Efficient EV Mode Driving Techniques

[Fat Fusion]

In general, the faster you stop, without using the friction brakes, the more regen you get.  Your average speed will be less when you stop faster, and hence, the energy consumed by friction (aerodynamic drag, tire rolling resistance, internal frictions, transmission losses, etc.) will be less.  

 

Don't you mean the opposite here?  The faster you stop, the longer you're waiting to stop, which means the longer you are driving at a higher average speed, right?

[larryh]

Don't you mean the opposite here?  The faster you stop, the longer you're waiting to stop, which means the longer you are driving at a higher average speed, right?

 

My statement was not well worded.  I was trying to be concise, but ended up being too concise.  In this case, I am measuring the amount of energy loss due to friction starting from the time the brakes are pressed until the car has stopped.  I am not considering the additional energy required to maintain speed longer while waiting to begin the stop. 

 

So a fast stop might require 20 seconds to come to a complete stop and slow stop might take 60 seconds.  At all times, after pressing the brake, you will be going slower for the fast stop than the slow stop.  Hence, there will be less energy lost due to friction for the fast stop, which is then available for regen.  After 20 seconds, you will have come to a complete stop with the fast stop and will no longer be losing energy due to friction.   After 20 seconds for the slow stop, you will have only slowed down slightly and will still be losing a significant amount of energy due to friction.   The longer it takes to stop, the more energy loss due to friction during the stop, which is not available for regen. 

 

For the fast stop, you might recover up to 80% of the kinetic energy during regen.  For the stop stop, you might only recover 45%.  The difference is due to the greater energy loss from friction during the slow stop. 

Edited October 12, 2014 at 11:03 AM by larryh

[Rexracer]

When resuming speed after the stop sign, you want to accelerate slowly.  The faster you accelerate, the more energy consumed.  If you accelerate faster, your average speed will be faster, and hence more energy will be required to overcome friction.  In addition, the motor operates more efficiently during acceleration than when maintaining constant speed.  By accelerating slowly, the motor will operate more efficiently longer.

 

When cruising, you want to maintain constant speed.  You don't want to unnecessarily speed up or slow down.   Mechanical energy from the motor is lost during acceleration and deceleration. 

 

The final thing to note is that the car operates more efficiently when it is warmed up versus when it is cold.  When you first start the car in cold weather, it will require 65% or more power to overcome friction than when it is warmed up.  So you want to keep the car warm. 

 

Maintain constant speed: This goes against the hyper-miler "Pulse/glide" method that they proclaim to. I agree with your research, as the pulse/glide has never made sense to me

http://www.instructables.com/id/How-to-Become-a-Hypermiler/step6/Pulse-and-Glide/

"This method has been proven many times over"

 

One technique that did seem to work in my old Focus, was when pulling away from a stop sign, is to start in 2nd and to floor it (with manual transmission), but short shifting, always keeping the engine below 2k RPM. Theory being that by flooring it, your telling the engine to create as much power as possible, but by keeping the RPM, your never using much fuel. Maybe it was the fact that i was effectively always accelerating slowly but I did notice improved mpg. And maybe it was when using it I was more aware of all my driving.

 

 

 

So a fast stop might require 20 seconds to come to a complete stop and slow stop might take 60 seconds.  At all times, after pressing the brake, you will be going slower for the fast stop than the slow stop.  Hence, there will be less energy lost due to friction for the fast stop, which is then available for regen.  After 20 seconds, you will have come to a complete stop with the fast stop and will no longer be losing energy due to friction.   After 20 seconds for the slow stop, you will have only slowed down slightly and will still be losing a significant amount of energy due to friction.   The longer it takes to stop, the more energy loss due to friction during the stop, which is not available for regen. 

 

For the fast stop, you might recover up to 80% of the kinetic energy during regen.  For the stop stop, you might only recover 45%.  The difference is due to the greater energy loss from friction during the slow stop. 

