Coasting to a stop

[DaveloMA]

Hi. When you talk about coasting to a stop before applying the brakes to regen, do you just mean "foot off the gas" or "put the transmission in neutral"?

 

Thanks!

[scottwood2]

Not sure what post you are referring to but in general the transmission is left in "D" and not move to neutral.  I coast in mine when I can.  You always loose some energy when moving energy around so a slower coast to a traffic light is more efficient than driving up faster and using the regen to capture the energy.  It might say 100% recaptured but there are some losses. 

 

I hope this answers your question. 

[plus 3 golfer]

Agree with scootwood2.  I think most people when referring to "coasting" means leaving the transmission in "D".

 

Coasting in "D" -- foot off the gas pedal, the traction motor (MG1) and the generator (MG2) are spinning, ICE does not spin if energy can be stored in the HVB.  If there is room in the HBV to store energy, the traction motor acts as a generator to simulate engine braking like in a conventional vehicle.  This is know as regeneration where some of the kinetic energy of the moving vehicle is used to generate electric power for use / storage in the HVB.  If the HVB is full, ICE spins and provides  "normal" engine braking.

 

Coasting in "N" -- foot off the gas pedal, the traction motor (MG1) and the generator (MG2) are spinning, ICE does not spin.  There is no regeneration taking place.  There is no engine braking.  

 

So, the $64 question is:  is it more efficient to coast in D or N when coming to a stop?  Since there are electrical related losses associated with regeneration, IMO it's more efficient to coast in N.  But, given that coasting in N in normal driving in traffic may "excite" drivers behind you because one likely has to start coasting in N well before a stop, and thus it is not very considerate / practical. 

 

Then, the question becomes: is it more efficient to coast in D when coming to a stop for as long as one can or should one coast in D to maximize regeneration using the brakes?  This is a little tougher question to answer. But I believe (and I've modeled it), that coasting with minimum regeneration (brake pedal not depressed) is more efficient than using the brakes even if one scores 100% brake score.  But, I found the difference between the two of marginal value though for normal driving in my modeling.  In other words, unless one is trying to set a hypermiling record, the value of not using the brakes in terms of increasing FE when one is getting 40 mpg is small (maybe a few % points increase in FE) if one has many, many frequent stops.

[drdiesel1]

If you shift out of D into N, it turns off the regeneration capability.  Don't do that.

[DaveloMA]

Great! Thanks all.

[BIG ROCCO]

I agree with most of what Plus 2 had to say - especially:

 

Since there are electrical related losses associated with regeneration, IMO it's more efficient to coast in N.

 

I have been coasting down long, moderately steep hills in Neutral (for example, Davelo, back in the day when I lived in the Boston area, down Rte 2 Belmont Hill toward Fresh Pond) since the 70's, and I still do it with the C-Max and my Buick Lacrosse (mild hybrid).  If I can coast, instead of feathering the throttle to maintain speed. that has to be a gas saver.  If the hill is too steep and I will pick up too much speed, then of course I use D or even hill descent mode   I do not coast in Neutral up to stop signs or red lights.

[SPL Tech]

In many states it is illegal to be in N during the operation of a vehicle. It can only be used for towing.

[ptjones]

I used coasting to improve MPG's in 07 Focus 5-speed manual trans. with good results, but don't see a good reason to do it in CMAX.  I will say that filling up the HVB going down hill is like driving in Neutral, scary, Hill Assist sure comes in handy. :) 

 

Paul

[scottwood2]

I have done the coasting thing with a manual trans vehicle for years.  I do not do this with the C-Max.  What I do find that helps somewhat is coasting but push on the gas pedal just enough to start discharging the battery.  That little bit of energy can push the C-Max a long ways.  I use this in the morning when no one is on the road (5am) and I am timing traffic lights.  I would rather be driving slow waiting for the light than sitting at the light waiting. 

[fbov]

Actually, you can't coast in a C-Max without pressing the gas...

 

To my mind, "coasting" occurs when neither the drive train, nor the brakes, are connected to the road wheels. No energy in, purely parasitic losses out.

 

When a C-Max is moving and your foot is off the pedals, a C-Max is regen braking, not coasting. That little charge-up arrow is the tell-tale. You're only coasting to my definition when neither charge nor discharge arrow are shown, achieved by a very, very gentle press on the gas, and dang hard to maintain!

