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Towing a Tesla at 70 MPH replenishes battery at fast charger rates (insideevs.com)
306 points by danboarder on June 21, 2021 | hide | past | favorite | 383 comments

The most spectacular example of regenerative braking are trains that are used in Scandinavia, heavily loaded with iron ore that is transported to the coast:

"In Scandinavia the Kiruna to Narvik electrified railway carries iron ore on the steeply-graded route from the mines in Kiruna, in the north of Sweden, down to the port of Narvik in Norway to this day. The rail cars are full of thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts of electricity by regenerative braking, with a maximum recuperative braking force of 750 kN. From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border. Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity." (via https://en.wikipedia.org/wiki/Regenerative_brake#Conversion_... )

The heavy iron ore is essentially a large battery storing gravitational energy. In a way it's just another natural energy source. Maybe in the future clean power can be generated from pulling down mountains.

Years ago I visited the Seneca Pumped Storage Generating Station [0] reservoir in person. It's a massive man-made lake on the top of a small mountain in Pennsylvania, USA.

They pump water up to it at night (when there is excess energy in the power grid), and let the water out in the day when the demand for energy is high (turning some turbines on its way down).

Prior to visiting I had an intellectual understanding of the concept of pumped storage [1], but I have to admit that it's one heck of an experience when you see it up close and personal. My thoughts standing at the edge of this massively perfect-circle deep lake full of water: "somebody built that... and it's one BIG BATTERY".

If you get a chance to visit one of these, I highly recommend it!

[0] https://en.wikipedia.org/wiki/Seneca_Pumped_Storage_Generati...

[1] https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

The idea of pumped storage is pretty simple, but compared to lithium batteries the energy density is ridiculously small. If you already have an area that can be used for it (abandoned mine or reservoir) then it's pretty simply technology to build, but otherwise it's probably not worth it.

As an example of how low density it is, imagine having a 1000l IBC tank, filled with water, on your roof at a height of 10m. That water (~1000kg) has a potential energy of 98000J or 27Wh - less than a laptop battery :D

True, but pumped storage can be made ridiculously large given favorable terrain. I've been to one that pumps 5 million tonnes of water up 800 meters (half a mile of head!), generating up to 1GW on the way back down. It probably costs less to maintain that thing than to charge and discharge an equivalent amount of lithium batteries every day.

Density isn’t everything. It’s likely a lot cheaper and less environmentally destructive to dig a big hole in the ground than to manufacture a lithium battery.

That is not entirely a given. Large eathworks have important knock effects on the environment and wildlife. We tend to think that dams and such are harmless but they change their surroundings radically. I had experience living next to one which modified the local climate and made a large contribution to the desertification of the place.

Stripmining was not exactly "environmentally beneficial" ... and that is literally digging a big hole.


> The power plant, rated at 451 MW

That's a damn big battery. I love pumped storage, too bad it's not used more.

> I love pumped storage, too bad it's not used more

Pumped storage is awesome and the most cost-efficient way to store energy. It's used as much as possible, but...

I attended a USGS [1] talk about dams (in California) a few years ago.

The key takeaway is: all places worth damming have been dammed. There are no more dammable places, hydrologically and geologically speaking.

When I drive through the central valley, I see a lot of roadside signs about "build more dams!" It's sad to see the miscommunication...

[1] https://www.usgs.gov/science-support/communications-and-publ...

(I cannot immediately find a link to the specific talk. USGS is fun: full of crusty geologists, who even in the heart of the Silicon Valley aren't particularly technologically sophisticated (a nice reminder of how niche we all are) )

With pumped storage you don't need anywhere near the amount of water or storage needed for straight Hydro. In a 24 hour cycle, you can empty the top dam into the low dam, running the generators flat chat (unlike typical hydro which is limited by water supply), then pump it back up again for reuse. If you do the numbers (E=m.g.h) the amount of water required to store energy for a city of millions is quite tractable, assuming a decent head.

Australia is doing exactly this with "Snowy 2.0" (by connecting existing dams).


350,000 megawatt hours of energy storage, which is enough to power 3 million homes for a week, or (if there was enough generation/transmission capacity to get the energy in/out fast enough) the entire nation for 12 hours.

Another recent example in 2021. Kauai is supposed to be up to 80% powered by renewables with their pumped storage install. Small scale, but great example.


They are only adding 2GW of generation capacity, so I guess the 350GWh stored would take a week (175 hours) to be consumed by Snowy 2.0 that way.

I think you are describing pump-back hydro dams which are only a subset of pumped storage. Strictly speaking pumped storage only requires an elevation change and a water supply

If not near a water supply or lack of intent to use some of the pumped water for irrigation anyway, as the parent post said, (nearly) everywhere it was worthwhile to do is already doing it.

The alternative is another form of gravity battery, often involving railway on a hillside and cars loaded with stone.

Bah - Bath County can eat Seneca for breakfast and again for lunch and for dinner and then some. Max storage is 24000 MWh and can generate up to 3 GW

As Swiss friend once told me that Switzerland have super cheap electrical power by buying surplus power from the french nuclear power plants in summer and pumping into reservoirs high in the mountains.

Never verified the story, but geography checks out.

Interesting. The first thought that comes to mind when I see that circle is Apple HQ.

I just measured on Google Maps:

Apple HQ - around 1,600ft diameter

Seneca Pumped Storage Lake: around 2,400ft diameter

Extra power at night? Where's it coming from?

Demand is low at night, but many types of power plants are not easily/efficiently throttled down for lower supply (e.g.: nuclear, wind). So it makes sense to use the extra supply to pump water up.

They generate the energy probably all of the time. At night, however, fewer customers are using and so they can store the excess. That stored energy can then smooth out large changes in the daytime operating hours, or it can just supplement the supply as necessary.

At least in the case of https://www.electricmountain.co.uk/Dinorwig-Power-Station it provides near-instantaneous peak power (especially at the top of the hour when everyone turns on their kettle for tea), and is recharged through the early am via nuclear and other power plants that can't be shut down that quickly.

I believe the main power generation is hydroelectric, so there is some flow through the turbines at all times.

Not extra, excess. Demand is lower at night.

Apparently there is already the concept of rail train batteries: https://www.vox.com/2016/4/28/11524958/energy-storage-rail

You run the trains up the hill when energy is cheap or, for example, when the sun is out and solar works. Then when you need it, you run the trains down hill to generate electricity. Similar to pumped hydro where they do the same by pumping water up hill and then draining it downhill later. Super cool!

It's interesting as a thought, but I don't think it's likely to be large-scale practical.

Suppose you wanted to run a normal electric train up a mountain. It would certainly take a decent amount of power but not so much that it would be a big challenge for a city-scale power grid. So far you're not yet talking the scale of power where storing it would really be interesting to a grid.

One option to store more power would be to make the train much, much heavier. Sounds simple enough -- fill all of the cars with concrete and now hauling it up the mountain will store a lot more power. However now the rails and the trains themselves will need to be far sturdier than a normal railway, and will wear out quickly.

The other option will be to simply scale up -- start the day with a hundred trains in a rail yard at the bottom of the mountain and over the course of a day move them all to a rail yard at the top. Now you have successfully stored a decent amount of juice.

But hold on a minute... you've now built two large rail yards, meaning you'll need a lot of relatively flat real estate at both altitudes. How about instead you just dig a hole on each side, called it a reservoir, and put a pipe between the two? Certainly it must be a lot easier to store and move mass in the form of water than it is in the form of trains!

That is why I don't see much potential in rail-based storage: if you have the geography to build one at-scale, probably you could build pumped hydro there cheaper. Even if you were in a water-scarce area where you would need to enclose both reservoirs to avoid evaporation loss it still sounds simpler to me than building and maintaining a hundred heavy trains would be. Also, routing a pipe between two reservoirs is a lot more flexible than building a railway.

Yeah the idea is the trains are filled with tons of weight.

