Tag: Prius

Post #1926: A Prius driver takes a pass on Chevy Bolt “one-pedal driving”.

 

Do electric vehicles (EVs) get rear-ended more often than conventional cars do?

They certainly should.

That’s my conclusion after trying out the “one-pedal driving” mode on my new (used) 2020 Chevy Bolt.   And working through the logical consequences of it.

The practical bottom line of this post is that you should think twice before you tailgate an EV in traffic.  Because the chances are good that they can stop a whole lot faster than you can.   And may give you less warning when they do.

Not convinced?  Keep reading the parts in red, below.

Words do not do it justice: An accurate description of one-pedal driving mode.

Source:  Yeah, I know it’s a front-wheel-drive car.  The Gencraft AI doesn’t, though.  Almost all pictures here are courtesy of Gencraft.

Here’s your typical bland one-sentence description of one-pedal driving mode:  “With one-pedal driving, the car has enhanced regenerative braking, and will begin to slow as soon as you ease up on the gas (accelerator).”

Before I bought a Bolt, my reaction to that was, big deal.  Almost all modern cars do that, to a degree.  Anything with an automatic transmission slows when you take your foot off the gas.  All hybrids use regenerative braking, that is, they slow down by generating and storing electricity, reserving the friction brakes (pads pressing on rotors) as a last resort.

Some EVs can now do it more?  Whoop-te-doo.

Now that I own a Bolt, I know that description is missing a key word:  Abruptly.  Or, rapidly. Or, with great force.  Take your pick.

Taking your foot off the gas in “one-pedal” mode is nothing like taking your foot off the gas in a normal or hybrid car.  You don’t coast, at all.  You stop, pronto.  Not quite a wheels-locked panic stop.  But far faster than I normally stop, and far faster than anyone would reasonably expect me to stop in traffic.  In the Bolt, in one-pedal model, take your foot off the accelerator and you pull a few tenths of a G worth of deceleration.  Enough to pull you forward in your seat.  Enough that there’s no way I would engage that mode in snowy or icy roads.  Enough that I’d think hard about it before I turned one-pedal driving on in a driving rain.

Enough, already.  You get the point.  Here’s a more accurate description of one-pedal driving mode:

The act of lifting up on the accelerator, in one-pedal driving mode, is equivalent to pushing the brake pedal.  Hard.  Your (lack of) accelerator pedal is your brake pedal.  It’s not 100% as much force as you can get, if you actually do mash down the brakes.  But it’s an appreciable fraction of it.

You may again think, so what?  So you can, in effect, actuate the brakes, without hitting the brake pedal.  What’s the big deal?

Keep reading.

Brake lights?  We don’t need no stinkin’ brake lights.

But wait, it gets better.

Prior to mid-2023, some EVs would do that — stop fairly abruptly, in one-pedal mode — without turning on the brake lights.  And no, I’m not kidding about that.  (Reference).

The worst of those were fixed via software update, so now, all EVs on U.S. roads will now show brake lights, at some point, during some level of deceleration, in one-pedal driving mode.

As an afterthought.  Does that make you feel better about it?

But even now, an EV manufacturer’s decision on when, exactly, to show brake lights, during rapid braking in one-pedal driving mode, is entirely voluntary, and entirely up to the manufacturer, here in the U.S.A.  And for all of them, those lights turn on after the car has started slowing down.

Oddly enough, if you see this brought up on-line, you’ll see nothing but apologists for it.  Ah, cars have always had ways of slowing down without showing brake lights.  Let off the gas, in an automatic-transmission car.  Downshift in a manual.  Or, if you’re a jerk, hit the parking brake to stop, to fake out the folks behind you.

But those events were either mild in nature (automatic transmission), or rare and mild (nobody in the U.S. drives a manual these days, and nobody in the last 50 years has been dumb enough to wear out their clutch rather than brake pads by routinely slowing the car by downshifting).  Or required outright malice, like using a hand brake to stop.

Now, by contrast, you’re putting out a whole fleet of cars, for Joe and Jane Driver, all of which are designed to be driven without touching the brakes.  Designed to allow for substantial rates of deceleration without using the actual brake pedal.  And for which the decision about whether, or when, to turn on the brake lights at some point during that one-pedal deceleration, is an option for the manufacturer to decide. 

Let me offer a clear contrast to what you are used to, in a traditional gas car.  There, the brake lights are designed to light the instant you rest your foot on the brake pedal.  Brake lights are actuated by a switch that typically sits directly above the metal bar holding the brake pedal.  That switch has a fine adjustment on it.  You literally fine-tune-it so that the tiniest movement of the brake pedal closes the switch.  Even the lightest possible braking pressure will turn on your brake lights.  Properly adjusted, you literally turn on the brake lights before the brake pads make contact with the rotors.