 

And there are times I'd like to actually coast...

 

Have fun,

Frank

Edited January 22, 2015 by fbov

[ptjones]

Actually, you can't coast in a C-Max without pressing the gas...

 

To my mind, "coasting" occurs when neither the drive train, nor the brakes, are connected to the road wheels. No energy in, purely parasitic losses out.

 

When a C-Max is moving and your foot is off the pedals, a C-Max is regen braking, not coasting. That little charge-up arrow is the tell-tale. You're only coasting to my definition when neither charge nor discharge arrow are shown, achieved by a very, very gentle press on the gas, and dang hard to maintain!

 

And there are times I'd like to actually coast...

 

Have fun,

Frank

Frank would you consider coasting when the HVB is full and your feet off the pedals?  :)  I have seen no arrows with about 3/4 full HVB on the FWY with some ICE power for quite a distance on fairly level ground. :shift:

 

Paul

[fotomoto]

  What I do find that helps somewhat is coasting but push on the gas pedal just enough to start discharging the battery.  

 

In hypermiling terminology, this is what is called a glide; as in Pulse and Glide.  Coasting is not a glide.  Hybrids allow the driver to legally coast in neutral without actually shifting into neutral gear by this gliding process and the gains are huge in fuel economy.  Prius gauges are better at showing this.

[plus 3 golfer]

If I understand it correctly, coasting in D with no throttle will provide regeneration when HVB is not "full" (ICE off) or normal engine braking when HVB is "full" (ICE is spinning).  Coasting in D with slight, correct throttle will provide just enough regeneration (ICE off)  to offset normal car electrical loads (like DC/DC converter) such that the HVB is being neither charged nor discharged (no battery arrows).  So, this would be very similar to an ICE vehicle when shifted into neutral where ICE would remain running at virtually idle rpm and turning the alternator which would supply normal electrical load.  In both cases, energy is being used to supply normal car loads (KE for the hybrid and ICE for a conventional vehicle).  In both cases, ICE engine braking is not contributing to slowing down the vehicle when coasting.

 

So, for the hybrid one can, in essence, coast in N legally - there is no ICE real or simulated engine braking via regeneration.  This should improve overall FE because one is reducing the electrical losses associated with regeneration.

 

Does this sound correct?  I'd love to run tests and record data to estimate the benefits of keeping the SOC "constant" by feathering the throttle while coasting in D to simulate coasting in N vs foot off the throttle with engine braking regeneration and also compare it to coasting in N.  But it just takes so much time to do properly and analyze the results.

[fotomoto]

Yup, you've got a good grasp of how it works.  P&G masters roughly strive for a 1/3rd pulse to a 2/3rd's glide distance ratio.  In my experience, I also found adding a touch more throttle to blend in a slight amount of EV really helped glides without aggressively draining the battery.  It's a fine line.  

 

The prius has an economy mode/button that deadens the throttle input making modulation much easier to find and hold the glide position.  One would park the throttle on their bar graph display right at 50% for a glide. ( <50% was regen, >50% was power)

[plus 3 golfer]

But doesn't it to an extent boil down to trading time for FE.  At what speed does one have to pulse up to take full advantage of 2/3 glide such that ones overall time covering a distance is the same as simply setting eco-cruise to say 55, 65 or 75 mph.  Has any Prius owner conducted a test keeping average speed the same (distance / time) using P&G and then eco-cruise to quantify the "real" benefits of P&G?  Aero drag will eat into benefits if one has to pulse to very high speeds.

[fotomoto]

I actually pulse above the speed limit by 5mph or so and glide back down to about 5 below and not focus on distances so fairly time efficient plus I'm not a rolling roadblock. :shift:   LOL   

 

I don't try to P&G at highway speeds because I, too, think aero drag would be too great.  Best to use aero mods.

[ptjones]

But doesn't it to an extent boil down to trading time for FE.  At what speed does one have to pulse up to take full advantage of 2/3 glide such that ones overall time covering a distance is the same as simply setting eco-cruise to say 55, 65 or 75 mph.  Has any Prius owner conducted a test keeping average speed the same (distance / time) using P&G and then eco-cruise to quantify the "real" benefits of P&G?  Aero drag will eat into benefits if one has to pulse to very high speeds.

