OBD II Data for HVB

[larryh]

I tried a simple experiment this morning.  Starting from the same stopping point going about 55 mph before the stop sign, I tried two different methods to stop. 

 

- Lifting my foot off the accelerator with the car in Drive about 0.4 miles before the stop sign then using the brake as I approached the stop sign.   Regen occurred slowly over a long period of time. 

 

- Shifting the car into neutral until I got closer to the stop sign (to stop consuming/adding energy from/to the HVB), then shifting into Low to brake, and then shifting back to neutral, and eventually back to drive and applying the brake.  Regen occurred rapidly over a short period of time.

 

With the slow regen method, the motor/generator produced 0.0761 kWh of electricity.  With the fast regen method, the motor/generator produced 0.0896 kWh of electricity.   For fast regen method, the motor/generator produced 18% more electricity.  This takes into account the power used by the car's accessories.

 

With the slow regen method, the car claims that 0.062 kWh of energy was added to the HVB.  With the fast regen method, the car claims that 0.078 kWh of energy was added.  However, the slow regen method took much longer than the fast regen method.  The car's accessories used twice as much energy during the slow regen method than the fast regen method.  Taking that into account, the fast regen method added 19% more energy to the HVB. 

 

So it appears to make a difference in the amount of regen for slow vs. faster braking.  You can get about 18-19% more regen with faster braking if you do it right.  However, if you continue to press the accelerator to maintain the car's momentum until the last possible moment, and then brake at the maximum charge limit, you probably won't realize much, if any, of the 18-19% additional regen.  It may be wiped out by the additional time you were using energy by applying the accelerator. 

 

Note that you don't want to apply the brakes when the car is in neutral.  You will get no regen from the brakes driving in neutral.

 

One would have to experiment a lot more to determine the optimal braking strategy for various situations.

[meyersnole]

I just wish there was an easier way to tell the difference between regen braking friction. I can usually tell when I hit the friction, but then it is too late - messed up my brake score. 

 

Also good news about the shorter braking being more efficient, as when I try to take a longer approach at the light or sign I am often thwarted by a car or two diving in front of me. 

[larryh]

The only reliable way that I know of to make sure that I don't stop too quickly is to shift into Low.  You can then very gently apply the brakes while in Low to get maximum regen up to the 35 kW charge limit of the HVB.  The only other way is to have a scanner and monitor the power going into the HVB, making sure it does not exceed 35 kW.  But the readings are delayed, so again, it may be too late to make the required corrections and avoid exceeding the charge limit.

 

The cars pulling in front of you are helping you improve your regen efficiency--they're reminding you to brake harder ;) .

 

I never shift the car into neutral except to conduct experiments.  I just brake normally and try to stay within the 100% regen limits by being conservative with my braking. 

[Bills_Fusion_Energi]

The only reliable way that I know of to make sure that I don't stop too quickly is to shift into Low.  You can then very gently apply the brakes while in Low to get maximum regen up to the 35 kW charge limit of the HVB.  The only other way is to have a scanner and monitor the power going into the HVB, making sure it does not exceed 35 kW.  But the readings are delayed, so again, it may be too late to make the required corrections and avoid exceeding the charge limit.

 

The cars pulling in front of you are helping you improve your regen efficiency--they're reminding you to brake harder ;) .

 

I never shift the car into neutral except to conduct experiments.  I just brake normally and try to stay within the 100% regen limits by being conservative with my braking. 

 

I found that braking "HARDER" rather than "SLOWLY" braking when coming to a stop provides much more charging energy to the High Voltage Battery as larryh has indicated, even though the BRAKING % display in the vehicle drops well below the 100% value. Braking slowly only seems to provide about 10 amps of battery charging (on average) but when braking harder I've seen the charging amperage as high as 55 amps while the display in the vehicle indicates around 80%.

 

This would indicate that even though the friction brakes are being used somewhat much more energy is supplied to the battery for charging.

[Bills_Fusion_Energi]

larryh:

 

Have you ever tried to investigate/document if the "Grade Assist" feature (button on the left side of the gearshift) provides any benefit as far as EXTRA battery charging is concerned? 

 

I've tried several times to determine if the battery charging amperage increases (even a SMALL amount) and I cannot see any increase in amperage when it is enabled, but I'm only using it for a short period, maybe it only provides a benefit when used for a long period of time, I'm not sure...

 

What are your thoughts?

