Container

This section looks at the container portion of the vehicle, i.e. the semi-trailer of an articulated vehicle or the body of a rigid vehicle and any trailer. The performance figures for the aerodynamic aids are based on various studies and experiments carried out by a variety of research organisations and universities. It should be noted that the fuel savings stated in this section for a drawbar vehicle relate to the motor vehicle, not the trailer. Any fuel savings from fitting aerodynamic aids to a trailer towed by a rigid vehicle are mentioned separately.

Fairing Vortex stabilisers Eco Liner Teardrop roof Sloping roof Rotating cylinders Flatbed container Side panels Wedge High-momentum mud flaps Airwedge

Aerodynamic under-body Rear mud flap Underride guard Boat tail Inset boat tail SDR Vanes Vortex generators Vortex strakes Blowing slots

Fairing

A relatively simple way of reducing the drag coefficient of the semi-trailer is to round the edges of the rectangular body. The pieces that can be fitted to a semi-trailer to round the edges are called ‘front fairings’. There are a number of types. Fitting front fairings yields fuel savings of 4%, 2.1% and 1.9% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively. Fitting a fairing to the front of a trailer being towed by a rigid vehicle reduces fuel consumption by 0.8%.

Another aerodynamic aid that falls under the same heading is the aircone, a convex piece that can be fitted to the front of the tractor or container. It ensures that the air flows over the tractor or container without becoming detached.

The function of the aircone is in fact the same as that of a roof fairing/deflector, and the fuel savings that can be achieved with it are thus of the same order of magnitude: 5.9%, 4% and 2.8% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively. An aircone can also be fitted to a flatbed semi-trailer used to transport e.g. sea containers.

Vortex stabilisers

Solutions to reduce airflow in the gap between the cab and the semi-trailer were put forward in section 1. One solution is to modify the rear of the cab, i.e. by fitting tractor side panels. There are also ways of reducing drag on the semi-trailer. An oblique incident flow creates powerful unstable vortices in the gap between the cab and the semi-trailer, which cause drag. These vortices can be kept under control using vortex stabilisers, vertical fins over the entire height of the semi-trailer. They can also be fitted to a trailer. The fins contain the vortices between them, as it were, thus reducing the pressure on the frontal area of the semi-trailer, resulting in less pressure drag. The fuel savings are 0.7% and 1% for an articulated vehicle and a drawbar vehicle respectively.

Researchers at the National Research Council of Canada have shown using wind tunnel tests that the dimensions (height and depth) of the fins and the distance between them have a major effect on performance. Six fins are fitted to the front of the semi-trailer, with a height of 244cm and a depth of 30cm. They are distanced evenly across the width of the semi-trailer. No reduction in drag was measured in the wind tunnel tests. It subsequently emerged that the fins were unable to contain the airflow because the dimensions were incorrect. It should be noted that the tests were conducted on an American truck, and these articulated vehicles have a larger gap between the cab and the semi-trailer than their European counterparts. In another study fitting vortex stabilisers resulted in fuel savings of 3.5-8.3%: this shows that the test conditions have a major effect on the results. This configuration is marketed by SOLUS in the United States.

Eco Liner

Krone, a semi-trailer manufacturer, has developed a fuel-saving semi-trailer known as the ‘Eco Liner’, which will soon come onto the market. It differs from a standard semi-trailer in such things as the side panels and the closing system for the tarp covering. The new pneumatic system makes the side of the semi-trailer more streamlined, partly because it tensions the tarp more tightly so that it behaves like a rigid wall, and partly because there are fewer protuberances at the sides. The inward tapering side panels guide the airflow to the rear of the semi-trailer, creating a vortex there that reduces the wake behind the semi-trailer. The improved streamlining, in conjunction with the semi-trailer’s lower rolling resistance and lower weight, yields fuel savings of 5-7%.

