GM-VOLT : Chevy Volt Electric Car Site

I had a discussion with Alex Cattelan, the Chevy Volt’s chief powertrain engineer about the engineering design and operation of the Chevy Volt’s charge sustaining mode.  This is the mode that occurs after the car has depleted the first 40 miles of  range and the gas generator has begun providing electric power.

When you first unveiled the Volt and it was a math model, the car was promoted as getting 50 MPG in generator mode. Now that there are real world parts and parallel hybrid like the Prius verse series. Can you speak about the efficiency difference between series and parallel hybrid operation?
We’re tuning our fuel economy right now. From an architectural perspective there are differences between series and parallel hybrids , there’s absolutely no doubt about that. The issue you mentioned the Volt is a series hybrid when we go into charge sustaining mode or when the engine comes on. We like to think of it not as a hybrid. You’ve got to understand that all of the decisions that we’ve made around this product are made because its an EV. That is the first and foremost thing that it needs to be. So because it is an EV some of the decisions that we’ve made around engine operation will be different than what Toyota makes in its parallel hybrid. For them they are always operating in hybrid mode so they need to optimize everything for engine operation.

In our case we’re optimizing everything for EV operation and the secondary is certainly going to be better than conventional vehicles, but were not necessarily totally optimizing the system for charge sustaining mode because we don’t want to compromise electric vehicle mode.

So to be optimally efficient in charge-sustaining mode you might compromise EV performance?
In the electric vehicle mode, and its not just performance, its efficiency in electric vehicle mode that we’re optimizing.

You mean those first 40 miles?
Right, so you’ve got to remember our principle promise is this is an EV and our engine is there as a range extender and so even when the engine is on, we operate as through we are in EV. All the primary propulsion is satisfied by the electric motor. The engine is really there to supplement power to keep the battery sustained. Now there are a couple of tricks of the trade that we do since we have the engine on more, but for example we don’t want to do a whole lot of gearing that you would do in a parallel hybrid, because none of that is beneficial to you in the EV state.

But doesn’t the fact that you could keep the engine at fixed RPMs also allow you better efficiency?
Actually we don’t keep it at a fixed RPM, we have a window of operation that is optimized. We have been able to optimize the engine for a window of efficiency but it is still best to charge your power and torque levels within that window as the customer torque request varies. We don’t want to always be operating at one state because really you may be putting too much energy into the battery or drawing too much energy out of the battery. It is still good to vary that engine power and torque. Not to follow exactly what the accelerator pedal does, but to optimize efficiency.

We actually have a very sophisticated efficiency calculator in our model within our software. It calculates on a very very short time scale what the driving conditions of the car are. Which mode you are in, whether you turning you engine on or off, and what power and torque you want to run than engine always with the optimization of efficiency in mind as well as managing trade offs for driveability and noise.

We took all of the model that’s in the two mode hybrid and we’ve basically been carving out pieces we don’t need, adding in pieces we do need for this architecture and optimizing that model for this particular vehicle. We didn’t build this from scratch, this is also software that we are using that is also on the 2-mode but we are modifying it for optimizing this architecture.

I’ve driven the 2-mode and notice you can see the switched in mode of operation without feeling it in the car.
Which is the goal, you don’t want you to feel it in the car, we don’t want the customer to know these transitions are taking place, but we need to be able to enable them for efficiency.

With the Volt, once you’re in CS mode you will have a few different windows of operation or just one window for the generator?
We’re optimizing the generator to have different power generating levels. But the beautiful piece of being able to decouple the engine or generator from the axle torque requirement is we can travel along and hit those power levels that we need to optimize the system for battery charging and discharging, we can maneuver across them at any rate of change we so choose.

So think of it as the beauty of being able to decouple the engine is we have a degree of freedom that we don’t have to follow the pedal at all. We can pick and choose the points that are most efficient, we can go between those point on the best path and the most pleasing path to the customer. Actually this is a lot of the work we are doing even on a Prius hybrid every hybrid does it to some extent but every engine is required to follow the pedal. It is much more coupled to the axle torque request than in our vehicle.

It seems to me then you should make CS mode even more efficient then in a car where the engine always has to turn the axle?
Right and it is more efficient than a conventional vehicle because they do have to have that engine coupled. Again were optimizing some of those efficiency point puts we are really doing is focusing on the optimization of the EV. There are trade offs because we absolutely consider this product an EV by nature.

On the same day the Today Show ran its piece on test driving the Chevy Volt integration prototype, Lindsay Brooke, a reporter from the New York Times published his.

Brooks was the only other journalist so far besides CNBC’s Phil Lebeau to test drive the Volt in generator or charge-sustaining mode and has written about it.

His brief article entitled Life After 40 tells us what we’ve been waiting to hear, but maybe not exactly what we wanted to.

