REx is not a panacea!

[ultraturtle]

- 10% increase in weight = 10% more energy required to push it around! Especially at low speeds, at higher speeds, wind resistance takes over as the dominant force to be overcome

Incorrect. Not even close. If you were to claim that a 10% increase in weight would require 10% more energy to lift it vertically, I would agree, but you will be unable to find a mathematical model anywhere that suggests the force required to move a rolling object horizontally increases by the same percentage as the increase in weight. The worst case models I'm aware of suggest the increase might be as high as 50%. That tracks reasonably close to BMW's own measurement of a 4.65% increase in energy consumption for a 10% increase in mass.

 

[Surge]

I guess you better buy a different car then :twisted:

Not at all, I'm buying the i3 BEV!

 

[Surge]

Yes, the last 6.5% the REx will start. But still, those 6.5% are available und you can and will use them if the fuel runs dry.

So on this point - the big question is whether or not the REx will keep the 6% charge at 75 mph in -15C weather. I'm not so sure… certainly the UK Telegraph reporter wouldn't think so.

So if that's the case, and I'm now "limping" in REx, or forced to travel below the avg speed limit and risk getting rear-ended… I would much rather have the BEV and keep going at 120 kph for another 12% more miles, thank you very much!

 

[Surge]

- 10% increase in weight = 10% more energy required to push it around! Especially at low speeds, at higher speeds, wind resistance takes over as the dominant force to be overcome

Incorrect. Not even close. If you were to claim that a 10% increase in weight would require 10% more energy to lift it vertically, I would agree, but you will be unable to find a mathematical model anywhere that suggests the force required to move a rolling object horizontally increases by the same percentage as the increase in weight. The worst case models I'm aware of suggest the increase might be as high as 50%. That tracks reasonably close to BMW's own measurement of a 4.65% increase in energy consumption for a 10% increase in mass.

Right. I said so, so now we are at 12% difference.

That's (still) a lot.

And the other thing is that the extended range, in the conditions of highway 75 mph and sub-zero outside temp., is not very large… so you are gaining about 30-50 miles in extended range when you need it,
but giving up 10-12 miles on every charge!!

 

[ultraturtle]

The REx needs 10% more power to push it down the road at 75 mph, vs. the BEV.
This is due to weight and Cd.
It's just a fact.

Not a fact, simply an incorrect assertion.

First off, the difference in Cd between the two vehicles is due solely to the width of the rear tires. The Cd quoted for the BEV applies only to the standard wheel set. Any upgraded wheel set, wether 19" or 20" will include the same width rear tires as the REx, and therefore the same drag performance throughout the entire speed range. I would venture a guess that the majority of BEVs sold have an upgraded wheel package, so a fair observation would be that there is absolutely no difference in drag between the REx and most BEVs on the road.

Again, the difference in energy consumption between the REx and the skinny tire BEV is 4.65%, as measured by BMW. This is a fact. That the difference in consumption between the REx and most BEVs is probably less than 4.65% is an assertion I'll be happy to make based on real numbers.

 

[ultraturtle]

... so now we are at 12% difference.

I cannot follow your math. BMW measured a 4.65% difference, and I have no reason to disagree.

 

[Surge]

That does not include the heat pump.
And if you never need cabin heat, then you can omit it.
Otherwise, it's another 5-10% penalty in range.
So you're at 10-15% lower range than the BEV.

 

[I33t]

At 75 mph you need 22.16 hp to keep the i3 REx moving,
vs. 21.16 hp to keep the BEV moving. That's a difference of 10%.

21.16 + 10% = 23.276

I don't know where you got your hp figures from but your math is way off! The difference is a bit less than 5%. In real life, it would probably be less than that (if the hp figures are accurate)

Perhaps consider a hybrid rather than a Rex?

You are correct, it is more like 5%. I stand corrected!

But why 'in real life it would be less than 5%'…? I don't see how it would be less.

Because... Maths

4.7% is less than 5%

Several of the mathematical propositions put forward to support your ideas of range and efficiency predictions have been shown to be incorrect. You cannot put forward a credible scientific/mathematic argument against the Rex unless you take care with your maths.