Teardrop roof

DON-BUR in the United Kingdom have developed an aerodynamic teardrop-shaped roof to reduce the drag on articulated vehicles. Not only does the roof profile make for a continuous airflow around the semi-trailer, it also ensures that the flow at the rear is directed in such a way as to reduce the wake. The airflow around the ‘Teardrop’ semi-trailer remains continuous, in contrast to that on a standard semi-trailer. The convex roof yields 10% more load capacity. These semi-trailers are fitted with side panels as standard. Altogether, the modifications yield a fuel saving of 16.7%. These semi-trailers are not permitted on the Dutch or continental European roads under the current legislation because of their height (5m).

Sloping roof

The drag generated by the rear of the semi-trailer or body can be reduced by having a roof that slopes down at the rear. The ideal is a long, slightly sloping roof profile; short, steeply sloping roofs do not perform as well. The angle between the horizontal and the inclined plane must not exceed 12º. The taper should be as deep as possible, taking into account the loss of storage space in the semi-trailer and access. The fuel savings for a sloping roof are 0.6%, 0.4% and 0.2% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively. A trailer with a sloping roof towed by a rigid vehicle yields a fuel saving of 0.3%.

Rotating cylinders

Two rotating cylinders can be fitted to the top of the semi-trailer, at the front and rear respectively. These are known as ‘Magnus rotors’. An airflow that comes into contact with a rotating cylinder gains energy. The additional energy imparted by the first cylinder is used by the airflow to remain attached to the roof of the semi-trailer. The second cylinder at the rear, like the first one, imparts energy to the airflow, which this time is used to guide the flow around the corner, thus reducing the wake behind the semi-trailer. Rotating cylinders are a promising technology: a drag reduction of 20% has been measured in a study.

Flatbed container

The ultimate shape of a truck with a flatbed semi-trailer or body is determined partly by the load. Fuel can be saved by placing the load in the right place and pointing in the right direction. You must ensure when positioning the load that the maximum permitted axle weights are not exceeded.

Side panels

Side panels, also referred to as ‘side skirts’, are fitted in order to close off the sides, so that as little air as possible flows under the semi-trailer, especially in crosswind conditions. This reduces drag and makes the vehicle more stable. Side panels are also useful in that they enable storage lockers to be fitted. A disadvantage is that they are vulnerable to stone chips and kerbstones as they are so low off the ground, so it is essential to select panels of the correct material. Side panels yield fuel savings of 1.2%, 0.5% and 0.8% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively.

Extensive wind tunnel research has been done in recent years at TU Delft into various side panel configurations. To ascertain how efficient the existing panels are, flat panels with straight or sloping leading and trailing edges were tested. The effect of uncovered wheels on drag was also investigated.

Side panels that left the wheels uncovered performed worst, yielding a drag reduction of 8% with no crosswind. With a standard side panel a drag reduction of 11% was obtained in experiments with no crosswind. Wind tunnel tests in which the model was rotated in order to simulate a crosswind showed that performance is virtually unchanged with this type of side panel.

A further reduction in drag is obtained if the panel has a sloping trailing edge: a drag reduction of 12% was measured with no crosswind. This configuration is also not sensitive to crosswind, so the same drag reduction was obtained with a crosswind. If both the leading and trailing edges of the side panel are sloping, a drag reduction of 13% is obtained with no crosswind; with a crosswind a maximum reduction of about 14% was measured.

The best performance was obtained from a side panel rounded on the inside: compared with the first two it was able to reduce the drag by 11% in wind tunnel tests with no crosswind, as against 7% for the first two configurations. In wind tunnel experiments with an increasing incident flow angle the efficiency of the side panel rounded on the inside increased compared with the ones rounded on the outside. For this application a semicircular body is not the ideal shape from the aerodynamic point of view, so the semicircle was replaced with a wing profile, a more aerodynamic shape. Wind tunnel tests with no crosswind showed a drag reduction of 14%; a maximum of 17% was obtained with a crosswind.

The aerodynamic body, known as a SideWing, guides the airflow towards the rear of the semi-trailer. By covering the wheels it reduces the amount of road spray. Various road tests carried out in collaboration with PART showed that fitting SideWings yielded a fuel saving of up to 15% in a strong crosswind at a travelling speed of 80 kmph. The SideWing is marketed by Ephicas, a technostarter of TU Delft.

Wedge

Researchers at the Lawrence Livermore National Laboratory conducted wind tunnel tests a few years ago on a simplified 1/16-scale truck, attaching a large thin-walled wedge under the semi-trailer.

