Efficiency and performance meet in BMW’s i3 | RenewEconomy

Efficiency and performance meet in BMW’s i3

BMW’s all-electric i3 has been rated the most efficient car in the U.S. market.




rsz_bmwi3The 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.


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.


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 completelyavoid 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.


Source: RMI. Reproduced with permission.

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  1. Miles Harding 6 years ago

    This is one of the most impressive electric car efforts I have seen.

    But, I really hate auto writers using a bullsh*t unit like MPGe to describe the energy needs – it’s utterly meaningless. Much more useful is kwh/100km, but watch for the dealer exaggerating wildly, as mitsubishi does.

    Here’s some useful info:
    Energy consumption 12.9 kwh/100 km (about $3 of electricity in australia)
    Max range 190km (driven slow on flat roads)
    Typical range 160km to empty
    battery size 18.8kwh (will charge from a normal outlet in a few hours, but may require a 3rd party charge cable)

    Given the battery capacity and the demonstrated performance of the Mitsubishi i-Miev, 160km is probably a bit optimistic and 140 is more likely in AUS cities.

    The petrol range extender option makes for a heavier vehicle, but allows for something like 400km operation from the tiny 9-litre fuel tank, motor and battery. The RE version also consumes 13.5 kwh/100km due to the increased weight.

    • Ronald Brakels 6 years ago

      Getting 7.75 kilometers of range per kilowatt-hour is excellent. And it would be nice if car writers would start expressing the energy consumption of electric cars that way – kilometers per kilowatt-hour. Generally people know what a kilometer is and they often have a rough idea of what a kilowatt-hour is, even if it is only about how much they pay for them, so I would guess that’s the most helpful way to put it.

      • Miles Harding 6 years ago

        The first two questions I get asked are invariably how far will it go on a charge and how much did it cost. The cost of charging vs petrol usually impresses. The concept that every house and business is potentially a filling station is a harder concept to get across.

        Using Kwh/100km provides a way of readily estimating the running costs.
        MPGe assumes that the total chemical energy in the fuel is converted to work. Car engines can only convert about 30% of this to useful output and a best new build power stations, around 50%.

        One thought experiment I like is to determine how long would it take to charge your EV from a bicycle generator. The answer is about 30 hours per 100km! A few solar panels can provide useful energy, though. An extra 1kW of panels will generate sufficient for 35 or more km of guilt free driving per day.

        If the EV was a bicycle, the same 1kw of panels would propel it more than 400km. As mentioned in RenewEconomy, 30 million electric bicycles were sold in China in 2012. The EV of the future!

        • David Osmond 6 years ago

          Thanks for the useful comments. So does an electrical bicycle get about 90 km/kWh (or ~1.1 kWh per 100km) ?
          It would be great if more people started using these here. I’ve heard someone say it’s even more efficient than human powered cycling, as the extra food you’d likely consume to ride your bike is likely to be more energy intensive than the electricity the electric bike would use.
          I think I’ll mostly stick to human power though 🙂

          • Ronald Brakels 6 years ago

            Very roughly muscles are about 25% efficient at turning the chemical energy in food into work. This is better than most petrol cars. However, over time humans lose out on efficiency as you can’t turn them off. Or if you do, you have to turn them back on again within a few minutes otherwise you’ll never get them started again. Some people think of defibulation as a jump start for humans, but that’s not really right. It can only throw a human back into gear if their gears are grinding. If everything has stopped then it’s useless and the only thing you can do is thump them in the hope they start up again by themselves. Believe it or not, human repair manuals actually recommend this method.

            A 25% efficiency figure is worse than the efficiency of an electric bicycle even if the electricity comes from fossil fuels. But the manuals say that some inefficient use of muscle power a few times a week can put off the day when your human body shuts down for good.

