Volt does not recharge its batteries while driving...

Well its clear that not all the information coming out on Volt is consistent. I don't think that means GM does not know what its building, but clearly not everyone that talks and writes about it does.

 

I really want to see their MPH-versus-MPG graph after the grid charge is gone.

You've drained the battery so far that the ICE kicks in, but you still want to go 75MPH. Whatever the watts required to go that fast, it has to be put back into the battery at LEAST as fast as the drive motors are taking it out. Fill, drain, fill, drain: will the ICE keep the car going at 75MPH, or won't it?...

 

I'm skeptical that ICE power *must* go through the battery, in fact I doubt it very much. This information was posted at a GM site as coming from an email from Lutz, but that just makes it more suspect. He is a salesman, not an engineer.

 

In the same vein how 'bout an EV range vs. speed chart.

From the data I have seen GM's claim of 40 mile range is based on the city cycle of the epa fe test. That test averages 21.2 mph. Is the Volt really a range extended nev with enough burst ability to make it hwy legal?

 

As long as 1 gallon of gas can generate enough electricity to move the car 33 miles at a decent speed. We can get maybe 2 miles @ 30MPH before even a green battery gets depleted and the ICE kicks in. Whatever that is, multiply it by 15 to get (anywhere near) their requirements, then factor in the kind of speed used in EPA MPG testing to push that even higher.

Here's a home generator I found:

Code:

[LIST]
[*] 2 cycle gas / oil mix (30/1)
[*] 120 volt 60Hz 7 Amp AC output
[*] Recoil handle (manual) start
[*] 2.0HP two stoke 63cc EPA certified
[*] 12 Volt 8.3 Amp DC output
[*] 900 running watts 1000 peak watts
[*] Circuit protecting device, circuit breaker
[*] 1.2 gallon fuel tank
[*] Up to 6 hours running time per full tank
[*] Lowest fuel consumption rate: 0.14 gallon per hour
[*] Uses capacitor discharge ignition system
[/LIST]

Anyone know if the watts per hour here would simply scale up when you adjust the engine size, or would the design of the generator in the Volt be too different. It sounds OK to get 900 watts per hour for 6 hours out of a little more than a gallon, but I'm not sure how far that'd move a Volt, or how much less you'd get generated by the Volt's possibly (probably?) less-efficient ICE...

 

I don't think thats correct. GM has already stated that the peak power out of the ICE is 72hp as I recall. When the battery hits its lower limit, it will be of no help, so 72hp is all you get. Really you get less than that, as the you will loose 10-20% converting to electricity and back since the ICE can't drive the wheels directly. So, let say about 61hp. If they are smart they will "sustain" at an SOC a few point above their hard minimum, leaving some extra power flowing back and forth to allow for for electric assist on acceleration. If not, the Volt will be a real dog off the line once the battery is low. Even if they do that, it will only have 61hp available on a long uphill with the battery depleted as there will have to be some hard bottom limit. At that point the battery can't help and the only power available to drive the motor is that being generated by the ICE.

Using one of the EV calculators out there I put in a reasonable vehicle, a 1990 Ford Probe, and loaded it to get the weight to 3250 where some claim the Volt to come in. I then did a chart of HP requirement vs. speed and grade incline. The results are below:

Spd/Grd -- 0% -- 2% -- 4% -- 6% -- 8% - 10% - 12% - 14%
-10mph -- 1.8 - 3.7 - 5.6 - 7.5 - 9.4 -11.3 -13.1 -15.0
-20mph -- 3.8 - 7.7 -11.5 -15.3 -19.1 -22.9 -26.6 -30.4
-30mph -- 6.5 -12.3 -18.0 -23.7 -29.4 -35.2 -40.6 -46.3
-40mph - 10.1 -17.7 -25.3 -33.0 -40.6 -48.3 -55.5 -63.1
-50mph - 14.8 -24.4 -33.9 -43.5 -53.0 -62.6 -71.6 -81.2
-60mph - 21.0 -32.5 -44.0 -55.4 -66.9 -78.3 -89.2 -100.7
-70mph - 29.0 -42.4 -55.8 -69.2 -82.5 -95.9 -108.6-121.9
-80mph - 39.1 -52.4 -69.7 -85.0 -100.2-115.5-130.0-145.3
-90mph - 51.6 -68.7 -86.0 -103.2-120.4-137.5-153.9-171.0
EV Calculator

If 61hp is in the ballpark of the Volt's depleted pack sustained max power output, then its max speed on a 6% grade should be about 65mph, and about 48mph on a 10% grade. Bear in mind though, that this means you have exactly enough power to maintain speed. To actually accelerate you need more power. For example on a 6% grade, I calculate the max acceleration as 3.4mph/s at 40mph, 1.6mph/s at 50mph, and 0.41mph/s at 60mph. Those are all rough numbers but you get the idea.

