This section looks at the aerodynamic aids available to reduce drag on a tractor (i.e. the motive power unit in an articulated vehicle, or the cab of a rigid vehicle or a drawbar vehicle):
These are designed to guide the airflow at the front of the cab by helping the air to flow around the corners. As the airflow round the corners remains attached, the vanes prevent dirt being deposited on the cab windows and sides, thus improving the driver’s visibility and the vehicle’s appearance. Well-rounded corners also works for a continuous airflow. The first prerequisite for an aerodynamic cab, then, is rounded corners. If cab side-edge turning vanes are fitted incorrectly they can increase the drag. They yield fuel savings of 0.6%, 0.4% and 0.3% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively.
Tip: To keep the airflow attached to the outside of the vanes they need to have a minimum radius of 75mm.
The solution that yields the biggest fuel saving is replacing the side mirrors with cameras, which can be built into the bodywork, thus reducing the frontal area, hence the drag. Cameras can provide a wider field of view to the rear, and those equipped with infrared technology can penetrate fog and rain, thus improving road safety. Aerodynamic side mirrors have been fitted to new trucks for some years now. Fitting aerodynamic side mirrors affects fuel consumption: the saving (compared with conventional side mirrors) is 0.3%, 0.2% and 0.2% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively.
Additional lights and horns are often fitted to the cab roof edge of trucks. They disrupt the oncoming airflow to the extent of causing a 0.1% increase in fuel consumption.
Tip: Keep the truck as tidy as possible. For this reason avoid having lights, horns, aerials and other objects exposed to the airflow.
An air dam is effectively a downward extension of the tractor bumper. It stops the air from flowing along the underside of the vehicle, guiding it along the more aerodynamic surfaces, i.e. the sides and the roof, thus reducing the wind pressure on the irregularities on the underside of the vehicle, hence the drag. The reduction in the drag coefficient depends on the vehicle’s under-body: if this is smooth, an air dam will even have the opposite effect, by increasing the frontal area. An example of an air dam is shown (see the white arrow). An air dam yields fuel savings of 0.9%, 0.4% and 0.3% for a rigid vehicle, an articulated vehicle and a drawbar vehicle respectively.
Eco-flaps and Vortex splash guards are perforated mud flaps that replace the traditional mud flaps fitted behind the wheels. They allow about 75% of the air to pass through them, resulting in less drag. As well as reducing fuel consumption by about 1,5%, they also reduce road spray.
These are used to bridge the gaps between the front and rear axles of the tractor. They are particularly useful in crosswind conditions, as they prevent air entering under the tractor. They also have road safety advantages: they reduce the amount of road spray, as well as improving safety for cyclists and pedestrians, who are less likely to be caught under the wheels. Fitting side panels to the tractor of an articulated vehicle yields a fuel saving of 0.7%.
ATDynamics hub caps are one of the quickest and easiest ways of saving fuel on a truck. They streamline the airflow around the wheels, thus reducing the drag. The manufacturer has not yet released data on the precise fuel savings that can be achieved. The disadvantage is that these hub caps prevent airflow through the rims which is needed to cool the brakes.
These panels have most effect on trucks that have large gaps between the various parts of the chassis behind the cab. They prevent air flowing through the tractor chassis, thus reducing drag. They also prevent the low pressure under the tractor chassis drawing in a larger amount of air in the gap between the tractor and the semi-trailer, as well as facilitating access to the rear of the cab and improving the appearance of the tractor. There are no figures available on the precise fuel savings.
Tip: Fitting a filler panel to the top of the tractor chassis prevents turbulence between the chassis members and reduces dirt deposition.
The primary function of a sun visor is of course to ensure that the driver does not suffer from glare. Its rounded edges can also have an aerodynamic function, reducing drag on a cab with a sharp horizontal roof edge, by guiding the airflow around it better. Fitting a sun visor yields a fuel saving of 3% for a rigid vehicle, and 1.9% and 2.3% for an articulated vehicle and a drawbar vehicle respectively.
Tip: The minimum radius of rounding of the cab’s horizontal roof edge required to achieve a drag reduction depends on the vehicle’s average speed. As a rule of thumb:
- To achieve a drag reduction at all speeds a radius of 150mm is best. This particularly applies to trucks that travel at low average speeds.
- A radius of 75mm will suffice to achieve a drag reduction at speeds of 80 kmph and higher. This applies to trucks that travel mainly on motorways.