The way I read this, was what you describe here, but then later in your earlier post you say once you add in the energy to maintain that speed longer (the extra 40 seconds) you are now using more power then if you had just done the slow stop.

Am I reading that correctly?

 

On the going down hill, is the energy loss due to the vehicles in-ability to recover the energy as quickly as needed?

 

And thanks for the simplified break down.

[Hybridbear]

When the car is cold, the internal friction is significantly higher than when warm.  The following table shows the motor power output to propel the car at 20 mph vs. transmission fluid temperature (TFT) in degrees Fahrenheit during a short trip.  The outside temperature was 30 F.

 

TFT (F) Motor Power (kW)

41      2.32

46      2.00

50      1.78

53      1.78

82      1.42

 

So at the start of the trip, the power required by the motor was 65% greater than at the end.

That's really fascinating. This must be a very viscous fluid then!

[HotLap]

larryh, wonderful analysis, you must be a Calculus or Algebra professor? :)  JK, but thanks, your research makes sense and I'm going to have to re-read several times to pick up on the fine points, but my takeaways (so far) are:

 

1) Maintaining a constant speed uses less HBV power than accelerating/decelerating

2) Driving at a faster top speed uses more power than at a slower speed (common sense I know, but there it is)

3) Regen is not as efficient as maintaining a constant speed, (an example would be if you are approaching a traffic light try and gradually slow and make the light vs. approaching the light at full speed and using the regen brakes)

4) Low gear regen can <sometimes> provide more regen, than slowing in Drive but not in all cases...

[larryh]

Maintain constant speed: This goes against the hyper-miler "Pulse/glide" method that they proclaim to. I agree with your research, as the pulse/glide has never made sense to me

http://www.instructables.com/id/How-to-Become-a-Hypermiler/step6/Pulse-and-Glide/

"This method has been proven many times over"

 

One technique that did seem to work in my old Focus, was when pulling away from a stop sign, is to start in 2nd and to floor it (with manual transmission), but short shifting, always keeping the engine below 2k RPM. Theory being that by flooring it, your telling the engine to create as much power as possible, but by keeping the RPM, your never using much fuel. Maybe it was the fact that i was effectively always accelerating slowly but I did notice improved mpg. And maybe it was when using it I was more aware of all my driving.

 

 

 

The way I read this, was what you describe here, but then later in your earlier post you say once you add in the energy to maintain that speed longer (the extra 40 seconds) you are now using more power then if you had just done the slow stop.

Am I reading that correctly?

 

On the going down hill, is the energy loss due to the vehicles in-ability to recover the energy as quickly as needed?

 

And thanks for the simplified break down.

 

Pulse and Glide (P&G) requires that you shift into neutral during the glide.  If you don't shift into neutral, you will get very poor results--see post #18.  You can also see the posts I have made regarding P&G starting at:  http://www.fordfusionenergiforum.com/topic/1683-obd-ii-data-for-hvb/?p=15551.

 

P&G relies on the fact that the motor operates more efficiently during acceleration than cruising at constant speed.  Motor efficiency might be around 84% during acceleration and 72% for constant speed.  So the motor is 84% / 74% = 1.14 times more efficient.  Unfortunately, as seen in post #18, their is a severe penalty for accelerating.  You may gain 14% in motor efficiency, but you also lose about 10% of the energy used to accelerate in the drivetrain components during acceleration.  The net improvement might be around 5% if you are lucky.  It takes skill and experience to use P&G effectively.  Quite likely you will end up with worse mileage than maintaining constant speed.

 

Yes, you will end up using more energy from the HVB by waiting to brake.  You may recover 80% of the kinetic energy with fast braking vs. 45% with slow braking.  However, maintaining speed until closer to the stop sign requires more energy than you will have gained through fast braking.

 

When going downhill, close to 100% of the mechanical energy applied to the motor is converted to electrical energy.  I measured it at 101%--obviously there is some measurement error.  The problem is that a significant fraction of the potential energy difference from descending the hill is not making it to the motor for some reason.  I don't know why. 