Brooks writes:

Like other reporters, I had already driven Volt prototypes in the battery-powered mode, and they were predictably smooth and silent. But for eventual Volt owners, a crucial — and so far unanswered — question is how the car will perform when the battery’s charge is depleted and all electricity is provided by an onboard generator, driven by a gasoline engine, that has no mechanical connection to the wheels.

Then as he runs the car’s electric range meter dramatically down to the zero mark, the moment we’ve all been waiting for occurs:

With the dashboard icon signaling my final mile of range, I point the Volt toward a hill and wait for the sound and feel of the generator engine’s four pistons to chime in. But I completely miss it; the engine’s initial engagement is inaudible and seamless. I’m impressed.

Good so far.  He finds that as he pushed the accelerator, the sound of the engine didn’t change.

But later as he’s accelerating around the test track he gets a little shock that he calls disconcerting.

A few hundred yards later, as we snake through the track’s infield section, the engine r.p.m. rises sharply. The accompanying mechanical roar reminds me of a missed shift in a manual-transmission car. For a moment the sound is disconcerting; without a tachometer, I guess that it peaked around 3,000 r.p.m.

He asks GM’s Tony Posawatz driving with him what just happened.

“The system sensed that it’s dipped below its state of charge and is trying to recover quickly,” apologizes Posawatz. “The charge-sustaining mode is clearly not where we want it to be yet.”

Huh?

He goes on to write:

Immediately the engine sound disappeared, although it was still spinning the generator. A few times later in our test, the generator behaved in similar fashion — too loud and too unruly for production — but there is time for the programmers to find solutions.

So there we have it, the first reporter in history to write about the Volt’s operation in generator mode, and its a bit of a mixed picture.

It seem to go on flawlessly but throughout driving apparently has spikes of on/off engine roars that he finds disconcerting and unruly.

Though not exactly a happy report, he concludes:

Throughout my test, the prototype behaves admirably. At its current state of development, the Volt is an extremely refined vehicle.

Looks like GM has a little more work to do.  But then again the experience may be a bit subjecttive.  We’ll wait to see what others have to say.

Source (New York Times)

Volt in Tennessee

I had the following discussion with Alex Cattelan who is the Chevy Volt’s chief powertrain engineer. She has a very deep understanding of how the car operates and recently drove in a fleet of integration Volt prototypes through rugged mountains of Knoxville Tennessee.

Do you feel the charge-sustaining (CS mode) experience is now pretty solid and are you pleased with it?
We are definitely pleased with the level of progression we’re at. We are right on track to where we need to be in terms of the next stages of development we need to do to meet our target. Is it commercial right now, no, but that’s why were taking the time to get it to be commercial. Does it have the capability of being commercial, absolutely. We’ve proven that to ourselves.

We just took our leadership through a test drive in Warren (Michigan) and comments are that they cant tell when the engine is coming on or off. Those are the kind of things we like to hear.

You drove the IVs around the mountains of Tennessee. What about those big hills and the generator. It sounded like it drove very briskly powerfully and effortless is that true?
Yes, absolutely. Some of our control capability to utilize the battery, the engine, and the motors and to be able to optimize for high load and low load situations we’ve been working on developing that stuff for the last year and half an I couldn’t tell you how happy I was when I was in Knoxville because that’s when I had the opportunity to see it all come together. A lot of those bits of software all came together on one trip and it was a joy to drive.

So you took it up some steep hills and mountains?
Absolutely. We were in the Cherokee area taking it up through those hills and a lot of situations following it. It was a very touristy area. Following the speed of traffic, absolutely no problem, and where I had opportunities I certainly like to push the limits of the vehicle, and we did on those mountains, and I could not get it to degrade in performance.

That’s in charge sustaining mode?
Certainly in charge depleting, we have no issues because we have battery power, I’ve got it all at my fingertips. Now in charge sustaining we know that the engine power is slightly less than the peak vehicle power but we have controls ways to manage that and to utilize the engine in conjunction with the battery to get a little bit of extra power when we need it, and replenish it back when we don’t.

I know the battery runs down to roughly the 30% level before for the engine comes on. Is that 30% itself the whole potential buffer band?
We certainly don’t utilize the full 30% but there’s a portion of that that we utilize as a buffer.

You wouldn’t go to zero ever?
No. When the engine cannot meet peak load requirements we’ll suck a little bit out of that buffer and as soon as we have a situation when we can, we’ll put it right back in. All the controls that we work do that to optimize not only the driveability but the efficiency as we’re doing it, NVH (noise vibration harshness) as we’re doing it, the total driving experience as well as to protect the battery from a life experience. This is what allows us to give really good warranty life on our batteries as we’re doing I all in a controlled fashion.

NEW: Discuss this story in the GM-Volt Forum

Chevy Volt Powertrain

I had the following discussion with Alex Cattelan who is the Volt’s Chief Powertrain Engineer. It explains for the first time anywhere in more depth how the Volt’s two separate electric motors function.

