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Discussion Starter #1 (Edited)
The attached model is a simulation of the Dynamics and Range of the GM Volt. You can also design your own EV by using the model with different design parameters.

The model estimates that the Acceleration Performance of the Volt is 0 to 60 mph in 8.6 seconds :) and the Single Charge "Highway" Range @70 mph cruise is 26 miles. :(

See model details at http://www.leapcad.com/Transportation/GM_Volt_Simulation.pdf.
 

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Discussion Starter #2
Road Grade Drastically Reduces Single Charge Range

A road grade of 3% drastically reduces Single Charge Range @70mph from 26 to 14 miles. Range @40mph is reduced from 46 to 19 miles :( See attached range curves. Grades of 6% are not uncommon. The Tour De France has an average grade of 7%.
 

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The Tour De France has an average grade of 7%.
So in the mountains, should we shoot a little EPO or testosterone in the battery pack? Will we have to have random Volt doping tests to ensure valid mileage runs?

Actually, pro cycling may have finally really cleaned up their act, though it took ruiniing dozens of careers and losing tens of millions of dollars in sponsorships to get ther. Even without doping, those guys can crank out 400 watts continuously for 4-5 hours, every day, for 21 days. Amazing athletes. I know that during July, my Dish receiver is programmed to VS every day.
 

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Well, the test mule just got 40 miles on a charge, in the wrong body, with a beta software set. I wonder what kinds of grades they're putting that thing through?
 

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my guess is that they are useing a different electric motor that we don't know about. GM did not says any thing about the motor at all.
 

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Discussion Starter #7
The size of the motor determines acceleration, not battery range.

my guess is that they are useing a different electric motor that we don't know about. GM did not says any thing about the motor at all.
Motor size and mass primarily determine how fast you get from point A to B. It needs different amounts of energy depending on the profile of changes from one speed to another. Once you get to a given cruise velocity, the battery drain is determined by mechanical and efficiency factors: mass, drag, tires, road surface, wind, road grade, regen, efficiency of drive electronics and motor and battery resistance. Changing the motor shouldn't much effect on battery range. :)
 

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Tom,

Thank you for this excellent document. I'm writing an article about EV range, especially as it is reduced by higher speeds and accessory loads, so this is perfect!

A couple of questions: I see you used a Cd of .22. That seems optimistic. The Prius is .26, and I understood that was the target for the Volt. And what did you use for the frontal area?

Are you an engineer; and in the EV field? Can I feel confident about using this chart in an article?

Thanks for any further answers.

Paul
 

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Discussion Starter #9
Volt Drag Coefficient Assessment

GM has not disclosed the value of the Volt Cd. As you know, the EV-1 had a Cd of 0.19 and GM was aware of the value of the Prius Cd. I originally used a value of 0.25 for my simulation. Later I saw a number of articles about GM engineers doing fine tuning of Cd in wind tunnel testing and concluding that Cd must be minimized to get high speed performance. From this, I hypothesized that they wanted to aggressivly best the Prius Cd. I tried simulations with Cd of 0.25 and could not get the claimed Volt high speed performance. I considered the above and guessed at a Cd of 0.22. I know that there are tradeoffs between performance and body styling (mirrors, sharp corners are "in", etc.). My purpose in publishing my model and parameters was to subject it to review and correction. If you or anyone has a technical rationale for changing any of my parameters I would be more than delighted to improve the model. What are your thoughts about 0.22?

The frontal area came from the height and width "Specifications" on the GM-Volt home page. I suspect the actual number is less than this. Once again, if you have a better number give me a shout.

Tom
 

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Tom,

the height and width may not be accurate for determining total frontal area, as there is substantial "tumblehome" in cars like the Volt (wider in the midsection than at the top, and perhaps bottom). Also, it appears from the model that the final Volt will be tallere than the very low-slung concept.

I distinctly remember reading that Lutz said that they had attained their goal of equaling the Prius' cd number. I can't find it now.

For modeling purposes, I would use the Prius's total CX (cd x frontal area), because I suspect it will be close to that. I think a cd of .22 is overly ambitious.

Would making those changes alter the results much?

Also, I assume I can link/insert your charts in an article about EV range I'm writing?

Paul
 

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While we're talking about Cd, I recall a comment that after some wind tunnel testing, GM engineers concluded they needed to add a spoiler to improve the Volt's Cd. I had always assumed the spoilers on the tails of Celicas, Eclipses, and other high mpg sporty cars were just for looks. Could they actually be designed to guide some air over the tail end and reduce high speed flow separation and turbulence in the rear? I know that when I put my bike rack and a bike on the rear of my wife's Celica, the increased drag reduces mpg by 20% at freeway speeds.
 

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hvcman: spoilers can be for looks, downforce (racing cars), and improved aerodynamics (look at a gen1 Prius). It all depends on how they're designed.
 

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HAVACMAN,

I believe the small rear spoilers trip the laminar flow to induce turbulance. Turbulent flow reduces separation of flow drag at the rear, at least to my foggy recollection.

Tom & Pnieder,

I remember GM's comments regarding Cd to have been that they wished meet or exced the Prius Cd. Looking at the prototype and the EV1, I think your original assumption of 0.25 is more likely. The 7MPH headwind doesn't make sense to me. I can see this to be the case for two lane roadway situations, but most two lane roadway driving is at low city speeds. Most higher speed driving is on divided highway driving, and I'ld bet that if there is a divided roadway only study it will indicate that there is tailwind. Not that the following has a place in your calculation but should be considered: in reality, much commuting for work is done in heavy traffic which has even better aerdynamics. The value of plug-in EV's will be amplified in this type of driving, mostly from regen and no ICE idling.