 

So far I have only discussed EV driving.  I'm not sure what the best way to accelerate is when using the ICE. 

Edited October 13, 2014 at 10:49 PM by larryh

[larryh]

That's really fascinating. This must be a very viscous fluid then!

 

During the winter, it requires about 2.5 kWh of electricity for my 8 mile commute to work in subzero temperatures.  During the summer, it takes 1.6 kWh.  So it takes 56% more electricity in the cold.  I think much of that additional energy is due to the additional drivetrain frictions when cold.  Only a fraction of it is from higher aerodynamic drag and increased rolling resistance with colder temperatures.

 

I suspect that if I had a heated garage, it would take much less energy.  I tried using the Engine Block Heater (EBH), but that did not warm up the transmission fluid very much.  You need a heated garage in the winter.   If your commute is longer than the EV range, a good strategy might be to start the ICE at the beginning of the commute to warm up the car and reduce the internal drivetrain frictions.

 

[larryh]

larryh, wonderful analysis, you must be a Calculus or Algebra professor? :)  JK, but thanks, your research makes sense and I'm going to have to re-read several times to pick up on the fine points, but my takeaways (so far) are:

 

1) Maintaining a constant speed uses less HBV power than accelerating/decelerating

2) Driving at a faster top speed uses more power than at a slower speed (common sense I know, but there it is)

3) Regen is not as efficient as maintaining a constant speed, (an example would be if you are approaching a traffic light try and gradually slow and make the light vs. approaching the light at full speed and using the regen brakes)

4) Low gear regen can <sometimes> provide more regen, than slowing in Drive but not in all cases...

 

I am a software engineer, but I do have undergraduate degrees in math and physics so I should be able to figure figure out basic mechanics.  I only use low when I intentionally want to slow down quickly and recover as much regen as possible.  Yes there is a lot to digest, but it should help improve driving efficiency.

 

You should try to achieve the highest driving score possible via the driving coach.  It is giving correct advice on how to improve driving efficiency.  In city driving, you should be able to achieve scores in the mid 90's.  For my commute to work on city streets with a speed limit of 55 mph, my score is usually 94 - 97.  If you have highway driving, you are going to be penalized for high speeds and your score will not make it to the 90's. 

Edited October 13, 2014 at 10:59 PM by larryh

[SteveEnergi]

For some reason, regen going downhill seems to be significantly less efficient than regen on level surfaces.  I have no idea why.  I have repeated the measurements many times on different hills and always come up with the same results.  Going uphill, energy losses are in line with what I expect--the same as driving on a level road.  But going downhill, I observe more than double the expected losses.

 

The hill I am considering in this post has an elevation change of 264 feet and an average grade of 8-9%.   I am using 1 meter resolution LiDAR Data to determine the elevation of the road with an elevation accuracy of better than 0.1 meters.  The Potential Energy errors in my measurements should be very small.  I use cruise control to go up the hill and grade assist to go downhill, both at 30 mph.

 

The motor mechanical power loss when going uphill is 3.07 kW (about 13%), which is what I would expect for 30 mph on a level road at 45 F due to friction.  The power loss when going down hill is 6.79 kW (about 32%), more than twice as much.

 

Considering the middle portion of the hill where speed was constant for both the down and up hill trips, the change in Potential Energy was 0.28 kWh.  During the uphill, the motor output 0.31 kWh of energy.  Thus there was a loss of 0.03 kWh.  During the downhill, the motor was supplied with 0.19 kWh of energy.  Thus there was a loss of 0.09 kWh of energy. 

 

The large loss of regen during downhills is going to make driving on a hilly road significantly less efficient than a level road if you maintain constant speed.  In this instance, it requires 35% more energy to go up and down the hill vs. driving on a level road. 