[larryh]

I have only used grade assist when descending a hill.  You can see the results here:

 

http://www.fordfusionenergiforum.com/topic/1683-obd-ii-data-for-hvb/?p=10850

 

I did not get all that close to the max charge limit going down the hills. 

Edited April 26, 2014 at 07:56 PM by larryh

[tseibel76]

I always drive in L.  I wonder if it makes a difference?  It sure is much easier than always trying to feather the brakes to get a 100% score.  It seems to me that if you can drive in L, do it.  You can get some pretty sweet numbers.  I drove around yesterday, 29.8 miles, 220 MPGe.  I like that.

[larryh]

I used grade assist today to see how much energy I could capture in the HVB going down a hill.  The hill was about 0.8 miles long with a descent of about 220 feet.  I estimate the potential energy difference to be about 0.353 kWh (assuming the car and contents weigh 1900 kg).   The car was traveling at about 40 mph at the top of the hill and I stopped at the bottom.  The kinetic energy difference was about 0.09 kWh.  The total energy generated by the motor was 0.355 kWh.  So it converted 0.355 / 0.443 = 80% of the available kinetic and potential energy to electrical energy.  The car reports the HVB charge increased by 0.308 kWh.  Accessories required about 0.010 kWh of energy. 

 

Going back up the hill, the total energy output from the HVB was 0.710 kWh.  Accessories required 0.008 kWh.  The car reports the HVB energy decreased by 0.694 kWh.  So it took twice as much energy to go back up the hill than was recovered going down the hill. 

 

The max power I observed to charge the HVB during the descent was -28.6 kW.  The max power I observed coming from the HVB for the ascent was 32.3 kW.

Edited May 10, 2014 at 04:56 PM by larryh

[Bills_Fusion_Energi]

I used grade assist today to see how much energy I could capture in the HVB going down a hill.  The hill was about 0.8 miles long with a descent of about 220 feet.  I estimate the potential energy difference to be about 0.353 kWh (assuming the car and contents weigh 1900 kg).   The total energy generated by the motor was 0.355 kWh.  So it looks like it was very efficient in converting the potential energy to electrical energy.  It basically converted it all--I don't have accurate enough data to determine the true precision, but it is close to 100%.  The car reports the HVB charge increased by 0.308 kWh.  Accessories required about 0.010 kWh of energy. 

 

Going back up the hill, the total energy output from the HVB was 0.710 kWh.  Accessories required 0.008 kWh.  The car reports the HVB energy decreased by 0.694 kWh.  So it took twice as much energy to go back up the hill than was recovered going down the hill. 

 

The max power I observed to charge the HVB during the descent was -28.6 kW.  The max power I observed coming from the HVB for the ascent was 32.3 kW.

larryh:

 

If you get a chance, try the same hill WITHOUT grade assist to see how much more energy grade assist provides.

 

Bill

Edited May 10, 2014 at 02:44 PM by Bills_Fusion_Energi

[larryh]

I forgot to account for the kinetic energy of the car.  I stopped at the bottom of the hill.  That amounted to an additional 0.09 kWh of energy.  I fixed my post above.

[larryh]

larryh:

 

If you get a chance, try the same hill WITHOUT grade assist to see how much more energy grade assist provides.

 

Bill

I would get regen by applying the brakes instead of from hill assist.  I would expect very similar results.

[larryh]

I analyzed the OBD II data for my commute home the other day during regen.  I observed the following for various stops taking into account the altitude difference between when the stop began and when it completed (gravitational potential energy), along with the initial speed (kinetic energy):

 

Initial Speed  Deceleration  % Mechanical Energy  % Energy Captured by Motor

               Rate          Captured by Motor    supplied to HVB

30 mph         0.6 mph/sec   50%                  68%

30 mph         1.7 mph/sec   86%                  92%

30 mph         1.6 mph/sec   83%                  95%

43 mph         1.5 mph/sec   82%                  96%

53 mph         1.7 mph/sec   70%                  96%

 

The faster you stop, without exceeding the maximum 35 kW regen limit, the more efficient the regen (probably because aerodynamic drag and rolling resistance have less opportunity to siphon off the potential/kinetic energy).  You capture more of the potential/kinetic energy and the energy captured by the motor is converted more efficiently to electricity--motor efficiency is about 95% for the faster deceleration rates.   With the slow deceleration, the efficiency of the motor was 68%.  The overall efficiency of regen was about 79% for the faster stops from slower speeds. 

