Basic Physics...

[mindmachine]

Just like an ICE, if you're following what's going on, you don't, or shouldn't try to go that last mile to get to your destination if you're likely to run out of go-power. I just don't see the difference with the i3, REx or not...when you see it getting that low, you MUST take corrective actions. If it were an ICE and you ran out of fuel, you'd lose your power steering, your assist on your brakes, and maybe other things. With the i3 and the REx, if you haven't run both the battery and the generator out of fuel, you CAN keep going with full safety systems and assist. Is an ICE dangerous because it could run out of fuel? Be sensible.

As I've said before...even if you could turn the REx on at 75%, if you continue to use more power than the REx can generate, you will run into an even worse situation...no REx, AND no battery left.

You know I don't see any benefit to arguing with you anymore, you see it one way I see it another. If you can engage the Rex at say 75% you can go a heck of a lot further without worrying about power loss, plain and simple.

 

[jadnashuanh]

If you can engage the Rex at say 75% you can go a heck of a lot further without worrying about power loss, plain and simple.

Not really...it all depends on the terrain, how you're driving, what creature comforts are turned on, and when it occurs. IF that hill is the last thing before you destination, and you've exhausted the REx fuel, and get the battery down as well, it will do exactly the same thing, but when it dies, it's dead.

 

[mindmachine]

If you can engage the Rex at say 75% you can go a heck of a lot further without worrying about power loss, plain and simple.

Not really...it all depends on the terrain, how you're driving, what creature comforts are turned on, and when it occurs. IF that hill is the last thing before you destination, and you've exhausted the REx fuel, and get the battery down as well, it will do exactly the same thing, but when it dies, it's dead.

Wow whatever.

 

[juniorbeep]

considering you would have had the rex on from 75% charge you have to go pretty mad to get the state of battery to zero. I think if I were you ... I'd try and stop worrying so much about what might or might not happen under extreme circumstances and relax a little. ... I'd say "go take a drive"... but that might add to the anxiety again

 

[KurtEndress]

If you can engage the Rex at say 75% you can go a heck of a lot further without worrying about power loss, plain and simple.

Not really...it all depends on the terrain, how you're driving, what creature comforts are turned on, and when it occurs. IF that hill is the last thing before you destination, and you've exhausted the REx fuel, and get the battery down as well, it will do exactly the same thing, but when it dies, it's dead.

The emphasis is not on the "go further" it's on the "without worrying".

The REx is suppose to almost double the range of the i3 and eliminate range anxiety. That's how it's designed and that's how it's marketed. After driving 70 some miles on pure electric the REx owner starts driving on gas. Now there is going to be a worry that the REx may not b keeping up with power consumption because of a long hill or a headwind, or just driving too fast. This could go on for the next 70 miles using gasoline, and if you stop and refuel you can worry for even further. The way you tell if you are in trouble is if too much of the remaining eighth of an inch of the battery meter disappears. Likely you won't be able to tell until you lose power.

The solution is either better reporting of the state of that remaining few percent of the battery used as the buffer for the REx or allow us to use more of the battery for the buffer. If you could switch the REx on at even 25% there would be a lot less worry about losing power while you still had plenty of gas left.

 

[Chrisn]

The emphasis is not on the "go further" it's on the "without worrying".

The REx is suppose to almost double the range of the i3 and eliminate range anxiety. That's how it's designed and that's how it's marketed. After driving 70 some miles on pure electric the REx owner starts driving on gas. Now there is going to be a worry that the REx may not b keeping up with power consumption because of a long hill or a headwind, or just driving too fast. This could go on for the next 70 miles using gasoline, and if you stop and refuel you can worry for even further. The way you tell if you are in trouble is if too much of the remaining eighth of an inch of the battery meter disappears. Likely you won't be able to tell until you lose power.

The solution is either better reporting of the state of that remaining few percent of the battery used as the buffer for the REx or allow us to use more of the battery for the buffer. If you could switch the REx on at even 25% there would be a lot less worry about losing power while you still had plenty of gas left.

