BMW i3 — Great Efficiency & Performance Thanks To Advanced Lightweight Materials

Rocky Mountain Institute

By Greg Rucks

The EPA recently rated BMW’s new all-electric i3 at 124 MPGe, making it the most efficient car in the U.S. market. While this is only nine MPGe better than the comparably sized Nissan LEAF, the i3 upholds BMW’s reputation for manufacturing luxury cars that offer a superior driving experience. How BMW created a car with greater efficiency without sacrificing performance or functionality is mostly due to its holistic design approach and use of advanced lightweight materials.

DESIGNING FROM THE GROUND UP

Unlike many car manufacturers, BMW didn’t take an existing platform and adapt it to electric drive, but designed the i3 from the ground up. Its design in many ways reflects a shift away from traditional car design due to the fundamentally different way an electric vehicle operates. There is no longer an engine or radiator, usually housed under the hood, so there is no need for extra length at the front of the vehicle. Similarly, there is no longer an exhaust system or traditional transmission and thus no need to accommodate these systems under the passenger, sometimes done today through the mound on the floor that runs the length of the interior and separates driver from passenger. But there are a large and relatively hefty battery back, new power electronics, and an electric motor to put somewhere.

To accommodate and best take advantage of these many differences, designers started from scratch. That’s why the battery pack essentially forms the foundation of the i3, creating a battery platform on which the rest of the vehicle sits and giving it a low center of gravity, contributing to good handling and stability. Simplified EV componentry perhaps made the i3’s flat floor, with no center console, easier to execute, giving the i3 more interior space. It also provided a surprising safety benefit—the driver can more easily exit or enter from the passenger door while parked on high-traffic city streets. The occupants are also shifted forward relative to a standard design and the footprint is very small considering the amount of interior volume it provides. To put things in perspective, BMW’s i3 offers as much interior space as the automaker’s 3-series sedan, but the i3 is a full two feet shorter.

As for performance, the i3 is legitimately quick, accelerating from 0 to 60 in 7.4 seconds.

ADVANTAGES OF LIGHTWEIGHTING

The i3 weighs 20 percent lighter than a Nissan LEAF. Our analysis indicates this level of weight reduction would normally provide an efficiency increase equivalent to 18 MPGe over the LEAF’s 115 MPGe, but the performance benefits partially reduced the efficiency benefit. Nonetheless, use of materials that enable low weight with equal or better structural performance allowed BMW to produce a high-performing vehicle while still hitting the impressive mark of 124 MPGe. In addition to the efficiency and performance benefits of lightness, use of lightweight carbon fiber composite in the i3 gave the car some benefits that are not seen in other vehicles.

Since carbon fiber composite is much stiffer and stronger than steel, it was easier for the i3 to eliminate the B-pillar, the vertical support between the front door and rear door. The front doors open normally while the rear doors are hinged at the back. These “coach doors” make it easier to load child car seats, groceries, and more, and contribute to a sense of spaciousness and accessibility.

Another interesting fact about the i3 is its low embodied emissions—the sum of all the emissions during production of the car. Carbon fiber poses a challenge from this standpoint because it is produced from a relatively energy-intensive process. Composite materials are also inherently difficult to recycle and part manufacturing currently produces a lot of excess scrap material that is difficult to reuse. However, BMW has at least partially mitigated these challenges by powering its carbon fiber production plant in Moses Lake, WA, with clean hydro-derived electricity, allowing it to completely avoid combustion of natural gas during the production process.

Of course the embodied emissions go up when the i3 is equipped with its optional “REx,” or “Range Extender,” as most are. We’ll discuss the implications of this in a future blog post.

It is unclear whether BMW’s i3 is actually a moneymaker given the much higher cost associated with its carbon fiber composite construction. Its selling price in the low $40,000s is already outside the price range of most car buyers. Whether BMW will be able to bring that price within range of the mainstream (assuming they want to do so in the first place) remains an open and important question for the industry.

While BMW could have gone further on the efficiency front, and while questions remain about cost, it still achieved the highest efficiency on the market while designing a car that appeals to BMW’s performance-oriented customers, has some very functional and unique features, and is extremely fun to drive. As the first production city vehicle to make extensive use of carbon fiber composite in the body, the i3 represents just the first step on a path toward fundamentally transformed and cost-effective vehicles that potentially take advantage of carbon fiber composite’s unique properties while perfecting all-new production processes and supply chains. There is a lot of room for even further improvement. The future can only be lighter.

Image courtesy Intrepix / Shutterstock.com

Source: Rocky Mountain Institute. Reproduced with permission.