 

 

Thanks for all of the great info.  My question is have you tried the downhill testing using Low rather than hill assist?  Further down in the posts you mentioned using Low to slow more quickly and recapture as much energy as possible (this is what I tend to do when I having too much speed coming into a sudden stop).  Maybe Low recaptures the energy better than hill assist?

 

Steve

[larryh]

I would expect grade assist and low to work about the same going down hill.  But I can try it out and see if there is a difference. 

[Fat Fusion]

I've determined that the ECO Cruise is the 2nd biggest waste of energy behind speed!  Seriously, if I get behind someone on the freeway they inevitably start speeding up and slowing down, over and over.  The ECO Cruise tries to maintain a certain distance instead of a constantly expanding/contracting distance.  I end up turning it off and maintaining a fairly constant speed that doesn't try to match the car in front of me.

[Hybridbear]

Fat Fusion, on 14 Oct 2014 - 6:01 PM, said:

I've determined that the ECO Cruise is the 2nd biggest waste of energy behind speed!  Seriously, if I get behind someone on the freeway they inevitably start speeding up and slowing down, over and over.  The ECO Cruise tries to maintain a certain distance instead of a constantly expanding/contracting distance.  I end up turning it off and maintaining a fairly constant speed that doesn't try to match the car in front of me.

Adaptive Cruise I think you mean. I agree about it. I use it selectively for that reason.

 

larryh, on 13 Oct 2014 - 5:17 PM, said:

During the winter, it requires about 2.5 kWh of electricity for my 8 mile commute to work in subzero temperatures.  During the summer, it takes 1.6 kWh.  So it takes 56% more electricity in the cold.  I think much of that additional energy is due to the additional drivetrain frictions when cold.  Only a fraction of it is from higher aerodynamic drag and increased rolling resistance with colder temperatures.

 

I suspect that if I had a heated garage, it would take much less energy.  I tried using the Engine Block Heater (EBH), but that did not warm up the transmission fluid very much.  You need a heated garage in the winter.   If your commute is longer than the EV range, a good strategy might be to start the ICE at the beginning of the commute to warm up the car and reduce the internal drivetrain frictions.

Having a heated garage which is heated to 55 F in the winter I'll have to see what sort of changes we see. I'll start paying more attention to TFT in the future. I have noticed for a frequent 6 mile trip that we make that our trips home often have higher consumption. With current temps this is likely due to drivetrain friction with the colder temps. The TFT is always 70+ when leaving home but when coming home we might see the TFT as low as 50 F in these recent weeks. Early in September when we first got the Energi there was no noticeable difference in consumption with TFT temps 70+ all the time.

[Fat Fusion]

Adaptive Cruise I think you mean. I agree about it. I use it selectively for that reason.

 

Having a heated garage which is heated to 55 F in the winter I'll have to see what sort of changes we see. I'll start paying more attention to TFT in the future. I have noticed for a frequent 6 mile trip that we make that our trips home often have higher consumption. With current temps this is likely due to drivetrain friction with the colder temps. The TFT is always 70+ when leaving home but when coming home we might see the TFT as low as 50 F in these recent weeks. Early in September when we first got the Energi there was no noticeable difference in consumption with TFT temps 70+ all the time.

 

Yes, Adaptive Cruise.  Thanks for clarifying that.

[rbort]

Larry, good stuff!

 

If you shift to L doing down hill, the difference between that and hill assist is that hill assist is "variable L" where L is full power L.  What would happen in L is that the car will go to max regen and slow down as much as possible.  Sometimes the hill is too steep for the car to slow down, but if its shallow enough you will be slowing down constantly in L.

 

In hill assist the L strength will be adjusted so that if the hill is shallow enough then you will maintain your speed and not slow down.  If the hill is too steep then hill assist will go to max regen power which would be equal to L.

 

-=>Raja.

[larryh]

Larry, good stuff!

 

If you shift to L doing down hill, the difference between that and hill assist is that hill assist is "variable L" where L is full power L.  What would happen in L is that the car will go to max regen and slow down as much as possible.  Sometimes the hill is too steep for the car to slow down, but if its shallow enough you will be slowing down constantly in L.