In my head it still seems like it would be cheaper than digging out massive reservoirs for pumped hydro. Rail seems relatively cheap even if you do have to replace it regularly because of the wear. It also seems like you could put the rail in all sorts of geographies, big and small. Maybe it isn't worth it unless you go big though... and at that point why not hydro.

It's good questions though. No idea how the economics of it will work out vs pumped hydro.

Perhaps the trains could partly be passenger trains, costing amusemant park rates.

Then collected money could eventually pay off the cost of construction, and eventually maybe even offset the maintenance cost.

If nothing else, it would certainly boast the advantage of being more fun than water storage.

Ideally have the transportees walk up the hill by themselves, summer sledding style. That way you are by some strange definition guaranteed to be net positive ;-)

I think you are on to something here. Make all elevators in hi-rises down-only and all stairwells up-only. Convert falling excess desk-jockey blubber into usable electricity while lowering rate of CV disease and generally improving fitness. Downside is probably a lot of BO that didn't exist before.

Clean power and mountaintop removal are mutually exclusive. That is neither clean nor renewable. Among other things, the water that flows through mineshafts, collects in quarries, or over strip-mines is often toxic as hell. When you dig into the rock like that, you expose a lot of water soluble toxic shit to the elements which poisons everything downstream.

> Maybe in the future clean power can be generated from pulling down mountains.

it takes a lot of power to dismantle mountains and load them onto trains. and folks get real grumpy about mining operations leveling off mountains.

And it sounds like a rather visible ... race to the bottom, as it were. :-)

>Maybe in the future clean power can be generated from pulling down mountains.

With added bonus efficiencies from combusting the coal that fortuitously falls out!

A thousand (1e3) metric tons (1e3 kg) of anything, up a thousand meters (1e3), has 1e9x(10)=1e10 Joules of potential energy on earth (g=~10 m/s2).

At 80% conversion (8e9 J) and assuming 1/8 of the energy is "payload" after paying for the train to go up, that's still ~1e9 or a GigaJoule.

At a -20% grade, you're looking at a 5km train ride, which reasonably might take a half hour or less (1800s). So, you're generating GJ/1800s = 555 KW by pulling down the mountain, assuming the rocks magically teleport into and out of your hoppers. (Thanks @calvinlh )

That's approximately 100-200 households per train pair, which might generate 10 K$ / month in electrical sales?

A 20% grade is probably unachievably steep for ordinary railways. Even a 5% grade is considered remarkable.

Wikipedia says (https://en.wikipedia.org/wiki/Saluda_Grade):

Saluda Grade is the steepest standard-gauge mainline railway grade in the United States.[1] Owned by the Norfolk Southern Railway as part of its W Line, Saluda Grade in Polk County, North Carolina, gains 606 feet (185 m) in elevation in less than three miles between Melrose and Saluda. Average grade is 4.24 percent for 2.6 miles (4.2 km) and maximum is 4.9% for about 300 feet (91 m).

Unless you're gonna build that mountain style with gear drive and toothed tracks, you're probably looking at 5% grade and a 20km ride, which drops you down to ~140kW for 2 hours.

Ion the other hand, it seems you can get 11 thousand tonnes in a coal train:


So on my 5% grade track I could get 1.5MW and if you can get your 20% grade to work that'd be just over 6MW

per train. Long track: more trains, right?

Great points though!

Then you also need hefty wires and adjacent infrastructure on whole track to collect all this electricity.

Sure, and magical robots that take no energy loading / unloading the trains.

If they are only moving blocks of concrete, loading/unloading is optional.

You're off by a factor of 3600-- 1 watt is a Joule/second, not a Joule/hour. 1 GJ / 1800s ~= 555kW, not 2GW.

I knew it seemed high. The GW description didn't match the wikipedia article.

This would make for a great oatmeal comic material.

Scifi novel idea: A habited planet without a sun (these actually exist) with heavy elements and high gravity. The surface unevenness is the best source of energy. After millions of years of extracting energy from flattening, the planet becomes a perfect ball and all life ceases as there is no energy to be found anywhere.

Geothermal energy is the way to go for this kind of worlds.

If there is a live core.

This idea is sort of being explored already: https://www.youtube.com/watch?v=itbwXMMkBQw

There's other ways of doing this too. Rolling a ball up a hill, inflating a balloon under water, etc.

That looks like a real life battery level indicator.

My take: it's cheating at jenga.

Energy Vault is a startup that stores energy using big concrete blocks. Pretty close to your idea: https://en.wikipedia.org/wiki/Energy_Vault

A similar concept is the Snowy Hydro project in Australia - essentially it is a battery for clean energy generators such as wind and solar when they cannot generate. There are two lakes - one at the top of a mountain and one at the bottom. When the wind is blowing or the sun is shining the wind generators and the solar panels power pumps that pump water from the bottom lake up to the top lake. When the sun is down or the wind is not blowing and electricity cannot be generated from panels or wind, water is run using gravity from the top lake to the bottom lake through a turbine, generating electricity that way and ensuring constant supply.

Melting down glaciers that subsequently pass through turbines on the way down should come close enough I guess? That future is here.

The iron ore was pulled up from the ground, so "some" energy is used there, to lift it into the trains. Now the mine is not quite as deep as ocean level, so there is still a net potential energy gain (you could say).

Literally destroying a mountainous environment is not clean energy.

But endless fields of solar panels and wind turbines is?

That seems like a somewhat arbitrary, fine line to walk.

neither of them strike me as great but taking down the solar farm is probably cheaper and easier than rebuilding a mountain.

Those sources of energy are both renewable and can be deconstructed with no permanent environmental impact, which makes them "clean" in my mind but I agree it's an ambiguous word.

This is fascinating. So they are essentially harvesting the gravitational potential energy of the iron ore at altitude to charge the batteries (and then some) for the cargoless return trip. Outstanding.

Reminds me of this dam operation around here that pumps water up to a mountain top reservoir during the day when power is cheap and then lets it go at night when it can generate and sell the electricity for more money. I was always in awe of the lake size battery they created.

Pumped hydro is really cool. Also really bad when it fails:


> ... catastrophic failure of a triangular section of the reservoir wall and the release of 1 billion US gallons (3,800,000 m3) of water in 12 minutes. The sudden release sent a 20-foot (6.1 m) crest of water down the East Fork of the Black River.

> A broad swath of dense forest was washed away and scoured to bedrock by the escaping flow.

A tidbit that will be interesting to any programmer or engineer: one of the causes of the failure was that the high-water gauges were moved to above the height of the dam wall because someone was annoyed by false positives.

Taum Sauk is unusual in that the dam fully surrounds the upper reservoir (i.e. the lake was made from scratch), although certainly all types of dam can fail.

All types of dams can be shittily maintained:


Sounds like they hired Homer Simpson to operate the plant.

Pumped hydro is one of the cheapest ways to store electricity iff you have the topography available to create the uphill lake and plenty of water available to compensate for evaporation.

> I was always in awe of the lake size battery they created.

This is exactly the feeling I had when visiting the Seneca Pumped Storage Generating Station that I mentioned in one of my other comments that I just posted before seeing your post. Truly awe inspiring. (Links provided in my other comment). Do go visit one of these!

Maybe this could be applied to water towers? That's a lot of potential energy just sitting there.

So I did the math and the average water tower holds something like 1/2 kWh worth of power. (50m tall, storage of 1m gallons, efficiency of 90%)

Hydro power storage is fantastic but needs truly ridiculous amounts of water and height deltas to make sense.

Must have been a bad calculator. It claimed 1000L/s was 400W.

You’re telling me I could fill a typical water tower in 1 hour with a 500 W pump running off a kitchen outlet?

500W is a lot, it is hypothetically enough to move 10 liters of water 50 meters up every second. A motor on a cycle an electric bike is 250w.

A kitchen tap provides like what, 0.2 liters per second?

