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Discussion Starter · #1 · (Edited)
I was curious about a direct comparison of the Volt to Prius vehicle dynamics. I did a macro model analysis comparison of their extremum conditions and plotted the results. This nicely illustrates some aspects of the series vs. parallel hybrid tradeoffs. One of the plots is shown below. The details of the analysis are at http://www.leapcad.com/Transportation/Macro Model Performance Comparison - Volt EREV vs Prius.pdf. The analysis reveals that the Prius makes very efficient use of its tractive components. Efficiency with respect to ICE duty cycling, is of course, depends upon miles between plug-ins. I welcome all additions/corrections, particularly about the Prius assumptions and Volt All-Generator-Mode considerations.


Tom
 

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It's here:

http://www.leapcad.com/Transportation/Macro%20Model%20Performance%20Comparison%20-%20Volt%20EREV%20vs%20Prius.pdf

I've only glanced at the paper, but it seems to have some major assumptions incorrect.

In the charge sustaining mode the Volt is limited to 53 kW continuous power. Because the Prius is series/parallel it has 100 kW of traction power in its charge sustaining mode. With 88% more charge sustaining power, we expect the Prius to be much better at sustained accelerating, climbing hills, and driving into the wind.
The Volt has an ICE generating 53 kW = 71 hp.
The Prius has an ICE generating 73 kW = 98 hp (in 2010)
That is the limiting "continuous power", it doesn't matter how the Prius juggles the battery power, ICE power, electric motor operations. It's only power source is gasoline.
The Volt has a good chance at getting very good MPG because it uses a smaller ICE, combined with a much larger battery buffer than the Prius.

They also assume the Volt's ICE gets 30 mpg, and thus CS mode gets 30 mpg, with no consideration for running the 1.4 liter at very low RPM, buffering energy for later bursts, regen brake power, turning the engine off when not needed, etc, all the techniques hybrids use for improving MPG. The paper assumes that the longer the Volt drives without recharging, the closer the MPG gets to 30, which indicates a very confused understanding of the Volt system architecture.

It also guesses that the condition of a depleted battery buffer, while in Charge Sustain mode, will come up 5% of the time, which seems awfully high.

I'll look at this in more detail in the future: it looks like serious effort was put into trying to understand the Volt, which is respectable.
 

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Discussion Starter · #4 · (Edited)
Conditions for assumptions

Geronimo
The Volt has a good chance at getting very good MPG because it uses a smaller ICE, combined with a much larger battery buffer than the Prius.
The paper explicitly calculates this for the extremum Generator-Only-Mode condition.


They also assume the Volt's ICE gets 30 mpg, and thus CS mode gets 30 mpg, with no consideration for running the 1.4 liter at very low RPM, buffering energy for later bursts, regen brake power, turning the engine off when not needed, etc, all the techniques hybrids use for improving MPG. The paper assumes that the longer the Volt drives without recharging, the closer the MPG gets to 30, which indicates a very confused understanding of the Volt system architecture.
The 30 mph was arbitrary. I thought this was what the EPA used to calculate the 230 mpg composite number.


It also guesses that the condition of a depleted battery buffer, while in Charge Sustain mode, will come up 5% of the time, which seems awfully high.
The stated conditions were "What is the probability of this occurring with 100 miles between charging, 10 mph headwind, median road grade of 9% for 4 miles, and with aggressive driving?" I thought 5% under extreme conditions is reasonable, but your guess is as good as mine.

Regards,
Tom
 

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is there really any point to doing these analysis without any hard data on how the Volt operates?.. we really know very little about the Volt, and daily (seems that way) we get conflicting statements from different GM managers.

Here are the known "facts" or specs:

50mpg for CS mode
100mph max speed in CS mode
Serial configuration
e85 1.4l 4 cyl engine
16kwh lithium pack
53kw generator
112kw traction motor
230mpg using some weird epa calculation
40 mile electric range in both city and hwy epa cycles

Did I miss anything?

Even if all the facts were present it would be hard to compare both cars.. the Prius is intended to be an ICE powered car with a very efficient electric transmission, and with some regen braking ability .. the Volt is totally different, it is a BEV with a part-time genset built in.

A Prius with a high quality CVT transmission and a hydraulic brake regen but no batteries would be indistinguishable from the existing model.

When the Prius if fitted with a plug-in pack (and its coming) then these comparisons will be more reasonable.

Why not do a 3-way comparison?.. do a Leaf, a Volt and a Prius.. tough aint it?
 

