BMW i3 Electric Car: ReX Range Extender Not For Daily Use?

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The upcoming 2014 BMW i3 electric car is generating a lot of quiet excitement--especially for its optional "ReX" range extender, a tiny two-cylinder engine that fits under the rear deck to power a generator.

The i3 will be only the third high-volume dedicated battery electric car on the market, after the Nissan Leaf and Tesla Model S.

And it's the first one that will offer the option of a gasoline-powered range extender like the one built into every Chevrolet Volt.

The company has shown prototypes of the BMW Concept i3 five-door hatchback (with no center pillar and rear-hinged rear doors) and an i3 Concept Coupe more recently.

The production version will debut at the Frankfurt Motor Show this September. First deliveries will begin in Germany around November, with U.S. deliveries following early in 2014.

Defined as zero-emission

The BMW i3's 21- to 22-kilowatt-hour lithium-ion battery pack is expected to provide a range of 70 to 100 miles.

Combined with a gas tank of 2 to 3 gallons, the ReX engine and generator almost double the i3's range--though a tank that small offers less range than the battery itself provides.

That's deliberate, because (pending final verification) it will keep the BMW i3 designated as a zero-emission vehicle under California's arcane emissions laws.

Buyers seem interested in the idea of an electric BMW with up to 100 miles of electric range that also happens to offer an optional range extender for longer trips.

And BMW has promised the i3 will perform and handle as well as any other "ultimate driving machine," regardless of its electric powertrain.

BMW i3 Concept MkII

BMW i3 Concept MkII

Enlarge Photo

Electric vs gasoline

But those potential i3 owners need to understand a critical point: The BMW i3 may not operate like a Chevy Volt.

That is, its performance in range-extending mode may be more limited than when it is running on battery power.

One of the core principles behind the design of the Volt was that its performance should be identical whether in battery-powered or range-extending mode.

BMW says the range extender of the i3, on the other hand, is designed not for long-distance travel but purely as a short-term stopgap to get drivers to the next recharging location.

Will performance vary?

And while the BMW i3 is expected to have an electric motor producing 125 kilowatts (170 hp) of peak power output driving its rear wheels, the little range-extending inline twin (derived from a motorcycle engine) is likely to produce considerably less.

BMW has not yet given the precise displacement or power output for the ReX, nor the capacity of the generator it will drive.

BMW i3 Coupe concept

BMW i3 Coupe concept

Enlarge Photo

But a good metric will be to see whether the generator develops at least half the output power of the traction motor.

The Volt's 83-hp 1.4-liter four-cylinder range extender runs a generator with a peak output of 55 kW (74 hp), which is half the peak power of the 111-kW (149-hp) traction motor that turns its wheels.

The Volt "buffers" power draw through the battery pack, so except under the most extreme circumstances, the performance of the Volt with its engine running a generator to power the traction motor is essentially the same as one running on battery power alone.

BMW's highest-performance 800cc vertical twin motorcycle engines carry ratings of 85 to 90 horsepower, so it's possible that a ReX range extender adapted from one of those engines could provide the same ratio.

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Comments (59)
  1. Great article, John. Thanks for pulling all this information together. To add another data point, Phil Sadow, who is well-known and respected in the LEAF community, has built a 30 kW Capstone turbine REx for his LEAF. His experience with it has been good, and he thinks that more power is not necessary. The i3 is expected to be about the size of the Nissan LEAF, and have significantly less weight:

  2. I'm sure the added weight of the ReX option will have some negative effect on the i3's electric range. I'm disappointed that BMW seems to have more range anxiety then it's potential i3 customers do. I'm looking to buy an i3 and I know I can live without the ReX option.

  3. We'll really have to wait for the official specifications to come out and see just how capable the car is in charge sustaining mode. However if what has been said is true and it will have a 2.3 gallon tank will add about 90 miles of range, then I can't imagine we're talking about some kind of low speed limp mode because nobody would limp for 90 miles. I suspect it will be highway capable, perhaps fine to cruise along at 70-75 mph on relatively flat terrain. I don't expect it to be able to drive 70mph up steep mountains for a prolonged time though, but again, we'll see.

