[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.

 

[BigRedFed]

I believe it's been mentioned in some of Lyle's blogs that GM used the EPA City Cycle.

 

[J in MN]

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.

Tom, what do you use for mass, drag area (or factor), motor efficiency, charging efficiency?

I plug in:
kerb mass (of Prius) + half a tank of fuel + driver = 1432 kg;
drag factor (emperical for Prius) = 0.5 kg.s/m;
motor efficiency = 90%;
charge efficiency = 90%,
and get US06 cycle energy consumption of 484 J/m or 7.4 km/kWh. That gives an AER of 59.5 km on 8 kWh, which is closer to 40 miles than 28 miles.

 

[Tom]

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?

 

[J in MN]

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?

Ok, thanks.

While thorough, it is very hard to follow and to compare to my Excel based calculations because of the hodgepodge of units of measure.

The primary difference is in the assumed efficiencies. I was assuming much higher values for what you call GPE and TInvE.

But, can you please explain more clearly the difference between TInvE and IPEE? And where do you account for traction motor efficiency?

 

[Tom]

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.

 

[gripperdon]

Whatever the method it needs to be more

Can't people get the message GAS is TOO High, Longer range is the key to selling more, Volume is what GM needs DESPERATELY. Volume will cure most ills. Quit fighting it and making excuses get more range.

 

[Koz]

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.

I believe the 8KWh that has been bantered about is for energy out of the battery and not totak diminished. Not that 2% will make a huge difference but every little bit helps. GM has said 40 miles end of life is their goal and beginning of life will be more like 50. Hopefully that pans out and must mean more than 8KWh in the beginning. I can't wait for them to start divulging more information, but this tidbit will probably not be divulged until late in the game. I do think they could tell us the cross sectional drag (Cd*A) without giving away too much. Maybe Lyle could ask for this in his next interview with GM

 

[Tom]

Average I^2R Losses

Koz,

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.

 

[Rooster]

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.

 

[Koz]

Koz,

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.

I thought you were referring to internal losses in the battery. 2% seems like a reasonable swag for line and terminal losses.

 

[Tom]

Delayed Reaction

Koz,

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.

 

[Tom]

2008+ MPG-Based AER

Koz,

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.

 

[Tom]

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.

 

[pennor1]

Here's a completely unscientific but real world response based on my personal observation...

Today I had a busy day. Busier than usual. I drove to town. I went to Lowe's, Costco for gas, the grocery store, the dentist, and a sporting goods store. I stopped and had lunch at Burger King, then I drove back home. Like I said, this was a busier day than usual. I drove 31 miles and burned about $8.50 worth of gas in the process. So give me 40, or give me 28 miles, I don't care. If I had the Volt today I would have used about 80-cents worth of electricty at my utilities current rate. I would have saved $7.70 and gotten everything done that I needed to do.

 

[Koz]

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.

My wife calls it something else, but it wasn't me this time. Blame the Rooster crowing.

 

[Rooster]

Worms

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.

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]

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.

 

[omegaman66]

.

Quit fighting it and making excuses get more range.

I disagree. Right now range is too expensive. It just isn't cost effective to pay more for range that few will really need.

Think about it from GM's and an unbiased outsiders view. Initially you will offer ONE car. Key words being initially and ONE. This car will NOT be a good fit for all. Actually it would be better to offer the Chevy Volt with a 20 mile range instead of 40 initially. Because with a 20 mile range they could sell twice as many cars... INITIALLY.

The world will get more benefit from twice as many people driving a car with half the electric range because more gasoline is displace this way. And although it isn't going to cut out all of peoples gas cost it will give a huge benefit but cutting out most of it.

The only reason that the range is being set at 40 miles is to show people that electric is viable, can give you the range you need and not leave you stranded when the battery is exhausted at a reasonable price.

Give it a time and I am sure more battery options will be offered as your choice of range extended electric vehicles grows.

If you are clamoring for more electric range that means you will use more than 40 miles in your commute. That would displace over 10,000 miles of gas driving each year. For someone to drive 20,000 miles a year that is a lot, and the volt would still displace over half of your gasoline cost assuming you don't use the electric on the weekends at all.

 

[Rooster]

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.

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.