calculation details of Volt's 230mpg city estimate

[john1701a]

Anyone else tired of the vague & misleading claims from the Volt enthusiasts?

Now that GM is pulling the same nonsense, the time has come for us to do something about it.


Here's a summary of my attempt...




What I did was create a spreadsheet which allows you plug-in (pun intended!) keys values of DISTANCE-TO-WORK and CHARGE-SUSTAINING-MPG along with individual entries to represent how you drive throughout the week. In others words, all the numbers are displayed for maximum clarity.

After entry is finished (the purple cells), totals automatically calculate and the graphs generate. Here's a sample...





And this the spreadsheet itself: 2007 format & 97-2003 format


I also created downloadable documents: 2007 Word & PDF format


The estimate results vary dramatically based on some rather critical values that will be far from consistent from owner to owner... as you can clearly see by playing with the spreadsheet.

Anyone have detail of their own to contribute?

.

[Glider]

You could argue that MPG only truly only makes sense when the car uses only gasoline (or another liquid fuel). But since people apparently want to know the "MPG" for plug-in (+gasoline) cars, the only way I ever saw it done that makes sense to me is to assume a total distance between charges. Then, you input how far you go on EV mode and the remaining distance (= d) on gasoline at a certain MPG(gas). Calling the combined MPG (for both EV and gas) = MPG gives

MPG = MPG(gas)*(d+40)/d

In the beginning, GM said MPG(gas) = 50. So then

MPG = 50(40+d)/d

Solve this for d = 2000/(MPG - 50) = 2000/(230 - 50) = 11 1/9 miles.

So, if every day you average 40 miles on electricity and then 11 1/9 miles on gas (at 50 MPG), you will get a total of 230 MPG. GM apparently thinks that 51 1/9 miles represents the average daily distance driven by Americans, and they are probably pretty close. The problem is that 230 MPG only works when you average exactly 51 1/9 miles per day and recharge every night.

I know the EPA has an energy equivalent way to do it, but I think it comes out the same thing as above. Not completely sure about that.

[daniel]

That's all kind of information overload for me. I would like two simple numbers:

1. Watt-hours per mile when operated on batteries.

2. MPG when operated on gasoline.

Giving "equivalent" mpg for electric usage is meaningless since there are different ways to consider it. And since different people will have different ratios of electric usage to gas usage, any system that tries to average them will be inappropriate for most drivers.

Just tell us wh/mi for electric and mpg for gas.

Of course they probably don't want to admit that once the charge runs out it'll get something like 20 mpg.

[john1701a]

Originally Posted by daniel

2. MPG when operated on gasoline.

It's been a month since the "230 MPG" announcement and well over a year since some of us starting begging for the answer to that question.

After all that time, the MPG during CS mode (Charge Sustaining) remains a complete mystery. GM simply does not what to tell us. Why?

It would indeed make matters easier.

After all, the decision whether to keep driving or run errands later focus will on that MPG value, not the overall efficiency. But then again, purchase decision is overall based.
.

[Glider]

Originally Posted by daniel

That's all kind of information overload for me. I would like two simple numbers:

1. Watt-hours per mile when operated on batteries.

2. MPG when operated on gasoline.

Giving "equivalent" mpg for electric usage is meaningless since there are different ways to consider it. And since different people will have different ratios of electric usage to gas usage, any system that tries to average them will be inappropriate for most drivers.

Just tell us wh/mi for electric and mpg for gas.

Of course they probably don't want to admit that once the charge runs out it'll get something like 20 mpg.

Your points make sense. You are trying to figure out the compound energy used in both electric and gas modes. I don't know if GM has released the Wh/mi number, but they originally said the MPG on gas was 50 MPG (maybe they later dropped this to the upper 40's, but I've always use the higher number = 50 MPG).

What I was saying (and what GM originally said) was you go a certain distance and use a certain amount of gasoline, and that's how you get MPG. So, simplifying what I said before boils down to:

Distance travelled = 40 miles on EV + 11.11 mi on gas = 51.11 mi

Gas used = 0 gals on EV + 11.11mi / (50 mi/gal) on gas = 0.2222 gals

MPG = miles / gallons = 51.11 mi / 0.2222 gals = 230 MPG

NOTE - that this method gives you a free ride on the EV part, because that doesn't use any gas. Of course, it DOES use energy in the form of electricity obtained by burning coal + renewables. If you go any distance other than 51.11 miles between charges, you don't get 230 MPG. Is this calculation a bit phony (because the electricity is not figured in) - YEP !!! but that's what it boils down to....

[daniel]

John says GM will not tell us the mpg on gasoline. Glider says that GM claims 50 mpg.

