Is Constantly Accelerating and Regenerating More Efficicent?

[thefuturenow]

Since I decided to drive efficiently as BMW intended and make liberal use of the regenerative braking, the i3 REX has surpassed my expectations as far as range is concerned netting about a 170-mile range uncoded on my trip back. I'd like to partly attribute this to my driving style, which involved minimal acceleration and aggressive regenerative braking.

Simple question: is it more efficient to cycle between accelerating lightly and aggressively regenerating to charge the battery even if braking is unnecessary, or is the amount of charge and the amount of regen utilized on the display disproportional? I.e. is it better to keep a steady speed or does regen braking 5-10mph every once in a while then slowly accelerating back provide more efficiency?

 

[Schnort]

The most efficient is to keep it 'in the middle' and do neither accelerate or regen.

You lose energy regenerating, so accelerating to a speed faster than you need is going to end up in some amount of energy being shaved off as losses to slow down to your desired speed.

It's best to slowly approach the speed you want to be at and keep it as close to the middle as you can to keep the steady speed. When you need to stop, take the foot off the "gas" at the exact time so that it stops where you want by only using regeneration.

 

[Elkjaer]

Schnort is right, and a simplified explanation is to see the batteries as a bank account. Acceleration is then a withdrawal from the account and regeneration is a deposit.
For every withdrawal the bank will take a small fee and the bank will also take a fee for every deposit you make.
So by accelerating and regenerating you will only lose money (energy).

The most effective driving style is then to convert all the withdrawn energy into distance, meaning no regeneration at all. Should you need to decelerate then there will be a fee, but this is still much better than other vehicles where regeneration is not possible and you will throw away all the withdrawn energy because you cannot make a deposit.

 

[jadnashuanh]

Learn how to coast in the i3. Let friction slow you down if you have time verses regen. Keep going at speed so you can maximize regen when you need to stop means you were using energy too long that you cannot fully recover during regen - there are conversion losses when trying to make power to charge the batteries.

 

[websterize]

One way to confirm coasting is to set the on-board computer to current consumption. In the U.S., it'll say 19.9 mi/kWh when you're coasting. After some practice, you can start to feel when you're coasting without looking at the instruments. It could be my imagination, but the coast "zone" seems wider in Eco Pro mode. You can still coast in Comfort mode, but it seems to require more precise foot input on the go pedal.

 

[jadnashuanh]

Get the power use meter to the center and keep it there to coast. It takes a little practice, but after a fairly short time, you'll recognize it without having to look. Unless you have some outside source (say gravity when going down a hill), regen will never be as efficient as just maintaining a reasonable speed and coasting. As I said, regen just means that you've used too much 'go' power, and need to scrub it off. You never regain as much as you use to accelerate and then slow intentionally. It's certainly more efficient than an ICE, but by no means as efficient as the i3 can be.

 

[bwilson4web]

Funnily enough I was asked the same referencing 'Pulse and Glide' which I throughly disapprove of unless on a closed track. Using a speed range of 25-42 mph, I measured a 10-15% improvement over the equivalent constant speed in a Prius. But it is as foolish in traffic.

I am still benchmarking our i3-REx but being predictable in traffic avoids 'Pulse and glide and crash'.

Bob Wilson

 

[jadnashuanh]

If you're driving at a steady speed, depending on the slight variations in the road, you may end up in coasting some of the time. Some people cannot keep a steady speed in those circumstances, and certainly would likely use more power as the car gently accelerates when there's a dip. Because of the conversion process, acceleration will never be as efficient as regeneration. If you really want to save as much energy as possible, use the cruise control and let it do the saving for you.

 

[thefuturenow]

I figure that perhaps you get more energy overall with i.e. aggressive regeneration to 0 as opposed to a conservative regen to 0 but that may be incorrect and if so a conservative regen may be better for the tires. I hypothesize that a conservative regen would put less wear on the tires, in the same way conservative acceleration puts less wear on the tires. Fair hypothesis?

