[ad#post_ad]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.