Timpo
12-02-2009, 09:50 AM
2011 Chevrolet Volt: If Not 230 MPG, Then What?
By Dan Edmunds | November 30, 2009
The involuntary eye-rolling began mid-sentence as we read GM's first press release claiming that the fuel economy of the 2011 Chevy Volt would be about 230 mpg.
The method by which this number was obtained was not clearly stated, and GM made vague references to a "draft EPA procedure". But we spoke to engineers at the EPA (Environmental Protection Agency), and they weren't clear on the formulas GM used to generate that much-touted figure, either.
Here's the problem: plug-in hybrids like the Volt are designed to be refueled with two different sorts of energy--electricity from a wall outlet sold in kilowatt-hours (kWh) and gasoline from a gas pump sold in gallons. No agreed-upon methodology exists to describe the fuel economy of a vehicle that uses two external fuel sources in the normal course of events.
Our resident engineers decided to take a crack at it, and the mathematical exercise that followed led to a far different conclusion. At today's national average prices for gasoline and residential electricity, the 2011 Chevrolet Volt will cost the same to refuel as a 59-mpg gasoline-powered car: both require $55.85 to cover the 1,250 miles that make up a typical month in a 15,000-mile driving year.
Details on how we arrived at these figures follow after the jump.
To begin, we need to understand how the Volt works. GM calls the Volt an "extended-range electric vehicle", but we dislike that term because it sounds like an electric car with a humungous battery for lots of range. But the Volt is an electrically-driven car that pairs a relatively small battery with 8 kWh of useable storage that provides 40 miles of range with a gasoline engine that generates additional electricity for extending the driving range for as long as you need--so long as you understand it's burning dinosaur juice until you plug in again.
There's a well-established word for cars that run on gasoline and electricity--we call them hybrids. Sure, the Volt can be plugged-in to pre-charge the battery and lessen the amount of time the engine runs, and that makes it a plug-in hybrid, a much-anticipated variation on the hybrid theme. A pure electric car with just 40 miles of range would be impractical in the extreme, but a plug-in hybrid with that much electric range looks impressive indeed.
The trick to estimating a plug-in hybrid's monthly fuel cost involves knowing the percentage of total miles the car will run on electricity (charge-depleting mode) and on gasoline (charge-sustaining mode). The Society of Automotive Engineers (SAE) has spent a lot of time studying this very question, and they have come up with a standard procedure for making such an estimate, SAE standard J2841.
They call the percentage of electric-only miles the Utility Factor (UF). To determine the UF of a given plug-in hybrid, we simply insert the vehicle's electric range into the SAE's chart. The Volt's 40-mile range returns a UF of 0.62, which means the average Volt will spend 62% of its life running on electricity that came in via the plug and 38% on electricity generated by the gasoline engine. Armed with these numbers, we can properly weight the costs of our two fuels; electricity and gasoline.
No clear-cut fuel consumption figures have been released, electrical or otherwise, so we applied some more math and made some careful estimates based on specific GM performance claims. We start with the preliminary specs GM has given us: a useable battery storage capacity of 8 kWh and an electric range of "up to" 40 miles. Dividing one by the other returns a best-case electric-only consumption rate of 20 kWh per 100 miles.
But you'll actually pay for more electricity than that because battery charging is an imperfect process. Batteries generate heat when they're being charged, so a fan may come on or a pump may circulate fluid to keep things cool. These losses represent electricity you pay for, so they have to be included as consumption even though none of it makes it into the batteries. For this exercise we're assuming a fairly common charging efficiency of 85%. Put another way, the electric consumption seen from your wallet's point of view rises by 15% to 23.5 kWh per 100 miles.
Our typical driving month is 1,250 miles, and 62% of that is 775 miles. At our electric consumption rate of 23.5 kWh per 100 miles, the Volt will consume 182.4 kWh of electricity during the month. The national average cost of electricity is 12.05 cents per kWh, so this equates to a monthly electricity cost of $21.97.
We've made further estimates on gasoline consumption in charge-sustaining mode because GM hasn't released specifics. Volt Chief Engineer Andrew Farah told us the "unadjusted" development target is 50 mpg in the so-called "FTP" city mileage test. But for our purposes we require EPA combined with the most recent 2008+ window sticker adjustments included because our own fuel economy testing has shown them to equate well to the real world. We have the conversion formulas, and 50 mpg city (raw unadjusted ftp) equates to 37 mpg city on a 2008+ window sticker.
The 38% of the month that is spent on gasoline amounts to the other 475 of our 1,250 miles. A gasoline fuel economy of 37 mpg means this distance will require 12.8 gallons, which costs $33.88 at the current national average price of $2.64 per gallon.
Taking gasoline and electricity together, the total monthly cost for 1,250 miles in our Volt is $55.85 - the same as a standard car that achieves 59 mpg on gasoline alone over the same distance.
Of course these costs can range lower for those who have solar panels or can take advantage of lower off-peak charging rates when they plug in, but we're not trying to account for the best-case scenario here. This calculation is attempting to figure the average fuel cost for the average consumer using current national average prices.
