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Re: Use of REx engine

Thu Oct 31, 2013 6:58 pm

MarkN wrote:Empty going 100 km/h up 5% grade: 18.9 kilowatts
With 100 kg load at 105 km/h up 6% grade: 25.5 kilowatts
Full car going 120 km/h up 7% grade: 39.5 kilowatts

No sweat. The i3 's 125 kW (170hp) motor is capable of more than twice the power of your most demanding scenario. Using the range extender, the 77hp required would place a load on the battery in excess of the REx output, which would, of course, be recouped during descent. A smart driver would engage the REx early in the drive in order to maintain plenty of battery energy for the climbs.

Beauty part? Electric motors do not lose power at altitude.

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Re: Use of REx engine

Thu Oct 31, 2013 8:21 pm

ultraturtle wrote:Sounds like a pretty good car for the job.

Keep in mind that you will only be able to recoup about 4 to 5 miles of range per hour charging at 12 amps, 120v AC using the BMW supplied EVSE cable. Should your company choose to install "fast" chargers (220v AC, 30 amp) at the outlying offices, that would make a huge difference - you can charge in "approximately 3 hours" (my guess is that means a 3 hour charge from depletion to ~80% capacity). Based on that, I would estimate a real world recouping of nearly 20 miles of range per hour of charge.

My company is an electrical distributor and we sell EVSE's. However in the ultimate irony we do not have any deployed at our branches. I hope to change that. However we do have 30 amp 240v outlets at each branch. I have a portable 25a (20 continuous) EVSE I carry in my current EV so I will use it for now at the branches.

My current EV can be seen here: www.EVThing.me
My other EV is a 1974 VW Thing http://www.EVThing.me
I was going to to get a REx i3, but decided that a Tesla Model S was a better fit for my daily driving needs.

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Re: Use of REx engine

Fri Nov 01, 2013 1:19 pm

ultraturtle wrote:A smart driver would engage the REx early in the drive in order to maintain plenty of battery energy for the climbs.

I hope BMW lets me engage the REx early. I'm worried that because of California regulations all i3s sold in the US won't allow that. According to http://www.greencarreports.com/news/108 ... ine-limits, to qualify for single-occupant HOV access in California "The APU must not be capable of switching on until the battery charge has been depleted".
2015 i3 REx

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Re: Use of REx engine

Sat Nov 02, 2013 5:24 am

I read somewhere it switches on automatically at 20% charge - but you can select it manually below 80% charge.

I guess 80% must count as "depleted".

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Re: Use of REx engine

Tue Nov 05, 2013 9:27 am

This is my first post, and by no means do I even pretend to be an electrical engineer!

I've been following the range extender option with some interest, especially as BMW states it's only intended for "Emergency" or "limited" use.

From what I can see reports from some BMW representatives that you can drive indefinitely at motorway speeds by constantly refueling might be false.
The figures just don't stack up. Of course final judgement should be reserved to official long term or economy run tests!

So let's start.


BMW i3 technical specs:

http://www.bmw.com/com/en/newvehicles/i ... _data.html

Electric range 190km (118 miles)
Electric range (mean customer value) 130-160km (80-99 miles)
Max. total range (most efficient mode) 190km (118 miles)
Capacity of lithium-ion battery in kWh 18.8
Fast charging, e.g. at DC fast-charging station: DC; 125 A; 50 kW (80 %) Under 30 min.
Charge time of high-voltage battery in h at 16 A (80%); 6.4 KW 6-8 hours


The REX engine has a max output of 34hp
As far as I understand this does not mean that it will supply 34hp/25KW to the battery, due to efficiency losses.

Take a highly efficient 30hp commercial generator as an example:

http://www.eg-electric.com/catalog/item ... 426153.htm

After energy losses it's only able to supply 15KW of continuous electrical power.

At best I would expect the REX to only supply 17KW to charge the battery.
That's not taking any other efficiency losses into consideration during actual charging, so it's a very optimistic figure.


Here's energy usage examples of the Nissan Leaf and Mini-E at motorway speeds:


The LEAF uses less power at 45 mph than the Volt (9.2 kW vs 10.1 kW). At 70 mph they are basically the same, but the Volt edges out the LEAF (22.8 kW vs 22.7 kW). The Volt is clearly more aerodynamic.

From the battery (from the wall):
45mph = 4.85mi/kWh (3.94mi/kWh)
60mph = 3.70mi/kWh (3.04mi/kWh)
70mph = 2.92mi/kWh (2.48mi/kWh)



Constant Speed Range @65mph
Range: 104.15 miles
Energy Used: 29.344 kWh
Efficiency: 281.7 Wh-DC/mile
Specific Energy: 112.9 Wh/kg
Charging Energy: 35.40 AC kWh

So by my calculations that would give the Mini-E 18.3 kW power usage over 1 hour @65mph


In summary, cruising an i3 at 70mph you will require around 20kW per hour.
The BMW i3 battery only has 18.8 KW capacity.

