OBD II Data for ICE

[larryh]

I suspect that the Positive and Negative Split modes of operation are equally efficient and there is no overwhelming reason to choose one over the other. 

 

In Positive Split Mode, the generator is consuming mechanical power and producing electrical power.  The motor then converts that electrical power back to mechanical power.  The generator steals power from the ICE powering the wheels, but the electric motor adds that power back to the wheels.  If the generator and motor were 100% efficient, then the wheels are effectively receiving all the power the ICE is producing.

 

In Negative Split mode, the roles of the generator and motor are reversed.  Now the motor takes power from the ICE powering the wheels and generates electricity.  The generator then converts the electrical power back to mechanical power which effectively adds the power back to the wheels.  Again, if the generator and motor were 100% efficient, then the wheels are effectively receiving all the power the ICE is producing.

 

In either case, the motor or generator generating electricity can generate more electricity than the motor or generator that is consuming it, and the excess power can be used to charge the HVB.  So I'm not sure why in the MFT power flow screen, Positive Split mode is labeled as "Charging HV Battery" and Negative Split mode is labeled as "Hybrid Drive".  Either mode can charge the HVB. 

Edited April 27, 2014 at 10:31 PM by larryh

[larryh]

A schematic for the HF35 Powersplit Transaxle used in the Energi can be found here in this series of charts:

 

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2010/power_electronics/apearravt024_poet_2010_p.pdf

[Hybridbear]

Regenerative Braking

 

During regenerative braking, the ICE is usually off.  So again, the generator and motor rotate at the same RPM, but in opposite directions.  Only the motor is used to generate electricity.  Ideally, the generator is just spinning freely.  But in actuality, as in EV mode, the generator is resisting rotational speed due to friction and other losses, resulting in less power to the motor for regen. 

 

If the generator were to generate electricity during regenerative braking, since the RPMs are negative, the torque would have to be positive, slowing down the negative rotational speed.  But if the generator torque were positive, then ICE torque would have to be negative.  The ICE can only rotate forwards and provide positive torque.  The only other alternative is to lock the ICE and planet carrier driving the ICE in place so that they don’t rotate and the generator could function as a generator.  But that doesn’t appear to be what the car is doing. 

 

When the car is in neutral, the electrical connection to the motor and generator is broken.  Neither can consume nor generate electrical power.  When the ICE is off, I observe the torque at the generator to be solely a function of generator rpms and resisting the rotation, which is most likely due to the friction and other losses.  I observe the same torque at the generator as a function of generator rpms no matter what gear the car is in or whether I am applying the brakes or not.  If the car were using the generator during regenerative braking or while in Drive or Low, then the torque on the generator should be different from what it is in neutral, but this is not the case.

Have you tested regen with the hill descent button on the shifter engaged to see how that affects regen braking?

[larryh]

Have you tested regen with the hill descent button on the shifter engaged to see how that affects regen braking?

No.   I have only used hill assist to go down hills at a constant speed.  I have not generally applied the brakes with hill assist. 

[larryh]

It appears that given the current speed of the car and current power requirements (based on acceleration), the car is free to operate the ICE at any rpm and torque it chooses.  If the ICE does not produce enough power, it will come from the HVB.  If the ICE produces excess power, it is used to charge the HVB.  This is very different from normal cars where the rpm and torque of the engine are highly correlated with the current speed and acceleration.  I have observed this behavior in the winter when the car is warming up.  Regardless of speed or acceleration, the ICE operates at a constant 1500 rpm and within a limited torque range.  The car is free to jump to any location (within reason) that it wants to on the Engine Map that I have previously posted.  For a given power output, it can choose to operate anywhere on the appropriate line of constant power (the curved lines shown on the Engine Map).  So it is free to choose the most efficient operating point given the current demands of the car, something not possible with normal cars. 

 

Also, the current drive ratio of the car is determined as follows:

 

E / W = (M+G)/3.55/W = (10.394*W+G)/3.55/W = 2.91 + G/(3.55*W),

 

where the variables are defined as in the previous posts.  So when the generator rpm is positive, the drive ratio is greater than 2.91 (low gear).  When the generator rpm is negative, the drive ratio is less than 2.91 (high gear). 