Exactly. +1.

Thank you.

 

[MarkN]

I've posted this before, but I'm reposting since it is relevant to the topic:

About once a year we need to take two cars when we go skiing in Summit County, CO from our home in Fort Collins. The total distance is 130 miles, almost all on interstates. The first 65 miles is nearly level on interstate 25, mostly with a 75 MPH speed limit. The next 35 miles is nearly constant steep high speed uphill on interstate 70 with mostly a 65 MPH speed limit and mostly a 3 to 4% grade, but getting above 6% in sections, and gaining nearly 6000 feet in altitude. Finally, the last 30 miles is a long downhill followed by a moderate uphill.

A European spec i3 REx could probably make this trip with one stop for gas. Engaging the REx at 75% and cruising just below 75 MPH on I-25 would keep close to a 75% charge in the battery to use during the climb. Climbing a 4% grade at 105 KPH with a 100 kilogram payload uses 17 kilowatts of power ignoring all friction, lighting, and heating, and it takes 7.3 kilowatt-hours of energy to raise the car the 1790 meter altitude gain, again ignoring friction, etc.

With an American spec i3 REx the only way this would be possible is to stop and recharge the battery.

 

[Chrisn]

A European spec i3 REx could probably make this trip with one stop for gas......With an American spec i3 REx the only way this would be possible is to stop and recharge the battery.

Yes. Exactly. Again, thank you. Common sense seems to be gaining some traction here.

 

[Chrisn]

Here is the elevation profile from my trip where I "hit the wall." A Euro-spec car could have made this trip WITHOUT stopping to refuel. In fact, with a full charge at my destination, I am sure I could do the round trip without stopping for gas. In fairness, I can make the return trip without problem on a full charge since the hill is "front-loaded."

From http://www.doogal.co.uk/RouteElevation.php

 

[fdl1409]

For quite some time I scratched my head why you in the US would complain so much about that 6.5% threshold. 23.5 kW even on a 5% slope would be enough to maintain a speed of some 50 mph. 50 mph and 5% slope means that you can climb 2.5 miles in one hour, and there is no such mountain in the US. So what.

However, the REx does not work that way. When power is needed most, on a slope, it does not produce those 23.5 kW, but much less than that. Resposible for this behaviour is the operating strategy as described in a document which is available here:
http://www.bimmerfest.com/forums/showthread.php?t=787263
Open ducument 12 about the W20 Engine pdf
You find the operating strategy on page 55 ff.

The REx can operate at 2.200 - 4.300 rpm. At least here in Europe models from March or later to 5.000 rpm and 28 kW power.

Also in this document, you find a curve rpm vs. power on page 2. Here some relevant points:
- 11 kW at 2.200 rpm
- 13 kw at 2.400 rpm
- 17 kW at 3.000 rpm
- 25.5 kW at 4.300 rpm
This is before conversion losses, net power is 94% of those values.

Now what happens if the REx is needed?
For a full 6 minutes it will run in warm up mode at low rpm, starting with 2.200 and then going up to 2.400 rpm, regardless of driving speed. So only about 12 kW are available in those 6 minutes. It becomes clear that if you start an ascent at 60 mph and a power demand of say 40 kW, your 6.5% SOC - which is about 1.2 kWh - will have been used up in less than 6 minutes. If the car needs 40 kW and the REx delivers only 12, it takes less than 3 minutes to drain the battery.

Even if you drive at a more moderate speed, SOC will drop rapidly in the warm up period.
Now warm up is over and the REx could produce 23.5 kW. But now the speed characteristic curve as seen on page 55 will determine - and limit ! - the rpm and power output of the REx.
For example at 80 km/h ( 50 mph ) and 3.5% SOC the REx is reduced to about 3.000 rpm and can thus produce only 17 kW instead of possible 23.5.
If your speed goes down due to power loss as a result of that operating strategy rpm goes down as well. This is a catch 22, less speed means less power which results in even lower speed and even lower power output... In the end you are stuck with only 12 kW, your car will slow down to 25 mph and there is nothing you can do to speed it up again because the REx is crippled by this operating strategy, which may make sense on flat ground, but is lethal on a slope.