The results of the measurements are shown in the figure. The dotted line represents the measurements without the thin-walled wedge, and the unbroken line represents the situation with the aerodynamic aid. The grey areas show where the wedge produced a lower drag coefficient. The biggest gain was founded in a headwind (ΔCD = −0.03); drag was also reduced with a lateral incident flow, but to a lesser extent (ΔCD = −0.015).

High-momentum mud flaps

The research team of Richard Wood and Steven Bauer have developed a technology to reduce a semi-trailer’s pressure drag using two contoured surfaces attached to the under-body and rear of the vehicle. The convergent duct accelerates the airflow coming from under the vehicle towards its wake, thus raising the pressure in the wake and reducing the pressure drag. The accelerated flow is shown by the red arrows in the figure.

The tests the research team conducted on an articulated vehicle showed fuel savings ranging from 0.8% to 3.3%.

Airwedge

Airman Inc. have developed a wedge that improves the airflow under the semi-trailer. The higher speed of the flow under the semi-trailer causes the air flowing along its side to be drawn in at the rear: this reduces the wake behind the vehicle, thus lowering drag and fuel consumption. The airflows with and without the Airwedge are illustrated in the figure. Airman Inc. claim that this system produces a fuel saving of at least 5%, but this has not yet been demonstrated in independent tests.

Aerodynamic under-body

In addition to investigating various side panel configurations, TU Delft have also done research in recent years into the design of semi-trailer under-bodies with the aim of guiding the airflow around the obstacles on the underside (i.e. struts, pallet box, axles and wheels), thus reducing drag. The first experimental setup was a fully aerodynamic under-body with the wheels uncovered. The results of the wind tunnel tests showed that with no crosswind 8% less drag was created; the performance improved with a crosswind, with a drag reduction of 12% at an incident flow angle of 6º. Covering the wheels resulted in an additional drag reduction of 3% compared with the previous configuration. This shows that uncovered wheels have a major effect on drag.

Rear mud flap

Often a single mud flap is fitted across the entire width of a rigid vehicle, semi-trailer or trailer behind the rear axle. This provides additional advertising space and reduces road spray to the rear. This wide flap, however, induces additional pressure drag and increases lateral spray, with the result that a rigid vehicle, an articulated vehicle and a drawbar vehicle use 0.7%, 0.6% and 0.4% more fuel respectively.

Wind tunnel tests conducted at TU Delft have shown that placing a barrier to the airflow on the underside at the rear adversely affects the overall drag. In an airflow with no crosswind this configuration generated over 20% more drag. This shows that the flow along the underside of the semi-trailer is critical to the vehicle’s overall drag.

Underride guard

An underride guard is required at the rear of a semi-trailer or rigid vehicle for safety reasons. Often the gap between the underside of the semi-trailer and the underride guard is filled in so as to fit the necessary lighting and the number plate, and/or to improve the appearance. From the aerodynamic point of view it is better to leave the gap between the underside of the semi-trailer and the underride guard open, as this creates less drag. The required lighting and the number plate can be incorporated in the underride guard.

Boat tail

A boat tail is a tapering – possibly hollow – rear end.

Mathematical simulations at TU Delft on a tapering rear end showed that the airflow follows the bodywork, resulting in a smaller wake and higher pressure at the rear, lowering the drag considerably. With a conventional rectangular rear end the airflow becomes detached, creating an enormous wake and a lot of drag.

Wind tunnel tests conducted at TU Delft have shown that a tapering hollow rear end can produce a drag reduction of 12%, which equates to a fuel saving of about 6%. Three different boat tail configurations (three different lengths) were road-tested in 2008. These road tests showed fuel consumption savings of 7.5% for a two meter boat tail (without extra bumper) at a velocity of 85km/h. Adding the extra bumper costs 2% fuel saving. Shorter boat tails (1.5m and 1m) all indicated lower fuel savings as expected: 6% and 3% respectively (all without extra bumper).

A tapering rear end can take various forms, namely collapsible thin fins, an inflatable body or a flexible body. With these types it is important to consider access to the load space and the safety aspects.

