Optimum speed using Rex for very long journey

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amateurish

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Joined
Dec 16, 2013
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102
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Inspired by the other thread other use of Rex vs range, I wonder what the optimum speed for Rex is to get the shortest journey time when travelling a long distance i.e. more than say 500 miles. Let's say it takes 7 minutes to fill up again, and that you use the full 9 litres each time. Anyone want to have a go at the maths?
 
Now that's a good question :)

I haven't done enough running with the REx on to figure out the mpg's at the different speeds. I do know that 75mph seems to be the magic number for me where the battery will slowly start to deplete (generally flat roads, warm, dry).
 
This blogger has posted some quite detailed information on consumption at different speeds.

(edit) Oops - just realised that the blog I refer to was looking at the electric power usage rather than petrol consumption.

http://bmwi3owner.com/2014/05/range-for-speed-2/#more-244
 
Using those figures it looks like the faster you go the better. Assuming journey is 500 miles (800km)

At 120 km/h you make 6 stops, total journey time = 6h40 + 0h42 = 7h22
At 100 km/h you make 4 stops, total journey time = 8h + 0h28 = 8h28
 
The other issue which could come into play and affect amateurish's conclusion is that each time you run out of petrol you will use up some battery capacity from then until the next filling station, which could be 15 miles or more on some parts of the UK motorway network.

With the 'higher speed but more stops' option you are more likely to get to the point where you can't use the full (!) 9 litres because you have little or no buffer of electric range left to get you to the next filling station if you run the tank dry.
 
That's a good point Roger, you would need to "ration" your remaining charge carefully.

120 km/h might be a natural limit since once you start going 130km/h+ then you will start eating into battery charge anyway.
 
This is quite an interesting infographic looking at something similar with the Tesla (i.e. what speed to drive at on a long journey for fastest result). They tried 3 speeds and the middle speed was best. The i3 was in there too but obviously has to charge a lot more so total time was large in comparison. It would've been interesting if they'd also tried a REx for this to see how it compared.

http://www.sauliala.fi/electric-mobility-and-high-speed-roadtrips/
 
Until the USA has a large network of DC fast chargers, long distance travel on any electric is restricted. And, with the design of the i3, people that expect it to act like an ICE, with nearly unlimited miles available, will be disappointed unless they console themselves to the reality of what it was designed for.

The Tesla was designed for longer distance travel, thus they have up to over 3x the battery capacity (but not 3x the range of the i3). Unless you have access to one of their superchargers, it will take a much longer time to recharge the vehicle...different designs, different results.

I can see the allure of the i3 being an only vehicle, and it may work for some people. I do not think that is true for most, though. You'll either need to have access to another vehicle for those longer trips, or take a very long time getting there and back.
 
The difference in range between the i3 rex and standard Model S is not that great. 150 miles vs 208 miles on the EPA rating.
 
amateurish said:
The difference in range between the i3 rex and standard Model S is not that great. 150 miles vs 208 miles on the EPA rating.
Click to expand...
Yeah, but the difference in efficiency is significant. At 117 MPGe, the i3 REx turns electricity into miles 23% more efficiently than the 60 kWh Model S at 95 MPGe (and 31% more efficiently than the 85 kWh model at 89 MPGe).
 
Sranger said:
Of course once the Rex kicks in, the tesla kind of kills it in efficiency for the next 150 miles or so....
Click to expand...
Yeah, but the Tesla has to carry its inefficiency for every mile of the first 80 or so - that's a 31% consumption increase over the i3 REx for your 85 kWh Model S. As we all have heard time and time again, statistically only the tiniest fraction of daily driving in the USA exceeds 40 miles, much less 80. Better to carry a 265 lb backup for that tiny fraction and suffer the 6% efficiency hit for every electric mile driven, than to haul around an extra 2,013 lbs of battery capacity and structure (that's more than 3/4 the weight of an additional i3!) that will seldom be used by the majority of us who do not drive long distances every day.

The Tesla Model S is an incredible car, and you have thoroughly documented on this site the wisdom of your purchase, as your daily drive requirements far exceed the i3's electric only capabilities. Please understand, however, that as the popularity of EVs grow, there remains a core group of us that care far more about efficiency and the environmental impact of our driving behaviour than we do about bragging rights of owning a "pure" EV.

I make only the efficiency case, here. If electric range and straight line acceleration are important to you, the Telsa Model S obviously wins. Interested in efficiency, cornering, controls, visibility, sophistication of safety and entertainment electronics, seat comfort, the ability to seat 4 tall adults, or fitting a large package in the hatch area with the rear seats folded flat? The i3 holds the advantage.

Horses for courses. Your Telsa Model S suits your needs as well as our i3s suit ours. I can only speak for myself, but the i3 REx hits on all cylinders (all two of them, anyway).
 
ultraturtle said:
amateurish said:
The difference in range between the i3 rex and standard Model S is not that great. 150 miles vs 208 miles on the EPA rating.
Click to expand...
Yeah, but the difference in efficiency is significant. At 117 MPGe, the i3 REx turns electricity into miles 23% more efficiently than the 60 kWh Model S at 95 MPGe (and 31% more efficiently than the 85 kWh model at 89 MPGe).
Click to expand...