 

I'm not sure why the stop from 53 mph did not capture as much of the potential/kinetic energy as did the stops from slower speeds.  I will have to examine more stops. Also, I am not sure how accurate my GPS altitude is. 

Edited May 22, 2014 at 12:22 AM by larryh

[larryh]

I analyzed the OBD II data for my commute home the other day during regen.  I observed the following for various stops taking into account the altitude difference between when the stop began and when it completed (gravitational potential energy), along with the initial speed (kinetic energy):

 

Initial Speed  Deceleration  % Mechanical Energy  % Energy Captured by Motor

               Rate          Captured by Motor    supplied to HVB

30 mph         0.6 mph/sec   50%                  68%

30 mph         1.7 mph/sec   86%                  92%

30 mph         1.6 mph/sec   83%                  95%

43 mph         1.5 mph/sec   82%                  96%

53 mph         1.7 mph/sec   70%                  96%

 

The faster you stop, without exceeding the maximum 35 kW regen limit, the more efficient the regen (probably because aerodynamic drag and rolling resistance have less opportunity to siphon off the potential/kinetic energy).  You capture more of the potential/kinetic energy and the energy captured by the motor is converted more efficiently to electricity--motor efficiency is about 95% for the faster deceleration rates.   With the slow deceleration, the efficiency of the motor was 68%.  The overall efficiency of regen was about 79% for the faster stops from slower speeds. 

 

I'm not sure why the stop from 53 mph did not capture as much of the potential/kinetic energy as did the stops from slower speeds.  I will have to examine more stops. Also, I am not sure how accurate my GPS altitude is. 

GPS Altitude is not accurate enough to do very precise computations.  Unfortunately, the result accuracy is very sensitive to accurate altitude.  Will have to do measurements on level roads. 

Edited May 24, 2014 at 03:29 PM by larryh

[larryh]

I used grade assist today to see how much energy I could capture in the HVB going down a hill.  The hill was about 0.8 miles long with a descent of about 220 feet.  I estimate the potential energy difference to be about 0.353 kWh (assuming the car and contents weigh 1900 kg).   The car was traveling at about 40 mph at the top of the hill and I stopped at the bottom.  The kinetic energy difference was about 0.09 kWh.  The total energy generated by the motor was 0.355 kWh.  So it converted 0.355 / 0.443 = 80% of the available kinetic and potential energy to electrical energy.  The car reports the HVB charge increased by 0.308 kWh.  Accessories required about 0.010 kWh of energy. 

 

Going back up the hill, the total energy output from the HVB was 0.710 kWh.  Accessories required 0.008 kWh.  The car reports the HVB energy decreased by 0.694 kWh.  So it took twice as much energy to go back up the hill than was recovered going down the hill. 

 

The max power I observed to charge the HVB during the descent was -28.6 kW.  The max power I observed coming from the HVB for the ascent was 32.3 kW.

 

Now that I have more accurate elevation data from Google Earth, I tried using grade assist on this hill again.  The descent is 316 feet (96 meters) over 0.67 miles or an average grade of 9%.

 

Assuming the car weighs 1875 kg, the potential energy difference is 9.81*96*1875 J or 0.49 kWh.  The speed at the top of the hill was 27 mph or 12 meters per second.  I stopped at the bottom of the hill, so the difference in kinetic energy was 0.5*1875*12*12 J, or 0.04 kWh.  The total energy available for regen was 0.53 kWh.  The total electricity generated by the motor was 0.32 kWh.  The regen efficiency was thus 60%.

Edited May 25, 2014 at 08:56 PM by larryh

[larryh]

I analyzed the OBD II data for my commute home the other day during regen.  I observed the following for various stops taking into account the altitude difference between when the stop began and when it completed (gravitational potential energy), along with the initial speed (kinetic energy):

 

Initial Speed  Deceleration  % Mechanical Energy  % Energy Captured by Motor

               Rate          Captured by Motor    supplied to HVB

30 mph         0.6 mph/sec   50%                  68%

30 mph         1.7 mph/sec   86%                  92%

30 mph         1.6 mph/sec   83%                  95%

43 mph         1.5 mph/sec   82%                  96%

53 mph         1.7 mph/sec   70%                  96%

 

The faster you stop, without exceeding the maximum 35 kW regen limit, the more efficient the regen (probably because aerodynamic drag and rolling resistance have less opportunity to siphon off the potential/kinetic energy).  You capture more of the potential/kinetic energy and the energy captured by the motor is converted more efficiently to electricity--motor efficiency is about 95% for the faster deceleration rates.   With the slow deceleration, the efficiency of the motor was 68%.  The overall efficiency of regen was about 79% for the faster stops from slower speeds. 

