Tractor-trailer design is significantly influenced by legal conditions with regards to the vehicle dimensions and the provision of a maximum transportation volume. These boundary conditions bring about brick-shaped trailer outer geometries, especially with the rear ends. That is why investigations of aerodynamic optimization of commercial-vehicle (CV) trailers are predominantly tied to detail measures.
Researchers from Graz University of Technology studied the aerodynamic characteristics of general modifications for the outer contour of long-distance haulage trailers, including a new approach for the realization of any variable trailer rear end. The variable adjustment of your sidewalls and the top of the trailer’s rear end might take the actual space requirements in the transportation load into account, thereby resulting in optimal transportation efficiency.
According to the European legislation the aerodynamic studies were based on truck models. Because the investigations focused on the back end in the trailers, the findings are applicable for the aerodynamic characteristics of trailers in different markets, including the U.S.
The initial step of the research work entailed the derivation of a generic virtual reference vehicle, which was created being a 3D-CAD model. Therefore, several cab-over-engine semi-trailer tractors for long-distance haulage from six different European truck manufacturers were evaluated and compared. With this investigation, the configuration and dimensions to have an average tractor were determined. The same procedure had been performed to collect information of typical semi-trailers with curtain-side configuration for long-distance transport.
The semi-trailer truck optimization process was split into two main parts: a coarse geometrical concept study by use of a simplified vehicle model, plus a detailed study based on a very high-resolution reference truck model.
The coarse concept study was based on a simplified reference model, developed in the shape of your single-volume model. During this study, principal geometrical modifications were performed with the outer contour of the semi-trailer. Following that, the air drag of each simplified model variant was assessed by putting on 3D-CFD simulations by utilization of a commercial software.
For the coarse concept study, 10 different trailer variants with varying payload-space V were created. The simulation results consider the payload space in comparison to the reference configuration R. The variant models 1 to 8 were modified by making use of different vertical tapers on the semi-trailer. Unlike those, the semi-trailer variants 9 and 10 ended up being equipped with horizontal boat tails as additional modification. On account of this study, each geometrical modification decreased air drag coefficient, but also reduced more or less the semi-trailer’s payload space.
The coarse study demonstrated that geometrical modifications of the trailer had a big impact on the aerodynamic resistance. The researchers selected two variants (models 9 and 1) having a significant lowering of air resistance for further detailed aerodynamic studies. Besides, those two variants were selected because of the comparative small decrease of payload space, the reduction of air drag.
The detailed study was performed to validate the results of your coarse concept study on account of higher simulation resolution as well as to compare the simulation results with the characteristics of present market-available semi-trailer trucks. The geometry of your detailed generic reference truck model was modified to make two optimized truck configurations with a high degree of geometrical information.
Out of the shape of selected model 1, the optimized Truck A was designed using a vertical taper of 20 in (508 mm) at the semi-trailer’s rear end. Model 9 (with an additional boat tail) was used as a template for the detail study Truck B. The visible difference between the geometries of these two configurations was the horizontal boat tail of 10 in (254 mm) on each side on the semi-trailer of Truck B. To enable an assessment of the modification’s impact on the trailer rear end, the tractor model remained the same during all variations.
Finally, the aerodynamic drag characteristics of Truck A and Truck B were assessed in detailed 3D-CFD simulations by application of the same boundary conditions because they were applied during the simulation of the generic reference truck.
In contrast to Truck A, the reference truck shows a large irregular dead water area behind the rear end from the semi-trailer for its brick-shaped outer contour. The aerodynamic-optimized form of Truck A enables the fluid an even changeover for the road surface, which leads to a reduced dead water area behind the rear end. As a result of Truck A trailer’s bent roof, the flow is subjected to a deceleration that further increases the static pressure at the back panel as opposed to the reference. Simultaneously, the influence of your static pressure was reduced by the downsized effective vertical area at the rear end. The result can be a significantly lower air resistance force from the driving direction.
Results-and the variable trailer rear-end concept
The fuel consumption of each variant was calculated in longitudinal vehicle dynamics simulations by use of predefined engine characteristics of a typical semi-trailer truck engine. The optimized semi-trailer trucks, Truck A and Truck B, obtained a declination in the aerodynamic drag coefficient between 15% and 23% associated with the reference truck. On account of these significant aerodynamic improvements, a reduction of fuel consumption up to 6.5% by Truck A and 10.2% by Truck B could be reached. In contrast to the decrease of fuel consumption, the payload space was comparatively slightly reduced, between 3.2% (variant Truck A) and 6.1% (variant Truck B).
The transport efficiency of CVs is determined by their payload and different cost factors. Regarding the results of these two optimized truck configurations when it comes to aerodynamics, a reduction of fuel consumption and thus a reduction in the operating costs may be reached. On the other side, the payload space shrank, because of the geometrical modifications with the trailers’ rear ends. In this manner, an improvement of the transport efficiency depends on the particular payload, or more exactly on the freight density.
A large number of trips is carried out having a payload that does not require the entire trailer volume. A tremendous share of trips is even performed without payload (in the United states, the range is 10-13%). This leads to the cognition the overall transport efficiency might be raised significantly by use of a variable trailer rear end.
A fresh approach has become developed to support high transport efficiency of future long-distance haulage trailers. With this approach, it is easy to adjust the trailer outer contour in accordance with the required transportation volume, combining both advantageous aerodynamic behavior and improved transportation efficiency.
The researchers presented a lowered trailer rear end containing an exemplary load configuration with prismatic packets. The complete mechanism may be built into the vertical stakes of a typical trailer and does not enlarge its dimensions. In this configuration, 95% of the total payload space is available for transportation purposes. At the same time, the aerodynamic drag of the truck combination is reduced about 6.5%.