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40 or 28 miles? Which AER driving profile did GM choose?

18083 Views 35 Replies 12 Participants Last post by  Tom
What is your normal driving profile? It will have a huge effect on the Volt’s Average Electric Range, AER. Compare three common driving profiles the EPA75, HWY, and US06. We note that these profiles are dynamometer profiles. They are not done in the wind, rough roads, or on road grades, all of which lower AER. Nor are they done with max power (209 motor hp) to simulate passing. The goal of these profiles was to check and compare emissions, not evaluate EV performance, such as AER.

The EPA Federal Test Procedure, EPA75, is called the City Cycle. It consists of the Urban Driving Cycle, UDDS, followed by the first 505 seconds of the UDDS. It has a top speed of 56.7 mph. It uses a maximum of 37 hp road power. See attachment.

The EPA Federal Test Highway Procedure, HWY, has a top speed of 59.9 mph. It uses a maximum of 30 hp road power. See attachment.

The US06 Supplemental Federal Test Procedure (SFTP) was developed to address the shortcomings with the FTP-75 test cycle in the representation of aggressive, high speed and/or high acceleration driving behavior, rapid speed fluctuations, and driving behavior following startup. It represents an 8.01 mile (12.8 km) route with an average speed of 48.4 mph, maximum speed 80.3 mph, and a duration of 596 seconds. It uses a maximum of 89 hp road power. See attachment.

I did a detailed second by second Volt simulation with these three profiles. The results were an AER of 40.2, 39.6, and 28 miles for the EPA75/UDDS, HWY, and US06 profiles, respectively.

Does anybody have a recommendation for a representative EV driving profile? Google did some work for improved EV fuel economy profiles. I'll check into that.

The simulation is attached below.


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US06 Cycle

All my assumptions, data, and methdology are in the attachment.
Did you account for the energy needed for the second by second acceleration and deceleration?
AER: Comments to J in MN

The primary difference is in the assumed efficiencies. I was assuming much higher values for what you call GPE and TInvE.
As the Section “Find the Single Charge (@SOC = 50%) Cruise Range for a given Velocity”, in my attachment states, TInvE, is defined as the Traction Inverter Efficiency and GPE is the Gear Power Efficiency. Their product is 83.2%.

Consider all the Power Train elements in this product. Let’s take the highest estimates for these Power Train components: Battery IR losses (98%), Buck Inverter Efficiency (97%), Motor Efficiency (92.5%), and Gear Power Losses (95%). This product is 83.5%. Without any evidence to the contrary, I don’t think it is reasonable to assume higher efficiencies.

It has been stated that GM uses the EPA city cycle for their AER determination. My analysis gives an AER of 40.2 miles for this profile and is in agreement with the published data. At this time, I don’t see a compelling argument to question this analysis, but I am open to correcting or improving the model as needed. Don't hesitate to comment or make constructive criticism.

Note: I am assuming that there is a separate low voltage battery for accessories. I also assume that when the vehicle is stationary and “ignition” on, that there is some drain from the high voltage battery for electronics. Let’s call this the Idle Current and the associated efficiency, Idle Power Electronics Efficiency, IPEE, and give it a value of 95%. In my analysis I assume this high voltage battery idle drain is 100W. The typical running power is in the range of 5KW. Idle is just 2% of the total. It is of little consequence to the total power.
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Average I^2R Losses


With the large currents (peak ~ 400A, cruise @ 50mph ~ 30A), I would think that I^2R power losses in general, (cabling, battery terminals, connections) could easily be 2%. 100milliohms and 50A average current ~ 3% power loss.
Delayed Reaction


I thought you were referring to internal losses in the battery. 2% seems like a reasonable swag for line and terminal losses.
When I first wrote the post I was thinking just in terms of the battery IR loss. When I reflected on it afterwards, particularly after thinking about the discussion of cable losses in the Tesla PT 1.5 blog, I realized that IR losses in addition to the battery would be significant relative to the stated 2% estimate.
2008+ MPG-Based AER

typical road conditions that can affect fuel economy, such as higher speeds, cold temperature, and use of air conditioning.
I did have a 2 mph headwind cranked into my estimate. I removed it, recalculated, and then used the 2008+ MPG approach. I get an mph based AER of 30 miles. This method may not be fairly applicable to the Volt. The volt may use a separate 12V battery for accessories like A/C, which would not affect AER. Also temperature will have a different affect on an EV than an ICE. These correlations may not be applicable to an EV.