So in an hour that's 36000L of water pumped up into the hypotheticals 1 million gallon tank. Right on the three orders of magnitude short that a previous commenter pointed out.

Sounds like they confused W and kW then?

Really good sanity check concept!

Calculator said 400W for 1000L/s and my mental sanity check missed the duration units and thought 400W can move a ton of water.

You probably made a mistake somewhere.

3.8Mg * G * 50m ~ 1700M Joules which is about 500kWh.

I suspect the k in kWh leads to the occasional factor of 1000 error.

Water towers are already used that way- water is pumped in during off-peak time and used during peak time.

Right, but that's about maintaining pressure and supply when everyone wants a shower at the same time in the morning, not about harvesting the energy of it all flowing downhill.

Pressure is energy


Energy is force times distance. You need to know more than just pressure to calculate energy.

(A bicycle tire at 120psi has a lot less energy than a truck tire at 90psi.)

pumped storage systems are so large as to be geographic features.

there isn't much energy in a water tower. and that water is already doing work, pressurizing the water system.

Before electricity there were many household appliances that operated on water pressure.

Water wheels were used in industry for over two thousand years.

Not enough volume.

And you have to maintain water in the tower in order to keep the water distribution system pressurized.

The same as a quarry truck in Switzerland: https://www.autoblog.com/2019/08/26/edumper-electric-mining-...

Which has a nearly infinitive range because because of how the quarry is set-up! In a way the perfect use case for electrification. The truck is able to drive anywhere and much more flexible - e.g. compared to a conveyor belt or cable car solution.

Same as this truck..

This Huge Electric Dump Truck Never Needs to Plug In


ARES (Advanced Rail Energy Storage) has been trying to commercialize purpose-built facilities of this type, with the first currently under construction in Nevada. They've gone through a number of design revisions but what they're building right now seems just just be multiple-unit trains full of gravel on straight lines. They've also proposed a system of autonomous trains loading and unloading concrete blocks via switchyards, but I imagine they are holding off on this more complex design for a larger installation, and the current site is a gravel quarry so they're using native materials. Advertised efficiency is over 90% at 5MW per rail line.

I’ve seen something similar to this with large dump trucks made for hauling loads down mountains. They generate so much power fully loaded on the way down that they can make the trip all the way back up.

An older strategy was to connect ore cars together so that the full train going down hill would pull the empty train going up hill. It was depicted in The Railway Series

What does the equation look like when you subtract the energy used in the actual mining, though?

On the other hand it would be nice to count in that the iron ore gains value when it's moved from inside the mine to the port.

thats not really a factor because the ore is being mined for other reasons, not strictly to generate power

probably not net positive if you factor in the energy required to load the ore onto the train at the top.

Regenerative braking is so neat. I drove up a steep windy road to go on a hike with my plugin hybrid. On the way down I tried to let gravity do all of the acceleration and regenerative braking do all of the deceleration. At the bottom of the hill I had racked up +3 miles in total. Another fringe benefit is that brake pads wear out slower and produce less pollution.

You can get this sort of benefit with a gas car with a manual transmission too. Engine braking uses no gas, its just your wheels turning your pistons and the friction from all of that slows the car down. Most of the time I only hit the brakes to go from 5mph-0mph in a manual car thanks to this.

This is commonly called power braking, usually called up in an emergency. It's a good technique that preserves handling. If you have RWD, your front suspension will remain more unsprung, in any case you can quickly apply the throttle to maneuver.

The friction isn't what's working here, it is the compression of the cylinders in the engine draining momentum from the wheels, think of it as negative torque.

As the forward momentum of the vehicle causes the rotation of the drivetrain to begin to overcomes the speed of the engine output shaft at the clutch or torque converter, torque is effectively being reversed. Instead of engine output pushing the rotation of the transmission, the forces invert causing the transmission to drive the engine speed upwards. Most ECMs will recognize the zero throttle position and cut off fuel, but older vehicles may not have this capability and especially carburetor fed motors are mostly incapable of reducing fuel delivery in this manner.

If you need to stop quickly and have the time, you can downshift even in an automatic, just throw it into 3 or maybe 2 while braking. If you have an electronic dual-clutch transmission with a "manual" mode, the electronic management controls will prevent the engine from over revving and causing damage.

Its important to keep the RPMs low if you do this a lot, if you redline, you redline no matter what your intentions are. I'd wager that running your ICE at 80-90% redline at low speeds for extended periods is also going to affect longevity.

There are other effects you might notice, such as additional bump and over/under steering as your suspension geometry responds to the unusual load. Also, other components will become loaded in usual ways during this maneuver causing some possibly unexpected handling characteristics to present themselves. Its good to practice this in a safe location, before employing it in the field.

If I may venture to say, it gets even more interesting; some vehicles are constructed in a manner where this happens all the time, the old school Porsches were notorious for an effect known as "Throttle Steer", a description of which exceeds the scope of this post, but a curious person will find several references to this in automotive literature.

I had a VW Golf with a DSG (DQ250) which I regularly engine braked on hills. However, I noticed that the instantaneous fuel economy meter would show some consumption on downshifting versus no consumption on coasting in drive.

It turns out that the logic will try to rev match to be in a lower gear before cutting off the fuel again.

Rev matching wasn't perfect either, as going down more than one gear would cause the engine to lazily rev up until it got close enough in speed before the clutches fully engaged.

The DSG offered a Sport mode (obviously to be used when late for work) that would downshift on deceleration, offering a mild form of automated engine braking. I could never quite tell whether would wearout the clutch packs or the brakes faster as it seemed to be a real push-pull scenario between the two.

Ouch, lots of the VW fans I've known were die hard due in part to the ergonomics and feel of the manual transmission. Something about a cable I think.

Too bad VW mechatronics doesn't read the throttle position for a clue here. It all just seems so obvious.

I think the rev matching to minimize the strain on the clutch pack. The car also had standard hill hold assist to minimize the risk of wearing out the clutch over a long period of time while living in a hilly area.

The design of the VW Golf Mark VII was absolutely brilliant. The EU model got a few why-didn’t-I-think-of-that extras that the NA market didn’t get, particularly around towing. There was a hidden tow hitch that popped out when a button on in the trunk was pressed, as well as reversing assist that used the electric mirror control knob (!) as steering control.

Dang, I might take one for a spin if I get the chance.

I talked to a mkVII GTI owner today at work, I asked him about his DSG. He said he liked it a lot, but mentioned that he found the shifting too fast and got a DSG tune and either paid or on his own was able to modify the shifting, but I didn't have time to ask him what he meant or for more details about the tune.

He's a chemistry PhD, and is into German stuff. Hes also got a w124 e320 that he's un-rusting (!) so I'd say he has pretty good taste.

APR is one tuner that provides chip mods for VW engines and transmissions. A base 1.8L 184 hp up engine can be wrung out to 239 hp with flash mods with practically no changes in reliability.

DSG tunes are also a thing but then it becomes a rabbit hole and a money pit…

Looks like a lot of fun, never got into VW much, so I don't know the ins and outs. He was out today so I didn't have a chance to chat. But, given that he's un-rusting a 1994 e320, I'd say hes pretty deep in the pit already.

VW transmissions haven't used a cable in many years, 30 plus.

> It's a good technique that preserves handling.

When traction is plentiful, yes. When approaching the limits of traction (in snow, or while turning, etc.), regular braking (or for curves, you could say trail braking) is usually best because the jerk of engine braking right when the clutch is engaged can upset the balance just enough to break loose.

That said, it does have an anti-lock effect in snow which is handy for cars that lack ABS, so as long as the jerk is accounted for, it can be a net win.

> This is commonly called power braking, usually called up in an emergency.

I've never heard it called power braking, always engine braking or j-braking.

It isn't just useful in emergencies but also in steep descents with heavy loads to make sure your brakes stay cool enough to be useful. It may be obvious, but the other technique that really helps keep your brakes cool is descending more slowly.

