Volt does not recharge its batteries while driving...

What happens to the MPG when the small engine has to move the car and "sustain" the battery charge? That can't be good.

 

Makes sense to me. Once the battery is depleted the most efficient way to move the car would be to have the engine generate only the juice needed. Using the engine to charge the battery is a lot less efficient than waiting for the grid power. This would only come into effect after 40 (claimed) miles so for most drivers and most usage it wouldn't matter. In my case I commute 46 miles a day so for 6 miles the engine would fire up and then when I got home the grid would do all recharging, thus minimizing gas used.

 

Despite the hype, this is what many have believed for some time. Its not exactly shocking, this is how most plug in hybrids operate. Just one more way thtat the volt is just another hybrid, not a "range extended electric vehicle." Charge sustain mode is the classic term for the mode a pluggable hybrid runs in once its battery is depleted. In that mode it may either use the battery a little to aid in high power demand situations, then replenish back to the sustaining value (as the prius does), or it may just not really use the battery at all at that point.

While it sounds bad, this is probably just as well. In order to offset as much gasoline as possible you generally want to end the day with the battery as low as possible. That way you have as much room in the battery to fill from the grid as possible. Any gas spent charging the battery would in that sense be a waste. The rub is that the ICE may be able to run more efficiently at cruising speeds if more heavily loaded. Charging up the battery is a great way to take advantage of that fact. This is one reason some folks are looking at adding a lot more smarts to PHEV systems. By using the NAV system, and either knowing (from user input) or predicting (from past driving experience) your driving route the car would try to balance these too situations. It would recharge the battery just enough to make sure you still end up at home with an empty battery, but would operate the ICE at higher efficiency as much as possible reducing overall fuel consumption.

Rob

 

I think that is correct. It will not go into electric mode anymore. What it may do is use a little power from the battery to give you a little boost during acceleration for example, then either put it back with a bit of extra juice from the generator during cruising, or from regen. Since the ICE/generator is much less powerful than the battery, if they don't do this the car will be much slower once the battery is depleted. I can't think of any reason they wouldn't do this as the amount of energy required will be trivial compared to the size of the battery. On the other hand it does complicate the battery management system significantly, and they are trying to rush this thing out the door. A good hard acceleration probably wouldn't even take 1% off the SOC. If I had to guess I'd expect they will run charge sustain mode kind of like the Prius does. They'll hold at 30%, but allow it to drop to 28% or so during assisted accelerations or small hills, and allow it up to 32% or something to allow for regen. This will be of no good on something like a long hill climb though. Then you will really be stuck with what the ICE can put out, 72hp max minus the generator and motor efficiencies. Of course they may just not have time to implement and test something like that, and may just blow the whole thing off. That would seriously impact the MPGs after discharge I would think.

Rob

 

Cutting to the chase. the Volt is just a local vehicle? It wouldn't be a car to go to Fl. from Pa. like we do with our Prius?

 

Rob,

That is my interpretation as well. They have indicated 30 mpg in their extended range mode. That's not very good, but if the ICE is running the whole time it is probably unavoidable with the slightly larger cross sectional area and probably 400 pounds of excess weight (plus I wonder what the efficiency will be of the path: ICE-generator-motor.) On hill climbs past ~40 miles they are going to have problems compared to the Prius (which can have its own problems.)

 

It's not going to be nearly as fuel efficient on long trips if the current info proves accurate. And it will suffer if you can't recharge it. (Perhaps not ideal for traveling salesmen?) I wouldn't take it on a trip to the mountains--ironically a place where sports cars are usually the most fun and the segment it has targeted to some degree.

But at least it has the capability of doing long trips. And trips on the flat shouldn't be a problem.

 

Here is the question that needs to be asked... What if Volt climb 20 miles long mountain and runs out of battery juice at the top? Ok, it will start to operate on CS mode. Now you go back down the mountain. Will the regen brake recharge the battery pack? I would say yes. Ok, then once you reached the bottom of the hill, the pack would have recapture majority of the energy back. At this point, will the Volt operate on CS or CD mode?

 

Hi All,

Edmunds is blowing smoke at us. They apparently are devoid of technical training, and do not realise, even with the battery at 40 percent charge, or whatever the low limit is, it still has 150 HP . Some editors should just not allow some journalists to write about cars!!!!!