- Ideally the sun visor should be built into the – rounded – horizontal roof edge, as this guarantees minimum airflow disruption while at the same time providing the cab with the rounding required for a continuous airflow around the roof edge.
A common aerodynamic modification made to cabs is the roof fairing/deflector, often wrongly referred to as a spoiler. These come in two types: (a) the roof deflector, which is a flat or curved (two-dimensional) plate with an adjustable angle; (b) the roof fairing, which is three-dimensional.
Roof fairings are more effective in crosswind conditions than roof deflectors, which is why deflectors are not used much nowadays; the majority of manufacturers opt to fit fairings.
The body or semi-trailer must be larger than the tractor cab, however, otherwise fitting a roof fairing/deflector will be detrimental, as it increases the frontal area. It is important to fit an adjustable type, as if it is too high it creates additional drag owing to the larger frontal area. If it is too low, on the other hand, it creates additional pressure drag on the top of the body or semi-trailer.
For a roof fairing/deflector to be fully effective at all incident flow angles it must line up well with the cab. The setting will depend on the height, shape and location of the fairing/deflector in relation to the roof edge of the body or semi-trailer. A good initial setting can be achieved by extending an imaginary straight line from the trailing edge of the fairing/deflector so that it passes above the roof edge of the body or semi-trailer. The smaller the distance G between the trailing edge of the fairing/deflector and the body or semi-trailer, the smaller the vertical distance H should be between this straight line and the roof edge. In other words, if the fairing/deflector extends to the container, its horizontal trailing edge should be in contact with the front roof edge of the container. A properly adjusted roof fairing/deflector yields a fuel saving of 5.9% for a rigid vehicle; a saving of 4% can be achieved for an articulated vehicle; and a drawbar vehicle uses 2.8% less fuel with a roof fairing or deflector.
Tip: A good way of optimising the initial setting of the roof fairing/deflector is to check the dirt deposits on the body or semi-trailer. Once the fairing/deflector has been in use for a while you should be able to see a darker band on the body or semi-trailer: this shows where the airflow reattaches to the body or semi-trailer. If there is no dark band, the fairing/deflector has been set too high. The biggest drag reduction is obtained when the dark band is barely visible at the centre of the roof edge and slightly more visible at the edges.
If the tractor is often used with semi-trailers of differing heights it is useful to mark the correct setting, so that the roof fairing/deflector can be adjusted quickly and accurately. When the tractor is running without a semi-trailer, or with a flatbed semi-trailer, it is important to set the fairing/deflector to the minimum height: this minimises the frontal area, resulting in less drag and lower fuel consumption.
It is important to minimise the gap and ideally to bridge it, but this is not possible in the case of some rigid vehicles because of the cab suspension. If the cab is unsprung it is advisable to fill the gap. The solution is what is known as a ‘collar’ (a streamlined piece bridging the gap between the cab and the body) in conjunction with a roof fairing. Like the roof fairing, the collar reduces the drag coefficient at all incident flow angles. This can make for an 8% fuel saving for a rigid vehicle and a 3.7% saving for a drawbar vehicle. It also improves the vehicle’s stability and appearance.
An articulated vehicle often has a large gap between the cab and the semi-trailer to enable the latter to turn, but this also creates a lot of pressure drag. We therefore recommend minimising the gap or ensuring that the air is guided past it. To reduce the pressure drag, tractor side panels can be fitted to the cab. Side panels are most beneficial with an oblique incident flow (crosswind). They prevent the air from entering the area between the cab and the semi-trailer, thus substantially reducing the drag coefficient. Fitting tractor side panels can yield a fuel saving of 0.7% for an articulated vehicle.
The tractor side panels must not interfere with the semi-trailer turning. To minimise the gap nevertheless, rubber strips can be fitted to the trailing edge of the panels, thus extending them as it were and reducing the gap to the maximum extent. Iveco have developed a concept for an inflatable collar that completely fills the gap between the tractor and the semi-trailer.
Tip: Flexible materials and inflatable structures guarantee the operational freedom of the semi-trailer.
The effect of the gap between the cab and the semi-trailer of an articulated vehicle can also be reduced by blowing air into it. This can be done by blowing air across the entire rear of the cab at low speed through a porous material. Recent numerical simulations and wind tunnel tests have shown that this technology can be more efficient than tractor side panels. There are some major issues with its practical application, however. To start with, it still needs to be investigated how much energy is consumed by the blowing system. There are also some uncertainties regarding the ultimate cost of the system and how often it needs to be serviced.