An Explanation of the Chevy Volt Generator Mode

Here is an interesting interview with GM executive Tony Posawatz as concerns the Chevy Volt operating in generator mode.

From GM-Volt.com:

When the generator goes on will it come on gradually?
We’re still playing with it. The initial transition we like very much, it is almost imperceptible. We want to tune it and exercise it more for production readiness, but we have an algorithm of software that feathers it in so that its almost imperceptible.

To many people it is imperceptible, to those that are a little bit more tuned in…Frank (Weber) says its not quite perfect, he wants it so that no one can hear it.

That transition isn’t handled in most hybrids that well, particularly when you stop at a light an the engine shuts off and you get that shudder. We have this benchmark we set, that we’re going to blow them away.

By the way, our motor is so much bigger than theirs, the power output of the electric motor, that effectively is the starter. We’re in the point of tuning it and it’s a very very complicated tuning exercise if you want to make it perfect.

It’s a balance equation. Here lies the challenge. You will get some feedback from the car about its environment, or the road load. You’re demanding so much power from the car, and you will get some feedback. That’s one input that you have to be able to respond to to deliver the right amount of power. The other aspect that we have to play with is this area of OK when I respond to it, the engine generator will run in different modes or RPMs. How do I transition from mode to mode, a timing issue, do I smooth it, do I try to react immediately to that response? If I act immediately to that response how will customers receive that? such as a wild of swing of RPMs?

There’s the responding to the car’s demands based on the loads number one, then there’s the affect on the customer, we want that to be really pleasant. The transition to charge sustaining, we’ve got that down, it’s the operating in the very different modes we’ve got to figure out. The next leg is making sure the engine is running the most efficiently that it can.

Generators are typically run at a sweet spot that’s very efficient. We could do that. The question is, does the amount of output of power that results from that manage or balance with what the car needs?…hmm how do you do that? That’s another reason we have this battery buffer reserve.

How often do you turn the engine off and on is another question. These are the factors that have to be balanced.

People ask why can’t you show it to me? Because we want it just right.

This requires a lot software. The other interesting challenge is the regulatory issues. Theoretically you could run the engine enough to build a surplus of energy. The regulatory guys are not going to let us do that, it’s a five legged stool.

Is the EPA sitting on a committee while you engineer the car?
This is more CARB than it is EPA. EPA is interested from the perspective that they need to understand how to label it. CARB really doesn’t want you to burn fuel if you don’t have to.

The message on charge sustaining mode is we’ve got a good beat on it, its the four or five things that have to be balanced together and the team needs time to work.

Will the engine rev higher when you step on the accelerator?
Yes, but the question is, if I have a little battery reserve at that time, do I turn the engine on right away to follow that or is it a smoothing function?

Also lets say you just went up a monster hill and the engine feathered up a little to support you and you get to the top of the hill and hit a light. Should the engine keep on running to allow you to recover or do you turn off the engine because the customer expects it too because they’re stopped at a light?

Technically are these difficult challenges? No. Its hard development work balancing the calibration of a lot of software.