So we now have a mixed fleet of cars on the road.  For 99% of them, the brake lights illuminate as soon as the driver puts on the brakes.  For the remaining 1%, the lights may come on at some point, after the driver has “put on the brakes”, assuming the rate of deceleration exceeds the manufacturer-specified threshold.

Yeah, what could possibly go wrong with that?

Braking distance versus stopping distance.

Definitions:  Both terms apply to panic stops.  Braking distance is how far your car travels, from the moment that you’ve firmly stomped on the brakes, until you reach a complete stop.  Stopping distance, by contrast, is that, plus the distance you travel during your “reaction time”, that is, the time it takes to say “oh shit”, move your foot off the gas, and hit the brakes.

Honking the horn is optional, but highly recommended here in Northern Virginia.

Now for just a bit of math.

1:  It takes about three-quarters of a second to lift your foot off the gas, and put it on the brake, in a panic stop.  That’s in addition to the initial reaction time — the time it takes you to realize you need to stop quickly.  (Estimates vary, that’s my reading of the literature on the subject.)

2:  At 30 miles per hour, in that amount of time, a car moves about two car lengths.  (Calculated as (30 MPH *5280 FT/MI *(0.75/(60*60) HOURS) = ) 33 feet.

3:  EVs in one-pedal driving mode can initiate an abrupt stop without moving their foot to the brake pedal.

My takeaway from all that is that EVs in one-pedal driving mode should be able to panic-stop somewhere around a couple of car lengths shorter than traditional cars.  That’s not due to better brakes, or better drivers.  That occurs because they begin to brake rapidly before they even move their foot to the brake pedal.

Yeah, what could possibly go wrong with that?

Summary

Shorter stopping distance is just dandy if you’re driving an EV in one-pedal mode.  But maybe isn’t such a plus for the person in a standard vehicle, tailgating an EV in one-pedal mode.

If you are in traffic, behind an EV in one-pedal mode, and the EV in front of you makes a panic stop, you need to be aware that, compared to a conventional car or hybrid:

1: That EV is inherently capable of stopping faster.

2: That EV will give you less time to realize it is stopping.

And nothing about that car will give you the faintest hint that those two factors are in play.

You’ve been warned.

Background:  Regenerative braking the Toyota way, or why Bolt one-pedal mode does me more harm than good.

We changed the brake pads on my wife’s 2005 Toyota Prius sometime around 140,000 miles.  Up to that point, the brakes hadn’t needed any attention.

The crazy thing is, that’s not even brag-worthy.  Going 100K miles between brake jobs is normal for any car with regenerative braking.

The Prius has regenerative braking.  To the greatest extent possible, the car slows down by turning itself into an electric generator.  It converts the forward momentum of the car to electricity, which then charges the traction battery.  Cars with regenerative braking routinely go 100,000 miles between brake jobs.  So says the U.S. DOE.

No material efficiency gains — for me.

The reason for the low brake wear in a Prius is that almost all the braking energy is done electrically.  In an ideal gentle stop, the friction brakes only kick in below about 5 MPH.  (If your rotors have surface rust, and your windows are open, you can hear that happen until you knock the rust off the rotors.)

In an idealized stop from 30 MPH to zero, you can easily calculate the fraction of braking “power dissipation” accounted for by electrical generation versus friction brakes.  Kinetic energy goes as the square of the speed, so, in a hypothetical gentle stop from 30 MPH to 0 MPH, where the friction brakes only handle the part below 5 MPH, the fraction of braking energy is:

Friction fraction of braking energy = 5-squared/30-squared = 25/900 = ~3%

Electrical Fraction of braking energy = 1 – friction fraction = 97%.

In other words, with a reasonably gentle stop, in typical suburban traffic, regenerative braking (Toyota-style) converts about 97% of the car’s forward momentum to electricity.  You don’t get to keep all of that, because there are losses in the electric motor/generators, the wires and charging electronics, and in charging the battery.  Maybe you keep 80% of that, or so.

One rationale offered for EV one-pedal driving is that it improves efficiency by recapturing more of the potentially available energy from braking the car.  That’s because you can literally bring the car to a full stop, and so, in theory, capture 100% of the car’s forward momentum and convert it to electricity.  Of which, again, you might be able to keep and use maybe 80%, after all the relevant losses are factored in.