Also known as "jake braking". It's useful if you're a dump-truck driver who thinks midtown Manhattan is too quiet.

A real "Jake brake" (Invented by Clessie Cummins) slows a truck down by turning the engine into an air compressor. It does this by modifying the exhaust valve timing to open at top dead center while cutting fuel. Once turned on all you do to engage it is to take your foot off the accelerator. That staccato popping sound is the compressed air blasting out of the exhaust valves.

On old rigs like my '61 Mack the jake brake is switched manually. You have to be sure to turn it off once you get close to engine idle rpms otherwise the engine will stall.

A quieter version called a bleeder brake which does the same thing though it leaves the exhaust valve slightly open during the entire compression stroke eliminating the popping sound. There is also the exhaust brake which puts a butterfly valve on the exhaust line creating back pressure when closed. Though the exhaust brake is known to cause problems with some valve train setups.

Jake Brake is engine braking, its really similar but not the same. Theres a difference between the kind of power braking/engine braking that a car does and a commercial truck.

I typed a lot about this elsewhere, but essentially trucks have crappy brakes because of reasons (thermal ones, in large part) and as a result, in order to gather enough brake performance to not be extremely hazardous, they often must apply their engine brakes/jake brake/engine compression release brake in order to safely navigate with their loads in normal traffic or an emergency braking scenario (such as being cut off in midtown by some a-hole).


Talking a little bit out of my ass here making some assumptions, but I'd guess that in this context, 'power braking' refers to grabbing an low gear and letting out the clutch to assist the front brakes when you're trying to stop really fast and maintain the handling dynamics of the car. It's hard to explain, but if you've driven a manual sports car for a while it's easy to learn. And it isn't the same as simple engine braking, which is usually done in higher gears to add moderate resistance, not to actually try to slow the car significantly.

And you do have to be careful not to grab too low a gear and/or don't let the clutch out too fast, or you can mechanically overrev the engine.

IMO compression braking (jake brake) would be a better analog.

> And you do have to be careful not to grab too low a gear and/or don't let the clutch out too fast, or you can mechanically overrev the engine.

A.k.a. the "money shift" as it results in spending lots of money, typically from grabbing 2nd instead of 4th (each being relatively down and to the left).

Your front end still takes the weight under any deceleration event. That's why FWD cars still squat in the rear on acceleration.

yes, it will always rock fore and aft to some extent, but consider the difference between applying a drag to the front of the vehicle or the rear. Try this on a bicycle if you have the time.

The "squat" you see on accelerating FWD vehicles is something different called wheel climb I think and is related to the effects of braking and center of gravity that we are discussing. When you gun the throttle, the wheel torque applies an opposite force as the wheels are turning in relation to the car. This causes a lever-like interaction forcing the front up and the rear down. During the engine brake maneuver the situation is reversed, as the torque is reversed in this scenario.

Imagine you had a car with independently activated front and rear brakes: Its trivial to understand that in a front-brake it will cause the front of the vehicle to tip forwards, and the rear will respond by tipping upwards. This unloads the rear suspension causing excessive camber, reducing tire contact, and if you were applying the brakes here, braking performance.

If you apply only rear brake, it will cause the rear wheels to pull their suspension downwards tipping the car rearward somewhat. As the stopping vehicle weight then pulls on the rear axle load, it applies a weak lever-like corresponding downward force on the front axle, reducing your center of gravity over the front-brake scenario. This will improve your handling performance, like I said.

Additionally, highway vehicles are designed with proportional braking that applies more of the braking force to the front rather than the rear. This, among other reasons, is why you can fishtail when you slam on the brakes. If you want to try it out, pull your ABS fuse the next time it rains, snows or there is some other slippery conditions and tool around in an empty parking lot.

This is why power braking is more suitable for RWD vehicles. It alleviates some of the effects of proportional braking and maintains a more favorable center of balance and suspension geometry.

It isn't that it doesn't work on FWD vehicles (It works well, and I did not mean to imply that it did not) only that RWD vehicles preserve more of their handling with this maneuver because of the facts regarding how a vehicle's suspension and center of gravity respond to braking and suspension load.

> If you apply only rear brake, it will cause the rear wheels to pull their suspension downwards tipping the car rearward somewhat.

Are you sure? I'm trying to draw a free body diagram of that, and I get a net moment tipping the car forward. Can you explain it using physics principles?

> Try this on a bicycle if you have the time.

You can't easily compare because the rear wheel easily skids with a fraction of the braking force the front wheel can take.

> Are you sure?


It's pretty confusing, I did the same thing and came up with the same answer as you. Then I typed a sentence about "even on fully independent multi-link rear suspensions" before going off into the weeds. If the suspension were rigid, it seems like it should do the opposite, climbing upwards, but there is more going on in the car than the assumptions we typically make with a model.

A rear suspension is of course not rigid. It is, in short, a vertical damped spring mounted on a lever attached to the frame in front of it's axis. The axle/wheel hub is attached to the bottom of this damped spring at the end of the lever.

During braking, the rear will deflect rearwards and also be drawn towards the centerline of the vehicle following the arc of a circle drawn on the ground. The suspension will control the forces through the trailing arm, directing them towards the rear along a second arc drawn on the side of the vehicle, centered on the rotational vertex where the trailing arm is bolted to the car in front of its axle. The sway bar will control both sides to the same level which will be the apogee of the circle, closest to the rear bumper.

You can test this by pulling your parking brake below 5mph. If this doesn't work, call your mechanic :)

>You can't easily compare because the rear wheel easily skids with a fraction of the braking force the front wheel can take.

That is because braking the front bike wheel causes the lever to apply an upward force on the rear axle, pulling the front wheel rearwards, causing absurd instability and poor handling. This also happens to the rear wheel, it wants to hop upwards in response to the brake forces. That's why the proportional brakes exist in highway vehicles, to stabilize the braking forces into the most efficient braking scenario. If the front wheel were mounted in a reverse mounted trailing arm suspension (This doesn't exist, so I don't know what else to call it.) it would drop the front of the bike as happens to a car instead of flipping you off the bike. The side effects of this are atrocious understeer and a nonsensical center of gravity.

The reason the wheels move like this is to control for a characteristic called caster. Bikes have negative caster, if you let go of the handlebars at low speed, they will flop to one side or another at random, causing you to crash. Cars have positive caster which is the most stable configuration for the steering to operate. This is also why your steering wheel returns to center when you let go of the wheel while moving forward, and it is why the braking forces can be stabilized in this manner.

I see. It's specifically an effect of the trailing arm type of suspension. Damping, the wheel's rotational inertia, the sway bar and lateral movement of the wheel aren't needed to explain this, are they?

No, I suppose the sway bar can be ignored for the conceptualization, the only thing is cars have 4-wheels and they all affect each other. If you want to simplify it, you gotta invent a 2-wheeled conveyance. The shocks and sway do a lot of work. If that satisfies you, don't read the rest:)

Some vehicles don't have swaybars, or instead they might have a "solid rear" rear subframe to which both knuckles are attached, and moves as one large unit. They always have shock absorbers though. I'll just keep considering only the rear axle here.

I can tell you are away of, but do note, that even without a trailing arm configuration but something else, the wheels will still always want to go back and towards the centerline of the vehicle during braking. The suspension geometry will determine how this is controlled, usually the move is to go down. I've never seen one that goes up unless something is broken, usually the shocks. People normally call this excessive movement "nose dive". It happens because a functional shock absorber momentarily control the climb, allowing the suspension to control the wheel characteristics. Without it, the forces will yank the wheel out of configuration.

In most vehicles, without the sway bar controlling both sides of the suspension, or the shock absorber damping, you will experience many varieties of reduced braking and handling performance unless both wheels on the axle are reacting in exactly the same way, which is very unlikely.

If the spring is undamped, aside from the "nose dive", it will start to oscillate in an uncontrolled manner during braking, causing hopping. This will cause the wheels to both not brake well and also lock up. This will still affect the solid rear, causing wonky camber.

