GB2063799A - Drag reducing devices for road vehicles - Google Patents

Abstract

In devices for reducing aerodynamic drag of bluff road vehicles, the devices operate by suppressing flow separation and vortices and three types of the devices are disclosed: 1) Corner vanes which inject high energy air into the boundary layer at the rear corner of a cab (not shown). 2) Plate devices 28, 29 Fig 12a for suppressing vortices in the gap space of a tractor-trailer combination. 3) Duct devices for use on a cab roof (not shown). Preferably, all three are used on a vehicle simultaneously. <IMAGE>

Description

SPECIFICATION Drag reducing devices for road vehicles This invention relates to devices for attachment to road vehicles, particularly but not exclusively box type goods vehicles, to reduce aerodynamic drag.

Fuel saving for road vehicles is a very important aspect of energy conservation. The power used by a road vehicle, apart from transmission losses has to overcome the rolling friction of the tyres and the air resistance.

The power required to overcome the rolling resistance is approximately proportional to the vehicle speed and weight, whereas that required to overcome wind resistance is proportional to the cube of speed and the size and shape of the vehicle. It is clear that at higher speeds of travel, the power and thus the fuel consumption is required largely to overcome air resistance. Fuel saving for road vehicles at high speed can be effectively achieved only through aerodynamic drag reduction. The devices to be described below are particularly suitable for goods vehicles rigid trucks and tractor-trailer articulated trucks.

When a streamlined body moves through the air the air resistance experienced by the body is mainly the skin friction drag which is comparatively small. If, however, the body is not streamlined but bluff, airflow can no longer be expected to follow the entire contour of the solid surface. The flow will separate from the surface at locations where the flow is too weak to overcome the adverse pressure gradient. The result of separation is the creation of large pressure differences over the bluff body. Aerodynamic drag in this situation is largely attributed to the aerodynamic pressure over the body. Trucks and lorries commonly used for city delivery and for inter-city transport of goods are, from the aerodynamic stand point, bluff bodies of the worst kind. They are usually of a box form with sharp edges.Positive pressure forces are found over the front part of the vehicle and negative pressure forces on the rear portion of the vehicle. As much as 80% of the aerodynamic drag is caused in this way. Flow separation can also occur over the side faces of the box-shaped trucks especially in yaw conditions. Although the amount of fuel saving provided by a given drag reduction depends on the fraction of the total power required to overcome the aerodynamic drag, it is true that a drag reduction can be readily realised by lowering the frontal pressure and by raising the pressure on the back of the vehicle.

Corner rounding is an effective drag reducing technique, but is likely to impose structural alterations on existing vehicles.

The present invention relates particularly but not exclusively to the tractor-trailer type of vehicle.

The tractor and trailer are articulated for manoeuvrability with a gap-space between them for turning purposes. An ordinary lorry with a box shaped cargo compartment has aerodynamic characteristics similar to the tractor-trailer vehicle when the gap-space is reduced to zero. Owing to flow separation from the tractor sides and roof and owing to a second stagnation streamline impacted on the exposed face of the trailer and to flow separation from its leading edges, bubbles, circulations and large scale vortices are created which account largely for the high aerodynamic drag. Well over 80% of the total aerodynamic drag is produced in this way. It is reckoned that the average distance of travel by this type of vehicle is no less than 100,000 miles a year. From the fuel saving standpoint, the reduction of drag over the forebody is most important.As the tractortrailer gap-space increases, the aerodynamic drag increases as well. Research has been carried out on the effects of blocking up the gap-space and good improvement on drag has been found. But such an approach is very impractical for many obvious reasons. Another type of device which has been extensively studied is the so called added-on device on the roof of the driver's cab to deflect the airflow. This type can improve the aerodynamic drag but not as much as has been claimed, especially with vehicles having a large tractor-trailer differential height and in yaw conditions. For small differential heights, air flow can be thus deflected over the roof of the trailer to transform the double-stagnation flow field into a single stagnation streamline.