You cannot indefinitely drive at 70-75mph motorway speeds, as the REX will only provide a maximum of 17KW in an hour.
At some stage your battery will be completely depleted, even if you continue to refuel.
Being able to engage REX at 80% charge will go some distance to alleviate it, but at some stage you're simply going to run out of energy at the battery.

So guess we'll have to wait and see whether one is restricted on power during REX operation.

I really like the i3 and will still go for a test drive as soon as it's available.
But at the very best it only looks to be suitable as a very expensive 2nd car for families, or for someone who only ever drives in and around city boundaries.

I've got a sneaky suspicion that driving from London to Edinburgh (400 miles) is just not something that's easily within reach for the i3, even with the REX option.
(And let's not start the whole flying/train debate - I do use it when it makes sense).

I hope that battery capacity improves at least two-folds in the next couple of years, or that the price of these cars are halved.


Just some more info, a Nissan Leaf only does 80 miles at 62mph:

http://insideevs.com/nissan-leaf-side-b ... 2-vs-2013/

That's with a 24KW battery capacity (roughly 21KW usable)

Another more worrying post, with someone only managing 53 miles driving a Leaf at almost motorway speeds:



Tesla Model S 60KWh at 74mph seems to average roughly 320 wh/mile, or
Thus you can expect 180 miles at most out of a 60KwH battery pack.

http://www.greencarreports.com/news/108 ... 60-kwh-car

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Re: Use of REx engine

Tue Nov 05, 2013 4:23 pm

Interesting points WT.

I certainly don't plan on any 400 mile trips - but I'd like to think I could comfortably manage the odd 150 miler.

Assuming starting on a full charge and a battery-only range of around 80 miles on a predominantly motorway journey.

20% battery useage would get you about 16 miles which would take me to my nearest motorway junct - then I could engage the REx motor. If that supplied 17kW and I was consuming 20kw at 70mph cruise I'd be draining 3kW from the battery.

My typical 150 mile journey would require around 100 miles or 90 minutes on the motorway - which would draw around 5kWh from the 80% charge of around 15kWh.

So I'd still have about 10kWh (or nearly 50% charge) left for my remaining 30 miles of pure electric urban driving.

Sounds as if that sort of 150 mile trip would be fine.

Do you agree?

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Re: Use of REx engine

Tue Nov 05, 2013 4:35 pm

Here is my thought.

BMW is being very selective with the details of how the REx can be used.

It might be because they have not got a definitive answer about the rules of US regulations and want to keep their info private for now.

On the other hand, the REx, might limit power significantly and put the car is a limp mode with limited power. This type of information would hurt sales at the moment.

I think we are just going to have to wait a month or two until either BMW clears this up or a independent test can clear it up for them. Anything else seem to be more speculation than fact...
My other EV is a 1974 VW Thing http://www.EVThing.me
I was going to to get a REx i3, but decided that a Tesla Model S was a better fit for my daily driving needs.

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Re: Use of REx engine

Tue Nov 05, 2013 8:51 pm

All this has been rehashed over and over ever since the REx details started coming out. We won't know anything until someone independent of BMW does a thorough REx test drive, perhaps 17 days from now in Los Angeles. But even then, the results of that test drive must be taken with a grain of salt because the REx car won't be out just yet. They'll have another few months to tweak it.

Don't waste your time analyzing it to death, it's been done and it's pointless. Just wait a couple weeks.

Excuse me, what was a thinking?! It's an internet forum. Go nuts :)
Chevy Volt Dec 2010 - Dec 2013 (first Volt in Georgia) / Ford Focus Electric Mar 2014 - Oct 2014, stopgap until i3 / BMW i3 (BEV) Oct 2014 - present / Tesla Gen3 Oct 2017? / www.ElectrifyAtlanta.com (includes a checklist for the perfect electric vehicle)

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Re: Use of REx engine

Wed Nov 06, 2013 12:02 am

Exactly Chris.

There is already a fleet of REx cars on a tour in France giving public test drives so I'm sure we don't have to wait much longer for real data.

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Re: Use of REx engine

Wed Nov 06, 2013 4:41 am

Yes it has been beaten to death but there are new people here that haven't read a lot of it out there. The REx will definitely not put the car in an emergency only limp mode. That has been absolutely confirmed by me personally at the i3 premier when I interviewed Oliver Walter and later confirmed it with Jose Guerrero of BMW. Walter is the head of product management for the i3, and Guerrero is the i3's top product manager in the US.

Both confirmed there will be slightly less power when the range extender is on, barely noticeable unless you are driving hard and that you'll be able to continue along driving for as long as you need to. But don't take my word of it, as said above it shouldn't be long before BMW has i3's with the REx available for test drives. They are still fine tuning the REx now and completing some final software revisions, it won't be long.
Tom Moloughney
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