Like a hot knife through butter

The science of aerodynamics is crucial in ensuring good economy and comfort from a truck. Needless to say, the application of it has evolved somewhat over the years. GAVIN MYERS explores the topic

The world has long been in the age of efficiency and environmental friendliness. Fewer and fewer countries tolerate old rust buckets that belch smoke and disturb the peace as they wheeze towards their next breakdown…

(Sadly, South Africa is one country that seemingly sees nothing wrong with this scenario, but that’s a topic for another day.)

Stricter legislation has meant engineers have had to push the limits of what is viable for market consumption, but it is not only about making engines more frugal, cleaner and quieter. The design of the vehicle is a huge contributor to the ease with which it moves through the air – advanced aerodynamic design will mean the vehicle can more easily part the air, which means it uses less fuel to do so, making it more environmentally friendly and quieter for the occupants.

The concept of aerodynamic drag reduction is not new. Streamlined vehicles were all the rage before the Second World War.

In his paper entitled Commercial Vehicle Aerodynamic Drag Reduction: Historical Perspective as a Guide, Kevin R. Cooper of the National Research Council of Canada notes the example of the 1947 Labatt Streamliner.

The Streamliner was created by the Labatt Brewing Company, mainly for advertising purposes and to provide larger capacity and higher cruising speeds. The truck could cruise at 80 km/h (at a time when most trucks were comfortable at 56 km/h), with a 50-percent larger load.

Over time, an increasing number of studies were conducted on the effects of aerodynamics on commercial vehicles and how to reduce drag. In their report: A Study on Aerodynamic Drag of a Semi-Trailer Truck, Harun Chowdhury, et al, of the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australia, state that heavy commercial vehicles are considered aerodynamically inefficient compared to other ground vehicles, due to their un-streamlined body shapes.

“Aerodynamic drag with a semi-trailer truck typically accounts for 75 to 80 percent of the total resistance to motion at 100 km/h. A large commercial vehicle travelling at 100 km/h consumes approximately 52 percent of the total fuel to provide power to overcome the aerodynamic drag. Therefore, reducing aerodynamic drag contributes significantly to the fuel economy of a truck as well as the reduction of greenhouse-gas emissions,” the report states.

Over time, remedies have been sought. Both recently and in the past we’ve seen some magnificent-looking, purpose-designed vehicles – like the famed Walmart Advanced Vehicle Experience, or the Labatt Streamliner.

A more common approach is to add aerodynamic devices to a standard vehicle. The prime mover is optimised to part the air, while deflectors, skirts and extensions on the cab and trailer help close gaps and manage the airflow; allowing the air to pass uninterrupted around and past the combination as it moves along.

In some cases these devices alter the size and shape of the trailer, for example, to a teardrop shape. This returns significant improvements – although at the sacrifice of cargo volume, which many operators will not overlook.

In their paper entitled Aerodynamics of Commercial Vehicles, Thorsten Frank and James Turney note: “Today, the optimisation of aerodynamics for commercial vehicles must not be restricted merely to the installation of a device on the roof of the cabin. Rather, the vehicle must be regarded as a complete system and optimised accordingly. Aerodynamic optimisation must be balanced against such considerations as styling, ergonomics and soiling (the water and dirt kicked up by and/or deposited onto a vehicle).”

Chowdhury, et al sought to quantify this. They placed a 1:10 scale model of a semi-truck in a wind tunnel and experimented on the effects of six different aero kit configurations on the truck as well as trailer.

“The results indicated that partially covering the gaps can enhance the performance for drag reduction, whereas full covering indicated the maximum drag reduction. The aerodynamic fairings have a notable impact on aerodynamic drag. The front fairing alone can reduce around 17 percent of drag.

“Further drag reduction of up to 26 percent is possible using various combinations of aerodynamic fairings in different parts of the truck body. Thus, the semi-trailer truck with maximum amount of surface area covered can enhance the drag-reduction performance,” they state.

However, in designing mass-market trucks for everyday use by every customer, original equipment manufacturer (OEM) engineers face many restrictions and compromises…

“OEMs spend vast sums on making their vehicles more aerodynamic, but they are hamstrung by legislation,” comments Cameron Dudley-Owen of Aero Truck. “I have no doubt that their vehicles would look radically different if they had carte blanche to design solely for aerodynamic efficiency.

“This is clear if one looks at the configuration of the Tesla Semi truck tractor compared to current designs of European OEMs. Significant advances in the effectiveness of aerodynamics, and consequent fuel savings, can be made if legislation were to be amended to accommodate such advances,” he adds.

This quandary also applies to OEM aerodynamic kits, which need to be designed to accommodate a broad spectrum of applications. “OEM aerodynamic additions are a compromise, because it is impossible to design the kits to cater for every aftermarket application for which a truck may be used,” Dudley-Owen says.

Similarly, in South Africa, the maximum length of an articulated superlink configuration is 22 m. This means that the addition of boat tails to the rear of the trailer, which create a significant reduction in drag as the air separates from the trailer body, is illegal.

“Trials have shown that these can add a further two-percent reduction in aerodynamic drag, which forms in the vortex behind the trailer, but again, these are not yet legal on European or South African roads. This is a pity since this vacuum causes significant drag once the vehicle reaches operating speeds of 80 km/h,” Dudley-Owen explains.

A report by the Chalmers University of Technology references on-road tests by the Society of Automobile Engineers (SAE) in 2006, which showed fuel savings of around ten percent from a semi-trailer fitted with a similar box section at the rear. The same report references a study of active and passive airflow control, in which the former returned a five-percent reduction in fuel consumption over the latter.

According to Dudley-Owen, “active-aero” is a real buzzword in the industry right now. However, such devices are hardly viable in an everyday transport application. There are many which are, though.   

Aerofins can be fitted at the rear of trailers to reduce the vacuum (drag) that forms behind the trailer, while under-trailer panels can act to smooth the airflow under the vehicle and around axels and any other protrusions.

“The Tesla truck tractor, for example, has a completely smooth underbelly. These have not yet been adopted by any prime movers, due to concerns with OEM warranties,” comments Dudley-Owen.

For aerodynamics to truly advance to the point where maximum benefit can be derived with the least amount of payoff, it will, however, be up to legislators to allow for improved configurations to be put on the road. According to Dudley-Owen, this would yield the most dramatic effect.

“In the interim, operators need to take cognisance of the aerodynamic needs of their specific applications. A truck tractor costs in the region of R2 000 000, whereas a custom-designed wind-deflector kit comes in at around
R18 500. This one percent extra spent on fitting aerodynamics will save many thousands of rand on fuel over the life of the vehicle. This is not a decision to be made on upfront cost, but rather on the return on investment that can be expected over the life of the vehicle,” he suggests.

The good news is that Dudley-Owen has noticed a trend of more operators accepting the benefits of fitting aerodynamic aids to their vehicles. Hopefully this attitude also means fewer belching rust buckets on our roads.