If one wheel locks up, the ABS will kick in. This is normally fine, and increases performance in all categories, but only if the wheels are in contact with the ground and exhibiting correct camber. If they aren't and are instead hopping because of the undamped spring, the ABS won't be able to control much, it will in fact suck.

If the sway bar is absent/broken/ the linkage or bushings damaged, and one wheel has different handling or braking performance than the other wheel on the axle, (such as in all the time or during hopping or ABS engagement) the ride height will change as the wheel reacts along the second circle, causing body roll which will alter camber and other factors on every other wheel, especially the opposing wheel, which will experience much worse performance in every category.

All this stuff will show up on the tire. You can often tell whats wrong just by glancing at the tire tread.

>it is the compression of the cylinders in the engine draining momentum from the wheels

Does it really? After all, while it will take energy when cylinder goes up and air is compressed, immediately after that the compressed air will be instead pushing cylinder down, returning that energy to crankshaft. There will be loss from compressed air heat migrating into cylinder walls, but it doesn't seem to be large enough to be responsible for majority of engine braking. It would also mean that diesel engines should engine brake much stronger due to higher compression ratio, but this isn't the case.

It seems to me that engine braking is just various friction losses from all parts of the engine, without single main source.

I got pulled over for this. I was driving home from a friend's at about 2am so nothing but clear roads, it was on about a 10 mile stretch of dual caradgeways with a few roundabouts along the way. At the 3rd roundabout I got the flashing lights and dutifully pulled over. Turns out they pulled me over because they thought my break lights were out. After a quick demonstration of my fully working break lights and a quick chuckle between myself and the cop I was off again.

I do wish the Tesla would turn on the brake lights when slowing down at a certain rate (due to regenerative braking). I've nearly been rear-ended several times by cars that didn't expect me to "coast" that slowly or, conversely, slow down that fast without brake lights.

They do

Yeah, I was worried about this and so I watched the lights on the visualization: at least per the visualization, the brake lights are turn on for regenerative breaking more or less like they would turn on for applying the brakes.

It doesn't refill your gas tank though, you just use the momentum, friction and heating up of compressed air of your engine to slow down, wasting all the energy of going downhill into heat.

EVs on the other hand get a partial refund for the energy they spent going uphill in the first place.

Most mechanics will quip "Remember, brake pad replacement is a lot cheaper than a transmission replacement".

Don't over do it.

I've always heard this, but when I drove a manual I always downshifted frequently and kept that car for 150,000 miles. I did have a clutch replacement somewhere in the middle, but blame that on starting on San Francisco hills more than downshifting.

There's no harm in engine braking if you shift correctly and mate your RPMs to the next gear.

Diesel engines in large trucks have a similar feature: https://en.wikipedia.org/wiki/Compression_release_engine_bra...

However, unlike regenerative braking, you won't end up with more fuel in the tank afterwards.

The only downside is it can be really loud as you shift down the gear and engine RPMs bounce around. Hence, all the signs when you approach a town that say, "No engine braking".

When correctly muffled, engine breaking is not really any noisier than the engine operating normally.

Truckers will sometimes install a valve to bypass the muffler, so they can let someone know that they just cut them off, and that they should try moving out of the way.

Engine braking is definitely always much louder than regular braking. Don't piss off your neighbors.

The bypass valve on semi's you're referring to is not the exhaust. The huge BRAAAAAAAP that you hear coming from trucks when someone cuts them off aren't an exhaust bypass valve, its them slamming on their brakes to avoid a tragic 20-car pileup.

It is a release of the compressed air-fuel mixture momentarily before combustion called a Jake Brake or Compression Release Engine Brake[0], that Semis and large commercial vehicles are equipped with that provide for additional braking in an emergency.

What they're doing is releasing the compressed air-fuel into their exhaust, just prior to combustion. This causes the engine speed to drop instantly, slowing the truck down. It will surprise you to learn that semi trucks have pretty terrible braking performance, loaded or unloaded. They all use drum brakes.

It is strictly prohibited to bypass a muffler or emissions control device in the US. While cars get away with it all the time, a commercial truck carries additional registration, inspection, and license restrictions which makes this not only dangerous, annoying, needless and harmful, but extremely expensive.

Commercial Driver's face much steeper fines for equipment that is out of specification. Many commercial drivers are required to inspect their vehicles before they depart for even so much as a top indicator light which is burnt out, or they may face harsh penalties and fines that would make your skin crawl. $10,000 citations are levied daily.

[0] - https://en.wikipedia.org/wiki/Compression_release_engine_bra...

do you know how does those volvo semis stop on a dime? are there some sort of destructive braking mechanism to perform emergency brake?

I'm not sure what you're referring to, sorry

There's a video I've seen of one slamming the brakes on for a kid that runs into the road, and it stops super quickly.

just saw another one, really black magic to me https://www.youtube.com/watch?v=P-giC24SxwE

That's actually not surprising at all for air brakes in good condition; braking pressure is supplied entirely by air, and drum brakes are used because they have a much larger area of friction material than disc brakes, and thus can provide a higher stopping force in the same volume. Their main disadvantage is brake fade, which is why heavy trucks use their engine brakes very often.

Another fact that might seem counterintuitive at first is that a heavily loaded truck can stop faster than an empty one. This is because it has more weight on the wheels, which means more force before they start to skid, and the brakes themselves are not the limiting factor.

Depends on the size of the engine and the design of the brake. Big engines are louder than small engines. Jake brakes are louder than regular exhaust brakes.

My Toyota car, with an automatic transmission, does this automatically after it detects I've been riding the brakes for some time while going down a hill.

Coming from manual transmission, I was super excited when it happened the first time.

My 17-year-old Honda minivan does the same. A few seconds on the brakes while going down a steep hill will trigger a downshift.

Now try regenerative braking in an electric car. The 70kW regenerative braking ability of the Chevy Bolt is amazing on any downhill stretch. To the point that I can set a cruise control going down a mountain and actually maintain that speed without applying friction brakes.

I recently changed from a 20 yo diesel to a 10 yo gas/benzine car ... on the old car "motor braking" got me reliable from 100 km/h (over land) to 50 km/h (city) speeds ... the "new" car apparently has less losses and motor braking doesn't seem to do me any good anymore :-/

It’s not quite the same, once you’re down to level ground with stop lights, or stopped engine breaking doesn’t let you drive 3-5 miles for free or get back up to 25mph for “free” if it was just a stop light. Quotation due to the penalty of accelerating a heavier car or lugging it up hill.

Show HN: lapetitejort goes hiking, discovers perpetual motion

That would be a neat test. Find a steep hill, preferably as straight as possible, and test how many times an electric car can traverse it before running out of batteries. Would the distance traveled exceed that on a flat road with no generation? By how much? I'm sure someone in San Fran has already tested this.

Would the distance traveled exceed that on a flat road with no generation?

No. Climbing the hill the car uses extra energy (compared to a flat road) because its fighting gravity. On the way back down the hill regen will recover some of that gravity-fighting energy but nowhere near all of it.

This scenario is a little different than a car but a Swiss company is experimenting with a Komatsu Dump Truck that basically recharges its battery using regen braking on it's trip down so it has enough power for the trip up. It actually generates a surplus of an extra 10kwh because the truck is carrying a full load down.


And going up it is empty. Its basically exploiting the fact that rocks at the top of a hill have higher potential energy than at the bottom. If you charged the trucks with wind/solar at peak times and used them to carry the rocks back uphill you could have yourself a very convoluted and mechanically fraught battery!

A good visual demonstration of entropy. Heat death occurs when all of the mountains have been leveled and there are no altitude differences to exploit.

That's excellent! I'm stealing that.

That's actually pretty cool. It's kinda like a dam... harnessing potential energy stored by geologic processes and turning that into power? I'm sure there's some fancy physics word for it that I don't know.