Edited April 28, 2014 at 08:56 PM by larryh

[larryh]

Determining whether less gas will be consumed by constantly running the ICE in “Hybrid Drive”, or by repeatedly running the ICE in “Charging HV Battery” mode and then turning off the ICE while allowing EV operation to consume the energy generated while in “Charging HV Battery” mode is a very complex task. Hopefully, the car has been programmed to do the right thing.

 

Assume the car is going 66 mph on the freeway.   At that speed, the ICE is producing about 20 kW of power and consuming gas at a rate of about 0.027 gallons/minute.  So, the mileage is about 66/(60*.027) = 40.7 MPG.    At that speed, the ICE should be running at a very efficient operating point.  If we want to charge the HVB, the ICE will have to generate more power.  The best we can hope for is that the ICE will be equally efficient when generating the additional power. 

 

Suppose the ICE generates 10% more power.  If efficiency remains the same, it will use 10% more gas, i.e. 0.0297 gallons/minute.  If the efficiency of generating and storing electricity is 80%, then after one hour, we will have stored 0.8*(10%*20) = 1.6 kWh of electricity.    If we want the mileage of the car to be better if the car charges the HVB than when it doesn't, we need to exceed 40.7 MPG.  We will have used 60*0.0297 = 1.782 gallons of gas after one hour.  So we will need be able to travel 40.7*1.782 = 72.5 miles on that gas, i.e. we will have to go 72.53 - 66 = 6.53 miles with the ICE off using the energy stored in the HVB that was accumulated over the one hour the ICE was on. 

 

The equivalent gallons of gas for 1.6 kWh is 1.6/33.705 = 0.0475 gallons.  We will need to get at least 6.53 / 0.0475 = 137 MPGe in EV mode going 66 mph to exceed an overall average of 40.7 MPG.  The actual MPGe in EV mode going 66 mph is closer to 120 MPGe.  The car cannot generate electricity and power the motor from the accumulated electricity efficiently enough to achieve higher mileage. 

 

At slower speeds, the situation will most likely improve.  The ICE will be less likely to be operating at its most efficiency operating point, so charging the HVB would allow it to run at a more efficient operating point.  In addition, MPGe increases significantly with decreasing speed. 

Edited April 28, 2014 at 10:08 PM by larryh

[Hybridbear]

Assume the car is going 66 mph on the freeway.   At that speed, the ICE is producing about 20 kW of power and consuming gas at a rate of about 0.027 gallons/minute.  So, the mileage is about 66/(60*.027) = 40.7 MPG.    At that speed, the ICE should be running at a very efficient operating point.  If we want to charge the HVB, the ICE will have to generate more power.  The best we can hope for is that the ICE will be equally efficient when generating the additional power. 

 

Suppose the ICE generates 10% more power.  If efficiency remains the same, it will use 10% more gas, i.e. 0.0297 gallons/minute.  If the efficiency of generating and storing electricity is 80%, then after one hour, we will have stored 0.8*(10%*20) = 1.6 kWh of electricity.    If we want the mileage of the car to be better if the car charges the HVB than when it doesn't, we need to exceed 40.7 MPG.  We will have used 60*0.0297 = 1.782 gallons of gas after one hour.  So we will need be able to travel 40.7*1.782 = 72.5 miles on that gas, i.e. we will have to go 72.53 - 66 = 6.53 miles with the ICE off using the energy stored in the HVB that was accumulated over the one hour the ICE was on. 

 

The equivalent gallons of gas for 1.6 kWh is 1.6/33.705 = 0.0475 gallons.  We will need to get at least 6.53 / 0.0475 = 137 MPGe in EV mode going 66 mph to exceed an overall average of 40.7 MPG.  The actual MPGe in EV mode going 66 mph is closer to 120 MPGe.  The car cannot generate electricity and power the motor from the accumulated electricity efficiently enough to achieve higher mileage. 

 

At slower speeds, the situation will most likely improve.  The ICE will be less likely to be operating at its most efficiency operating point, so charging the HVB would allow it to run at a more efficient operating point.  In addition, MPGe increases significantly with decreasing speed. 

