Archive for the ‘efficiency’ Category


Which car would you rather drive?

September 19, 2010

Which of these two cars would you rather drive… 

Smart Car

Edison 2

The top car is the mis-named “Smart Car” from Daimler AG.  For somewhere between $12,000 and $20,000 you get a vehicle that carries two people gets 33 mpg city and 41 mpg highway for a combined 36 mpg.  That’s almost 85% of the fuel economy of a Honda Civic – fifteen years ago.    

The bottom picture is of the  Automotive XPrize winner,  the Edison2.  The Edison2 seats four, gets over 100 mpg, has a top speed of of 110 mph and a range of over 600 miles on a single tank of gas.  It will travel 50 mph on a mere 3.5 horsepower. and will go from 0 to 60 in less than 10 seconds.  And it would cost half as much as a Chevy Volt.   Edison2 is headed up by Oliver Kuttner, and according to consumer reports Kuttner says the Edison2

has plans for a car that is closer to being production ready, with bodywork that sounds more substantial. Should it progress to production, the car could be offered in the $20,000 range.

The call is yours, which would you rather drive? 


I was (partially) wrong

August 20, 2009

I recieved  comment form the GM spokesman, Rob Peterson, about my last two posts lambasting the supposed 230 mile per gallon Chevy Volt.  Here is Rob’s comment  in its entirety.

This is Rob Peterson from GM.

Although the Volt has a 16 kWh battery, only 8 kWh is used. This will significantly impact the rest of your calculations and your synopsis. Please post a correction based on this fact.

As for the Volt’s city fuel efficiency rating of 230mpg – this is based on the EPA’s draft methodology. The same methodology which will be used for all other vehicles of this type.


I responded to Rob with two comments, which you can read here.  One of those comments questions his sincerity about “blaming” the 230 mile per gallon claim on the EPA.  However, he is essentially right about the the charging cycle of the 16 kWh battery only using about half of that.  He has asked me to “Please post a correction based on this fact.”  I have done so, but the final numbers for the vaunted Volt are still underwhelming.

Here is a table comparing miles per gallon, kWh per mile, and pounds of CO2 per mile for the Chevy Volt, the Toyota Prius, and the a couple of ancient Honda Civics.  You can read the details of how I derived the numbers for the Volt, using Rob’s partial capacity charge cycle scheme in the text below.  Note that the prices for the Honda Civics have been adjusted for inflation to 2009 dollars for a fair comparison.

milage chart copy

Now that I have posted a correction, can I expect Rob Peterson to post a retraction of GM’s preposterous 230 mile per gallon claim?  Not Likely.

I  have not yet been able to find an official specification for the number of kilowatt-hours per mile for the Volt.  I am hoping Rob will provide one.  I have found Rob’s description of the charging scheme for lithium-ion batteries to be essentially correct.  That is, the battery is typically charged by the electrical grid to around 90% of total capacity.  Then the car is propelled entirely off of battery power until it reaches about 30% capacity.  This is known as the “charge depletion” mode.  When the battery gets to about 30% of capacity the gasoline powered generator kicks in and maintains the charge at about 30% capacity.  This is known as the “charge sustaining” mode.  

Then, when the battery is plugged into the electrical grid it is recharged with grid energy from about 30% capacity back up to about 90% capacity    That is a range of about 60% of the total capacity.  So, for a 16 kilowatt-hour battery, a complete charge off the electric grid puts about 9.6 kWh (0.6 x 16 kWh) into the battery.  But an extra 10% or so is lost due to transmission line and battery conversion losses.  So the amount of power taken from the electrical grid will be about 10.6 kilowatt-hours. If that charge will propel the car for 40 miles, then that works out to 3.8 miles per kWh (or about 0.27 kilowatts per mile) 

I cannot find the value of about 0.27 kWh per mile anywhere in the specifications for the Volt, but I did find this somewhat cryptic statement at

“Under the new procedure, the EPA weights plug-in electric vehicles as traveling more city miles than highway miles on only electricity. The EPA procedure would also note 25 kilowatt-hours/100 miles electrical efficiency in the city cycle.

So, lets accept the value of 25 kilowatt-hours/100 miles (0.25 kWh per mile) for the moment.  What is the affect that this will have on the numbers I reported for CO2 emissions?

The number of pounds of CO2 emitted per mile while powering the car with gasoline (known as the “charge sustaining” mode) will remain unchanged.  There are 19.4 pounds of CO2 produced per gallon of gasoline burned, and GM claims 50 miles per gallon in “charge sustaining” mode.  So:

( 19.4 lbs of CO2 / gallon) / (50 miles / gallon) =
0.39 lbs of CO2 per mile

This 0.39 lbs of CO2 per mile for the Volt running on gasoline (charge sustaining mode) is the same as for the Toyota Prius, because it also gets 50 miles per gallon.

