Turning off regen and power limiting for increased range

[Krazaak]

Coasting doesn't improve range. If you've positioned the accelerator such that zero torque is being applied at the motors, then the car is being slowed by wind and other resistance and eventually you're going to have to accelerate again to get back up to speed. When I'm a passenger, there's nothing more nauseating than a driver who constantly cycles between coasting and accelerating. The most efficient method of driving at any given speed, is by keeping the motors providing just enough torque to overcome the opposing resistance.

The best method to accomplish that is feathering the accelerator and avoiding unnecessary acceleration and deceleration. Some people just don't seem to have the fine motor control for the accelerator and it's probably more efficient to use TACC on the open road and just best effort to avoid slowing and speeding up while in traffic.

 

[mxnym]

Coasting doesn't improve range. If you've positioned the accelerator such that zero torque is being applied at the motors, then the car is being slowed by wind and other resistance and eventually you're going to have to accelerate again to get back up to speed.

The argument of the OP wasn't that coasting improved range, it was that regen reduced it. In reality, coasting does improve range and regen can reduce it. Any time regen turns on, the vehicle is being slowed down by regen to gain less electricity than it would take to get back to where the vehicle would be without regen. When you need to slow down, having regen is great, but any other time, it is less efficient than not having it, this includes maintaining speed, coasting in a tailwind, coasting down a hill, and probably many other scenarios. Moreover, coasting and gently accelerating is a known hypermiling method, so it may improve range even outside of those given scenarios. Finally, feathering the accelerator isn't de facto the best way to accomplish maximum range, it's simply the best way currently available and legal (to the extent you believe you could be ticketed and lose a court battle for shifting to neutral while driving a vehicle that is not a manual transmission).

ETA: Many hypermiling techniques are dangerous and/or illegal. I wouldn't recommend that most drivers attempt hypermiling or shifting to neutral while driving, as it it can be very dangerous to shift into neutral even in a vehicle that isn't a manual, for instance, if you forget you're in neutral and do not shift out when you need to.

 

[Krazaak]

The argument of the OP wasn't that coasting improved range, it was that regen reduced it. In reality, coasting does improve range and regen can reduce it. Any time regen turns on, the vehicle is being slowed down by regen to gain less electricity than it would take to get back to where the vehicle would be without regen. When you need to slow down, having regen is great, but any other time, it is less efficient than not having it, this includes maintaining speed, coasting in a tailwind, coasting down a hill, and probably many other scenarios. Moreover, coasting and gently accelerating is a known hypermiling method, so it may improve range even outside of those given scenarios. Finally, feathering the accelerator isn't de facto the best way to accomplish maximum range, it's simply the best way currently available and legal (to the extent you believe you could be ticketed and lose a court battle for shifting to neutral while driving a vehicle that is not a manual transmission).

ETA: Many hypermiling techniques are dangerous and/or illegal. I wouldn't recommend that most drivers attempt hypermiling or shifting to neutral while driving, as it it can be very dangerous to shift into neutral even in a vehicle that isn't a manual, for instance, if you forget you're in neutral and do not shift out when you need to.

I didn't say that unnecessary use of regen wasn't wasting energy. It obviously is, because the motors are not 100% efficient in either direction, so there's a loss associated with expending or regenerating electric power.

What I said, is that coasting does not inherently save any energy and putting the vehicle in neutral does not achieve any of the benefits associated with doing the same thing in a vehicle where a portion of the drive train actually disengages.

The OP was talking about coasting on the highway. Ignoring wind speed and elevation for just a moment, coasting (zero torque at the motors) is just going to result in the vehicle slowing down due to the various resistances being applied to the vehicle. Assuming you intend to keep moving, you're going to have to accelerate again, expending at least as much energy as saved. A tailwind reduces the energy required to maintain forward momentum, but nothing short of storm force winds are going to negate all of the motor torque required to maintain speed.

Now, on a downward slope, the vehicle can actually accelerate while coasting and ignoring the issue of speed limits, there's going to be a point where you'd be better off letting regen capture some of that potential energy instead of bleeding it all off to wind resistance. I don't pretend to know the calculations for that sweet spot.

