How would regenerative braking work for a long downhill?

[UG_Passat]

The long downhill run regeneration is something I have experience with in both the 2010 Prius and 2017 Prius V. Granted these vehicles have very small capacity in contrast to a true EV, but here is what I have experienced. There is a long and steep grade on VT route 9 approaching Bennington from the east that I've run many times. The battery always completely tops up, stops charging and then the cooling system kicks in. I imagine that something similar would happen descending a large mountain with a pure EV, provided that the battery had the capacity to accept the renerative load,

At least on my HiHy, on steeper downhills, the engine will kick in also, assist with engine braking.

How do I know the engine is on? My HiHy has a HUD, and one of the options for configuration is a tachometer, instead of the power meter, and you can hear the engine roaring also

 

[IndyIan]

What do you mean the cooling system kicks in and how do you know?

Maybe a resistor is used to take care of the excess energy?

I guess in the early EV's or hybrids with resistive heating you could slam on the heat to full and open the windows, for a small draw on the battery, but a 3-4klbs of car rolling down a steep grade would need a big big resistor to change to absorb excess regen. It would be far cheaper and easier to just put bigger brakes on it I think.

 

[Jetronic]

It would still need a differential, no?
The reduction gearing would be that gearset.

The same one that the electric motor uses, so with that reasoning all EV with 1 motor per axle have a gearset.

 

[fdcg27]

The same one that the electric motor uses, so with that reasoning all EV with 1 motor per axle have a gearset.

So a dif isn't geared and doesn't have a reduction ratio?

 

[Jetronic]

So a dif isn't geared and doesn't have a reduction ratio?

Not all, but the one I'm thinking of was a special case... the reduction was inside each wheel hub. Mercedes O405

 

[fdcg27]

Not all, but the one I'm thinking of was a special case... the reduction was inside each wheel hub. Mercedes O405

FWIU, the early VW Type 2 used a similar arrangement with a little gearbox at the wheel end of each driveshaft.

 

[Jetronic]

Well, in he outlander, the engine drives a generator, and that drives both electric motors. But with a clutch the engine can also drive the front wheels directly (connecting to the electric motor), but the speed (thus engine rpm) needs to be high enough.

Driving the wheels directly when possible can be more energy efficient than making electric and transforming that back to mechanical energy.

I would call it a transmission as it is used to tranmit energy from 1 component to another, but it hasn't got selectable gears so not a gearbox. the driveshafts have differentials and the electric motors have reduction gears (like a transaxle, all in one).

 

[fdcg27]

Well, in he outlander, the engine drives a generator, and that drives both electric motors. But with a clutch the engine can also drive the front wheels directly (connecting to the electric motor), but the speed (thus engine rpm) needs to be high enough.

Driving the wheels directly when possible can be more energy efficient than making electric and transforming that back to mechanical energy.

I would call it a transmission as it is used to tranmit energy from 1 component to another, but it hasn't got selectable gears so not a gearbox. the driveshafts have differentials and the electric motors have reduction gears (like a transaxle, all in one).

So we are on the same page.
It's pretty obvious that the gas engine can't drive the wheels at a ratio of 1:1 at any speed.

 

[Cruisin'n5thgear]

There's not a ton of excess energy when going downhill. With a fairly aerodynamic Mazda6 MT, only the steeper grades above 4 percent will allow it to coast at highway speeds or even gain speed. EVs are heavier, so that would result in more excess energy, but still wind resistance is a big factor sapping the excess energy.

Would be nice if predictive management could enable the battery to lower state of charge before starting a long downhill. Plug elevation data into the navigation system and tie that into the power management software.

 

[y_p_w]

There's not a ton of excess energy when going downhill. With a fairly aerodynamic Mazda6 MT, only the steeper grades above 4 percent will allow it to coast at highway speeds or even gain speed. EVs are heavier, so that would result in more excess energy, but still wind resistance is a big factor sapping the excess energy.

Would be nice if predictive management could enable the battery to lower state of charge before starting a long downhill. Plug elevation data into the navigation system and tie that into the power management software.

I'm not necessarily thinking of capturing that energy in and of itself. It's certainly nice, but my concern would be safety. With a traditional car with a transmission, it's possible to put it in a low gear for engine braking. If you're going continuously downhill at a place like Pikes Peak or Haleakala, that can be pretty hairy just relying on friction brakes for such a long downhill. At least with an EV, the general idea would be that regenerative braking can help where there is no engine braking.

 

[Astro_Guy]

What do you mean the cooling system kicks in and how do you know?

The battery or inverter cooling fan kicks in. It'spretty obvious when it happens.

 

[PandaBear]

I'm not necessarily thinking of capturing that energy in and of itself. It's certainly nice, but my concern would be safety. With a traditional car with a transmission, it's possible to put it in a low gear for engine braking. If you're going continuously downhill at a place like Pikes Peak or Haleakala, that can be pretty hairy just relying on friction brakes for such a long downhill. At least with an EV, the general idea would be that regenerative braking can help where there is no engine braking.

