Efficient EV Mode Driving Techniques

[stevedebi]

When the engine spins you know it, you feel something different about the car like if your battery is full and you shift it into L going down a hill.

 

I don't think it spins the engine if you are in EV mode and the battery is not full, I think it just charges to the max possible and slows the car as much as possible.  If you need to slow down even more, then you need to use the brake.  Spinning the engine doesn't make sense unless the battery can't take the charge that's when its done.

 

-=>Raja.

The manual says that L provides "Maximum engine braking", page 154. I think it spins the engine when it is on, although to be honest I've not used L except when braking. I don't drive it that way under normal circumstances, only when I really need to slow down. Maybe when you are accelerating it will not engage the engine.

 

I know on my 2008 Escape Hybrid, it just changed the regeration to a higher state, until the SOC got too high - then it spun the engine. But with the 2009 and later, my understanding is that it spun the engine when engaged at speed.

[murphy]

I use L all of the time driving in EV Now mode.  The only time the ICE spins is if the HVB is fully charged.  Since I am driving on the HVB it is never fully charged so the ICE never spins.

[rbort]

Steve, spinning the engine is for another vehicle maybe like the Escape older 2008 as you're talking, in the Cmax Energi it does not spin, only if the battery is full then it spins and starts the engine, at least this is what I believe from feeling and listening to the car with no radio windows shut unless someone proves me wrong.

 

OK, I'm back, wow, spent over an hour doing this, and burned up 95% of the battery down to 5% when I got home.  Here is what I did:

 

First, from the park accelerate at 1.5 bars to 25 engage cruise, take a right before the shallow hill which turns back uphill (less of an altitude drop), two stop signs, and then start from 0mph accelerate to speed, engage cruise and climb the steep hill.  Close to the top disengage cruise and pull off into a parking lot on the left, power off the car and check trip summary MPGe numbers.

 

Cruise control at 25mph,  111.7 MPGe, battery from 93 to 86%.  (note battery percentage is not super accurate as its only 1 significant figure)

Cruise control at 30mph,  107.6 MPGe, battery from 84 to 76%.  (down hill back to park, got 555 mpge) - no particular routine here

Cruise control at 35mph,  106.2 MPGe, battery from 75 to 67%.  (down hill back to park, got 661 mpge) - no particular routine here

 

Manual driving speed around 30, 1.8 bars up the steep hill,  110 mpge, battery 65% to 58%.   (down hill back to park, got 626 mpge)

Manual driving speed around 30, 2.2 bars up the steep hill,  108.8 mpge, battery 56% to 48%.   (down hill back to park, got 615 mpge)

 

Next decided to not turn right and instead go all the way down the first shallower hill to a stop sign.  Then turn right and climb further to the point where I would have arrived (higher elevation), and turn left and go up the steep hill again to the parking lot.

 

Cruise control at 25mph,  111.6 MPGe, battery from 47 to 39%.  (down hill back to park, got 670 mpge)  (this one I used HA to maintain speed and/or braking)

Cruise control at 30mph,  107.4 MPGe, battery from 38 to 29%.  (down hill back to park, got 617 mpge)  (this one I used L after letting the car run away - no brake)

 

Notice that the CC tests at 25 and 30mph gave very close results whether I decended lower with the car and back up or took a side street to keep the car at higher elevation and go through an extra stop sign to get to the same place.  This basically tells me the elevation change didn't matter much and no reason to take that side road I've been doing late at night with an extra stop to save battery.  Maybe the extra stop hurts, and the other gets more regen I just need to keep the climb slow at 1.8 bars and 20mph back to the same point where I turn left.

 

After this I ran tests on the other end of the hill, the longer and more steep downhill part.  So from the parking lot, I took off at 1.5 bars and get up to speed, engage cruise, and then:

 

Cruise control at 35mph, let the car runaway down the hill up to 40 mph, and at some distance later disable cruise at the same point, brake to a stop, and power cycle the car after backing into someone's driveway to turn around.  224 MPGe.  

