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The Volt's gasoline capacity is being cut back

The Chevy Volt plug-in electric car from GM is one of the most desired new cars coming to the market.  Everyone is talking about the car's stylish design and relatively affordable price tag of around $40,000; not bad compared to past electric cars totaling over $100,000 like the Tesla Roadster.  With sales planned to commence in 2010 and with a production model possibly being unveiled later this year, the buzz surrounding the new vehicle is intense.

One fact many people didn't know is that while the Volt is an electric car, relying first on charge, it also sports hybrid capabilities.  It features a 12 gallon gas tank, which will only kick in after the lithium-ion battery charge is depleted.  This could take the car 600 miles, thanks to an impressive 50 MPG projected fuel economy.

However, recent reports have revealed that GM, in a surprising development, has decided to cut down the fuel tank for the Volt.  They won't say what the new capacity will be, with the source only saying, "We're working on that." 

The original goal for the Volt was to equip it to drive 40 miles on a charge, without using any gas.  This goal remains, and appears to be within GM's reach.  Additionally, since 40 miles isn't long enough for road trips or longer business travels, GM planned on adding the gas tank.  The gas does not power a traditional engine, but rather a generator, which provides enough charge to keep going, and actually performs more efficiently than most gas-engines.

GM has revised the target range to 360 miles, which would seem to indicate a fuel tank of 7.2 gallons.  The reasoning behind the cut is research that cars, on average, travel less than 40 miles a day.  While GM wants to provide a comfortable extension to this range, it decided that 600 miles was a bit excessive.  Many older sedans don't get much more than 300 miles on a 12 gallon tank, so the new range seems relatively reasonable.

Jim Hossack, vice president of AutoPacific, an automotive research and consulting firm states, "Most cars today have a range of more than 300 miles and less than 400 miles before refueling.  GM didn't need a longer range because most bladders can't go 600 miles.  By going with a smaller tank it means GM can take weight and price out and make Volt a little lighter and a little cheaper, and that's what you call making an improvement."

While GM's move should help it cut costs and cut down slightly on the weight of the vehicle, it may come as a disappointment for fans of ultra-long treks across the country.  Still, it is dubious that such minor gripes will be able to do anything to quell the PR frenzy surrounding the Volt.

At its price tag, even with possible subsidies, the Volt will likely fall into the luxury car class.  However, with its wild popularity and sleek looks, it seems likely to thrive in this class, or whatever class it may enter as prices drop.

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RE: Hybrid
By pauldovi on 7/9/2008 1:45:59 PM , Rating: 1
Yes, in a series hybrid system you can operate the gasoline motor at higher efficiency. However you neglect to point out that all power generated by the gasoline motor is undergoes additional energy conversions before ever powering your car, you lose more if not all of the efficiency here. Your energy starts out as mechanical energy produced by the gasoline motor. Then it is turned into electrical energy by the electric motor (generator). Now your electrical energy in transfered into chemical energy stored in the battery. Finally the energy is again transfered into electrical energy and powers the car. At each stange there is an energy loss, and thus a reduction in efficiency.

I don't see how the engine can be made any smaller than an engine in a parallel hybrid system? Any simplicity gained by from the lack of a transmissino is recomplicated with the additional electric motor and motor controller required.

Small battery packs are not an inherent property of parallel hybrid powertrains. The batteries are not solely powered by regenerative braking! They can be kept topped off just like in a series hybrid powertrain, except they do not require the additional electric motor to be a generator.

My statement about 1/3 of the car total power being used is accurate. If you intend to have the gasoline motor keep the batteries charged after the initial battery charge has exhausted you need a gasoline motor with equal power of the drive electric motor. This gasoline motor is not connected to the driveline. Furthermore you need another electric motor acting as a generator connected to the gasoline motor, again with equal power output as the gasoline motor. So your car has a total of 3x the power, while only x is being used to propel the car! Series hybrid systems like the Volt do not connect the gasoline motor to the powertrain, like you suggest.

RE: Hybrid
By jRaskell on 7/9/2008 3:39:53 PM , Rating: 2
First, Lithium Ion batteries are very efficient. Depending on the charging circuitry, anywhere from 96-99% efficient, so you lose VERY little efficiency going from electric to chemical back to electric.

Second, the Volt is designed to run directly off the ICE generator as much as possible once it switches to that mode of operation (which actually occurs when the battery drop below 30% charge). It'll only switch back to battery when the driver demands greater acceleration than the ICE generator can provide. The Volt has a 120kw electric drive motor (160hp) and a 53kw generator tied to the ICE. For any cruising or light throttle driving, the motor can easily be powered directly by the generator, with any surplus current being used to recharge the battery. It's only when hard acceleration is required that the motor falls back onto the battery.

