Semi-truck: working aerodynamism to perform

“Revising the aerodynamics of a semi-trailer has an impact on the vehicle’s consumption and operating costs, and therefore inevitably on the total cost of transport.” Olivier Dutrech, Director of Innovation and Fraikin Business Solutions.

Because a vehicle moving through the air inevitably leaves a drag, this resistance requires greater fuel consumption, so it is necessary to work on its aerodynamics to perform well. To find out the aerodynamic coefficient of a semi-trailer, you need to calculate its Cx coefficient and multiply it by the size of the surface of the vehicle entering the air. Once the coefficient of a semi-trailer has been determined, it is improved in order to reduce it.

“For long-distance journeys, it is interesting to place fins -flaps- at the end of the trailer, on the doors, at the rear of the body and on the sides thanks to an aerodynamic side skirt. This spoiler solution has a real potential as the costs are reduced in terms of installation and the impacts on fuel consumption are very quickly beneficial. To measure the impact of these devices, we are working in the field by running courses with and without fins, in the same driving and weather conditions, to note the differences. Our full-scale test proves that fuel consumption with fins is reduced by 3 to 4%!”explains Olivier Dutrech.

The drag coefficient, whose symbol is Cₓ, CA or CD is part of the family of aerodynamic coefficients. It is a dimensionless number that is used to quantify the drag or resistance of an object in a fluid, such as air or water

Each innovation must be confronted with the regulations in force, Olivier Couderc, product and service manager at Fraikin, moderates: “Everything must be studied because by adding length to the semi, especially with the flaps, we realise that this increases the length of the vehicle. Indeed, despite a recent evolution, the regulations related to vehicle dimensions are strict and must be respected.

In order to limit the creation of air vortexes at the wheels of the semi-trailer, “skirts” can also be fitted to cover the sides of the semi-trailer. A fairing that immediately has a positive impact on the aerodynamics of the semi-trailer.

The shape of the body itself can be worked on so that the air flows are better directed and the air drag is reduced. This is known as a teardrop shape at the front of the semi-trailer. “The air is guided by the hump at the front of the trailer and follows the streamlined shape of the body. It is necessary to keep the same useful volume as for a ‘normal’ semi, taking care that the ‘front’ part is larger than the ‘rear’ part to maintain this sufficient volume,” adds Olivier Courderc.

Bodywork: reduce the weight of the semi-trailer for optimised fuel consumption and ensure perfect insulation

The aerodynamics of a semi-trailer also involves reducing the weight of the body and therefore re-designing its bodywork. There are two major challenges: to lighten the body so that the road tractor saves fuel, but also to maintain the interior temperature of refrigerated semi-trailers.

The insulation coefficient “K”, which expresses the temperature management ratio, is very important for refrigerated trailers. The better the K-value, the less energy is consumed by the refrigeration unit that supplies the box.

Olivier Dutrech explains how Fraikin’s partner bodybuilders, such as Chereau, are working on the technology of the body walls: “The new generation of bodywork puts forward a vacuum insulation technique which allows to obtain a very good quotient but also better results in terms of environment. This technique is revolutionary because it not only achieves an excellent K-value but also reduces the width of the walls and increases the payload. Traditional techniques that rely on layers of different materials to keep the temperature down actually generate much more volume within the walls of the box.”

The Chereau Group also explains that the external colour of the walls plays an important role: “Radiation has a warming effect on bodies exposed to the sun, which leads to overconsumption of the refrigeration unit. To limit the effect of this radiation, the basic colour of the bodywork is bright white, which gives it excellent reflectivity. We recommend that no dark coating is applied to the bodywork to avoid impairing its performance.”