GM’s Voltec mysteries revealed

Award –winning Chevrolet Volt is an electric vehicle

Published: February 22, 2012, 6:00 AM
Updated: October 3, 2014, 12:02 AM

It’s more than four years since I was first exposed to whatGeneral Motors now calls its Voltec Propulsion System – the combination of gasoline engine, electric motors and lithium-ion battery pack that serves asthe powertrain for the Chevrolet Volt.

And I’ve had much more exposure to the system since then.But, although I thought I did, I’ve never truly understood how it works, in allits intricacies – until now.

Of course, I knew the basics: the Volt is driven by twoelectric motors, powered by a sophisticated, liquid-cooled battery pack that ischarged by a combination of plug-in power from the electric grid and on-boardpower from a generator driven by a small gasoline engine.

The engine’s sole purpose, as initially stated by GM, was to drive a generator that charges the batteries – the classic definition of aseries-hybrid powertrain, not unlike that in a diesel locomotive, in concept.

The company, however, wanted to separate the Volt fromexisting hybrids in the marketplace – not an unreasonable desire because it isa very different animal from the others. So GM chose to call it an Extended Range Electric Vehicle, not only downplaying but ignoring the hybrid label altogether.

Some auto journalists, myself included, called GM on thatapparent spin-doctoring – although, in truth, it didn’t matter what it wascalled for it is a pioneering technology. But little did I (we) realize thenjust how much a hybrid it really is.

That realization began to occur later, when GM acknowledgedthat, under some conditions, the gasoline engine could transfer some powermechanically to the wheels, which would mean, in those conditions, that it wasacting as a parallel hybrid.

So just what kind of animal is it, really?

I finally got a full explanation from Dan Mepham, productmanager for the Chevrolet Volt at GM of Canada, who made a full presentation ofthe Voltec system’s workings to a panel of technical auto journalists gatheredto vote on the Automobile Journalists Association of Canada’s (AJAC) Best NewTechnology award.

What Mepham explained to us is that the Voltec system hasnot just one or even two but four separate operating modes – two pure electricand two that combine use of the electric motors and gasoline engine.

The key component that enables those different operatingmodes, and seamless transitions from one to the other, is a very sophisticatedcontinuously-variable transmission (CVT) module, although it is much more thanthat.

Unlike most other CVTs, it comprises primarily a singleplanetary gearset – sun gear, five planetary gears and their carrier, and aring gear – not that different in concept from those used in conventionalautomatic transmissions.

A sophisticated computer control system engages and releasesthree separate clutches to permit power-flow through the gearset in fourdifferent ways, corresponding to the four modes of operation.

Mode 1 is pure electric drive, using the system’slarger (111 kW) electric traction motor. This mode is used for startup, initialacceleration and low-speed operation, where it is most efficient. An electricmotor produces maximum torque at startup and its efficiency drops off as speedincreases.

Mode 2 adds input from the second electric motor(55kW), which also serves as a generator.

Being smaller, it is more efficient at high speeds than theprimary traction motor, so at higher vehicle speeds overall efficiency isimproved by supplementing power from the larger motor with that from thesmaller.

In both Mode 1 and Mode 2 the Volt operates the same way asa pure battery electric vehicle.

Mode 3 also employs the second motor, but as agenerator, driven by the gasoline engine.

Its power output is applied to recharging the batteriesrather than driving the wheels – series-hybrid operation – and only the largetraction motor drives the wheels.

Mode 4 engages the clutch connecting the smallmotor-generator to the gearset, while keeping it connected to the engine.

The engine, in that case, is mechanically connected to thegearset so can provide power directly to the wheels, supplementing that fromthe large traction motor – parallel hybrid operation.

The system typically operates in this mode at highway speedswhen additional power is required, such as for hill-climbing or passing.

So, as GM initially called it, the Volt is is an EV; but aswe initially maintained, it is also a series hybrid; and, as we just learned,it’s a parallel hybrid, too. All of which means that GM’s Extended Range EVlabel fits as well as any.

Whateverone calls it, it truly is a pioneering technology. Sufficiently so that it wonAJAC’s Best New Tecnology Award for 2012.