Also, I'm a little confused at exactly what the recuperation per kWh is stating. On my maiden voyage home where I was cycling between an aggressive regen and steady acceleration I averaged over 20 mi per kWh in recuperation, and now driving more by-the-book average around half that.

 

[thefuturenow]

Funnily enough I was asked the same referencing 'Pulse and Glide' which I throughly disapprove of unless on a closed track. Using a speed range of 25-42 mph, I measured a 10-15% improvement over the equivalent constant speed in a Prius. But it is as foolish in traffic.

I am still benchmarking our i3-REx but being predictable in traffic avoids 'Pulse and glide and crash'.

Bob Wilson

What about in comparison the equivalent constant speed in an i3?

Of course, only "pulse and glide" when there's nobody behind/around you to avoid the crash step :lol:.

 

[jadnashuanh]

Accelerating means getting the entire mass of the vehicle up to speed. Constant speed only has to deal with drag. Constant speed is way more efficient than accelerating. Watch the power consumption meter on the screen...while cruising at a constant speed, it may barely be above the zero point (more, as you go faster or up a hill). Using power longer then using maximum regen to slow down will never be as efficient as coasting to a stop. Using some round numbers for the i3, where E=1/2*m*v^2, and using 60mph, it takes about 124W (or 446512 joules) to do it. say you do that at near max, that's about 7-sec. Given that the car can easily get 4-miles/kw, and four miles at 60mph is 4 minutes of traveling, that's only 250w/mile, or 250/60-seconds, 4W/second (I'm rounding numbers here - you can be more exact if you really want to yourself). Accelerating for 7, takes 124, but cruising would only take 28W, or nearly 5x more power to accelerate verses just maintaining. Regen is nowhere near as efficient as draining the battery, the conversion losses (much of that in heat) just doesn't add up to not driving the vehicle smoothly.


Note, I made an error in the above calculation...I used pounds rather than Kg to determine energy use. I'm not going to redo the numbers, but even if I did, acceleration still would be taking at least 2x what driving the same time at steady state speeds would be.

 

[psquare]

Also, I'm a little confused at exactly what the recuperation per kWh is stating.

In the column display for ePower and Recuperation over time, the ePower graph shows the kWh/mi. The Recuperation/mi shows how many miles it would take to recuperate 1 kWh. The aim is to have small columns at the lower graph (recuperation) and larger ones on the top (consumption).

 

[thefuturenow]

Also, I'm a little confused at exactly what the recuperation per kWh is stating.

In the column display for ePower and Recuperation over time, the ePower graph shows the kWh/mi. The Recuperation/mi shows how many miles it would take to recuperate 1 kWh. The aim is to have small columns at the lower graph (recuperation) and larger ones on the top (consumption).

Ah, I understand. So x miles of regenerative braking would net 1kWh. The description BMW provides was slightly unclear.

 

[thefuturenow]

Accelerating means getting the entire mass of the vehicle up to speed. Constant speed only has to deal with drag. Constant speed is way more efficient than accelerating. Watch the power consumption meter on the screen...while cruising at a constant speed, it may barely be above the zero point (more, as you go faster or up a hill). Using power longer then using maximum regen to slow down will never be as efficient as coasting to a stop. Using some round numbers for the i3, where E=1/2*m*v^2, and using 60mph, it takes about 124W (or 446512 joules) to do it. say you do that at near max, that's about 7-sec. Given that the car can easily get 4-miles/kw, and four miles at 60mph is 4 minutes of traveling, that's only 250w/mile, or 250/60-seconds, 4W/second (I'm rounding numbers here - you can be more exact if you really want to yourself). Accelerating for 7, takes 124, but cruising would only take 28W, or nearly 5x more power to accelerate verses just maintaining. Regen is nowhere near as efficient as draining the battery, the conversion losses (much of that in heat) just doesn't add up to not driving the vehicle smoothly.

Thanks for the in-depth analysis!