down hill coasting

[Marc Smith]

Hey folks.  question about hills.

 

Another post got me thinking.  FWIW  I use the engage display with instant MPG showing as well

 

When going downhill  we have several options at the top of the hill.

 

1. We can coast down the hill and let the car build up momentum/speed with out applying any brakes

2. We can coast down the hill using the brakes to maintain our speed

3. we can have the downhill button depressed and allow the compression of the ICE to help slow down and maintain speed.

 

on my drive today  I noticed that when using options 1 and 2  the instant mpg bar  stayed in the blue and the EV icon stayed lit.

 

So I hit the downhill button  and I noticed that on the more gentle slopes the car seemed to slow a bit  but the EV stayed lit and the mpg bar stayed blue.  But on a steeper slope  the mpg bar turned white and the EV disappeared, and the ICE came online.  and by coming on  I mean that it acted like it was a manual car with the transmission in gear while coasting down.    IE  that the resistance of the engine compression was slowing the car down.

 

So  my question is.

 

Which is best for fuel economy....  I'm thinking that #3 is probably the worst for FE

[hybridbear]

Ideally, drain the HVB as low as possible before the start of the descent so that you can recoup as much energy as possible. Once the HVB is full, the ICE will be forced on. If the ICE comes on via the downhill button before the HVB is full, this means that the gravitational force that is trying to accelerate your descent is greater than what can be resisted just through the traction motor working to do regen. Speeding up will avoid wasting energy via the brake pads or the ICE, but can be dangerous if you start going too fast. You can also end up wasting energy by increasing wind resistance. Wind resistance is calculated using your speed squared, so increases of speed have an exponential impact on wind resistance.

 

It should be noted that the ICE uses no gas when it comes on via the downhill button. It just works as an air pump to slow the car.

 

The possible uses of this gravitational energy are (listed from best to worst):

1. charge HVB via regen
2. increase speed of car
3. turn into heat (waste) via brake pads or turn into heat/pumping losses by spinning the ICE

We would usually try to drain the HVB as much as possible before beginning a descent & also reduce our speed so that more energy could go through options 1 & 2, and less through 3.

Edited December 15, 2015 by hybridbear

[Marc Smith]

yeah I know the engine is not "running"  just spinning

 

my commute has a brief stint flat at 55.  but everywhere esle 35-40 and some hills.

 

I usually just set the eco cruise at 40 and just roll.   let the car coast and generate as much momentum when going down hill such that I can make it up the next hill/incline with out the ICE coming on.  so i wont use the hill descent button.  Some time  if I'm in some traffic  I'll have to kick the ICE on sooner  to keep the speed up.

 

So,  the next question.  if you apply the brakes  lightly  are you just increasing the regen more than straight coasting?  

[SnowStorm]

So,  the next question.  if you apply the brakes  lightly  are you just increasing the regen more than straight coasting?  

From my observations with OBDII data coming through the Forscan software, straight coasting (regen active but no pressure on brake) will not go above 10kW into the battery.  Regen braking, however, can send power into the battery up to the maximum allowed of 35kW.  So the answer is yes.  I have no idea if using the "downhill assist" button on the side of the shifter changes the 10kW limit.

[BIG ROCCO]

Ideally, drain the HVB as low as possible before the start of the descent so that you can recoup as much energy as possible. Once the HVB is full, the ICE will be forced on. If the ICE comes on via the downhill button before the HVB is full, this means that the gravitational force that is trying to accelerate your descent is greater than what can be resisted just through the traction motor working to do regen. Speeding up will avoid wasting energy via the brake pads or the ICE, but can be dangerous if you start going too fast. You can also end up wasting energy by increasing wind resistance. Wind resistance is calculated using your speed squared, so increases of speed have an exponential impact on wind resistance.

 

It should be noted that the ICE uses no gas when it comes on via the downhill button. It just works as an air pump to slow the car.

 

The possible uses of this gravitational energy are (listed from best to worst):

1. charge HVB via regen
2. increase speed of car
3. turn into heat (waste) via brake pads or turn into heat/pumping losses by spinning the ICE

We would usually try to drain the HVB as much as possible before beginning a descent & also reduce our speed so that more energy could go through options 1 & 2, and less through 3.