So, all that power does NOT go through additional energy conversions to drive the car, and even what power does go through additional energy conversions, does so at a very high efficiency and still maintains a much higher overall efficiency than a standard direct drive ICE is capable of.

RE: Hybrid
By ChronoReverse on 7/9/2008 3:51:51 PM , Rating: 2
Wouldn't Toyota's HSD be the best of both worlds?

It uses a CVT with an Atkinson's Cycle engine in its drive, thus fulfilling the "running in the band" part of the equation. The HSD is capable of routing the gas engine output to either charge the batteries or to power the wheels (parallel hybrid).

The Prius is capable of running fully on electricity, fully on the gas engine or both at the same time depending on the situation and to varying degrees. If it had a better energy storage system (i.e., lithium ion batteries), it pretty much could run as a serial hybrid if that's the best choice for the situation.

RE: Hybrid
By pauldovi on 7/9/2008 4:31:26 PM , Rating: 2
The total peak charge efficiency is no where nere 99%. It may reach the upper 90% in ideal conditions, which means very slow charge, but it doesn't do that in the Volt's application. However, even at an unrealistic 96% charge / discharge efficiency, you are losing almost 8% of the energy. This also does not include the loss of energy from the conversion of mechanical energy to electrical energy in the generator.

What you are describing is a parallel hybrid system, and the Volt does not have this. The volt is driven soly by the electric motor (160HP) and the ICE is not connected to the powertrain. The ICE is only connected to the generator (71HP) and is completely independent of the powertrain. The car cannot be driven by the ICE and it cannot be driven by the electric motor connected to the ICE.

Any power generated by the ICE must first be converted from mechanical to electrical energy, then from electrical to chemical energy, then from chemical to electrical energy, then from electrical to mechanical energy. Unless of course the Volt is capable of bypassing the battery pack when it is depleted (this is unclear and seems like it is not by details on wikipedia).

RE: Hybrid
By jRaskell on 7/9/2008 5:52:20 PM , Rating: 2
Nothing about what I described is a parallel system.

RE: Hybrid
By randomly on 7/9/2008 5:26:10 PM , Rating: 2
Yes in the series hybrid the ICE drives a generator which drives the electric motor which drives the car.

Yes there are additional steps of energy conversion. However they are highly efficient steps, in the +90% range.

NO the loss in efficiency from the added steps does not use up all the gains made up by the increase in efficiency of the ICE. The increases in ICE engine efficiency are very large.

In fact serial hybrids have long been in use in large cargo ships where they can attain efficiencies as high as 56%.

Don't forget than in the parallel hybrid you have to go through a transmission which is a similar +90% energy conversion stage.

The serial hybrid engine can be made smaller than a parallel hybrid because
1) it does not need to supply the peak power requirements needed by a parallel hybrid system.
2) the increased efficiency gains from running the engine at a constant load/RPM also increases the power density of the engine allowing you to get the same power out of a significantly smaller engine.

Your statement about only 1/3 of the cars power being available to drive the car is incorrect. There is no requirement to recharge the batteries once they are depleted enough to trigger the use of the ICE. All the power of the ICE can be used to drive the car. Batteries can be recharged or not charged at any rate you desire.

The motor being used as a generator, is just that. a generator. It's not another motor available to do work. Electric motors and generators are compact efficient pieces of gear. I wouldn't be surprised if the motor and generator combo in the volt is smaller than the engine in a prius.

If you feel your are so right about your analysis you should contact all those GM engineers and tell them they are wasting their time and hundreds of millions of dollars.

RE: Hybrid
By pauldovi on 7/9/2008 9:35:00 PM , Rating: 1
No ICE can obtain an efficiency of 56%. Have you ever studied the Carnot cycle? The most efficient engine in the world was a large steam power plant that achieved efficiency in the low 40's.

The ICE is not connected to the powertrain in the Volt! How can it drive the car?

If you have a 100HP electric motor that drives the car, a 100HP gasoline motor that powers the generator that charges the batteries, and a 100HP generator to charge the batteries you have 300HP total onboard but only 100HP actually drives the car. This is a huge waist!

A electric motor and a generator are the same thing. That is my point that it isn't another motor available to do work. That is why you are wasting available power on the vehicle.

We all know that every vehicle GM has ever made has been perfect! I guess that is why they lost $35 billion this past year.

RE: Hybrid
By randomly on 7/9/2008 10:11:51 PM , Rating: 3
Yes I'm fully aware of the Carnot cycle limitations on heat engines.