This is fantastic information! On the occasions where I've driven my parents' Energi I have tried to pay attention to the instant MPGe in different scenarios to get a feel for this kind of info because I can then apply it to the hybrid. I believe that part of getting good MPGs in the FFH or getting good MPGs in the Energi in hybrid mode/EV Later is about making the best use of your time spent in EV mode. In the FFH I try to never drive in EV mode when the demand exceeds 1 bar on the Empower screen because above that point your MPGe will drop precipitously. I also try to avoid accelerating in EV mode unless I'm only accelerating up to 25 MPH or less and I have a high SOC for the same reason.

 

If you were to selectively use your EV at points where you could get higher than 137 MPGe then you could do better with the ICE charging the HVB and then periodic stretches of EV. That reasoning must be what moved Ford to increase the EV threshold for the FFH/C-Max up to 85 MPH like the Energi instead of the original 62 MPH limit. They must have determined that people could get better MPGs at those high speeds by allowing the ICE to shut off at low power demands and use up some of the charge in the HVB and then using the ICE to charge the HVB later on.

[larryh]

Actually, if you go through the calculations, the MPGe threshold is:

 

MPGe threshold = 33.705*v/(P*e),

 

where v is the speed of the car, P is the power from the ICE in kW at the given speed when it is not charging the HVB, and e is the efficiency of the motor/generator.  So plugging in the numbers from my previous post:

 

MPGe threshold = 33.705 * 66 / (20*0.8) = 139, same as before, accounting for roundoff errors.

 

This assumes the ICE is as efficient when generating the extra power to charge the HVB than when it is only generating enough power to drive the wheels.  In general, that is not true,  It may be more or less efficient when generating more power to charge the HVB. 

Edited April 30, 2014 at 09:51 AM by larryh

[larryh]

The following chart shows the power being supplied to the HVB during a 60 mile commute as a function of the car's speed.  Negative power means the HVB is being charged.  As you can see, during most of the trip, the HVB was being charged.  A 240 V charger supplies about 3 kW of power to the HVB.  The maximum power for charging the HVB is 35 kW during regenerative braking. 

 

So it did not matter whether the car was operating in positive or negative split mode, or whether the power flow screen said "Hybrid Drive" or "Charging HV Battery", during most of the trip the car was charging the HVB when the ICE was on.   The motor supplied the majority of the electricity. 

 

If I compute the value for the MPGe threshold equation in the previous post, most of the time the threshold was below 140 MPGe, and a significant fraction of the time, indicative that it would be advantageous to charge the HVB. 

 

Edited May 2, 2014 at 02:53 PM by larryh

[lolder]

Really nice research. About the significance of "N"; regulations require a transmission selection position where the vehicle can not be powered. That is "N". Power is disconnected from the motor and from the generator except if you put the vehicle in "N" going downhill and the max EV ( generator with ICE off ) speed is exceeded, the generator will motorize the ICE to limit the generator speed. This is the case in 2010-12 models and I expect it to be the same in others to protect the generator from overspeed. The Prius plug-in has an additional planetary gear to accomplish this as their generator was not capable of as high a speed as Ford's.

[larryh]

Really nice research. About the significance of "N"; regulations require a transmission selection position where the vehicle can not be powered. That is "N". Power is disconnected from the motor and from the generator except if you put the vehicle in "N" going downhill and the max EV ( generator with ICE off ) speed is exceeded, the generator will motorize the ICE to limit the generator speed. This is the case in 2010-12 models and I expect it to be the same in others to protect the generator from overspeed. The Prius plug-in has an additional planetary gear to accomplish this as their generator was not capable of as high a speed as Ford's.

That brings up an interesting question.  How does the car implement deceleration fuel shutoff?  From what I have read, the ICE is connected to the planet carrier via an overrunning, one-way clutch.  So I'm not sure how the ICE could supply negative torque to slow the car down.

Edited May 7, 2014 at 12:48 AM by larryh

[murphy]

The computer stops injecting fuel.  One of the electric motors has the ability to spin the ICE to start it.  Feed the regenerated power to that motor and it turns the ICE which becomes an air compressor.  Compressing air uses up the extra energy.

[larryh]

I wonder how fast the ICE spins in deceleration fuel shutoff mode when providing negative torque to slow the car down. The generator must supply positive reaction torque in order for the ICE to transmit negative torque to the wheels to slow the car down. If the ICE is not spinning fast enough, the generator rpm will be negative. To supply positive torque, the generator will have to act as a generator to try and slow the rpm toward zero. But if the HVB is full, I don't think we want to do that. If the ICE is spinning fast enough, the generator rpm will be positive. Then the generator will have to act as a motor, consuming electricity from the HVB, to try and increase rpm for positive torque.

