MPG or performance: It's a trade with turbos

Some automakers say turbochargers offer high fuel economy and quick acceleration, the best of both worlds.

Well, yes and no.

Turbocharged vehicles allow automakers to keep power output high while they downsize engines for better fuel economy. But much of the mpg gain is lost when American drivers stomp the gas pedal and enjoy turbos' high performance.

Turbos will still grow in popularity as automakers reduce gasoline engine sizes to meet increasingly stiff corporate average fuel economy standards, reaching an estimated 25 percent of new vehicles in North America in 2017, LMC Automotive predicts. But turbos' limits as fuel savers leave plenty of opportunity for diesel, hybrid and electric powertrains in the coming years.

A glance at some turbo advertising from Ford Motor Co. shows the company is eager to promote the potential fuel savings of its turbocharged EcoBoost engines.

In the current EcoBoost Challenge campaign, one commercial shows an F-150 briskly towing a large boat as the pickup's EPA rating of 22 mpg on the highway flashes on the screen. Another spot boasts crisp acceleration for the Escape crossover and 33 mpg on the highway.

The EcoBoost name itself implies that a turbocharger combines high fuel economy (eco) with performance (boost).

At Chevrolet, a large banner on a brand Web site recently touted the Cruze Eco with a 1.4-liter turbocharged engine as offering "42 mpg and sacrificing nothing," suggesting that the car can deliver that mileage even if the driver uses the turbo frequently.

Real world intervenes

But despite the marketing for turbos' high fuel economy, the numbers in the real world often fall short.

"When people think of turbos, they think of performance, but the faster you drive, the worse the fuel economy," says Mike Omotoso, senior manager of powertrain forecasting at LMC Automotive.

Consumer Reports recently tested 11 turbocharged vehicles from seven automakers to see if the vehicles delivered on fuel economy claims. The combined city-highway fuel economy for all of the vehicles came in lower than the EPA estimates by a few miles per gallon.

In addition, consumers typically pay at least $1,000 more for a turbocharged engine. So, buyers not only pay more but are likely to get lower fuel economy than with a regular engine, the magazine concluded.

Moreover, in some cases nonturbo engines can get higher fuel economy than turbocharged engines.

Honda quietly dropped the 2.4-liter four-cylinder turbo engine in the 2013 Acura RDX and replaced it with a bigger nonturbo engine that gets better fuel economy. The turbo engine, EPA rated at 19 mpg city/24 highway/21 combined, was replaced by a 3.5-liter V6 that is EPA rated at 20 mpg city/28 highway/23 combined.

The editors at pickuptrucks.com put two Ford F-150s in a towing test, one with the V6 EcoBoost twin turbo engine, the other with a 5.0-liter V8. They got better fuel economy with the larger engine, 9.4 mpg for the V8, compared with 7.2 for the V6 EcoBoost.

In online forums, many buyers of vehicles with turbocharged engines say they are satisfied with their fuel economy. But some are grumbling.

On Green.Autoblog.com, one turbo-charged Cruze Eco owner wrote that his fuel economy has varied from 29 mpg highway in cold weather to 35 mpg, lower than the car's 39 mpg highway rating. But another owner tested his car extensively on the same roads at the same speeds and claims his Cruze achieved 47 mpg.

Industry analyst Jim Hall of 2953 Analytics in suburban Detroit says that the EPA window label number is frequently inaccurate. Calculating accurate real-world fuel economy figures that apply to most drivers of turbo vehicles, Hall said, is like asking: "How much does a bag of groceries cost?"

On F150forum.com, an EcoBoost F-150 owner named Detox wrote, "I get 17 city and 20 highway with light foot. Real driving, I get between 15 city and 17 highway mpg." The F-150 EcoBoost, the top-selling turbocharged vehicle in the United States, is EPA rated at 16 mpg city/22 highway.

Tkevin1 wrote on Edmunds.com: "I purchased a truck with the EcoBoost engine a couple of months ago and am now approaching 3,000 miles. I have had zero issues and in mixed driving have averaged around 19 mpg and have gotten as good as 24 mpg at 75mph on the interstate."

Ford spokesman Mike Levine says F-150 customers are satisfied with turbos. "Fuel economy is the top purchase consideration for EcoBoost customers," he says. "Data we have indicates that of all F-150 drivers, those with EcoBoost engines are most satisfied with their fuel economy, rating themselves 'very satisfied.' In fact, F-150 leads fuel economy satisfaction among all light-duty, nonhybrid pickup truck customers."

Ford has been the most aggressive with turbos. About 80 percent of its North American nameplates are available with an optional EcoBoost engine, which combines a turbocharger with direct fuel injection and other technologies.

Ford has sold more than 600,000 vehicles in North America with small turbocharged engines under its EcoBoost brand name. More than 400,000 of those have been one model, the F-150.

Small engine, heavy vehicles

So why do turbocharged vehicles often fall short of their EPA ratings?

The turbocharger, mounted in or near the exhaust manifold, is powered by exhaust gases that spin a small turbine at very high speeds. The turbine runs an air pump that blasts a denser mixture of fuel and air into the engine's cylinders, enabling a smaller engine to deliver more power.

But when the turbocharger is running, fuel economy declines, sometimes dramatically.

In addition, Omotoso says some automakers are gaming the EPA's mpg rating system by taking advantage of the agency's test procedures. He said some vehicles are being calibrated to deliver the highest fuel economy label number based on EPA test procedures, which may not match real-world driving conditions.

The EPA's highway fuel economy test, for instance, is run for about 12 minutes, has a top speed of 60 mph and an average speed of 48.3 mph. Such low speeds would produce a high mpg rating for a turbocharged engine.

"You get into a game that the only numbers that matter to car companies are the EPA numbers because if you don't get good numbers you don't sell cars," Omotoso said.

Also, small engines in large vehicles, such as the 2.0-liter turbocharged four-cylinder in the 4,600 pound Ford Explorer, often lack the quick acceleration that drivers expect when the turbo isn't running. So drivers use a heavy foot, run the turbo frequently and erode fuel economy.

Automakers could help drivers improve the fuel economy of their turbocharged vehicles with an educational campaign about driving with a light foot. But so far, no automakers are attempting such a campaign. Ford, GM, Chrysler, Hyundai, Volkswagen, BMW, Mercedes-Benz and others are offering turbo four-cylinder engines in mid-sized vehicles in place of V6s.

Analyst Hall sums up the inherent dilemma of turbochargers this way: "EcoBoost is great. The customer gets eco or boost. And he gets to choose which one with the throttle pedal."