  2. Malcolm Scott 6 years ago

    Yes the design has resulted in quite impressive efficiency and acceleration. However, with US EPA rating of 81 miles (130 km) it is only slightly better then the Nissan Leaf at 75 miles (121 km). The EPA test cycle for EVs produces quite conservative outcomes and can be easily bettered in normal day use, but not on that cold winter’s day when you want to have a day trip and lunch at the wineries, or other destinations on weekends. For the many enjoyable places 100km away, home charging doesn’t get you there and back.
    Whilst the I3 REx extends the range with a noticeably reduced performance envelope, basic low cost charging infrastructure at many trip destinations and some public fast charging infrastructure to create the EV highway is much needed for this revolution, and BMW and Nissan Leaf sales.
    I’m looking forward to seeing what charging standards BMW will use in Australia – the European standard as Tesla has chosen for Australia, or will we get the solution for US markets that has dominated here thus far?

    • Miles Harding 6 years ago

      A lot better than the Japanese test cycle that is impossible to get near outside the test track.

    • Ronald Brakels 6 years ago

      Well surely we will get the European style charging seeing as we have souped up European style current? I assume that being a German car the i3 charges at about 2 kilowatt-hours an hour from a normal European power point rather than the much slower rate of the Japanese/US standard of the Nissan Leaf?

      • Malcolm Scott 6 years ago

        The BMW I3 on-board AC charger has a capacity of 7.2 kW whereas the Leaf’s is 6.6 (did Australia get any of the 3.3 kW ones given you speak of the Leaf as being a slow rate charger?). Whilst there might be a BMW solution using 10 amp home power points for the 2.4 kW you speak of, it will take a quite a long time to fully charge the I3s battery – most of the 11pm – 7am cheap tariff. For home charging it will be interesting to see what permanently wired 7.2 kW home charging station and installation partner that BMW will use. Will Bosch finally get in on the full system/service act in Australia?
        The Japan/American charging standard (AC Type 1) is not slower than the AC Type 2 used in Europe – the speed is determined by the car’s on-board charger. Some of us with 3.3 kW charging cars of course wish for 6.6 kW charging rates as waiting at charging stations is not a practical way to use time unless you are also visiting Bunnings or the like, and if so you are probably close to home. You might be referring to the 3 phase AC fast charging that the European Type 2 can employ, but who will be deploying such systems in Australia and again will BMW be using the physical Mennekes plug/socket (Tesla? We need to wait and see)
        I sort of hope that BMW goes with the European standard and aligns with Tesla’s plans for Australia. That seems more strategic to me, even if high power AC charging like the Renault Zoe uses does not happen in Australia. If you are a Type 1 J1772 user (Volt, Leaf, iMiev, Outlander), adapting cables seem to be readily available for a Type 2 Mennekes charging station. I’ve not found the other way around.
        Also, if you want a Tesla owner to come to your business, you might attract that customer with a Type 2 charging station. Is BMW going to get on board with this?
        I should stop rambling

        • Ronald Brakels 6 years ago

          Rambling is fine. Or at least this rambling is. I like the Leaf but I don’t like the fact that it can’t take advantage of Australian current and charge at 2+ kilowatts from a normal power point. In a country with little in the way of quick charging facilities that ability would be a useful selling point for me. Also I wouldn’t need to install a charger at my place since two kilowatts is enough for me and if it ever turned out that wasn’t enough I’d have one of my servants drive it out to a quick charger. In fact, since Australian cars are only driven an average of about 40 or so kilometers a day, a lot of people could easily make do with just charging from a normal power point overnight or through the day if it’s parked at home as most private passenger vehicles are.

          But if I’m stuck charging at the speed of a Japanese power point, as the Leaf currently is if no charger is available, then that’s going to be a tad inconvenient with the way things are arranged at the moment. No recharging overnight anywhere that has a grid connection and a powerpoint. And no getting a full charge while at work either (about 19 kilowatt-hours or so is a full charge for a Leaf). So I’m hoping the i3 and other European electric cars will be able to take full advantage of current from a normal Australian/European power point.

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