    How many Volt owners speak about driving 80-90% on electric, and that's with a 35-40 mile AER. The i3 will more than double that so how much will it really even be used?

  4. Tom - I think you are at the heart of the discussion with your post. The i3 has the potential and on average will immediately post more Electric miles than the Volt. I believe that this will bring on another band of high(er) mile drivers that can keep more of those as Electric miles. More choices and more Electric miles are a GOOD thing.

  5. Exactly Paul. It doesn't have to be a do-all car to be a perfect fit for many people that want an plug in car. If you frequently need to drive 150+ miles than it's probably not the right fit, a Volt would suit your needs better. However if you are looking for a plug in car with about 100 mile range & great performance than the i3 is a good choice. If you want the added security of never running out of charge, and being able to run the battery flat without being stranded, then the range extender option might be a good choice. Yes, more choices are a good thing, there is nothing really like this out there yet and I'm guessing it will do very well.

  6. Well, I can speak for my own use case. Most of the time that 40 miles is fine with the Volt. But when I do use the gas engine, it is for range more than 200 miles.

    So, in this case, the extra electric miles only help for trip between 40 and 100 miles if the ReX is NOT for daily use.

    40 miles for daily driving and 200 miles for weekend or long road trips...

  7. If something is unpleasant to use, it won`t be used. They could have reduced the complexity and added a few more kilometers of emergency battery capacity.

  8. As far as I understand, BMW sees the i3 essentially as a metropolitan car. Semantically, that is how it started. The range extender is adding a suspender to the belt on the pants - extra safety. Just like a spare tire in the car.
    Anyway, it is a small car. There is no room for a bigger engine that would also suck those 9l faster. And as har as I know, it is the engine of the C 650, not of the F 800, so forget 90 HP. That would also be much too loud.

  9. Great report, John! I'm with you and Tom -- this sounds a bit like a solution in search of a problem! Toyota and Ford are challenging the Volt with shorter range, not longer!
    (By the way, I and others who drive Volts say it feels BMW-like in its power, in both modes.)
    People who buy a BMW want its performance -- a "limp-home" mode doesn't really add much. And explaining this car will be agonizing. I hope BMW builds great EVs and PHEVs.

  10. Especially if it can't "limp" on the hwy with full passenger...

  11. Doesn't seem THAT hard to explain...80 miles of EV range with nopossibility of being stranded during an 80 mile trip. BEV w? training wheels.

  12. SO it follows that the BMW i3 with ReX will be considerably cheaper than a Volt as the Volt will out perform the BMW with ease. I did have to drive 75+ MPH over the grapevine here in CA, a famously long steep grade, with both the battery and the ReX pulling the car in mountain mode. It was exhilarating that I still had plenty of passing power and that I could stay ahead of traffic that would surly rear-end me if I went any slower.
    If I spend over $30K for a car it better perform and I suspect most car consumers will think that too. I'd hate to see BMW fail at this.

  13. This seems like tricky math:

    "The BMW i3's 21- to 22-kilowatt-hour lithium-ion battery pack is expected to provide a range of 70 to 100 miles.
    Combined with a gas tank of 2 to 3 gallons, the ReX engine and generator almost double the i3's range--though a tank that small offers less range than the battery itself provides."

    I thought elsewhere BMW had talked about a total range of 180mi--which would put the EV and gas ranges nearly identical. Has BMW revised the REx range?

  14. Nothing's changed Adam. BMW has stated the i3 will go 80 -100 miles(conditions permitting) on electric and the REx option would add about another 90 miles. The electric range and the range extender range is just about identical at 90 miles each. That's how you get 180 miles total.

