Thursday, June 20, 2013

BMW i3 REx: A Hobbled Horse or a Galloping Thoroughbred?

The BMW i3 range extender engine which is made by Taiwanese manufacturer Kymco for BMW.
There is still a lot of unknown information about the i3. BMW has been extremely conservative about what information they offer. While they have stated it will have an electric range of 80 to 100 miles and cost about the same as a nicely equipped 3-Series, the two biggest questions still remain: What will the EPA range rating be and exactly what will it cost? However right behind those two questions for many people is: What will the performance be when the range extender is in use?

The range extender engine will be a 650cc Kymco engine borrowed from the BMW C650 GT scooter. It will be highly optimized to lower noise & vibration and will only put out 35hp, compared to its 65hp when used for the scooter. The gas tank will be a miniscule 2.8 gallons and BMW has stated it will extend the range of the car about another 90 miles, effectively doubling the cars range. However many journalists are wondering will that added 90 miles be an ultimate driving experience or the ultimate disappointment?

The REx will fit nicely next to the e-motor
The Chevy Volt is the closest vehicle to compare the REX i3 to, even though it's not exactly an apples to apples comparison. The Volt has a 1.4 liter gas engine that puts out 80hp to power the generator, more than double the output of the i3's gas range extender engine. However the Volt weighs 1,000lbs more than the i3 will and isn't nearly as efficient so it would naturally need a larger, more powerful range extender engine. The Volt's robust range extender allows it to drive uninhibited while the range extender is maintaining the car's charge. The million dollar question is will the same be said about the i3? That answer looks like it will be no, it won't. BMW's Product and Technology Communications spokesman, Dave Buchko said of the range extender: "The i3 range extender is meant to enable the car to go a little further than the pure BEV on those rare occasions when driver needs to go a little further. It is not intended for daily use. We've said that the REx [range extender] will double the effective range of 80-100 miles. I have not seen anything to suggest that it would not be capable of doing so.". Then BMW North American CEO Ludwig Willisch said while the range extender is running: "The car will not be as agile," Willisch said. "It will not have full power when it runs on the combination."

I'm not exactly sure what "not intended for daily use" means though. That sounds like you'll break if you use it every day and that's obviously not going to be the case. Will the cars performance be so inhibited while in REx mode you wouldn't want to drive it that way every day? I find it difficult to believe BMW would add the range extender if it was so weak it offered terrible performance while in use. Willisch must have confidence it will perform admirably in REx mode though because he predicted at least 80% of US i3 buyers will order it with the range extender. I've also heard it described by some as a "limp home mode" by some(not BMW people) and I also don't believe that is accurate either.

An i3 with the range extender was caught at a gas station refueling in this spy photo. The gas filler door will be located on the front passenger quarter panel and the charge port will be on the rear passenger side quarter panel, just above the wheel.
A big determination of how it will perform in range extender mode is how much of the battery the car holds as a buffer when the range extender turns on. Lets say the i3 normally only uses 90% of its 22kWh capacity. That means the all electric i3 will utilize about 19.8kWh's. If the REx i3 holds another 10% as a buffer to help with short bursts of needed energy than the range extender will turn on after using only 17.6kWh's of the 22kWh pack. This will allow the car to perform better when in REx mode but it will also mean the car will drive about 10 miles less on electricity. The larger the buffer the better the REx performance but the shorter all electric range. I hope BMW allows you to turn off the REx for those times when you could make your destination on pure electricity if you only need a few miles after the REx would have turned on. That way you could utilize the full ~19.8kWh that a BEV i3 has available, without reserving the buffer needed for the REx since you don't intend to need it. Of course the percentages of the battery pack utilization I'm using here are just guesses on my part, but I expect them to be pretty close to reality.

My guess is it will be pretty much in between the hobbled horse and "ultimate driving EV" when the range extender is in use. I think it will drive fine on flat terrain, and can probably even drive along at 70mph in those circumstances. However introduce an extended incline or times when you need to go 75 or more to keep up with traffic and the car could quickly find itself gasping for enough energy to maintain its motion. It definitely won't be a "It'll get you home but you'll have to crawl along at 30mph" but lets just say you'd lose if you try to autocross in REx mode. Then again, if BMW does reserve a fair amount of the battery back as a buffer, it could perform very well for the first 20 miles or so, before the stored energy gets to a minimum and the REx is working at maximum output to sustain the charge level.