Here is the same calculation for my ancient 1988 Honda Civic hatchback that got 47 miles to the gallon:

( 19.4 lbs of CO2 / Gallon) / (47 miles / gallon) =
0.41 lbs of CO2 per mile

And for the 197887 Honda Civic Coupe HF, which got 57 miles per gallon:

( 19.4 lbs of CO2 / Gallon) / (57 miles / gallon) =
0.34 lbs of CO2 per mile

Lets assume now that the Volt uses 0.25 kilowatt-hours per mile (“25 kilowatt-hours/100 miles’) when running off of power provided to the battery by the electric grid (known as the “charge depleting” mode).  On the average the grid yields 1.34 pounds of CO2 per kilowatt-hour. The grid transmission losses and grid to battery conversion losses  add up to about 10%.  So the amount of CO2 yielded per mile will be:

(1.34 lbs of CO2 per grid kWh) x (0.25 kWh per mile)  x 1.1 =
0.37 lbs of CO2 per mile

Almost identical to the CO2 emitted when it is running off of gasoline (0.39 lbs of CO2 per mile).  And it is also nearly identical to the amount of CO2 per mile as the  much cheaperPrius generates while running off of gasoline.

But here it the rub.  If the Volt is driven in an area where the electricity is predominantly generated with coal (by far the most common source or electricity generation in the US), then the CO2 emissions go way up.  That is because Coal emits about 2.1 pounds of CO2 per kilowatt-hour generated for the electric grid.  So again we can asume 10% for the sum of the grid transmission losses and grid to battery conversion losses.  Then the amount of CO2 that the Volt yields per mile driven in a region where coal is the primary source of electricity will be:

(2.1 lbs of CO2 per grid kWh) x (0.25 kWh per mile) x 1.1 =
0.58 lbs of CO2 per mile

If we really concerned about reducing CO2 (I’m not), saving energy (I am), creating American jobs (I am), and saving money (I am), then we should support the production of an American car that is similar to a 1988 Honda Civic.  Why argue the merits of a $40,000 car that few people will ever be able to afford?  A $15000 dollar car that gets as good or better mileage and generates as little or less CO2 would be bought by millions and have a much greater impact.


More eye opening facts about the Chevy Volt

August 18, 2009

OK, so maybe the Chevy Volt doesn’t really get 230 miles per gallon.  Are such exaggerations justified because they serve a greater cause?  The Chevy Volt will help save the world, after all, by reducing  Co2 emissions, right?


In fact, in some cases the amount of CO2 generated per mile for the Chevy Volt is the same as a conventional automobile getting only 21 miles to the gallon.  Read on…

When running on gasoline (known as “charge sustaining operation”) the Volt will get 50 miles per gallon.   According to the EPA burning one gallon of gasoline yields 19.4 pounds of CO2.  That means the CO2 emitted per mile driven while running on gasoline will be 0.39 pounds.

 ( 19.4 lbs of CO2 / Gallon) / (50 miles / gallon) = 0.39 lbs of CO2 per mile

How much CO2 will be emitted per mile when the Volt is powered by energy from the electrical grid that has been stored in its battery?  That depends on how the energy on the grid is generated.  If you live in an area where the power on the grid is generated primarily with coal, then the amount of CO2 per kilowatt-hour generated is fairly high.  If you live in an area where the power on the grid is generated primarily from nuclear, then the amount is fairly low.  On the average, though, there are 1.34 pounds of CO2 pumped into the atmosphere for every kilowatt-hour of energy generated for the electric power grid in the United States, according to the Department of Energy (2000).

The fully charged lithium-ion batteries hold 16 kilowatt-hours of energy and will propel the Volt 40 miles.  That works out to 0.4 kilowatt-hours per mile.  So that means on the average, 0.54 pounds of CO2 will be put in the atomosphere for every mile that the Volt drives on energy drawn from the electrical grid, assuming perfect charging efficiency.

(1.34 lbs of CO2 per grid kWh) x (0.4 kWh per mile) = 0.54 lbs of CO2 per mile

But charging a lithium-ion battery off the grid is not 100% efficient.  There are grid transmission losses and grid to battery conversion losses which add up to about 10%.  So running your Volt  off of electric grid power will yield closer to 0.59 pounds of CO2 for every mile your drive.  That is 151% of the CO2 put in the atmosphere by the running the Volt off of gasoline.

How many miles per gallon must a conventional automobile get in order to put the same amount of CO2 into the atomsphere per mile as a Chevy Volt does when running off of grid power?  That’s easy- about 33 miles per gallon.  Here are some cars that will do better.

( 19.4 lbs of CO2 per Gallon) / (0.59 lbs of CO2 per mile) = 33 miles per gallon

If you drive in an area where the electric grid is primarily powered by coal, then the numbers are even worse.  Burning coal to power the electric grid yields about 2.1 pounds of CO2 for every kilowatt-hour generated.  Driving your Volt with grid generated power will yield about 0.92 pounds of CO2 for every mile driven (when 10% conversion inefficiencies are added in).

(2.1 lbs of CO2 per grid kWh) x (0.4 kWh per mile)  x 1.1 = 0.92 lbs of CO2 per mile

That is the same amount of CO2 per mile as a conventional automobile that gets only 21 miles per gallon!

( 19.4 lbs of CO2 per Gallon) / (0.92 lbs of CO2 per mile) = 21 miles per gallon

So don’t be fooled by astronomical claims of miles per gallon for the Chevy Volt.  And if you are worried about CO2 (I’m not), then don’t count of the Chevy Volt to save you - it won’t.


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