 

In hill assist the L strength will be adjusted so that if the hill is shallow enough then you will maintain your speed and not slow down.  If the hill is too steep then hill assist will go to max regen power which would be equal to L.

 

-=>Raja.

 

You need to set cruise control when driving in L down a hill.  That will maintain constant speed down the hill similar to grade assist.  You will average slightly slower speed down the hill (and get slightly more regen) using L and cruise control than with grade assist. 

 

Edited October 15, 2014 at 08:49 AM by larryh

[larryh]

I do not have adaptive cruise control.  I manually use the steering wheel controls to adjust cruise control and maintain distance to the cars in front of me. 

Edited October 15, 2014 at 09:10 AM by larryh

[larryh]

Having a heated garage which is heated to 55 F in the winter I'll have to see what sort of changes we see. I'll start paying more attention to TFT in the future. I have noticed for a frequent 6 mile trip that we make that our trips home often have higher consumption. With current temps this is likely due to drivetrain friction with the colder temps. The TFT is always 70+ when leaving home but when coming home we might see the TFT as low as 50 F in these recent weeks. Early in September when we first got the Energi there was no noticeable difference in consumption with TFT temps 70+ all the time.

Make sure you are taking elevation into account when comparing the outbound vs. return trip energy consumption. Go to this web site to determine the origin and destination elevations using the elevation tools: http://arcgis.dnr.state.mn.us/maps/mntopo/. The site provides elevation accurate to better than 0.1 meters. You can use it to generate an elevation profile for a trip. If the destination elevation is greater than the origin elevation, then an additional m*g*h Joules of energy is required, where m = mass of car in kg (I use 1871 kg), g = 9.81 (Earth’s gravitational constant), and h is the elevation change in meters. To convert Joules to kWh, divide by 3600 seconds in a hour and by 1000 watts in a kilowatt.

Edited October 15, 2014 at 05:10 PM by larryh

[rbort]

That elevation site seems to only work in the area it starts in.  I tried it in MA but there is no data.  I tried it even in Indiana nearby and it seems to have no data either.

 

Is it only good in MN?

 

As far as hills are concerned, when you climb a hill you would like to lose some speed you indicated on the up side, and then on the downside gain the speed back.  Do you want to let the car "run away" on the down side and get excess speed and then slow back down to the cruise speed or do you want to try to hold it at the cruise speed by shifting into L going down the hill?  

 

So to lay it out, the questions are:

 

If not on cruise control, do what?

 

a) climb hill at 1.8 to 2 bars and let the car lose speed, maybe from 35 down to 25 and then on the other side of the hill stay at 1.8 to 2 bars until the car gets back to 30 and then get off the throttle and hope it gains back to 35 and not overshoot.

 

b) climb the hill at 2.2 bars and let the car lose speed maybe from 35 down to 30, and then do the same on the other side?  Better to lose more or less speed?  I think anything more than 2.2 bars and you start getting dinged in the acceleration coach.

 

If on cruise control then,

 

Let the car climb the hill by itself, lose whatever speed it does, and on the downhill side, do you:

 

a) do nothing let the car run away faster than cruise speed and slow back down by itself before drawing from the hwb

b) put hill assist on and let the car get down the hill maybe gain a little speed and disengage when close to the bottom.

c) shift the car into L and back into D close to the bottom of the hill.

 

What works best and at any point of these options down the hill, do you disengage cruise so you don't overshoot on the speed?

 

-=>Raja.