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Discussion Starter · #6 · (Edited)
Did I miss anything?
For the last several decades Engineering Design has been done exclusively through simulation. Nuclear weapons, large aircraft, Integrated Circuits (e.g. microprocessors, memory, etc.) can only be designed through simulation. LaGrangian equations for nuclear weapons and BSIM physical models for Integrated Circuits dictate the dynamics. Just about anything that is manufactured these days is designed by 3D modeling to drive numerical controllers. Simulation is used seamlessly from Engineering to Production. Suppliers are required to submit their products as design files. The motion of the Volt must follow the laws of Physics. If we know the equations, we know how it must move. We may not know the control algorithms for the Volt, but we do know its extremum (max/min) behavior.

You may not be able to trust what the GM Executives tell you, but you can always trust what the simulations say.

P.S. We could do a three way comparison to the power, torque, velocity, and acceleration of a mosquito (if we assume there is no wind).

Regards,
Tom
 

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Discussion Starter · #8 ·
and dont forget global warming.. if it wasnt for the modeling we would never know that the planet is warming up..
The Global Warming stuff was not based on first principles. They were based on correlations (subsequently proven to be incorrect) and do not include important factors such as solar effects. The new PC term is no longer Global Warming, it is now Climate Change (warming, cooling, ice ages, etc.).

Tom
 

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hermperez: "and dont forget global warming.. if it wasnt for the modeling we would never know that the planet is warming up.."

False. We have temperature records showing a lot of warming over the last 150 years.

Tom: "The Global Warming stuff was not based on first principles. They were based on correlations (subsequently proven to be incorrect) and do not include important factors such as solar effects. The new PC term is no longer Global Warming, it is now Climate Change (warming, cooling, ice ages, etc.). "

Let's see... False, false, false and true. You got the new terminology right. Still, only 1 in 4... you sure packed a lot of wrong into a single paragraph.
 

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hermperez: "and dont forget global warming.. if it wasnt for the modeling we would never know that the planet is warming up.."

False. We have temperature records showing a lot of warming over the last 150 years.

Tom: "The Global Warming stuff was not based on first principles. They were based on correlations (subsequently proven to be incorrect) and do not include important factors such as solar effects. The new PC term is no longer Global Warming, it is now Climate Change (warming, cooling, ice ages, etc.). "

Let's see... False, false, false and true. You got the new terminology right. Still, only 1 in 4... you sure packed a lot of wrong into a single paragraph.
By changing the politically-correct terminology from "global warming" to "climate change," the True Believers can whine and cry about anything - warming, cooling, raining cats and dogs, etc. - and not be subject to being pinned down. Brilliant.
 

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Discussion Starter · #13 ·
The Title Of This Thread Is

"Volt Vs. Prius Vehicle Dynamics."

On The GM-VOLT FORUM.

Thank You.
 

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Geronimo

The paper explicitly calculates this for the extremum Generator-Only-Mode condition.
But you already misunderstand how often that situation arises:
To get performance in the Generator-Only-Mode comparable to the Charge Depletion Mode, the ICE/Generator must be sized to match the battery peak power,
The Charge Depletion mode will have a battery buffer to assist in power to the traction motor 99% of the time. The whole point of having a genset that generates half of the maximum power of the traction motor (what you call the battery peak power) is to burn less gasoline, to use the battery to average out the power demands. This is the whole beauty of a serial plugin hybrid, the large battery that you have available for a CS mode battery buffer. The only downside to a large buffer is battery lifetime - the deeper the discharge, the faster the battery ages, but if 80% down to 20% can last 10 years, that gives a battery buffer more than 3 times the size of the Prius (1.6 kWh).

Generator-only-mode is a critical condition, which can also arise in the Prius, by the way (which I have been driving over 5 years) - it's when the battery is depleted and can no longer assist the ICE. My 2004 Prius has a 76 hp ICE, only slightly larger than the Volt ICE. The battery in the Prius is to smooth out power demands as well. It is only 0.52 kWh. During the rare times it is depleted, the Prius has only 76 hp (the new 2010 is 98 hp). The whole point of the software and battery buffer is to make that condition very rare.

The 30 mph was arbitrary. I thought this was what the EPA used to calculate the 230 mpg composite number.
The asymptotic MPG as you get farther and farther from the last charge (also known as the Charge Sustaining Mode MPG) is a topic of some controversy on this site - some people think it will be close to 50 mpg. Others say in the 30's.

First, the EPA has not yet tested the Volt - GM used their preliminary, proposed testing methodology for plug-in vehicles, and came up with the 230 mpg figure for city driving. I don't know if the details of the proposed EPA test have been released yet, but I've seen analyses where the test is criticized because:
the first 40 miles use no gasoline
If CS mode gets 50 mpg, then if the EPA test is for 50 miles city driving, the last 10 miles uses 0.2 gallons of gasoline, for a mpg of 250.
If the city driving test is 80 miles, total fuel economy drops to 100 mpg.
If the city test was 300 miles, mpg would drop to 57.7
http://money.cnn.com/2009/08/11/autos/volt_mpg/?postversion=2009081108
That 230 mpg is highly dependent on the exact details of the test.
Better info would be: all electric range and charge sustaining mode mpg.