To compensate for slope, the rpm / power curve would have to be adapted, but it isn`t.

BMW made a serious mistake here. This could be overcome by a simple software update. BMW won´t do that because of those stupid CARB rules. Think they will need some pressure, because this is not acceptable in my view.

Frank

PS: can you please send some of that Californian heat over to us? Had only 2.8°C ( 37°F ) yesterday morning, low 50ies in the afternoon. Yes, this is the northern hemisphere.

 

[mindmachine]

The emphasis is not on the "go further" it's on the "without worrying".

The REx is suppose to almost double the range of the i3 and eliminate range anxiety. That's how it's designed and that's how it's marketed. After driving 70 some miles on pure electric the REx owner starts driving on gas. Now there is going to be a worry that the REx may not b keeping up with power consumption because of a long hill or a headwind, or just driving too fast. This could go on for the next 70 miles using gasoline, and if you stop and refuel you can worry for even further. The way you tell if you are in trouble is if too much of the remaining eighth of an inch of the battery meter disappears. Likely you won't be able to tell until you lose power.

The solution is either better reporting of the state of that remaining few percent of the battery used as the buffer for the REx or allow us to use more of the battery for the buffer. If you could switch the REx on at even 25% there would be a lot less worry about losing power while you still had plenty of gas left.

Exactly. +1.




Thank you.

Exactly what I have been trying to say, but wasn't getting my point made clear enough I guess.

 

[KurtEndress]

Now what happens if the REx is needed?
For a full 6 minutes it will run in warm up mode at low rpm, starting with 2.200 and then going up to 2.400 rpm, regardless of driving speed. So only about 12 kW are available in those 6 minutes. It becomes clear that if you start an ascent at 60 mph and a power demand of say 40 kW, your 6.5% SOC - which is about 1.2 kWh - will have been used up in less than 6 minutes. If the car needs 40 kW and the REx delivers only 12, it takes less than 3 minutes to drain the battery.

Now we’re actually talking basic physics.

During that 6 minute warm up phase at 2400 RPM from the charts it looks to me like the Rex generator would produce 12.4 kW. It would start at 6.5% SOC (relative) and you’d be in trouble at 1.9% SOC because you have “restricted drive power” below that. That 4.6% of the battery is 0.865 kWh which could be depleted in 6 minutes at a deficit of 8.65 kW. Add that to the 12.4 kW from the Rex and you get 21.05 kW or greater to drop you from 6.5% to 1.9% SOC in 6 minutes of warmup.

So how hard is it to exceed 21 kW of power draw for 6 minutes?

If you’re driving at 60 mph and getting 4 mi/kWh you are averaging a power level of 15 kW. So reasonable highway driving like that isn’t going to be a problem. Is it possible to drive the i3 at 80 mph and maintain an average efficiency of better than 3.8 mi/kWh? Because that would come out to 21 kW.

Note that once it’s warmed up the Rex will run at about 3400 RPM between 3.5% and 6.4% SOC above 55 MPH. At that RPM it looks like it generates 18.3 kW or so. If you are drawing more than that on average the SOC will drop below 3.5% and then the REX will go to 4300 RPM and 23.5 kW. If you can't keep your average draw below 23.5 kW and your speed above 43 mph (where the REx reduces power) you will run out of power at some point.

Mind you, I appreciate that BMW used a small ICE rather than going for overkill. It seems just about the right size for normal highway travel. It just needs to be able to use more of the battery than 6.5 % as a buffer for some situations and give the driver more information than an eight of an inch bar to show how it's doing.