And that’s the main reason that Bolt one-pedal driving does more-or-less nothing for my driving efficiency.  Because, despite what you may read, the Bolt’s regenerative braking does more-or-less the exact same thing as the Prius, during moderate stops.  In normal (not one-pedal) driving, when I take my foot off the gas, the car begins to recapture energy through regeneration.  And when I push gently on the brake, it begins to capture even more energy through regeneration.  Just like a Prius.  (All you have to do is look at the dashboard, as you brake, to see that this is true.)  And in a normal, gentle stop, with rusty rotors, you can hear the Bolt friction brakes engage at about the same speed as the Prius — about 5 MPH.

I guess if you drive like a bat out of hell, regenerative braking can improve your efficiency somewhat.  Plausibly, those who routinely make quick stops can benefit from converting more of the stop to electricity, before the friction brakes kick in.

But my driving habits were formed during the Arab Oil Embargoes/energy crises of the 1970s.  And I’m cheap, to boot.  So I try to avoid rapid stops.

My gut reaction, from reading about this, is that the real fan-boys for one-pedal driving are, in fact, those who want to drive like a bat out of hell.  They like it for the “sporty” feel, and how it lets them zip around all that much faster.  Which, to me, makes the whole “efficiency” argument kind of silly.  If you drive that way, clearly efficiency isn’t your goal.  You’d get more miles per KWH by not trying to drive the Bolt like a sports car.

So, from my perspective, as far as efficiency goes, one-pedal driving provides a marginal improvement in efficiency, for those with habitually inefficient driving styles.  Turning that around, if you’re a laid-back driver by nature, you ain’t going to get much additional efficiency out of one-pedal driving, beyond what you get from regenerative braking in “normal” driving mode.

Extras for experts, 1:  There is one weird final twist on this, in that, in a hybrid, regenerative braking doesn’t much matter.  It might typically add just 2% to the vehicle’s overall efficiency.  That’s from a combination of factors.  First, even with the efficient Atkinson-cycle engine of a Prius, you start off by wasting 60% of the energy in the gasoline.  Second, with relatively small electric motor/generators, and most importantly a relatively small battery, the amount of regenerative braking force — the amount of current you can safely generate and squeeze into the battery, without damaging anything — is highly limited.   So for the U.S. EPA drive cycle, with its extended periods of fast stop-and-go driving, you tend to show only a modest amount of energy recapture, as a fraction of the total energy used by the vehicle.

In an EV, by contrast, regenerative braking is a much higher contributor to overall vehicle efficiency, as the Federal government measures it.  First, unlike a hybrid, all the inefficiency in converting fossil fuels to electricity is “off the books”, so to speak.  That occurs at your local utility, not in your car.  The calculation of overall car efficiency starts with charging it, so as a whole the vehicle appears to have vastly less total wasted energy, than a hybrid does.  Second, with large motors and much larger battery, you can safely put more current into the battery.  Thus, in a hard stop, an EV can likely capture more of the energy than an hybrid can, prior to applying the friction brakes.

Old dog, new trick — look ma, no brakes!

The first thing about Toyota-style regenerative braking is that it’s absolutely seamless.  In the best case, you wouldn’t even guess that the car had this feature.  Only if you listen very closely, and brake very slowly, can you discern the point at which the friction brakes are engaged.

The second thing about Toyota-style regenerative braking is that hybrids with regenerative braking behave exactly the same as any non-hybrid car with automatic transmission.  Take your foot off the gas, and the car begins to slow just a little bit, just like any other automatic-transmission car (then) on the road.  The harder you push on the brake pedal, the more braking force you get.

Regeneration in the Bolt, by contrast, feels nothing like a normal car in this regard.  It is far more aggressive, even in normal (not one-pedal) mode.  Take your foot off the gas in a Bolt, and you slow much faster than you would in a standard car with automatic transmission, or in a typical hybrid.  I have already had to break myself of the long-learned habit of lifting my foot from the gas when I see a red light ahead.  On the roads around here, If I were to do that in a Bolt, I’d come to a dead stop long before I make it to the light.

But I can live with that.  I lift my foot, eyeball the dash, and look for the something close to zero KW going into or out the battery.  It’s hardly a life-changing difference in driving technique.  Not after I had to re-learn driving for the Prius Prime, and its preference for constant-power (instead of constant-force) acceleration (Post #1618:  There ain’t no disputin’ Sir Isaac Newton).

But switching to one-pedal driving has one potentially life-changing difference:  You may lose the instinct to put your foot on the brake.  If you never need to panic stop, you can literally drive the car in one-pedal mode and never touch the brake.   (Some one-pedal fans brag about doing exactly that.)