 

Next test, same thing Cruise control at 35mph, this time use Hill Assist, car speeds up to 38mph down the hill before slowing back down to 35 where I disable hill assist.  Disengage cruise, brake to a stop and back in, power cycle, 229 MPGe.

 

Third and final test, Cruise control at 35mph, use L down the hill, car speeds up to only 36mph down the hill before slowing back down to 35 when it levels off and I go back to D, disengage cruise at the cone (same place), brake to a stop, and back in.  Power cycle, this time 232 MPGe.

 

I didn't intend to go in the person's driveway was going to use the park on the right but it came up too quick the constant pressure braking stopped right past this person's driveway and it became the point where I back up.  Luckily it seemed no-one noticed or was home, and I was dreading someone behind me, didn't happen on the first two attempts, on the third one there was a guy behind me and I figured he is going to force me into the park and mess up the test, but he turned off at the cone into his driveway and I was able to repeat the same exact routine.

 

Looks like the moral of this hill story is maintain a constant speed down the hill and don't let the car run away.  Use hill assist if the hill is shallow enough, or shift to L if its too steep.  With experience you'll know which hills hill assist is sufficient for, and which hills need L.

 

The other side of the story is slower is better, and cruise control is alot easier to drive than manual control.  This way you don't have to watch your speed as much to maintain it constantly, and you can relax and enjoy the ride.  Need more range, slow down.  Have enough battery, speed up to get home sooner.  Watch the battery remaining miles compared to the distance to home, and make sure it remains the same or higher, if getting lower slow down to compensate - even 2 to 3 mph make a difference over a 10 mile distance.

 

Lastly, on the way back from the parking lot to the park, I accelerated to 20mph and let the car roll down the hill.  At 35 I used hill assist to hold it and then released in time before the uphill to the stop sign.  From there its a right down the hill to a left turn.  I used a combination of braking, hill assist and L.  While I wasn't as specific about this in the beginning, later I gathered from my driving tonight that using hill assist or braking to hold a constant speed is better than letting the car run away and using L at the right moment to generate alot of power back into the battery before the turn or stop.  Is using L, I didn't get the best MPGe score - notice 617 above versus 670 best score.

 

-=>Raja.

Edited October 16, 2014 at 12:50 AM by rbort

[larryh]

I don't think it spins the engine if you are in EV mode and the battery is not full, I think it just charges to the max possible and slows the car as much as possible.  If you need to slow down even more, then you need to use the brake.  Spinning the engine doesn't make sense unless the battery can't take the charge that's when its done.

 

-=>Raja.

 

I tried Low on a very steep hill (70% max grade according to Google Earth) going less than 15 mph--I didn't dare go much faster.  I had to use the brakes to slow the car down with brake score in the lower 80s.  The ICE did not come on.  The max regen was only about 21 kW. 

Edited October 17, 2014 at 02:03 PM by larryh

[larryh]

OK, I'm back, wow, spent over an hour doing this, and burned up 95% of the battery down to 5% when I got home.  Here is what I did:

 

First, from the park accelerate at 1.5 bars to 25 engage cruise, take a right before the shallow hill which turns back uphill (less of an altitude drop), two stop signs, and then start from 0mph accelerate to speed, engage cruise and climb the steep hill.  Close to the top disengage cruise and pull off into a parking lot on the left, power off the car and check trip summary MPGe numbers.

 

Cruise control at 25mph,  111.7 MPGe, battery from 93 to 86%.  (note battery percentage is not super accurate as its only 1 significant figure)

Cruise control at 30mph,  107.6 MPGe, battery from 84 to 76%.  (down hill back to park, got 555 mpge) - no particular routine here

Cruise control at 35mph,  106.2 MPGe, battery from 75 to 67%.  (down hill back to park, got 661 mpge) - no particular routine here

 

...

 

-=>Raja.