At the end of the day, it doesn't worry me, I have a BEV on order. You may not be intending it, but your post looks like a troll post to fire up the Rex owners/orderers. I suggest you re-post corrected maths and see how your efficiency ideas look then.

I agree that more weight = less efficiency, especially when accelerating. Once up to speed, power is only required to overcome resistance and run accessories. At city speeds, the Cd is pretty much irrelevant, and I don't think there is any difference between the Cd of the BEV and Rex? Also agree re heat pump for those times when heating is required which in most markets is not all day every day so heater hours of use need to be taken into account for overall efficiency.

 

[I33t]

That does not include the heat pump.
And if you never need cabin heat, then you can omit it.
Otherwise, it's another 5-10% penalty in range.
So you're at 10-15% lower range than the BEV.

IF the heater runs continually. Your maths seems to assume that the heater runs the whole time the car is in use. Not so - heaters have thermostats, the heater will run until cabin temp reach the selected temp and then cycle off. So although the heatpump uses less power than the resistance heater, you need a run profile (outside temp, selected heater temp, + radiant input from sunshine) to calculate the real load and therefore the actual resulting reduction in range.

 

[bxb40]

For all it matters, the only winter tire option is 155 (narrow, front and rear) so the Cd difference between the two versions should be the same at -15C if indeed the Cd was due to the rear tires difference.

 

[jimmies]

This is a slightly silly thread....so I will add to the silliness

One factor in favour of the Rex (with regards energy usage) i the fact that yes it takes more energy to get up to the same speed of the Bev....but once there it has more potential energy for recuperation and gain even more when going down hills. So this must add a % point back for extra battery charge

 

[fdl1409]

Let us do some more calculations.

My assumption is that at -15°C you drive 75 mph in a BEV with a power consumption of 22 kW - 15 kW to overcome air resistance, 4 kW for rolling resistance, 2 kW for the heat pump, 1 kW for other users.

Now we do the same with a REx, but reduce speed to 70mph. Heating without heat pump now needs 3 kW, other users remains the same 1 kW. Rolling resistance is equivalent to speed, but also to weight. Instead of 4 kW we now need 4.05 kW ( +8.6% for extra weight, - 6.7% for speed ). Air resistance power goes up square for Cd ( in Germany it is cw ), but down cubic for speed driven. Thus power to overcome air resistance will go up 7% for Cd 0.30 for the REx against 0.29 for the BEV and will go down 18.7% for lower speed of 70 miles. Power for air resistance then is 13.05 kW.
Total power 21.1 kW. So the REx at 70mph needs less power than the BEV at 75mph. Did some iterations - at 71.5mph the REx will equal the 22 kW of the BEV at 75mph.

As you take the same power out of the battery, driving time will be the same, only range a little shorter due to reduced speed. This is a penalty of 4.7%.

If driving 71.5mph is too much of a sacrifice, you continue with 75mph. Then power demand goes up and your battery will be empty earlier. You then need 4.344 kW for rolling resistance and 16.050 kW for air resistance. Total power demand 24.394 kW, 10.9% more than the BEV.

Conclusion: 11% reduced range including extra losses for heat pump, weight and Cd if you maintain 75mph, 5% if you reduce speed slightly from 75 to 71.5%. You will get equal range if you reduce speed further to about 68 mph.

Frank
Germany

 

[Stevei3]

Hi Surge,

to add even more to the mix, to ensure that the differences cannot be established with a calculator :

#1: the efficiency of any heat pump deteriorates if it gets colder. So much so that at some point, the conventional heater needs to run to keep you warm. As I don't know enough about this system and how the heat pump and heater work together, I can only guess that at any point in the near (sub) zero centigrade region, the heat pump system will be losing efficiency or stops working all together.

#2: don't forget that the heat pump system weighs something as well.

