Friday, September 11, 2009

What is the Volt's MPG?


In August '09 GM launched the "230" marketing campaign. They claimed that the Chevy Volt will get 230 MPG. Soon after GM launched the campaign, the EPA said that it could not support these claims, and Nissan pointed out that using the same calculation scheme the all-electric LEAF would rate at 367 MPG.


Plug-in hybrid cars are scheduled to be on the market in 2011. They promise to have significantly better gas mileage than traditional hybrids. These will be partially 'fueled' from the electrical grid with the balance of propulsion coming from gasoline (or ethanol). If a car uses electricity and gasoline, how do you rate its MPG when not all of the Ms are using Gs? How can an all-electric car even have an MPG?

PHEV, EREV, REEV; what are they?
The above questions are further complicated by the fact that PHEVs come in two different types. There are parallel-series plug-in hybrids such as the plug-in Prius and BYD cars and there are series plug-in hybrids such as the Chevy Volt or the Chrysler Envi line. Plug-in series hybrids are referred to as extended range electric vehicles (EREV) by GM and range extended electric vehicles (REEV) by Chrysler.

A series plug-in is propelled exclusively by an electric motor. The electricity comes from batteries for an initial range and then after the batteries are exhausted, the electricity comes from an on-board generator.
  • AER-All Electric Range
  • CSM-Charge Sustaining Mode (i.e. the flex fuel generator provides most of the electricity required to propel the car.)
For a parallel plug-in, it is not as clear when the gasoline engine will run. It will run when the batteries are low just like a series plug-in but it can also run when the vehicle is at high speed, during acceleration, or when climbing hills.

Given that multiple fuels will be used, how can you measure fuel efficiency? Some methods include: energy equivalence, pollution, and cost. Since the focus is currently on the MPG ratings, we'll use energy equivalence.

disclaimer
For the rest of this article, let's examine the 2011 Chevy Volt as presented in the sticker above from gm-volt.com. It is unofficial but matches all the data that is known at this point. I must point out, that despite the big 230 MPG marketing campaign, real answers about the Volt's performance are still being held close to the vest, so many of the assumptions below may turn out to be incorrect. I'll leave comments below if/when I am made aware of any discrepancies.

assumptions
The first thing to note is that GM claims a 40 mile electric range for the Volt. However, to ensure the battery life they are only going to use ~10kWh of the 16kWh pack for the electric mode. This means that you can only get the promised 40 miles in city driving (or congested freeway driving). Aerodynamic drag and rolling resistance reduce the efficiency at higher speeds. For this exercise we'll assume that all 40 miles can be traversed with just 10kWh using the better city performance values. Note that if your first 40 miles of daily driving includes some high-speed driving, you might not get the full 40 miles before the generator kicks in.

Since we are already dealing with the complexity of two fuels, let's assume a 50/50 split of city and highway driving for the charge sustaining portion of the drive. Averaging the 50 city and 45 highway yields 47.5 miles per gallon after the battery is depleted. Given that the car changes operating modes at the 40 mile mark, we'll need to examine different distances. I'll use 25, 50, 100, & 200 miles assuming each starts out fully charged.

While these single trip numbers are helpful illustrations, a month of driving with commuting and an occasional longer trip is more interesting. For this example month, Mon-Friday driving will be 30 miles (all-electric). On most weekends, we'll use 25 miles per day unless we go for a longer drive to the beach or the mountains. For these long weekend journeys, we'll use 200 miles round trip and assume that you can charge at your destination.

Example Month:
23 weekday commutes = 690 miles electric
6 short weekend trip = 150 miles electric
1 long trip weekend = 200 miles = 80 miles electric + 120 miles gas
Example Total = 1040 miles = 920 miles electric + 120 miles gasoline


Miles



Electricity
(kWh)


Gasoline Used
(gallons)

25

6.25

zero

50

10

0.2

100

10

1.3

200

10

3.3

1040*

230

2.5

* one example month

energy equivalence
This method of comparison converts all fuel sources, be it watts-hours or gasoline, to an energy unit such as Joules. One gallon of gasoline is 132 mega-Joules. A kilowatt-hour of electricity is 3.6 mega-Joules. With this you can convert the battery energy and gasoline used on a given drive to get a Joules per mile rating. Most people don't have an intuitive feel for Joules per mile, so this would generally be converted back into MPG and noted as MPGe, regardless of the energy source or how silly that result sounds. While this does give you an interesting efficiency number, it hides the sources of the energy. It is an over simplification that often adds confusion especially when the 'e' is missing. The energy source is an important factor in both the cost and pollution of those miles. And in cases where majority (or all) of the 'fuel' is electricity, stating a MPG rating is just not right, as Nissan illustrated when they stated the all-electric LEAF would rate at 367 MPG.

These are not exchangeable items; it is like reporting how many miles per banana it gets. But MPG is a unit that the public is used to hearing and anything over 100 MPG is attention getting. That is impossible for the marketing types to resist. Don't be fooled by this, demand to know the consumption numbers in both modes and do your own math for your driving patterns.