So do I, as a 65-year-old guy, now want to train myself to drive in one-pedal mode?  This, when the approach to driving is so different from our other car (a Prius).  And this, where driving in this new style means basically to ignore the brake pedal.

Short answer, no.  Sooner or later, in NoVA traffic, I’m going to have to do a panic stop.  And when that happens, that panic stop happens on instinct.  It took me close to 20 years to get used to ABS, and to lose the instinct to release the brakes in response to a skid, and just keep my foot mashed to the floor.  I really, really don’t want to lose the instinct that tells me to hit the brakes in an emergency-stop situation.

So, it’s not that I couldn’t learn this new trick.  It’s that I probably shouldn’t.  Not with driving two different cars.  And not with my recent entry into geezerhood.  Better to leave sleeping dogs lie.

The Prius Gene

This is a true story.  We bought our first Prius in 2005.  The same week that we bought ours, hundreds of miles away, with no communication between us, one of my brothers also bought his first Prius.

We’re now a two-Prius family.  I think my brother and his wife have been a three-Prius family, with one going off to Prius heaven as a result of a freak highway accident.

My brother says the exact same thing about his Prius, as I say about ours:  It pushes all my buttons, in just the right way.  From the super-smooth acceleration with no gear shifts, to the dashboard feedback on mileage, pretty much everything about the car says “relax, chill, enjoy the drive”.

Maybe we both like that because that’s pretty much the way my dad drove.  Maybe we inherited the genes that give us that bent.  In any case, it seems to run in the family.

It takes some work to drive a Bolt as if you were puttering along in a Prius.  But for whatever reason, by golly, that’s how I choose to drive it.

So, no one-pedal mode for me.  It’s insufficiently Prius-like.

Post #1714: Ah, crap, another 80 MPG trip.

 

I am presently recovering from a severe shoulder sprain.

It was self-inflicted, the result of patting myself on the back too hard.

The problem starts with my wife’s Prius Prime.  It has more-than-met our expectations in every respect.  In particular, as-driven, it typically exceeds the EPA mileage rating, either on gas or electricity.

Lately, I’ve been trying a few techniques to try to squeeze some extra gas mileage out of the car.  Just some around-town trips, driving it to try to keep the gas engine in its most efficient zone.  Which, per Post #1711,  boiled down to fast starts on gasoline, followed by coasting on electricity.  Below, that’s an attempt to stay on the top of the green efficiency “hill”, followed by keeping the gas engine off while driving in the aqua “EV carve out” zone.  (The labels on the contour lines are “efficiency”, the percent of the energy in the gasoline that is convert to motion.)

Results were encouraging.  A couple of test trials showed mid-70-MPG for a series of trips and test runs, entirely on gasoline (using no grid electricity).  Given that the car has an EPA rating of 55 MPG for city driving, I figured I was doing something right.

But at some point, it dawned on me that

  1. the current EPA mileage test is based on the typical U.S. driver (i.e., somebody who drives like a bat out of hell, whenever possible), and
  2. I have no idea what my “typical” city mileage is, because I almost never drive the car, around town, on gasoline.

In short, I made a classic mistake of trying to do an experiment without a control.  I had no baseline to which I could compare my results.  I literally didn’t know what mileage the car would get if I wasn’t fooling around with the accelerator pedal.

I decided to find out.  Yesterday we took a trip out to my sister-in-law’s and back.  About 15 miles, mostly on 35 MPH suburban roads, rolling hills, no traffic to speak of.  Gas only.  Didn’t need the AC or the heat.  Relatively few stop lights.  Driving normally.  (But acknowledging that I’m a light-footed driver by nature, and that monitoring the car via a Scangauge 3 has done nothing but increase that tendency.)

In short, reasonably close to ideal conditions for a trip.

Results:  The odometer clicked over to 80 MPG for the trip, just as we were returning to our driveway.

I am reminded of the following medical advice:  If untreated, the common cold will last a week.  But with proper medical attention, you can expect a full recovery in just seven days.

Thus it would appear, for urban hypermiling in a Prius Prime.  As-driven, 80 MPG, for my suburban area.  No fancy footwork required.

Post #1712: The Balkanization of EV battery recycling

 

Background:  I can’t get rid of the damned thing.

My wife and I have been believers in electrically-powered transport for some time now.