  15. @Adam: BMW execs and engineers also tend to quote miles on the European test cycles--which, as you know, are considerably more lenient than the U.S. cycles. They tend to produce gas-mileage ratings and electric-car ranges that can be as much as 20 percent higher than U.S. test cycle figurs.

    The "70 to 100 miles" was my way of covering everything from a low EPA range rating up through what BMW has quoted.

    And until we see EPA ratings--and then compare them against how the car actually performs in real-world usage--it's all a little hypothetical.

    BUT, if BMW is saying 100 miles on battery and 180 miles total, then that says to me 80 miles from the ReX. That fits with the CARB ZEV mandate.

  16. But we can do some "guessing" work, can we?

    BEV efficiency vary from 3.11 miles/KWh in EPA rating (Tesla S) to 3.8 miles/KWh (i-Miev). So, let us assume the i3 can do the same, then the 22KWh is good for 83 miles. But with the new 80% rating, then that is 67 miles EPA rating.

    Of course, that is without weather derating.

    What type of Lithium battery it uses can and will impact its real world range.

    I think the REx will give the owner some lower range anxiety level so its battery will end up be cycled more deeply and more often... I hope BMW designs its battery accordingly...

  17. My ideal would be able to have the range extender be removable and possibly rentable from the dealer for a long trip rather than carry the weight of the IC engine and fuel around all the time.

    A range of 70-100 miles would be great for my usage as a 2nd vehicle or in the case where I rented a car for those rare longer trips.

    I really like the Tesla Model S but hope that the i3 will be more affordable.

  18. The reports are that BMW will have a 25kW generator.

    Phil who built a range extender for the Leaf runs says the 18kW it puts out is "fine".

    I think 25kW is a little underpowered for using i3 to go over passes that are close to Seattle. Normal freeway travel should be fine. On level ground we should be able to go at 75mph or so - and short bursts of acceleration should be provided by the battery.

  19. Well, I seriously doubt that is "fine" since his max speed was capped at 37mph on a hill. That is NOT including acceleration or loading... If you think that would work, try go up a hill in SF or Berkeley and see if that 30HP is enough...

    Anyway, people usually mix up the understanding of power required to sustain a speed and power required to reach that sustain speed in "safe" and "reasonable" amount of time...

  20. 100mi electric range and 100mi gas range is perfect IMHO. Just coming from an ActiveE perspective, it manages 95% of my daily driving needs and would be capable of nearly 100% if it just had a quickcharge port. Having a rex means you have instant access to additional range with a quick stop to an established infrastructure. As for power, I think most times even a 70HP generator will do if you are using the battery as a buffer, it makes perfect logical sense. For example, the ammeter on my ActiveE has 8 power notches respectively showing 0-125kw or 0-170HP, even at 85MPH on a flat level surface, the guage only shows right around the 3rd notch. Which is 46.875kw or 63.75HP. Given that the i3 will be 1000lb ligher, it wont need that much.

  21. Again, there is a major difference between sustain a speed vs. getting to a speed with loads on a hill... I seriously doubt the REx has even 70HP...

  22. The i3, like most pure EVs, is designed as an intra-city car where most daily trips are 100 miles or less. Great as a commuter car or driving around town where you're bound to end up totaling less than the range of the battery--and near a charger, hopefully.

    But what BMW learned from its pilot Mini-E program of 500 "real-world drivers of all-electric Mini Coopers is that we WISHED we weren't limited by range--for those (rare) times we have to go inter-city (New York to Albany, say) or didn't carefully calculate the range or need to run intra-city errands that take us beyond the charged limit.

    Hence, i3's ReX really is like an "emergency tank" and not really for daily use a la the Chevy Volt's setup.

  23. What an interesting time to be able to watch as electrics morph in front of our eyes. Soon, a car manufacturer will achieve just the right combination of electric and assisted power, and sales will skyrocket. So far, the Volt seems to have come closest to achieving the perfect combination. The BMW is on the right track, also.

