Using aerodynamics to reduce fuel use
The ongoing quest to achieve seemingly impossible fuel-efficiency targets that are scheduled to tighten progressively from now until 2025 is reshaping the cars we drive. Literally. Huge progress is being made through weight reduction measures and improved powertrain efficiency — both have been well documented and widely advertised.
But it won’t be long until gains in both those areas are well into the realm of diminishing returns, where smaller and smaller gains are incurring greater and greater costs. We may be there already.
So what avenues are left? There’s just one with the potential for significant payback: aerodynamics.
While the impact of aero efficiency is minimal in urban driving situations, it becomes a significant factor at highway speeds, in terms of both fuel-consumption rating determinations and real-world fuel economy.
According to some calculations, aerodynamics may account directly for less than 5 per cent of fuel used in city driving but for more than 60 per cent of total consumption at expressway speeds.
It was an error in calculating aerodynamic drag that forced the muchpublicized correction of advertised fuel-consumption figures by Hyundai/Kia back in 2012, and subsequently prompted some other automakers to do the same.
So the importance of reducing aerodynamic drag as a tool for reducing fuel consumption is well-recognized by automakers and most, if not all, are already well down that path.
There are two key figures that quantify the aerodynamic drag of a vehicle — its coefficient of drag (Cd) and its cross-sectional area when viewed from the front.
Cd is a dimensionless number that defines the vehicle’s slipperiness, independent of the vehicle’s size. Its total drag is the product of the Cd multiplied by the frontal area, so smaller is better.
In most cases, frontal area is effectively determined by the vehicle’s type and size class, so reducing Cd is the primary target when it comes to improving aero efficiency.
To put today’s progress in perspective, in 1984 the newly-introduced Mercedes-Benz E-Class boasted a Cd of 0.29 — arguably the first sedan to break the 0.30 barrier.
Thirty years later, the Mercedes-Benz CLA (in Euro spec 180 Blue-Efficiency form) sets the benchmark with a Cd figure of 0.22 (it’s just 0.23 in North American trim).
“We fight for every third decimal place,” says Dr. Teddy Woll, head of the aerodynamics/wind tunnels department at Daimler AG. “It’s a very simple matter. If we are able to reduce the Cd figure by ten-thousandths (.010), fuel consumption across the customer average falls by one-tenth of a litre, and at very fast motorway speeds by up to 0.4 litres per 100 kilometres.”
Achieving the same fuel saving with lightweight construction methods, Woll says, would require a mass reduction of at least 35 kilograms.
While we may think of aerodynamics in terms of what we can see, Woll says the three areas of greatest progress in recent years have been in airflow through the engine compartment, airflow around and within the front wheel wells and airflow beneath the vehicle.
He cited the example of an eleven-thousandths Cd reduction in the front wheel arches of the CLA-Class cars thanks to the combined effect of serrated wheel spoilers, slits in the wheel arches and optimized aero-wheels.
Such is the detail level where many aero gains are being made today. Several vehicles, including such low-priced models as the Chevrolet Cruze and Dodge Dart, incorporate active grille shutters to reduce drag when extra cooling is not required.
Ford’s monstrous Atlas pickup truck concept, introduced at the 2013 Detroit auto show, featured active wheel shutters that closed at highway speeds to seal gaps between its spokes, reducing turbulence around the wheels and tires.
While wind tunnels can provide an overall measure of aerodynamic drag, much detailed development is now being done in the virtual realm of Computational Fluid Dynamics. CFD combines the use of applied mathematics, physics and fluid flow theory with powerful computation- al software and detailed component mapping to visualize how air flows over, around or through objects, based on equations that describe how the velocity, pressure, temperature and density of a moving fluid are related.
It sounds complex. It is. But it’s opening doors to incremental reductions in drag in areas where aerodynamics weren’t even considered in the past. Those gains are adding up, a thousandth at a time, to enable hard-won but meaningful improvements in fuel efficiency.
Gerry Malloy is a regular contributor to Toronto Star Wheels. To reach Wheels Editor Norris McDonald: nmcdonald@thestar.ca