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

MPG-Based Approach

Tom,

Thanks for checking your model against the EPA MPG test profiles. In my earlier post I assumed GM would just use the US06 profile to calculate the range. Upon further investigation, I agree this is incorrect as US06 is too aggressive. Although I do find it encouraging that your model predicts the Volt would still achieve almost 30 miles all electric using that profile.

Anyway, inspired by your results, I dove head first in to the EPA Final Technical Support Document EPA420-R-06-017, which details the revisions to improve the calculation of fuel economy estimates that became effective with model year 2008. www.epa.gov/fueleconomy/420r06017.pdf


If I were GM, I would base my range estimate for the Volt using a “combined range estimate” extrapolated from the technique the EPA uses to calculate the combined MPG. The EPA calculates the combined fuel economy by weighting the city at 55 percent and the highway at 45 percent using the following equation:

FE combined = (55% x City FE) + (45% x HWY FE)



Using this equation and substituting range for MPG, results in the following “combined range” equation:

Range Combined = (55% x city range) + (45% x hwy range)

Plugging in the raw range numbers your model predicted for City and Hwy range yields a combined range of 39.9 miles.

Range Combine = (55% x 40.2) + (45% x 39.6) = 39.93.

For pre-2008 model year vehicles, the city and highway tests (I understand these are the 2 profiles you ran in your model, in addition to US06) were adjusted downward to account for typical road conditions that can affect fuel economy, such as higher speeds, cold temperature, and use of air conditioning. The city results were adjusted downward by 10 percent and the highway results were reduced by 22 percent. I know this is deep into the specifics, but didn’t you state your model includes allowances for headwinds, grades, A/C load, etc? I believe the answer is yes, and if correct, your model’s range predictions would appear to agree with GM’s claims!

HOWEVER, for 2008 and later model year vehicles, the fuel economy label estimates are determined by either performing the entire set of five tests on the test vehicle (see http://www.epa.gov/fueleconomy/420f07066.htm#calc), and using that data to calculate city and highway estimates, or by applying mathematical derived linear regression equations to the basic data from the city and highway tests to obtain estimates that simulate the effects of all five test cycles. The second approach is referred to as the mpg-based approach, since the resultant label adjustment will vary depending on the measured fuel economy (i.e., mpg) of a vehicle over the FTP and HFET tests, and will not require any additional tests. (See pages 2 and 5, EPA420-R-06-017)

The mpg-based approach is the following:

MPG based city FE = 1 / (.003259 + 1.18053/FTP FE),
where FTP FE = City test/profile

MPG based highway FE = 1 / (.001376 + 1.3466/HFET FE),
where HFET FE = Highway test/profile

For example, substituting ranges for fuel efficiencies, and using the range values predicted by your model yields the following ranges:

MPG based city range = 1 / (.003259 + 1.18053/CITY RANGE)
MPG based city range = 1 / (.003259 + 1.18053/40.2)
MPG based city range = 30.7 miles

MPG based highway range = 1 / (.001376 + 1.3466/HWY RANGE)
MPG based highway range = 1 / (.001376 + 1.3466/39.6)
MPG based highway range = 28.3 miles

And using these values to calculate a MPG based combined range yields

MPG based combined range = (55% x 30.7) + (45% x 28.3)
MPG based combined range = 29.6 miles

As a check, it would be interesting to remove the "typical road conditions" (headwinds, grades, A/C load, etc.) parameters in your model , and recalculate the HWY and City Ranges. Then plug these values for HWY and City ranges into the MPG based approached as a check? I’m guessing this is what GM is doing, and predict it should also equal approximately 40 miles. If correct, this result would be additional confirmation that your model and assumptions are pretty darn good, thus it would be reasonable to assume the ranges it is predicting at different sustained speeds is also accurate.

Worms

Tom said:

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.

Click to expand...

Couple of thoughts/theories based on your results:

(1) GM may intend to use more of the battery capacity for EV only power, say 55-60% vice the claimed 50%. Perhaps the battery will be charged to roughly 85% of capacity and allowed to drain to 25% before the ICE starts to bring the SOC back to 30%. That would equate to more range, not sure what it would do to battery life?

(2) The EPA MPG-Based correction is on based linear regression calculated from the real world data of ICE powered cars. The correlation fits the real world MPG data well, R^2 values > 95%, and represents additional power (work) required to overcome real world conditions vice the work required during the two tests on a dynameter. An ICE powered car is less efficient than an electric vehicle, so perhaps you are correct that this correction is not applicable to an EV. I do believe the increase in work it represents is likely accurate, but the amount of additional battery power required to overcome the real world losses is a function of the overall vehicle efficiency (power to the wheels). Since EVs are more efficient, my assumption that EV,RangeHwy/City = MPG,Hwy/City is probably not correct. I think the trick is to going to be figuring out how much additional work the MPG-based correction represents, and correlate it to increased Electric Power. EVs are more efficient at putting power to the road than an ICE powered vehicle, so that must be factored into the correction.

(3) The EPA conducts its Hwy/City profile testing on rollers, and I noted the friction values in your model are based on road friction. I would assume steel rolls have less friction than the values used in your model, so the friction value may need to be adjusted. Road friction would then be taken into account by the MPG -based correction, which needs to be correlated to a net power increase to represent real world driving conditions as discussed in (2) above.

Tom said:

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.

Click to expand...

Tom,

Just re-read this post. Based on statements by GM since this post, I understand the 40 mile range is based on City Driving. Thus I assume the Hwy range, and the real world range, will be closer to the estimate your model is providing.