 

[Stevei3]

Hi,

in my experience, at least with my car, it runs far less than 6 minutes @ low revs when cold. More like two. 6 minutes could be an absolute maximum in colder temperatures.

Nevertheless, it would be great (or less bad) for you guys if the REx would just start a little before 6.5%, so it is able to deliver a meaningful amount of oompf when 6.5% is reached. Could be implemented without upsetting the CARB balance, one would guess. Just run the REx warm without taking electrical power when > 6.5%.

Regards, Steven

PS: if true, the negative feedback loop based on the cars speed seems quite daft...

 

[ultraturtle]

...in my experience, at least with my car, it runs far less than 6 minutes @ low revs when cold. More like two. 6 minutes could be an absolute maximum in colder temperatures.

The warm-up time is temperature dependant:

The period for this warm-up phase is roughly 360 seconds depending on the ambient temperature.

 

[Chrisn]

Is the speed-related rpm limit purely "aesthetic" ( to limit noise), or might it be related to cooling/airflow somehow?

 

[pokiri]

A European spec i3 REx could probably make this trip with one stop for gas......With an American spec i3 REx the only way this would be possible is to stop and recharge the battery.

Yes. Exactly. Again, thank you. Common sense seems to be gaining some traction here.

The bizarre part to me is that there's hysteresis. As in, I might be able to take a trip from A to B with no problems, but I can't take a trip from B to A because the REx won't be able to keep up with the last part. I still have range anxiety, but it's directional!

I don't think I can explain it to my wife, so I don't know how BMW would explain it to the larger population.

 

[nostatic]

on a related note, I think that Direction Range Anxiety is playing Lollapalooza this year...

 

[fdl1409]

I think rpm limitation ist because of noise only.

This strategy works fine on even ground - at least we haven´t heard complaints - but not on a slope. During warmup and down to SOC 3.5% not even the full power is available once the REx starts. And after that, power is so short that you simply don´t have any reserve to speak of. Once speed begins to drop, the game is over as power output follows falling speed. No way to stop the downward spiral.

It would be quite easy to ease the situation with a simple software update. They could implement a slope adjustment in addition to speed. One percent slope could for example have the same influence as 5 mph speed. So if you are driving with 40mph on a 5% slope power output would be the same as with 65 mph on flat road, in effect full power.
As it is now, you only get full power if SOC drops below 3,5%. And then at SOC 2.5% only if driving 60 mph or above. Even at SOC 0.7% you only get those 23.5 kW if you can maintain a speed well above 40 mph.
With grade adjustment, you would always have full power on significant slopes. The other way round, power output could be reduced downhill.

Frank

 

[jadnashuanh]

That's one of the better ideas posted here! The gps in the car, with a little logic, should be able to determine the current road's grade. There are small, cheap accelerometers (used in cellphones to determine orientation and acceleration) that could be used as well, but that's a hardware change, and much less likely to happen in the near term verses a software tweak.

 

[Chrisn]

That's one of the better ideas posted here! The gps in the car, with a little logic, should be able to determine the current road's grade. There are small, cheap accelerometers (used in cellphones to determine orientation and acceleration) that could be used as well, but that's a hardware change, and much less likely to happen in the near term verses a software tweak.

Why complicate it? Simply suspend the speed-based RPM limits with SOC is below, say, 4%. For bonus points, car could observe that the recent load (whether from speed or slope-- who cares?) is such that the driver may need all the REX juice he can get, so fire up the REX's warm-up cycle at say 9% SOC. You could optionally pop up a UI prompt saying "I'm about to warm up the REX early due to high power use. Delay REX?"-- In case you are about to reach a charger, for example.

There are numerous similar approaches that would deal with some edge cases but NOT (arguably) run a foul of promised may to CARB. As further evidence to support this framework, I note that the car will occasionally start the REX for maintenance even though (gasp) the battery is not fully depleted. This would be a similar common sense fix that does not threaten the normal operating behavior.

Obviously a normal mountain mode would be better (ideally along with the fixes above).