Regenerative Braking Physics Question

[TromboneAl]

During a test drive of a Nissan Leaf, the salesman said, "Of course you get better range for city driving, due to the regenerative braking."

That seemed to make sense until I thought about it later.

Assuming you get back less energy when braking than you use when accelerating, his statement can't be right, can it?

Compare:

1. Accelerate 0 to 30, drive 50 miles, decelerate 30 to 0.

2. Drive 50 miles in the city, accelerating to 30 and decelerating to 0 multiple times.

I'd expect situation 1 to use less energy.

 

[gauss]

Perhaps what he was trying to say was: "You will get more of an advantage of an electric vehicle over a conventional vehicle with city driving".

-gauss

 

[Animorph]

They can actually get better milage in the city than on the highway. The regen braking would be a contributor, but I think higher highway speeds and wind drag are major factors in lowering highway milage.

 

[ArkTinkerer]

... but I think higher highway speeds and wind drag are major factors in lowering highway milage.

This. Regen will never recapture the energy 100%. You never get back all the energy you put in to accelerate. So, bottom line, regen can't gain energy. The faster you go the more the wind load increases. I've read that load can be high enough on some cars that running the AC is more efficient than opening the windows!

 

[soupcxan]

Drag increases with the square of speed. So 0 to 30 to 0 with a good brake regen may use less total energy than flying along at 60 with no braking. I think he could be right, but it's not as simple as he's making it sound.

 

[jazz4cash]

During a test drive of a Nissan Leaf, the salesman said, "Of course you get better range for city driving, due to the regenerative braking."

That seemed to make sense until I thought about it later.

Assuming you get back less energy when braking than you use when accelerating, his statement can't be right, can it?

Compare:

1. Accelerate 0 to 30, drive 50 miles, decelerate 30 to 0.

2. Drive 50 miles in the city, accelerating to 30 and decelerating to 0 multiple times.

I'd expect situation 1 to use less energy.

Yes situation 1 uses less energy, but the salesman was saying you would get better range because without regen, the brake energy is converted to heat and wasted to the atmosphere. With regen, some of the energy can be stored and used for propulsion. City driving (e.g. high stop density) increases regen utlilization compared to highway driving.

 

[Koolau]

My understanding is that hybrids often get their best mileage in town. I'm assuming it is the combo of region. PLUS the lower speeds leading to lower drag. I'm guessing there are forums on the Leaf and/or all electrics. I know there are on hybrids. You might see what they have to say. Enjoy the process.

 

[LRDave]

Yes situation 1 uses less energy, but the salesman was saying you would get better range because without regen, the brake energy is converted to heat and wasted to the atmosphere. With regen, some of the energy can be stored and used for propulsion. City driving (e.g. high stop density) increases regen utlilization compared to highway driving.

^^^ He means this.^^^^^^^ You get better mileage given the same number of stops/slow downs that would require conventional braking.

 

[NW-Bound]

+1 on what others said.

To paraphrase, lower speed travel should result in better fuel mileage due to lower aerodynamic drag, whether the engine is internal combustion or electric. However, city driving is also stop-and-go, and that hurts the fuel performance of the ICE car relative to the electric car because the latter recovers some of that energy with regenerative braking. In addition, while stopping, the ICE still burns fuel while idling, while the electric motor is shut off.

From what I have seen, ICE cars have lower city MPG than highway MPG, but electric or even hybrid cars can have better range in city than on highway.

 

[ERD50]

Other posters have covered it, but let me express it just a bit differently...

...

Compare:

1. Accelerate 0 to 30, drive 50 miles, decelerate 30 to 0.

2. Drive 50 miles in the city, accelerating to 30 and decelerating to 0 multiple times.

I'd expect situation 1 to use less energy.

Yes, but if you are comparing to highway driving the comparison should be:

1 (highway). Accelerate 0 to 55 mph, drive 50 miles, decelerate from 55 to 0.

2 (city). Drive 50 miles in the city, accelerating to only 30 mph and decelerating to 0 multiple times.​

So as others have pointed out, the conventional ICE car and the EV face pretty much the same situation on the highway.