In 2008, we bought an aftermarket battery pack to convert my wife’s 2005 Prius into a plug-in hybrid electric vehicle.  At the time, the manufacturer (A123 systems) assured us that the battery pack would be fully recyclable, and that they had partnered with Toxco, Inc. to guarantee that.

To be honest, that retrofit never worked very well.  It wasn’t the battery’s fault.  The main limitation was that a Prius of that generation wasn’t really built to function as an electric vehicle.  That placed a lot of limitations in driving in all-electric (“EV”) mode.  Gasoline savings were modest, at best.

Fast-forward to 2012.  A123 had gone bankrupt.  Toxco was no longer in the battery recycling business.  We had a problem with the charger on that battery pack, and decided to have it fixed, in large part because, at that time, there was no way to get rid of the damned thing.  Far less hassle to fix it and keep using it.

At that time, the word was that infrastructure for EV battery recycling was just around the corner.  But from a practical perspective, here in Virginia, we couldn’t find someone to take that off our hands and recycle it.

Fast forward to 2018, and the original nickel-metal-hydride traction battery in that Prius died.  We thought about scrapping the car at that point (177K miles), but everything else was fine, we dreaded the thought of buying a new car.  So we we paid to have the dealer install a new Toyota nickel-metal-hydride (NiMH) traction battery.  (Toyota recycles the dead NiMH batteries recovered through their dealerships.)   But, in part, the decision to keep the car was driven by that A123 battery pack.  We looked around for recyclers, but there was still no way to get rid of the damned thing.

Apparently, EV battery recycling was still just around the corner.

Jump to 2023.  It now looks like that 15-year-old A123 pack has finally given up the ghost.  It will no longer charge.  And at this point, we have no interest in trying to get it fixed, even if we could.  Any money spent on that would be better invested in getting a new purpose-built PHEV, such as a Prius Prime.

I’m sure you’ve guessed the punchline.   I just looked around for recyclers, and yet again, there is even still no way to get rid of the damned thing.

Now, that’s not 100% true.  There’s an on-line ad for a company that, if I give them all my information, might be willing to offer me a quote on how much they’ll charge to recycle my particular battery.  There might be a shop as close as North Carolina that might take it, if I could prepare it properly.  I haven’t bothered to inquire.  My wife’s going to call the dealer who installed it originally, after this three-day weekend, and see if they’ll remove it and dispose of it for us.  (Last time we asked, that wasn’t an option.)

My point is there’s no place within, say, 200 miles, that I can just call up, make and appointment, and drop off the battery for recycling.  It’s all either a custom, one-off service, or requires crating and shipping the battery, or required driving at least hundreds of miles, round-trip, if I can find a place that will take it.

On the plus side, I’m in no hurry.  A fully-discharged lithium-ion battery isn’t a fire hazard.  I’ve checked several sources on that, and that’s the overwhelming consensus.  A completely discharged lithium-ion battery is just dead weight, not a death trap.  You definitely don’t want to try to recharge one and power it up, once it has been over-discharged, as it can easily form internal short-circuits in an over-discharged state.  That can lead to a big problem in a short amount of time.  (And chargers in general will not allow you to try to charge a lithium-ion battery with excessively low starting voltage, for exactly this reason.)  But as long as you don’t do anything stupid — don’t bypass the charger, don’t puncture it, don’t roast it — it’ll remain intert.

On the minus side, it looks like the U.S. EV battery recycling industry is in no hurry, either.  I sure don’t perceive a lot of forward motion since the last time I looked at this.  Worse, what seems to be happening is that the industry is going to get split up along manufacturer lines.  Tesla will recycle Tesla batteries, Toyota will recycle Toyota batteries.  And if you fall into the cracks — with some off-brand battery — there will still be no way to get rid of the damned thing.

My impressions of the EV battery recycling market

I’ve been tracking this market for more than a decade now.  With the personal stake described above.  I thought I might take a minute to offer my observations.  In an unscientific way, without citation as to source.

First, it doesn’t pay to recycle these.  At least, not yet.  That was surely true a decade ago, and my reading of is that it’s still true.  So you’ll see people talk about the tons of materials saved, for ongoing operations.  But I don’t think you’ll hear anybody say what a cash cow lithium battery recycling is.

Second, EV battery recyclers start up and fail at an astonishing rate.  Near as I can tell, none of the companies involved in it, when I looked back in 2012, are still in that business.  I just looked up a current list of companies that cooperate with GM dealers for EV battery recycling, and all the names were new to me.  This “churning” of the industry has been fairly widely noted by industry observers.

Third, we’re still just around that damned corner.  The Biden infrastructure bill appears to have about a third of a billion dollars earmarked for development of EV battery recycling (source).