 

If you are making significant elevation changes, as in your tests, then the elevation difference between the origin and destination is what is going to dominate your results.  Speed and the elevation profile will only be minor factors affecting the results since most of the energy is spent providing the necessary potential energy to get up the hill vs. overcoming friction.  You did not observe much difference in MPGe for the various speeds.  The same is true when you went down the hill. 

 

If there is some alternative path that avoids the hill completely, you would want to compare the MPGe for this route vs. one the goes up and down the hill to see just how expensive hills are. 

 

Rather than considering just one steep hill, another interesting case would be how to tackle a road with many hills to provide the best MPGe for a given average speed.

Edited October 16, 2014 at 11:12 PM by larryh

[rbort]

 

 

If you are making significant elevation changes, as in your tests, then the elevation difference between the origin and destination is what is going to dominate your results.  Speed and the elevation profile will only be minor factors affecting the results since most of the energy is spent providing the necessary potential energy to get up the hill vs. overcoming friction.  You did not observe much difference in MPGe for the various speeds.  The same is true when you went down the hill. 

 

If there is some alternative path that avoids the hill completely, you would want to compare the MPGe for this route vs. one the goes up and down the hill to see just how expensive hills are. 

 

Rather than considering just one steep hill, another interesting case would be how to tackle a road with many hills to provide the best MPGe for a given average speed.

 

Hey Larry:

 

The origin and destination are the same elevation change every time, except in the path that I took one path decends deeper and comes back up where the other does not before climbing the big hill.  I looked at google maps again and the path that doesn't decend as deep is 0.4 miles where the other is 0.3 miles.  So going deeper may have some negative effect, but its slightly shorter and only features one stop sign instead of 2.

 

There is a third path, it climbs to the same road after the big hill, but is a longer shallower climb.  I tried that way before way back when, but couldn't get up the hill with less than 7% battery used.  It is also definitely longer, not the direct way there so its not worth it due to the extra distance.  I just checked the maps:

 

Path a) goes the deepest down the hill and then back up the steep hill, 1 mile from just before the right turn to the arrival of the long shallow hill up.

Path b) goes to the right through 2 stop signs, not as deep downhill, and back up the steep hill to the same point, 1.1 miles.

Path c) goes the deepest down the hill, then turns left instead of right and goes a little further before turning right for a long ride up the hill in a shallower angle, this path is 1.4 miles.

 

Seems to be for my situation option a) which is the shortest option is the most ideal provided I keep the speed at 25mph for the best mpge.

 

Sadly there are no other ways to get from point a to b without going way out of my way.

 

 

Rather than considering just one steep hill, another interesting case would be how to tackle a road with many hills to provide the best MPGe for a given average speed.

 

Using your information and my tests, I think I may have already answered this one.  Stay on cruise control, use L to prevent runaways down the hills (i.e) keep your selected speed as tight as possible around that speed.  Hill assist can be used if the hills are shallow enough to serve the same purpose.

 

After this strategy, then the only other thing is your average speed - the answer to that, the slower the better the mpge, i.e. 25 is better than 30 which is better than 35 which is better than 40.  Go as fast as you need to and base it on how much battery you have to make it.

 

-=>Raja.

[larryh]

There are more techniques to take into consideration.   Since energy conversion between potential and kinetic energy is 100% efficient, it would be interesting to measure the impact of slowing down going up the hill and then allowing gravity to assist speeding the car back up again going down the hill.

[rbort]

Well in a way in manual mode I sort of did that.  Going up that steep hill at 1.8 bars the car slowed down to 25mph or so.  Going up the hill at 2.2 bars the car was holding speed and slowly accelerating to 34 mph at I came slower to the top of the hill (from 30).

 

Keeping it simple means using CC, as you have to watch the power bar like a hawk if you're driving it in manual mode.

 

-=>Raja.