In other vehicles, shifting to neutral while rolling down hill can save energy because it disengages the engine and a chunk of the drive train from the wheels, letting the engine idle and lowering mechanical resistance. In the Tesla however, nothing disengages and it's already possible to control motor torque with fine accuracy from the accelerator.

Anyone can hypermile in an EV, just maintain 26-30mph to ride the sweet spot between rolling and wind resistance. Accelerate slowly and don't brake (regen or otherwise) unless you absolutely have to. Coasting up hills and regaining speed going downhill doesn't magically save any energy, it just lowers your average speed and hence the energy required to overcome wind resistance.

Now, if you can coast to a stop instead of using regen, that's more efficient; but it's also possible using only the accelerator. Not practical in most cases though.

 

[mxnym]

What I said, is that coasting does not inherently save any energy and putting the vehicle in neutral does not achieve any of the benefits associated with doing the same thing in a vehicle where a portion of the drive train actually disengages.

Shifting to neutral disengages regen completely. No matter how good you are at feathering the pedal, there is going to be some energy expended and some regen used in a lengthy and/or downhill coasting scenario. In a downhill scenario, your point about it being more efficient to regen at some certain speed may be true, but at the same time, energy expended to accelerate at such a speed would be greater, too, so neutral solves for this while regen isn't smart enough to be as efficient as possible and your foot isn't an chip AI connected directly to the vehicle's sensors and inputs.

Anyone can hypermile in an EV, just maintain 26-30mph to ride the sweet spot between rolling and wind resistance. Accelerate slowly and don't brake (regen or otherwise) unless you absolutely have to. Coasting up hills and regaining speed going downhill doesn't magically save any energy, it just lowers your average speed and hence the energy required to overcome wind resistance.

When I refer to hypermiling, it includes the more dangerous aspects like driving on the painted lines to reduce traction and following very closely to a semi in addition to shifting to neutral. What you describe, I would only refer to as hypermiling if the speed limit was 45 or higher while it was being done. That having been said, coasting up a hill reduces your speed when it would take more energy to maintain it, and regaining that speed for free by coasting on the downward side saves energy again. It's not magic, but you would have to drive slower than the average you end with to get the same efficiency at a consistent speed.

Now, if you can coast to a stop instead of using regen, that's more efficient; but it's also possible using only the accelerator. Not practical in most cases though.

Not only impractical, but arguably infeasible without a lot more energy loss than you'll experience coasting in my X. I did this (down to creep speed) in my previous ICE vehicle sometimes, but only because it engine braked so hard in D.

 

[Krazaak]

Shifting to neutral disengages regen completely. No matter how good you are at feathering the pedal, there is going to be some energy expended and some regen used in a lengthy and/or downhill coasting scenario. In a downhill scenario, your point about it being more efficient to regen at some certain speed may be true, but at the same time, energy expended to accelerate at such a speed would be greater, too, so neutral solves for this while regen isn't smart enough to be as efficient as possible and your foot isn't an chip AI connected directly to the vehicle's sensors and inputs.

I assume you meaning accelerating downhill in neutral, then switching to drive to regen down to some lower speed, then repeat the N and D cycle between whatever two speeds you consider too slow and too fast? There's nothing inherently wrong with that, but I'd argue that it's more work for the driver, annoying to those behind you and likely no more efficient than just letting the car regen slightly to just maintain whatever speed you consider comfortable.

I doubt it's the case, but if you meant shifting into neutral and using the friction brakes, to keep you from driving dangerously fast, then I'd disagree completely.

My last trip to Boone, coming down the mountains, my 15 mi consumption was -150 Wh/mi and 30 mi was -15 Wh/mi. 6-8 miles of downhill regen was enough to negate the remaining 22-24 miles of flat or uphill driving and it obviously would have been impossible to navigate those hills without braking.

Now, I will concede that very fine control of the accelerator pedal seems a little more challenging for some people and I suspect it has a lot to do with past vehicle history as much as anything else.

