Turn on Rex early on freeway?

[Mamaci3]

Looking for the best strategy on the freeway.

Will the car be more efficient turning on Rex while on the freeway?

 

[TheMK]

Usually if I'm going on a >100mi trip, then I use REx on the freeway if I can. You need to code "hold state of charge", what this will do is allow you to turn the REx on and off with the push of a button as long as the car's battery is below 75%.

I like to avoid using the REx in the city if possible.

 

[redbarn]

I do a lot of freeway driving in our Rex.
Firstly, you need to hack your I3 so that you can turn the Rex on after reaching 75% SOC.

I use the battery for non freeway running such as between home & the freeway entrance.
Once it is below 75% SOC, if I am on the freeway, I turn on the Rex & run at 70-73 mph.
This uses about 10% of the battery for each full tank of Rex running which is circa 80 to 90 miles.
I run the Rex until the Rex is down to 0 miles left & automatically shuts off.
I then fill up at the next available gas station, stay on battery until back on the freeway & then repeat the cycle.
I have done 500 miles like this.

I rarely use the Rex in town or even on 2 lane roads.

 

[jadnashuanh]

FWIW, the most efficient way with the i3 is to never use the REx! But, if you must, it's better to use it on the highway to help from the emissions in the densely packed city area. The REx seems to get about the equivalent of around 34-40mpg while the REx is about 113mpge...If you need the REx a lot, there are other cars that can get better mileage when their ICE is running.

 

[CharonPDX]

REx solely generates electricity to feed to the battery. REx is no more or less efficient on any given driving conditions - it's just sending power to the battery pack, and your efficiency driving on the battery pack is what matters.

Note that if you know you're going to be digging in to REx on your drive, it can be more convenient to use range-hold mode to use up the gas earlier in the drive, so you have more flexibility on when to refill the gas tank, than letting the battery drain, and having to plan your gas stop in a very short REx range. (I mean, what's the point in refilling at 1/2 tank repeatedly, you'd be refilling every ~30 miles...)

 

[jadnashuanh]

If you look at how far your REx extends your range, it's fairly easy to assign a MPG value to the energy the REx can produce. Yes, it never directly, mechanically drives the wheels, but the energy it produces does move the vehicle, and that distance is fairly easily measured, and it tends to be less than 40mpg. If you can use the entire 2.4g, and figure out how far that adds to your EV only range, those numbers reveil it's not super efficient.

 

[jfran2]

If you can use the entire 2.4g, and figure out how far that adds to your EV only range, those numbers reveil it's not super efficient.

You're absolutely right; the Rex on its own is not super efficient. But taken as a whole with the rest of the battery range, it's excellent! I use my Rex at least a little every week. Last week I averaged over 160 mpg. That, in my opinion, is what makes the i3 Rex brilliant, and so much better than any other PHEV out there. Nothing else can touch it!

 

[jadnashuanh]

Figuring how many miles you go when it is with the REx off is not going to give you an indication of how efficient the REx engine is by itself...you need to see what the incremental range you get while the REx is running. If you do that, then the solely EV range versus the REx augmented range when separated, can give you your mpg...it's not great, there's more than a few that can beat the i3 REx's mileage gain.

Say running the REx adds maybe 80-miles to your range of the battery alone. 80/2.4=33.3mpg gained...IOW, to go those 'extra' 80-miles, you had to burn 2.4g. Now, as a convenience if you must travel beyond the pure EV range, it extends the usefulness of the vehicle, but it does not represent super efficiency from burning that liquid fuel. By far, the most efficient mode is to use the vehicle solely on batteries and never use the REx at all. ANd, it's less than the BEV's mostly because of the extra weight you're carrying around all of the time for those infrequent uses. IOW, it could be 3-4% better all of the time if it didn't have the REx in there at all.

Since it can save the need for a second vehicle for some users, it extends the market for the vehicle, but it is not the most efficient use, and the engineers didn't want to include it...the marketeers did (and were probably right, but not for an efficiency reason). WHen the engineers sweated grams, why would they then throw in many tens of thousands of them by the addition of the REx? It wasn't by choice.

 

[jfran2]

I'm sure you're correct as usual. We're coming at this from two different angles. From my point of view, I don't really care if the Rex is heavy, noisy, and inefficient. Thanks to its existence (and the ingenuity of BMW engineers), I can drive 160 miles of variable-length trips in a week and burn one gallon of gas. I don't have to worry that charging stations are few and far between in my part of the country, or that winter puts a pretty sizeable dent in my range. As my car gets older and the battery loses more and more efficiency I'll still be able to take it wherever I want - I'll just have to use a little more gas in the process. And if at some point I've lost so much range due to age and wear on the battery, perhaps BMW will have a battery swap for a reasonable price, and I'll be able to give the car a whole new lease on life. Meanwhile, I'll continue to enjoy my little Rex and the freedom it gives me to go wherever, whenever, and be dependent only on my own level 2 charger and the occasional gas station. I applaud all of you who drive BEVs, and I would love to see a world where a BEV could be a practical reality for nearly anyone. But I love my Rex, and I'm thankful BMW created it! (climbing down from the soap box now...)

 

[jadnashuanh]

WHen the X5 hybrid is re-released with the new configuration and nearly triple sized battery of the original one, I may get rid of my i3 and GT. THat should have enough battery capacity so my day-to-day needs can be entirely done on batteries, but I can then hop in, tow a trailer across the country only stopping when I want to fill up the large tank. As battery tech gets better, more and more uses for EVs and hybrids will open up with no significant user compromises and, at a better overall efficiency than what's generally available out there today.

Even Tesla's superchargers mean following a fairly rigid path going cross-country, and probably a 45-minute stop periodically. In my GT, I can go the same distance, anywhere I want, and not have to stop while traveling 500-miles or so. WHen I do eventually stop, I can be ready to go again for another 500-miles after a less than 10-minute stop and I can do that nearly anywhere.

CCS units as currently planned, with up to 350Kw outputs will help, but there are over 100K gas stations in the USA, many with LOTS of pumps, and less than a thousand CCS units with more than 50Kw, if that.

The infrastructure will expand as the demand increases. It will be a different environment in 5-years or less, with no help from the current US administration. The multi-billion $ fine on VW will end up helping all EVs.