[stevedebi]

I use L all of the time driving in EV Now mode.  The only time the ICE spins is if the HVB is fully charged.  Since I am driving on the HVB it is never fully charged so the ICE never spins.

OK, so it works like my 2008 FEH; it just uses the regen to slow down, rather than spins the engine. Good to know!

[larryh]

The following chart shows Regen during a stop.  At distance 0 miles, the car is going 40 mph.  The stop sign is at distance 0.32 miles.  The plot shows three different stops.  For the red curve, I coast for a long time in Drive starting at distance 0.04 miles and then use the brakes starting at distance 0.3 miles to come to a complete stop at the stop sign.  For the orange curve, I continue at 40 mph for 0.14 miles, coast in Drive until distance 0.26 miles, and then apply the brakes to complete the stop.  For the purple curve, I maintain 40 mph until distance 0.22 miles and then shift into Low.  I apply the brakes at distance 0.32 right before the stop sign to complete the stop.

 

Each curve shows the energy consumed from the HVB vs. distance.  The car consumes energy from the HVB to maintain 40 mph until coasting begins or I shift into Low, after which regen commences.  The curves now start falling as the motor generates electricity from the kinetic energy of the car which is supplied to the HVB. 

 

For the purple curve, 0.04 kWh of energy is consumed to maintain a constant speed of 40 mph until I shift into Low (you can see the the value of the purple curve at 0.22 miles is 0.04 kWh).  While in L, regen supplies about 0.067 kWh of energy to the HVB and the curve falls to -0.027 kWh at distance 0.32 miles.  The kinetic energy of the car at 40 mph is approximately 0.08 kWh.  So regen captures approximately 84% of the kinetic energy. 

 

For the red curve, 0.008 kWh of energy is consumed to maintain constant speed until coasting begins.  Regen supplies about 0.045 kWh of energy to the HVB and the curve falls to -0.037 kWh at the stop sign.  This time regen is only 56% efficient.

 

The orange curve is in between the other two.  We end up with the most energy in the HVB with the long coast (the red curve) at -0.037 kWh.  We end up with the least amount of energy in the HVB when using Low to stop (the purple curve) at -0.027 kWh.  The difference is 0.01 kWh.  While there is more regen when using Low to stop, 0.067 kWh vs. 0.045 kWh of energy, the extra energy from the HVB required to maintain constant speed of 40 mph until closer to the stop sign more than offsets the additional 0.022 kWh gained from regen. 

 

Edited October 17, 2014 at 07:14 PM by larryh

[rbort]

Thanks Larry for the great post, I linked it to the Cmax group :worship:

 

-=>Raja.

[larryh]

The following plot shows Energy Loss vs. Distance on a hilly road.  The road is about 1.4 miles long.  The tops three curves show the energy loss (presumably due to friction) at 20, 29, and 33 mph.  The bottom two curves show the kinetic energy of the car (for the trip at 20 mph) and change in potential energy (due to the hills). 

 

For the top three curves, I have plotted the mechanical energy output of the electric motor, subtracting out the kinetic and potential energy that must be supplied by the motor to get the car up to speed or to slow down, and to climb or descend hills.  The curves are not what I would expect--something is missing. I would have expected them to be fairly straight lines.  There should be no bumps in the curves.  I am plotting energy loss.  There is no way to reclaim lost energy.  However, at about 1.2 miles, the energy loss decreases.  That is impossible.  The car has just reached the top of a hill and is now beginning to descend and gain speed.  There has to be some other mechanism that is storing and releasing energy other than kinetic energy and potential energy.  That mechanism seems to be related to the rate of change of speed of the car.  It probably has something to do with changes in the moment of inertia of the various rotating components inside the car, such as the wheels, planetary gear system, motor, and generator.  But I have no idea what it is.  Without understanding this mechanism, I don't have a good way of analyzing efficient driving techniques for hills.

