Optimum speed using Rex for very long journey

[FrancisJeffries]

If you need to drive more than about 160 miles, the i3 is not the car for you. The REx does not replace an ICE car.

That's definitely true in the US. But not in the ROTW.
I would not have bought an US-version i3 Rex, because I sometimes do 150, 300, 600-mile round trips. I definitely don't want a second ICE car just for those. And I decided stopping with a BEV for a 20-minute recharge every 60-70 miles at maybe-available CCS chargers compared with five minutes every 90-100 miles for petrol was no contest!
For the long journeys, I use the i3 Rex as a petrol car with a small tank, and a get-out-of-jail-free battery backup when it runs out. I set out with full battery and fuel, and switch Rex on as soon as the battery is down to 75%. Doing that, I found that I can do over 100 miles between refills - with very little battery depletion - and arrive 700 miles later still with half a battery!
I don't bother with keeping the speed down on the motorways - quite the reverse - and still get 90-95 miles on a tankful.

 

[Stevei3]

For the long journeys, I use the i3 Rex as a petrol car with a small tank, and a get-out-of-jail-free battery backup when it runs out. I set out with full battery and fuel, and switch Rex on as soon as the battery is down to 75%. Doing that, I found that I can do over 100 miles between refills - with very little battery depletion - and arrive 700 miles later still with half a battery!

Hi Francis,

a bold +1
been there, done that. Works like advertised. Arrived in Geneva with 69.5% battery after >850 km

Regards, Steven

 

[Surge]

Absolutely agree - in Europe the REx is well worth it.

My comments above apply to the US and Canadian models only.
(In Canada we get the same hobbled REx as in the US, although I don't believe we have any of the US CAFE/California Bullsh!t requirements)
Don't forget that our fuel tank is physically diminished too!

 

[nowtta60]

The arugments about i3 vs Tesla are all irrelevant anyway. The Tesla costs almost double the cost of an i3 rex. Efficiency is more than Wh/mile, its also £/mile. And the i3 works out cheaper per mile due to having a lower purchase price and hence depreciation costs. The high price also affects the fact that probably 10 times as many people can afford an i3 than a Telsa. That's judging by the number of £30-40,000 cars on the road(A4/3 series/C class etc) , Vs £50,000+ cars(V8 Luxury cars/performance cars). So overall the i3 will have a bigger impact on reducing emissions. But I'd still prefer a Telsa when I can afford one ;-)

Back to the original question about shortest journey time... Given the REX is an ICE and they are most efficienct at maximum power (as the throttle is wide open and not creating a vacuum) it might be feasible that driving as fast as possible for short bursts with Maintian SOC on, then dropping back to reduce drag and stop the battery depleting while the REX is still running flat out for a bit as it catches up to where it should be. That's based on the assumption if you set it at 75%, then it drops to 65% it will keep generating flat out till it reaches 75%. - mine's not due till March 2015 so can't try it out. :-( Once you get back to 75% SOC hit the gas, er, I mean electron pedal. Do you get any indication of what the REX is doing other than how loud it hums? Like revs, or power output gauge?

That's quite hard to model - even for me and I'm an excel whizz.

[edit to add...]
I forgot to say 7 minutes for a fill up? You obviously don't have a 20 litre jerry can That's 7 minutes every 3rd stop, and 1 minutes 50 seconds for the other 2.
15 to get out the car and open the hatch
15 to get the can and open the fuel flap
50 seconds pouring (maybe less)
15 to put can back
15 to get back in the car.
;-)

The question now is do you make a stop to use the last 2 liters in the jerry can? Probably, for 1m50 you get 20 more miles. Call it your fuel station finding reserve after you run the rex dry for the 3rd time in a row.

What you really need is a roof rack with 4 jerry cans, all linked to a gravity fed auto filler upper that replaces the standard fuel flap. sod the aero, you want to get there fast. It's what the guys doing long distance overland trips do when doing things like trying to set the London to Cape Town records.

http://www.4x4lifestyleshop.com/shop-equipment/expedition-accessories/fuel-tanks/90-aux-fuel-tank.html

Hmmmn... there's an idea.

 

[jadnashuanh]

FWIW, the fuel system in the i3 is pressurized...you probably could not just run fuel into the filler port.