 

I have not observed the generator actively supplying positive torque. I have only observed positive torque generated passively in EV mode due to friction and other forces. Ideally in EV mode, the generator is spinning freely with negative rpm. But friction and other forces in the generator result in a drag on this negative rpm trying to reduce it towards 0, resulting in positive torque. The resulting positive torque is more than I would expect and is purely a function of generator speed. It results in losses of 1.5 kW or more. I haven't quite figured out what is going on.

 

The generator torque as a function of rpm is as follows, where y is the generator torque in Nm (positive) and x is the generator rpm (negative):

 

y = -1.3x/2000, -2000 <= x;

y = 2.5e-8x^2 – 0.0001x + 1, -2000 <= x <= -6000;

y = x/6000 + 3.5, -6000 <= x

 

It seems strange that torque is such a perfect piece-wise linear/parabolic function with round and non-arbitrary constants. It looks like the torque is being controlled by one of the control modules for some reason rather than spinning freely. This happens in EV mode in all gears.

Edited May 7, 2014 at 11:11 AM by larryh

[larryh]

For my recent 56.5 mile commute, MPG was 82.0.  During the commute, accessories consumed 0.724 kWh of electricity.  The ICE had to burn extra gas to provide this energy.  If I assume the energy content of each gallon of gas is 33.705 kWh and that the car is 33% efficient in converting the energy from the combustion of gas to electricity, then the car burned

 

0.724/33.705/.33 = 0.065 gallons of gas

 

to produce the energy required to power the accessories.  Had the accessories consumed no power, MPG would have been 90.4.  The hit on MPG by accessories was approximately 9.5%.

[lolder]

I don't think there is any clutch between the ICE and carriage gear in the 2010-12 FFH. In this model, when you brake down a long hill over max EV speed, once the HVB is full, I think the gen provides torque coupling from the wheels to the ICE to spin it and cause braking. The torque split is always the same determined by the CVT geometry but the power ( torque X RPM ) split is determined by the gen which essentially controls the CVT in addition to generating. Remember that the Gen can operate as a generator or motor in either direction to apply whatever torque is needed for system management.

Edited May 18, 2014 at 12:00 PM by lolder

[larryh]

For my recent 56.5 mile commute, MPG was 82.0.  During the commute, accessories consumed 0.724 kWh of electricity.  The ICE had to burn extra gas to provide this energy.  If I assume the energy content of each gallon of gas is 33.705 kWh and that the car is 33% efficient in converting the energy from the combustion of gas to electricity, then the car burned

 

0.724/33.705/.33 = 0.065 gallons of gas

 

to produce the energy required to power the accessories.  Had the accessories consumed no power, MPG would have been 90.4.  The hit on MPG by accessories was approximately 9.5%.

 

The following is a more complete summary of the trip:

 

The motor/generator consumed a net 5.10 kWh of energy from the HVB.  Accessories consumed 0.72 kWh.  That means 5.82 kWh of plug-in energy was used.  The ICE produced 8.33 kWh of mechanical energy.  It consumed 0.71 gallons of gas.  That means the overall efficiency of the ICE for the trip was 8.33/(33.705*.71) = 35%. 

 

The motor/generator generated 1.63 kWh of electricity powered by the ICE.  The motor/generator generated 1.39 kWh of electricity through regenerative braking.  Overall, they generated 3.01 kWh of electricity during the trip. 

 

The motor/generator consumed 3.09 kWh of mechanical power from the ICE, and 2.4 kWh of kinetic energy during regenerative braking. 

 

The motor/generator consumed a total of 7.39 kWh of electricity and produced 7.39 kWh of mechanical power.  It may look like 100% efficiency, but it was not.  I didn't separate out the mechanical power consumed during EV mode, when the ICE was on, and during regenerative braking. 

Edited May 18, 2014 at 05:25 PM by larryh

[larryh]

Assume the car is going 66 mph on the freeway.   At that speed, the ICE is producing about 20 kW of power and consuming gas at a rate of about 0.027 gallons/minute.  So, the mileage is about 66/(60*.027) = 40.7 MPG.    At that speed, the ICE should be running at a very efficient operating point.  If we want to charge the HVB, the ICE will have to generate more power.  The best we can hope for is that the ICE will be equally efficient when generating the additional power. 