For long gradual descents, I still prefer "N" - that seems the most direct and efficient way to recapture the potential energy at the top of a hill. When approaching a peak, I let my speed drop, then with a constant pedal position, the car will automatically speed up up, then shift to Neutral, and let gravity speed up the car.  So, let's say the speed limit is 65 MPH - if there is an upslope coming up, followed by a downslope, I let my speed drop to maybe 55 MPH going up the hill...or 2 bars...or 3,000RPM; then, at the top of the hill, I keep my foot on the throttle until ~60 MPH, then let off the throttle and shift to N and let the speed increase to 70 or maximum 75MPH.  I am not a hypermiler by any means, but our '13 lifetime average for 30K miles was 45.9 MPG, and a lot of that was highway and mountain driving.  The '15 does not have a lifetime average, and it's not even broken in yet, so that's why I mentioned the '13

[hybridbear]

From my observations with OBDII data coming through the Forscan software, straight coasting (regen active but no pressure on brake) will not go above 10kW into the battery.  Regen braking, however, can send power into the battery up to the maximum allowed of 35kW.  So the answer is yes.  I have no idea if using the "downhill assist" button on the side of the shifter changes the 10kW limit.

The downhill button on the shifter will get you up to the maximum 35 kW of regen. If the HVB cannot accept that much energy (because it is full or too hot or something else) then the ICE will turn on. If the motor cannot generate enough energy to maintain your speed (because the descent is steep enough that 35 kW is not enough or your speed is too low so the motor can't do 35 kW or something else) then the ICE will turn on. Not using the downhill button should keep the ICE off, but your speed will rise. Using only cruise control will keep the ICE off, but your speed will rise.

 

For long gradual descents, I still prefer "N" - that seems the most direct and efficient way to recapture the potential energy at the top of a hill. When approaching a peak, I let my speed drop, then with a constant pedal position, the car will automatically speed up up, then shift to Neutral, and let gravity speed up the car.  So, let's say the speed limit is 65 MPH - if there is an upslope coming up, followed by a downslope, I let my speed drop to maybe 55 MPH going up the hill...or 2 bars...or 3,000RPM; then, at the top of the hill, I keep my foot on the throttle until ~60 MPH, then let off the throttle and shift to N and let the speed increase to 70 or maximum 75MPH.  I am not a hypermiler by any means, but our '13 lifetime average for 30K miles was 45.9 MPG, and a lot of that was highway and mountain driving.  The '15 does not have a lifetime average, and it's not even broken in yet, so that's why I mentioned the '13

I have done this too, but some people will express concern about shifting to N being illegal and/or dangerous, so I resisted mentioning it. You can achieve the same effect by gently pressing on the accelerator pedal to reduce the amount of regen drag, without reaching the point of consuming power from the HVB to accelerate the car.

[stevedebi]

The usual note applies for CA residents. It is illegal to coast in "N" here in CA.

[Adrian_L]

Driving in "N":  It's both illegal and dangerous in British Columbia, too.

[ScubaDadMiami]

N produces no charge to the HVB.

 

On rolling hills (not steep enough to increase or maintain speed without some assistance when going downhill) at highway speed, I use ICE on the way up, and then I use HVB on the way down.

 

On steep downhills, which will usually not be at highway speed, I use Downhill Assist. I never noticed that I would see white come up in the Empower threshold icon. It's been a while, but I thought that it was blue. Once the HVB will maxed out, ICE would "dry fire," meaning the cylinders were pumping without using fuel.

[BIG ROCCO]

OK, folks, I expected your reaction, but how would a police officer know if you are in N - does anyone know someone who was cited for that?

 

I suppose the suggestion to press on the accelerator ever so slightly to eliminate regen without engaging EV propulsion is valid, but maintaining that is hard work - it's so much easier to put in N.

 

Scuba Dad Miami - when you write :

 

On rolling hills (not steep enough to increase or maintain speed without some assistance when going downhill) at highway speed, I use ICE on the way up, and then I use HVB on the way down

 

you describe the exact scenario I like to us N, except using HVB uses more energy than N, right?  If I start at the top of the hill doing 50MPH, put the gear selector in N, and max out at 58MPH, before finally slowing back to 50 MPH a the road levels out, is that not more efficient than feathering the "gas" pedal to maintain speed?   As far as not producing a charge, I think you would only capture a portion of the kinetic energy and potential energy of a moving vehicle, once converted into electrons and then later back to kinetic energy, no?

[ScubaDadMiami]

OK, folks, I expected your reaction, but how would a police officer know if you are in N - does anyone know someone who was cited for that?

 

I suppose the suggestion to press on the accelerator ever so slightly to eliminate regen without engaging EV propulsion is valid, but maintaining that is hard work - it's so much easier to put in N.

 

Scuba Dad Miami - when you write :

 

On rolling hills (not steep enough to increase or maintain speed without some assistance when going downhill) at highway speed, I use ICE on the way up, and then I use HVB on the way down

 

you describe the exact scenario I like to us N, except using HVB uses more energy than N, right?  If I start at the top of the hill doing 50MPH, put the gear selector in N, and max out at 58MPH, before finally slowing back to 50 MPH a the road levels out, is that not more efficient than feathering the "gas" pedal to maintain speed?   As far as not producing a charge, I think you would only capture a portion of the kinetic energy and potential energy of a moving vehicle, once converted into electrons and then later back to kinetic energy, no?

