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Ford C-Max vs Prius after 1 year

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#41 OFFLINE   stevedebi

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Posted 06 November 2014 - 07:09 PM

Correct CVT infinitely variable within hardware constraints (includes MG1 and MG2 rpm constraints and the final drive gearing.  The best one could do (tallest overall gearing) is to put a holding torque on MG1 so that it does not spin when the engine is running.  Then, all engine rpms would be directed to the output shaft.  So, a numerically lower final drive ratio would yield lower engine rpm than a higher final drive ratio at the same vehicle speed. For highway cruising one wants to run ICE at the most efficient area of the Brake Specific Fuel Consumption map which is generally low rpm, high torque.  It's like up-shifting a  conventional transmission to a higher overdrive gear to lower engine rpm to improve FE.  This should be achieved in Negative-Split Mode Operation (from the Ford OBD System Operation Summary for Plug-in and Hybrid Vehicles):

 

 

Prior to a PCM update of the Hybrid to increase the EV top speed from 62 mph to 85 mph, it was quite easy to get into negative split mode  by controlling the throttle and to stay in this mode for some time.  With EV operations now up to 85 mph, it harder to get into the mode (keeping the battery charged near its upper limit) as any slight decrease in power requirements can trigger EV operation at higher speeds.  

 

The different final drive ratio for the hybrid and NRG supposedly was to improve performance of the NRG given it's extra weight (larger HVB) such that both vehicles would be very similar performance wise.  My guess is the PCM algorithms for both the NRG and Hybrid are the same once the NRG enters Hybrid mode operation.  So, by numerically increasing the NRG final drive over the Hybrid ratio, the NRGs low end performance would be improved somewhat. 

From what I can see the Negative Split Mode is bleeding off excess energy because the battery is full? It says "the generator consumes electrical energy". Also, they list a "parallel mode", but don't describe it with diagrams and an explanation such as they do for the Split, Series, and Electric modes.

 

I notice that the document diagrams of the Negative and Positive Split modes are identical from what I can tell.









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#42 OFFLINE   Plus 3 Golfer

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Posted 06 November 2014 - 09:42 PM

From what I can see the Negative Split Mode is bleeding off excess energy because the battery is full? It says "the generator consumes electrical energy". Also, they list a "parallel mode", but don't describe it with diagrams and an explanation such as they do for the Split, Series, and Electric modes.

 

I notice that the document diagrams of the Negative and Positive Split modes are identical from what I can tell.

I've monitored HVB State of Charge while in negative split mode and the SOC stays virtually the same. MG2 can generate the energy required to run MG1 as a motor with virtually no change in SOC.  See this post.  Also, attached is a good pdf on Hybrid transmissions.

 

This is how I summarize the various modes:

 

Series - speed is zero and engine is on.

Split - Positive - engine is on, engine power is split between charging battery via the generator and driving the car

Split - Negative - engine is on, engine power is split between the generator consuming power and driving the car

Parallel - Generator is stopped (not spinning), engine power and battery assist power (traction motor) driving vehicle.

 

The point I was making is that when ICE has to run because the HVB is above a threshold level, there is a physical lower rpm limit and it can be reached when MG1 acts as a motor slowing down ICE rpm.   A numerically lower final drive ratio should allow ICE to operate at a lower rpm and likely increase FE at highway speeds (even though MG1 may be consuming power). There is simply no other way that I am aware of slowing ICE rpm to put ICE at higher efficiency with the hybrid split transmission.   I think many of us were hoping that Ford would lower the final drive ratio and that might allow an improvement in highway FE perhaps at the expense of a drop in performance.  

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#43 OFFLINE   stevedebi

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Posted 07 November 2014 - 11:58 AM

Thanks for the explanations. I think this confirms what I have been saying - that if one gets to the negative split mode, the system bleeds off excess energy from the engine because the SOC is at "maximum".  However, I've yet to see this situation - in general, I see the SOC going up and down while at LA traffic speeds, even when the speed is consistent. I think I've seen it on a recent trip with a long downhill, where I forgot to switch to Auto mode in my Energi; the SOC got up to a certain state and didn't go higher, even though the energy was definitely positive.

 

For the Energi, I believe this is also correct, which is a waste, since Ford could have recharged the larger battery, but instead treats the Energi as if it were the normal C-Max while on the highway.



#44 OFFLINE   Jus-A-CMax

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Posted 07 November 2014 - 01:43 PM

wait - r u saying in the energi it is not recharging the larger battery to its full capacity when downhilling? I always thought that the energi had a huge advantage when downhilling a super long grade to build & store more charge than a regular CMax. 

 

also in a negative split mode, the cmax is supersensitive to the slope of the road and will charge or use the batts on what, to the eye, appears to be a perfectly flat road. 

 

it was also so much better and higher in the ice high mpg mode aka negative split mode, when the limiter was on at 64mph. the 13b07 change to push the EV to 85mph seems to reduce the effectiveness of the ice high mpg, based on what i am seeing.


Edited by Jus-A-CMax, 07 November 2014 - 01:46 PM.