A number of manufacturers have developed off-the-shelf boat tails. One example is the ATDynamics TrailerTail. Collapsible thin fins are fitted to the doors of the body. When the doors are opened, the fins collapse within 8 seconds. Expanding them takes 6 seconds. Road tests conducted in the United States have shown that the TrailerTail yields a fuel saving of 5.1% at a speed of 100 kmph.

Another example is Aerovolution Corp.’s inflatable boat tail, which is fitted to the rear corners of the body. It is in two sections, enabling one door to be opened. Once inflated, with the doors closed, the inflatable boat tail retains its shape. Unlike the TrailerTail, the Aero-Tail does not have a hollow rear end. Inflating it takes about 30 seconds, using the on-board compressor or an external one. The manufacturer of this boat tail claims a fuel saving of about 3.5% at 100 kmph. The Aero-Tail is illustrated in the figure.

Inset boat tail

An inset boat tail comprises thin plates attached to the rear of the body. The inset arrangement at the rear creates vortices that reduce the wake and increase the pressure in the wake, thus lowering the pressure drag.
This solution produces a drag reduction of about 15%, resulting in a fuel saving of a few percent. The inset boat tail can easily be made retractable so as to guarantee access to the container.

SDR

SDR (System Drag Reduction) can be regarded as a wind deflector that is fitted to the roof at the rear of the body. It guides the airflow on the roof towards the wake behind the body. It can reduce the wake by to 50%, resulting in less drag. The black arrows in figure show the direction of the airflow, and the green arrows point to the SDR. Various firms are already using the system and report fuel savings of 4-6%.

Vanes

Another simple way of reducing the wake is to fit small wings to the rear of the body. These vanes guide the incident airflow behind the body inwards, thus reducing the wake. A study in the 1930s showed a 50% drag coefficient reduction in a highly simplified two-dimensional wind tunnel model. Mathematical simulations of this two-dimensional system at TU Delft produced the same result. Research at TU Delft has shown that the vanes are also effective in a three-dimensional configuration: the initial results are promising, and fuel savings of 5-10% are thought to be feasible. The results of this research are expected some time in 2009.

Vortex generators

One way of reducing drag easily and cheaply is to fit vortex generators to the body (and if necessary the tractor). These ensure that the airflow along the sides and top develops into vortices that expand behind the body, resulting in less drag.

This principle enables over 5% less drag to be generated, resulting in fuel savings of 1-2%. Vortex generators can also be fitted to the cab and tractor side panels, again having a beneficial effect on drag, as they reduce the airflow through the gap between the cab and the body, thus reducing the effect of crosswind on overall drag.

Vortex strakes

A variation on vortex generators is vortex strakes. These are attached to the sides of the body at the rear and in an angled configuration on the roof. They generate large numbers of vortices that impart energy to the passing airflow: this additional energy enables the air to flow around the corners at the rear of the body more readily, resulting in higher pressure in the wake, thus reducing drag. The angle of the strakes ensures that the airflow under the body combines better with the wake behind it, resulting in a smaller wake. Although in theory this method can produce over 10% less drag, this has not yet been demonstrated in road tests. The fuel savings at present are of the order of 1-2%.

Blowing slots

These are slits at the rear of the body from which air is expelled. The air then flows over curved surfaces (the Coanda effect) and the flow along the vehicle is drawn around the corner, resulting in a smaller wake and higher pressure, hence less drag.

This pneumatic technology produced a drag coefficient reduction of over 35% in the wind tunnel. Allowance has to be made for the fact that it uses energy, however. A fuel saving of 4% was obtained in trials. By gradually changing the wind tunnel model to make it increasingly resemble a real truck – which is less aerodynamic – the researchers have been able to ascertain where the losses arise. The research is still ongoing, and a fuel saving of 16% is expected without the blowing slits; the fuel saving is expected to be 23% with the blowing slots in operation. This technology could also improve safety: blowing could be used to generate more drag when braking so as to reduce the braking distance. Differential blowing could be used to offset crosswind, so that the truck driver experiences less yawing. Mathematical simulations of this technology have been carried out at TU Delft during the past year, and the results are expected some time in 2009.