 

I'm not sure why the stop from 53 mph did not capture as much of the potential/kinetic energy as did the stops from slower speeds.  I will have to examine more stops. Also, I am not sure how accurate my GPS altitude is. 

I redid these computations using more accurate Google Earth altitudes.  The first column is the initial speed when the brakes were applied.  Each time I came to a full stop.  The deceleration shows how quickly I stopped.  The next two columns show the percentage of available regen energy reaching the motor and the percentage of the regen energy being converted by the motor to electrical energy.  The fifth column shows the percentage of the total regen energy which is potential energy from the gravitational potential difference due to a change in altitude from the beginning of braking to when I stop.  The rest of the regen energy is kinematic energy from stopping the car.  The sixth column is the percentage of regen energy stored in the battery.  Part of that regen energy is used by accessories and is thus not available to be stored in the HVB.  This is shown in the last column.

 

Initial   Deceleration   Regen Energy  Regen Energy  Regen Energy    Regen    Regen Electrical

Speed     (mph/sec)      Input to      Output from   from Altitude   stored   Energy used

(mph)                    Motor         Motor         Difference      in HVB   by Accessories

14.5      1.7            82%           80%            0%             55%      21%

30.9      0.6            44%           35%           45%             22%      31%

29.2      1.4            82%           75%           27%             57%      15%

29.8      1.5            73%           72%           14%             67%       8%

43.5      1.5            72%           70%           25%             63%       5%

52.9      1.6            65%           63%           10%             58%       5%

26.8      1.1            59%           50%            4%             36%      15%

26.1      1.1            69%           67%           43%             55%       9%

46.5      2.2            76%           74%            4%             61%       4%

16.3      1.1            52%           51%           44%             41%      18%

 

The percentage of regen energy converted to electrical energy by the motor varies from 50% to 80%, depending on the rate of deceleration and initial speed among other factors.  The percentage of regen energy actually stored in the HVB varied from 22% to 67%, depending on the total amount available from the motor and the amount used by accessories. 

Edited May 25, 2014 at 01:50 PM by larryh

[larryh]

I repeated the same experiment, this time the stops were from 55 mph and I used Low to brake in all but the last case.  Unfortunately, Google Earth is not perfect either (the altitudes are off by a few feet), so the percentages may be off by 5%. 

 

Initial   Deceleration   Regen Energy  Regen Energy  Regen Energy    Regen    Regen Electrical

Speed     (mph/sec)      Input to      Output from   from Altitude   stored   Energy used

(mph)                    Motor         Motor         Difference      in HVB   by Accessories

53.3      2.2            79%           79%            6%             65%      2%

52.8      2.1            80%           80%            4%             69%      2%

53.1      2.2            83%           83%            6%             68%      2%

53.5      2.3            83%           79%            2%             67%      1%

 

52.7      1.7            50%           50%            2%             62%      2%

 

It looks like if you brake near the max charge limit, you capture about 80% of the potential/kinetic energy available for regen.  The HVB stores about 70% of that.

If you brake slower, you will get less regen as evidenced by the last row (presumably because it takes longer to stop and hence there is more opportunity to lose energy due to rolling resistance, aerodynamic drag, friction, etc.).  Something is wrong with the HVB energy measurements during that stop.  You can't store more energy in the HVB than is supplied to it--the percentage should be less than 50%.

 

From this and the previous post, if you want more regen, you need to stop faster, up to the 35 kW charge limit for the HVB.  However, I don't know a good way to implement that observation.  There is no way to accurately guess how far before the stop you need to start applying the brakes at the maximum charge limit so that you arrive exactly at the stop sign.  In addition, if you wait to brake, you are using additional energy from the HVB to maintain speed.  It looks like the best option is to brake immediately at the max regen limit to slow down to around 30-40 mph, and then you can brake more slowly until you reach the stop sign. 