But in this case the drag produced on the trailer with a small differential height is low anyway. For large differential heights, the add-on device has to be tall which is likely to cause a higher tractor drag because of the inevitable circulation formed behind the device and of the low tractor base pressure.

Fluctuating flow will also result.

The present invention seeks to improve the aerodynamic drag of conventional road vehicles with a minimum of structural change.

Streamlining is thus precluded, and instead the invention is based on energy controlling the effects of flow separation by means of energy exchange and/or prevention of the formation of vortices.

According to one aspect of the invention there is provided in an articulated vehicle having a tractor and a trailer with an intervening gap space, drag-reducing plate means arranged in said gap space to reduce or prevent the occurrence of circulating air flow in said gap space.

Another aspect of the invention provides, in a vehicle having a cab and a bluff body behind and extending above the cab, a drag reducing device mounted on the roof of the cab and comprising through-flow duct imme diately above the cab roof and extending across substantially the whole width thereof, and means above said duct shaped to conform substantially to the potential flow stream line of the vehicle.

A further aspect of the invention resides in a road vehicle having one or more bluff frontal surfaces defined by relatively sharp edges, and in which one or more of said edges is provided with a drag-reducing device, the or each drag-reducing device comprising a duct or ducts arranged to accelerate incident airflow and inject the accelerated airstream into an area of low pressure.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures la and 1b are respectively a schematic side view and a schematic plan view of a conventional articulated goods vehicle showing a typical flow field around the forebody; Figure 2 is a schematic perspective view illustrating the flow field in yaw conditions; Figure 3a is a schematic perspective view of a tractor-tailer forebody showing corner vanes according to the present invention; Figures 3b and Sc are scrap plan views of the corner vanes on the trailer and tractor respectively:: Figures 4a and 4b are perspective and plan views of corner vanes on a trailer forebody in another embodiment; Figure 5 is a schematic plan view illustrating a further form of corner vane; Figure 6 is a schematic perspective view of a tractor-trailer forebody embodying plate devices in accordance with the invention; Figure 7 is a schematic side view of another form of plate device in conjunction with an air duct device; Figures 8 and 9 illustrate alternative embodiments similar to those of Fig. 7; Figures 10a-10d are schematic cross-sections of air duct devices; Figures 1 la and 1 ib are schematic side and plan cross-sections, respectively, of another air duct device; Figure 12a is a schematic side view of a tractor-trailer combination embodying a com bination of devices in accordance with the invention;; Figure 12b is a front view on the line a-a of Fig. 12a; and Figures 13a and 13b show a simplified embodiment similar to that of Fig. 1 2.

Referring to Fig. 1, positive pressure forces are normally found over the front part of the vehicle, the front face of the tractor and the exposed portion of the trailer, and negative pressure forces on the rear portion of the tractor and trailer. As much as 80% of the aerodynamic drag is caused by the pressure differences in this way. In a potential flow regime, the pressure difference does not exist.

In a real flow situation, any action to streamline the body can reduce the pressure difference to a minimum. The fact that a bluff body suffers from high pressure drag is attributed largely to flow separation. Flow separation results in the separation of vortices and an associated low pressure. Separation and vortex formation lead to a high loss of energy and these are undesirable phenomena occurring when, in the present case, a fluid flows round a sharp corner or a surface discontinuity. With a tractor-trailer transporter the air which impinges on the driver's cab splits up and flows round the cab undergoing separation as it encounters the front edges.The air which impinges on the trailer above the cab splits up, some of it flowing over the top and sides of the upper portion of the trailer and some of it flowing downward along the trailer front face through the gap-space and under the bottom and the sides of the lower portion of the trailer. 'Bubbles' are usually formed after separation of the flow at the edges and reattachment may occur only at the rear end of the trailer where the boundary layer (or the wake) becomes thick and turbulent. The high velocity down-flow in the gap-space is vorticular and unsteady creating a very unstable wake to the tractor. The pressure in this region is low and fluctuant. Fig. 1 shows the general flow pattern in this region observed by flow visualisation.