#45 OFFLINE   stevedebi

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Posted 07 November 2014 - 03:51 PM

wait - r u saying in the energi it is not recharging the larger battery to its full capacity when downhilling? I always thought that the energi had a huge advantage when downhilling a super long grade to build & store more charge than a regular CMax. 

 

also in a negative split mode, the cmax is supersensitive to the slope of the road and will charge or use the batts on what, to the eye, appears to be a perfectly flat road. 

 

it was also so much better and higher in the ice high mpg mode aka negative split mode, when the limiter was on at 64mph. the 13b07 change to push the EV to 85mph seems to reduce the effectiveness of the ice high mpg, based on what i am seeing.

Once you go into EV Later mode, the reserve % doesn't change. The Energi will max out at 1.6KW. If you want to add to the main battery, you have to manually switch back to Auto or EV Now, at the appropriate time for charging conditions.

 

I've heard folks say that the main battery can "come back" from depletion on an extremely long downhill, but I've not seen that, and it must be some quirk of the system if that happens.


Edited by stevedebi, 07 November 2014 - 03:52 PM.


#46 OFFLINE   Plus 3 Golfer

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Posted 07 November 2014 - 06:55 PM

Once you go into EV Later mode, the reserve % doesn't change. The Energi will max out at 1.6KW. If you want to add to the main battery, you have to manually switch back to Auto or EV Now, at the appropriate time for charging conditions.

 

I've heard folks say that the main battery can "come back" from depletion on an extremely long downhill, but I've not seen that, and it must be some quirk of the system if that happens.

 

Thanks for the explanations. I think this confirms what I have been saying - that if one gets to the negative split mode, the system bleeds off excess energy from the engine because the SOC is at "maximum".  However, I've yet to see this situation - in general, I see the SOC going up and down while at LA traffic speeds, even when the speed is consistent. I think I've seen it on a recent trip with a long downhill, where I forgot to switch to Auto mode in my Energi; the SOC got up to a certain state and didn't go higher, even though the energy was definitely positive.

 

For the Energi, I believe this is also correct, which is a waste, since Ford could have recharged the larger battery, but instead treats the Energi as if it were the normal C-Max while on the highway.

No, that is not a correct characterization of what happens. There is no bleeding off of ICE energy.  

 

Don't confuse energy with power or torque. Energy input is a function of fuel burned.  Energy output is the integrated instantaneous power over time.  The torque / power requirement can remain the same over time, but lowering ICE rpm can reduce fuel input. So, the efficiency of ICE improves - output / input increases.  Again MG1 acts as a motor using energy, MG2 acts as a generator producing energy.  The power requirements of ICE hasn't changed.  But what changes is the rpm of ICE and thus ICE operates more efficiently (uses less fuel because it's operating at a more efficient point on the BSFC map). If the torque / power requirements change (road load changes)  MG2 makes up the difference by either producing or using power.  

 

Again, this is no different that shifting a manual transmission from say 5th gear to 6th gear.  Road load hasn't changed.  But ICE efficiency likely improves and ones FE goes up running in a higher gear.  There is no bleeding off of ICE energy.  What changes is rpm goes down (same as Hybrid in negative split) and fuel use goes down because one is likely operating ICE on a more efficient point on the BSFC map. 

 

Again, the generator MG1 will use energy to slow engine rpm to increase ICE efficiency (road load requirements haven't changed) and the traction motor MG2 can use energy or produce energy to cover load variations to keep ICE at an efficient point on the BSFC.  SOC as I've observed, recorded, and graphed in negative split mode over about a 4 mile stretch with eco cruise at 55 mph started at about 56% and leveled off at 58% in the first 1/2 mile or so and remained at about 58%.  I couldn't drive any further in negative split because I was approaching a red light.  

 

What's important is for the negative split algorithm to operate the vehicle most efficiently. The efficiency of the Hybrid transmission can and likely goes down in negative split mode.due to additional energy losses in MG1, MG2, Inverter and so forth.  But an improvement in ICE efficiency by lowering rpm should more than offset these energy losses.   FE thus can go up.

 

Also, I think it was member Valkraider that would partially fill his Energi "big" battery when going down a mountain from skiing. He had pics that showed starting at the top with energy only in the "little" battery.  After 16 miles of mostly downhill IIRC he had around 4 kWh in the big battery.



#47 OFFLINE   stevedebi

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Posted 07 November 2014 - 07:12 PM

No, that is not a correct characterization of what happens. There is no bleeding off of ICE energy.  

 

Don't confuse energy with power or torque. Energy input is a function of fuel burned.  Energy output is the integrated instantaneous power over time.  The torque / power requirement can remain the same over time, but lowering ICE rpm can reduce fuel input. So, the efficiency of ICE improves - output / input increases.  Again MG1 acts as a motor using energy, MG2 acts as a generator producing energy.  The power requirements of ICE hasn't changed.  But what changes is the rpm of ICE and thus ICE operates more efficiently (uses less fuel because it's operating at a more efficient point on the BSFC map). If the torque / power requirements change (road load changes)  MG2 makes up the difference by either producing or using power.  

 

Again, this is no different that shifting a manual transmission from say 5th gear to 6th gear.  Road load hasn't changed.  But ICE efficiency likely improves and ones FE goes up running in a higher gear.  There is no bleeding off of ICE energy.  What changes is rpm goes down (same as Hybrid in negative split) and fuel use goes down because one is likely operating ICE on a more efficient point on the BSFC map. 

 

Again, the generator MG1 will use energy to slow engine rpm to increase ICE efficiency (road load requirements haven't changed) and the traction motor MG2 can use energy or produce energy to cover load variations to keep ICE at an efficient point on the BSFC.  SOC as I've observed, recorded, and graphed in negative split mode over about a 4 mile stretch with eco cruise at 55 mph started at about 56% and leveled off at 58% in the first 1/2 mile or so and remained at about 58%.  I couldn't drive any further in negative split because I was approaching a red light.  

 

What's important is for the negative split algorithm to operate the vehicle most efficiently. The efficiency of the Hybrid transmission can and likely goes down in negative split mode.due to additional energy losses in MG1, MG2, Inverter and so forth.  But an improvement in ICE efficiency by lowering rpm should more than offset these energy losses.   FE thus can go up.

 

Also, I think it was member Valkraider that would partially fill his Energi "big" battery when going down a mountain from skiing. He had pics that showed starting at the top with energy only in the "little" battery.  After 16 miles of mostly downhill IIRC he had around 4 kWh in the big battery.

I'm still not getting it. The generator kicks in to slow engine RPM? That is what the negative split mode is about? I don't see how that helps, as opposed to lowering the RPM by kicking in electric assist, since the SOC is full anyway. As you said, since the new hybrids will go EV up to 85 MPH, maybe it isn't that big a deal any more...



#48 OFFLINE   Plus 3 Golfer

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Posted 07 November 2014 - 09:02 PM

The generator is run as a motor and thus speeds up.  Watch this video on the Prius hybrid transmission.  See if that clears up anything. The equation for rotational speed (from the document I attached and neglecting the internal gear ratios inside the transmission) is:  Speed MG1 + Speed MG2 = Speed ICE

 

So, if MG2 is spinning at wheel speed and doesn't change and one wants to decrease ICE rpm, MG1 needs to speed up so that ICE  rpm drops (look at Fig 1 in the attachment). MG1 is attached to the sun gear, MG2 to the ring gear, and ICE to the planet carrier.  MG2 will act as a generator / motor to balance power requirements out as road load change to maintain speed. To much change in road load can kick in EV mode.  Of course one can also ease up on the throttle slightly to try to stay in negative split should road load increase say due to a slight increase in elevation.

 

To tie this into the Prius vs C-Max, the Prius EV mode operation is limited to around 40 mph (? IIRC)  So, the Prius should run in negative split a lot more than the C-Max. 

 

One more point I forgot to mention is that why not lower ICE rpm by kicking in electric assist? Because when ICE rpm drops due to electric assist so does load on ICE.  Thus, ICE is not operating as efficiently.   I think what is overlooked is that all energy in the hybrid comes from the fuel burned.  One does not want to use electric assist unless conditions require it or it improves overall  efficiency like lower speed acceleration from stop, larger changes in road load, harder acceleration and so forth.  Using electric assist to deplete the HVB for no efficiency gain doesn't make sense when one can take advantage of the hybrid transmission and keep ICE load up and lower rpm to improve overall efficiency.  You need to understand the BSFC map. 


Edited by Plus 3 Golfer, 10 November 2014 - 07:46 AM.

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#49 OFFLINE   Jus-A-CMax

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Posted 07 November 2014 - 09:49 PM

ok guys - friendly reminder to get back to the original topic at hand but feel free to start a new thread or piggy back off an existing thread on negative split mode discussions....:)



#50 OFFLINE   jimdel13

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Posted 14 December 2014 - 07:14 PM

I am a 2005 Prius owner considering a 2013 CMax Hybrid.  After reading this forum I am not sure if I shouldn't look for a newer Prius.   If the battery issue was solved, I would definitely go for the CMax.

Jim



#51 OFFLINE   stevedebi

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Posted 15 December 2014 - 12:43 PM

I am a 2005 Prius owner considering a 2013 CMax Hybrid.  After reading this forum I am not sure if I shouldn't look for a newer Prius.   If the battery issue was solved, I would definitely go for the CMax.

Jim

I have not seen any reports of problems with the 2015, and few with the 2014, in either the C-Max or C-Max Energi forums. I think they have solved the basic issues. Of course, and individual car may have a problem not related to design, but that is true of any vehicle. I'd certainly shop both and see which one fits your needs, without worrying about the battery.

 

But really, the two vehicles are pretty different. The Prius seems more oriented to MPG, and hence is lighter and drives that way. The C-Max is oriented to good gas mileage if driven right, but with more solid ride and comfort - completely different design concepts, and appropriate for different folks. I would not expect the same ride from the Prius and I would not expect the same MPG from a C-Max.


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