  24. Volt is far from perfect. Very low EV miles (can't even go to Seattle downtown and come back in EV mode). Too comples. No 5th seat. Not even talking about non-drive train stuff.

    I definitely prefer i3 & Tesla's approach. Best would be to combine the approaches. Let there be options in the same car for
    - Multiple battery sizes
    - Multiple REx generator power sizes

  25. Well, a Tesla S can't even stay "afloat" during extreme cold as in all BEVs with standard 120V outlet.

  26. I own a Tesla Model S and your statement makes no sense whatsoever. Why would anyone attempt to stay "afloat" using a standard 120V outlet when so many other options are available? Actually, being able to plug in anywhere is a plus but never intended as a primary charging solution.

  27. @Ken Cas,

    Well, staying "float" means that you can keep battery warm enough so it doesn't lose any range or charges. If you can't even keep warm enough with 1.4KW power, then I think it is a design issue. Or at least the design is NOT ready for extreme cold weather.

  28. What car is perfect ? A $75,000 Tesla ? A Volt costs less to operate than a new Honda Civic and I average 160 mpg . Today I used .53 gallons on my round trip . 45.1 in EV/43.3MPG. I average more than 800 EV miles per month and use around 4 gallons of gas monthly .

    The I3 sounds like a good idea ,
    but we won't be sure until it we know the price and real world capabilities . Tesla would be great if they could provide a 50Kwh vehicle with DC charging infrastructure for less than $40,000 .

  29. I'm getting 1000 in mine

  30. A larger 60EV miles Volt would be ideal. But for the price and all the capability, I think it is probably one of the best...

  31. I am very interested in the i3, because of the aluminum frame, carbon body panels and tall LRR tires. It makes EVs that are converted steel ICE vehicles look pretty antiquated.

    The 70-100 mile range is fine for my daily commute. But, I drive to San Diego twice a month and it's a 140 mile round trip on hilly freeways. I would purchase the REx, but I wish it could be manually engaged before the battery runs down. I would definitely prefer a dense 40KWh battery to a REx, but we will probably have to wait a couple of years for that.

  32. Do you think the unique platform with aluminum and carbon and a 22KWh battery will end up cost you less than $50k?

    I seriously doubt it... Of course, we won't know for sure until BMW release it. BMW can barely make a car that sells for less than $40k in gasoline version. Why does anyone think the electrical version will cost less than $50k?

    I think the cost will be between a Ford Focus and a Tesla 40KWh version. That is about $50k starting price.

  33. I have to add something to this comment, which is... "BMW can barely make a car that sells for less than $40k in gasoline version... in the US". There are plenty more affordable models in Europe :)

    However, I see your point. The i3 won't be cheap.

    But then, I don't think Bret was implying it *would* be cheap, so unless you meant to reply to someone else, your comments seem a little odd.

  34. Well, he mentioned all the "fancy" features of the i3. I was just pointing the cost associated with those "fancy" features...

  35. Thanks Antony. The i3 won't be cheap and I didn't expect it to be. After all, it's not just a converted Cruze, Versa or 3-Series. It was built from the ground up with better materials to be an efficient EV. That costs money and has benefit.

    My hope is the technology BMW has created for the Mega City platform will be adopted by other EV manufacturers. It will definitely drop in price as it becomes the norm, instead of the cutting edge.

  36. I agree. One should be able to start a long trip with the Rex running to keep the battery charge as high as possible. That way the battery would retain a lot of charge for a much longer distance, and the performance of the car wouldn't suffer. Of course, one would have to stop frequently to fill up the mini-tank, but that's not a a bad idea anyway, for safety's sake. If you take frequent breaks, you're less likely to fall asleep at the wheel.

  37. Does the ReX provide cabin heat?
    Does the ReX provide any battery warming capability for extreme cold?

    If NOT, then it is really like BMW said, nothing more than something to get you to the next charging station...

    I want a 60 EV miles Volt with more space and better performance. I guess BMW didn't make one for me....