If GM used this method and got 40 miles AER, then I have a 25% difference in my AER calculation methodology. For greater accuracy using an analytical approach, I may need a time scale finer than 1 second, i.e. I may need 10 Hz speed sampling. I will examine results with 10Hz sampling.
2008 and Later, MPG-Based AER with More Accurate 10Hz Data Files

I ran the program with 10Hz data files and required program changes for 10 Hz sampling. I got the same numbers as the 1 Hz files within 0.05%. Apparently, with a large enough 1 Hz data sample, the 1 Hz sampling errors average out.

I increased the Regeneration Efficiency from 80% to 90% (I doubt if it's this efficient). I got a Pre2008 City and Highway AER of 44.4 and 40.3 miles, respectively, and a post 2008 City (33.5) and Hwy (28.7) AER for a combined of 31.4 miles, an 21.5% difference from 40 miles.
The results imply that when wind speed, road grade, road roughness, etc. are considered, that the Volt cannot meet the 40 mile AER per post 2008 EPA testing specs.

What is the source of the discrepancy with the specified 40 mile AER for the Volt? Koz, you opened up a can of worms. But I suspect you have a habit of doing this.
The 8kwHr EPA 5Cycle AER Discrepancy


Thanks for the interest in the calculated AER issue. My concern is that the press would skewer GM and the Volt if it did not meet this highly publicized 40 mile spec per the post 2008 EPA spec.

Nice catch on the road resistance. It drops AER by 3.4%, More than I would expect. But this is only roughly 1/10 of the 25% AER difference. It can't explain the major portion of the difference.

One fact that is still disconcerting is that one of the components of the new EPA test, the USO6 profile, consists of just aggressive driving (max speed: 80 mph, max acceleration: 0.34 g) under normal conditions (room temp, no wind, no A/C). Thus, it can be directly calculated. No fleet average correlations to factor in "environmental" conditions are required.

I calculated AER for US06 as 30 miles. You can't factor in 30 miles and get a 40 mile AER. One of the other profiles factored in for A/C use. I still wonder if A/C power comes off the high voltage or a 12 (48?)V accessory battery, which would then not affect AER.
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AER vs Curb Weight


Results are in attachment.

I used Mathcad. Any programming language with numerical integration will work.


Compared to an electric motor, an ICE has many functional factors (e.g. air intake/exhaust, octane rating) that need to be modeled to get a reasonable MPG estimate. Additionally, control parameters for parallel operation would also have to be known.



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Alternative Viewpoint


I guess I'm still looking for validation of the model against some published numbers.
GM’s PUBLISHED data gives no information on the US06 or other, e.g. constant velocity, driving conditions, or effects of wind conditions, accessory power loading, climbing grades, increased AER with decreased mass, estimated top speed, or effects of regen for different driving profiles. If you want answers to any of those questions, you have to do an (accurate physics based) model.

There is enough EV1 history, published Volt specs, current motor, converter, and battery tech info to make intelligent guesses about values of the Volt parameters. Obviously, there are many subtleties of behavior that are not known, but their quantitative effects are small compared to known major parameters like mass, motor behavior, frontal cross section/drag, and battery energy.

With regard to the stuff we design and manufacture, in today’s world of integrated circuits and nanotechnology, instruments are incapable of making accurate measurements of some critical parameters at the device’s scale because the measurement greatly perturbs what you are trying to measure. Designers can only get/infer accurate numbers for some critical design parameters with computer models. (You just have to be sure that you model all of the relevant phenomena at that scale. Parameter extraction then becomes a critical design step.) In today’s manufacturing world and beyond, you can’t design an airplane, automobile, new chemical, drug, or nuclear weapon (Lagrangians) or power plant without a computer model. The designer validates the parameter's measured/extracted/inferred behaviour against the well tested and known physics of the computer model, not the other way around. With today's enterprise scale systems, the major present worth of a company may be tied up in its parameter extraction, design, process, and production control models. Potentially, hundreds of millions of dollars of intellectual property in a flash drive.

If you had such a flash drive (or a portion of it) what would you do with it? Your country's national security or economic health might depend on the answer.
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Carnegie Mellon Study Applied to Volt

I did the Carnegie Mellon simulation for the Volt. The results of the simulation are shown at The simulation is based on the work found in . The CM restudy was done with $700/kWhr and an ICE cost equivalent to the Volt’s performance. The results are that for a 12 year lifetime the Volt’s NPV Average Lifetime Cost is less than that of an equivalent ICE. However, most buyers do not consider the lifetime cost. Research has shown that they base their buying decision on the perceived cost saving only over the first two or three years. Based on this criterion, the typical buyer will not purchase a Volt.
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