Most energy efficient travelling speed

[Blue20]

We know "highway speed" drains power very quickly. Is there a "sweet spot" on speed that is most efficient? 30 mph? 55 mph?

 

[jadnashuanh]

Drag goes up at the square of the velocity...so, the faster you go, the faster you'd drain the batteries. As a result, unlike an ICE, I'm not sure there is a 'sweet' spot that keeps the engine in an efficient spot without excessive drag. It's more of a function of how far do you want to go, and how fast do you want to get there, then figure out if you can actually make it under those conditions! Slow and steady will be the most efficient, but may create hazards if you're creating a moving roadblock, or you get there too late to do what you wanted!

 

[AviatorMan]

That was an interesting question.
An article that relates is http://bmwi3owner.com/2014/05/range-for-speed-2/#more-244
Using the information from that, converting to miles per hour, and then assuming the i3 has a usable capacity of 15KWh (leaves a 20% safety margin, 18.8KWh x .8 = 15KWh)
results in the following range at speed:

mph range
25 89
37 78
50 63
62 52
74 41

From that I am starting with the assumption that on the highway I will go 60 mph with a range of 52 miles and be left with a reserve of 13 miles at 62 mph.

 

[DeJay58]

The i3 has 19kWh usable capacity!

 

[Bunter]

18.8 kWh is the useable battery capacity, the actual battery capacity is 21.6 kWh so your range numbers are lower than I would expect. Unless you are factoring an a extra bit of battery comfort zone?

Bill

 

[roperld]

I have seen a curve of range vs speed for the Nissan LEAF. The peak is at 15 mph. I once had to drive slowly to make it home in the LEAF I leased; my experience was that 15 mph was an optimum speed.

 

[RJSATLBA]

Below a certain speed range becomes bladder dependent! :lol:

 

[noakey]

Below a certain speed range becomes bladder dependent! :lol:

Lol... excellent...

My speeds and modes are usually dependent on how far I am going and how quick I want to get there...

With a 56 mile round trip on the m4 commuting, I can do 70mph all the way and get home with 30 miles left..

I pretty much always drive with eco pro.. makes for a more comfortable ride for passengers and I.

Plus I like the blue text in the dash.

 

[jadnashuanh]

Keep in mind that the roads you are traveling on may or may not provide opportunities to either coast or to provide some significant amount of regeneration, and those two things can increase your overall distance traveled considerably, as can a long grade diminish it (unless you can regain some of it going back down the other side!). The i3 app shows how much power you've used and how much you've produced via regeneration, and sometimes, the regen numbers are pretty significant (best if you never touch the brakes).

 

[M2140]

I don't think the figures above are accurate.

I regularly get 75 - 85 miles per charge driving 90% on the freeway at 60-65 MPH in Eco Pro mode with AC on. I keep and eye on my current mi/kWh so that I know I'm driving Efficiently but still try to stay at around 65 mpg.

I average 4.5 mi/kWh over the last 2000 miles.

My best range so far was 85 miles on a single charge with a set departure time and the guage read 12 miles left. The trip was from my home in Menifee, CA to the California Science Center in Downtown LA.

I did not set a departure time for the way back so the Range Exterder kicked in at 75 miles, 10 miles from home. What I'm finding out is that without a departure time set the battery only goes to 80 or 90% of the battery capacity. Even though the car states 100%, you cannot safely leave the battery at 100% for an extended period. So the car probably charges to 90%. When I set a departure time I get more range because I'm guessing the charges charges the batteries at a higher percentage closer to 100%.

Other EVs like the tesla let you have more control on the percentage of the battery. BMW took a different approach with the departure times to protect the battery and make it easier for the non Techy people out there.

 

[ecoangel]

At 100km/h (62mph) the i3 BEV creates 326 Newtons of drag. The REX model a little more at 336 Newtons.

More meaningful to the average punter is BHP absorbed by drag. Here we are not considering the drag from the drive-train / single gear or tyre friction and this is for a flat road with nil wind.

At 40 mph the i3 consumes a minimum of 3.33 BHP (2.5kW) in drag alone. Not a lot! But accelerate to 60 mph and it goes up a factor of 4 to 12.5BHP (9.317kW). Get on the freeway in Montana or de-restricted Autobahn and at 93 mph the i3 requires 42.14 BHP (31.44 kW)!

It is no wonder that i3 drivers have noticed a massive reduction in range when driving on faster roads. If you add in tyre resistance, wheel well turbulence, and a less than optimal gear ratio (BMW optimised this for acceleration rather than cruising) then expect about 60 mile range from a BEV and a bit less all electric from the REX in fast ICE like driving style.

I would say the i3 is most efficient around town below 40 mph where aero effects are minimised.

 

[kevinb61]

At 100km/h (62mph) the i3 BEV creates 326 Newtons of drag. The REX model a little more at 336 Newtons.

More meaningful to the average punter is BHP absorbed by drag. Here we are not considering the drag from the drive-train / single gear or tyre friction and this is for a flat road with nil wind.

At 40 mph the i3 consumes a minimum of 3.33 BHP (2.5kW) in drag alone. Not a lot! But accelerate to 60 mph and it goes up a factor of 4 to 12.5BHP (9.317kW). Get on the freeway in Montana or de-restricted Autobahn and at 93 mph the i3 requires 42.14 BHP (31.44 kW)!

It is no wonder that i3 drivers have noticed a massive reduction in range when driving on faster roads. If you add in tyre resistance, wheel well turbulence, and a less than optimal gear ratio (BMW optimised this for acceleration rather than cruising) then expect about 60 mile range from a BEV and a bit less all electric from the REX in fast ICE like driving style.

I would say the i3 is most efficient around town below 40 mph where aero effects are minimised.

Yep, as is evidenced by the 138 city / 111 highway eMpg rating.

 

[Rower]

What would the expected round trip range be on reasonably flat interstate at 75mph?

Thanks

 

[i3atl]

What would the expected round trip range be on reasonably flat interstate at 75mph?

Thanks

It will depend on ambient temps/accessory use, but this has been verified in the BEV - 66 miles @ 75mph w/ the following conditions:

Ambient:60F
Elevation:Mostly flat with some mild hills
Test length:33 miles
Climate Control: Heat 70F
Drive Mode: Eco Pro
Other Notes: Headlights On

From: https://docs.google.com/spreadsheet/ccc?key=0AvITyz2R1UpYdFJlbU5QYktlZDk4eXNvYnlMRVZZQ3c#gid=0

 

[wayne325]

What would the expected round trip range be on reasonably flat interstate at 75mph?

Thanks

It will depend on ambient temps/accessory use, but this has been verified in the BEV - 66 miles @ 75mph w/ the following conditions:

<snip>

Yes except.... wind. Rain. etc etc etc.

Add a 20 MPH wind and you get half a trip at 95 MPH aerodynamic load plus half a trip at 55 MPH - and probably worse than that because wind won't be exactly in the direction of travel (or 180 degrees away).

So I'd say even that 66 range calculated is quite optimistic.

Now... if you have a charger at your destination then the calculus changes quite a bit.

The i3 is really meant as a city car. It's not for going on the freeway at 75 MPH for any extended time, as this exercise shows.

 

[Boatguy]

The i3 is really meant as a city car. It's not for going on the freeway at 75 MPH for any extended time, as this exercise shows.

Precisely. The problem for BMW is that most of those megacity dwellers don't have a reliable parking/charging location. In California we've got a combined $10K of government rebates and much better charging infrastructure, but too much freeway driving.

The i3 is fabulous driving in San Francisco, but the range melts away when I hit the freeway.