In the city, the stop and go requires a lot of energy from the ICE to accelerate, and to idle, and the energy to stop is wasted. For the EV, less idle wasted (unless the heater or AC is on?), and recovered energy when stopping.

If the EV gets better city range than highway range, that must mean the wind drag is a bigger effect than the acceleration/regen/idle.

-ERD50

 

[ERD50]

Yes situation 1 uses less energy, but the salesman was saying you would get better range because without regen, the brake energy is converted to heat and wasted to the atmosphere. With regen, some of the energy can be stored and used for propulsion. City driving (e.g. high stop density) increases regen utlilization compared to highway driving.

But that doesn't fully answer his question.

Assuming you get back less energy when braking than you use when accelerating, his statement can't be right, can it?

Yes, the regen helps, but as T-Al mentioned, it isn't even 100% efficient, so stop/go alone can't increase the range. That would take greater than 100% efficiency (perpetual motion machine talk).

You need to add in wind drag and compare stop/go at city speed to constant highway speed to see why city range can be greater than highway range.

-ERD50

 

[NW-Bound]

Very good, ERD50. I, along with others I believe, missed T-Al talking about driving 30 mph on the highway.

What T-Al was thinking is that between the constant 30-mph speed and case 2 of repeatedly accelerating to 30mph then slowing down (thus having an average speed lower than 30mph), the constant speed case will be more energy efficient, even with regenerative braking for the varying speed case. This is likely true.

We missed his leisurely 30-mph drive on the highway because while he might be able to do it where he lives, it would incite road rage in most locations.

 

[Winemaker]

I used battery equipment for over 20 years in an underground environment on track haulage, with both hydraulic and dynamic braking. Track haulage is considered close to frictionless; but I still had to deal with elevation changes. Trust me, traveling in and out same track for up to 2 miles each way, was not easy on batteries.

I chuckle when I hear about battery use/life on some present and future transportation. If any industry needs and requires better, stronger, lighter, faster to charge, cheaper, efficient batteries, it is the underground mining industry. A LOT of money is and has been spent on battery technology since the 80's, and has little to show for it.

 

[NW-Bound]

Yes, please tell us more.

I have read that this new Lithium Iron Phosphate (LiFePO4) type is wonderful and outperforms other batteries in the long run, even though the initial cost is higher. Have you seen it tried in your application?

Been thinking about putting that into my RV, but I am not a full-time RV'er hence cannot justify the cost of $1500 to replace a couple of golf-cart batteries.

 

[lawman3966]

The high mileage of a hybrid was explained to me as follows by a former colleague. He was describing the Prius, but I believe that the explanation applies to electric vehicles as well.

He said that the principal reason the Prius got better mileage than other vehicles, at any speed, was the gas engine was kept running at a substantially constant speed, and was supplemented at times by the battery. This is in contrast to gasoline engines which operate at a huge range of engine speeds many of which suffer low energy efficiency.

As others have said the reason any particular hybrid vehicle runs more efficiently at lower speeds arises from the energy loss to wind drag at higher speeds.

As to regen, braking my former colleague was deeply skeptical. He indicated that braking causes energy to dissipate at a much faster rate than the rate at which batteries output energy during acceleration. As a result, the batteries and electrical circuits simply can't absorb the electrical current generated from the mechanical braking energy. As a result, much of the braking energy is dissipated as heat, even in the presence of a regenerative braking system.

Returning to the original question: in all likelihood, Priuses and electrical cars get better mileage at city speeds than at highway speeds. However, this result is likely due to avoidance of wind drag and avoidance of the gasoline engine energy efficiency variability at a range of engine speeds, and not mostly due to regen braking.

 

[clifp]

Here is nice video which shows the measurement for a Tesla on regenerative breaking.

 

[Winemaker]

We did not use lithium batteries, whether it was prohibited by fed or state for underground use yet, I do not know. I am unaware of any equipment manufacturer even using them; even on the surface. We use a lot of battery equipment, diesel is prohibited in most cases, and equipment with cables are a HUGE pita. There is a great need, and I'm sure if anything was practical it would be in use. An extreme amount of downtime changing batteries, and an extreme amount of power/time requirements to recharge even at 480V 3 phase power.