But surely you realize what that means.  See “First” above.  The fact that the Feds have to subsidize EV battery recycling is pretty much proof that it just doesn’t pay to recycle these big lithium-ion EV batteries.  At least not yet.

Finally, car markers are developing their own captive recyclers, for their own batteries.  Tesla has its own systems.  GM has contracts with a limited number of vendors, plausibly to serve GM dealerships.  Toyota has its own system, for batteries recovered by its dealerships.

That last move makes perfect sense.  Because recycling is a net cost, and yet a significant consumer concern, manufacturers are pledging to take care of their batteries, if they are recycled via their dealers.  But, so far, I’m not seeing any generic recycling capability for (say) any hybrid or EV showing up at a junkyard.  Let alone for my oddball A123 batteries.

Per this article, it currently costs Tesla more than $4 per pound to recycle its lithium-ion batteries.  At that cost, you can see why they might be willing to deal with their own, but they’re sure not going to take anybody else’s batteries for recycling.  It’s not clear that other processes — with less complete recycling of all the materials — are as costly as Tesla’s.  As of 2021, at least one company was in the business of simply warehousing used EV batteries on behalf of vehicle manufacturers, handing batteries replaced under warranty.   The theory is that right now, it’s cheaper to store them and hope for lower recycling costs down the road (reference).

I’m sure that big junkyards and scrap yards have some way of dealing with these, at some cost.  Surely plenty of the (e.g.) Generation 3 Toyota Prius hybrids with lithium-ion batteries have now been scrapped.  I don’t know if they can recycle via Toyota’s internal system, or if … well, I just don’t know.

Conclusion

All I know, at present, is that if I can recycle that totally dead 5 KWH A123 lithium-ion battery pack, it’s going to be either a hassle or a major expense or both.  As long as I can get it recycled, I will.

But, the fact is, until that 2005 Prius actually dies, I won’t have to face up to it.

And, in a nutshell, that characterizes the American market for lithium-ion EV battery recycling.

I’ve decided just to let that dead battery be, and let the 2005 Prius continue to haul around that 300 extra pounds of dead weight.

Because, as we all know, readily-available EV battery recycling is just around the corner.

Post #1624: 80 MPG?

 

Not quite.  But I think I’m finally figuring out how to drive my wife’s Prius Prime.

Above is the gas mileage on my wife’s Prius Prime, after a round trip from Vienna VA to Harper’s Ferry WV.  This is all after resetting the odometer once the battery was depleted.  So it’s straight-up gas mileage.

This trip contained a short section of high-speed driving, but was mostly hilly primary and secondary roads in western Virginia and West Virginia.  And I think I finally understand how I’m getting such great mileage.

The Prius Prime loves hilly roads.  It is an excellent car for a particular style of pulse-and-glide driving.

Continue reading Post #1624: 80 MPG?

Post 1621: Look ma, no battery!

 

Or, “why I truly don’t give a 💩 about high gasoline prices in the U.S.”, the sequel.

Back in June of this year, in Post #1454, I explained why I didn’t give a 💩 about the price of gas.  In a nutshell, I don’t use much.  I drive my wife’s Prius Prime.  The 30-mile battery range covers essentially all our local travel.  One we’ve run through that, the gas mileage is outstanding.

The genesis of the prior post was our annual trip to Ocean City, Maryland, where the car got 72 MPG on the highway.

I figured it was a fluke.  There were no hills to speak of.  We probably caught a tailwind.  Unlikely to be repeated.

Today we went leaf-peeping, driving from Vienna VA to Sperryville, VA and back.  There is just something about the autumn scenery in central Virginia that my wife and I both love.

(Best sign seen on the trip:  “God Allows U-Turns”.  This, at the exit of the parking lot of a little church in Sperryville where we were — yeah — making a U-turn.)

The trip was a combination of interstate highways, then primary and secondary highways traversing hilly terrain. It was a nice drive — once we got off the interstate.  I reset the odometer after the battery was depleted so that I could check the gas mileage.

Lo and behold, in round numbers, 72 MPG.  Straight-up gasoline-powered transport, no battery.  Completely different terrain, time of year, and driving conditions compared to last time.

So, no fluke.  I’m not drafting trucks. I’m not doing 35 in the right-hand lane.  I’m  just keeping up with traffic, and paying a bit of attention to instrumentation on the dashboard that offers guidance for best fuel economy.  (And it didn’t hurt that we didn’t need AC or heat for this trip.)