 

Edited October 17, 2014 at 08:13 PM by larryh

[rbort]

Larry, here is an easier test to do without having to "figure out" exactly how things are happening:

 

Put the car in cruise at 35mph on a level road that approaches a long downhill section:

 

a) let the car speed up down the hill, then slow back down to 35mph and continue onwards.  

b) use hill assist to hold the car back some, say at about 38mph, and disable it when the road levels off and the car slows to 35 and continues onwards.

c) use L to hold the car at close to 35mph, maybe 36 or so, and disable it when the road levels off and the car slows to 35 and continues onwards.

 

In all 3 cases look at the HVB charge level at the top of the hill, and then again after some point at the bottom of the hill after the road levels off.

 

The idea is to find out which method out of the above 3 travels the most distance by the time the battery has drained X amount, or is a net zero.  

 

You don't have to stop, just make note at the moment the battery consumption is the same for all 3 choices and see which choice went the furthest and by how much between all of them.

 

Should be an easy test for you with your SG equipment, I did it using MPGe its not as easy for me to see how far I go as 1% on the battery display or 0.1kwh shown on the trip display is probably not accurate enough for me to tell as easily.

 

-=>Raja.

[larryh]

I don't have a Scan Gauge.  The Scan Gauge is not useful for the analyses I have been doing.  I have the OBDLink MX scanner. 

 

It makes no difference whether you use the brakes, grade assist, or Low and Cruise Control to maintain constant speed going down the hill.  The car does the same thing and you will get identical results. They all use the motor for regen and hold back the car in exactly the same way.  I have tried it.

 

The only thing that could make a difference is how much you allow the car to slow down when going up the hill and how much you allow it to speed up when going back down.  I don't have any good hills without stop signs, too much traffic, or too many curves to try that on at the moment.

 

I did try going down a relatively steep hill at 20 mph vs. 30 mph.  Regen was about the same.  For 20 mph, it was 0.491 kWh / mile.  For 30 mph, it was 0.476 kWh / mile.  There is a little more energy lost due to friction at the higher speed. 

Edited October 18, 2014 at 04:56 PM by larryh

[rbort]

I experienced different results with D, Hill assist and L meaning that L held the car speed closest to the original speed where HA was in the middle and D let the car run away the most.

 

My point was if you run away and then let it burn the same amount of end result power versus if you hold it back and then use the regen power to burn the same amount of battery, which one goes further?

 

-=>Raja.

[larryh]

Note that I maintained the same constant speed with each of the different methods.  If you go down the hill faster, you will loose slightly more energy from friction and it will not be a fair comparison between the methods to determine which yields the best regen. All the hills I use have stop signs at the bottom. 

Edited October 18, 2014 at 04:35 PM by larryh

[rbort]

Its not about which yields the best regen, but about which yields the best range.

 

If you didn't have to stop going down the hill at the bottom that is.

 

-=>Raja.

[larryh]

Looking more closely at the data I have collected on a 0.5 mile section of a hill at 8% grade and constant speed, I observe the following conversion efficiencies from the motor mechanical energy or the HVB electrical energy to potential energy:

 

Speed  Direction  Mechanical  Electrical

(mph)             Efficiency  Efficiency

20     Uphill     87%         74%  

30     Uphill     86%         72%

20     Downhill   76%         75%

30     Downhill   72%         72%

 

So when going up the hill at 20 mph, 87% of the mechanical energy output from the motor is converted to potential energy which propels the car up the hill.  The remaining 13% is lost to friction (aerodynamic drag, tire rolling resistance, etc.).  The motor is 85% efficient converting electrical to mechanical energy, so 87% * 85% = 74% of the electrical energy consumed from the HVB is converted to potential energy. 

 

When going down hill at 20 mph, 76% of the potential energy from descending the hill is converted to mechanical energy via the motor for regen, the remaining 24% is lost due to friction.  The motor is 99% efficient during regen, so 76% * 99% = 75% of the potential energy is applied to the HVB.