The design of the electric motor, is there a separate generator or does the motor itself just turn the other way and act as a generator?

Very interesting question. There are two motors. One is considered the traction motor and the other one is the generator. However, and they are two motors, the traction motor is  higher-powered and designed specifically to meet the traction requirements.  The generator is designed to efficiently couple to the engine to generate what we need and match the efficiency band of the engine as much as possible in all the operating modes. So we look at that motor as coupled with the engine in system and then we also have a traction motor.

Some of the interesting pieces though of this are, for example, in EV operation I have two motors on board and I typically use the traction motor only to drive the vehicle. However, I do have some mechanisms to couple those motors and in some points of operation these two motors can be coupled and have a more efficient state.

Does that produce more power if they’re coupled?

It’s actually not additive for power, it’s actually the way it’s architected, and a lot of this is proprietary so I can’t get into the full architecture, but what it does is optimize the rotating speed and the losses of the motors so in certain states its better to operate both to propel the vehicle and in some states its better to utilize more of the generator and less of the traction motor. In some states its more efficient to use more of the generator and have more of the traction motor actually be a generator. That would be for example in coast down situation often we use our traction motor as a generator on regen.

We do have the ability to utilize both motors in propulsion mode.

The issue is and the direct answer to your question is we do have a primary traction motor and a primary generator motor and they are designed specifically for those levels of operation. However, we have a little bit of flexibility in exactly how we use them.

GM vice-chairman Bob Lutz was interviewed on Fox Business News.  The entire interview can be watched at the bottom of the post.

Just as vehicle line director Tony Posawatz was asked by Fox the other day, Lutz was also asked what the Chevy Volt’s MPG will be when it is running in charge-sustaining mode, after the first 40 all-electric miles have been driven.

As someone who has followed this Volt story since day one, along with many of you, I have found that Bob Lutz cannot help but be honest.  GM has never officially acknowledged this number though at the time of the initial concept revelation they mentioned 50 MPG was the goal.  However, in those days the generator was to be a 1 L turbocharged 3 Cylinder, and not the normally aspired 1.4L 4 cylinder it turned out to be.

When now asked what the Volt’s fuel economy would be in charge sustaining mode, Lutz replied as follows:

We haven’t published it and it’s not finalized.  Once it’s running on pure gasoline it will be like a highly economical vehicle in that size class. It will be exceptionally good mileage but it obviously won’t be comparable to what it is when it runs on electric.

The vehicle is conceived primarily for urban or suburban use. It’s for that 80 percent of Americans who travel 40 miles or less per day; they will never use a drop of fuel.  If you have a 60 mile commute, you’ll have 40 miles purely electric, the remainder of the mileage on very good gasoline mileage, and your average fuel economy will be somewhere between 120 and 150 miles per gallon.

So let’s do the math.

On a 60 mile commute, the first 40 miles are electric and the next 20 are on gasoline.  At an average of 120 mpg, that would mean 0.5 gallons are used in those 20 miles (40 mpg).  At 150 mpg, it would mean 0.4 gallons are used in those 20 miles (50 mpg).

Thus the Volt will average between 40 and 50 mpg in charge sustaining mode.  Good enough for me, how about you?

Thanks to Philerup for the tip!

The Chevy Volt’s 1.4 L 4 cylinder flex fuel generator garners a lot of attention and discussion online.

It is this range-extender that make the car so unique. Although the Volt always operates as an EV, once the battery’s state of charge reaches roughly 30% this generator powers on, providing electricity to the motor. Energy is also obtained from regeneration and the battery buffer when power demands are high.

GM has yet to publicly demonstrate the car operating in this charge-sustaining mode, but most accounts indicate it is unnoticeable.

In spite of many months of rampant speculation here, GM’s lead Volt engineer Andrew Farah has finally disclosed some details about the engine’s operation. He said the generator would “operate from 1200 to 4000 RPMs and from a 30% to 100% load.”

Further explanation comes from Volt Powertrain Engineer Alex Cattelan:

“We don’t keep it at a fixed RPM, we have a window of operation that is optimized. We have been able to optimize the engine for a window of efficiency but it is still best to change your power and torque levels within that window as the customer torque request varies.”

“We don’t want to always be operating at one state because really you may be putting too much energy into the battery or drawing too much energy out of the battery. It is still good to vary that engine power and torque. Not to follow exactly what the accelerator pedal does, but to optimize efficiency.”

Finally Volt vehicle line director Tony Posawatz explains it this way:

“In charge sustaining or range extender mode, the Volt will not follow the throttle position. The challenge is to select the right operating points (RPMs) that are 1.) efficient, 2.) pleasing to the driver, and 3.) meet regulatory requirements.”

And he teases us, “we are about ready to expose people to this experience.”