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

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.

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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.

The 8kwHr EPA 5Cycle AER Discrepancy

Rooster,

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.

AER vs Curb Weight

Rooster,

Results are in attachment.

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


pdt,

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.

Regards,
Tom

Alternative Viewpoint

pdt,

I guess I'm still looking for validation of the model against some published numbers.

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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.