Edited October 15, 2014 at 02:24 PM by rbort

[Hybridbear]

larryh, on 15 Oct 2014 - 07:37 AM, said:

Make sure you are taking elevation into account when comparing the outbound vs. return trip energy consumption. Go to this web site to determine the origin and destination elevations using the elevation tools: http://arcgis.dnr.state.mn.us/maps/mntopo/. The site provides elevation accurate to better than 0.1 meters. You can use it to generate an elevation profile for a trip. If the destination elevation is greater than the origin elevation, then an additional m*g*h Joules of energy is required, where m = mass of car in kg (I use 1871 kg), g = 9.81 (Earth’s gravitational constant), and h is the elevation change in meters. To convert Joules to kWh, divide by 3600 seconds in a hour and by 1000 watts in a kilowatt.

Starting elevation: 838 feet (848 reported by site, car parked one story underground or about 10 feet lower than reported)

Ending elevation: 838 feet

 

The link didn't work for me. Here is one that should. Thanks Larry for sharing the great link!

[larryh]

That elevation site seems to only work in the area it starts in.  I tried it in MA but there is no data.  I tried it even in Indiana nearby and it seems to have no data either.

 

Is it only good in MN?

 

As far as hills are concerned, when you climb a hill you would like to lose some speed you indicated on the up side, and then on the downside gain the speed back.  Do you want to let the car "run away" on the down side and get excess speed and then slow back down to the cruise speed or do you want to try to hold it at the cruise speed by shifting into L going down the hill?  

 

So to lay it out, the questions are:

 

If not on cruise control, do what?

 

a) climb hill at 1.8 to 2 bars and let the car lose speed, maybe from 35 down to 25 and then on the other side of the hill stay at 1.8 to 2 bars until the car gets back to 30 and then get off the throttle and hope it gains back to 35 and not overshoot.

 

b) climb the hill at 2.2 bars and let the car lose speed maybe from 35 down to 30, and then do the same on the other side?  Better to lose more or less speed?  I think anything more than 2.2 bars and you start getting dinged in the acceleration coach.

 

If on cruise control then,

 

Let the car climb the hill by itself, lose whatever speed it does, and on the downhill side, do you:

 

a) do nothing let the car run away faster than cruise speed and slow back down by itself before drawing from the hwb

b) put hill assist on and let the car get down the hill maybe gain a little speed and disengage when close to the bottom.

c) shift the car into L and back into D close to the bottom of the hill.

 

What works best and at any point of these options down the hill, do you disengage cruise so you don't overshoot on the speed?

 

-=>Raja.

 

That site only has elevation data for Minnesota.   I don't know if any other states have LiDAR data available.  The elevation data provided by Google Earth is not very accurate.

 

I do not have enough experience with hills yet to answer those questions.

Edited October 15, 2014 at 05:13 PM by larryh

[stevedebi]

One thing to note is that the engine always spins (but does not use any fuel) in "L". On the grade assist it does not spin when the battery is being charged.

 

I've not tried to measure the difference between grade assist and "L". I generally use L when I need to slow down suddenly. I never use either unless the battery is showing charge (^).

[larryh]

One thing to note is that the engine always spins (but does not use any fuel) in "L". On the grade assist it does not spin when the battery is being charged.

 

I've not tried to measure the difference between grade assist and "L". I generally use L when I need to slow down suddenly. I never use either unless the battery is showing charge (^).

 

I would not expect the ICE to be used to slow the car down unless regen exceeds 35 kW or the HVB is full.

[rbort]

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.

Edited October 15, 2014 at 09:50 PM by rbort

[rbort]

Larry, there is a hill (the steep one) about 3 miles from my house.  My car is charging back up now from a trip to HD, the post office and back.  I may take a ride to it and try to climb it like 5 to 10 times and see if the different methods yield different results.  I don't have a SG but I will look at the battery % level.  Not sure if I should power off and look at the trip meter for mpge numbers after each try or just go around and around and look at the battery display.  MPGe will be another good indicator I think.

 

-=>Raja.

[larryh]

Use the trip odometers and observe MPGe. Record the results for each method.  You might want to reset the trip odometer at the beginning of each method.   But I think the differences may be small and hard to detect.  You need to make sure you execute each method very precisely. 

Edited October 15, 2014 at 10:23 PM by larryh