50 mpg has been suggested in various articles for charge sustaining mode, but my understanding is the final number depends on the details of their software and design decisions: battery buffer size, how aggressively they refill it, how many different RPM optimized-efficiency points they tune the engine for, etc.

The stated conditions were "What is the probability of this occurring with 100 miles between charging, 10 mph headwind, median road grade of 9% for 4 miles, and with aggressive driving?" I thought 5% under extreme conditions is reasonable, but your guess is as good as mine.

Regards,
Tom
Why does the distance between charging have anything to do with the battery buffer state ? In the Prius, you drive hour after hour after hour, and the state of the battery charge could be anything - it has no relation to distance, only to the exact details of the drive up to that point.
Same for the Volt. At 100 miles since plug-in, the battery buffer could be 100%, or it could be 0%.

Yes, if you are flooring it up a 9% grade for minute after minute with a 10 mph headwind, eventually you will deplete the battery buffer.
At that point, you should slow down.

If one finds himself driving up windy mountains 5% of the time, the Volt is probably not a good choice of primary vehicle :)

I appreciate all the work you've put into the pdf file - perhaps after reading some of the archived articles and comments at gm-volt.com, you will change some of your assumptions as to the design.
Anyway, a lot of the details are proprietary or not finalized, so it's hard to get an accurate picture of how the Volt will operate.
 

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Discussion Starter · #15 · (Edited)
Half a loaf

As I stated in my paper, I do not know the control algorithms that GM will be using.
The only thing that I can meaningfully calculate are the extremums.
The only thing that I can meaningfully calculate are the extremums.
The only thing that I can meaningfully calculate are the extremums.
The only thing that I can meaningfully calculate are the extremums.
The only thing that I can meaningfully calculate are the extremums.
The only thing that I can mea...

Got it?

Tom
 

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The Global Warming stuff was not based on first principles. They were based on correlations (subsequently proven to be incorrect) and do not include important factors such as solar effects. The new PC term is no longer Global Warming, it is now Climate Change (warming, cooling, ice ages, etc.).

Tom
The Title Of This Thread Is

"Volt Vs. Prius Vehicle Dynamics."

On The GM-VOLT FORUM.

Thank You.
Either you are publicly chastising yourself for making an off-topic comment, or you think it's reasonable for you to make an off-topic comment but not reasonable for others to respond even if the comment is erroneous and/or misleading. Either way it doesn't seem right.
 

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Discussion Starter · #17 ·
I am publicly chastised, also:

The Title Of This Thread Is

"Volt Vs. Prius Vehicle Dynamics."

On The GM-VOLT FORUM.

Thank You.
 

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You should spend some time doing a comparison on the Volt and the BYD F6DM..

Here are the known facts about the F6DM:
50kw 1.0l engine
25kw generator
50kw traction motor
62 mile all electric range, 258wh/mile
0-60mph is 10.5 seconds
top speed 93mph
1800kg weight

20kwh LiFePo battery, packaged similar to the Volt.

The FD6M starts out in EV mode. At medium speed it will shift to range-extending series hybrid mode, and at high speed it will shift to Prius-like full parallel mode.

http://jalopnik.com/344806/detroit-...chinese-hybrid-through-cobo-arena?mail2=true/

http://green.autoblog.com/2008/01/15/detroit-2008-byd-exposes-the-f6-dm-plug-in-hybrid/
 

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PHEV-40 breakeven analysis

One of the key points of the document discussed in this thread is the economic viability of the EREV-40 platform (Volt) vs. an HEV (Prius) platform. I think this is an important and interesting topic. It really comes down to battery price vs fuel price.

I posted an analysis of this some time back:

http://gm-volt.com/forum/showthread.php?t=2554

I think in the long term (10 years), the EREV platform will win as battery prices to auto makers will come down into the $300-400/kWh range and fuel prices will be sustained above $3/gallon. I think choices in all-electric range will also help this, as the economics look best when your battery is sized for your daily driving.

In the short term people will not buy the Volt for economic reasons, just like the early versions of the Prius.

In the very long term (50 years) the question will shift to EREV vs pure EV. This is largely about infrastructure investment.
 

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Discussion Starter · #20 · (Edited)
You should spend some time doing a comparison on the Volt and the BYD F6DM.
Need to know drag coefficient, frontal area, ICE torque and power curves, motor torque, ICE/motor parallel gearing, tire radius, and peak battery power vs. 10 sec to do dynamics.

Tom
 
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