#3: also, keep in mind that a large part of the i3 community will use the winter tyres in winter conditions. These are 155's all around, even for REx equipped cars. I don't think, au contraire Ultraturtle, that the whole cd difference is caused by the wheels, for some will be caused by, for instance, the radiator of the REx, but the tyres will cause the biggest difference. It will be a difference of near 0% if everybody uses the winter tyres.

I, for myself, am very interested in the efficiency differences between BEV and REx. I know we're consuming more energy with our REx. I'm not happy about that (love to have had the oppotunity to have the heat pump), though I *am* very happy that we've got that bee buzzing in the back if the need arises.

Regards, Steven

PS: question to BEV users, really meant as a serious inquiry, which maybe OT here: are you sitting comfortably when the remaining charge drops below 6% and you are not driving homeward ? What safetycushion do you keep in your battery ?

 

[ultraturtle]

That does not include the heat pump.
And if you never need cabin heat, then you can omit it.
Otherwise, it's another 5-10% penalty in range.
So you're at 10-15% lower range than the BEV.

Okay. Although I find your estimate of heat pump energy savings to be unrealistic, at least I now understand where you are coming from. You believe the mile or two of range savings afforded by occasional use of the heat pump to be a panacea but the additional range afforded by REx is not. Fair enough.

 

[Surge]

That does not include the heat pump.
And if you never need cabin heat, then you can omit it.
Otherwise, it's another 5-10% penalty in range.
So you're at 10-15% lower range than the BEV.

IF the heater runs continually. Your maths seems to assume that the heater runs the whole time the car is in use. Not so - heaters have thermostats, the heater will run until cabin temp reach the selected temp and then cycle off. So although the heatpump uses less power than the resistance heater, you need a run profile (outside temp, selected heater temp, + radiant input from sunshine) to calculate the real load and therefore the actual resulting reduction in range.

In below zero weather, the heater runs continually!
Especially -10C or below!

 

[I33t]

That does not include the heat pump.
And if you never need cabin heat, then you can omit it.
Otherwise, it's another 5-10% penalty in range.
So you're at 10-15% lower range than the BEV.

IF the heater runs continually. Your maths seems to assume that the heater runs the whole time the car is in use. Not so - heaters have thermostats, the heater will run until cabin temp reach the selected temp and then cycle off. So although the heatpump uses less power than the resistance heater, you need a run profile (outside temp, selected heater temp, + radiant input from sunshine) to calculate the real load and therefore the actual resulting reduction in range.

In below zero weather, the heater runs continually!
Especially -10C or below!

Maybe, we would need someone to verify that applies to the i3 for us.

So you have -10C or below year round where you are?

Steve is correct about heat pump efficiency at low temperatures, typically they reach a COP of 1 (equal to resistant heater) at around -18C. Above 0C they are typically around COP 3. We'd have to see a temperature/COP chart for the i3 heatpump to know where it sits because no two heatpump systems are the same, but let's make a reasonable guess of COP 2 at -10C.

Cold weather like you are suggesting might effect heating efficiency and reduce the heat pump benefit, but all that pales into insignificance compared to the range impact of those sort of temperatures on the battery.

 

[Surge]

Good point about heat pump efficiency. It's not an easy comparison, is it?!

Now, different topic - perhaps should go in a new thread - but I wonder if you can manually retrofit the heat pump into the REx?!

Here's why it may be possible:
BMWi3Owner reported here: http://bmwi3owner.com/2014/02/heat-pump-in-front/
that the heat pump is actually in front and is quite small:
http://realoem.com/bmw/showparts.do?model=1Z22&mospid=56088&btnr=64_1958&hg=64&fg=18&lang=en

Now, in the REx, that space is taken by the fuel tank, but why not modify the front storage bin and put the heat pump there?!

 

[Surge]

Cold weather like you are suggesting might effect heating efficiency and reduce the heat pump benefit, but all that pales into insignificance compared to the range impact of those sort of temperatures on the battery.

It would be great to have someone do a range experiment in -10C or colder temperatures. Here in the north-east, it was quite a brutal winter, with temperatures certainly at -10C and below for a large part of the season. Anyone who lives in Chicago, Buffalo, NY, Mass., Maine, Vermont, certainly knows what I'm talking about.