 

This seems very strange.  Conversion efficiency is asymmetric with respect to mechanical energy. but symmetric with respect to electrical energy.  At 30 mph, the conversion efficiency from mechanical to potential energy is 86%.  The reverse conversion from potential to mechanical energy (during regen) is much lower at 72%.  However, the conversion in either direction between electrical and potential energy is the same at 72%.  The same holds true for 20 mph. 

 

For kinetic energy, the symmetries are reversed.  The conversion efficiency between kinetic and motor mechanical energy is the about the same in either direction, but the conversion efficiency from electrical energy to kinetic energy is much lower than the conversion efficiency from kinetic energy to electric energy via regen.  Motor efficiency is about 85% when accelerating and is greater than 95% during regen.  

 

Note that when stopping quickly, the conversion efficiency from kinetic to electrical energy is at most 80%, i.e. 80% of the kinetic energy is converted to electrical energy by the motor.  This is comparable to the 75% conversion efficiency from potential to electrical energy observed going down the hill. 

 

Since the conversion between electrical and potential energy is around 75% efficent and the conversion between potential and kinetic energy is 100% efficient,  this suggests that you want to slow down going up the hill, converting some of the kinetic energy to potential energy, and let the car speed up going back down the hill, converting some of the potential energy to kinetic energy. 

Edited October 18, 2014 at 09:40 PM by larryh

[Hybridbear]

Motor efficiency is about 85% when accelerating and is greater than 95% during regen.

Ford.com has said since before these cars were released that regen is greater than 90% efficient. Good work to get more precise data. I believe Ford claimed early on that their regen was more efficient than Toyota's.

[larryh]

I found a hill quite some distance away on a road with very little traffic.  I made measurements going up the hill and back down again.  The hill is about 0.8 miles long with an average grade of 4%, a typical hill one might encounter on a highway (unlike the steep one I considered previously).  I included a tenth of a mile in either direction in the measurements after the hill levels out at the top and bottom.  The hill has a shallow dip in the middle.  So you go half way up the hill, then descend a small distance, and continue on the way up to the top. 

 

Uphill Measurements

 

Method   Avg Speed HVB Energy

         (mph)     (kWh)

CC       39.3      0.48

 

CC       48.3      0.54

Manual   40.8      0.47

 

I used cruise control the first two times.  The third time I allowed the car to slow from 50 mph to 35 mph at the top and then sped back up again to 45 before the end of the trip.  The last trip used the least energy, slightly better than cruise control at 39.3 mph.  The average speed was slightly higher at 40.8 mph.

 

Downhill Measurements

 

Method   Avg Speed HVB Energy

         (mph)     (kWh)

CC in L  38.8      -0.05

CC in L  48.8      -0.02

CC in D  50.1      -0.01

Manual   50.6      -0.03

 

Using CC in while in Low at 38.8 mph produced the best results.  You would expect better results at slower speeds since there is less energy loss from friction. 

 

Using CC while in Low yields the most constant speed while going downhill.  The car adjusts regen (up to 35 kW) to maintain the speed set with CC.

 

Using CC while in Drive yields more variation in speed.  The car limits the max regen to 10 kW.  If the hill too steep, and 10 kW of regen is not sufficient to hold the car back, the car will speed up.

 

In the manual method, I limited regen to a fairly constant 3 kW by pressing the accelerator.  So the car sped up even more than it did using CC while in Drive going down the hill.  This method had the fastest average speed and yielded the second best results.  The reason this method was more efficient than the other two methods with an average speed around 50 mph, was because for the other two methods, the car consumed energy to speed back up again when coming out of the dip in the middle of the hill.  In the manual method, I maintained a constant 3 kW of regen throughout the entire dip and did not waste energy accelerating the car to get up out of the dip--I simply let the car slow down.  That was a lot of energy wasted to accelerate the car coming out of the dip only to continue regen down the remainder of the hill.

