adopted for trailers, but many more remain at the research phase. One area where possible gains exist with the addition of aerodynamic treatments is the underbody of the tractor.
As reported in research [ 45 ], there were measurable improvements in aerodynamic performance when smoothing plates were added to the underside of the tractor. These benefits were measured to be in the range of a 0. Using the methods found in Leuschen and Cooper [ 29 ], these reductions would equate to an annual possible fuel saving of up to litres of fuel per tractor. This is an area of the vehicle for which the relative motion between the vehicle, the ground and the main airflow is critical. As yet unpublished experiments at NRC have shown diverging results for techniques applied to the lower body depending on the motion, or lack there-of, of the wind-tunnel floor.
Implementation of such devices could pose operational issues with regards to maintenance, and increased weight, however, some benefits may be observed in that critical under cab components would be better protected, and fewer locations may exist for the accumulation of dirt, snow and ice. The flow behaviour in this gap region affects directly the pressures on the back-face of the cab and the front face of the trailer, both of which are large surfaces perpendicular to the vehicle motion and therefore strong contributors to the overall drag on the vehicle.
When prevailing winds impinge on the tractor-trailer at even moderate oblique angles, the cross flow through the gap modifies the pressures on the cab and trailer faces resulting in an increase in overall vehicle drag [ 46 ]. This is a dominant region for which a wind-averaged-drag evaluation is required to ascertain the benefits of drag reduction devices. To minimize the effect of the gap flow on drag, completely sealing the gap would eliminate its drag contribution under cross-wind conditions. However, due to operational requirements, a minimum gap distance is required to allow the tractor to articulate relative to the trailer to facilitate manoeuvring at loading facilities and vehicle depots.
Typical tractor-trailer gaps are in the range of about 1. It has been shown that the gap begins to have a significant impact on vehicle drag once it is greater than about 0. Research by Landman et al. There are two primary types of devices aimed at reducing aerodynamic drag in the tractor-trailer gap. These are tractor side extensions and devices in the gaps.
Most OEMs offer side extenders as standard options for tractors. Additionally, the gap area is also often used to store ancillary tractor equipment. It is envisioned that a stowage solution could be designed that would not only allow the secure and efficient stowage of the ancillary equipment, but could also serve as a cab mounted gap filler device. It has been reported in the research [ 43 ], [ 45 ] that significant opportunity exists for aerodynamic improvements in the treatment of the surfaces immediately behind the tractor cab.
By covering the area in a horizontal fairing and blending it into the wheel fairings, it has been observed that significant aerodynamic benefits are possible. A gap splitter large vertical plate is a technique often promoted for trailers see next section. Cooper demonstrated [ 16 ] this as viable technique for cross-wind conditions. A tractor-mounted gap splitter would behave similarly while minimizing the cost of implementation. A final tractor-mounted technique to reduce the drag associated with the gap is to reduce the distance between the rear of the tractor and the front of the trailer.
This method is limited by the need of the operator to retain sufficient turning radius to enable loading and unloading at constrained dock areas or to negotiate tight right turns. This concept would likely require low energy consumption that would be more than outweighed by the benefit of a smaller gap in highway conditions. Additionally, for tractors without roof fairings, or those with fairing not optimized for the trailer being pulled, gap reduction may not provide any significant benefit.
Many of the proposed devices intended to reduce the aerodynamic losses in the gap region are trailer-mounted devices.
Consequently, there has not been a high level of adoption of the current gap-seal devices, with the exception of the tractor mounted side extensions discussed below. Further, gap splitters typically come in two varieties: center plane boards, and devices that attempt to fill the actual gap. The percentage savings are, however, highly dependent of the test procedure chosen, including initial gap size, and test speed. Some of the gap-filling devices look similar to refrigeration units. It may be possible to optimize the shape of such refrigeration units to take advantage of the gap reduction effect.
It is however appropriate to investigate first the theoretical maximum benefit of completely closing the gap. These are important findings, as they are in line with quoted manufacturers numbers. There does exist, however, a significant opportunity to refine and optimize the traditional gap treatment devices to attain the maximum possible fuel savings. Additionally, little has been done to study the effects of towing a variety of trailers behind an aerodynamically streamlined cab.
There exists the possibility that some combinations of tractor-trailer may exhibit reduced aerodynamic performance with the addition of certain gap treatments.
In a similar manner to the gap between the tractor and trailer, the open area below the trailer provides a greater drag detriment under cross-wind conditions. The general approach to minimize drag associated with this region is to prevent air from entering. The trailer makes use of air dams, trim panels, side skirts, wheel fairings, and a boat tail to reduce the overall vehicle drag. The concept is a complete package and does not consist of individual add-on components. A thoroughly evaluated combination of add-on devices may provide the same level of drag reduction.