 

[mxnym]

I assume you meaning accelerating downhill in neutral, then switching to drive to regen down to some lower speed, then repeat the N and D cycle between whatever two speeds you consider too slow and too fast? There's nothing inherently wrong with that, but I'd argue that it's more work for the driver, annoying to those behind you and likely no more efficient than just letting the car regen slightly to just maintain whatever speed you consider comfortable.

You could do that, and you probably wouldn't have to alternate most of the time if you used the pedal to reduce regen and took longer to get back down to the "too slow" speed (once you're on the fast end of the spectrum, there is minimal risk of accidentally expending energy). However, I'm not sure exactly which of my comments you were making this assumption based on, so I'm not going to try to elaborate on what I meant. To be clear, my main point was that there are many scenarios where coasting can be more efficient. I agree that switching to neutral is more work; that is the OPs complaint and reason to want to disable regen completely. I also agree that driving in these more efficient ways is annoying for drivers behind you, and in fact, now that I have regen, I just use TACC except in rare circumstances. For instance, there is a back road I sometimes traverse where I can shift to neutral at 35MPH at the crest of one hill and remain in neutral until I'm nearly to the crest of a third hill where I shift back into D at 35MPH. My speed reaches 60MPH in those hills, and I personally haven't been able to achieve anything close to the same result (in terms of efficiency / I can't keep the energy consumption marker in the IC at 0 through the roller coaster) with my X in D with standard regen.

I doubt it's the case, but if you meant shifting into neutral and using the friction brakes, to keep you from driving dangerously fast, then I'd disagree completely.

You are correct that this is NOT what I meant. Brake pads touching the disc when regen is available is always a waste of energy. To be clear, that benefit of that waste can outweigh the cost, and does at different points for different people. for instance, a hypermiler may never touch the pads to the disc in a Tesla above 4 MPH except to avoid an accident due to behavior of a third party while a rushed individual may use the pads even when regen would exceed 50kWh in order to be going faster further and stopped sooner.

 

[SSonnentag]

I prefer to simply charge once more each month than to fret over miniscule power savings.

 

[ThosEM]

Let's try a thought experiment.

You are driving down a long, steady and rather steep incline from the mountains. In cruise control, you observe that regeneration is generating 20-30 kW on this incline. Now you increase the setpoint speed for your cruise control a few clicks. Will the amount of regeneration increase? Of course not; it will decrease, because more energy will then be going into drag than into the battery.

Now decrease the setpoint speed of your cruise control a few clicks. What happens to the amount of regen? Of course, it goes up, because less is going into aerodynamic drag.

So where is the supposed advantage of letting the car freewheel up to the speed that causes drag to balance the component of force driving the car downhill? Do you want that energy back going downhill or do you want to spend it on going faster? That's always the choice, and overall energy consumption is always enhanced by going slower, uphill or downhill makes no difference.

 

[mxnym]

Let's try a thought experiment.

You are driving down a long, steady and rather steep incline from the mountains. In cruise control, you observe that regeneration is generating 20-30 kW on this incline. Now you increase the setpoint speed for your cruise control a few clicks. Will the amount of regeneration increase? Of course not; it will decrease, because more energy will then be going into drag than into the battery.

Now decrease the setpoint speed of your cruise control a few clicks. What happens to the amount of regen? Of course, it goes up, because less is going into aerodynamic drag.

So where is the supposed advantage of letting the car freewheel up to the speed that causes drag to balance the component of force driving the car downhill? Do you want that energy back going downhill or do you want to spend it on going faster? That's always the choice, and overall energy consumption is always enhanced by going slower, uphill or downhill makes no difference.

If you are coasting down a hill where speed can increase beyond your set point and there is another hill to climb at the bottom of the hill in question, the more speed you have at the bottom of that hill, the less energy you have to use maintaining the aforementioned set speed on the way up the next hill. As I mentioned in my post just before yours, I have a real life example:

For instance, there is a back road I sometimes traverse where I can shift to neutral at 35MPH at the crest of one hill and remain in neutral until I'm nearly to the crest of a third hill where I shift back into D at 35MPH. My speed reaches 60MPH in those hills, and I personally haven't been able to achieve anything close to the same result (in terms of efficiency / I can't keep the energy consumption marker in the IC at 0 through the roller coaster) with my X in D with standard regen.