 

If you do it correctly, you can go up a hill and come back down again without using significantly more energy from the HVB than driving on a level road.  In fact, if you used the manual methods above you would use less energy from the HVB going up over the hill and down again than traveling on a level road with the same average speed.  If you blindly use CC, grade assist, or Low, you will end up using more energy than you would use on a level road.  However, if the hill is too long or steep, I think you will end up using a lot more energy than driving on a level road not matter what you do. 

Edited October 20, 2014 at 01:13 AM by larryh

[rbort]

Thanks Larry, this is great info. From what I gather reading all this if I'm in EV mode it sounds like cruise control less than 40 miles an hour with L gear on the hills is a pretty good deal. Keeps things simple and consistent over time you'll probably win out this way than trying to manually do it perfect to do better.

 

In my test going down my hill I did it seems to agree with you in the fact that low gear gave the most consistent speed and the most MPGe.

The only thing that I would change with cruise control is disabling it's going up the hill and letting the car lose speed until I get to the top, then leave in the power at one point five bars and waiting for the car to speed back up before we enabling CC again. Only do this though if battery is low and I need to get somewhere otherwise if this plenty of battery don't even worry about it.

 

I keep thinking I need to start a cheat sheet for this car with all the hints and tricks i've learned over the past year driving and figuring out things, plus reading here and such. Would be good to have all the pieces on information in one document.

 

-=>Raja.

[Hybridbear]

In the manual method, I limited regen to a fairly constant 3 kW by pressing the accelerator.

I'm going to be driving to Wisconsin for work this week so I'm very interested in maximizing my highway cruising efficiency on the rolling hills. How do you use the accelerator to maintain 3 kW of regen? If you just release both pedals is regen greater than 3 kW? What's the advantage of maintaining only 3 kW of regen versus 5 or 6, for example?

[larryh]

I'm going to be driving to Wisconsin for work this week so I'm very interested in maximizing my highway cruising efficiency on the rolling hills. How do you use the accelerator to maintain 3 kW of regen? If you just release both pedals is regen greater than 3 kW? What's the advantage of maintaining only 3 kW of regen versus 5 or 6, for example?

 

You control regen via the accelerator.  With your foot off the pedal, you get full regen during coasting (up to 10 kW in Drive).  If you press the accelerator lightly, you will get less regen.  As you press it more, eventually you will get the same effect as if you shifted the car into neutral, i.e. no regen and no power to the wheels.  Pressing the accelerator further provides power to the motor to propel the car.   Unfortunately, the car's display does not show regen.  You have to guess how much regen you are getting.  Until you press the accelerator far enough and the blue bar in the Empower screen shows the power level above 0, you are getting regen.  You need to use a scanner to see the amount of regen you are getting. 

[larryh]

The following plot shows Electrical Energy Losses for a segment of a commute.  This is the energy output from the HVB minus potential and kinetic energy changes plotted vs. distance.  You can recover potential and kinetic energy (via conversion between the two or via regen), so what is left is unrecoverable energy losses due to friction and conversion losses by the motor when converting electrical to mechanical energy. 

 

The blue line shows potential energy, i.e. elevation.  You can see the drive is mostly downhill.  The red line shows kinetic energy, i.e. speed.  I start out at 0 mph, make one stop at about 1.23 miles, and stop again at the end of the trip at 1.72 miles.

 

The green line shows the actual energy losses.  The purple line shows the energy losses due to friction alone.  It assumes an average motor efficiency of around 70% while traveling at constant speed on a level road.  The slope of the line increases with speed since there is greater friction with greater speeds, i.e. the slower you go, the less the energy loss due to friction.  You can see a slight decline in the slope at distance 1.23 miles when I stop because I am traveling at a slower average speed. 