The magic number on speed is 56mph. At that speed, the REx can keep up, slow down, and the generator slows down, too. The generator will not run fast enough to keep the battery level up or be able to recharge if you go much faster than 56mph, and if you go slower, the output drops to match the vehicle load. IOW, most of the time, at speed, it's barely keeping up with current loads, and only has a small surplus to recharge the batteries - it's pretty much that the output goes almost directly to drive the wheels rather than recharge the batteries (but it still needs to go THROUGH the batteries). After all, it is only a 34Hp motor, and that's when it is running full tilt, which doesn't happen when you slow down below 56mph. Drag goes up at the square of the speed, and that's a good portion, but by no means all, of the drain on the batteries. So, at 30 verses 60, at 60, you're drag component is 4x as much as at 30. Move that up to 90, and drag is 9x as much as at 30. The i3 won't go much more than 90mph (close to 130km/hr).

 

[FrancisJeffries]

The magic number on speed is 56mph.

I'm sure that's right... but I still get 90-95 miles on a tankful on fast roads and I'm not doing 56 mph! I'm doing 70-75mph (actual - my speedo over-reads by 2mph according to my GPS).
I can't hear it, but I guess the Rex is earning its living because the battery doesn't get depleted.

I daresay I could get much further at 56mph maximum, but I won't bother - life's too short, and it's much more fun keeping up with the other traffic

 

[nowtta60]

FWIW, the fuel system in the i3 is pressurized...you probably could not just run fuel into the filler port.

The magic number on speed is 56mph. At that speed, the REx can keep up, slow down, and the generator slows down, too. The generator will not run fast enough to keep the battery level up or be able to recharge if you go much faster than 56mph, and if you go slower, the output drops to match the vehicle load. IOW, most of the time, at speed, it's barely keeping up with current loads, and only has a small surplus to recharge the batteries - it's pretty much that the output goes almost directly to drive the wheels rather than recharge the batteries (but it still needs to go THROUGH the batteries). After all, it is only a 34Hp motor, and that's when it is running full tilt, which doesn't happen when you slow down below 56mph. Drag goes up at the square of the speed, and that's a good portion, but by no means all, of the drain on the batteries. So, at 30 verses 60, at 60, you're drag component is 4x as much as at 30. Move that up to 90, and drag is 9x as much as at 30. The i3 won't go much more than 90mph (close to 130km/hr).

I asked about what happens when the the REX output on another thread. Can't remember the name of the energy management unit, but it can route power either to the battery or direct to motors. I asked as I thought it would be silly to charge the battery just to run it down immediately after - more efficient to use the power direct in the motor. Meaning the i3 is the car I always wanted (till I can afford a Tesla) - a true series hybrid. As far as I know - the only one in existence. All the others are parallel hybrid (even the Volt).

Again from other threads, the REX can keep the EV motor running without depleting the battery at speeds up to 75 mph, on flat roads on calm dry days. Don't forget running the motor slower isn't necessarily better. It's about the balance between aero, motor electrical efficiency and REX electrical output efficiency. You never know - they may have set up the system so that its' more efficiency at 75mph than 56. Despite the higher drag. Only experimentation will find results though.

 

[ecoangel]

I use the i3 Rex as a petrol car with a small tank, and a get-out-of-jail-free battery backup when it runs out. I set out with full battery and fuel, and switch Rex on as soon as the battery is down to 75%. Doing that, I found that I can do over 100 miles between refills - with very little battery depletion - and arrive 700 miles later still with half a battery!
I don't bother with keeping the speed down on the motorways - quite the reverse - and still get 90-95 miles on a tankful.

Good to know Francis.

So the i3REX needs about 8 refills of the 2 imp Gallon tank plus 9.4kWh electricity to achieve 700 motorway miles at national limit? That's a lot of energy consumption for a small car!

AE magazine ran a test with accurate data logging eqm using an old 2008 spec. Vauxhall/Opel Astra:

"We set the Vauxhall Astra 1.9 CDTI SRI up on Millbrook’s high-speed bowl to do a single lap at 60, 70, 80 and 90mph. Our first run gave a very creditable 52.4mpg, but as our speed built, it soon tailed off.

At 70mph the average was down to 42.3mpg, at 80mph it was 39.7mpg, while 90mph saw it down to 36.1mpg.

Read more: http://www.autoexpress.co.uk/car-news/31861/mpg-mythbusters#ixzz3FZtk2Ycj
"

So the old ICE Astra did 2.7 less miles per gallon but used zero electricity. Hmmm, come on BMW surely this is 2014 and technology will improve efficiency even at the lowest autobahn speeds?!

 

[FrancisJeffries]

So the i3REX needs about 8 refills of the 2 imp Gallon tank plus 9.4kWh electricity to achieve 700 motorway miles at national limit? That's a lot of energy consumption for a small car!