If range is reduced to 30-40 miles on battery power, that's something to know & be aware of.
This will push most into getting a REx, I would assume. At least then you have 60-80 miles of total range.
Probably won't be fun being stranded on a very cold day waiting for roadside assistance to give you a battery charge…

This is the biggest drawback, to me, of BEVs: the wide variability in range. No one really seems to know what the range will be in very cold conditions on the highway… all we can do is assume and guess. Even if someone does post their real-world experience, it won't mean that you will get the same range.

So I certainly understand the decision to get a REx, especially in cold climates where highway travel is required.

 

[ultraturtle]

... at least one member here advocates choosing the option that will work in the worst possible scenario. I respectfully disagree. If we chose all of our possessions for the 0.0001% outcome, we would be living in concrete bunkers.
More realistically, apply the 80-20 rule -- figure out how you do 80% of your driving and plan for that. Don't let the 20% dictate the 80%. (Or choose 90-10, if you feel more comfortable).

I was unable to find a quote by anyone on this forum advocating decisions based on worst case, or 0.0001% probability, but did find one that is based on a minimum range requirement:

A decision to purchase a BEV would best be based on the minimum range requirement considering the worst case scenario, which would be the coldest temperature on the last day of either ownership or battery replacement.

It would be foolish to establish a minimum range requirement based on a slim probability event. Better to use an 80% / 20% criteria, as you suggest. Let's take a reasonable scenario based on these numbers and see how that works in the real world.

Say a driver's minimum range requirement is a 5 day/week commute of 60 miles that can (based on a given speed, temperature, driving mode, and ignoring heat pump use for now) be accomplished on battery power by either the BEV with 7 miles of battery range remaining, or the REx with 4 miles of battery range remaining. Let's further assume that once every two weeks this driver needs to make a side trip that increases the range needed to 135 miles under those same conditions of speed, temperature, and driving mode. The 9 x 60 mile trips make up 80% of the total 675, and are accomplished using battery power by both the BEV and REx. The 135 mile trip, however, cannot be accomplished by the BEV, and whatever ICE vehicle gets used lowers the total EV useage of this 675 mile scenario to 80%. A REx, on the other hand, can not only eliminate the need for a second car, but increases the share of miles driven on battery power to 89%, since 60 of those 135 miles are driven on battery power . Some folks have a difficult time wrapping their head around the concept, but a range extending gasoline engine option will nearly always increase the percentage of total EV miles driven over the combination of a BEV and seperate ICE vehicle.

This scenario conservatively presents the point. The more trips taken by an ICE vehicle that just exceed a BEV's range, the smarter the choice of a REx.

 

[fdl1409]

Absolutely true. I very rarely drive distances longer than 50 miles. Still I kept my Audi A2 as a backup when I had my Mitsubishi i-MiEV. Only used the Audi twice in one year, but those were emergencies and I really needed to drive immediately. My brother fell down a ladder, broke an arm and needed transport to hospital. What do you do then with a BEV if the battery doesn´t provide enough range? Now with the i3 REx, I will sell the Audi. My electric driving will probably exceed 98%. Looking from a purely economic view cost per REx mile, this may not make sense, but I simply need and want the extra range.

Today I did a 144km trip ( 90 miles ) with 26km remaining ( 16 miles ). That is one trip I never did with the i-MiEV. With that I could also do up to 95 miles in perfect conditions. That would have been too close a shave, never dared to try that.

How close can you go to zero with a BEV? Last year I wanted to drive home with a comfortable rest of some 5 miles according to range estimate. Then there was an unexpected roadblock because of a traffic accident. All traffic had to use a bypass, much longer journey. I only very narrowly made it home.
I do not suffer from pathological range anxiety, have emptied the i-MiEVs battery very often to below 0km range indication - there comes a "turtle reserve" after zero, and I have seen the reptile many times. However, I feel much more comfortable now with the REx.

Frank
Germany