 

Suppose the ICE generates 10% more power.  If efficiency remains the same, it will use 10% more gas, i.e. 0.0297 gallons/minute.  If the efficiency of generating and storing electricity is 80%, then after one hour, we will have stored 0.8*(10%*20) = 1.6 kWh of electricity.    If we want the mileage of the car to be better if the car charges the HVB than when it doesn't, we need to exceed 40.7 MPG.  We will have used 60*0.0297 = 1.782 gallons of gas after one hour.  So we will need be able to travel 40.7*1.782 = 72.5 miles on that gas, i.e. we will have to go 72.53 - 66 = 6.53 miles with the ICE off using the energy stored in the HVB that was accumulated over the one hour the ICE was on. 

 

The equivalent gallons of gas for 1.6 kWh is 1.6/33.705 = 0.0475 gallons.  We will need to get at least 6.53 / 0.0475 = 137 MPGe in EV mode going 66 mph to exceed an overall average of 40.7 MPG.  The actual MPGe in EV mode going 66 mph is closer to 120 MPGe.  The car cannot generate electricity and power the motor from the accumulated electricity efficiently enough to achieve higher mileage. 

 

At slower speeds, the situation will most likely improve.  The ICE will be less likely to be operating at its most efficiency operating point, so charging the HVB would allow it to run at a more efficient operating point.  In addition, MPGe increases significantly with decreasing speed. 

 

During my commute this morning, I tried forcing the ICE to charge the HVB and then after it had finished charging, run in EV mode to use up the accumulated charge.  I was traveling at 65 mph with cruise control on.  I forced it to run in EV mode by temporarily reducing cruise control speed and then restoring it back to 65 mph.  I did this for two cycles.  The MPG over these two cycles was 41.7.  Simply running with the ICE on the entire time on, the MPG in the past has been around 44.  As predicted, this technique does not appear to improve mileage at 65 mph. 

 

For each cycle, the HVB charge ranged from 3.5 kW to 3.7 kW.  The ICE was off for about 40 seconds each time.  When charging the HVB, the instantaneous ICE MPG started at around 25 MPG and gradually increased to 42 MPG.  ICE power started out at 30 kW and gradually reduced to 19 kW.  The power supplied to the HVB was about 10 kW when the ICE output was 30 kW, and reduced to less than 1 kW when the ICE output was 19 kW.   

 

At 30 kW, the ICE consumed 0.0863 gallons for each kWh of mechanical energy produced (34% efficiency).  At 19 kW, the ICE consumed 0.0832 gallons of gas for each kWh of mechanical energy produced (36% efficiency).  The ICE was about 4% more efficient producing power at 19 kW.

 

During the first cycle while the ICE was on, the ICE supplied 0.743 kWh of mechanical energy.  The generator produced 0.141 kWh of mechanical energy and the motor consumed 0.317 kWh of electrical energy.  This implies that 0.567 kWh of energy was required to propel the car.  The HVB received 0.142 kWh of electrical energy (if no power was consumed by the accessories).  Assuming the generator and motor are equally efficient, this also implies that they were 93% efficient in converting between mechanical and electrical energy. 

Edited May 23, 2014 at 10:04 PM by larryh

[jeff_h]

I was traveling at 65 mph with cruise control on.  I forced it to run in EV mode by temporarily reducing cruise control speed and then restoring it back to 65 mph.  I did this for two cycles.  The MPG over these two cycles was 41.7.  Simply running with the ICE on the entire time on, the MPG in the past has been around 44.  As predicted, this technique does not appear to improve mileage at 65 mph. 

 

 

No, but a variation of this is has helped me get the HVB recharged on those higher speeds to be more useful at lower speeds.  This is something I was experimenting with last month during a trip up to PA in my car, where I-83 between Baltimore MD and York PA has several uphills and downhills that are about 5-7% grades and .3-.6 miles long each way.  Here is what I found, though I don't have hard data to show as you normally have and display.