And what we come up with for the 2011 Chevy Volt is still impressive compared to the class-leading Toyota Prius. But it's nowhere near 230 mpg.
http://blogs.insideline.com/straightline/assets_c/2009/11/2011_volt_det_chevrolet_fd_5-thumb-717x477.jpg
By Dan Edmunds | November 30, 2009
The involuntary eye-rolling began mid-sentence as we read GM's first press release claiming that the fuel economy of the 2011 Chevy Volt would be about 230 mpg.
The method by which this number was obtained was not clearly stated, and GM made vague references to a "draft EPA procedure". But we spoke to engineers at the EPA (Environmental Protection Agency), and they weren't clear on the formulas GM used to generate that much-touted figure, either.
Here's the problem: plug-in hybrids like the Volt are designed to be refueled with two different sorts of energy--electricity from a wall outlet sold in kilowatt-hours (kWh) and gasoline from a gas pump sold in gallons. No agreed-upon methodology exists to describe the fuel economy of a vehicle that uses two external fuel sources in the normal course of events.
Our resident engineers decided to take a crack at it, and the mathematical exercise that followed led to a far different conclusion. At today's national average prices for gasoline and residential electricity, the 2011 Chevrolet Volt will cost the same to refuel as a 59-mpg gasoline-powered car: both require $55.85 to cover the 1,250 miles that make up a typical month in a 15,000-mile driving year.
Details on how we arrived at these figures follow after the jump.
To begin, we need to understand how the Volt works. GM calls the Volt an "extended-range electric vehicle", but we dislike that term because it sounds like an electric car with a humungous battery for lots of range. But the Volt is an electrically-driven car that pairs a relatively small battery with 8 kWh of useable storage that provides 40 miles of range with a gasoline engine that generates additional electricity for extending the driving range for as long as you need--so long as you understand it's burning dinosaur juice until you plug in again.
There's a well-established word for cars that run on gasoline and electricity--we call them hybrids. Sure, the Volt can be plugged-in to pre-charge the battery and lessen the amount of time the engine runs, and that makes it a plug-in hybrid, a much-anticipated variation on the hybrid theme. A pure electric car with just 40 miles of range would be impractical in the extreme, but a plug-in hybrid with that much electric range looks impressive indeed.
The trick to estimating a plug-in hybrid's monthly fuel cost involves knowing the percentage of total miles the car will run on electricity (charge-depleting mode) and on gasoline (charge-sustaining mode). The Society of Automotive Engineers (SAE) has spent a lot of time studying this very question, and they have come up with a standard procedure for making such an estimate, SAE standard J2841.
They call the percentage of electric-only miles the Utility Factor (UF). To determine the UF of a given plug-in hybrid, we simply insert the vehicle's electric range into the SAE's chart. The Volt's 40-mile range returns a UF of 0.62, which means the average Volt will spend 62% of its life running on electricity that came in via the plug and 38% on electricity generated by the gasoline engine. Armed with these numbers, we can properly weight the costs of our two fuels; electricity and gasoline.
No clear-cut fuel consumption figures have been released, electrical or otherwise, so we applied some more math and made some careful estimates based on specific GM performance claims. We start with the preliminary specs GM has given us: a useable battery storage capacity of 8 kWh and an electric range of "up to" 40 miles. Dividing one by the other returns a best-case electric-only consumption rate of 20 kWh per 100 miles.
But you'll actually pay for more electricity than that because battery charging is an imperfect process. Batteries generate heat when they're being charged, so a fan may come on or a pump may circulate fluid to keep things cool. These losses represent electricity you pay for, so they have to be included as consumption even though none of it makes it into the batteries. For this exercise we're assuming a fairly common charging efficiency of 85%. Put another way, the electric consumption seen from your wallet's point of view rises by 15% to 23.5 kWh per 100 miles.
Our typical driving month is 1,250 miles, and 62% of that is 775 miles. At our electric consumption rate of 23.5 kWh per 100 miles, the Volt will consume 182.4 kWh of electricity during the month. The national average cost of electricity is 12.05 cents per kWh, so this equates to a monthly electricity cost of $21.97.
We've made further estimates on gasoline consumption in charge-sustaining mode because GM hasn't released specifics. Volt Chief Engineer Andrew Farah told us the "unadjusted" development target is 50 mpg in the so-called "FTP" city mileage test. But for our purposes we require EPA combined with the most recent 2008+ window sticker adjustments included because our own fuel economy testing has shown them to equate well to the real world. We have the conversion formulas, and 50 mpg city (raw unadjusted ftp) equates to 37 mpg city on a 2008+ window sticker.
The 38% of the month that is spent on gasoline amounts to the other 475 of our 1,250 miles. A gasoline fuel economy of 37 mpg means this distance will require 12.8 gallons, which costs $33.88 at the current national average price of $2.64 per gallon.
Taking gasoline and electricity together, the total monthly cost for 1,250 miles in our Volt is $55.85 - the same as a standard car that achieves 59 mpg on gasoline alone over the same distance.
Of course these costs can range lower for those who have solar panels or can take advantage of lower off-peak charging rates when they plug in, but we're not trying to account for the best-case scenario here. This calculation is attempting to figure the average fuel cost for the average consumer using current national average prices.
And what we come up with for the 2011 Chevy Volt is still impressive compared to the class-leading Toyota Prius. But it's nowhere near 230 mpg.
http://blogs.insideline.com/straightline/assets_c/2009/11/2011_volt_det_chevrolet_fd_5-thumb-717x477.jpg