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Chevy Volt Delivers Novel Two-Motor, Four-Mode Extended Range Electric Drive System; Seamless Driver Experience Plus Efficiency

by Mike Millikin and Jack Rosebro

The Volt drive unit combines two motors and three clutches to deliver four distinct operating modes to maximize efficiency and provide a seamless driving experience. Click to enlarge.

During the serial media launch events for the Chevrolet Volt, GM provided more detail (subsequent to the completion of related patent work) on the novel drive architecture applied in their first extended range electric vehicle to enhance the efficiency of both the battery electric and extended-range driving modes.

The complex system leverages GM’s learnings from its two-mode hybrid system in a number of areas, including the efficiency benefits of a multiple-motor approach to meeting the full range of electric drive operating requirements; synchronous clutches; and the vehicle’s control software and architecture.

The Volt is an electric-drive vehicle, powered by a 16 kWh Li-ion battery, incorporating an internal combustion engine and generator to serve as a range extender; an electrical plug is intended to be the primary source of stored energy used to deliver motive power. (The Volt could also be called a plug-in series hybrid, depending upon your taxonomic preference.)

Given that GM had committed to a combustion engine and a generator as a range extender for the battery, the engineering team set out to develop a drive system that could maximize the combined efficiency of all the components under the different driving modes (all battery electric, and extended range). Put another way, the GM team wanted to extend the range of the vehicle as efficiently as possible, while maintaining quality driving dynamics and experience.

The story of the electric drive is really a story about efficiency. How do we take all this battery energy...and very efficiently and effectively drive the wheels. That’s ultimately what the customer is looking for, is to maximize this notion of electric range, to maximize this notion of efficiency of the generator when we have to use it.

—Pam Fletcher, Global Chief Engineer for Volt and Plug-In Hybrid Electric Powertrains

Mechanical losses such as friction and windage dictate that the efficiency of an electric motor declines somewhat as motor speed increases. Recognizing the benefits derived from their two-mode approach in their earlier hybrids, the engineering team also observed that they had a second motor—the generator motor—“going along for the ride”, as Fletcher said during her presentation at the launch event in Detroit. They thus decided to exploit the generator motor more thoroughly than it would otherwise have been if used exclusively as a generator in extended-range driving.

The resulting Volt drive unit consists of two motors—a 111 kW main traction and 63 kW (at 4800 rpm) generator motor (55 kW generator output)—as well as three clutches and a planetary gear set tucked in the end of the traction motor that improve overall efficiency by reducing the combined rotational speed of the electric motors as needed.

This engine obviously has the capability of revving much higher and producing much more output. But this is really a study in rightsizing, rightsizing an internal combustion engine for this extended range capability. We’re almost repurposing an internal combustion engine to provide this very unique type of propulsion. That’s how much power output we determined we needed for this car that has a very large battery and almost a half-size engine in terms of displacement to provide you with the average power required to provide an urban and highway type commute.

—Pam Fletcher

This configuration reduces battery drain at steady state, cruising speeds in a window ranging from around 30 mph to more than 70 mph (48 to 113 km/h), adding up to two miles (3.2 km) of additional all-electric range. The Volt delivers a pure-electric range between 25 and 50 miles (40 and 80 km)—depending on terrain, driving techniques, driver comfort requirements (e.g., HVAC), and weather. The range extender pushes that to approximately 350 miles (563 km).

The Volt’s drive unit uses an on-axis configuration; motors and gear-set are mounted in an in-line with the range-extending internal combustion engine. Two of the clutches are used to either lock the ring gear of the planetary gear-set or connect it to the generator/motor depending on the mode. The third clutch connects the internal combustion engine to the generator/motor to provide range extension capability.

The 111 kW traction motor is permanently connected to the sun gear, and the final drive (gear reduction, differential) is permanently connected to the planetary carriers. The planetary carrier gears are used to modulate gearing ratios between the vehicle’s electric motors, its internal-combustion engine, and its 2:16 final drive.

The Volt has two primary driving modes:

  • All battery-electric (charge depleting), in which the battery is the sole source of power for the motors; and
  • Extended-range (charge sustaining), in which the battery and engine work together in different operating modes to power the traction motor and to improve overall efficiency.

Each of these two driving modes is supported by two drive unit operating modes: a low-speed, 1-motor mode, and a high-speed, 2-motor mode.

Mode 1. Click to enlarge.