To be clear, remaining at 35 MPH (or the higher posted speed limit) would be less efficient, because the less than 100% efficient regen gains would immediately be spent on the inefficient hill climbs that could have been free.

 

[David99]

Just another data point to this discussion. I'm using 'Scan My Tesla' app to read the CAN bus. The app has a trip meter that calculates energy used and energy regenerated. On a typical road trip going on the freeway the amount of regenerated energy is around 5%. Regen is about 80% efficient including battery round trip efficiency. So we are talking about 1% of losses. At this point the whole discussion should end. But for the sake of it....

Now the question is can these losses be avoided? You can put the car in neutral half a mile before the exit and let it coast so it comes to a stop when you are at the end of the off ramp, but if anyone is behind you, you you might get rear-ended. I highly recommend not doing that. If the off ramp is a downhill (most likely) then you can't use coasting. You will have to use regen. Going through some surface streets from and to the Supercharger and you can't let the car coast down to a red light or stop light either. Again people will get really mad at you for slowing down half a mile before the traffic light. In other words we are talking about a theoretical 1% difference but when looking at the real world you can't drive in a way to to avoid regen by coasting.

Once and for all, regen isn't bad, it is good! Driving too fast and then having to slow down a lot is an inefficient driving style. You can't blame that on regen. You need to adjust your driving style so you use less acceleration followed by deceleration. Drive as smooth as reasonably possible and then use regen is a way that you hardly use the friction brakes.

 

[wdolson]

I have experimented coasting by putting the car into neutral or just one pedal driving and keeping the energy in the 0 KW zone when I want to coast. The latter takes a bit more skill, but leaving regen set to high and learning how to one pedal drive is drastically more energy efficient in the end.

We live up a hill and every time I leave home I am going down hill. I note the WH/Mi at the bottom of the hill every time. Having regen on max is 2X more energy efficient than putting the car in neutral and coasting. I've gotten as low as 50 WH/Mi (average is about 90 WH/Mi) at the bottom of the hill with regen on max. Coasting as much as possible in neutral was never below about 120 WH/Mi.

I just noticed this is a year old. The new "Chill" mode should reduce power consumption when accelerating.

 

[_jmk]

I had Ioniq EV before. By far the best highway efficiency was always done by setting the regen to zero. Give up speed tiny bit going uphill, speed up a bit going down. Never let it regenerate unless you are going around the city. Tesla is really missing few basic features in terms of maximising the efficiency, this is certainly not the only one.

 

[flar]

(Begin massive "IMHO" section - i.e. the rest of this post...)

I think the analysis of "is coasting better than regen at losing speed" is a red herring that ignores the most important factor in getting more range. When you arrive at that decision, you've already made a prior mistake that you didn't even notice and discussing the best way at recovering from that mistake is less interesting than avoiding that mistake in the first place.

The real problem is having speed that you don't want in the first place. Avoid that and you will rarely/never need to coast or regen or brake in the first place.

The main comment I see that is most telling is the recurring reference to "when I take my foot off the pedal" above. If you do that, you've already failed the game, so solving for that case is not interesting.

The mistake is this - your goal should be to continue down the highway at your desired speed. Ignoring tailwinds and downhills for this first part of the discussion, then if you solve for the energy requirements for that situation then the answer will always be a positive expenditure of energy. At that point coasting vs. regen is irrelevant - you need to gauge the controls to apply that amount of motive force and stay there. In an ideal world under those constraints that analysis of "the lifted foot" would never be needed and all energy expenditures would be positive (i.e. greater than zero as in not coasting).

Personally, my driving foot never moves in an open highway setting (though I tend to use TACC nowadays anyway, but in the few times when I want to do it myself, I don't find myself varying speed much at all in any way that would require coasting or decelerating. If you find it hard to maintain a fixed speed, then I'm not sure the rest of this really applies, but I'll outline it all anyway.