What if you did this with passengers instead of a truck load of rocks? Carry passengers down a mountain, go up the mountain with an empty tesla.

I don't think the passengers would appreciate getting back up via plate tectonics.

I mean, you provide a service to bring skiers down the mountain. You can let regular taxis bring them up

I’ve tested this on an extremely steep block in seattle. This was about 8 years ago in a plug-in Prius. Distance around the block was about 0.25 miles, one side generating a lot of range, the other side eating it up because I’m going back uphill. On each cycle, I would lose an extra 0.1miles of range over what I experienced on flat ground.

It’s actually very interesting to me that while driving that car, I developed a sort of feel for how trading potential and kinetic energy affected my available range, and I had an imaginary boundary drawn in my head that defined all the places where I’d be able to make it home “for free” that was kind of like a topo map. In particular, if I managed to crest the hill where Canlis sits on highway 99, I knew that despite all the ups and downs in between me and home, I’d be able to make it there without having to fire up the gas engine. :D

In theory they would be identical, but in practice a flat road would be better. There are more real world inefficiencies with going up and down a hill.

My intuition would say you'd never have longer range driving up and down a hill compared to driving flat in a straight line (and never braking).

I would imagine that at best, you could maybe match the range. If there's any scenario where going up and down a hill would yield more range than driving flat, I'd be very interested in how/why.

There is a potential benefit, and it is not really due to regenerative braking. It would require that your traction system is more efficient at converting stored charge into kinetic energy at high loads than at medium loads and that it also has a very efficient "coasting" mode to cut losses when almost no output is required.

Cruising on level terrain is a mediocre load on the traction system to offset the rolling and aerodynamic friction. Climbing a steep grade at the same speed will increase the load significantly, but that extra kinetic output energy is being stored as gravitational potential energy rather than lost like the baseline cruising load. Then, when you descend the mountain on the other side, you recover that gravitational energy to offset the rolling and aerodynamic friction. You want to be "falling" down the mountain grade at your terminal velocity where no braking and no traction force is required to maintain your cruising speed. It will not work well for winding descents where you need to brake for turns.

I've seen this work with turbocharged ICE cars with elevation gains of 4k foot or more and hundreds of miles distance. I've repeated on many trips to prove to me that it isn't simply a fluke (such as extreme headwind or tailwind). Going over a pass yields me a better trip MPG than covering the same several hundred miles on relatively flat ground.

I could see this benefit for a hybrid car too, since I believe they mostly handle highway cruise on ICE power. However, it seems unlikely for a BEV car unless there is something about battery and power electronics that I do not understand, that would give them a significant efficiency boost at high power.

It seems possible in theory. The air is thinner at higher altitudes, and wind resistance is a big part of fuel economy, isn't it?

Suppose you're going to make a 100 mile (160 km) trip. You have two possible routes. One is a straight shot on level ground. The other is straight, too, but it takes you up a 10% grade for the first 10 miles (gaining 1 mile in altitude), then you continue on level ground for 80 miles, and then you descend a matching 10% grade at the end.

It seems like the 80 miles of cruising on level ground at high altitude would use less battery than 80 miles of cruising at sea level (if the speeds are the same).

There will be some losses due to climbing and descending. Maybe climbing and descending is a little less efficient. Also, you're definitely traveling a very slightly longer distance. But those losses might be made up for spending the bulk of your trip in thinner air.

Going up/down at 20 MPH and going flat at 80 MPH might be this scenario. If the velocity difference is big enough, wind resistance will have a bigger impact on range than the thermodynamic inefficiency of regen braking.

Yeah, rethinking it, assuming you start and end at the same spot, the milage at the beginning is the max you can achieve, full stop. Simple conservation of energy. So the question becomes, how many miles do you lose at the end?

+3 miles hopefully didn’t include the energy spent going up, or the law of energy conservation would have been broken.

Maybe he picked up several large people who were already at the top.

Energy trolls?

Maybe lapetitejort had better lawyers.

probably the first time a comment on HN has made me laugh.

I hope the prosecutor has enough leverage with the court! Justice truly is blind!

I imagine there might also be an effect in play, in which the available miles are calculated using the energy used in the past x miles. Although it should usually take a fairly long distance to change that approximation... I think. Maybe something about using negative energy though regenerative braking skews the approximation.

Depends entirely on the vehicle. The lookback on some is fairly long, others not so much (Bolt EV).

New EV users often forget that unlike 'gallons of gasoline' or 'kWh', 'estimated miles of travel' are not a true unit of energy.

I think the wind at his back is key here. His car was acting like a wind turbine

I think OP meant winding and not windy, but even if not, Teslas (and any other car) are designed to be aerodynamic, not the opposite, so it's quite impossible to yield more energy than it cost in that scenario, even ignoring motor efficiency, regenerative braking efficiency, and every other real world inefficiency.

> Teslas (and any other car) are designed to be aerodynamic

Why not design them to be aerodynamic for headwinds, and maximize wind-resistance for tailwinds? I wonder how much energy would be gained.

Windy is a perfectly valid descriptive word for a twisting road.

the confusion in this thread is that windy, as written, could mean there's lots of wind (said "win-dee") or lots of bends (or "winds", said "whine-dee"). "winding" is less ambiguous

Obviously. But `croon` seemed to think that windy was was incorrectly, not just ambiguously: "I think OP meant winding and not windy"

Not necessarily. The range is a function of energy over rate of consumption. It's possible the car changed the denominator giving it a further range estimate with less total energy remaining.

He used a portal at the top and bottom of the hill and has now solved the world’s energy problems, including powering the portals

+3 miles is precise enough it could easily be measurement errors

Climbing was probably done by the ICE not the EM.

That's just pseudoscience: The earth is flat so there's no such thing as going up hills.

My regular (non plug-in) Hybrid only lasts a few miles on downhills until the battery is 100% charged. After that it uses the generator as a motor to spin the ICE engine to burn up energy. It's not enough on long steep downhills, so I still need to brake.

+3 miles electric, -20 miles gas (or something like that).

The Mercedes got 5 MPG while towing the Tesla at freeway speeds. [1]

1: https://youtu.be/nILM_DEdBqM?t=336

Makes sense. A MB C200 has 135 kW max from the engine, the Tesla’s battery is taking half of that, and losses in the energy transfer would be extra.

I don't think you have much losses in the transfer, the strap is probably efficient. Although they would be extra so you are correct.

GP almost certainly meant losses in the transfer from mechanical energy to chemical energy by way of the cars motors being used as generators, then charging the batteries.

Don't forget the double loss from sending the rotational energy to the ground and back.

Unsprung mass always surprises.

I'm surprised it was that good. I get 8 MPG from my F250 when towing a travel trailer that doesn't have the brakes engaged.

Your travel trailer makes up for it by punching a very big hole in the air.

Yeah, I liken it to what it would be like towing an open parachute behind you.

I'm disappointed at the terrible efficiency we're getting from this rig, to be honest. Based on experience I was expecting more like 10mpg. It doesn't make a big difference in cost, but Ford only gave the 6.2L F250 a 30 gallon tank. For something that gets such abysmal gas mileage, that is a really questionable decision. Our last truck, an F150, had a bigger tank even with a shorter bed. I have no idea what Ford was thinking here.

I too exist where the vent diagrams of HN reader and F250/travel trailer owners intersect.

Why the down votes?

Strangely, the intersection of HN and F250 owners is a smaller set than the intersection of HN and F250 haters.

This seems like an interesting solution to the "what to do if my battery runs out in the middle of nowhere" problem.

Normally, you'd either have to have to call a tow truck or have someone with a generator come along and recharge. However, towing for awhile (probably at much less than 70 mph for safety reasons) to recharge the battery enough to get to the next town is something that could presumably be done by just about any passing car if you have a tow strap (which could be stored in the EV for such an occasion).