  38. A 60 EV mile Volt sounds good , but oddly it would only save me 2 gallons of gas per month . I usually use around 4.5 gallons . The I3 with ReX would get me down to around 5 gallons per year , a savings of about 50 gallons annually . BTW, today my EV range was 45.1 miles . The performance is quite nice and addictive to drive . I don't miss my 2009 CTS or the 2007 530i a bit.

  39. BMW has another way to tackle range anxiety. It will now loan gasoline vehicles to i3 EV buyers for longer trips:

    I think the i3 with a range extender would work best for buyers who live in regions where public charging network is relatively much less adequate. For such people, the ReX would be actually uesful. With the loan program, the occasional longer trips are also taken care of. However, to some extent, the loaner vehicle feature could undermine the usability of ReX since a typical EV with 100mile range and the ease to loan a gas car for longer trip would be more attractive...

  40. So, $10 to fill Rex with liquid fuel, vs what $5 to fill it via a charge cable?

  41. Has anyone seen the new Free-Piston Linear Generator?

    It has opposing pistons and replaces an ICE engine's rotational mechanism with a linear generator. It is compact and efficient and should make a great REx.

  42. The Ecomodder website has a good vehicle energy calculator. Most people here will know about it, but if you don't then if you search for this in Google you'll find it:

    "Aerodynamic & rolling resistance, power & MPG calculator"

    I just used it to see how much energy you need for a range extender for the BMW i3. It's highly possible I have both got the specs for the car somewhat wrong (since they are not all known) and that I have used the calculator incorrectly. That's just a disclaimer. :-)

    Specs I used:

    Car weight = 2800lb (1271kg)
    Coefficient of drag= 0.26 (mostly a guess in the dark)
    Frontal cross sectional area = 2m(width) X 1.5m(height) = 3 m^2
    Engine efficiency = 85% (rough guess)

    Continued in part 2...

  43. Seems like a range extender really doesn't have to produce that much power if you have a light and aerodynamic car.

    I was going to produce the results, but they are so low I'm doubting them now.

    18KW @ 70mph. Does that seem right?

  44. @Dilbert: Power of 18 kW for steady-state cruising at 70 mph should be fine. The issue is whether an 18-kW range extender will provide acceleration from a stop along a freeway on-ramp with metering lights that is speedy enough to join 75 mph traffic. That requires far more power.

  45. I kinda of thought that was the batteries job?

    Even with just 10% charge the batteries can handle peak power needs. You rarely drive constant anyway letting off the throttle for traffic, lights, breaks, etc where it catches up. Unless open uncrowded freeway you are off the gas as much or more than on.

    Check yourself out on the way home and see what % of the time you actually use fuel other than idle/coasting/braking. I bet it won't be 25% of the time. A generator will only be recharging during that other 75%.

  46. Well, sure battery will help that somewhat. But what if you are in Rex mode and you are using heat or A/C? And if Rex is already maxed out, then it doesn't get too much spare for acceleration.

  47. The motor is 130KW in the BMW i3 and the 0-60 mph time is supposed to be less than 8 seconds. Using 130KW of power for 8 secs consumes roughly 0.3 KWhrs. You can very quickly get 0.3KWhrs back with a 20-30KWh ish range extender.

    Hills area bit of a problem if they last a long time, but I guess you just go slower.

  48. First of all, you are assuming that 130KW is 100% efficient. During heavy load or max power, the efficiency will drop. It will be probably around 60% at best case. (80% in the motor and 80% in the battery and controller). So, that 0.3 KWh just become 0.5KWh. If 20-30KWh was barely enough to keep it going at Hwy speed, then when do you ever get to charge it? Also, you are assuming there are no A/C/Heat usage, no headlights, no wipers, no radio or any other accessory usage.

    Sure, they can always go slower. But that is just another "limp" mode...