It could. When I was driving my Prius down grapevine (1500 ft 6% grade, 5 miles), after the battery is full it can no longer regen and must rely on either engine braking or brake pads. The brake would "shudder" after a few mins every single time. Personally I would rather slow the car down early enough so I have more time for this distance and height, and therefore more time for the brake to dissipate the heat, but it does seem to not work very well as I cannot slow it down enough to not get rear ended.

Opening windows and turn on heating or AC would probably be the best you can do, if your passengers are ok with it. The parachuting effects would help a lot.

 

[PandaBear]

Would be nice if predictive management could enable the battery to lower state of charge before starting a long downhill. Plug elevation data into the navigation system and tie that into the power management software.

The problem is, it wouldn't be able to predict which way you will go unless you are at the peak in all direction you are traveling to. If you leave your house everyday going downhill then obviously it would work, except the time you are not going your usual route.

What an EV charging station can do in the future, may be to also "drain" your car enough to let it regen braking downhill, and maybe give you some charging credit for later. Other drivers going the opposite direction can pick up your power, or sell your electricity back to the grid.

 

[Sequoiasoon]

Where are the REGEN pads in most of these vehicles? Are they inside the hat of rotor like a parking brake shoes? If they are at the wheels do they heat up the rotors also like having the regular pads being applied a long time on a downhill?

I have no experience on any of these and will probably need to up my knowledge game in the future. My only electric braking time is with trailer where the magnet drags against inside of drum apply brake shoes.

 

[Torrid]

Where are the REGEN pads in most of these vehicles? Are they inside the hat of rotor like a parking brake shoes? If they are at the wheels do they heat up the rotors also like having the regular pads being applied a long time on a downhill?

I have no experience on any of these and will probably need to up my knowledge game in the future. My only electric braking time is with trailer where the magnet drags against inside of drum apply brake shoes.

It uses the motor as a generator. If it’s not under load it creates drag slowing the car and produces power which feeds back into the battery. The brakes are fairly standard disc designs.

 

[Torrid]

The problem is, it wouldn't be able to predict which way you will go unless you are at the peak in all direction you are traveling to. If you leave your house everyday going downhill then obviously it would work, except the time you are not going your usual route.

What an EV charging station can do in the future, may be to also "drain" your car enough to let it regen braking downhill, and maybe give you some charging credit for later. Other drivers going the opposite direction can pick up your power, or sell your electricity back to the grid.

Maybe but even with extreme grade for long distances it’s not making a ton of power. On a road trip where you’d be using public charging you’d be using well more than you’d recover and very few are charging to full when stopping. It’s an interesting concept. Sure it could be a benefit if you left full and immediately were headed downhill, but I’d likely save my time and just use a bit of brakes when necessary and just enjoy that I was at 100% at the bottom of the hill to continue my journey.

 

[y_p_w]

Maybe but even with extreme grade for long distances it’s not making a ton of power. On a road trip where you’d be using public charging you’d be using well more than you’d recover and very few are charging to full when stopping. It’s an interesting concept. Sure it could be a benefit if you left full and immediately were headed downhill, but I’d likely save my time and just use a bit of brakes when necessary and just enjoy that I was at 100% at the bottom of the hill to continue my journey.

In my original examples (Pikes Peak or Haleakala) it's going up to the top and then going down. As far as I know, there is no charging at those peaks. I think Haleakala Highway has a 45 MPH speed limit. Not sure about Pikes Peak, but it doesn't look like it. When I researched it in the past, they had a strict requirement that any car must have a "first gear" available that can be manually selected - for engine braking. That could have been interesting with rental cars where sometimes the renter is stuck with whatever is available. My wife's car is a Honda Civic that only has D(4)/D3/2 selectable. It obviously has a first gear, but that can't be forced. Then there are CVTs with pseudo gears, but I'm thinking that would be acceptable.



But the important thing in my mind is whether or not it can be slowed down without smoking the friction brakes. Along with the times where I was driving a Tesla and all of a sudden the limited regenerative braking indicator popped up. It would suck to see that happening when it's needed.

It's not exactly the same, but I used to ride bikes a bit and bought a "trainer" to allow use in the house. There were two basic types - friction and magnetic to provide resistance. It's my understanding that the magnetic type was similar to regenerative braking, although it didn't necessarily produce electricity, but would heat up.

 

[PandaBear]

In my original examples (Pikes Peak or Haleakala) it's going up to the top and then going down. As far as I know, there is no charging at those peaks. I think Haleakala Highway has a 45 MPH speed limit. Not sure about Pikes Peak, but it doesn't look like it. When I researched it in the past, they had a strict requirement that any car must have a "first gear" available that can be manually selected - for engine braking. That could have been interesting with rental cars where sometimes the renter is stuck with whatever is available. My wife's car is a Honda Civic that only has D(4)/D3/2 selectable. It obviously has a first gear, but that can't be forced. Then there are CVTs with pseudo gears, but I'm thinking that would be acceptable.