44mpg on the Rex didn't seem too bad to me! I never seemed to get better than 45mpg in my MB B180, either, and the i3 is much the same frontal area. Don't forget, I have only one car, and using the i3 Rex this way saves all the hassle of either hiring a car for long journeys or having a second ICE car just for them. It's a no-brainer.
And the cost of fuel pales into insignificance as compared with depreciation!

 

[BrianStanier]

You can't compare a single constant speed lap with real world driving. IIRC on the standard test cycle (USA one?) the i3 Rex does very well, only hybrids consistently beating it. My real world dricing in Rex mode gets a little over 50 mpg (imperial). My baby Hyundai i10 barely manages 47. Our Prius managed 57.4 over its 4 years,

 

[Lecram]

Last weekend I did a 250 km trip, starting with a full battery and full fuel tank. After 110 kms at 100 km/h with 21% soc left, I stopped for diner at a CCS charger location. Unfortunately, this charger was occupied by a Tesla and I had to use the 3.7kW charger. After diner, I left with 27% soc with 151 km to go. I decided to drive at 120 km/h to test it's range. After 30 km, the Rex turned on (at 6.5% soc) and 30 km before home, I dropped speed to 90 km/h to be able to get home without a stop. 3 kms before home, I stopped at a gas station as the tank was empty and I had 2 kms electric range left.
I had 3 passengers with me so the car was pretty heavy loaded.

 

[jadnashuanh]

34Hp with conversion losses is all the REx can supply. Depending on what accessories you have on, heater, air conditioning, lights, stereo, wipers, etc. whether it will keep up with your use depends on that plus your speed and the road you are on. Certainly, cruising on fairly level roads is one thing, running up and down steep hills or charging up a long grade is another. Doing it in the rain with the wipers going full tilt and the lights on up a long grade is entirely a different one. 56mph is the slowest you can go and have the REx run at full output...slow down, and the REx slows down...depending on your current consumption, that may or may not be enough to maintain, or charge the batteries. If you let the battery get nearly depleted, you'll lose max speed and many of your creature comforts. A REx with an empty fuel tank is slightly worse than a BEV version, since it's carrying around an extra person in the back all of the time - in USA EPA testing, that extra weight accounts for about 7mpgE for the REx verses the BEV.

Certainly, if the REx fits into your needs, it's a good choice, but it's not an ICE where you have full performance anytime as long as you still have some fuel. The i3 starts to degrade when it starts to run out. ANd, like any vehicle, run it out of energy (whether that's liquid or electrons), it will stop.

 

[WoodlandHills]

I picked up my i3 a few days ago and just about the first thing I did was to run the battery to zero to see how the REX performed. The range went from 1 to blank and I suppose the REX kicked in a some point although I could not hear it over the wind at 60. I did hear it when I slowed down to 45, but it shut off at every stoplight and let me pull away on battery before it started up again. I also had no trouble cruising at 65 on the freeway and climbing hills. This was in 99 degree weather with a/c on and drive in Comfort.

At this point, I can see or feel no difference in performance although I look forwards to trying the same experiment at night with the lights on and a/c running.

 

[jadnashuanh]

You may not notice any differences until the SOC reaches somewhere just below 2%. At that point (don't remember the exact number), you WILL notice, as it will shut off the heat and a/c and limit top speed considerably. Many people will never experience this situation unless they are trying to deal with long grades at high speeds under heavy load once the batteries are run down.

 

[ultraturtle]

Regarding the very real, but massively hyped limitations on REx operation, Jadnashuanh sums it up nicely:

Many people will never experience this situation unless they are trying to deal with long grades at high speeds under heavy load once the batteries are run down.

Engineering a vehicle involves tradeoffs. BMW could have tweaked any number of gasoline engine platforms to create greater or lesser electrical output, but settled on what I and others believe to be a very good balance. Greater output would require greater weight and volume, detracting from efficiency. Anything less might not be able to sustain typical highway speeds of 75 mph or so with enough grunt in reserve to sustain full power accelerations and reasonable climbs.

All for the weight of an average American teenager? Pretty sweet.

Not sure why folks keep comparing the i3 REx to the i3 BEV. They are completely different animals for completely different tasks. The i3 BEV is far and away the most efficient and practical BEV on the planet, whereas the i3 REx is far and away the most efficient and practical PHEV on the planet, beating out all but the i3 BEV and a couple of compliance BEVs. Just look at the numbers: