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Discussion Starter #1
We did a simulation of the "Dynamic Performance" of the Volt in an earlier thread. We can now use this model to simlulate the Single Charge Cruise Range. (We increased the Gear Ratio to 7.595.)

We want to determine how many miles the battery will allow for a single charge (50% discharge) at a given vehicle speed. We use the following assumptions. The vehicle stops and starts starts every 15 minutes. The battery supplies the energy required to accelerate to the cruising speed every 15 minutes from a stop and also for losses for air drag and tire resistance at the cruising speed. The power conditioner and gears each has a 90% power efficiency.

We will find the cruise range for three conditions: (1) idle power consumption is 200W, (2) a high efficiency heat pump/air conditioner consumes ~ 700W plus idle power, and (3) a resistance heater consumes 2000 watt plus idle power. Obviously, power needed for A/C and heating is dependent on environmental factors such as ambient temp, wind, humidity, snow, etc. We just want some baselines. The resultant curves are attached. Summary: the 40 mile range for cruising speeds given the three above conditions are 56, 53, and 41 mph, respectively. Looking at the attached curves makes interpretation of the above results easier. The bottom line is that you can cruise with A/C at 53 mph.
 

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An interesting conclusion is - the faster you go, the less the parasitic loads affect your electric-only distance. Too fast, though, and dynamic resistance forces drag your EV-only distance down more than your AC. The sweet spot is between 45-55 mph for any AC. AC/heat energy consumption makes the biggest difference in miles covered in low speed stop-go traffic, where it might take you 2-3 hours to cover 40 miles. This is why you will want to make sure you're in the HOV lane.

One question - I have heard that opening the windows fouls up the aerodynamics so much that it is more efficient to keep them closed and keep the AC on. Any hard data on that?
 

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One question - I have heard that opening the windows fouls up the aerodynamics so much that it is more efficient to keep them closed and keep the AC on. Any hard data on that?
I don't know about hard data, but rolling the windows down definitely will affect drag and probably disproportionately higher for a car with a Cd as low as the Volt's is supposed to be. I can't imagine hard data that would mean too much. It is dependant on so many variables: ambient temp, humidity, velocity, how far the windows are lowered, air speed and direction, AC temperture setting, etc, etc. I believe using the vent is best for drag (and cracking your windows a little if needed) if it is comfortable.

Tom,
Thank you for the analysis. They are well thought out and very relevent. What are the other assumptions (or parameters) you used? Such as KWh for 50% discharge and what % recovered with regen. Is the rolling resistance factor for low rolling resistance tires? The curves don't look like they account for 15 minute stops and starts. I would expect a drastic change at the points where volicity dictates an additional stop/start cycle. It would be interesting to see city cycle only (1 1/2 minute start/stop) and a highway cycle only (constant speed) graphs. I don't have a feel for automobile A/C and heat loads. Where your assumptions based on average loading or full load? If full, then using a "typical" % may be more meaningful since the A/C and heat do cycle.
 

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Discussion Starter #4
More Realistic Simulation of Single Charge Cruise Range

The Single Charge Cruise Range Simulation was originally done with a Drag Coefficient of 0.25. With all the wind tunnel work GM is doing, a value of 0.22 for an EV seems a more likely target. Research has shown that vehicles typically encounter a headwind of about 7 mph. We re-ran the simulation with these more realistic parameters. The results are shown in the attachment. The single charge range for a 900W load (High Efficiency A/C plus 200W idle load) @ 55 mph is only 35.4 miles. Running with a 2000 watt heater drops this down to 31.4 miles. (Anybody have any idea what the power losses are when driving through snow?) Running at night with head lights will drop this further.

There is some data to indicate that opening windows degrades the drag coefficient by about 5%. Since drag increases with the square of velocity opening windows has a greater effect at higher speeds. Using this 5% value we find that the Single Charge Cruise Range for a 900W load (A/C plus 200W idle load) @ 55 mph decreases the single charge range by about 3%.
 

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So the conclusion from the open-window simulation may be: In slow traffic, open the windows and turn off the AC. On the interstate, it just depends on how much AC you need.

Your methodology, assumptions, and numbers all looked valid. I doubt you'll be proved wrong, unless the physics of aerodynamic drag suddenly changes. The driving dynamics on the highways may dramatically change in the next few years. Major trucking companies are already starting to tweak their speed limiters to slow down their drivers a few mph and improve mileage.
 

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Tom,

Your "cruise range simulations" include a stop and start every 15 minutes?!

How about a chart with continuous steady speed? I am curious as to how that would look.
 

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Discussion Starter #8
Number of Stops

I did my model based on our particular driving situation for number of stops. Do you have a suggestion for a better number? The Single Charge Range curves for 30/55 mph are extended by about 10/4%, respectively, with no stops.

I just had a thought. Changing the SOC of the battery for recharge has a huge effect (+50% from 0.5 to 0.25 for generator turn on SOC) on range. How will GM detect SOC for generator on/off? Battery [email protected] temperature, cumulative "speed/accessory-charge vs. mileage" or motor power/accesory-charge vs. mileage algorithms? The method they use to detect SOC will have a significant effect on Single Charge Range. They may even choose to reduce SOC for generator turn on to meet their stated 40 mile goal.

Tesla runs their battery down to 20% SOC to get their range, but their warranty is 3 years/36,000 miles.

Tom
 

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With respect to open window drag, I think the Aptera guys have thought this one through a little bit. They provide a front scoop and rear facing vent to allow for air flow that does not degrade the aerodynamics of the sealed shell of the vehicle. Here is a picture of the rear vent:
http://www.aptera.com/about.php

They also plan to provide a solar panel to vent the car and keep the cabin heat down while parked. I don't know where the front scoop is but I suppose any air flow from front to back is more efficient than forcing the air around the shell.
 

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We did a simulation of the "Dynamic Performance" of the Volt in an earlier thread. We can now use this model to simlulate the Single Charge Cruise Range. (We increased the Gear Ratio to 7.595.)

We want to determine how many miles the battery will allow for a single charge (50% discharge) at a given vehicle speed. We use the following assumptions. The vehicle stops and starts starts every 15 minutes. The battery supplies the energy required to accelerate to the cruising speed every 15 minutes from a stop and also for losses for air drag and tire resistance at the cruising speed. The power conditioner and gears each has a 90% power efficiency.

We will find the cruise range for three conditions: (1) idle power consumption is 200W, (2) a high efficiency heat pump/air conditioner consumes ~ 700W plus idle power, and (3) a resistance heater consumes 2000 watt plus idle power. Obviously, power needed for A/C and heating is dependent on environmental factors such as ambient temp, wind, humidity, snow, etc. We just want some baselines. The resultant curves are attached. Summary: the 40 mile range for cruising speeds given the three above conditions are 56, 53, and 41 mph, respectively. Looking at the attached curves makes interpretation of the above results easier. The bottom line is that you can cruise with A/C at 53 mph.
Tom,

I can't help myself - curiosity has the better of me and I simply must ask.

Who is we?

You and Jim Bean and Jack Daniels late on a Friday night?!?
 

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Another potential tweak for the model:

I think KE at the end primary discharge cycle has value. Since the car started at rest and the KE came from battery energy, I believe this energy should be included in the range. Since the car will be traveling at 70mph at end cycle,then all curves should be shifted up about 1.5miles (additional range for amount of KE).
 

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Discussion Starter #12
Total KE = Number of Start/Stops * KE

Koz,

Another potential tweak for the model:
The model includes KE "drain" in the range calculation. We assume a start/stop every 15 minutes. The number of start/stops for a given cruise velocity is then calculated iteratively, i.e. make an estimate for number of stops, crank it back into the model and do the iteration three times. The KE drain to cruise velocity is then multiplied by the number of stops.

Did I miss something?
 

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Koz,


The model includes KE "drain" in the range calculation. We assume a start/stop every 15 minutes. The number of start/stops for a given cruise velocity is then calculated iteratively, i.e. make an estimate for number of stops, crank it back into the model and do the iteration three times. The KE drain to cruise velocity is then multiplied by the number of stops.

Did I miss something?
No, you've got it right.

My bad, I thought I saw in the analysis that a standing start was assumed, but that was only for 0-60 analysis. I see now the start and stop energy is paired together in the number of starts equations.
 

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Tom,
You may have seen this http://www.teslamotors.com/blog4/ already. It gives the latest details for Tesla's Powertrain 1.5. They really do a good job with their engineering posts, eventhough these posts are few and far between these days. Their efficiencies are a bit higher than the Volt simulation model uses. It also discusses gear ratio a bit. I am especially encouraged that this iterative development added 4.5% to the range and this wasn't even the core purpose of the redesign. I can't wait for EV's to be produced in earnest. Their devlopement will be be on a steep improvement slope, quickly leaving traditional ICE's in the dust.
 

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Discussion Starter #15
Tesla PT 1.5 Engineering Update

Koz,

Thanks for sharing the Tesla 1.5 development site. It is really exciting to share in the details of Tesla's historic powertrain development. The photos are beautiful! I was surprised by the 33% performance improvements (160 milliOhm?) from better IGBTs. The implication is that this comes from lower resistance, but single 600V 100 KHz Buck Chopper SOT-227 IGBTs are available with resistance slopes of 10 milliOhms. There doesn't seem to be much room for a dramatic current improvement. It was also interesting to hear about their thermal management and peak power strategies.
 

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Tom,

Glad to hear you found the Tesla post interesting. Like you point out, the efficiency gain from the IGBT's alone was probably very minor. I think the greater importance to the newer component was in being able to keep the rest of the PEM design intact. Sounds the like the more tangible efficiency gains came from the other changes mentioned.

By the way, they had a previous post on controls that I had found very interesting and is relevent to the discussion on 2-wheel regen.. here http://www.gm-volt.com/forum/showthread.php?p=4563#post4563
 

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Discussion Starter #17
Update: AER Simulations now available for EPA Drive Profiles

I did a detailed second by second Volt simulation to find the All Electric Range, AER, with three major EPA profiles. The results were an AER of 40.2, 39.6, and 28 miles for the EPA75/UDDS, HWY, and US06 profiles, respectively.
Plots of the profiles and details of the simulation are available at http://gm-volt.com/forum/showthread.php?t=581.
 

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40 Mile EV range assumptions

Tom,

I recently read on the GM Volt webpage that the 40 mile all electric range is base solely on the the EPA city cycle. Thus, your estimates for the range at highway speed are likely accurate.

Cheers!
 
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