Especially in City Driving being very aware what is going on in front of you(driving style) can save you time by catching green lights instead of stopping for red ones. Drafting on the FWY doesn't necessarily mean you are losing time, but you are gaining MPG's. It comes down to how much effort (Driving Style) you want to put into it, most people don't want to bother with it. IMO  For me it's FUN. :)

Paul 

 

P

[C-MaxSea]

I actually pulse above the speed limit by 5mph or so and glide back down to about 5 below and not focus on distances so fairly time efficient plus I'm not a rolling roadblock. :shift:   LOL   

 

I don't try to P&G at highway speeds because I, too, think aero drag would be too great.  Best to use aero mods.

 

Double Yup on that - not from experience, but from knowing the stars on this forum do exactly that - P & G city, aero mod for highway speeds.  :hat_tip:

 

For us it is ECO Cruise for both; otherwise it is maximizing the PE/KE opportunities on all of our hills. Too lazy for P & G in city on 'flat' arterials (very few), just ECO at or above the speed limit. :skateboard:  (47+ MPG all around)

 

BTW, today is our 2 year mark with our C-Max, which means we can now start 'buying in' to the aero mod game for highway speeds.    Hello Paul !  :flyaway:  (Time to learn better throttle control (Glide) as well - an exciting new year)

 

Nick

 

(2 years, $0 maint. cost other than gas, 3 oil changes (300, 5,000, 10,000), low miles, 47+ mpg wherever we go, brilliant car) 

Edited January 23, 2015 by C-MaxSea

[scottwood2]

In hypermiling terminology, this is what is called a glide; as in Pulse and Glide.  Coasting is not a glide.  Hybrids allow the driver to legally coast in neutral without actually shifting into neutral gear by this gliding process and the gains are huge in fuel economy.  Prius gauges are better at showing this.

I did not know that.  I thought gliding was using EV to try to keep the car going without using ICE.  I have not tried this on the highway but I think the speed would drop much faster than SOC with just pushing on the pedal enough to just start discharging the battery?  Something new to play with.  Learned something new again today.  Thx

[fbov]

Yes, Paul, that's coasting.

 

Scott, I'm going to disagree with fotomot... glide is coasting by definition.

 

Keep in mind that hypermiling greatly predates when hybrids hit the market, but some techniques are so much easier with a hybrid drivetrain they're become associated with hybrids. Pulse and glide is one of them.

 

Have fun,

Frank

[ptjones]

Yes, Paul, that's coasting.

 

Scott, I'm going to disagree with fotomot... glide is coasting by definition.

 

Keep in mind that hypermiling greatly predates when hybrids hit the market, but some techniques are so much easier with a hybrid drivetrain they're become associated with hybrids. Pulse and glide is one of them.

 

Have fun,

Frank

I'm alittle confused by the terminology of coasting, so gliding with regen is the same as gliding without regen? :) 

 

Paul

[ScubaDadMiami]

Under my definitions, pulse is running ICE up to or above your intended speed, and typically something in the neighborhood of a two bar burn; gliding is running with EV (including while slowly losing speed); and coasting is keeping the car in D while not applying any pressure to the pedal. (BTW, do not shift into Neutral when you are not using the gas pedal. In that instance, you will not provide any charge to the HVB while the wheels are turning.) I'll add another category to running ICE: recharge. That is when I do a slower burn to charge up the HVB, after which I can then glide for a bit, and, with a still decently full HVB, I can then pulse back up to speed. So, I distinguish categories of: pulse, glide, and recharge burns.

 

For stopping, your choices are to roll to a stop without touching the brake pedal, to stop by applying just enough pressure to capture regen, or to brake hard enough to use the actual brakes. I think that it is better to roll to a stop than it is to approach with a fast enough speed to require applying the brakes, even if that is lightly enough to get the benefit of regen. Of course, such a perfect execution is almost impossible to achieve, given the unpredictable nature of the traffic flow. Typically, I stop by coasting (see above definition) until moving slowly enough to finish up the last stage of coming to a complete stop by applying just enough force to the brake pedal to trigger regen until a stop. If I have timed it perfectly, I stop just at the end of the regen period, without either requiring letting up on the pedal (because I have applied the pedal too soon), or without having to apply extra pressure to the pedal (because I failed to get the maximum regen distance). Coincidentally, this is how you get your 100 percent braking score.