Mode 1: Low-speed EV Propulsion (Engine Off). In this mode, the ring gear is held (locked) by clutch C1. With clutch C2 and C3 disengaged, the generator-motor is decoupled from the engine as well as the planetary gearset. As the traction motor is permanently coupled to the sun gear, the planetary carriers must rotate when the traction motor rotates. Since the planetary carriers are permanently coupled to the final drive, the traction motor propels the vehicle. The generator-motor and the engine are idle during this mode, although the engine is free to start if necessary (example: engine maintenance mode).

Virtually all of the vehicle’s motive power is therefore delivered by the traction motor in this mode, including hard accelerations, using power supplied by the battery pack. With this configuration, the traction motor can produce up to 111 kW (149 hp) and deliver up to 370 N·m (273 ft-lb) of torque.

Mode 2. Click to enlarge.

Mode 2: High-Speed EV Propulsion (Engine Off). As vehicle speed increases, motor speed and losses also increase. To engage both motors and preserve motor efficiency, clutch C1 is disengaged, allowing the ring gear to rotate. At the same time, clutch C2 is engaged, connecting the ring gear to the generator-motor. The generator-motor is then fed current from the inverter, and runs as a motor. The engine remains disengaged from the generator-motor.

This mode allows the two electric machines to operate in tandem at a lower speed than if the traction motor alone was providing torque. The speed of the traction motor in this mode drops to about 3250 rpm from 6500 rpm in the 1 motor mode, according to Fletcher.

This strategy allows the Volt to wring out as much as two extra miles of all-electric operation out of its battery pack, depending on operating conditions. However, switching from low-speed to high-speed EV mode requires the simultaneous operation of two clutches. GM’s experience with simultaneous clutch operation in their two-mode transmissions and transaxles was key to the development of the Volt’s transaxle control strategy.

Mode 3
Mode 3. Click to enlarge.

Mode 3: Low-speed Extended-Range Propulsion (Engine Running). Once the Volt’s battery pack has reached its minimum state of charge (SOC) (which varies depending on operating conditions), clutch C1 engages, locking the ring gear, and clutch C2 disengages, decoupling the generator-motor from the ring gear. At the same time, clutch C3 engages to couple the Volt’s 1.4 liter Ecotec range-extending engine to the generator-motor, so that it may be operated in generator mode.

During low speeds as well as hard accelerations, the traction motor propels the vehicle. The engine drives the generator-motor, and power to drive the traction motor is delivered by the generator-motor as well as the battery pack via the Volt’s inverter. Under most conditions, the generator will provide enough power to maintain minimum battery SOC, and therefore allow the vehicle to remain in this mode until it is plugged in.

Mode 4. Click to enlarge.

Mode 4. High-Speed Extended-Range Propulsion (Engine Running). The blended two-motor electric propulsion strategy used at higher speeds in EV driving has also been adapted for extended-range driving. In this mode, the clutches that connect the generator/motor to both the engine and the ring gear are engaged, combining the engine and both motors to drive the Volt via the planetary gear set. All of the propulsion energy is seamlessly blended by the planetary gear set and sent to the final drive.

This novel mode—which GM calls “combined mode”—enables a 10-15% improvement in efficiency at steady state cruising speeds compared to a comparable single-motor mode, GM says. Under no circumstance can the Volt be propelled by engine torque alone; the traction motor must be operating if the vehicle is to move and the engine is to provide torque.

When we’re in this combination...on this planetary gearset we are driving the engine-generator combination onto the ring gear. We are utilizing the traction motor to provide the reactionary force so that we can ultimately drive the output. That is what happens in combined mode, that’s what allows us to get the 10-15% more efficiency.

—Pam Fletcher

In this mode, the generator still continues to produce electricity as well as deliver torque via the gearset, Fletcher said. The ratio of torque to power generation varies with operating conditions, and is, as the rest of the system, under the management of the control software, according to her. As noted earlier, the control software and architecture enabling this drive unit is critical to its overall success. We anticipate that additional information will be disclosed about the control mechanisms for Mode 4 as patents are awarded and SAE papers are approved for publication.

Packaging of the drive unit. The drive unit is quite compact, and includes the power electronics as well as the engine, motor generator, planetary gearset and traction motor. The power electronics unit includes three IGBT inverters: one for each motor, and one for the electric oil pump.

“If the Volt is in electric mode, we can accelerate the car wide open throttle to 100 mph—so you can have the full performance envelope of the car all electrically. That to me is a very important point.”
—Pam Fletcher

Driver experience. During the launch event (which GCC attended courtesy of GM), journalists paired off to drive pre-customer versions of the Volt on roads under different conditions for almost 200 miles. Based on that limited sampling, we can report that the transitions between modes are seamless and smooth; at one point, we had entered into range-extending mode without even knowing. The Volt accelerates crisply (and quietly), and handles snappily at moderately excessive interstate speeds—all on battery power.