 

I drove until the HVB was down to 1 mile and then switched to EV-later to save that single mile (about 5%)... drove in EV-later and then when going down those good-size hills would switch back to auto at the top and the adaptive cruise would regen the HVB on the way down, and then when the regen was done I'd switch back over to EV-later.  Each hill would get about 2-8% (greatly depending on length of hill and other traffic) added to the HVB by the time I was done with the interstate I was back to almost 50%... then when I left the interstate and on the small 2-lane rolling roads, I had 10+ miles of EV and don't think it really hurt the MPG on the big interstate uphills nearly as much as it helped with the EV on the smaller roads.  So when I got back from that weekend trip and finally recharged the HVB at home, that's when the full charge showed an estimated 39 miles (posted in another thread here somewhere) since that one cycle theoretically contained a lot more ups and downs than the typical 20-26 or so miles showing when the HVB is charged (is my theory anyway).  So now I do that on my daily commute since there are a few good-size hills like that and also when the 60-mph traffic slows to 10-mph in the distance (there are predictable places where everyone's brake lights come on about 1/4 mile ahead) and that makes for some nice switch to auto and even regenerating -- so now most of my commutes result in the miles saying 30-32 when the HVB is full, and also in that 116 MPG on the 58-mile commute the other afternoon where 45 of those miles were in EV (pic posted in a different thread).  So others can sample with this and see if it works out for them, I'm always looking for ways to use less gas and I think it has improved my results.

[SteveEnergi]

I'm not sure you would need to switch to AUTO going down the hill.  The car will still regenerate in EV Later mode and fill the battery.

 

Steve

[jeff_h]

I'm not sure you would need to switch to AUTO going down the hill.  The car will still regenerate in EV Later mode and fill the battery.

 

I've tried it both ways, and leaving it in EV-later recharges the hybrid portion of the HVB (for lack of a better term), while going to auto puts it back into the larger portion of the HVB.  Last summer I started that thread about the HVB not being gone once it gets to 0 and how going down a long hill eventually started filling the big portion back up - but I noticed that the display showed the hybrid portion at max and stayed there for a while before finally busting into the larger portion.  So in sampling back and forth on commutes I've found the same thing, that leaving it in EV-later and doing regen seems to hit a plateau but switching to auto regens directly to the larger portion, and in trying each method I've found that using the EV switch back and forth yields the better MPG results.

[larryh]

EV later mode is a charge sustaining mode.  It attempts to maintain the battery within a predetermined range of state of charge based on when EV later mode was initiated.  It does whatever is required to keep the state of charge within that range.  It would be interesting to observe what the car does when it is at the top of the range and you are going down a long hill providing an opportunity for significant regen.  I don't have any hilly routes that I regularly drive on to investigate what happens. 

 

You would probably get better mileage if you didn't have any hills to begin with.  In general, you want the ICE to do all the hard work and reserve the HVB to power the motor for slower speeds.  As indicated in many of my posts, regen based on potential energy (hills) or kinetic energy (regenerative braking) is about 80% efficient.  It requires about twice the energy from the HVB going up a hill as you get from regen going down the hill.  Unless you can make it entirely in EV mode, you would want to use the ICE to go up hills.   In addition, you want to gather as much regen as possible on the down hills.  And you want to use the regen energy (and plug-in energy) during slower speed driving.   Which I believe is what you are doing.

 

You could also try leaving the car in auto mode while going up the hills with the ICE on.  In auto mode it made attempt to do more opportunistic charging of the HVB while going up the hill rather than in EV later mode.  You will have to monitor the Engage screen to see if it is using the Motor to assist the ICE rather than to charge the HVB. 

Edited May 23, 2014 at 10:02 PM by larryh

[larryh]

You would probably get better mileage if you didn't have any hills to begin with. 

 

Actually, you may get better MPG on a hilly route. 

 

Assume the car is traveling at 65 mph, the car and contents weigh 1875 kg, and that there is a 2 mile long uphill and 2 mile long downhill at 5% grade.  The elevation change is then 2*5280*5% = 528 feet or 161 meters.  The potential energy difference between the top and bottom of the hill is 9.81*161*1875 = 2.96e6 J, or 0.82 kWh.  If we assume that the car is about 75% efficient in regen, then the car will capture about 0.62 kWh of that potential energy when going downhill.  At 65 mph, the MPGe is about 118, so that 0.62 kWh of energy from regen will power the car for about 2.2 miles in EV mode.