> However, towing for awhile ... is something that could presumably be done by just about any passing car

A Tesla model 3 weighs over 4000 pounds. A base model Toyota Tacoma isn't even rated to tow that weight, there's no way any passing sedan could. Now remember that for regenerative _braking_ to work, the tesla's brakes have been on! Definitely not something any passing car can handle.

The weight of the Tesla is moot. The majority of the load experienced by the pulling vehicle is in the incredible drag of the Tesla’s wheels. They’re extremely difficult to spin and this for pace far exceeds any others that would normally be encountered when pulling a traditional load.

Well, at 70 mph the load would be a lot. If the car were towed at 25 mph it wouldn't be as difficult, and I assume most EVs let you configure the regen amount. If you set it to light regen it'll be easier to tow, but charging will take longer.

Having at least some regen braking all the time seems like kind of a benefit, since there's less risk of the rear car hitting the front car if the front car stops abruptly and the driver in back doesn't step on the brakes fast enough.

I can agree though that if the EV being towed really thinks it's supposed to try to stop as hard as it can, it would take a pretty powerful engine/motor or low gearing in the lead car to drag it along.

The cool thing about EVs is that they have an instantaneous power meter in them, so there really is no need to speculate.

The following numbers roughly correspond to my 2017 Model S: At 30mph the car consumes approximately 200Wh/mi (6kW or 8hp) and at 70mph approximately 300Wh/mi (21kW / 28hp). This is the power required to maintain these velocities against all external factors, and therefore is exactly equal to the power required to tow the vehicle at these speeds.

If you have regen set to normal, the car will supply up to 50kW / 67hp through regen. TBH I don't know exactly what the minimum speed to achieve full 50kW is, but I am reasonably confident that the car will do it at 30mph since the tires have more than 10 times the required grip to do it.

Towing at 30mph = 56kW / 75hp, 89% charging efficiency, equivalent to a ~2000lb trailer Towing at 70mph = 71kW / 95hp, 70% charging efficiency, equivalent to a ~2500lb trailer

The trailer weight estimates are roughly basaed average of engine power / towing capacity figures for medium duty pickups. The tow vehicle to do this safely in my estimation should have a 250hp power plant or larger.

The biggest flaw with the linked article is that it's not necessary to go 70mph to achieve the maximum charge rate from regen, and indeed it is less efficient to do so.

> A base model Toyota Tacoma isn't even rated to tow that weight

Isnt this rating more about supporting and breaking force for the combined vehicle and load weight.

My understanding in this case tow means 'pull the vehicle' vs carry the front weight. So I think even a smaller car would be fine to pull a Tesla.

>Isnt this rating more about supporting and breaking force for the combined vehicle and load weight.

There are braking requirements in the test but the bottleneck is almost always (i.e. I want to say always but I don't put it past Dodge to show up with a 700hp vehicle) the acceleration and minimum speed up a grade portions of the test. Given the weight that can be accelerated and pulled up a grade per the test requirements basically every vehicle winds up passing the braking portion with room to spare.

But if you get all your vehicle performance advice on Reddit and HN I can see why you'd think brakes are always the bottleneck.

Being able to safely break is part of the towing test, as well as being able to haul the load up hill at highway speed.

The point is that an average sedan absolutely does not have the power to tow 4,000 pounds, let alone 4,000 pounds with it's breaks on.

Towing ratings are given in lbs but you have to remember that is a "full system" number which includes the load on the hitch, the tongue weight, and (the usual limiting factor) the ability for the vehicle to reasonably accelerate and stop the entire mass in a reasonable time and distance.

I estimated the required power and the equivalent "trailer weight" of towing a tesla model s under full regen elsewhere in this thread; it's roughly equivalent to towing a trailer of approximately 2000-2500lbs, depending on speed. A vehicle with a similar tow rating and a minimum 250hp engine should do the job.

All modern trucks are conservatively rated in the towing department because of the grade pulling requirement in the test all the OEMs use.

The SAE J2807 tow grade test is done on a real highway in Arizona, not some made up crazy scenario.

If you can only tow a certain load in ideal circumstances (no hills, no sudden braking, no wind, etc) then you actually cannot safely tow that load. The real world has hills and the occasional need to slam on your brakes.

>The SAE J2807 tow grade test is done on a real highway in Arizona, not some made up crazy scenario.

So? That doesn't make it representative of typical conditions.

>If you can only tow a certain load in ideal circumstances (no hills, no sudden braking, no wind, etc) then you actually cannot safely tow that load.

So someone in Kansas or Florida should be limited by grades that aren't even with a day's drive of them? Do you seriously believe this?

This line of reasoning is even more comical in light of how the current SAE test almost always results in engine power being the bottleneck.

There's a pretty massive amount of safe behavior you can do outside the confines of the magic rating number and even more options of unsafe things you can do within the number. At the end of the day some amount of good judgement and discretion is required because there is no shortage of variables that are static in the SAE test

Your position is only a stone's through from the pitch of the slimy RV salesman who says you can tow something anytime anywhere because it's less than the magic number. I think you (and the hypothetical RV salesman) need to take your appeal to authority and desire to substitute a nuance-free number with critical though and shove them somewhere unpleasant.

> So someone in Kansas or Florida should be limited by grades that aren't even with a day's drive of them? Do you seriously believe this?

I never said that. I think the current tow rating system does represent the safety margins needed in real world towing. If a truck can't tow X pounds up an actual hill on an actual US highway, then it shouldn't advertise being able to tow X pounds.

But, if you live in Kansas and you are sure that your brakes can handle a higher load, that's fine. As you said, it's up to you the driver to know what's safe and not. I also think you have it coming if you lose control of an overloaded truck and your insurance company refuses to cover damages, etc.

I view it similar to overclocking a server. Servers are clocked somewhat conservatively to ensure reliability. You are free to overclock a machine you own, but you run the risk of instability and you cannot complain to the manufacturer if you have downtime resulting from running your machine outside of spec.

Given the mileage of towing a vehicle with its brakes on, I can't imagine it wouldn't be more efficient to just tow the stranded vehicle to a charger.

When you're stranded, energy efficiency isn't the top concern: any solution that works could be an acceptable one. And it's probably a lot easier to tow a car on a long straight road for awhile then cut them loose than it is to tow a car through intersections and into parking lots to find a charger.

I work in automotive, I’ve seen some amazingly bad towing examples. Sounds like a decent solution to get two vehicles stranded.

I would think unless the tide is coming in, getting a generator and hanging out for awhile would be a safer for everyone solution.

>I’ve seen some amazingly bad towing examples

Are we talking genuinely dumb things like forklift loaded facing forwards on a small utility trailer or the typical online pearl clutching when someone is 100lb over their tire rating?

The former :)

Tongue weight, what is it and how does it work? I guess we’ll never know!

It probably means you need to get someone to tow you halfway to a town. But there isn't that much advantage over towing you all the way to a power outlet. And once you are asking strangers for help, just toe you to a residence that is happy to power you up.

The residence won't charge anywhere near as quickly as this, though.

The Tesla charger at my residence charges the car at ~30mph

I love this measure of charging speed. I'm wondering now why I haven't ever seen it used before.

It is getting more popular, also because vehicles charging at same energy input (say, 100 kW) can show larger kmh value (or mh) depending on how efficient the car is: the more efficient car can go further on the same energy input.

I imagine this might incentivize cars optimized for the maximum efficient speed rather than top speed, (so they can report the most miles per hour when charging).

This is pretty standard: the Tesla charging screen shows this while charging.

The tow charge was registering 65 kw. In a Model 3 or Y, this would be over 200mph.

Only if any of those passing cars are gas-powered. If the passing cars are themselves also EVs, then there's both more obvious forms of energy transfer, but also you'll be taking that much range (and more) from the EV doing the towing.

Cost asymmetry - if I had the spare battery capacity (assuming I'm only doing local drives) I'd happily donate it to someone stranded. It's like giving a fiver to fill up emergency gas if they're broke at a gas station.

