From Small Steps to Giant Leaps in MPG

5/29/2015

Spring 2015

The new-generation Subaru Legacy 2.5i has made dramatic gains in fuel efficiency. Beyond its enhanced SUBARU BOXER engine, the Legacy benefits from a sleeker design and reduced friction throughout its mechanical components. Here’s the inside story on these advancements.

 

It takes a lot of little changes to make a big difference. That’s how engineers and designers have been able to increase the Subaru Legacy’s EPA combined fuel economy estimates by 36 percent in the last six years.1 Assuming that you drive 12,000 miles a year, trading in your 2009 Legacy for a 2015 model could save you 145 gallons a year. Even at today’s prices, that’s a big savings. And keep in mind we’re talking about an All-Wheel Drive (AWD) vehicle, with all the traction and control for which Subaru is renowned.

So how did this dramatic increase in efficiency occur? One small step at a time.

RESISTANCE IS FUTILE – AND EXPENSIVE

Just about every automaker has done the “easy” stuff already – such as more fuel-efficient engines paired with wide-ratio transmissions. Increasing overall fuel efficiency now requires making incremental changes, mostly in two areas: reducing aerodynamic drag and cutting friction.

Let’s start with aerodynamics first. It’s a pretty heady scientific concept, but even a child with a paper airplane discovers the benefits of a streamlined shape.

With cars, of course, there are compromises to make so the car is safe, practical, and visually appealing. Your average family sedan would look a little ridiculous with a huge wing out back and a low, curb-smashing air dam in front.

However, more subtle changes in the 2015 Subaru Legacy have made it much more efficient at slicing through the air – which means it uses less power and fuel.

GO WITH THE FLOW

Compared to the previous-generation model, the new Subaru Legacy has reduced its aerodynamic “drag” by approximately 10 percent. Although it’s difficult to generalize, automotive engineers assume that every 10 percent reduction in aero drag can reduce fuel consumption by 3 percent.

Design details that help the Legacy do this easily are both obvious and subtle. Up front, the biggest change is the use of an Active Grille Shutter below the front bumper in 2.5i models. This shutter is open at low speeds and whenever extra engine cooling is needed.

But at highway speeds, it closes. With that opening in the lower bumper closed, the Legacy presents a more aerodynamic “face” to the wind.

It takes a practiced eye to detect the other changes, but each contributes its share. For instance, the alloy wheels on most Legacy models were designed to reduce air turbulence, the rocker sills that run below the doors are designed to help manage the flow of air alongside the car, and the body-panel gaps were tightened so air will pass more smoothly over the car. The exterior mirrors, which extend beyond the body, also were carefully designed to not only decrease aerodynamic drag but also to help reduce wind noise.

Even interior design changes can help improve fuel efficiency. Not only is the new Legacy roomier and more airy inside, but the front roof pillars (called A-pillars) have been moved forward to create a more steeply raked windshield. Combined with a slightly lower roofline, this makes the Legacy body more “slippery.”

Subtle, built-in spoiler. No WRX STI wing here, but even this little enhancement helps overall aero efficiency

There are similar types of changes underneath the car as well, since managing airflow there is equally important. Various underbody covers help protect specific components and present a smoother surface to the air.

All of these advancements have been accomplished while making the Legacy interior one of the roomiest in its class.

THE SCIENCE OF FRICTION

Cutting friction was the next goal. Even kids can understand this concept. For example, there was a large slide at my old elementary school that was beat-up and a bit rusty. But kids soon figured out that if they borrowed a sheet of wax paper from home, they could whiz down it much faster. That’s a perfect example of how reducing friction increases efficiency.

Now think about all the moving parts in your car, from the wheels and tires to the engine and drivetrain, including the Symmetrical AWD system.

Sure, there are bearings, grease, and oil to help everything rotate more easily, but this was also an area where Fuji Heavy Industries engineers, in their quest to improve fuel economy, saw room for improvement in the new Legacy.

Leaving aside the engine, which received its own program of friction-reducing measures, let’s start with the Lineartronic CVT (Continuously Variable Transmission) that’s standard on every 2015 Legacy.

Not only were specific bearings redesigned to help cut internal friction, but a special transmission “warmer” was added. This device helps heat up the transmission fluid, which helps the CVT warm up and operate more efficiently in less time, even in cold weather. Other friction-reducing enhancements in the CVT help minimize transmission noise.

From the CVT and front differential, power moves along a central driveshaft to the rear differential. Here, new low-friction bearings were fitted, making it easier for engine power to flow to the rear wheels. As an added bonus, these bearings were designed for longer life.

Finally, the wheels themselves require less energy to rotate due to new, larger, lower-friction wheel bearings. These new bearings are lighter and fitted with a new inner seal to help prevent water from entering the bearings.

The suspension bushings also were revised to cut friction. Even the brake-pad clips have a PTFE (think DuPont™ Teflon®) coating to reduce brake drag. Brake drag is the unnecessary contact between the brake pads and the disc-brake rotors when the brakes are not being applied. Along with all the other changes, this minor enhancement helps the Legacy cruise down the road more efficiently.

NEW SYSTEMS, NEW DESIGNS

Two other changes on the 2015 Legacy also helped it achieve its 26/36 mpg rating: Electric Power Assisted Steering (EPAS) and low-rolling-resistance tires.

EPAS replaces the traditional hydraulic power-steering pump and its hardware with an electric motor to help make steering easier. There are several advantages to this system. Because it replaces an engine-driven pump, the engine no longer has to work as hard just to drive the pump. The new system is also lighter in weight. Finally, because it's electronically controlled, the steering assist can be tailored to the car, so you have just the right amount of “road feel,” along with easy parking-lot maneuvering.

Legacy 2.5i models are equipped with the latest Goodyear® low-rolling-resistance tires. There’s some incredible engineering that's gone into modern tires because of all the tasks they must accomplish – especially in an AWD Subaru vehicle. The Goodyear tires on the Legacy are a modern all-season design, with great all-weather traction, but also with a special rubber compound in the tire tread that helps increase fuel efficiency.

TRACK YOUR OWN PROGRESS

With all of these amazing engineering changes, your fuel economy still depends on your driving. One of the ways the new Legacy helps you optimize your driving efficiency is via a standard ECO gauge and current/average mpg readout in the center display. So no excuses now – ease up on the throttle, anticipate stops, and generally think “green” as you drive along. The new Subaru Legacy will do its part to help you optimize your fuel efficiency.

 

1 EPA combined fuel economy estimates for 2009 Subaru Legacy 2.5i with automatic transmission compared to 2015 Legacy 2.5i with Lineartronic CVT. Actual mileage may vary.