 

The blue-green line shows the difference between the actual energy loss and the loss due to friction, which I have labeled Excess Energy Loss.  This is associated with acceleration and climbing hills (which does not happen much in this trip).  The motor has to supply the kinetic energy associated with an increase in speed, and since the motor is not 100% efficient, there is an electrical energy loss associated with acceleration.  The blue-green line is essentially showing this loss.  If I had gone at constant speed and there were no acceleration, the purple line and green line would coincide.  You want to avoid acceleration using the motor if at all possible (which means don't make unnecessary speed changes).  You can see the blue-green line increasing from distance 0.0 to 0.2 miles as the speed of the speed increases from 0 mph to 43 mph.  You can also see it increase from distance 1.23 to 1.3 miles as I again accelerate from a stop back up to speed. 

 

From distance 0.0 to 0.2 miles, the Excess Energy Loss increases from 0.0 to 0.023 kWh.  The kinetic energy increases from 0.0 to 0.104 kWh.  That means there was a 0.023/0.104 = 22% energy loss converting electrical to mechanical energy when accelerating.   From distance 1.23 to 1.3 miles, the Excess Energy Loss increases from 0.018 to 0.041 kWh, for an increase of 0.041 - 0.018 = 0.023 kWh--the same as at the beginning of the trip.  The kinetic energy increases from 0.0 kWh to 0.107 kWh (going slightly faster this time).  The electrical energy loss during acceleration is thus 0.023 / 0.107 = 21%, similar to before.  The two accelerations mean that I used 2*0.023 = 0.046 kWh more energy than I would have traveling at constant speed.  Note that the loss during regen is minimal since the motor is at least 95% efficient during regen. The blue-green line does not increase much during regen, i.e. stops or when descending a hill.

 

Normally, the blue-green Excess Energy Loss line associated with acceleration should only increase with distance since you cannot recover a loss.  However, it does fall a little at around 0.5 miles.  This is because the car is going up a slight incline.  The motor is more efficient when outputting greater power.  The slight fall in the blue-green line in simply indicating increased efficiency of the motor. 

 

Edited October 25, 2014 at 02:26 PM by larryh

[larryh]

The following plot is similar to the previous post for the same trip.  However, this time I allowed gravity to accelerate the car when going downhill rather than using power from the HVB.  From about 0.84 to 1.08 miles, during the downhill section, I allowed gravity to speed up the car from about 40 to 45 mph.  I slowed down to 40 mph prior to the hill rather than maintaining a constant speed of 45 mph as in the previous chart so I could use the hill to speed the car back up.  Similarly, from 1.27 to 1.47 miles I allowed gravity to speed up the car rather than using power from the HVB.  I accelerated from the stop sign at 1.23 miles in both cases.  But in this case, I only accelerated to about 30 mph using energy from the HVB rather than all the way up to 45 mph.  The hill did the rest.

 

I allowed the hills to speed up the car by reducing regen.  I switched to Low and pushed gently on the accelerator.  Make sure you don't press too hard or you will use energy from the HVB to increase the speed of the car.  See post 74.  I depressed the accelerator sufficiently so the car acted as if it were in neutral, i.e. 0 power going into or out of the HVB. 

 

From distance 1.23 to 1.55 miles, the Excess Energy Loss increases from 0.009 to 0.021 kWh, for an increase of 0.021 - 0.009 = 0.012 kWh--about half of the loss in the previous post.  This time, the energy loss to accelerate to 45 mph was only 0.012 / 0.108 = 11% as opposed to 21% in the previous post.  Conversion between potential and kinetic energy is 100% efficient.  There are no losses.  Conversion from electrical energy to potential or kinetic energy is not.  There are losses.

 

So for this trip, the overall Excess Energy Loss (above frictional energy losses) supplying the potential/kinetic energy from the HVB was 0.023 kWh.  In the previous post, it was 0.039 kWh.  So by using some of the potential energy from the hills to increase speed of the car (converting potential to kinetic energy), there was 0.039 - 0.023 = 0.016 kWh less loss, i.e. 40% less energy was lost. 

 

 

 

Edited October 25, 2014 at 05:56 PM by larryh