Last time I checked, a 150 hp engine weighs about 400 pounds. The battery is hardly a useless piece of stuff to haul around when down to its lower charge limit. Since its the primary source of power in the car!!!! Just like a 400 pound engine.

 

Hi again,

Well, the big problem with cars is partial throttle efficiency. So, when the Volt 1.4 liters is running, its running at an optimum load, for best efficiency, because it can run fast, even though the car is going slow. Then it can turn off.

I tell you what ain't good for efficiency, its taking your traditional automatic transmission car down a 30 mph road. This car will have better fuel economy at 30 mph, battery at low charge state, than almost anything out there.

 

Complete inability to charge the battery from the engine would be an awful design, due to ICE pumping losses at low power demands, and maximum power output of 70 hp at high demands. As much as I cannot stand GM, I just find that scenario really hard to believe, and unless one of our experts says otherwise, not integral to a serial hybrid design. The two-mode hybrids of GM tell us that they have learned to write battery managment software.

I'm more inclined to believe that GM will allow the battery to cycle between say, 5 - 6 kwh in 'depleted mode' much the same way HSD does. Prius owners know that there will be the occasional very long climb where the battery is drained, and passing other cars is out of the question. GM is going to have quite the PR compaign to explain why this is true in their $40,000 wonder-car, too

Actually, the only reason I can think of that the system would not operate this way is battery longevity, or at least GM's inability to test endurance in this mode in the time they have given themselves to push the vehicle out to market

 

No, donee, the smoke is coming from GM. If GM was technically competent they could explain it. So far they've been incapable of doing something that simple, which doesn't bode well for the final product.

I can't blame any writer for gagging on the crap that GM is feeding him/her/us.

 

There's no reason a plug-charge depleted Volt couldn't light
its engine, run it at an efficient point, and both push the car
and send a little charge into the battery at the same time. If
the car is steady-state then it isn't using all of the engine's
output capability anyways, so why not load the engine harder
and send that somewhere useful? The system could simply have a
lower target SOC and once that's reached, simply shut down for
a while under light demand and continue running from the battery.
That a> leaves plenty of room for regen, and b> arrives home with
a reasonably low SOC so plug-recharging still gives good benefit.
.
But GM isn't likely to understand this sort of simple logic,
let alone be able to educate its customers on why a system like
that works like it would and how to best utilize it...
.
_H*

 

I think behind the scene, there is a struggle between making the Volt a RE-EV and a PHEV.

I think Bob Lutz does not want to be a liar and want only the gas engine to power the Volt until you get home. In order for it to be an electric car, it can not combine two power sources (gas engine and battery) or else it will be a hybrid.

From the engineering point of view during the charge sustain mode, it would be more efficient to:

1) Buffer the extra ICE power and charge the battery in cases of going downhills, gust of wind, etc...
2) Battery assist the ICE during high power demands, in order to maintain in the sweet RPM spot

However, this will make the Volt a hybrid.... Will engineers triumph or will Mr. Lutz retire with honor? If the history is of any hint, the engineers are fighting an uphill battle.

 

Thanks for posting this.

I've been racking my brain trying to figure out what GM meant when they released a statement saying that the Volt would have a 400 mile range with a 12 gallon tank. It doesn't take a math genius to figure that that's not really exceptional gas mileage.

You would have to try pretty hard to REDUCE the range of the Prius to 400 miles on 12 gallons!

Quo Vadis, Volt?

 

It is interesting that Lutz has said that the Volt will push everything through it's batteries (via e-mail as noted in other thread.) This does not sound efficient. In driving the Prius efficiently we try to avoid charging and discharging the battery except where we must (warm up slow traffic, hills, etc.) Whenever the Volt relies on charging the batteries with its ICE it is going to get hammered compared to the Prius. Those 30 mpg indications on extended range were no accident.

 

I'm under the impression that a trip through the Prius battery carries about a 15% surcharge vs ICE to wheels. Shouldn't a serial hybrid manage close to 40% ICE efficiency just about all the time ? That would imply a Volt ICE to wheels of 34% if the electric drivetrains of the two vehicles are comparable.

 

So does the proposed Volt have a gearbox system.. or is it direct drive via electric motors..

Wheel hub motors ?

I'd imagine that hub motors would be more efficient. With the ice providing the power directly to the motors and not via the battery when it's out of juice.