 

The ICE will be on for only the first 2 miles.  At 65 mph, the car normally gets about 44 MPG.  So it will consume 2/44 = 0.0455 gallons of gas for the first two miles if the road were level.  However, the ICE must provide an additional 0.82 kWh of energy to get up the hill.  Assuming the ICE is 35% efficient, it will use 0.82 / 33.705 / 0.35 = 0.0695 additional gallons of gas to get up the hill.  The total fuel consumption will then be 0.1150 gallons.  On that 0.1150 gallons, the car will have traveled 2 miles up the hill, 2 miles down the hill, and then 2.2 miles in EV mode.   So the overall MPG is 53.6, which exceeds the normal 44 MPG that the car gets going 65 mph on a level road.

 

I would have to make actual measurements of the regen efficiency on such a downhill to verify my assumptions.  75% may be too high. 

 

In your case, you are saving the accumulated charge in the HVB until slower speeds.  That should produce even greater MPG. 

 

Note that during the downhill, the car will be generating power at the rate of 0.62 / (2/65) = 20 kW for 1.8 minutes. 

 

Using hills for regen seems to be much more efficient than having the ICE run the generator directly to generate electricity.  With hill regen, the ICE is on 1.8 minutes generating an additional 20 kW of power to climb the hill.  Then it is off 1.8 minutes during the descent down the hill while regen is taking place.  Then the car goes in EV mode for about 2.2 miles.  If the ICE runs the generator directly, it would supply an additional 20 kW of power for 1.8 minutes, and then run in EV mode for about 2.2 miles.  The ICE does not get the additional 1.8 minutes of rest during the descent down the hill.  The ICE is on significantly longer when directly running the generator vs. hill regen.  They basically accomplish the same thing, but when the ICE powers the generator directly, the ICE runs for a significantly larger fraction of the time.  That is why hill regen is much more effective.  So find the hilliest possible route for your commute. 

Edited May 24, 2014 at 05:42 PM by larryh

[larryh]

Looking at a 2% grade hill for about 0.8 miles, I see a regen efficiency of about 50%--it is difficult to tell since GPS altitude is not very accurate.  However, the ICE efficiency appears to be 31% for the uphill climb.  If the ICE efficiency for the climb drops from 35% to 31% and regen is only 50%, then the mileage is going to be worse with the hill than if there were no hill. The MPG was 45 with the hill vs 53 without the hill.

Edited May 24, 2014 at 10:50 PM by larryh

[larryh]

Using Google Earth I can get precise elevation data.  The change in elevation was 183 feet over 0.77 miles, or a grade of 4.5%.  The actual regen efficiency was only 31%.  The fuel consumed to go back up the hill was 0.040 gallons.  The amount of energy added to the HVB was 0.082 kWh.  MPGe at 55 mph is typically 145.  So I can go an additional 0.35 miles from the energy stored during regen.  The MPG is then (2*0.77 + 0.35)/.04 = 47 MPG.  Normally at 55 mph, I get 53 MPG. 

 

The ICE produced 0.415 kWh of energy to climb the hill.  So the ICE efficiency was about 31%.

 

In order for the car to get better MPG with hills than without hills, the regen efficiency has to be significantly better than 31%.  A steeper downhill would help. 

Edited May 25, 2014 at 12:11 AM by larryh

[larryh]

I examined the efficiency of the ICE in generating electricity for my weekend commute the other day.  I looked at two sections where the ICE was charging the HVB while traveling at 65 mph along relatively level sections of the freeway.  I plotted ICE power vs. power supplied to the HVB (the electrical power generated by the motor).  The results are as follows:

 

y = -0.7365x + 11.21 , R^2 = 0.7406

y = -0.7821x + 13.499, R^2 = 0.8133

 

y is the power (in kW) supplied to the HVB battery.  y is negative if power is being applied to the HVB.  x is the power (in kW) output from the ICE.  R^2 is a measure of how well the equations above fit the data.  1.0 is a perfect fit.  0.0 means no correlation.

 

That implies the efficiency of the motor in generating electric using power from the ICE is between 73.6% and 78.2%.  If the ICE were not generating any power for the HVB, then the power required from the ICE to propel the car at 65 MPH for the above equations would be:

 

11.21/0.7365 = 15.2 kW,

13.499/0.7821 = 17.3 kW.