If the hill is steep enough that you can maintain or increase speed without using the accelerator, then you are better off gliding and recharging while staying in gear than you are to glide in Neutral, where you will not charge the HVB. I guess that it is possible to be on a hill that is just the right steepness that leaving the CMAX in gear would cause you to slowly lose speed, while going in N would allow you go maintain speed. I guess that this could be an exception. Not sure how good that is for the vehicle and transmission, but I am not sure about that.

[plus 3 golfer]

IMO, the answer to "what is best for FE" (original question) is to use the potential energy of the hill in the order below without exceeding a safe speed. I think most posters have generally hit on the points on the list in the same order.  I will move up and down the list when going down steeper, longer hills especially on two lane mountain roads. I look ahead and anticipate my next move on the list with the goal of maintaining as much safe speed as I can at the bottom of the hill. I generally don't use "N". I find that on interstates, the speed limits (speeds I drive at) and grades of the hills are generally such that I just leave eco-cruise and grade assist on continuously.

 

1) coast in neutral (if one has no issues with it) and engage grade assist (can wait to step 3 but I usually leave it on in mountainous areas)

 

2) coast in gear - regenerate the minimum amount energy that one can.  This uses the least amount of the available potential energy of the hill.  There are mechanical to electrical to mechanical energy losses that one generally wants to minimize.

 

3) engage grade assist (if not already on) as it will vary the amount of regeneration to maintain speed beyond just coasting in gear.  Engine braking will be automatic if speed increases too much.

 

4) shift to "L" to increase engine braking or use friction brakes or both together.  One needs to slow down.  

 

5) use friction brakes.

[Marc Smith]

thanks all for the discussion...

 

I have done the "N" before I have a stretch that if I hit it at 45  (speed limit changed from 45 to 30) I can coast in N all the way to the intersection with out ever falling below the speed limit.   its a fairly constant slope for 200' that a bit over  a mile long with a stoplight intersection.

 

in N for the entire trip the HVB will never be full at the end of the run.

 

but if I leave it in D and feather the gas and maintain the speed and use the brakes on the final slope to the stoplight  I can get the HVB full which helps for the final stretch to work which is 3 miles of flatland

[stevedebi]

OK, folks, I expected your reaction, but how would a police officer know if you are in N - does anyone know someone who was cited for that?

 

....

Some folks like to follow the laws regardless of if they are caught or not. One might as well ask if we should go ahead and commit any crime if the probability of being cited is low.

 

But in fact, I suspect insurance would not pay off (for example), if it were determined you were in an illegal mode when an accident happened. Likely? No, but there you are.

 

For me, FE is not as important as other things.

[Adrian_L]

Took the words right out of my keyboard, Steve.  There's a little mom-and-pop convenience store two blocks from my house.  Every time I go in there I could EASILY pocket a candy bar and get away with it.  Does that make it OK to steal candy bars?  Seriously, Rocco..........

 

Personally I couldn't care less if you drive down hills in neutral while texting your wife and eating a cheeseburger.  Couldn't care less.  But since we're on a forum and you're proposing dangerous advice to members, I'll weigh in with my two cents. 

[stevedebi]

..

 

Personally I couldn't care less if you drive down hills in neutral while texting your wife and eating a cheeseburger.  Couldn't care less.  But since we're on a forum and you're proposing dangerous advice to members, I'll weigh in with my two cents. 

I'm not OK with that unless it is a bacon cheeseburger...

[ptjones]

Just a reminder that the HVB SOC keeps going down when you are in N so at some point the ICE will come on which I don't think is efficient. I think the ICE needs to be moving the car and charging the HVB at the same time. IMO   I don't feel right putting car in N and coasting.  I have gotten to 68.3 MPG without doing it. ;) 

 

Paul 

[fbov]

I've deleted posts before posting a couple times already...

 

This car is designed to Pulse and Glide. You just need a sensitive right foot.

 

There is a mode where it actually coasts, neither charging nor discharging. You can't get to it, and if you do, you can't stay there.

 

Instead, I use a zero-bar burn, a discharge carat with no blue bar showing. I can hold it past the point where the blue outline collapses. Very useful for stretching speed approaching a red light, gathering speed going down hills (speed you'll lose EV'ing up the other side of the valley, so why regen?). In fact, I see it as an alternative to regen in cases where the added aero losses might be less than the 10% regen penalty, and the road/traffic allows.

 

HAve fun,

Frank

[stevedebi]

I've deleted posts before posting a couple times already...