It’s odd how your expectations change.  These days, if I come in under 65 MPG for the gas portion of a trip, I’m disappointed.

This is not as clean as a pure EV of the same size.  At least, not as clean, at Virginia’s current electrical generating mix.  But it’s not bad for the latest refinement of a gasoline-based technology that Toyota put on the road more than two decades ago.  And the same drive train that gets us 72 on the highway allows us an effortless transition to electrical transport for all our around-town driving.

For us, this plug-in hybrid electric vehicle (PHEV) is absolutely the sweet spot in the spectrum of what’s on the market today.  After a year of driving this car, we have no regrets about buying it.

Post #1618: There ain’t no disputin’ Sir Isaac Newton: Efficient driving in an EV.



Driving an electric vehicle (EV) efficiently is forcing me to learn some new driving habits.  And, in particular, I have to un-learn some cherished techniques used for driving a gas-powered car efficiently.


When I look for advice on driving an EV efficiently, all I get is a rehash of standard advice for driving a gas car.  But the more I ponder it, and the more I pay attention to the instrumentation on my wife’s Prius Prime, the more I’m convinced that’s basically wrong.


An electric motor is fundamentally different from a gas engine.  With an electric motor, you want to avoid turning your electricity into heat, rather than motion.  That boils down to avoiding “ohmic heating”, also known as I-squared-R losses.


To minimize those heating losses, you want to accomplish any given task using or generating constant power.  That task might be getting up to speed after stopping at a red light, or coming to a stop for a red light, in some given length of roadway.


Here’s the weird thing.  Assuming I have that right — assuming that an efficient EV driving style focuses on providing or generating constant power over the course of an acceleration or deceleration — that implies a completely different driving style, compared to what is recommended for efficient driving of a gas-powered vehicle.


In particular, the standard advice for gas cars boils down to accelerating and decelerating with constant force.  When you take off from a stop light, aim for a constant moderate rate of acceleration.  When you are coming to a stop, aim for a steady rate of deceleration.  Constant acceleration or deceleration boils down to constant force on the wheels, courtesy of Sir Isaac Newton’s F = MA (force is mass times acceleration).


But power is not force.  As I show briefly, in the next section.  In a car, power depends on speed.  Constant force on the brake pedal (and so, on the brake rotors in a traditional car) generates far more power at high speed than at low speed.  Similarly, a constant rate of acceleration consumes more power at high speed than at low speed.


And so, there seems to be a fundamental conflict between the way I was taught to drive a gas car efficiently, and what seems to be the right way to drive an EV efficiently.


In a nutshell, to drive an EV efficiently, you should be more of a lead-footed driver at low speeds.  And taper off as the car speeds up.   Conversely, hit the brakes lightly at high speed.  And press the brakes harder as the car slows.


That’s the driving style that aims for production and consumption of power at a constant rate, over the length of each acceleration or deceleration. And that’s completely contrary to the way I was taught to drive a gas vehicle.


Think of it this way.  Suppose you apply a certain level of force to the brake pedal of a traditional car.  The resulting friction between brake pads and rotors will generate heat.  That rate of heat production is, by definition, power, as physicists define it.  You’re going to generate a lot more heat per second at 80 MPH than you are at 4 MPH.  (In fact, 20 times as much.)  Restated, for a given level of force, you are bleeding a lot more power off the car’s momentum at 80 MPH than at 4 MPH.  And those big differences in power, over the course of an acceleration or deceleration, are exactly what you want to avoid in an EV with regenerative braking, in order to avoid I-squared-R losses.


 




Force and power:  A brief bit of physics and algebra




1:  Two definitions or laws of physics


Work = Force x Distance


Power = Work/Time


2:  A bit of algebra


Substitute for the definition of work:


Power = (Force x Distance) / Time.


Re-arrange the terms:


Power = Force x (Distance/Time)


Distance/time = speed (definition)


Power = Force x speed.


For a constant level of force applied to or removed from the wheels, the rate of power consumption (or production) is proportional to the speed.


Upshot:  To accelerate or decelerate at constant power, the slower you are going, the heavier your foot should be.  The faster your are going, the lighter your foot should be.  For the gas pedal and the brake pedal.




Ohmic heating:  Why a long, hard acceleration trashes your battery reserve.




Anyone who drives a PHEV — with a relatively small battery — will eventually notice that one long, hard acceleration will consume a big chunk of your battery capacity.  On a drive where you might lose one percent of battery charge every few minutes, you can knock several percent off in ten seconds if you floor it.


Another way to say that is that getting from A to B by flooring it, then coasting, consumes much more electricity than just gradually getting the car up to speed.