But the important thing in my mind is whether or not it can be slowed down without smoking the friction brakes. Along with the times where I was driving a Tesla and all of a sudden the limited regenerative braking indicator popped up. It would suck to see that happening when it's needed.

It's not exactly the same, but I used to ride bikes a bit and bought a "trainer" to allow use in the house. There were two basic types - friction and magnetic to provide resistance. It's my understanding that the magnetic type was similar to regenerative braking, although it didn't necessarily produce electricity, but would heat up.

I don't think going on 1st is a good idea even for engine braking. As far as I know you probably can't drive on 1st for above 15 mph without reving it too high, and downshift to 1st for engine braking is really, really bad for the engine. I would say if you try to go that low you can risk over reving past redline if you are not careful. 2 really is the lowest you should go.

My understanding is, if you are not charging at the peak, you get up the hill by your battery power, and you will not overcharge your battery when you are going down if you are regen braking, and you are not going to overheat your cooling system more than you going up. The only exception is if your car is not designed right and it has smaller regen braking power circuit than the driving circuit. If you buy a reputable EV from a major company you should not have this problem.

My main concern would be whether the friction brake overheat in a short stretch of very steep slope, like grapevine on I5, with 6% grade and 1500 ft drop in 5 miles. If you see runaway truck lane like those on grapevine that when I would worry. I never had brake fade problem on my Prius coming back from Tahoe and the altitude is even higher, but over a much further distance.

 

[PandaBear]

Maybe but even with extreme grade for long distances it’s not making a ton of power. On a road trip where you’d be using public charging you’d be using well more than you’d recover and very few are charging to full when stopping. It’s an interesting concept. Sure it could be a benefit if you left full and immediately were headed downhill, but I’d likely save my time and just use a bit of brakes when necessary and just enjoy that I was at 100% at the bottom of the hill to continue my journey.

I think for a mountain top resort it can happen, so they probably should have charger stopping with enough capacity left for regen braking. What is more concerning to me is a midway stop, for undecisive drivers who doesn't know if he wants to go further uphill or downhill, and he just in case charge all the way to a full battery.

But still, if a hybrid has to deal with this (very small battery), an EV would have no problem dealing with it. This just means you still need a big enough friction brake like a gas car for the worst case scenario, but not so big you have to do emergency braking from 60-0 with all friction brake. EV that wants a small enough brake has to reserve some battery capacity for regen assist emergency braking, and apply friction brake for the constant braking need.

Off topic:
You know those highway induction charging? In theory you can put in a bunch of them on the downhill side to help drain the cars and then power the other direction for uphill boost. It would be costly but I think this might be a good idea for safety.

 

[y_p_w]

I don't think going on 1st is a good idea even for engine braking. As far as I know you probably can't drive on 1st for above 15 mph without reving it too high, and downshift to 1st for engine braking is really, really bad for the engine. I would say if you try to go that low you can risk over reving past redline if you are not careful. 2 really is the lowest you should go.

My understanding is, if you are not charging at the peak, you get up the hill by your battery power, and you will not overcharge your battery when you are going down if you are regen braking, and you are not going to overheat your cooling system more than you going up. The only exception is if your car is not designed right and it has smaller regen braking power circuit than the driving circuit. If you buy a reputable EV from a major company you should not have this problem.

My main concern would be whether the friction brake overheat in a short stretch of very steep slope, like grapevine on I5, with 6% grade and 1500 ft drop in 5 miles. If you see runaway truck lane like those on grapevine that when I would worry. I never had brake fade problem on my Prius coming back from Tahoe and the altitude is even higher, but over a much further distance.

I think the idea is that in some sections, it's really steep and there might be a low speed limit (like 10-25 MPH). I was recently driving on Pinehurst Road near Oakland, and there are some sections where it's steep with super tight turns. And the big thing would be a last ditch if the brakes fail. Damaging the engine might be preferable to losing control. I've personally gone down a hill in 1st gear. Especially Marin Ave. in Berkeley, which is really nasty. In my 1995 Integra GS-R, it didn't reach redline (8000 RPM) at my 20 MPH speed. My WRX doesn't reach redline in 1st at 25 MPH. It' might be around 5000 RPM and gets really loud, but I'm thinking it's not that bad if it's just compressing. At that point the fuel is going to be cut off anyways by the ECU.

I've also been present when someone clearly lost brakes (likely fluid boil) and I could hear the handbrake being applied. I talked to the driver, and she said once she lost it she pulled the handbrake and put it in 2nd, which was the lowest should could do. It slowed it down to about 30 MPH down the steep hill in front on my house where it finally stopped at this weird curb (hard to describe) and jumped over it. The occupants walked out but with the airbags deployed and with clear seatbelt bruises. I think 1st gear might have been preferable. On something like a long stretch, it might be enough time for the brake fluid to cool and the fade to go away.