The sole exception to the noise quality was on entering into mountain mode (driver-selected via the console); the engine races loudly.

The Volt offers three driver-selected modes: normal, sport, and mountain; mountain mode is designed to help the Volt traverse particularly steep and long grades—e.g., the Eisenhower Pass. This mode increases minimum battery SOC to around 45%. The driver will hear more engine noise during mountain mode, due to the higher rate of power generation required to maintain this mode. GM expects mountain mode to be required only under unusual power demand conditions.

GM engineers said that in the customer models, they are implementing a software fix to reduce the mountain mode noise somewhat. That said, GM wants the use of mountain mode to be exceptional—i.e., it doesn’t want customers running on mountain mode to recharge the pack. Power should come from the plug.



So the Volt isn't a pure series hybrid after all; it has far more in common with the Prius drivetrain than we knew, and the mechanical arrangements are constrained by the need to couple the engine to the final drive.

That does explain why the under-hood pictures look so completely conventional; it is quite conventional.


Not quite as elegant as Toyota's Hybrid Synergy Drive, but it looks different enough to not infringe Toyota's patents.


It seems like they lost track of some design goals somewhere and increased complexity in many cases even where it was not needed.

" This configuration reduces battery drain at steady state, cruising speeds in a window ranging from around 30 mph to more than 70 mph (48 to 113 km/h), adding up to two miles (3.2 km) of additional all-electric range. "

OR...they could have used another .5kWh of the battery pack and gotten another 2 miles of range without adding all the cost/complexity.

Designing these types of systems requires engineers to make judgement calls and tradeoffs. It seems like whoever was leading this team was erring on the side of trying to squeeze every little ounce out of the system and may have ended up making the system itself too complex and missed the mark.

Impressive...but did it have to be?


"...this notion of electric range, to maximize this notion of efficiency of the generator when we have to use it." Electric range & generator efficiency aren't abstract "notions" dreamt up by consumers, they're real performance specifications that people demand. Where do these ignorant, arrogant corporate types come from? Is it from being around the bureaucracy too much?

Account Deleted

The Volt is no less than 4 times better than the Prius in terms of fuel economics, emissions and noise pollution and this is no small thing as the Prius has lead the auto industry on these increasingly important attributes for over a decade now.

An example will illustrate

The average driver going 12000 miles per year in a Prius burns through 12000/50 = 240 gallons of fuel at a rate of 12000/240 = 50 mpg.

The average driver going 12000 miles per year in the Volt burns trough 10000 miles of electricity possibly generated by green hydro or wind power and 2000miles/(35mpg in extended range driving) = 57 gallons of fuel at a rate of 12000/57 = 210 mpg or over four times better than the average annual consumption of the Prius.

To conclude, the Prius does 50 mpg and the Volt does 210 mpg for the average US driver and this fact makes the Volt one of the most important cars in the past hundred years. However, it will only take a month before the competition arrives in terms of the Nissan Leaf. For a lot of good reasons, like national security, public health and the environment I hope demand will outstrip supply for both of these vehicles until these kinds of vehicles rule the market.


More parts mean more maintenance and more money for GM.

A simple electric car doesnt produce long term revenue.
No oil changes, transmission fluid, filters keeping you coming into the shop every 3000 miles. So the GM dealers wanted 3 clutches and a gas engine with oil changes.

They can make a rose red,white and blue but they cant make grass grow 2 inches. No long term revenue


"Is it cheap? New technology never is. Still, the Volt strikes us as the closest in concept to the winning formula of the Prius, albeit with the next generation of propulsion and the whole thing inverted. Nothing else has so successfully incorporated all of the key aspects of Toyota’s golden child—big fuel-economy numbers, a unique name and styling, and enough range and people and cargo space that it can be an only and everyday car. Those traits have enabled the sales of nearly 2 million Priuses worldwide since its 1997 debut. With the possible exception of a fairly cramped back seat and an undersized cargo hold, the Volt checks all the boxes, plus it outdrives the hybrid competition. This is without a doubt the most important new car since the advent of hybrids in the late ’90s, and GM has nailed it. Is this the handing off of the Prius’s very illustrious torch?"
Car and Driver - December, 2010


"Is this the handing off of the Prius’s very illustrious torch?"
Car and Driver - December, 2010"

No it's not and time will prove it.


What I see is a lot of friction loses in the gearing, weight loses from the ICE and it's related components, and efficiency loses from energy conversions.

Ridding the vehicle of these loses and converting to a BEV can be as simple as removing the ICE and its associated components, adding a larger battery pack and cleaning up the layout and you will have a great car.