When the vehicle moves through calm air, the flow direction is in line with the direction of motion of the vehicle. In gusty conditions when the wind makes an angle to the longitudinal direction, the resultant air flow direction is yawed relatived to the direction of the vehicle's motion. In the case of a tractor-trailer combination in yaw, the frontal area seen by the incident flow is increased and there will be flow separation occurring along the side edges as well as the front edges of the tractor and trailer. Due to the presence of the gapspace, the shielding effect of the tractor over the trailer is also reduced in yaw. Moreover, the side flow through the gap space will separate from the leeward vertical edges of the trailer, producing therefore a wide wake.

Fig. 2 shows the general flow pattern in yaw.

As these adverse effects are magnified by the resultant flow direction moving side-ways from the vehicle direction of motion, the aero, dynamic drag (and the associated tyre frictional drag owing to non-symmetrical pressure distribution on the vehicle) increases with the increase of the yaw angle.

The aerodynamic drag reducing devices in the invention can be divided into three catego ries, namely, curved vanes, plate devices and air duct devices. They are simple to manufacture and can readily be attached to existing vehicles in a way which will not impair the maneouvrability or the driver's vision. They can be applied individually. The best result can be obtained from the combined effects of the three types when applied appropriately.

We now describe the functions of the three said types of devices.

Curved Vanes The curved vanes are also known as corner vanes. They can be made of sheet metal, hard rubber or plastic materials. When applied to the side edges of a tractor 10 or the leading edges of a trailer 1 2 or such corresponding positions as shown in Fig. 3, the vanes 14 are to be fitted parallel to the edges and spaced therefrom such that in use a portion of airflow past the vehicle flows through the space between the vane and the corner, the said space having an inlet opening substantially wider than its outlet opening. The function of the vane 14 is to guide the flow round corners and to inject energy into the boundary layer behind the vane, thus reducing the effect of flow separation.The vanes 14 are preferably perforated or slotted over up to 15% of their surface area to reduce the frontal air pressure and to provide a bleed of air to prevent the formation of vortices behind the vane.

The corner vane 14 is intended to provide for high velocity flow ejection around the front corner of the body by drawing energy from the flow before it reaches the corner. The shape of the vane is such that the inlet width is very much larger than the outlet width so that the incident flow is compressed to give a relatively higher outlet velocity for the purpose of strengthening the boundary layer and thus reducing the effects of flow separation.

It is preferred that the cab vane is dimensioned such that a > d/80 and ss > d/250 and that the trailer vane is dimensioned such that d/30 < a < d/10 and d/120 < fl < d/40 where d is the vehicle equivalent frontal diam eter, which is equal to (maximum fron tal dimension X 4/s a is the width of the outlet, and ss is the width of the inlet.

Usually the drag coefficient of the vehicle increases with increasing angle of yaw (e.g. in side wind condition). With corner vanes fitted to the vertical edges of the trailer, yaw effect can be appreciably reduced.

The corner vane can also be applied to the top rear edge of the trailer (not shown), with the flow inlet opening larger than the outlet opening.

If structural alteration in existing vehicle is possible (or in a new design) the corner vanes can be integrated into the structure as shown in Fig. 4. The vanes 1 6 are then part of the trailer skin which extends forward to form curved lips which direct the airflow through slots 1 8 along the sides 22 and along the roof 20 to energise the boundary layer for the control of flow separation. The slot openings are preferred to be so oriented as to allow the air outlet direction to be close to the tangent of the surface.

In cases where the tractor width is small than that of the trailer, a pair of concave vanes 24 to be fitted onto the rear vertical edges of the tractor as in Fig. 5 is beneficial.

Their function is to divert the airflow from impinging directly on the front face 26 of the trailer. There is no opening space between the vane 24 and the corner and there is no need to perforate the vanes 24. These rear vanes 24 together with the corner vanes for the side edges of the tractor and the trailer provide a streamlining effect on the forebody.

Slotted corner vanes along the side top edges of the trailer (not shown) are also beneficial in crosswind conditions.

