calculation details of Volt's 230mpg city estimate

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.

 

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....

 

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.

 

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.

[ame="http://en.wikipedia.org/wiki/Chevrolet_Volt"]Chevrolet Volt - Wikipedia, the free encyclopedia[/ame]

Hope this helps.

 

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.

 

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.

 

That seems to be the Volt's possible downfall, or at least a symptom of it. It seems to me that GM was thinking, "we've made an EV, how hard is it to put on a generator?" Once they started, they then realized it will take more time to get the control system running at point of running at optimum efficiency and reliability for the generator and battery. And then the cost of doing so became and factor, and using off the shelf parts was decided upon.

The truly daring would have went with a wankel rotorary or microturbine that was further downsized for space and weight.

 

Steam. Steam cars failed for reasons of marketing, not technology. They have the advantage that external combustion can be much cleaner. And with a PHEV, startup time is not an issue.

But of course with GM you are guaranteed to get crap, designed to screw the consumer. Reliability? Forget it! Quality? Forget it! Energy independence? No way! Not as long as GM owns a chunk of Big Oil!!!

 

They would have been bad choices as they are so inefficient (especially the turbine).

The best choice is a reciprocating engine - sized to be able to efficiently generate the average power needed for high speed cruising. To meet this requirement you will need a 1-2 litre engine. Pretty much what they have ended up with in the Volt and the Prius.

kevin

 

John,

Any reason why vacation miles are not consuming gasoline?

 

In a series hybrid hybrid application, where the engine has no direct link to the drive train, you can have the engine operating at a much narrower rpm range, which makes it easier to design for efficiency. Remember, part of the Prius' success with fuel economy is downsizing the engine to the point where it spends most of time at its efficiency peak, instead of below it because the engine needed to be oversized to occasionally pass and climb hills. A series hybrid with battery can take further advantage of an engine's economic sweet spot by having it spin only at that rpm.

Ideally, a series PHEV would have a genset that had a very narrow set output. It would generate enough power for a set cruising speed. This depends on average speeds in the country of destination and amount of buffer from the battery. Likely 75mph in the US with the shorter EV range of the 1st gen vehicle. Once the charge of the battery hits a setpoint, same 50%, the genset kicks on. If you aren't going that set cruise speed, the excess power generated goes into the battery. Eventually the battery will be topped off, and the genset will shut off. Going above the set speed meanstapping into the battery. Eventually, doing so for extended periods will put the car into a turtle mode, a la the 1st gen Prius. But this is a 1st gen of our ideal series PHEV.

It doesn't sound like the Volt does this. Getting such a system right would take time to develope and research. Whatever the reason; pressed for time, greed, simply daunted, etc., GM went with an EV that happens to also be a standard series hybrid. Which means using an engine with a more variable output.

Back to our ideal S-PHEV, the reason to use a wankel or micro turbine is for weight and space savings. A reciprocating engine may have an advantage in efficiency. The narrow rpm range needed for the application should narrow that gap, but even if it doesn't, for most people the electricity for daily driving is coming from the grid. In which case, a smaller, lighter engine means more room for cabin and cargo space. Maybe a slight edge in battery range, or even more options for installing the genset.

This is going th be simplistic. The RX-8 is the only available wankel engined car. It's 1.3L gets 19mpg combined. The Yaris gets 31mpg from a 1.5L 4 cyclinder. That isn't a complete picture. The 1.5 produces 100hp. The 1.3, 230hp on premium, so lets say 200hp on regular. In a generator, the 1.5 will have to run longer to match the electricity generated by the 1.3. Which means burning about as much gas, maybe more, as the 1.3.

Efficient electric generation doen't necessarily require the same considerations as driving economy.

 

With different modes of generating power for cars the EPA should list the cars in a cost per mile to operate @ a set cost for the unit of fuel. For example
A pure electric vehicle cost $.03/mi city and .05/mile highway @$.10/Kw*H
A pure Gas vehicle cost $.07/mi city and $.05/mi highway @$2.50/gal
or any combination of the above.
Anything else and it leaves the consumer guessing about what the most economical vehicle is for their use

 

I'm a bottom line type of person. Here's my question GM. I plan on taking a 200 mile round trip to the Pocono mountains. What is my MPG ~ going to be on this trip. If you claim the car gets 240mpg that means I'll use less than a gallon of gas? DUH... of course not. U know something? GM deserves to have gone under. They're still a bunch of morons. Liars, swindlers etc. So tell me GM... what's it gonna cost me in fuel to go round trip to the Poconos from NYC. ...Pardon me??? Ok when are you going to get back to me?? Ok thanks.. thought so. IDIOTS!!

 

There is no way to give a cost per mile in a car like this without knowing how long the trip is, and the price today of both gas and electricity.

What we need, as I've said before, are two figures:

We need to know the watt-hours per mile, or (what amounts to the same thing) the miles per kWh, when the car is running on batteries, and

We need to know the miles per gallon when the car is burning gasoline.

Obviously a 20-mile trip will cost less per mile than a 300-mile trip. With the above two figures (but only if you know both the above figures) you can figure out the cost of a given trip based on current or projected fuel and electricity prices.

GM is being underhanded, disingenuous, and downright dishonest when it gives the 230-mpg figure without breaking down the separate electric and gas usages!!! Of course car makers never gave us FE numbers until the government forced them to, and they will not give us the numbers we need now unless they are forced to, because GM is run by criminals who will use every dishonest and underhanded means they possibly can to sell cars.

The government (in this case the EPA) needs to force them to provide separate electric and gas efficiency numbers.