What other constraints might come into play? If something comes up where you need to consider a slower speed, then consider how soon you need to be at the new speed. If the situation is immediate, you're going to need brakes and you might as well regen while you use them. If the situation is seen far in advance, then simply reducing (but not zeroing or reversing) power will get you down to the speed you need in an efficient manner. Going into a "coast" mode as your default reaction may also get you there in those cases and is probably more efficient than regen, but it would be a very odd situation where "coasting" is exactly the right amount of energy expenditure to get you to the desired speed within the desired time frame. As the "distance to new speed" goes down then the energy expenditure required to get there gradually will reduce from "just about as much energy as I was already using in the steady state" more and more until it hits zero and as the distance gets even smaller the energy required can also go negative (i.e. active deceleration is required). The amount of energy required to reach the goal in time will vary from "barely less than the steady state energy you had been using" to "full regen plus brakes". The cases of "OMG, zero energy expenditure is the exact solution to my delta-V!" will be nearly non-existant.

(One might argue that "slower speeds have less air resistance, and so getting to the slower speed faster is better, so I'll choose the most efficient method of coasting over regen", but um, if you are so willing to embrace that lower speed that getting there quickly at the earliest sign that you might need to is a viable solution, then why weren't you going that speed to begin with? If you want that speed, be already going that slow and you will use even less energy overall. Once you accept that you are encountering a situation where you are being forced to go slower than you'd like to due to circumstances out of your control, then you really don't need to be in a hurry to get there and so simply reducing positive energy flow will be the right trick.)

The key is not to worry about how much energy you are using, but how quickly you are approaching traffic and how deep your foot is on the pedal. Change your foot position very early on (as soon as you realize there is a traffic speed issue) until the "Uh, oh, I'm going to hit that" bells in your brain stop going off. And then as you slow to the speed where "Now I'm not even going to approach it at all" happens, then resume a position to maintain that new speed when you get there. If you've disable regen then there will be cases where "still too fast ... lift ... still too fast ... lift (repeat)" results in your foot leaving the accelerator and your only other choice is to use the brakes and that is a far worse situation to be in. If you leave regen enabled then you should only end up using the regen when you would have otherwise used the brakes.

But, if your focus and decision process is not on the closing speed but instead on whether you can easily default to a theoretical "coasting" setting then you will be like the carpenter with only a hammer. To him, all projects involve nails even when a screw or glue would work better. If all you have in your speed maintenance arsenal is a "coasting hammer" then all deceleration situations are nails and all you can do is slam them with coasting.

A carpenter with a full toolbox will figure out how much speed loss is needed and how quickly and simply move their foot to match.

Now TACC is really amenable to gaining speed that it ultimately will not want. In practice, humans can do this if they are impatient or they don't judge the speed of cars too far out, but TACC can do this while approaching slower traffic much closer and more aggressively than most humans would accept or notice. (I would say it has gotten better at adjusting following speed in the past year, but it still has a ways to go.) It will be great when TACC gets much better than this and adjusts speed without using coasting or regen, and instead by reducing forward power a ways before it gets to the traffic, but it isn't there yet.

As for downhill (or tailwind), the discussions of "when you reach the too-fast speed then it is better to ..." are on the wrong track. You should have reacted well before you reach that speed. Again, if you gain *any* speed in a downhill you are wasting energy. Ease your foot up until you maintain the speed you want and if you are still expending a small amount of energy because aerodynamic drag is stronger than the hill, then so be it. If the hill is stronger than the air resistance and you need to use regen to avoid speeding up then that is actually a good solution. At 80% efficiency there is probably a small sweet spot where allowing a small amount of speed gain might be desirable, but it is probably easier for most people to just move their foot to maintain speed (including allowing regen) than to do all of those calculations. Simply using coasting and letting the car gain speed with no check in play is not a good solution as you are simply redirecting your kinetic energy into the air rather than back into the batteries (even at an 80% efficiency, claiming it to the batteries is a superior solution than bleeding it into heated air and turbulence). A small allowance could be made to say that you will allow a +N MPH difference on a downhill, though, and then lift your foot in advance to avoid reaching that speed. Either way, if you are worried about the state of the drive system rather than the speed you are going and whether you've gained unwanted speed, you are monitoring the wrong thing.