Lastly, I doubt that power will have to go "through" the battery in charge sustain. If you are putting current into the battery at the same rate you are taking it out the battery will not charge and discharge. The incoming current will just directly satisfy the outgoing requirement of the motor. When hitting the hard limit, the motor controller will have to limit its current draw to equal that being produced by the generator. This is pretty much how the Prius handles it anyway. Offhand I can't see why it would be any different.

Rob

 

Its also interesting to consider, that if the Volt is not charging the battery significantly in charge sustain mode it doesn't make sense that the engine will run at a fixed power/efficiency point. Many have claimed this to be the case, and that the engine would be very efficient as it would be optimized for that one operating point. It would seem that the Volt's ICE will have to be throttled in steps just like the Prius ICE, otherwise there will be too much power generation under light load conditions and/or too little power generation under high load conditions. Does that make sense?

Rob

 

I agree, but its a tough call where to draw that line. Lets say the ICE is more efficient at full load, ie when cruising and charging the battery at the same time. On a long trip, cycling the battery up and down would yield the best efficiency. The problem I see is with trips in the range of 1.5-2X the EV range. If the ICE above runs at 30mpg while charging, it would still do better in the short term even at lower efficiency due to lower power output, say maybe 45mpg. So here are a couple of somewhat simplified scenarios I can envision:

1a. Long road trip, cycling: 30 miles EV, followed by 30 miles at 30mpg, repeat until gas tank is empty. Average long range fuel economy: 60mpg

1b. Long road trip, charge sustain: 30 miles EV, followed by 45mpg to tank empty. Average long range economy: ~45mpg

2a. 60 mile trip, cycling: 30 miles EV, followed by 30 miles at 30mpg. Average medium range economy: 60mpg

2b. 60 mile trip, charge sustain: 30 miles EV, followed by 30 miles at 45mpg. Average medium range economy: 90mpg

So cycling operation looks better for very long range trips, but worse for trips in the ~2X range. As these trips probably occur more often than the long rage, I'm not sure charge sustain operation is the wrong answer. A switch to decide between the two might be nice

Rob

 

Rob,

Interesting thoughts..

If GPS were to be used, just operate with the best strategy. If the trip is long, use the blended mode - use gas engine to supplement peak power demands.... etc...

Putting artificial rules (such as drain the battery first) is just counter productive to displace gas usage.

 

Hi Rob,

I came to the exact same conclusions you did. But now I keep asking myself: what does a Volt ICE running at 70 hp sound like ?

Joking aside, as the Prius battery heads towards it's absolute minimal SOC, battery assist power draw peters out. I've always guessed it was battery physics, but maybe I am wrong ? Certainly it would be a bad engineering choice to let a Volt driver scream up a hill at 150 hp until ... suddenly ... 70 hp.

 

The notion is ridiculous; the press release must be in error. Nobody would design a series-hybrid car so crippled.

 

I'm getting confused in thinking about power flows and the drivetrain. Please correct as needed--

ICE -> AC generator -> inverter -> DC motor
Battery -> DC motor
Pathways connect proximal to the DC motor.

 

Hi Miscrms,

The assumption that the Volt will always reach lower charge limit on a mountain climb is incorrect.

There are ways that the engineers can use to reduce the possibility of reaching the min-charge limit. Hysteresis (charging to an artificially high limit after a discharging event) and Predictive Charging (starting the charging well in advance of low limit based on the running average power and present SOC) are two. Most likely there are others.

Clearly this is a Prius issue too. And its relatively well handled. Not to say the Prius does not get into battery protect mode either. And during battery protect mode, even with the parallel hybrid structure, not all the power goes to the wheels. Some energy ends up in the battery to get it out of the low SOC as quickly as possible.


For example, if the car can predict it will reach low SOC limit at the present power level averaged over 3 minutes, within 20 minutes then engine just comes on. That gives an extra 20 minutes of engine power if that average power level is maintained (as in mountain climbing). Remember, that big battery can take a whole bunch of engine time and save it. I doubt GM will do this, but they could even make these parameters configurable. This technique avoids running the engine in hilly terrain, as the downhill negative power output will lower the running average power.

With this predictive charging, one could always pull over at a scenic outlook, and wait till the battery is up to the hysteresis limit, too.