I have look closer at the study, but if it calculates 29KWh draw with 500W accessory load and 60mph cruise as Tom posted in the public blog then there must be something off. Tesla with a bit higher Cd only draws about 14KWh at 60mph. At first glance, my guess is that the following assumptions are the problem: "The Traction Inverter, DC-DC Converter, and gear power efficiency are each 90%." This would be a combined efficiency of 72.9% for these components. Tesla claims their battery to wheel efficiency is better than 80% and this includes rolling resistance.
 

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Tom, try using an electric motor efficiency of at least 92.4%. According to:
http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html, electric motors constructed according NEMA Design B must meet the efficiencies below:

Power (hp)........Minimum Nominal Efficiency
20 - 49..............88.5
50 - 99..............90.2
100 - 124..........91.7
> 125................92.4

I would assume the motor in the Volt would meet this spec, and likely exceed it. The Volt EM could be closer to 95% efficient.
 

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Koz (and Tom),

The Tesla Cd may be higher, but don't forget, it's total aero drag (CD x frontal area) that counts. The Tesla is very low and small, and has a much smaller fronat area than the Volt or Prius. I would expect its Cx to be substantially lower than both. That's one of the reasons they chose the Elise to base it on, light weight and low total aero drag.

I will repeat what I said earlier; I am certain that Lutz said somewhere that they had "equaled the Prius' aero drag" with their redesign of the Volt. I think the safest number to use is the Prius' FA, which my notes show to be 2.16 square meters. Does that sound right?
 

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1999 Lotise Elise Cd*Af = 6.35 (couldn't find Tesla's Roadster info)
2004 Toyota Prius Cd*Af =6.05, Af= 23.25 sq ft
(Cd - drag coefficient, Af - Frontal Area)

Tom's study uses Af=25.28 for the Volt. This seems a little high since the Volt should end up a few inches shorter and perhaps a little narrower since it's only a 4 pasenger vehicle. I believe the comments about raising the Volt were in regards to increasing ground clearance in part to help reduce drag. I looked but can't find the article that had the comments about how the improvements in drag compare to the Prius. I pretty sure they inferred equal or better drag. I expect Cd*Af to be slightly better than the Prius' based on GM's comments about their Cd and what have heard and seen of the final design.

Tesla probably improved drag some for their roadster but it should be similar to the Elise. Thus, it is very plausible that the Volt's aerodynamics are similar and perhaps slightly better than the Roadster's despite a larger Af.
 

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Tom,

Here is a calculation someone made for the Tesla. Note the higher efficiency numbers assumed for the drivetrain.

Also, I have numbers for the total aero drag for the Tesla: between 5.7 and 6.12 feet square.



Robin wrote on May 16th, 2007 at 1:45 am
CdA and range for tesla roadster:

Dimensions:

1.127 x 1.7 (-mirrors estimate) = 1.9 m²
X 85% (standard shape correction factor) = 1.63 m²
Cd = 0.3 (mentioned in online article)
CdA = 0.3 x 1.63 = 0.49

(9.81 x 1350 x 0.011 + 0.49 x 50% x 1.293 x 26.5²)

x 75% / 3.6 = 136 wh/km

Efficiency: (batteries, charger, drive train) 0.93 x 0.93 x 0.9 = 0.78. minus 3% for lights, AC, heating gives a 75% average efficiency to work with

9.81 = gravity
1350 = weight in kg of roadster including 300 lbs (total = about 3000 lbs = 1350 kg) passenger and cargo average
Rolling resistance estimate: 0.011
1.293 = air density
26.5 = average speed in m/s that compensates for regen braking. Calculated this speed by analyzing EV1 speed and range data.
Divide by 3.6 to get wh/km

Range: 50 kW (battery pack energy estimate) / 0.136 = about 370 km or 230 miles
 

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Discussion Starter #18
Requested Modified Model Parameters - Thanks for the feedback

Guys,

I have modified the parameters per your posts. I also did a model for the Tesla (fixed first gear). The Tesla model appears to agree with performance specs. See attachments.

nieder
I have revised Cd upward. This decreased single charge range significantly. See below. Yes you can link/copy charts.

HAVACMAN
Changed Cd to 0.25. There is always an “average wind.” See discussion under Engineering, “EPA Ratings ...”
Bottom line: the data at http://www.awea.org/faq/usresource.html suggests 7 mph is reasonable. I will change it if anybody has statistical data to support rationale for a better value.

The Volt and Tesla power draw at 60 mph at 500W is ~ 14.9kW. See attachment for Tesla.

Rooster
Used 92.4% motor efficiency. The DC-DC converter is only used to supply accessory power. It’s efficiency does not affect traction power. If they use a separate 12V the DC-DC converter losses would not apply.

I don't know if the Li Fe battery 16/53 kWh energy spec includes battery losses. Battery loss, if not accounted for, could be a significant model error.

Pnieder, Koz
Used 2.16 m^2 = 23.25 ft^2 and Shape Correction Factor of 0.85. Changed tire rolling resistance to 0.011.

Tom
 

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Using the cruising power curve, a 65 mph and at 200-500 watt parasitic load, the power is 20 kW. Assuming 35% high efficiency engine-generator combination and 130,000 BTU/gallon gas, it works out to 43 mpg in generator mode. Just slowing down just 5 mph to 60 mph takes 15 kW and gets you 58 mpg. Conclusion - highway mileage will be extremely sensitive to speed.

Great work, Tom.
 
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