Here is our table of examples expanded to add energy consumption:


Miles



Electricity
(kWh)


Gasoline
(gallons)


Total Joules (MJ)


Miles per Megajoule


MPGe **

25

6.25

zero

23

1.1

147

50

10

0.2

62

0.8106

100

10

1.3

208

0.564

200

10

3.3

472

0.456

1040

230

2.5

1158

0.9119
** use MPGe with caution, and please don't drop the 'e'

You can see in the table above that there is no MPG equivalent that is close to the 230MPG that GM is reporting for the Chevy Volt, not even the all electric 25 mile trip. While I could not find the exact calculations that GM used, I think I found the cause of the discrepancy. The DoE formula includes a multiplier, called the Gasoline-Equivalent Energy Content of Electricity Factor (Eg), to compensate for upstream efficiencies in the fuel source. Currently, the government is promoting the idea of getting off of foreign oil (sounds good to me), so this compensation factor is very favorable for electrically powered transportation (you can read the full EPA explanation here).

The value being used for Eg is 2.42 in 2000 and it might be higher in the 2008 version of this specification currently in use. This means that the electrically powered mileage is getting extra credit at a significant rate.

Applying this factor and you can see in the table below that the 230 MPG number that GM is reporting is achievable for a trip just around 50 miles.

Miles

GM MPG

25

355

50

226

100

100

200

72

1040

267


This is much closer to what the 230 campaign is claiming (as incorrect as that might be). And you can see how the Nissan Leaf scored over 360 MPGe. My understanding is that the EPA test track is a 10 mile loop with various speed, acceleration, stop & start zones. For a plug-in car like the Volt, they will drive it until the batteries are depleted and then do one more lap around the track with the generator running. This corroborates that the 50 mile calculation is the one closest to the magic 230.

The 106 MPGe for the 50 mile trip in the second table or the one month of driving example at 119 MPGe would still be a very impressive number to launch a marketing campaign around and it would not have the backlash that quickly branded the 230 MPG campaign as unrealistic hype. You can see the reception that Conan O'Brien gave it on The Tonight Show in the video below.

If you are a regular reader of this blog, you know that I support plug-in vehicles. And I have discussed the importance of "well-to-wheel" consideration. Well-to-wheel is looking at the full life-cycle of the fuel that you use. However, hiding this in a single MPG number is not the right thing to do. This is really more of a pollution value than a vehicle fuel efficiency value. Presenting it simply as MPG, sets up unrealistic expectations. I like the idea of a pollution indication per mile, especially if were applied to all vehicles. On a regular gas powered car with a 32 MPG rating, when you look at all the energy used to process, refine, and move the fuel, you might see that it has a pollution rating of 120 pounds of CO2 per 100 miles. Whereas a grid powered 110 MPGe PHEV might have only 15 pounds of CO2. That would stop the "long tailpipe" argument quickly. And as crude moves from easy to extract light sweet crude to the to deeper and heavier forms, the gasoline numbers will get worse, whereas as the electricity grid will improve as new wind turbines, geothermal and solar thermal plants come online.

The consumer could decide how important each the fuel economy value and pollution factor value is to them and shop according to their own values. They might decide that only one of the numbers matters and simply ignore the other or they may try to find a balance.

Despite the fact that GM has greatly oversimplified things and shown the best possible MPG that they could (without using all electric), they just might be able to get away with this. Looking at the example month, there is 1040 miles of driving, using just 2.5 gallons of gas. If one were to ignore all the watt-hours used, 1000 miles on 2.5 gallons is 400 miles per gallon of gasoline used. Miles per gallon of "gasoline used" is not the same as MPG, but if this is the number that people are seeing, they may just feel like they are getting more that the 230MPG promised. And if your concern is the reduction of oil use (and not vehicle efficiency), this is a valid number to consider.

conclusions
With plug-in cars, the game has changed. People are used to a single MPG number and change can be difficult for some. The marketing departments are going to want to fly these great MPG(e) numbers on a high flagpole, even if they don't apply to the real world because people will ask and they are great numbers. GM and other companies making plug-in vehicles should strive for integrity and rise above the temptation to hustle these as facts.

Consumers are going to have to consider more than just a single MPG number. There is a learning curve. Auto companies can help by creating tools such as a webpage where prospective buyers could input their driving patterns and information such as can they plug-in at work, etc. Then this webpage could generate a custom report with cost, consumption, pollution and other relevant information. Hymotion has a web tool to do just this for people that are considering a PHEV conversion. I hope GM creates one for the Volt too.

Now you are more aware and can be an informed skeptic when confronted with only an MPG number for a plug-in vehicle. Decide for yourself what aspects are important and do the math.




1 comment:

CelticSolar said...

some progress
http://news.cnet.com/8301-11128_3-10364352-54.html