 

[NW-Bound]

Many industrial equipment pieces still run on lead-acid batteries I believe. Lead-acid batteries are well-understood and safe. LiFePO4 batteries are claimed to be a lot safer than earlier types of Li battery. I have seen some test batteries intended for aviation that can be punctured by a bullet and will not turn into a fireball like earlier types. Their price is still high, and may never get down to that of the lead-acid type.

 

[ERD50]

I used battery equipment for over 20 years in an underground environment on track haulage, ...

.... If any industry needs and requires better, stronger, lighter, faster to charge, cheaper, efficient batteries, it is the underground mining industry. ...

A bit of a sidetrack here, but I recently saw a Shark Tank episode where Pat Boone and another guy were trying to pitch a car that 'runs on air'. Of course it is only propelled by (compressed) air, it 'runs' on the energy that was used to compress the air, which is done by an electric motor at a 'charging station' and, since they were marketing these in Hawaii, most of that electricity is being generated by burning oil, and since there are inefficiencies all along the way , the 'air car' is very likely burning more oil than the average mid-size car (and the air car was a tiny clown-car style)!

But of course, there were many supporting comments on the web, many of them touting the brilliance of this 'new invention' (that the oil companies will buy up and bury in the desert or some other tin-foil talk!)!

Turns out their pitch was just licensing the MDI 'product', which was supposed to come to market just about every year since 2000. You can google MDI and air car for more.

But to your comment, reading more on the history of compressed air, compressed air vehicles have been in use in mining operations since the mid-late 1800's. So much for this 'new invention'! For mines, I guess the compressed air provided the advantage of no harmful exhaust, and no combustion. But it is inefficient (largely because you lose much of the heat during compression - that is wasted), so won't replace alternatives except in these niche applications.

But a lot of non-technical people sure were impressed! It runs on air! Crush the big, bad oil companies!

A LOT of money is and has been spent on battery technology since the 80's, and has little to show for it.

Seems to me that battery progress has been steady, but mainly in the lithium area, and it sounds like lithium was not used in mining, due to cost or safety?

So maybe what you are saying is that lead-acid batteries have not improved much? I guess I'd agree. A company in Peoria (former Caterpillar employees), were developing a lead-acid with some kind of carbon foam plates to replace the heavy lead plates. Was said to improve battery characteristics, and reduce weight. But of course there were some obstacles to overcome to get into production. IIRC, their goal was to replace both anode and cathode, I think one was easier (but far less total benefit), but I think they got some military contracts based on that first step.

Here they are: Firefly -

History Of Innovation | Firefly International Energy

-ERD50

 

[Winemaker]

In the auto industry, efforts have been made to decrease mass and therefore weight, in the powertrain and in the vehicle. (ie. aluminum engine and engine pieces, plastic and carbon fiber) These allow less power to be used in acceleration and braking, making autos glorified golf carts.

No, not much advancement in lead-acid batteries, other than smart charging systems. Lead-acid batteries also provide mass and weight to increase drawbar pull, traction and counterweight requirements, but again, if a modern battery could provide longer life, faster recharge times, and be safe, hell, concrete could substitute.

 

[ERD50]

I guess I'm so jaded by all the 'breakthroughs' that die on the vine, I was pleasantly surprised to see that this carbon foam lead-acid battery by Firefly has actually gone into production!

What is the stage of development is Firefly technology?

The Oasis battery (Group 31 size) is commercially available in 12v and 4 v models. These batteries use carbon foam negative plates. Larger format batteries are expected to be in production by the fourth quarter of 2015. Batteries with carbon foam positive plates will become available in 2016.

So these are the single plate foam versions. But the fact that they are actually shipping these gives me some confidence in their 2016 goal for replacing both plates.

The Firefly batteries have four times the life and two times the energy density of a traditional lead acid battery

It looks like that's still about half the energy density of Lion? I'll have to poke around to see how much better they expect the generation with two foam plates to be.