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Also, these devices have simple mechanical designs such that maintenance and reliability should not be a major concern. Some variants on side skirts have been proposed. Modified version of side skirts, wedge skirts and side wings [ 14 ] are claimed to be optimized to enhance further the under-body aerodynamics. There are operational issues that would need to be resolved, as well as significant weight penalties that may arise as well as changes to the breakover angle of the trailer; however there are benefits as well.
It is envisioned that these boxes could be developed for the stowage of binding straps, chains, spare tires and other accessories. Such boxes could offer side under-ride protection to vulnerable road users. Any such implementation would have to be well designed to ensure that no ground clearance operability issues arose. Leuschen and Cooper [ 29 ] found a measurable drag reduction associated with fairings around either the trailer wheels or around the full bogie.
However, basic side skirts provide a larger magnitude of drag reduction and are generally of simpler design and construction. Therefore it is not expected that fairings will be strong contenders for the transportation industry. The trailer base is one of the largest sources of drag for tractor trailers.
Low pressure on the trailer face due to the aerodynamic wake, combined with the high pressure on the front face of the vehicle, causes a net pressure differential that generates a force in the downwind direction. This front-to-back pressure differential is the primary source of drag for most heavy vehicles. Increasing the base pressure will reduce this differential and reduce the net drag on the vehicle.
Therefore, many drag-reduction technologies for the trailer are aimed at increasing this back pressure. Tapering the back end of a long vehicle will increase its base pressure by providing pressure-recovery of the surrounding flow before it leaves the sharp back edges and forms a wake. This increased base pressure provides a lowered overall pressure difference from front-to-back of the tractor-trailer combination. A collaboration between NRC and Transport Canada has identified some optimum configurations aerodynamically and operationally using a scale wind tunnel model.
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Demonstration in representative road conditions would be beneficial to provide further recommendations. Vortex generators are devices that, when placed on a surface, generate a vortex or array of vortices in the flow along the surface that can influence the flow-separation behaviour further downwind. Several concepts for implementing such devices on the sides and tops of trailers have been proposed as a means to reduce the aerodynamic drag of heavy vehicles.
A small-scale wind-tunnel study of a vortex-strake concept showed a small benefit [ 49 ]. In their study, Leuschen and Cooper [ 29 ] attempted but were unable to find a vendor or concept developer available to have their vortex generator devices systematically tested on a full-scale truck in a wind tunnel.
Therefore, they developed their own prototype vortex generator for testing purposes, which was based on the vortex-strake concept studied in [ 29 ]. Since the first publication of this report in , a privately funded track test program has demonstrated limited potential of a vortex generator concept applied to the roof and back side edges of a 53 ft trailer [ 36 ]. The frontal area of a tractor-trailer is dictated by the trailer and its cargo volume. This technology also requires a retractable tractor fairing, in order to be effective. The most significant barrier to the adoption of any of the aforementioned devices will not be from a technical point of view, but rather, from an operational stand point and industry acceptance.
It is believed however, that with an appropriate balance between operational concerns and aerodynamic benefit, significant enhancements can be made without unduly affecting operation or greatly increasing overall tractor or trailer acquisition costs. The results of aerodynamic testing on heavy duty front bumpers have been scattered, however other factors are driving operators to use such devices. The vehicle-road-wind relative motion is an important for evaluating bumper and lower-body technologies. Modest aerodynamic improvements may be achieved with the use of wheel covers and slotted mudflaps.
Cab underbody treatments have been shown to decrease the aerodynamic drag of tractors, however, testing should be performed using a rolling-road type wind tunnel to quantity the important vehicle-road-wind relative motions and hence actual performance gains. It is suggested that all the devices described in this section could be worthy candidates for further study with the exception of the base bleed devices and active flow control technologies.
Furthermore, an integrated study of all the devices could be made to ensure that the aerodynamic gains of one device do not reduce the aerodynamic performance of another device installed further downstream on the vehicle. The suggested process could involve scaled wind tunnel tests involving the sequential addition of each device until the vehicle was equipped with all of the above mentioned devices. Following that preliminary stage, full scale prototypes could be developed or purchased and tested in controlled real world driving situations.
The wind-averaged drag coefficient measurement, which was designed to account for angle variations in terrestrial winds, is the most effective evaluation criteria. In any of these future studies, the approach could first be to understand the operational concerns and barriers to commercial entry, prior to undertaking any aerodynamic experimentation or simulation.
In , approximately 35, intercity coaches were in operation in North America. The body of available literature on aerodynamic drag-reduction techniques and add-on devices for intercity buses is less extensive than for class 8 tractor-trailers. Several factors may account for this difference. The North-American bus fleet is much smaller than the tractor-trailer fleet and consequently, the annual fuel consumption and GHG emissions by intercity buses are significantly lower. Unlike tractor manufacturers, bus producers have control over the aerodynamic design of the entire vehicle and notwithstanding operational and regulatory constraints, they benefit from considerable design freedom to minimize aerodynamic drag through shape optimisation, among other things, with a direct impact on vehicle efficiency.