Edited May 25, 2014 at 05:07 PM by larryh

[Hybridbear]

From this and the previous post, if you want more regen, you need to stop faster, up to the 35 kW charge limit for the HVB.  However, I don't know a good way to implement that observation.  There is no way to accurately guess how far before the stop you need to start applying the brakes at the maximum charge limit so that you arrive exactly at the stop sign.  In addition, if you wait to brake, you are using additional energy from the HVB to maintain speed.  It looks like the best option is to brake immediately at the max regen limit to slow down to around 30-40 mph, and then you can brake more slowly until you reach the stop sign. 

What I have been trying to do recently, based on your tests, is put the car in neutral and coast (to not consume energy from the HVB to maintain speed) and then wait longer to brake. I've been learning how to judge based on my speed and the incline of the road how long I can wait before braking to get near the max charge limit while not losing potential energy to heating the brake pads. In the city I'm not getting near the 35 kW max charge limit, but I do try to brake hard enough to exceed 20 kW. I've found that at the low speeds the traction motor can't put out 35 kW of regen braking because the RPM is too low. I've had some stops with less than a 100% brake score even though my highest power level going to the HVB was around 22 kW.

 

I have seen a statistically significant increase in Regen miles from doing this. I don't have enough data to know if there's a corresponding increase in MPG or not since MPG is so much more variable. But I've been able to increase my Regen miles by about 25% in city driving by braking more efficiently.

 

There is definitely a difference between the 17-inch tires & the 18-inch tires. I can wait much longer to brake in our FFH with the 18-inch tires and still get 100% brake score than I could in our previous FFH with 17-inch tires. I also see the difference when I have driven my parents' C-Max Energi. The rolling resistance difference is noticeable when coasting in Neutral as well.

Edited May 26, 2014 at 12:29 PM by Hybridbear

[larryh]

When you put the car in neutral, you are still losing energy due to aerodynamic drag, rolling resistance, and internal frictions.  The loss is far greater at higher speeds than slower speeds.  To conserve as much energy as possible, you might want to reduce your speed some as soon as possible using regenerative braking to capture as much of the kinematic energy as possible before it is lost due to aerodynamic drag, rolling resistance, and internal frictions. 

Edited May 26, 2014 at 01:24 PM by larryh

[larryh]

I observed the following measurements during this morning’s commute to work:

 

Electrical Energy output from HVB:  2.40 kWh

Electrical Energy used by accessories:  0.18 kWh

Mechanical Energy output from motor used to propel car:  1.81kWh

 

Electrical Energy supplied to HVB by regen:  0.63 kWh

Electrical Energy used by accessories during regen:  0.04 kWh

Mechanical Energy captured by motor during regen:  0.69 kWh

 

This implies that 2.40 – 0.63 = 1.77 kWh of electricity is plug-in energy (came from the wall outlet)  And 0.63 / 2.40 = 26% of the energy came from regen.

 

The efficiency of the motor in converting electrical to mechanical energy to propel the car was then 1.81 / (2.40 – 0.18 + 0.04) = 80%.

The efficiency of the motor during regen was (0.63 + 0.04) / 0.69 = 97%.

 

I have observed the efficiency of the ICE to be 35% during my commutes.  So the motor is 80%/35% = 2.3 times more efficienct than the ICE. 

 

 

 

[larryh]

The car actually computes two different SOC values for the HVB, the actual SOC and the displayed SOC. The displayed SOC is what you see when looking at the battery display in the car, MFT EV information screen, or MFM. The car does not report the actual SOC to you via the displays or MFM--you need an OBD II scanner to see it.

When the displayed SOC is 100%, the actual SOC is between 95.5% and 100%. When the displayed SOC is 0%, the actual SOC is about 21.5% and you are now in hybrid mode.

So the actual SOC varies by 100% - 95.5% = 4.5% when the car reports the SOC is 100%. That explains why you can travel a mile or more in EV mode before the displayed SOC of the HVB falls to 99%. Until the actual SOC falls below 95.5%, the car will display that the HVB SOC is 100%.

While the car is off, the HVB slowly looses charge over time. The car initially charges the HVB to more than 99%. But if you let the car sit for a few hours, it will fall below 99%. Unfortunately, you can't observe the actual SOC without a scanner. So if the car displays 100% SOC, all you know is the actual SOC is greater than 95.5%. If you want to be sure that you have the maximum possible SOC before leaving for a trip, you will have to turn on the car and use up some of the SOC and then let the car recharge. Unfortunately, the car will not generally attempt to recharge the battery to 99%+ actual SOC unless the displayed SOC is below 100% or, equivalently, the actual SOC is below 95.5%.