-ERD50

 

[MBAustin]

FWIW, my Prius gets better mileage at moderate highway speeds (55-65) than in city traffic with frequent stops. The battery is not just charged by regen braking but also by the gas engine, so the electric engine is always providing some of the power. Even though you get significant battery charge from stopping at a traffic light, the acceleration back to 30-35mph is almost completely from the gas engine at a low MPG (at which point if you back off the accelerator, you can go some distance on electric only),

 

[ERD50]

FWIW, my Prius gets better mileage at moderate highway speeds (55-65) than in city traffic with frequent stops. The battery is not just charged by regen braking but also by the gas engine, so the electric engine is always providing some of the power. Even though you get significant battery charge from stopping at a traffic light, the acceleration back to 30-35mph is almost completely from the gas engine at a low MPG (at which point if you back off the accelerator, you can go some distance on electric only),

I'm not sure the bold parts are accurate.

so the electric engine is always providing some of the power. - Unless you are accelerating, or running under battery power only, the electric motor won't normally be providing any power. An exception to that would be if the regen braking had already topped off the battery, I think they will 'use up' some of that to propel the car, so that the next regen cycle can be stored rather than wasted.

There is no point in using a generator to charge the battery to run the motor in a Prius (as opposed to a serial hybrid), there is loss in the conversions. Other than acceleration boost (and the exception I gave above), it is more efficient for the gas engine to drive the wheels directly, and skip the conversions. Even the Chevy Volt, which is mostly a serial hybrid, does some of this for efficiency.

Another mode could be, the battery is very low due to little regen. But they need the electric motor boost to be ready to handle any required acceleration, they could use the gas engine to do some battery charging. The steady state of the engine won't be affected much, and then the battery will be ready to boost the acceleration for a short burst. But they wouldn't run the drive the motor at the same time (I think the motor and generator are the same piece of hardware in the Prius?).


the acceleration back to 30-35mph is almost completely from the gas engine - Acceleration is where the electric motor gives max boost. It might be true that most of the energy comes from the gas engine (especially at those speeds), but if it was 'almost completely from the gas engine', there would be little point in adding the electric motor and batteries!

Or maybe almost all the electric motor boost is below 30 mph? I don't know - I would assume it would come into play anytime you accelerated?

-ERD50

 

[MBAustin]

I'm not a Prius engineering expert, my comments were based primarily on my 5 years of Prius driving experience, including watching the displays that show roughly what's going on with the 3 motors (2 electric & 1 gas).

For those who really want to get into the details, there's a much better forum:

PriusChat

Two other interesting sites:

Explanation/demo of the Power Split Device (PSD) that determines which engines do what at any given time:

Toyota Prius - Power Split Device

Detailed explanations of "What's Going on As I Drive" a Prius

Graham's Toyota Prius
"The harder you press the accelerator, the more torque the ICE produces. This increases both the mechanical torque though the ring and the amount of electrical power generated by MG1 for MG2 to use to add still more torque. Depending on various factors such as the battery state of charge, the road grade and exactly how hard you press the pedal, the computer might draw extra power from the battery to boost MG2's contribution. This is how highway passing acceleration is achieved with only a 70 horsepower ICE in such a big car. On the other hand, if power demand is not that high, some of the electricity produced by MG1 may be used to charge the battery, even while accelerating! The important thing to remember is that the ICE both drives the wheels mechanically and drives MG1 forwards enabling it to generate electricity. What happens to that electricity and whether more electricity is taken from the battery depend on complex factors which may be beyond our ability to fully figure out."

 

[NW-Bound]

Here is nice video which shows the measurement for a Tesla on regenerative breaking...

I watched the energy consumption display, and it went from 1.7 kWh at rest to 2.9 kWh at the top speed of 100+mph, then dropped down to 2.4 kWh at rest again.

So, the car expended 1.2 kWh to get to that top speed and recovered 0.5kWh or 42% of that energy. That's not bad considering the energy irretrievably lost in the moving part friction, tire rolling resistance, and aerodynamic drag.

By the way, 1.2 kWh is the energy that can lift this 4,800-lb car a height of 660 ft. And in order to charge the 85 kWh battery from empty by regen braking, you need to coast the car from a height of no less than 47,000 ft.

The above numbers give me a better perspective on the amount of energy that is required to push a car at highway speed. It's huge.