Yeah, but the +31% difference is only good for the first 70ish miles. The rest is -44% (assuming ReX gets 40MPG on gas). Model S drives the whole distance on battery and uses less overall energy at the end of the same 150 miles.

--Woof!
 
woof said:
Yeah, but the +31% difference is only good for the first 70ish miles. The rest is -44% (assuming ReX gets 40MPG on gas). Model S drives the whole distance on battery and uses less overall energy at the end of the same 150 miles.
Click to expand...
Which is why the Tesla Model S is a wise choice for folks who drive 150 miles every day. Happily, many of us do not need to spend nearly that many hours on the road, and the i3 is far more efficient for the time we do.

Horses for courses.
 
ultraturtle said:
Sranger said:
Of course once the Rex kicks in, the tesla kind of kills it in efficiency for the next 150 miles or so....
Click to expand...
Yeah, but the Tesla has to carry its inefficiency for every mile of the first 80 or so - that's a 31% consumption increase over the i3 REx for your 85 kWh Model S. As we all have heard time and time again, statistically only the tiniest fraction of daily driving in the USA exceeds 40 miles, much less 80. Better to carry a 265 lb backup for that tiny fraction and suffer the 6% efficiency hit for every electric mile driven, than to haul around an extra 2,013 lbs of battery capacity and structure (that's more than 3/4 the weight of an additional i3!) that will seldom be used by the majority of us who do not drive long distances every day.

The Tesla Model S is an incredible car, and you have thoroughly documented on this site the wisdom of your purchase, as your daily drive requirements far exceed the i3's electric only capabilities. Please understand, however, that as the popularity of EVs grow, there remains a core group of us that care far more about efficiency and the environmental impact of our driving behaviour than we do about bragging rights of owning a "pure" EV.

I make only the efficiency case, here. If electric range and straight line acceleration are important to you, the Telsa Model S obviously wins. Interested in efficiency, cornering, controls, visibility, sophistication of safety and entertainment electronics, seat comfort, the ability to seat 4 tall adults, or fitting a large package in the hatch area with the rear seats folded flat? The i3 holds the advantage.

Horses for courses. Your Telsa Model S suits your needs as well as our i3s suit ours. I can only speak for myself, but the i3 REx hits on all cylinders (all two of them, anyway).
Click to expand...

I mostly agree with you...

Since you are primarily focusing the environmental impact, you do need to factor in the additional impact of the gas, oil in the hybrid system... They are not trivial...

In my case I think that the Model S probably has a lower over all carbon foot print that the i3 REx because I need more than 70 miles in a day often. In fact last week I would have had to visit the gas station 5 times with the i3 REx due to an unusual driving pattern where I drove about 150-160 miles every day...

I guess in the end either vehicle is a MUCH better alternative to a pure ICE or even a parallel Hybrid design. You just need to make suer the vehicle suits your driving needs well...
 
ultraturtle said:
Which is why the Tesla Model S is a wise choice for folks who drive 150 miles every day. Happily, many of us do not need to spend nearly that many hours on the road, and the i3 is far more efficient for the time we do.
Click to expand...

For the record, you brought up the 150 mile range, which doesn't happen unless using ReX.

And, also for the record, after alternately driving the i3 BEV and the Tesla Model S 85kWh on my daily commute, I find the i3 is 48% more efficient than the Tesla (as measured by both car's "energy usage" readings...not wall meters like MPGe measures). Same driver, same commute, several drives over several days, similar weather/traffic. In traffic or by myself I much prefer the i3. With passengers or on the open road, the Model S. As you say, horses for courses.
 
Going back to original post.

You cannot travel 120 kph over long distances. You will deplete the battery.
I tested this over 2 days on a REx. 120 kph (actual, not indicated) did not allow the car to travel sustainably on gas alone. The car would reduce power to enable the battery to gain charge.

I did not test the optimal speed, but it's below 120 kph. Maybe close though.

But you have to allow for grades, cold weather, and cargo.
My testing was on flat ground, no cargo, and cool summer weather.
 
ultraturtle said:
Better to carry a 265 lb backup for that tiny fraction and suffer the 6% efficiency hit for every electric mile driven...
Click to expand...

I disagree wholeheartedly.
First, as I've said before, it's more than a 6% effeciency hit. You are not considering the more effecienct heating system of the BEV, and that you can more easily use the last 6% of the battery, and other things as I posted earlier.

Second, the most efficient solution is not to carry a backup at all!

The REx only adds a narrow additional useable range of about 80 extra miles. Beyond that it's not not efficient and not practical.
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.

"Horses for courses", as you say, means accept that the i3 is a great city car, and the REx adds no real objective value - other than psychological.
 

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