You can force the car to fully charge the HVB by running the AC or heater with the car plugged-in and started. Initially, you will observe a down arrow below the battery icon while the AC/heater is running indicating power is being drawn from the HVB. When the battery has sufficiently depleted, the car will start charging the HVB and you will observe the up arrow above the battery icon (provided the AC/heater is not drawing more power than the charger can supply). You can now turn off the AC/heater while the car is charging the HVB. The car has completed charging when the up arrow is replaced by the down arrow and the car is now drawing power from the HVB again. The actual SOC will now probably be above 99%. You can't rely on the charging ring or the EV information screen to indicate the car is charging when the displayed SOC exceeds 100%.

Note that the car also computes Energy to Empty. This measures the amount of energy in the HVB. When the actual SOC is 100%, the Energy to Empty is 7.14 kWh. The lowest Energy to Empty that I have observed is 1.00 kWh with an actual SOC of 16.5%. The maximum capacity of the HVB is 7.6 kWh.

Also note, that If you turn on the car while it is charging, it messes up your lifetime MPGe, trip odometer MPGe, and estimated range. See the following post:

"http://fordcmaxenergiforum.com/topic/2794-video-explaining-what-happens-to-mpge-and-soc-charging-while-powered-on/?p=21363"


I got 999.9 MPGe doing that (the car couldn't compute the exceptional mileage I achieved--it was off the scale) ;). I could probably also get 255 estimated range on the HVB.


http://fordcmaxenergiforum.com/uploads/gallery/album_36/med_gallery_520_36_42627.jpg

Edited July 7, 2014 at 08:50 PM by larryh

[snedecr]

I'm wondering if you could use your OBD scanner to either confirm or debunk a myth.

 

Conventional wisdom is that you need an "eggshell" touch on the accellerator pedal to get the best mileage.

 

I don't know if that's true in electric drive mode.  In a conventional engine, you rev the engine, you get more slippage of the torque converter, which gets converted to heat, more loss of power, etc.  But in electric mode, there is no torque converter, no slippage as in a conventional engine.  Your scanner could help get some data to tell the difference.

 

Could you take the same distance, accelerate briskly to say 40 mph, then keep at 40 to the end, and count the kWh,

then, "eggshell' babysit the same distance, just barely getting to 40 mph by the end of the distance, and count the kWh.

 

At 40 mph, there shouldn't be too much difference in wind resistance, so that shouldn't play too much into the numbers...

 

I think it would be an interesting experiment, if you're up for it.  Perhaps you've already done it, or it may be academic, who knows...

[larryh]

You will most likely use less energy with slower acceleration.   Resistance increases with the square of the vehicle’s speed.  By accelerating slowly, your average speed will be lower than when accelerating faster.  I believe this will yield higher overall MPGe.
 
I estimate efficiency of the motor to be around 95% when accelerating fast and around 75% when accelerating slow.  The kinetic energy when driving 40 mph, assuming the car and its contents weigh 1875 kg is about 0.083 kWh.  Acceleration must provide this kinetic energy.  So accelerating faster may use about 20%*0.083 = 0.017 kWh less energy.
 
However, if you accelerate faster, you will be going faster sooner.  Going faster requires more power.  At the extreme end of acceleration, assume you can instantaneously accelerate to 40 mph with 100% efficiency and then travel at 40 mph for one mile.  At 40 mph, you will get about 189 MPGe.  So going one mile at 40 mph will use 33.705/189 = 0.178 kWh.  But you also need to provide the kinetic energy of the car going 40 mph.  So you will have consumed a total of 0.083 + 0.178 = 0.261 kWh.
 
If accelerate uniformly until you reach 40 mph, to overcome aerodynamic drag and rolling resistance, you will consume about 0.137 kWh of energy (going through all the math).  On top of that, you will use 0.083/0.75 = 0.111 kWh of energy for the 40 mph kinetic energy at the end of one mile assuming the car is 75% efficient in converting electricity to mechanical power to propel the car.  Thus accelerating slowly will consume 0.137 + 0.111 = 0.248 kWh.  You will save 0.013 kWh. 
 
I predict I am going to have a hard time measuring the difference.
 