Unwanted forward speed is the real thing to optimize, not which circuits are sending electrons in which direction.

 

[flar]

I suppose I should add that it is my understanding that ICE engines are rather finicky about how efficient they are at various RPM and so they may often find that "disengaging the transmission and switching the engine into a low energy not-stalling mode" is more efficient than a subtle variation in the amount of gas being fed to attempt to reduce output by a few HP/kW.

But, electric motors are much more linear in how much power they produce vs. consume and so switching between "power applied" and "no power applied" is not much different than simply applying the proportional amount of power between the two.

 

[mxnym]

Now the question is can these losses be avoided? You can put the car in neutral half a mile before the exit and let it coast so it comes to a stop when you are at the end of the off ramp, but if anyone is behind you, you you might get rear-ended. I highly recommend not doing that. If the off ramp is a downhill (most likely) then you can't use coasting. You will have to use regen. Going through some surface streets from and to the Supercharger and you can't let the car coast down to a red light or stop light either. Again people will get really mad at you for slowing down half a mile before the traffic light. In other words we are talking about a theoretical 1% difference but when looking at the real world you can't drive in a way to to avoid regen by coasting.

Once and for all, regen isn't bad, it is good! Driving too fast and then having to slow down a lot is an inefficient driving style. You can't blame that on regen. You need to adjust your driving style so you use less acceleration followed by deceleration. Drive as smooth as reasonably possible and then use regen is a way that you hardly use the friction brakes.

You're focusing only on electricity, and there is also added tire wear at the very least, hybrids and electrics are known to go through tires faster, and this is a cost you can't capture with your analysis. Additionally, you're referring to whole trips. I agree regen is good, I wouldn't want to turn mine off for a trip, but that doesn't change the fact that there can be parts of a trip where regen is inefficient and/or annoying. Speaking of annoying, there is no way you can drive that won't annoy someone. For instance, even if you turn on TACC, someone else's cruise control will behave slightly differently, so when you have the same speed set, someone is going to have to manually deal with that.

I have experimented coasting by putting the car into neutral or just one pedal driving and keeping the energy in the 0 KW zone when I want to coast. The latter takes a bit more skill, but leaving regen set to high and learning how to one pedal drive is drastically more energy efficient in the end.

We live up a hill and every time I leave home I am going down hill. I note the WH/Mi at the bottom of the hill every time. Having regen on max is 2X more energy efficient than putting the car in neutral and coasting. I've gotten as low as 50 WH/Mi (average is about 90 WH/Mi) at the bottom of the hill with regen on max. Coasting as much as possible in neutral was never below about 120 WH/Mi.

I just noticed this is a year old. The new "Chill" mode should reduce power consumption when accelerating.

In scenarios like the one I described, you really should be looking at the road, not the IC, and it's really not possible to keep it at the 0kW mark by feel in the particular scenario I described. I actually like single pedal driving and don't like that they reduced immediate regen when you remove your foot from the pedal, but that doesn't make the OPs desire any less valid (although, personally, I'd rather have regen on and use neutral even if it is more work). Also, regen is always better than brakes, but does have energy loss, so I have to assume you have to stop at the bottom of your hill; not every hill has a required stop at the bottom. Finally, chill mode is NOT an economy mode and won't necessarily increase efficiency per the manual.

I had Ioniq EV before. By far the best highway efficiency was always done by setting the regen to zero. Give up speed tiny bit going uphill, speed up a bit going down. Never let it regenerate unless you are going around the city. Tesla is really missing few basic features in terms of maximising the efficiency, this is certainly not the only one.

Only one data point, but this ^, really, exactly what the OP was on about.

If the hill is stronger than the air resistance and you need to use regen to avoid speeding up then that is actually a good solution.

