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which is aimed to make CO 2 emissions AERODYNAMIC EFFECT ority. The legal provisions must also be
from trucks and its components measur- observed.
able, comparable and assessable. As a The aerodynamic resistance is generated The European Commission has
result, CO 2 limits for commercial vehicles by the resistance surface c D × A (resist- already taken steps towards utilisation of
will be set, which will pose additional ance coefficient multiplied by cross-sec- the rear wing potential, and revised
challenges to the industry to offer corre- tional area) and the dynamic pressure Directive 96/53 [5] for weights and
sponding products. Corresponding limits ρ/2 v² (air density and airflow velocity). dimensions. According to this measures
are already mandatory in USA, Canada, While the resistance surface in passen- will be in future permitted Europe-wide
China and Japan. ger cars is typically 0.6 to 0.9, in semi- to increase the length of the vehicle by
Fuel consumption is determined in trailer trucks it is around 5. This is due 500 mm to the rear and the width of the
particular by aerodynamic, roll, acceler- to the large front surface (A ≈ 10 m²) and vehicle by a total of 50 mm. The initial
ation, braking and other resistances of the high c D value (c D ≈ 0.55). This is the task was to design an effective wing
the drive train as well as auxiliary result of the blunt shape of the whole with a length of 500 mm – unlike in the
equipment. To determine the fuel con- vehicle, which produces particularly USA, where wings with a length of 2 m
sumption or CO 2 emissions of trucks large amounts of separated flow and may already be used.
individually, the European Commission swirling at the rear, which consume a lot Frequent and exhausting loading and
has developed the calculation pro- of energy. Large vortex areas are formed unloading, often involving manoeuvring
gramme Vecto [3]. The aim is a calcu- which nutate and transiently oscillate damage to the outer rear edges of the
lation base for the subsequent type around their stagnation points and gen- vehicle, are all part of the daily routine
approval process. Its results will show erate ring vortex structure over time. in the forwarding sector. An examina-
the shares in the total consumption in It is known that the energy consump- tion of damage has come to the conclu-
the respective driving situation and the tion of this turbulent area can be reduced sion that the rear wing system should be
effective savings achieved with the by deflectors mounted along the rear installed above the ramps and offset
selected improvement action. edges of the truck. As a result, the flow from the edges to the inside. At the same
FIGURE 1 shows the resistance distribu- is guided at an entrance angle into the time, the higher point of attachment will
tion of a typical long-haul driving cycle. rear area of the truck, thus creating a protect the other road users.
We can clearly see that the rolling and more favourable ratio of kinetic vortex The rear wings should unfold their
aerodynamic resistance makes up a large energy and distance to the rear surface; aerodynamic effect while the vehicle is
share in the consumption. While the in addition, pressure variations at the in motion, and be closed when manoeu-
reduction of unladen mass and rolling rear base are reduced. Wind tunnel vring or parking, so that they are not
resistance coefficient is now restricted by measurements and CFD simulations have damaged. This should not pose addi-
economic and physical limits, the aero- shown that the installation of a specially tional inconveniences to the driver and
dynamics gives more potential. Besides rounded rear wing reduces the c D value the driver should be able to open the
improvements to the towing vehicle, the by 10 %, and further improvements might doors in the usual way. Therefore the
optimisation of the trailer offers great also be possible. This is done with aver- rear wing system was designed in such
potential. Combined measures such as age value obtained under wind influence a way that it can work automatically.
slot and side covers as well as rear wing and taking into account wind distribu- A GPS receiver sends a speed signal,
can bring about fuel savings of 4 to 6 % tions and boundary layer conditions. with which the system opens automat-
according to estimates of the ICCT [4]. ically when the speed is above a thresh-
Own research by Betterflow and Edag REQUIREMENTS FOR old of 50 km/h for a long time and
indicates that, in the medium term, A REAR WING SYSTEM closes when it is correspondingly
improvements of about 20 % in the below 30 km/h. In the event of a fault,
c D value can be achieved by combining Besides the aerodynamic efficiency, the driver should be able to operate
measures. robust design and usability have top pri- the system manually.
To date we have had no practical and
robust solutions with corresponding
acceptance in the market. The systems
must run smoothly and without obstruc-
tions in the everyday forwarding busi-
ness to keep their economic promise.
Therefore, many challenges are to be
overcome, starting with the design and
readiness for serial production through
to the proven aerodynamic quality and
reduction of consumption in practice. In
cooperation with Edag, Betterflow has
designed and tested a truck rear wing
system in preparation for a subsequent
optimisation, series development and FIGURE 1 Resistance distribution of a truck for a typical long-haul driving cycle – a calculation is possible
production. with the EU programme Vecto [3] (© Betterflow, Edag)
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