Edited January 24, 2015 by ScubaDadMiami

[plus 3 golfer]

Especially in City Driving being very aware what is going on in front of you(driving style) can save you time by catching green lights instead of stopping for red ones. Drafting on the FWY doesn't necessarily mean you are losing time, but you are gaining MPG's. It comes down to how much effort (Driving Style) you want to put into it, most people don't want to bother with it. IMO  For me it's FUN. :)

Paul 

 

P

I always try to time lights and have been doing that since the 1970s.   Time is virtually the same whether one has to stop for a light or coasts to a light.  

 

My point on time is using P&G on highways not in drafting on the highway.  If one accelerates from minimum speed to maximum speed such that speed / time is linear for 1/3 distance, then coasts for 2/3 distance, the coasting is not a linear function with respect to speed vs time.  Thus, the average speed over the distance is not the simple average of the maximum and minimum speed but something less.  So for a true test of the benefits of P&G.  One needs to compute their average speed over a route using P&G by distance / time  and then set eco-cruise at that average speed over the same distance.  Then IMO, one has has a good comparison of the difference in FE using P&G vs eco-cruise.

 

Also, I note in the Prius P&G link provided by Frank that the average speed is very low at 36.5 mph.  So, the effects of aero drag are not very great.  I'd like them to test P&G at say an average speed of 65 mph to see the results.  In addition, P&G likely result in a lower average speed and thus longer time than what one could do without using P&G due to driving constraints.  So, if one wants better FE, one likely spends more time on the road whether it's simply reducing overall speed or using P&G.

Edited January 24, 2015 by Plus 3 Golfer

[SPL Tech]

Especially in City Driving being very aware what is going on in front of you(driving style) can save you time by catching green lights instead of stopping for red ones. Drafting on the FWY doesn't necessarily mean you are losing time, but you are gaining MPG's. It comes down to how much effort (Driving Style) you want to put into it, most people don't want to bother with it. IMO  For me it's FUN. :)

Paul 

 

P

DO not draft on the freeway.... For one, Mythbusters proved it's completely ineffective unless you are right on them, less than 6' back. And for two it's dangerous, very dangerous. Even if it increased your fuel economy by 20%, the very mundane amount of fuel you saved would NEVER catch up to the amount of money you would need to spend on repair bills after causing an accident. You would need to draft for 1,000,000+ miles just to break even, and then if you totaled the vehicle, hurt someone or got sued, foreget it, you could draft all day everyday for life and never even come close to breaking even.

 

Do not suggest to someone they do something unsafe and illegal for marginal fuel gains. Most drivers cant drive for crap as it is, we dont need to make that worse.

Edited January 24, 2015 by SPL Tech

[plus 3 golfer]

DO not draft on the freeway.... For one, Mythbusters proved it's completely ineffective unless you are right on them, less than 6' back. And for two it's dangerous, very dangerous. Even if it increased your fuel economy by 20%, the very mundane amount of fuel you saved would NEVER catch up to the amount of money you would need to spend on repair bills after causing an accident. You would need to draft for 1,000,000+ miles just to break even, and then if you totaled the vehicle, hurt someone or got sued, foreget it, you could draft all day everyday for life and never even come close to breaking even.

 

Do not suggest to someone they do something unsafe and illegal for marginal fuel gains. Most drivers cant drive for crap as it is, we dont need to make that worse.

A little bit off in the number.  Mythbusters obtained a 20% gain in FE at a distance of 50 feet and a 11% gain at 100 feet  at 55 mph behind a semi-truck.   But I agree with the safety concerns of drafting - at 100 feet back, it takes about 1 1/4 seconds to close the 100 feet at 55 mph (80 feet/second).

 

This has been discussed in other threads and IIRC most believe that the two second rule is a safe driving distance behind another vehicle. So, two seconds at 55 mph is 160 feet which would be very little if any FE improvement from drafting.  Now having said that, driving the freeways around Phoenix, it's virtually impossible to maintain a two second rule.  At 70 mph in rush hour, I'd say the traffic is generally less than 100 feet apart.   At 100 feet cars will be filling the 100 foot gap.