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Maybe driving style doesn't matter?


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30 replies to this topic

#21 OFFLINE   markd

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Posted 11 January 2015 - 07:57 PM

If you drive 68 mph on the hiway in Indianapolis you'll get ran over. I attribute the poor mpg's to heat and idling.







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#22 OFFLINE   SnowStorm

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Posted 16 January 2015 - 11:16 PM

So back to "driving style" which, per original post, means fast starts, higher speeds and quick stops.  We know all the things that can make To/From mileage different - prevailing winds, altitude, engine warm-up, battery SOC and EV+.  Kelly expects he would get 66 on the return trip versus the wife's 56.  His speeds are about 5 mph slower so that accounts for about 5 mpg - at his wife's speeds he would get 61.  That leaves about 5 mpg (8%) to jackrabbit starts/stops based on our limited data.  (You can make your own decision as to whether that difference is "not much" or not!)

 

Regen braking is great but it's also over-hyped.  A couple of stops in 20 miles with a 50% score versus 100% just isn't going to make much difference.  Remember, kinetic energy is proportional to speed squared, so you can coast from 60 to 30 (using up 3/4ths of the energy), hit the brakes "hard" (50% score) and you still only "lost" 12.5% of the original energy.  It all depends on how many stops per mile and at what speed you hit the brakes.

 

With acceleration there may be even less to worry about.  Gasoline engines are not efficient at light loads - that's the main reason we have hybrids in the first place.  Very slow acceleration might not help at all (except that your average speed is less!).  The car has a ton of programming based on data none of us have.  Step on it and go!

 

All of which points to the fact that lead-foot driving simply does not negate the advantage of having a hybrid even if it hurts mpg a bit.  Your mileage might be "not great", but who cares at 55 mpg!  I love how Smiling Jack put it:

 

      "The cars care even if we don't."



#23 OFFLINE   Kelleytoons

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Posted 17 January 2015 - 04:47 AM

That's a nice analysis (although, as you point out, it's based on limited data.  Not sure I could or would want to conduct experiments involving my wife :>).

 

At the very least it makes me feel less worried about the times I need to sharply accelerate in order to perform "correct" maneuvers to get out of the way of folks or otherwise be safe, as well as the frustrating times I accelerate only to see the signal in front of me abruptly turn red even though that particular signal has never done it that way before (they seem to be shortening the yellow light time more and more here in Florida) so that my brake score is less than perfect (it's almost never less than 70% even then, as I'm pretty good at still not jamming on the brakes).



#24 OFFLINE   fotomoto

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Posted 17 January 2015 - 11:15 AM

 

Regen braking is great but it's also over-hyped.  

 

I realize this is the hybrid forum so I'll skip the details but we Energi owners have found a method to bank excess regen energy back into the battery and save it.  In the same scenario in hybrid mode, the cars' programming would seek to immediately burn off that excess to keep the battery SOC within set parameters.   On a short 45 mile hwy trip that I regularly take, I can bank almost 5 EV miles using this method.

 

At highway speeds, I've found (via scangauge) that it's best to use the brakes moderately hard for max regen (max brake scores) and not coast.  Too much kinetic energy is lost to wind resistance and tire friction while coasting that could be going into the battery.



#25 OFFLINE   shinytop

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Posted 17 January 2015 - 11:33 AM

Coasting further not only allows for more charge of the battery but often means longer periods with no gas being used, the best driving for low mileage you can have.  I have not used scangauge but the harder the stop the more difficult it becomes to stay out of friction braking which shortens the life of your brakes.  


Edited by shinytop, 17 January 2015 - 11:35 AM.


#26 OFFLINE   Plus 3 Golfer

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Posted 17 January 2015 - 11:42 PM

At highway speeds, I've found (via scangauge) that it's best to use the brakes moderately hard for max regen (max brake scores) and not coast.  Too much kinetic energy is lost to wind resistance and tire friction while coasting that could be going into the battery.

 

Coasting further not only allows for more charge of the battery but often means longer periods with no gas being used, the best driving for low mileage you can have.  I have not used scangauge but the harder the stop the more difficult it becomes to stay out of friction braking which shortens the life of your brakes.  

The question though is how efficient is the capture and reuse of the kinetic energy.  In my research and testing, I believe the following to be realistic:  the traction motor is likely between 90 - 93% efficient, the inverter efficiency is likely 90-95 % efficient, the battery is likely around 90% - 95% efficient.  So how much of the KE at the wheels can be captured and returned to the wheels.

 

wheel to wheel efficiency might be as low as = 0.90 * 0.90 * 0.90 * 0.90 * 0.90 = 60%

wheel to wheel efficiency might be as high as = 0.93 * 0.95 * 0.95 * 0.95 * 0.93 = 74%

 

So then, if losses in the regeneration process might be between 26 -40%,  is it better to use the losses even at higher speeds to offset road load by coasting or is it better to brake harder at higher speeds to store more of the KE to slow the car down quicker and reduce road load quicker?  I have modeled this before but am looking at it again.

 

Also, my guess is that when slowing down to a stop, many are seeing the PE of a slight drop in elevation also being captured.  Here's a quick chart showing the KE and PE of the C-Max. So, at 70 mph exiting an interstate down a 1/4 mile ramp with an elevation change of 10 meters, the C-Max has 0.25 kWh of KE and around 0.05 kWh of PE or 0.30 kWh of total energy.  So, what's the best course of action to capture the most energy in this 1/4 mile?  Or should one back off the throttle at 1/2 mile or more and coast in gear to scrub speed?  We know coasting adds time but it also saves fuel for 1/4 mile.

