Aerodynamic experts at General Motors are cautiously optimistic that a long-awaited leap in vehicle fuel efficiency is advancing toward near-term production: the replacement of the two exterior mirrors with camera-based streaming-vision technology—at no overall cost penalty.
The gains from eliminating the drag-inducing mirror assemblies “will be significant—on the order of nine to 10 counts of aero per mirror,” said Scott Miller, GM's Director of Global CO2 Strategy, Energy, Mass, and Aerodynamics, at the recent opening of a new reduced-scale wind tunnel at the company's Technical Center in Warren, MI.
Miller noted that about 1 mpg in real-world vehicle fuel efficiency is typically gained by each 12-count reduction of aerodynamic drag. So removing the two exterior mirrors used on new road vehicles would deliver, on average, a 20-count drag reduction, adjusting for vehicle frontal area. "That's worth an easy 1½ to 2 miles per gallon,” he explained.
Speaking with Automotive Engineering and other media at the wind tunnel event, Miller said he expects the exterior mirror will be replaced “fairly soon,” without adding specifics. When asked if going to vision systems will add cost, Miller replied that the vision systems can be brought in at “pretty close to cost parity” because the cost of current screen-based display technology is coming down steadily.
“From our [GM’s] standpoint, and from my area, it’s a win-win,” he said.
One “count” of aerodynamic drag is equal to 0.0001 Cd. Drag coefficient (Cd) is a dimensionless parameter used to quantify the drag or resistance of an object in a fluid environment such as air or water. For automobiles (and various other objects) the reference area is the projected frontal area of the vehicle. The product of the Cd and frontal area is the optimum measure of a vehicle’s aero efficiency.
In the book Aerodynamics of Road Vehicles published by SAE International (http://books.sae.org/r-177/), author Wolf-Heinrich Hucho notes that exterior side view mirrors contribute to the vehicle’s total aerodynamic drag by an average of 2 to 7%.
The vision-system technologies needed for a mirrorless driving future are increasingly being added to new vehicles as key elements of the new-generation safety and navigation systems, the experts acknowledge. Various OEMs and Tier 1 suppliers also have shown advanced concepts that eschew mirrors for onboard camera arrays and flat screen monitors.
At the 2015 Frankfurt Motor Show, Valeo and Continental unveiled camera-based vision systems designed to replace conventional side-mounted mirrors to help improve vehicle aerodynamics. Valeo claims its “Sightstream” system enables automakers to eliminate up to 1.3 grams of CO2 per kilometer, while reducing cabin noise levels. Continental had a demonstrator vehicle equipped with three cameras and an array of flat-screen displays to enable the driver to easily monitor the vehicle's sides and rear area.
Tesla’s new Model X, now entering production, was designed to have no exterior mirrors and used small cameras in the mirrors’ door locations. Only current federal and state requirements for traditional fixed glass mirrors kept it off the road, for now. And the most advanced production vehicles are approaching a transition phase: Honda uses an outstanding passenger-side camera that displays a view down the vehicle’s right side on the instrument panel screen when the right turn signal is switched on. GM itself has a new Rear Camera Mirror system that debuts on the 2016 Cadillac CT6, to be followed by the 2017 XT5 crossover.
Vehicle engineers have some cause for hope on the regulatory side. In North America, the Alliance of Automobile Manufacturers (AAM) last year petitioned the NHTSA to change the 1968 law requiring passenger vehicles to have a mirror on the door and another inside on the windshield and instead permit cameras to replace them, with the goal of improving side vision and increasing fuel efficiency. The AAM recommends that any change to the federal law would override state laws. The NHTSA has not yet made a ruling, but the advancing technologies and growing pressure on the industry to increase both occupant safety and fuel efficiency are major factors influencing this important front.