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GM’s Voltec mysteries revealed

Award –winning Chevrolet Volt is an electric vehicle, series hybrid and parallel hybrid all in one

Published: February 17, 2012, 10:00 PM
Updated: June 17, 2015, 5:05 PM

It’s more than four years since I was first exposed to what General Motors now calls its Voltec Propulsion System – the combination of gasoline engine, electric motors and lithium-ion battery pack that serves as the 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 all its intricacies – until now.

Of course, I knew the basics: the Volt is driven by two electric motors, powered by a sophisticated, liquid-cooled battery pack that is charged by a combination of plug-in power from the electric grid and on-board power 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 a series-hybrid powertrain, not unlike that in a diesel locomotive, in concept. 

The company, however, wanted to separate the Volt from existing hybrids in the marketplace – not an unreasonable desire because it is a 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 that apparent spin-doctoring – although, in truth, it didn’t matter what it was called for it is a pioneering technology. But little did I (we) realize then just how much a hybrid it really is.

That realization began to occur later, when GM acknowledged that, under some conditions, the gasoline engine could transfer some power mechanically to the wheels, which would mean, in those conditions, that it was acting as a parallel hybrid.

So just what kind of animal is it, really?

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

What Mepham explained to us is that the Voltec system has not just one or even two but four separate operating modes – two pure electric and two that combine use of the electric motors and gasoline engine.

The key component that enables those different operating modes, and seamless transitions from one to the other, is a very sophisticated continuously-variable transmission (CVT) module, although it is much more than that.

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

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

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

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 the primary traction motor, so at higher vehicle speeds overall efficiency is improved by supplementing power from the larger motor with that from the smaller.

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

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

Its power output is applied to recharging the batteries rather than driving the wheels – series-hybrid operation – and only the large traction motor drives the wheels.

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

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

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

So, as GM initially called it, the Volt is is an EV; but as we 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 EV label fits as well as any.

Whatever one calls it, it truly is a pioneering technology. Sufficiently so that it won AJAC’s Best New Tecnology Award for 2012.

ARIAL 11

It’s more than four years since I was first exposed to what General Motors now calls its Voltec Propulsion System – the combination of gasoline engine, electric motors and lithium-ion battery pack that serves as the 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 all its intricacies – until now.

Of course, I knew the basics: the Volt is driven by two electric motors, powered by a sophisticated, liquid-cooled battery pack that is charged by a combination of plug-in power from the electric grid and on-board power 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 a series-hybrid powertrain, not unlike that in a diesel locomotive, in concept. 

The company, however, wanted to separate the Volt from existing hybrids in the marketplace – not an unreasonable desire because it is a 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 that apparent spin-doctoring – although, in truth, it didn’t matter what it was called for it is a pioneering technology. But little did I (we) realize then just how much a hybrid it really is.

That realization began to occur later, when GM acknowledged that, under some conditions, the gasoline engine could transfer some power mechanically to the wheels, which would mean, in those conditions, that it was acting as a parallel hybrid.

So just what kind of animal is it, really?

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

What Mepham explained to us is that the Voltec system has not just one or even two but four separate operating modes – two pure electric and two that combine use of the electric motors and gasoline engine.

The key component that enables those different operating modes, and seamless transitions from one to the other, is a very sophisticated continuously-variable transmission (CVT) module, although it is much more than that.

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

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

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

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 the primary traction motor, so at higher vehicle speeds overall efficiency is improved by supplementing power from the larger motor with that from the smaller.

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

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

Its power output is applied to recharging the batteries rather than driving the wheels – series-hybrid operation – and only the large traction motor drives the wheels.

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

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

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

So, as GM initially called it, the Volt is is an EV; but as we 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 EV label fits as well as any.

Whatever one calls it, it truly is a pioneering technology. Sufficiently so that it won AJAC’s Best New Tecnology Award for 2012.