The bottom line is if you really just want the security of knowing you'll never have to call a tow truck to get home, then the REx will be worth its weight in gold for you. I've been there on the side of the road a couple times in the past four years falling short of my destination so I know it will be a valuable asset for those circumstances. However if you plan to use the range extender to drive hundreds of highway miles to make a far off destination, then I'm guessing this isn't the right EV for you. Horses for courses. Hopefully, we'll get the answers to these questions and all the technical specifications of the i3 soon. It launches in Europe in the fall so the wait is nearly over.


  1. Neither, It will be a draft horse. Always dependable and always getting the job done.

    Very nice article and well researched :)

    80,000 sunshine powered miles

    1. Thanks Peder. The range extender is generating a lot of interest and it's really not yet clear how well it will perform. At first BMW just said it will double the cars range but then quickly started saying it will have some kind of limited abilities so the speculation is running wild.

      I'm sure it will be a very capable performer in REx mode and able to do handle 90% of the circumstances other than climbing long, extended inclines and high speed driving. As long as it gets you to where you need to go and you don't get stranded it's doing its job in my opinion!

      BMW just need to be very clear to buyers what it can an can't do so there aren't issues down the road.

  2. Good article! I almost forgot that Darell Dickey documented a range extender AC Propulsion built for the 1st gen RAV4EV. It was based on a 500 cc motorcycle engine, and judging from the photo included in the write-up, it was powerful enough to go up Tajon Pass. The old RAV4 had a curb weight of 3440 lbs. A lot will depend on how the REx will be implemented in the i3, but hopefully Darell's post shows that there is plenty of potential:

  3. I really hope you will be allowed to turn the REx on earlier to preserve the charge for instances when you need it later. The Volt has a mountain mode for this. You will not need it often, but when you do it is very useful and works just as it was designed to.

    I also like your idea of having the ability to turn off the range extender when you know you won't need it. I think I would find it very annoying to have it turn on a couple miles before I arrived home knowing full well I would have made it on battery alone.

  4. surfingslovak: Yes, I really believe the range extender will provide admirable performance. Not quite as much as with it's running solely on battery, but it will have plenty of power for normal driving. I can't imagine BMW having a car that's underpowered or incapable of achieving highway speeds. I'm well aware of Darell's "Long Ranger" he and I are friends and we even participated together on a panel for a Green Drive Expo in Richmond a few years back.

    Phil: I would LOVE the ability to turn it on or off whenever I wanted to also. However I believe that may be against the new CARB rules that are under consideration and if the car could do so, it may not qualify for HOV access, which is imperative that it does. Still, BMW could make that a CA thing and allow the rest of us the freedom to turn in on and off if they wanted to .

  5. Tom,
    besides of some regulations in states like California, etc.. I hope the i3 will have 3 modes to choose.

    Mode 1: Full EV mode (forced by driver)
    The range extender does not start until the battery is nearly fully depleted. Reduced performance is acceptable for the last mile due to the drivers whish to have maximum range without gasoline.

    Mode 2: Full range mode (forced by driver)
    You plan to got to the limits of the combined range of battery and gasoline or even beyond...
    The range extender kicks in right at the beginning of the journey and keeps the battery state of charge as high as possible (some reserve for the regen in mind). Maybe you can do a short stop at a gasoline station with empty gasoline tank but battery still at good condition.

    Mode 3: careless mode, intelligent mode (standard)
    You tell the i3 via the navigation what is your planned trip today. Taking battery status, power consumption and range left to reach a selected charging point (home or any other)into account, the EV decides on its own when to start the REX. Goal is to have full car performance at any circumstances, but still use the battery as much as possible.

    1. I like your thinking Stefan. But you're German, and you know how German engineers think! They may decide they know what's best for us and only offer one mode ;)