It is a fine solution most of the time, but not always the most efficient solution, and not always the desired solution. There are plenty of scenarios where the free kinetic energy gained from gravity can be absorbed by gravity or wind without the efficiency loss. It is also arguable that, as in the rest of you post, one should have planned for the decline and been going slower when it started, but then we are potentially in dangerous hypermiling territory again.

 

[ThosEM]

If you are coasting down a hill where speed can increase beyond your set point and there is another hill to climb at the bottom of the hill in question, the more speed you have at the bottom of that hill, the less energy you have to use maintaining the aforementioned set speed on the way up the next hill. As I mentioned in my post just before yours, I have a real life example:To be clear, remaining at 35 MPH (or the higher posted speed limit) would be less efficient, because the less than 100% efficient regen gains would immediately be spent on the inefficient hill climbs that could have been free.

I agree that kinetic energy of the car is a possible place to store energy for the next hill, comparable in some ways to the battery. There are a couple of problems, though.

First, the aerodynamic drag can be accurately thought of as an inefficiency in storing energy in the kinetic energy of the car, and for the normal driving situation, it is a very significant inefficiency that increases with duration of storage, so it can be much greater than the 5-10% losses in the regeneration storage, depending on how long one tries to hold it before using it.

Second, the usual situation on highways is that there are long gradual grades being taken at highway speeds, rather than a roller coaster of ups and downs being taken at relatively low speeds, so energy stored in the cars extra speed is dwarfed by the amount of energy going into aerodynamic drag and will be gone before the beginning of the next hill. Maintain speed will consume it before there is a chance to climb a hill.

IMHO, there is no energy saving point in taking the car out of cruise control during highway driving. If driving a winding rollercoaster you may be more interested in enjoying the drive than saving energy.

 

[flar]

First, the aerodynamic drag can be accurately thought of as an inefficiency in storing energy in the kinetic energy of the car, and for the normal driving situation, it is a very significant inefficiency that increases with duration of storage, so it can be much greater than the 5-10% losses in the regeneration storage, depending on how long one tries to hold it before using it.

The inefficiency of kinetic storage also increases with the amount stored. The resistance goes up with the square of the velocity so the more you store, the less efficient it is.

 

[David99]

... there is also added tire wear at the very least, hybrids and electrics are known to go through tires faster, and this is a cost you can't capture with your analysis.

Never heard of that and certainly not my experience driving two EV for aprox 200k miles total and comparing it to the 4 ICE vehicles I owned.

... there can be parts of a trip where regen is inefficient and/or annoying.

Then use the foot to make it smooth and not annoying. It really isn't that hard. What you want, you can achieve with the foot.

Speaking of annoying, there is no way you can drive that won't annoy someone.

I don't disagree but that has nothing to do with the situation where you try to start coasting on a freeway a mile before the off ramp to be able to come to a stop without regen. That's just plain dangerous and very annoying to everyone else on the freeway. Same with trying to coast to a stop sign or red light. Those are the only times where you would use regen on a trip and you absolutely have to in order to move with normal traffic.

I think we all agree that driving smart, looking way ahead in traffic and adjust your own speed as early as possible (without becoming a traffic hazard) to avoid any manual slow donws is the most efficient way to drive. But turning off regen on the car will not help at all. The issue is the driving style. If you accelerate too much and have to slow down coasting won't cut it. If you drive looking ahead and adjust your speed so you don't have to slow down, you don't get into regen and all is good. It just makes no sense to turn regen physically ff in the car.

 

[flar]

If the hill is stronger than the air resistance and you need to use regen to avoid speeding up then that is actually a good solution.

It is a fine solution most of the time, but not always the most efficient solution, and not always the desired solution. There are plenty of scenarios where the free kinetic energy gained from gravity can be absorbed by gravity or wind without the efficiency loss. It is also arguable that, as in the rest of you post, one should have planned for the decline and been going slower when it started, but then we are potentially in dangerous hypermiling territory again.

Note that the bulk of my post was actually a reaction to reports that people were claiming that the most efficient travel on a long flat highway involved the use of coasting to manage speed and I completely disagree with that assessment. But you are focusing on my added thoughts on hills and I agree that the cases there are more mirky. But, to expound...