 

med_gallery_167_32_24967.jpg


Edited by Plus 3 Golfer, 18 January 2015 - 05:11 AM.


#27 OFFLINE   fotomoto

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Posted 18 January 2015 - 10:07 AM

  We know coasting adds time but it also saves fuel for 1/4 mile.

 

It's your last sentence that's the most significant for me (and I suspect most others).  I split the difference and use moderate braking from high highway speeds and coasting for lower speeds.   Coasting down from highway speeds with highly inflated tires isn't practical in most of my traffic situations as it takes too long (which is a GOOD thing from an efficiency standpoint).   The difference between the two at city speeds is usually only a tenth or two of a percent on the SGII; so a wash.  

 

So, I've found that banking the most regen I can in a short time frame (and thus not holding up traffic) is the best compromise in my Energi. NOTE:  It can bank more regen than the hybrid.



#28 OFFLINE   Plus 3 Golfer

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Posted 18 January 2015 - 11:05 AM

It's your last sentence that's the most significant for me (and I suspect most others).  I split the difference and use moderate braking from high highway speeds and coasting for lower speeds.   Coasting down from highway speeds with highly inflated tires isn't practical in most of my traffic situations as it takes too long (which is a GOOD thing from an efficiency standpoint).   The difference between the two at city speeds is usually only a tenth or two of a percent on the SGII; so a wash.  

 

So, I've found that banking the most regen I can in a short time frame (and thus not holding up traffic) is the best compromise in my Energi. NOTE:  It can bank more regen than the hybrid.

Yes, I've said similar before - traffic constraints and my time dictate what I do.  It simply takes too long to coast in gear from 70 mph to the exit ramp for it to be practicable for me given the traffic conditions I drive in.  So, I do brake fairly hard once I hit the exit ramp and then  apply moderate braking once speed is reduced trying to achieve a good score.  In suburban and city driving where speed limits are generally in the 35 - 45 mph range with lights every 1/2 to 1 mile, it's a lot easier to coast and time lights plus the amount of KE is so small (that would not be captured by coasting) that it matters little on braking technique. Increasing braking score amounts to very little additional energy captured. 

 

Bottom line: I'm not going to alter my braking techniques - that's why I quit trying to develop a regeneration model after Ford got the RLHP coefficients correct.  Also, we don't know the exact algorithm used in regenerative braking but can guess at the max regen kW  and the coasting regen kW to simulate ICE braking (difference between my coast down runs in gear vs in N).  Anyways it's fun to try to understand the physics behind regenerative braking.

 

Tying this back to the topic, regenerative braking style / technique IMO matters little to FE given traffic constraints most drive in.  Decrease ones overall driving speed and watch FE soar. :)


Edited by Plus 3 Golfer, 19 January 2015 - 09:56 AM.

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#29 OFFLINE   SnowStorm

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Posted 19 January 2015 - 11:15 PM

At highway speeds, I've found (via scangauge) that it's best to use the brakes moderately hard for max regen (max brake scores) and not coast.  Too much kinetic energy is lost to wind resistance and tire friction while coasting that could be going into the battery.

I can believe your scangauge will show more energy but I don't think that is the goal.  The goal is to use the least amount of total energy going from point A to B.  It is better to have kinetic energy go directly towards pushing the car rather than loose some through the regen process.

 

So, what's the best course of action to capture the most energy in this 1/4 mile?  Or should one back off the throttle at 1/2 mile or more and coast in gear to scrub speed?  We know coasting adds time but it also saves fuel for 1/4 mile.

 

Goal should be to waste the smallest amount of energy.  That means no friction brakes and the smallest amount of regen practical.  The 1/4 versus 1/2 mile comparison is nice - you burn no fuel for an extra 1/4 mile.  But then your average speed is slightly less so you have to cruise a tiny bit faster!  Probably offsets the whole thing.

Anyways it's fun to try to understand the physics behind regenerative braking.

 

Tying this back to the topic, regenerative braking style / technique IMO matters little to FE given traffic constraints most drive in.  Decrease ones overall driving speed and watch FE soar. :)

Agree - agree - agree.  Still think the simplest, most natural and "best" is:

  1. "Coast" down from high speeds for a bit.  It just seems "wrong" to hit the brakes at 70 mph (friction or regen).
  2. Brake the rest of the way so as to get a high score (80% to 90% or above).  Experience, available distance and grade dictate when to start braking.  I suppose the goal is to use max available regen without friction brakes.  I'm not obligated to do a panic stop just because a tailgater wants to save 2 seconds.
  3. If you have a choice on exits, choose those that are uphill.  Converting kinetic to potential energy is essentially 100% efficient.
  4. Otherwise, don't worry about it!

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#30 OFFLINE   fotomoto

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Posted 22 January 2015 - 08:51 PM

 The goal is to use the least amount of total energy going from point A to B.  

 

My goal is to get from point A to B in the most efficient manner in regards to both energy and time.  I can do my daily commute via two routes; one is more energy efficient and the other more time.   I choose the time route and try to manage it with the least amount of energy I can.  When I got my 894 mile tank, I chose the more efficient route but it cost me a whole lot of time (approx. 30 minutes per day).


Edited by fotomoto, 22 January 2015 - 08:52 PM.

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#31 OFFLINE   ptjones

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Posted 23 January 2015 - 10:07 AM

Back to the OP, you don't go 920mi on 13.5gal averaging 68mpg by accident, driving style is everything! :) 

 

Paul 








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