 
 

Edited July 9, 2014 at 10:37 AM by larryh

[snedecr]

You will most likely use less energy with slower acceleration.   Resistance increases with the square of the vehicle’s speed.  By accelerating slowly, your average speed will be lower than when accelerating faster.  I believe this will yield higher overall MPGe.

 

I estimate efficiency of the motor to be around 95% when accelerating fast and around 75% when accelerating slow.  The kinetic energy when driving 40 mph, assuming the car and its contents weigh 1875 kg is about 0.083 kWh.  Acceleration must provide this kinetic energy.  So accelerating faster may use about 20%*0.083 = 0.017 kWh less energy.

 

However, if you accelerate faster, you will be going faster sooner.  Going faster requires more power.  At the extreme end of acceleration, assume you can instantaneously accelerate to 40 mph with 100% efficiency and then travel at 40 mph for one mile.  At 40 mph, you will get about 189 MPGe.  So going one mile at 40 mph will use 33.705/189 = 0.178 kWh.  But you also need to provide the kinetic energy of the car going 40 mph.  So you will have consumed a total of 0.083 + 0.178 = 0.261 kWh.

 

If accelerate uniformly until you reach 40 mph, to overcome aerodynamic drag and rolling resistance, you will consume about 0.137 kWh of energy (going through all the math).  On top of that, you will use 0.083/0.75 = 0.111 kWh of energy for the 40 mph kinetic energy at the end of one mile assuming the car is 75% efficient in converting electricity to mechanical power to propel the car.  Thus accelerating slowly will consume 0.137 + 0.111 = 0.248 kWh.  You will save 0.013 kWh. 

 

I predict I am going to have a hard time measuring the difference.

 

 

 

Ok, I see your point.

 

In the ICE world, the Prius people using scanners have pretty much proven that the "pulse and glide" technique with brisk acceleration followed by coasting is better than continuous slow ICE usage to a speed setpoint, so I was just thinking about how it might apply to all-electric mode.  Hoping to get the light flashers and horn honkers off my tail!

[larryh]

The following is a plot of the voltage variation between the HVB cells for my 60 mile commute this morning.  The plot on the left shows the variation vs. SOC of the HVB.  The right plot shows the variation vs. power output of the HVB, where negative power results from regen.   I'm not sure how correct this is, but I suspect that large voltage variation is bad and is indicative of stress on the battery.  You want all the cells to be balanced with less than 0.01 volt variation.  According to the graphs below, large voltage variation occurs when the SOC is low and during high power output of the HVB.  I'm not sure how long it takes the cells to recover and become rebalanced after the balance is perturbed.  The worst thing you can do for the balance is to run the HVB at a low SOC.  It increases significantly below 17% SOC.  That is about 50% charge for the hybrid battery icon. Perhaps we should stay in EV later mode and maintain the charge level above 20% until the end of the trip.  At the end, then let it enter hybrid mode and use up the remaining charge of the battery.  Try to stay out of hybrid mode as long as possible.  This may help preserve HVB life.

 

[larryh]

I'm wondering if you could use your OBD scanner to either confirm or debunk a myth.

 

Conventional wisdom is that you need an "eggshell" touch on the accellerator pedal to get the best mileage.

 

I don't know if that's true in electric drive mode.  In a conventional engine, you rev the engine, you get more slippage of the torque converter, which gets converted to heat, more loss of power, etc.  But in electric mode, there is no torque converter, no slippage as in a conventional engine.  Your scanner could help get some data to tell the difference.

 

Could you take the same distance, accelerate briskly to say 40 mph, then keep at 40 to the end, and count the kWh,

then, "eggshell' babysit the same distance, just barely getting to 40 mph by the end of the distance, and count the kWh.

 

At 40 mph, there shouldn't be too much difference in wind resistance, so that shouldn't play too much into the numbers...

 

I think it would be an interesting experiment, if you're up for it.  Perhaps you've already done it, or it may be academic, who knows...

I accelerated from a stop to a maximum speed of around 40 mph.  I measured the total energy consumed after traveling about 0.25 miles.   The slow acceleration took the entire distance to reach 40 mph.

 

Acceleration Speed     Energy (kWh)

Slow                            0.136

Medium                       0.143

Fast                            0.137

 

There are too many different ways to accelerate to 40 mph over a distance of 0.25 miles.  There is not much difference in the energy used to determine which is the best way. 

Edited July 12, 2014 at 02:26 AM by larryh