For a steady state of a long downhill you are usually better reclaiming the energy into the battery before you increase wind resistance. Consider an exteme case where the car speeds up by 10MPH if you coast but then reaches a steady state where it won't get any faster and you are happy with maintaining that faster speed (safety/enjoyment-wise). In that scenario you have maximized your storage and are gaining nothing further. By introducing just a small amount of regen you will continually reclaim some energy that would have gone into increased friction (heat and turbulence) and be able to reuse it later. And you will keep reclaiming that energy, however imperfectly efficiently, for as long as the hill continues. If you simply continue coasting then you gain nothing. If you don't reach steady state, then you are likely going to reach a speed at which you no longer feel comfortable and then you will want to use the brakes. At that point you are then demonstrably and radically less efficient than using regen.

Looking at the less extreme cases, every MPH you gain reduces the efficiency of the speed gain because air resistance goes up quadratically. But, every second of regen has a consistent efficiency that lasts as long as you use the regen. I suppose there may be cases that depend on the size of the hill, its steepness, and how many MPH you gain where coasting would be a good solution too, but it would depend. And saying "I always want to use coasting because sometimes it might be the right answer" is a little short-sighted.

The presence of an immediate uphill right after the downhill also complicates things because not only do you spend the added speed immediately into a buffer against gravity, but it also reduces the amount of time you spend on the uphill.

I think the theoretical still almost never involves exactly zero energy. If you had a supercomputer analyzing the next 50 miles of roadway (and traffic?) ahead of you it could potentially model all variations of energy usage and it could come up with exactly the right amount of energy to put in or take out right at the moment for end-to-end efficiency and it would almost never be exactly 0.

But, humans can't calculate that fast and that fully, so coasting is an easy thing to comprehend that can get them more efficiency than guessing in many cases, but they aren't being optimal in the theoretical case, they are simply being better with their poor compute capacity than they could have if they just guessed at the amount of energy to use.

On a roller coaster road, humans will likely never be able to calculate any better than coasting on the downhills and spending the speed on the uphill. And they might not be far off from the efficiency of a supercomputer making carefully calculated use of regen.

On a long downhill, though, regen is probably a better answer than unchecked speed gain. If your local highways include more long slopes rarely paired with an immediate uphill then regen is probably a win. If you do have a lot of shorter back to back hills, then coasting might be an easier plan to achieve efficiency.

Also, my other point was that humans work better as a situational awareness calculating engine than as a numbers calculator. If you are looking at your numeric speed and kWh expenditure and the amount and direction of electron flow then you are using your brain for something that a computer would be better at. If instead you get your head out of the car's numbers and look ahead, even if what you see is a steep downhill leading into a steep uphill and how fast the highway is sliding beneath you, your brain will be able to do what it is naturally good at. You can train yourself as to how much speed in which situations is best so that you don't over-accumulate speed (whether the excess violates safety, or comfort, or what you can spend later on an uphill) and your foot will find 0-coasting when it needs to.

If you decide instead that "coasting" is the be-all and end-all solution as a control state, then you might get some added efficiency, but the ping-pong nature of intentionally entering that control state as a tactical end goal is limiting, and you then need a crutch to find that rigid coasting state (i.e. wanting to get exactly coasting by lifting your foot completely from the pedal).

 

[mxnym]

The presence of an immediate uphill right after the downhill also complicates things because not only do you spend the added speed immediately into a buffer against gravity, but it also reduces the amount of time you spend on the uphill.

Most of your post I agree with, and in fact, I pointed out in an earlier post that regen doesn't know/care/target that sweet spot you mention regarding wind resistance vs coasting on a long downhill. Regarding the quoted section, though, I must point out that efficiency is measured in kWh/Mi. The "less time" on the uphill is actually a good thing, as it is more Mi at a lower kWh. This is especially true if climate control or accessories are active, because they draw at a steady state, so when you aren't expending energy to gain speed or sacrificing it to maintain speed, the higher speed actually reduces the cost per Mi of your accessories.