 

You know we can discuss charging, range, power, fuel economy and whatever else you'd like. However, at the end of the day, it's still a GM.

Not falling for that one again.

... Brad

 

Brad,

I'm ok with GM ... after the Volt has proven itself reliable for 10 years, and the car is more than a competing Toyota offering. Less or the same, GM can rot.

 

I didn't mean to imply that I thought this would necessarily be a common occurrence, just one operating mode. On the other hand, due to its serial hybrid nature, and its reduced power ICE I'm not sure the car will have a whole lot of options as to how to deal with the situation. Much less so than the Prius does for example, and we know the Prius can still get into trouble in this way under the right conditions/driver inputs.

Your predictive idea is a good one, but I see a couple of flaws. First, I think you over estimate the battery capacity. We believe that the Volt's battery is 16kWh, and reaches charge sustain at 30% SOC. So usable capacity would be ~11kWh. The latest I've heard is that the actual range is coming in at about 33 miles, which would be about 3mi/kWh. If we assume this is true at 60mph steady state, the entire battery would have a 33 minute capacity so right off the bat trying to maintain a 20 minute reserve seems unlikely. Running any significant reserve seems unlikely IMHO. Even if the engineers felt it was the right thing to do, management seems so fixated on making the Volt seem as electric car like and un-hybrid like as possible I don't think they'd go for it.

Next, in charge sustain mode in particular by the time you know you are in trouble its too late. Based on the table above, at 60mph you would normally be cruising at about 21hp. Now you come to a long 10% grade, and power required to maintain speed goes up to 78hp, an increase of 3.7X. This is going to reduce the battery range by the same 3.7X, so even with a completely full battery you would only last 9 minutes or miles. That might get you over the hill, but I've certainly been on longer. If you were in charge sustain mode already its a much bigger problem. Lets say the Volt maintains a 5% SOC reserve, IE you hit charge sustain at 30%, then it recharges to 35% and cycles but 30% is the absolute minimum allowed. 5% SOC would normally last you about 2.3 miles or minutes, so thats the kind of duty cycle you'd normally see on the ICE. However, on a 10% grade the 5% SOC will only last 0.6 miles, or about 37 seconds. Using your idea of a 3 minute sampled average, the battery would already be gone before it knew what hit it. Then all you have left is the ICE, and its limited power output. Even if you fired the ICE at that point, I'm not convinced it would help much. If you went to 100% power on the ICE, you'd still be pulling power out of the battery just to maintain speed. The minute you dropped the pedal to try and pass or accelerate you'd be done. And remember this is just at 60mph, at higher speeds the situation gets worse. At 70 mph you'd need 108hp to maintain speed. With the ICE at 100% putting out an effective 61hp, you'd still be pulling 47 hp from the battery depleting a 5% SOC reserve in about 1 minute.

Since the Volt will have to operate in charge sustain mode on all long trips, I do think this is an issue people will run into from time to time. From a technical standpoint, I'm not sure its the wrong choice. Greater hill climbing ability would mean more surplus horsepower in the ICE, and a bigger generator. This would increase cost, and reduce mileage under normal conditions. Is it a show stopper? No, but I think its just one step further from GMs original promise of an aggressive, no compromises car. From a PR perspective it could certainly cause them some problems. I'm sure they'll take some crap from the automotive media if it turns out it really can only maintain ~50mph on a long mountain grade.

Rob

 

Wait for those cases where the Volt driver reaches the hill with a SOC of 30.1%

GM is gonna have have 'splaining to do why their $40,000 car had to putter along in the truck lane. And it is not like they could just add a turbo to the ICE; the generator would have to be resized too.

 

The 16 kWh pack in the Volt allows only 50% SOC cycling for longevity (and conservative) purposes. This means only 8 kWh is available for EV propulsion.

The test mule is easily managing 40 miles range on the 8 kWh window, meaning 5 miles per kWh from the mule. No-one from the test team has predicted how that figure will translate to the lighter / more aerodynamic Volt chassis.

 

40 mile range plugins for $40K aren't going to sell--even with a $7.5K tax rebate. You can buy a golf cart for much cheaper. And for short drives to the store every day, you need little more than a golf cart.

I like the Prius drive concept. The battery charges with kinetic energy while driving, and the car is powered by a battery/ICE combo.

Here's an idea: why don't they make plug-in charging an option? In other words, you can either charge the battery while driving, as now done--or charge it in your garage through a power outlet, to save gas. That might work--at least until they have batteries powerful enough to go 500 miles without a charge, the range equivalent of a tank of gas.