However, everyone needs to keep the big picture in mind. According to a Bureau of Transportation Statistics (BTS) Omnibus Household Survey, 78% of US commuters drive 40 miles or less each day, and 68% drive 30 miles or less each day.

http://www.bts.gov/publications/omnistats/volume_03_issue_04/html/entire.html

In 2003 U.S. passenger cars consumed 74,590,137,000 Billion gallons of gasoline which roughly equates to 3,825,135,200 Billion barrels of oil (i.e., 1 barrel oil (42 gallons) = 19.5 gallons of gasoline). In 2003, the US imported 4,101,870,000 Billion barrels of oil. If every passenger car in the United States had been a E-REV with a 30 mile electric range, then 68% of the demand for passenger car gasoline would be eliminated according to the BTS survey data. That equals 2,601,091,936 Billion barrels of oil, or 63.4% of all net imported oil for the year. Not a bad start for first generation E-REV car technology.

So why is everyone focusing on a 40 mile EV range as a target? Here's why:

If every passenger car in the United States was a E-REV with a 40 mile electric range, then 78% of the demand for passenger car gasoline would be eliminated according to the BTS survey. That equals 2,983,605,000 Billion barrels of oil, or 72.7% of all net imported oil for the year.

Furthermore, an additional 12% of daily commuters in the BTS survey drive between 40 and 60 miles per day. If they too drove an E-REV with performance like the Volt, their equivalent gas mileage would be 150 MPG or greater. That is nearly 7 times the 2003 average passenger car fuel consumption of 22.3 MPG. Thus the next 12% of American drivers who consumed approximately 459,016,220 Million barrel of oil in 2003, would have consumed less than 68,523,316 Million barrels of oil. That equals another 10% net reduction in imported oil. Ignoring the remaining 10% of American drivers who drive more than 60 miles per day, the first 90% have the potential to eliminate more than 82% of net imported oil by driving a E-REV like the Volt.

We could then make up the remain 18% with domestic renewable fuels, if we wanted which brings me to my second point.

I say "if we wanted" because it will require a floor on the price of a barrel of oil of around $70-80 a barrel. Americans drove 5.6% less last year according to the U.S. Department of Energy , and demand for petroleum in 2009 is not forecasted to increase significantly due to the global economic downturn. Consequently, oil has dropped from an average weekly high of $133.60/barrel on 4 July 2008 to $62.82 as of 24 Oct 2008. That’s a 47% reduction in price in only 3 months from a 5.6% reduction in US demand for petroleum.

Anyone care to guess what would happen to the price of oil if US demand for it is gashed by 10%, 20%, 30% -- 82%? Well, here's a clue, According to Paul Winters, a spokesman for the Biotechnology Industry Organization, investors think an oil price of $70 to $80 per barrel is the threshold at which cellulosic ethanol becomes viable. On Friday, 14 Nov 2008, oil closed at $57.04/barrel. Bye Bye celluolisic ehtanol.

BTW, a $70 floor is equal to an average price for regular gasoline of $2.88/gallon.

Favor to ask

Tom:

Would you mind halving the weight of the Volt in your simulation, to see what it does to EV range, against the 3 EPA test profiles?

The question I'm trying to answer is what percent improvement in EV performance would a light weight composite vehicle allow for in the Volt, all else being equal. If it's not too much trouble, maybe 3 runs using different % weights for the vehicle (50%, 60% and 70% of the 3140 lbs vehicle in your model), plus the weigh of the driver (170 lbs).

You're model, by far, has the most fidelity of any I've seen. I attempted to make one in Excel, but it is no where near as comprehensive as yours. So I trust your numbers more than mine.

BTW, what program are you using? I'm guessing it's not Excel.

Thanks

Tom said:

Rooster,

Results are in attachment.

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

Regards,
Tom

Click to expand...

Tom,

Mucho Gracias!

I'm going to have to bite the bullet and purchase Mathcad. When I need to integrate, I still use my trusty HP-48SX that I purchased in 1989! It still works great, but it can be a tad slow at times. ;-)

Anyway, your numbers are more in line with what I was expecting. I used a spreadsheet to try and measure the work performed in KWh (ignoring the EV component efficiencies). The bottom-line, my "model" underestimated the efficiency gains, which was my hunch.

The more I read, the more I believe GM is basing their estimates on the FTP and HWY tests alone -- so I think your model is about as accurate as it can be without having the actual performance data. In fact, the more I think about it, I'm not so sure the 2008 MPG regression corrections are going to be applicable to an EV anyway. Once you have the trons in the battery, an EV power-train should be more efficient that an ICE power-train. Consequently, the regression analysis, which is based on ICE power-trains, is likely overestimating the loss of efficiency.

Your model shows a 37% improvement in the CITY profile, and a 29% improvement in the HWY profile by halving the weight. Nothing to sneeze at. I do hope at some point in the future, GM attempts an all composite E-REV based on technology like Fibreforge -- maybe in a Cadillac? Assuming of course, they manage to stay in business.

Thanks again for running the numbers!

Cheers



Any,

Excellent Article, thanks for posting it. It follows very closely to the calculations in Tom's model of the Volt (see data below):


Volt Curb Wt (100%) = 3140 lb
Driver = 170 lb

EV Range in miles using 3 EPA Test Driving Profiles:
Curb Wt (%) / FTP / HWY / US06
50% / 63.8 / 56.3 / 32.3
60% / 56.5 / 52.1 / 35.8
70% / 51.7 / 48.5 / 32.97
80% / 46.5 / 45.3 / 31.3
90% / 42.9 / 42.7 / 28.9
100%/ 40.1 / 40.1 / 27.5