Do a little research on Serial hybrid systems for cargo ships. They are low speed diesel combine cycle systems and efficiencies do reach %56. These are not theoretical systems either but systems currently in use. Not really surprising considering fixed combine cycle power plants have efficiencies up to 60%.

I would never suggest that GM vehicles were even remotely perfect. But GM engineers are not stupid and serial hybrid technology has some significant advantages. This is why many other car companies are pursuing it including BMW, Mercedes Benz, Toyota, etc. Maybe they know something you don't?

Your reasoning that a 100hp gasoline engine driving a 100hp generator driving a 100hp electric motor means you have 300hp worth of power available is goofy. You obviously aren't an engineer. If it upsets you so much just put the generator and the electric motor in a black box and rename it TRANSMISSION and be happy.

RE: Hybrid
By pauldovi on 7/10/2008 11:30:21 AM , Rating: 2
Just because a company does something does not mean it is the best way to go about it. Having an vehicle that is an "electric" vehicle may have implications beyond the mechanical advantages, such as tax advantages and appealing to the "green" side of people.

I never said 300HP was available. But 300HP worth of engines is onboard. Meaning you are paying for 300HP worth of engines, carrying around 300HP worth of engines, but only driving the car's wheels with 100HP. That seems pretty silly to me.

I do not rebuttle personal comments so I will not respond to that.

RE: Hybrid
By Jim28 on 7/10/2008 10:59:58 PM , Rating: 2
You still don't get it.
The drive engine is 120Kw, the ICE/generator combo outputs 53Kw.

Obviously the drive engine can't deliver peak power on the ICE/gen alone but it is more than enough to drive the car BY Itself as peak power is only needed when you put your foot to the floor.
When that happens the motor pulls additional current from the battery pack. The car does not wait for the battery to be completely depleted to start the ICE/gen so that it has a little punch left.
In normal driving the drive engine may need say 40Kw to run the car, the extra 13Kw the ICE/gennie outputs is used to charge the battery and overcome the 10-12% conversion losses.(guessing here but it should be in the ballpark.). See! Energy is conserved, the Thermo gods are happy!
In engineering terms this type of power plant is called a turboelectric drive. (At least on ships and ships don't use batteries.)

RE: Hybrid
By Jim28 on 7/10/2008 10:49:11 PM , Rating: 2
You don't know the math/mechanics of power transfer apparently.

Motor generater sets are above 90% efficient for power transfer and are widely used on ships for this reason and because they are damn near bulletproof.
And like the poster above said, the battery conversion factors are very high.

The difference that you can't see is that the gas engine is tuned and running in it's most efficient band, producing peak HP for a given fuel consumption. This gives you the ability to use a smaller engine with higher power output that can produce enough juice to power the car. It does not require three times the power, it requires at most 1.2 times the power for a good series hybrid design.
Also you don't need a tranny so that weight is saved and is more than enough to cover the generator weight.

RE: Hybrid
By William Gaatjes on 7/13/2008 8:29:33 AM , Rating: 2
I agree totally.

Using series hybrid has only advantages.

to some it up for Paul :

1. No heavy transmission, cuts weight and you can use this saved weight to put more batteries in the car.

No direct powercoupling/distribution stages to combine output power of ICE and electric.

2. i may be wrong but i am guessing that the electric motor is a 3 phase type and the electronics to switch these motors are highly efficiënt too. 3phase motors are widely used because of the high efficiëncy( if i am right > 90%) simple structure and high reliability/ endurance.
There is a big library of information for these electrical motors.

The generator is 3phase as well.

However you first have to normalize the output to the ICE to kw , this normalized output is input for the generator.
some loss for the generator , some loss for the electronics , some loss for the motor and some loss in bearings and so on. Yes nothing is 100% efficiënt.


you get ICE > generator > electronics > motor > drivetrain > wheels.

current cars :

ICE > transmission > drivetrain > wheels.

You would say that there are less conversion steps, but the efficiëncy is so much lower for the ICE that the volt still wins.

I am wondering why they didn't go for motor driven wheels.
You know, where the motor is build inside the wheel it self.
This would give 4 smaller motors and a easier way of getting energy back when braking. It would also save some drivetrain weight cause you don't need that anymore too.

3. As has been sad above the ICE runs constantly at 1 constant rpm value where the efficiëncy is at it's highest.
But there are more advantages. 1 constant rpm means you can
tune the engine also for vibrations, weight, maintenance, reliability/endurance and as mentioned output power.
I am sure there is some electronics after the generator that monitors the ICE to make sure that the load the generator exposes to the ICE does stay within an upper ans a lower limit. The key to make this all work is the electronics to adjust and control the ICE. And the car manufacturers have a lot of experience with that.

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