 

This car is designed to Pulse and Glide. You just need a sensitive right foot.

 

There is a mode where it actually coasts, neither charging nor discharging. You can't get to it, and if you do, you can't stay there.

 

Instead, I use a zero-bar burn, a discharge carat with no blue bar showing. I can hold it past the point where the blue outline collapses. Very useful for stretching speed approaching a red light, gathering speed going down hills (speed you'll lose EV'ing up the other side of the valley, so why regen?). In fact, I see it as an alternative to regen in cases where the added aero losses might be less than the 10% regen penalty, and the road/traffic allows.

 

HAve fun,

Frank

The folks who really hypermile will shift into neutral while gliding - it provides the longest glide distance. I never saw the point myself.

[plus 3 golfer]

...In fact, I see it as an alternative to regen in cases where the added aero losses might be less than the 10% regen penalty, and the road/traffic allows.

 

HAve fun,

Frank

 

The folks who really hypermile will shift into neutral while gliding - it provides the longest glide distance. I never saw the point myself.

I think the point is the regenerative losses (penalty) exceed the "added" aero losses. So, shifting to neutral makes sense when using pulse and glide. Use the kinetic energy at the wheels against aero drag and glide farther and not scrub off energy for inefficient regeneration.  

 

I believe the 10% regen penalty comes from Ford in literature and a video. In Ford's videos on hybrids (discussed over a year ago) they say the car captures 90% of the available braking energy AND then sends it back to the battery to be stored.  So, apparently the 10% is really the losses in the traction motor and perhaps some additional transmission rotational losses. There are additional losses in the conversion of the AC output of the traction motor to DC, storage losses in the HVB, conversion of the DC back to AC and running the traction motor to drive the car.  

 

Several years ago I found a curve on the efficiency of traction motors. The most efficient in use were around 92% peak at the time.  The AC to DC converter is likely no more than 95-97% efficient.  The HVB likely has around a 1% charge / discharge loss.  So, the efficiency of the regeneration overall process (mechanical energy at the wheels to electrical energy in the HVB and back to mechanical energy at the wheels might be = 0.92*0.95*0.99*0.95*0.92 = around 75% . I also in a post over a years ago linked to a study on regenerative braking (older study) and IIRC the overall efficiency was between 50% and 60% based on several regenerative braking strategies.

Edited December 20, 2015 by Plus 3 Golfer

[ptjones]

I think I remember when I first got the CMAX that regenerative breaking captured up to 80% of the energy, so that sounds about right.  I still feel awkward putting the car in Neutral and the added attention required to monitor that function.  So maybe I could gain another 50miles per tank, It's just to much added work. we will see how it goes. :) 

 

Paul 

[plus 3 golfer]

I believe the Jetta Hybrid e-golf allows the driver to select the amount of regenerative braking to apply when coasting in gear ( 3 positions IIRC) Maybe Ford will implement this on its next generation. It would be nice to have one position to be no regenerative braking or at least very little regenerative braking (maybe just offset the HVB drain and no charging of the HVB.).

Early on I used neutral going down hills on the interstates until one time I forgot I was in neutral and tried to accelerate going back up the hill with traffic on my tail. I quit using it.

 

Edit: it's the e-golf not the Jetta Hybrid:

One innovative driving feature available in the E-Golf is driver-controlled regen levels. Slap the gear shifter to the left—once, twice or three times—to progressively increase the amount of regenerative braking applied, ranging from a no-regeneration (or “coast”) mode to heavy engine braking in B.

 

Edited December 20, 2015 by Plus 3 Golfer

[ptjones]

Well I've been trying it out on my 7-7.2mi. trips back and forth to work and home.  I have seen an improvement in MPG's, this morning I started out  side temp 47*F, garage temp 54*F, WT 110*F and SOC 34% and I ended up with 47.9mpg, usually I get 40 to 45mpg. :) 

 

Paul 

[plus 3 golfer]

Well I've been trying it out on my 7-7.2mi. trips back and forth to work and home.  I have seen an improvement in MPG's, this morning I started out  side temp 47*F, garage temp 54*F, WT 110*F and SOC 34% and I ended up with 47.9mpg, usually I get 40 to 45mpg. :)

 

Paul 

Are you trying P&G in neutral or something else (fbov suggestion)? :)

[stevedebi]

Well I've been trying it out on my 7-7.2mi. trips back and forth to work and home.  I have seen an improvement in MPG's, this morning I started out  side temp 47*F, garage temp 54*F, WT 110*F and SOC 34% and I ended up with 47.9mpg, usually I get 40 to 45mpg. :)

 

Paul 

Oh, from what I have read, if you are willing to do it, P&G will net exceptionally good MPG.