  49. Does that include tire resistance?

  50. Even if the range extender was half the power of the one in the Volt and produced 27.5kw, that's still more than enough for highway cruising under most circumstances. On long trips I would probably pop it into "hold" mode once I was on the highway, and then return to battery mode when I approached the city and not even notice the diminished performance.

    I love this idea. I would strongly prefer not to carry around a large engine that I rarely use. It would be even cooler if I could remove it when I didn't need it (most of the time), but that's probably complex and/or too heavy.

  51. Actually Volt's engine is running about half of the RPM most of the time during hwy cruising. But as soon as hard acceleration or hill is near, the speed of the engine goes up significantly.

    So, if you design the ICE to be only good for 27KW, then it will always run at that high speed. ICE's max power is usually at its higher RPM range. It will be noisy and has higher tear and wear.

    Also, sometimes a lot of power is needed even when the speed is low when the hill is still.

    Try the Berkeley Hill sometimes or some of the streets in SF.

    The last thing you need is an EV with its REx screaming at low speed on a hill in SF... I am sure that will help the EV image...

  52. If the BMW can't double it's 100 mile range with it's extender something is seriously wrong.

    Likely far too large generator. You only need enough generator to do 60mph on the flat and the battery pack handles the rest. That is about 15kw/20hp.

    In my lightweight EV only needs 4.5kw of generator to do 80mph at well over 100mpg for unlimited range and that ratio of 5kw/1000lbs is all a battery dominated, 60-100mile range EV's need.

    Not sure why the author was worried about a 10 mile grade as EV's are designed for it. And usually you get the energy back on the next down grade or going back home. So unless it's a short range EV it just doesn't matter.

    Doesn't the Volt's motor drive the diff.wheels on the highway? If so not like BMW's

  53. "Doesn't the Volt's motor drive the diff.wheels on the highway?"

    No. Its main traction motor always drives the main shaft. The 2nd generator can spin up to lower the speed of the main traction motor at speed above 60mph to increase efficiency of the main traction motor.

    In the extended range mode/gas mode, the ICE engine provide additional torque at speed above 70mph.

  54. Jerry, care to share with us what magical vehicle you have here?
    At 80mph, current production EVs (Leaf & co) burn roughly 5x the amount of power you quote mostly just to fight air friction. Weight isn't important at constant speed on flat ground.

    Re the Volt:

  55. I'd like to echo the suggestion that a ReX should be a removable, rentable option from a dealer for longer trips. You avoid paying the extra cost for it up front, you don't have to lug its useless weight around during your daily driving, and you get range security when you need it. They could even offer to rent you an extra battery pack to fit in the same space instead (most likely in the 'frunk'), say for 50-100% more range. Again, you only pay for the extra range when you need it.

  56. When I was young a VW bus had a 24hp, later, 36hp engine. They were slow and piled up going to, say, Lake Tahoe. Then the Japanese started putting 42-60 hp. engines in datsuns and corrollas. They were all slow. We didn't mind. This car sounds great and even at reduced power the REx system looks good. Too bad Calif. won't certify them with, say, a 6 gal gas tank and a range of 270 miles. Most driving would be electric, but the longer trip would be possible. Have a Prius which betters the mileage of a volt in many circumstances and can go 600 miles if you want. I routinely get 55mpg.

  57. @Kurt .... nice mileage. My Volts mpg is 247 for 8 months lifetime since new. Now on second tankful. Beats Prius out to 265 miles. ie. 84 mile RT = 1 gallon. Nice

  58. I haven't been able to find any information as to WHY the range extender isn't for "regular" use. Has BMW, or anybody else, given any reason? Would there be some negative consequences if the REx engine did receive regular use? We own an i3 REx, and other than the hassle of frequently refilling the tiny tank, it seems like it would just keep running as long as it has gas. Our longest trip between charges so far has been 200 miles, and the car performed very well throughout the trip. We don't expect to use the REx engine frequently or a lot, but it would be helpful to know why BMW apparently advises against it.

  59. @Paul: This may help explain the situation:

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