Fuel goals mean smaller cars

I suspect it’s going to be a lot more difficult for automakers to significantly increase fuel economy than we’ve been led to believe. Of course, being a paid cynic, my skepticism is not unexpected, just as I am sure the electric vehicle lobby is hardly surprised when I pooh-pooh yet another of its fairy tales. Nonetheless, the travails necessary to meet the Obama administration’s call for a 54.5 mile-per-U.S.-gallon (4.2 litres per 100 kilometres) Corporate Average Fuel Economy (CAFE average is going to shake the automotive industry — not to mention we consumers — right to the bone.

Even if that seemingly outlandish number, as explained in a recent Motor Mouth, really only translates (thanks to some truly cynical politicking) into a “real-world” 5.9 L/100 km rating, recent observations suggest that our desired fuel-sipping nirvana is not a mere efficiency improvement away. Smaller engines and lighter building materials will not, by themselves, deliver us from our reliance on fossil fuels. It is indeed going to be a tough slog, one that will require the average motorists to give up much more in creature comforts than protagonists currently purport.

Lost in my recent treatise detailing Land Rover’s seemingly outlandish claims of 6.3 L/100 km for its new hybrid-powered Range Rover was that the same car powered by its conventional supercharged V8 achieved but a 9% increase in fuel economy. Yes, despite losing an incredible 318 kilograms thanks to its revolutionary aluminum frame (the first ever for a production SUV), the big beast boasts but a 9% reduced fuel consumption. Making matters worse is that at least a part — most probably a large part — of that paltry gain has nothing to do with the weight loss but the lower highway revs attributable to its new eight-speed automatic transmission.

It’s impossible to know the transmission’s exact contribution (mainly because, in announcing the overall fuel economy improvements of their new automobiles, automakers hate crediting less marketable technologies such as multi-speed transmissions when they are trying to push sexier upgrades such as turbocharged EcoBoost engines), but a little sleuthing and some basic math suggests the increased number of gears is, by itself, good for about a 6% boost in highway fuel economy. That leaves but two or three points to be credited to the weight reduction, an incredibly small figure that illustrates that simply building lighter vehicles will not, in and of itself, result in huge fuel economy improvements — at least not in the highway cycle.

Nor will simply installing smaller engines in existing cars be the end all of fuel efficiency gains. Ford’s much ballyhooed substitution of a 3.5-litre twin-turbocharged V6 for its traditional V8 in pickups has been, from a fuel economy standpoint, at least, a flop. Not only were the Environmental Protectioin Agency (EPA) gains minuscule, but, according to one Consumer Reports test, there was no fuel consumption reduction whatsoever despite the displacement reduction.

Jaguar’s introduction of smaller- engine XJs in other markets further illustrates the limits of simple displacement reduction. While the new 3.0L supercharged V6 version (which we will get here in Canada) of its aluminum-bodied flagship sedan does indeed eke out some fuel economy savings (it averages a commendable 9.4 L/100 km in the EU’s overall test cycle), substituting an even smaller engine — the turbocharged 2.0L in-line four currently powering the Range Rover Evoque — reduces fuel consumption by just 0.1 L/100 km more (to 9.3 L/100 km). Yes, in reducing performance from the 340-horsepower V6 to the 240-hp four, one saves about three ounces of fuel for every 100 km driven. No wonder Jaguar has declined to bring the 2.0L XJ to North America (though the car is important in countries where taxation is based on engine size and not fuel consumption).

Why are the gains so paltry? One of the main reasons is that the most important fuel consumption criteria for highway fuel consumption is aerodynamics. While sheer weight is the determining factor in city fuel economy (and one presumes the new aluminum Range Rover will show greater gains in urban use), how efficiently a car cuts through the air at speed largely determines how much fuel it will consume. At a steady 100 km an hour, weight is hardly a determining factor. And, while engine efficiency is important, the largest gains are to be made with slippery shapes.

But here, too, there is more than meets the eye. Automakers have for quite some time misled us about the aerodynamics of their cars, not by outright lies but rather the classic fib of omission. While often bragging about the slipperiness of their cars — usually denoted as a co-efficient of drag — they have neglected to inform us that the c of d is but one half of the equation determining how efficiently their cars slice through the air.

The actual drag that air imposes on a moving car is actually a result of its drag co-efficient multiplied by the size (actually the frontal area) of the car. For those who’ve forgotten their high school calculus, that means that the size is as much a determinant of its highway fuel economy as its slippery roofline. Hence, a Toyota Yaris will always have less drag than a CLS 63 no matter how aerodynamically Mercedes-Benz crafts its coupe-like exterior and why, beyond a certain point (i.e., perhaps downsizing from a 5.0L V8 to a 3.0L V6), reductions in engine size alone have little effect on highway fuel economy.

On the other hand, weight is extremely important in the urban cycle. While pulling away from a stoplight, aerodynamic drag is hardly a factor. It is the act of accelerating its weight that is the determining factor (credit Sir Isaac and his force = mass x acceleration for exactly how much). It explains why a relatively heavy but sleek sedan such as the Audi A6 I tested recently can eke out a creditable fuel economy of 7.0 L/100 km while cruising the highway at 120 klicks yet suck back a miserable 14 L/100 km while crawling around the city.

Even if you haven’t followed all the physics (and, again, my apologies for sending you scurrying for calculators and textbooks), the fundamental message is that no singular change will garner the dramatic improvements in fuel economy we say we desire. To achieve the ambitious goals put forward by governments, automobiles will have to be dramatically smaller (for aerodynamic improvements on the highway), lighter (for less load during city driving) and powered by significantly smaller engines. Two out of three won’t do.

What the various proponents of the new fuel economy regulations are not telling us is that, to meet the new standards, the North American automotive fleet will have to look dramatically different than it does today if we are to achieve anything remotely like 4.2 L/100 km.

Indeed, if we do ever get there, I suspect the result will be that our typical car will much more closely resemble the European fleet, where the average car is a tiny 2.0L hatchback and where even what we call mid-sized (say, a BMW 5 Series) will seem gargantuan. Quite how we’ll get over our super-size-me automobile fetish, no one has explained.

Photograph by: Sean Gallup, Getty Images