Plate Devices One plate system is shown in Fig. 6 and consists of two horizontal plates 28, 29 and one vertical central plate 30 to be situated in the gap-space between the tractor 32 and the trailer 34. One horizontal plate 28 is extended from the roof 36 of the tractor 32 to meet the other plate 29 extended from the front face 38 of the trailer 34. The plates 28, 29 overlap each other in such a way that they do not interfere with the turning maneouvre of the vehicle and that there is an air gap 8 between them with the tractor plate 28 being placed above the trailer plate 29. In operation, the horizontal plates 28, 29 act as bulkheads to prevent downward flow and to reduce the exposure of the trailer face 38.Air bleed will take place through the air gap 8 to stabilise the flow below the plates 28, 29 and to increase the back pressure of the tractor 32. The vertical central plate 30 is situated below the horizontal plates. It is preferred to have it extended from the face 38 of the trailer to the same dimension as the trailer horizontal plate 29, providing also for the support of the horizontal plate 29. In operation the vertical plate 30 inhibits the fluctuation of flow in the tractor-trailer gap-space and suppresses periodic formation of vortices. The dynamic characteristics of the vertical plate is similar to a splitter plate which is well known for its drag reducing ability. In yaw conditions (e.g. side wind), the vertical plate 30 is particularly useful in improving drag. The plate 30 can be extended downward and upward as far as convenient.Both the horizontal plates 28, 29 and the vertical plate 30 should be properly supported (or should be thick enough) to prevent any local vibration from occurring.

Under certain circumstances, it is advantageous to have the horizontal plates 28, 29 curved and fitted at an inclined angle. An example of a combination of the three types of device is shown in Fig. 7. The curvature of the two plates 28, 29 and the air gap 8 between must be so arranged as not to foul with each other during turning operation of the vehicle.

It is preferred that 6 is not greater than 0.0 sod, where d is defined as above.

Air Duct Devices Most tractors have a flat front and a vertical rear which are the causes of high aerodynamic drag. The stagnation streamline occurs rather low down and most incident flow separates from the roof of the cab. The commonly known add-on air deflectors provide some protection for the trailer and thus reduce the trailer front pressure. Because of flow separation at the back of the deflector, the reduction of aerodynamic drag is mainly on the trailer.

The air duct devices illustrated in Figs. 7 to 1 2 provide an effective system for controlling flow separation from the roof of the tractor.

As shown in Fig. 8, it is preferable to have ducts 40 arranged in tiers with a decreasing size towards the top for the purpose of preventing the formation of a roof bubble and for directing the appropriate quantity of airflow to prevent circulation from taking place in front of the exposed face 38 of the trailer 34. The configuration of the ducts 40 can be arranged to follow the path of a potential flow streamline 42, as shown in Fig. 9. To avoid the unnecessary increase of viscous drag due to an increase in the number of ducts, packing fairings 44 can be arranged between the ducts 40 as seen in Figs. 5 and 8. By means of convex curving 46 or concave curving 48 of the fairings 44 at a location just forward of the duct entrance more or less quantity of airflow through the ducts 40 can be arranged, as illustrated in Fig. 8.A top fairing is used to prevent bubble formation there and to direct the flow over the trailer roof. Fig. 8 shows various possibilities for air duct arrangement.

Test results indicated that Fig. 8 (b) and (c) were slightly better than Fig. 8(a) and (d).

Air duct with fairings can also be arranged for lorries with zero gap space as shown in Fig. 9. In this case a wedge shaped fairing 50 to be placed within the duct 40 is necessary in order to divert the flow from the upper face 52 of the lorry to the sides 54. To make use of the diverted flow, corner vanes 56 can be incorporated to channel the flow around the side edges of the lorry which may have a larger width than that of the driver cab.

Curved or angled plates instead of horizontal plates can reduce the exposed area of the trailer especially when the differential height between the tractor and trailer is large. In this case the air duct system can be arranged to suit. The dimensions of the air ducts are determined according to the cab size, the differential height between the tractor and the trailer and the gap-space between them. Test results indicated that good performance can be obtained by terminating the rear ends of the ducts at the same vertical plane. To improve yaw conditions, it is preferable to have the widths of the ducts tapered smoothly from the base to the top in such a way as not to cause side component flow separation. The air duct system can be made from sheet metals or from moulded plastics and it can be either bolted or clipped on to the roof of the driver's cab.

