How Chevy coaxed Cruze diesel to 52 mpg

To earn the EPA's rare 50-plus highway fuel economy rating for the 2018 Cruze diesel sedan, Chevrolet engineers didn't just drop a fuel-efficient engine and transmission into the Cruze and call it done.

"There is no one silver bullet," says Craig Weddle, Cruze chief engineer. "There were a lot of little things we had to do to make it all add up."

It was a significant technical feat.

Last week when the EPA confirmed the Cruze diesel's 52-mpg highway fuel economy rating through its own testing at the agency's Ann Arbor, Mich., labs, the Cruze became one of only four non-hybrid cars to punch through the 50 mpg highway barrier since 1990.

And after the EPA revised its fuel economy test procedures in 2008 to better reflect real-world fuel economy, only one non-hybrid car since 1990 -- the Geo Metro XFi, also sold by Chevrolet -- has retained a 50-plus mpg rating. (The EPA has revoked certification and mileage ratings for all VW diesels affected by the emissions cheating scandal).

"Fuel economy is the reason for purchase, and we were looking for a wow factor," said Dan Nicholson, GM's vice president of Global Propulsion Systems.

The Cruze's results took a total General Motors team effort that drew in aerodynamicists, electricians, powertrain engineers, lightweighting specialists, friction gurus, calibration engineers and others.

The Cruze diesel, on the way to dealers now, even has a chief energy engineer, Eric VanDommelen, whose goal was to make sure every pound-foot of torque sent to the wheels delivered maximum efficiency and that few, if any, electrons were wasted.

Much of the technology used to boost the car's fuel economy, such as the smooth underbody, low-drag brakes and active grille shutters, came straight from hybrid cars.

GM will have its challenges marketing the fuel-sipping diesel. The car is part of a big push at GM to attract Volkswagen buyers leaving the brand in the wake of the diesel emissions cheating scandal there. That won't be easy. Gasoline is cheap and consumers are deserting cars for SUVs, crossovers and trucks. Diesel engines, thanks to VW's controversy, now have image issues.

It's also not a given that VW buyers will consider switching to Chevrolet.

Nicholson said in August that GM would be going after VW's diesel faithful, who once accounted for about 20 percent of the German brand's U.S. sales. A diesel version of the Equinox crossover is also on the way, and by the end of 2018, GM will offer 10 diesel models in North America. Nicholson said last week diesel vehicles represent a growth opportunity for Chevrolet.

But none of that mattered to the engineers who tuned and tweaked and prodded the Cruze diesel until it rolled over the 50-mpg goal line.

Unlike the cramped, stripped-down '90s-era cars that got 50 mpg or close to it, the Cruze diesel sedan is smooth, quiet, comfortable and fully equipped with safety features and electronic gear.

"Certainly having a number that started with 5 was enticing, but we didn't sacrifice the rest of the car to get that," Weddle says. He added there was no pressure from upper management to hit 50 mpg.

"It was really about producing a great car with fuel economy that we thought would excite our expected diesel customer base," he said.

AutoPacific analyst Dave Sullivan calls the 52-mpg highway fuel economy rating for the Cruze diesel impressive. He notes engineers struggle for every 10th of a mile fuel economy gain and that the new Cruze diesel not only beats the old model by 6 mpg, but it bests the highway rating of the 2015 Cruze Eco, a gasoline-powered model built for high fuel economy, by a full 10 mpg.

"Other cars need two powertrains to hit anything in the 50 mpg range -- I'm talking about hybrids," says Sullivan. "Now you can get hybridlike fuel economy without the stigma of being seen in an oddly styled hybrid."

The biggest parts of the Cruze's 52-mpg highway rating came from three areas: weight reduction, aerodynamics and the powertrain. 

To start with, the current-generation Cruze, introduced last year, weighs about 250 pounds less than the 2015 version, mostly because of increased use of lightweight high-strength steel in the body, and aluminum in the suspension components. 

"We put the mass where it matters to get the stiffness in the structure, but thinned it out anywhere where it is not absolutely needed," Weddle says. 

Engineers also booked a 33-pound weight reduction under the hood. The old car's 2.0-liter diesel engine had a cast-iron block and weighed 368 pounds installed. The 2018 diesel's 1.6-liter engine has an aluminum block with cast iron liners and weighs 335 pounds. 

Another key piece of the Cruze's fuel economy puzzle is its manual transmission, which requires less energy from the engine to power the wheels than the available nine-speed automatic. The six-speed manual also weighs less than the automatic.

Improved aerodynamics were another important part of the equation. Many of the aerodynamic tweaks GM engineers made to the car carry over from the gasoline version.

In front of each tire, for example, a black plastic deflector pushes air away from the wheels. The Cruze also has active grille shutters, which adjust to the engine's cooling needs. When the shutters are closed, the aerodynamic efficiency is improved as air is routed over, under or around the car. Headlights, taillights, door handles and exterior mirrors saw plenty of wind tunnel time.

The 2018 Cruze diesel's aerodynamic drag coefficient -- a measurement that quantifies aerodynamic efficiency -- is 0.28, down from the 0.30 of the 2015 Cruze diesel. The lower the number, the less energy it takes to push the car through the air. 

VanDommelen, Cruze's chief energy engineer, said other tweaks, including using low-friction bearings in the drivetrain and wheels, tires that have lower rolling resistance than those used on the first generation Volt, and managing energy usage also helped attain the 52-mpg rating. 

Even though GM's internal testing showed the Cruze well over 50 mpg on the highway, the champagne corks stayed in the bottles, VanDommelen said, until the EPA confirmed it. 

"We had some hopes, and we're looking forward to hitting that goal. But I didn't want to really believe it until I saw it from the EPA. Once they confirmed it, we felt pretty good," he said. 

Nicholson, GM's powertrain boss, said engineers weren't sure the Cruze would attain the 50-plus rating until late last year, near the end of the car's development cycle. "We were careful not to put pressure on the team," he said, "We wanted them to focus only on delivering the best car for the customer." 

VW diesel owners had routinely beaten the EPA label numbers in their actual driving. And Sullivan thinks Cruze diesel drivers can get even higher than the 52 mpg highway rating. 

"Most consumers have been able to beat the EPA numbers with small displacement diesels," Sullivan says. "If Cruze owners can beat the EPA number, it might make GM the new diesel leader in a market that has been leveled to nothing."