There are also few manufacturers of intercity buses and in this competitive industry, design secrecy is the rule and little information is made public. The popular belief is that buses, due to their characteristic shape, are not burdened by the large aerodynamic losses that plague tractor-trailers.
This assertion is inaccurate and must be confronted to physical arguments. Based on the analysis of the respective contributions of mechanical and aerodynamic losses to the energy consumption of road vehicles, it is possible to determine a 'cross-over' vehicle speed beyond which aerodynamic losses dominate. At highway speeds, a significant fraction of the energy expended is dissipated into aerodynamic losses.
The corollary is that for a given percent-reduction in drag coefficient, the net percent-reduction in fuel consumption is larger for a bus than it is for a tractor-trailer. Adapted from Cooper [ 55 ]. A typical intercity bus is characterised by a refined and enclosed shape and the absence of pronounced discontinuities.
This configuration has the potential to provide a high level of aerodynamic efficiency, provided that the design takes under serious consideration the physics that will govern the aerodynamic behaviour of the vehicle in its real operating environment. The aerodynamic losses can be broken down into four categories, each requiring a custom treatment:. A- The dominant contribution to the aerodynamic drag of an intercity bus is the pressure differential between the forward- and rearward-facing surfaces of the body, with a minimal contribution from skin friction.
According to Cooper [ 56 ], about 60 to 70 percent of the total wind-averaged drag of a bus is attributed to pressure loads acting on the vehicle forebody, making it the principal area for drag reduction strategies. This reduction was attributed to reduced static pressures on the corners, indicating flow attachment. Recent experiments by Newnham et al. Hence, a proper simulation aimed at minimizing the aerodynamic drag of buses must reproduce both the Reynolds number and the turbulence characteristics that will be experienced by the vehicle in real operating conditions.
Of significant importance, as well, is the presence of obstacles in the vicinity of a corner, such as rubber mouldings or joints, which may trigger premature flow separation. Notwithstanding the critical importance of forebody flow, gains can also be made in the tail region, where it is desirable to energize the wake and reduce the spatial extent of the separation bubble. Experimental and numerical work has been done on generic bluff-body shapes where parametric variations of a tapered tail were studied [ 59 ].
By combining corner rounding and tapering with an innovative tail comprising a "vortex trap", Fletcher and Stewart's wind tunnel investigation [ 60 ] reported a reduction of the aerodynamic drag coefficient from 0. More recently, the CFD experiments of Raveendran et al. Another wind tunnel investigation by Balakrishnan et al. These modifications allowed the coefficient of aerodynamic drag of the original box-like buses to be reduced from as high as 0.
The CFD experiment of Kim et al. In light of the large C D -reduction potential afforded by a sound aerodynamic design of the body shape, a bus optimisation process aimed at reducing fuel consumption ought to assign a high priority to aerodynamic shaping, early in the design process. As pointed out by Fletcher and Stewart [ 60 ], reducing aerodynamic drag by optimising body shape, besides diminishing fuel consumption, also provides substantial secondary benefits associated with a "cleaner flow", including:.
B - It is paradoxical that bus appendages such as rear-view mirrors, when fastened to an aerodynamically-optimised bus body, may cause a non-negligible deterioration of the vehicle's aerodynamic performance. Consider a pair of mirrors, each with a surface area of 0.
This is without considering that the presence of such mirrors in the vicinity of the forward lateral corners perturbs the local flow field and may induce detrimental flow separation coupled with a further drag increase. As the design trends move towards bus configurations with streamlined shapes and drag coefficients approaching 0. In this context, regulations authorizing the use of standard rear-view cameras to replace mirrors would be a step in the right direction, to minimize aerodynamic disturbances, as was discussed in Section 5 in regards to tractor-trailers.
This implementation could be coupled with the mandatory installation of standard mechanical pop-out mirrors that would deploy in the event of camera failure. C - Underbody aerodynamics is becoming increasingly important, in the quest to reduce fuel consumption of surface vehicles. The correlation between the wind tunnel and track tests were based upon the measurement of static pressures, distributed over the panels. The study revealed the importance of proper ground simulation and wheel rotation, to ensure the realism of the wind tunnel measurements of the underbody flow. A recent CFD parametric study conducted by Ortega and Salari [ 65 ] focused on the effect of an underbody fairing installed on the underside of a trailer attached to a heavy-duty tractor.
Each tapered fairing tested was located immediately downstream of the tractor wheels and extended as far as the trailer bogey. Its main purpose was to mitigate or eliminate the observed flow recirculation zone originating downwind of the tractor drive wheels and inducing negative pressures on downstream-facing elements. The best configuration tested longest fairing was found to reduce the drag coefficient by 0.
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