Fig. 1 2 shows one simple form of construction, with a single duct 40 and a deflector surface 58 formed from sheet metal, the interior 60 being hollow.

Preferred Practical Embodiment An embodiment which is preferred at present for practical purposes and simplicity is illustrated in Fig. 1 3. A central vertical plate 62 is provided on the front face 38 of the trailer 34 but runs, in this embodiment, the full height of the trailer 34. The cab roof is provided with a top fairing 62 forming an approximate streamline to the trailer roof, and with an air duct 40a arranged to inject air downwardly into the gap space.

As before, the vertical plate 62 is of particular assistance in crosswinds. The air duct 40a mitigates flow separation and vortices in the gap space, and effectively reduces drag on the tractor 32 when the air duct is provided with appropriate inlet and outlet openings.

The arrangement of Fig. 1 3 is preferred because of ease of manufacture and fitting.

When small corner vanes are also fitted to the frong vertical corners of the tractor, drag can be further reduced.

Claims (21)

1. In an articulated vehicle having a tractor and a trailer with an intervening gap space, drag-reducing plate means arranged in said gap space to reduce or prevent the occurrence of circulating air flow in said gap space.

2. The invention of claim 1, in which said plate means comprises a vertical plate attached to and extending forwardly from the front face of the trailer.

3. The invention of claim 2, in which said vertical plate is on the centre line of the trailer.

4. The invention of claim 2 or claim 3, including a horizontal plate extending across the front face of the trailer.

5. The invention of any preceding claim, further including a flow guide attached to the roof of the tractor cab, the flow guide including at least one through-flow duct immediately above said roof and an angled deflection surface above said duct.

6. The invention of claim 5, in which said duct is formed to discharge air flowing therethrough downwardly over the rear face of the tractor cab.

7. The invention of claim 1, in which said plate means comprises a first transverse plate extending rearwardly from the roof of the tractor cab and a second transverse plate extending forwardly from the front face of the trailer, said plates overlapping and defining an air gap therebetween.

8. The invention of claim 7, in which said plates are horizontal.

9. The invention of claim 7, in which said plates curve concavely and upwardly from the cab roof.

10. The invention of any of claims 7 to 9, further including a vertical plate attached to and extending forwardly from the front face of the trailer.

11. In a vehicle having a cab and a bluff body behind and extending above the cab, a drag reducing device mounted on the roof of the cab and comprising through-flow duct immediately above the cab roof and extending across substantially the whole width thereof, and means above said duct shaped to conform substantially to the potential flow stream line of the vehicle.

1 2. The invention of claim 11, in which said means comprises an angled deflecting surface.

1 3. The invention of claim 11, in which said means comprises a plurality of further ducts whose entrances are progressively stepped back.

14. The invention of claim 13, in which the ducts are separated by spacers having front surfaces shaped to achieve a desired division of flow through the ducts.

1 5. The invention of claim 1 3 or claim 14, in which the further ducts are progressively smaller transversely of the vehicle.

16. A road vehicle having one or more bluff frontal surfaces defined by relatively sharp edges, and in which one or more of said edges is provided with a drag-reducing device, the or each drag-reducing device comprising a duct or ducts arranged to accelerate incident airflow and inject the accelerated airstream into an area of low pressure.

1 7. A vehicle according to claim 1, in which the or each duct is formed by the combination of one of said edges and a curved vane, the vane being so arranged that the inlet:outlet ratio of the duct is at least 1.5:1.0.

1 8. A vehicle according to claim 17, in which the vane is perforated or slotted.

1 9. A vehicle according to claim 18 in which the perforations or slots occupy at least 15% of the area of the vane.

20. A vehicle according to claim 1 8 or claim 19, in which slotted vanes are also fitted to the rear and/or side edges of the vehicle roof.

21. A road vehicle substantially as herein described with reference to and as illustrated in any one of Figs. 3 to 9 or Fig. 11 or Fig.

1 2 of the accompanying drawings.