Transferring energy from one car to the other isn't quite that straightforward, even though it's something that sounds like it ought to be easy. (The simplest thing would be jumper cables, but there's not usually any easily-accessible high voltage terminals, and the batteries might be different voltages anyways. And you'd want to limit curren, and so on....) Maybe if the new Ford F150's 220-volt outlet becomes a common feature, then charging from one car to anther will be easier (though very slow).

I think the main towing advantage a typical ICE car would have over an EV with extra battery capacity is the ability to shift into 1st or 2nd gear. It's not really an intrinsic property of EVs that they can't have transmissions, but so far the major manufacturers have decided it's not worth it. (The Taycan is a notable exception.)

On the Long Way Up they did just this to recharge their Rivians

That is a fantastic show. Rivian and Harley (electric bikes) did great.

It’s actually really dangerous and generally illegal to tow a vehicle with a simple, easily stored strap.

It's more dangerous than sitting in an air conditioned office and posting on HN but if you're not trying to keep up with stop and go traffic it's not substantially worse than flat towing. You have to drive like a responsible adult and trust your buddy to do the same but that kind of goes without saying for any situation where you're moving double or more the weight you usually do.

This is really terrible advice.

It's super dangerous, especially at highway speeds (i.e., fast enough to charge a tesla). Granted, leaning on the brake regen makes pulling a tesla a little less dangerous than pulling a gas car around at speed.

There's a reason this is illegal most places. It's also, generally, pretty hard on both vehicles to tow on a non-rigid connection because there's a lot of shock transmitted between vehicles whether the rope goes slack.

You don't need to go 70mph to regen a Tesla. They went fast in the article in order to get the wattage up to an impressive article-worthy number. 20mph would work but more slowly.

There shouldn't be much if any shock loading because the towed vehicle will be on the brakes the whole time.

Obviously some good judgement is advised. Roads with hairpin turns or the shoulder of a major urban interstate is not the place for this.

Obviously a tow bar would be better than a rope but nobody winds up in a situation where they need to regen tow a vehicle because of an abundance of alternative good options.

I guess I was a little too generous to HN when I assumed I didn't need to include a bunch of "be reasonable about it" disclaimers in my original comment.

It seems like the tire wear wouldn't be worth it. You're pulling the tires at almost 100 horsepower of energy transfer. Sure, neat for a video gag though.

Hah, don't try it in Europe either, max towing speed in Europe is 40km/h or 25mph.

I guess a tow truck with a bed that has a rolling road would be an idea, prop up the car so the weight of the car isn't actually exerting force that makes the wheels harder to turn/cause extra wear. Or the "rolling road" can be replaced be a contraption that attaches to the wheels by e.g. its rims, and spins it.

This would work but it would probably make more economic sense to carry a generator to the car than to carry a wheel spinner gizmo.

Plus in Europe it's forbidden to tow with a rope like he does. You must use a rigid link, like a bar or a tube.

It's only kinda sorta legal in the US.

But like speeding as long as you show decent discretion when doing it you'll be fine.

Not true, a rope is fine.

I don't think so. Most vehicles have virtually no braking power with the engine off. It's definitely completely forbidden in France to tow with a rope, and I'm pretty sure this applies to all or at least most of UE too. In Germany, even tow dollies are forbidden.

Towing rope is legal in Germany, but you are only allowed to tow a car to get it out of the way and to the nearest repair opportunity after it has broken down, not for general transport - for that, it needs to be completely off the ground.

> Most vehicles have virtually no braking power with the engine off

You just need to press the brakes harder. Like 3 times harder, not 30.

If you park in a slope and turn the engine off you can try braking after pumping the vacuum out - it works just fine.

If you’ve always used power assisted brakes and you’re not expecting to have to press really hard on the brakes it could easily lead you to rear end the car towing you if it stops suddenly. Also, without power assisted steering, modern cars are surprisingly difficult to control.

Ye especially the steering. Small steering wheel and lower gear ratio on modern cars makes them kinda unsafe in the event of power steering loss.

Until the tow truck hits the brakes…

I was once involved in towing a car using a partially-inflated water bed as a 'bumper'. We got there!

Old tire not on a rim is the classic item to use for that use case.

First day of statics - you can't push a rope.

Not necessarily forbidden, but there's a good reason for not towing with a rope. Especially at 70MPH (which is 112 kph)

Though towing a Tesla with regerative braking is the best case. You definitely wouldn't want to tow anything that can't brake or steer with a rope.

@Peloton, here is your next big idea

Hah, they should integrate the bike into the car and make the modern Flintstones car.

It’s likely fine, 65 kW is only ~85 HP. Simply maintaining highway speeds is ~25HP, so as far as the tire is concerned it’s the equivalent of mild acceleration or breaking.

The regenerative breaking system is likely designed for long mountain roads, so it might overheat but probably not.

Teslas will switch to friction brakes when regeneration limits (peak current or battery SOC) are exceeded. I’ve sustained constant 50kw regen while downhill through Appalachia passes, no issues, with the caveat that regen current is limited if the pack is cold until it approaches operating temperature (yellow dashed lines on the regen current display indicate your reduced regen capability).

Does this mean that when the battery is full you must use the brake pedal? Or does the software automatically apply the friction brakes when you back off the accelerator?

Regen braking is noticeably less powerful when your battery is nearly full, and you have to manually use your friction brakes more. Sounds like it might be a safety issue, but in any scenario where you use regen brakes you have more than enough time to switch to physical brakes if you want to.

that's correct. You don't get one pedal braking if it's not charging.

I don't think the brake pedal was being used in this test, though.

My understanding is that above 95% SOC, even if you’re not using the brake, regen will not occur and an error is presented to the user notifying them about regen limits.

Sure, so the car stops charging and still won't waste/damage its brakes in the tow.

Teslas have no resistor banks?

A great omission.

Why would a resistor bank be better than mechanical brakes? Both get rid of excess power as heat, and you probably need the mechanical brakes anyway.

> Why would a resistor bank be better than mechanical brakes?

Resistors don't wear out like frictional brakes do.

What is implied by the GP is that the cost of ownership for regenerative brakes is lower per joule.

That's obviously true when that joule is "reused" and not so obvious when it's dissipated. Mechanical brake pads have to be replaced but I assume there is wear on the (more expensive?) parts in a regen system too.

Electric braking has close to no wear, and much cheaper EVs use it.

A resistor bank with liquid cooling will be few kilogrammes at most.

A resistor bank of the required wattage would simply be a big lump of dead weight most of the time. And it would need its own cooling fan which would add even more weight.

There's a free 1000+ lb steel heatsink - the chassis of the car.

It wouldn't be, all teslas already have liquid cooling.

Few kilogrammes at most.

It isn't worth the complexity for the rare cases when it might be useful. Even if you live at the top of a mountain, the easier solution is to just not charge to 100% to account for your "free" energy gains when leaving.

65 kW is a little over 87 horsepower (parent previously claimed ~80 HP, now edited to 85).

To get 65 kW into the battery, you have mechanical and electrical conversion losses on top of that. The tires are seeing pretty close to 100HP.

That really depends on what the 65 kW being displayed represents. If it’s the AC power from the regenerative breaks that should be extremely efficient, post AC/DC conversion things look worse etc.

The article claim is pretty clear:

> putting back electricity into the batter[y] at a rate of 65 kW

The article could be wrong, of course.

It is worth it if you run out of battery away from a charger.

I anticipate a future where mobile road repair will carry their own massive batteries by default to restart dead EVs on the side of the road.

It'd be nice if those vehicles also were hybrids that could use the gas power plant to regen the aforementioned batteries.

A Tesla will regularly put 100hp or more into just accelerating to highway speeds on an onramp. It's not exactly a tire-shredder.

For 10 seconds at a time, of course. Our old Honda does the same. Both of those actions do in fact wear the tires, but fortunately for drivers, neither car merges onto a highway for hours at a time continuously (such as charging a Tesla might).

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