I’m not exactly sure why that is.  But I am sure that it is universally attributed to I-squared-R or ohmic heating losses in the motors, batteries, and cables.


Any time you pass electric current through a wire or other substance, it heats it up.  From the standpoint of moving your car, that heating is a loss of efficiency.  The more current you pass, the more it heats up the wire.  And that heating is non-linear.  Watts of heat loss are proportional to I-squared-R, in the argot.  They go up with the square of the current that you pass through that wire.


Again, I’m not completely sure here, but my takeaway is that your heating losses, at very high power, are hugely disproportionate to your losses at low power.  At constant voltage, I believe those losses increase with the square of the power being produced by the electric motors.  In other words, ten times the power produced to move the car creates 100 times the ohmic heating losses.


And that’s how ten seconds of pedal-to-the-metal can use up as much electricity as 10 minutes of moderate driving.


That said, I have to admit that I’m relying on “what everybody says” for this.  For sure, hard acceleration seems to trash your battery capacity far in excess of the distance that you travel at that rate of acceleration.   Whether the root cause for that is I-squared-R losses, or something else about the car, I couldn’t say.


Either way, my takeaway is that if losses are proportional to the square of power consumed or generated, then to accomplish any given task (any fixed acceleration or deceleration episode), your aim should be to do that at constant power.  Because that’s what will minimize the overall energy loss from that acceleration or deceleration episode.




Drive like you are pressing on an egg — that was a real thing.  EV drivers should chuck the egg.




Source:  Duke University Libraries, via Internet Archive.


Those of us who grew up during the 1970s Energy Crisis will probably recall public service announcements that asked you to drive as if there were a raw egg between your foot and the gas pedal.  I managed to find a Texaco ad of roughly that era, laying out that egg-on-gas-pedal meme.  The picture above is from that video.


As kids, that was pretty much beaten into us.  Responsible driving means no jackrabbit starts, no tire-smoking stops.  Easy does it.  We’re in the middle of a prolonged gasoline shortage, after all.


So now I come to the part that is absolute heresy for someone of my generation.  If you’ve absorbed the prior two sections, you realize that this advice probably isn’t correct for an EV.  Why?  To consume power at a constant rate over the course of an acceleration, you should start off with a brisk rate of acceleration, then diminish that as the car speeds up.


In other words, if you drive an EV, drive with a lead foot.  Not all the time.  But at the start of every acceleration.  And the end of every deceleration.


If you drive an EV, chuck the egg.




The Prius Prime Eco display




Source:  Underlying picture is from Priuschat.


With that understanding in hand, I’m finally starting to make some sense of the “eco” display on my wife’s Prius Prime.


In theory, this little gauge is giving you guidance on how to drive the car most efficiently.  In practice, I could never make head or tail out of it, except that it seemed to be telling me to drive with a lead foot.


Which, I now understand, it was.


On this display, if you put your foot on the gas, it will show you your actual throttle (gas pedal) position, and the gas pedal position that will, in theory, give you greatest efficiency.


By contrast, when you put your foot on the brake, it doesn’t show you the brake pedal position.  Near as I can tell, it shows you the amount of power than you are generating.  That is, a constant brake pedal position will lead to a shrinking bar, as the car slows down and less energy is generated.


Watch what happens if I try to accelerate gently:




It’s possible that all the meter is actually telling me is that a very lightly-loaded electric motor will operate inefficiently.  I don’t think the Prius eco monitor is actually trying to get me to drive at constant power.


Edit 3/11/2024:  After driving around with a ScanGauge III, my conclusion is that accelerating at constant power is exactly what the eco-meter is trying to get you to do.  It wants you to start off with a heavy foot, and then, as you accelerate, it wants you to back off.  Near as I can tell, that bar is set up to keep you at around 23 HP of power output, or 50 amps of discharge current, or a “2 C” rate of discharge, for this battery.


In particular, my diagram above is labeled wrong. The two red arrows should be labeled “desired power output” and “actual power output”.  You will notice that if you don’t move the gas pedal, the “actual power output” line will creep up as you speed up.  The only way to keep that line in the same place is to back off the gas as your speed increases.  So that line isn’t the throttle position, it’s the power output (power = force x speed).


But no matter how I arrive at it — from theory, or from finally paying full attention to the Prius eco meter — the whole drive-like-there’s-an-egg-between-foot-and-gas-pedal is clearly obsolete.  Gentle acceleration may get you your best mileage in a gas-powered vehicle.  But it’s not the correct way to drive an EV.