Here's my thinking on "charge-sustaining mode":

Imagine two different kinds of series hybrid systems: Battery-buffered and unbuffered:

1. Unbuffered: This is what freight trains use: A diesel engine runs a generator which powers an electric motor (or several of them). You get the advantage of running the ICE at full speed when the vehicle is moving slowly as it starts out. You get lots of torque and the ICE does not lug. But the ICE has to put out all the energy or power that the vehicle needs at any given moment. This means that sometimes (under light load) the ICE is working well below its maximum efficiency.

2. Battery-buffered. The Prius is battery-buffered. Battery buffering allows excess energy produced by running the ICE at maximum efficiency to be stored for later use. If the battery is full and the power demand is light, the ICE can shut off and the car uses that excess energy for a while. This is a very important efficiency mechanism and is one of the great strengths of Toyota's HSD.

NOW, if the Volt allowed the ICE to actually recharge the battery, then it could (like the Prius!) be run always at maximum efficiency, with excess energy going to the battery, and once the battery is full enough, the ICE could shut off and use that energy for a while.

BUT if the Volt is merely "charge sustaining" then the ICE (like the ICE in a conventional car!!!) must always and at all times produce exactly the amount of power needed at that moment. This means that (like a conventional car!!!) the ICE must be big enough to provide acceleration (the Prius can use its battery & electric motor for acceleration) and (like a conventional car!!!) the Volt's ICE will be inefficient when the power demand is low, which is most of the time in normal driving.

This is why I say that a charge-sustaining PHEV will be less efficient than a "recharging" PHEV.

Of course, it is absolutely imperative, if the car is to be efficient, that there be a switch to tell the car either "I'm nearing home or a charging station, so use all the electricity you have" or "I'm on an extended trip, allow the ICE to keep the battery half-full so I have power for acceleration and headroom for regenerative braking." GM, of course, will not bother to engineer in such an option for drivers.

And that is why I predict a very poor fuel-efficiency rating for the Volt. Not 50 mpg, but maybe 20 or 30 mpg.

And the fact that GM will not release the number reinforces my skepticism.

As both John and Glider point out, purchase decision will be based on overall efficiency (gas plus electric), but overall efficiency will depend on specific driving habits, and the only way for a buyer to know her overall efficiency will be to know both wh/mi in electric and mpg on gas, so she can plug those numbers into her own specific situation.

Information is power. By withholding this information from the buyer, GM is intentionally and maliciously obfuscating the decision-making process. Does anybody wonder why I hate GM? This is just one more example of how GM does everything it possibly can to screw the consumer.

But then, it's all really a moot point, because they're never going to market the Volt. I'll go on record as saying that if GM markets the Volt, I'll eat a chocolate bunny.

[Glider]

Originally Posted by daniel

John says GM will not tell us the mpg on gasoline. Glider says that GM claims 50 mpg.

OK, here's one of many links specifying about 25 kW-hr / 100 mi of city driving

GM Media Online

There are many links giving 50 MPG when on gasoline. This one is in the Fuel Efficiency section of this Wikipedia article.

Chevrolet Volt - Wikipedia, the free encyclopedia

Chevrolet_Volt

Hope this helps.

[daniel]

It looks to me as though that Wikipedia article was written using early claims from GM, which I understand GM later backed down from, and which were in any case rather far-fetched.

Until they actually build a Volt and allow an independent laboratory to test it, all such numbers are speculation and hot air.

[john1701a]

Originally Posted by Glider

There are many links giving 50 MPG when on gasoline.

50 MPG was indeed the estimate once... upon a time... when the engine was still a 1.0 liter 3-cylinder. The switch to a 1.4 liter 4-cylinder rendered the original numbers obsolete.

We know the number is lower, based on the way an executive danced around the response when confronted by several die-hard supporter requests all asking about CS mode efficiency. It has become a really touchy subject. In fact, that's what contributed to the "230 MPG" marketing. They clearly wanted to draw attention elsewhere... hence this thread, to get past all that hype, propaganda, and side-stepping to find what's truly realistic.

Remember, it was GM itself that stated Two-Mode superiority from not having to convert to & from electricity. Now they have Volt stating essentially the opposite. Reality is somewhere in the middle.
.

[JamesBurke]

According to an engineering associate of mine who works for a federal agency this is how the test was described.

It was a dyno test.
The test loop program is an unpublished proposed epa method for conventional gas and diesel vehicles not hybrid specific.
It was one of many variations, city, hwy, mix, a/c, heater ect.
The results were legit for the Volt and the Nissan but also show the short comings of the current and proposed test regimes.

As for the viral marketing of a results that they know come from a flawed test. I have never bought a GM product and probable never will.