If Volts and Prius drive trains are much the same, will Toyota sue GM for patent rights or is Toyota working on the next generation and will leave GM use their older technology (at a very low cost)?


Danshl brings up some great points. Also remember, a major part of this equation is UAW jobs - many of which are ICE and ICE component dependent. Any possible way to increase the complexity and dependency of the Volt on the ICE will mean more UAW jobs and job security. Doing a "whisper quiet" efficient turbine instead of an ICE will mean fewer UAW employees are needed for the overall construction of the Volt.


Yep, this is essentially a variation on a plug-in Prius. Not that that's a bad thing.

Frankly, I'm surprised that a company like GM isn't focusing on range extenders with ultra-high power density. This is where I think Mazda has a leg up on everyone. They could use a turbocharged wankel genset weighing ~50lbs to replace the 300lb+ I4 beast in either the Prius or the volt.

Aixro makes a NA wankel that weighs ~32lbs and produces ~50hp

On the whole, wankle engines are clearly less efficient (even less durable) than piston based engines but their power density makes them attractive for a PHEV. The goal is NOT to use the engine, right?


The problem with BEV's is marketing
They could make a 50k BEV that get 200 miles but no one would buy it.

People cant figure out longterm expense
no oil changes, raditor fluid, transmission fluid
Plus the car engine would last longer, brakes would last longer. oh and i almost forgot the no gas

take all of that and you make a 20k car a 100k over it life time


Best comments I have ever red on greencarcongress. I have nothing to add.


Henrik, I have tried to sum up the Volts strengths and I thought I had done a good job, but your summation is better than mine. The Volt is a great car for getting us off foreign oil ASAP, and it sounds like it is fun to drive as well. But GM has priced it too high and the relatively less important CS mileage is just low enough that it can be used to attack the Volt. GM's engineers have built a great car, and GM's MBA's have priced it just high enough to make it seem unreasonable.
But on the positive side, many of the people attacking it now are the same people that claimed it was vapor-ware and would never be built at all, so hopefully they will be equally as misguided on the Volts future.


"A simple electric car doesnt produce long term revenue. No oil changes, transmission fluid, filters keeping you coming into the shop every 3000 miles. So the GM dealers wanted 3 clutches and a gas engine with oil changes"

Ding! Ding! Ding! I think we have a winner here. Good observation.


It's drivetrain similar to Prius, but with THREE extra clutches. Apparently based on GM's complex two-mode hybrid system which is probably too expensive for economy cars.

The most likely reason Honda avoids the use of a clutch in their IMA hybrid (to make it more efficient) is the price.


"But on the positive side, many of the people attacking it now are the same people that claimed it was vapor-ware and would never be built at all, so hopefully they will be equally as misguided on the Volts future"

Gm was doing electric cars with the EV1.

That is the point GM didnt need to make it this complex
just put a diesel or gas generator for long range. KISS rule applies, but then where would the service department go, complexity brings future revenue


Does GM not have an Atkinson cycle engine? Wouldn't that have been a better solution to use than a standard Otto cycle engine? With 50% better efficiency (20% for Otto vs. 30% for Atkinson) the mileage may have been comparable to the Prius. In fact, I think the real gains in hybrids like the Prius or the Ford Escape hybrid are more due to the Atkinson cycle engine, than the regen. I think the electrics help compensate for the lower torque output of the Atkinson cycle. And since the Volt mainly derives it's power from the electric motor, an Atkinson would have been ideal for the ICE.
But then I guess it would have been way too much like the Prius.


@ Henrik

Both the Prius and Volt are great engineering feats and I hope the Volt is very successful upon launch. However your statement, that the Volt is four times better than the Prius, is blatantly incorrect. That is like comparing apples and oranges.

Using mile per gallon statistics for plug in vehicles is just a play on numbers. A more universal and accurate statistic should be used, such as energy consumed per mile (kWh/mile). Looking purely at the fuel economy of each vehicle, once the initial charge has run out, will show that the Prius gets 50mpg and the Volt gets around 33 mpg.

Even a simple calculation to determine the annual gas consumed is not straightforward. The average annual vehicle mileage driven in the United States is a little over 12,000 miles. However, newer vehicles are driven much further annually than older vehicles. Disregarding these statistics, Volt drivers could be much more conservative driving to only accumulate all electric miles or they could completely ignore the fact.

Furthermore, charging a vehicle from electricity in the grid in the United States is far from green. With a loose classification on what is considered a renewable energy source, very few states or metropolitan areas currently have above a 10% energy consumption rate from renewable sources. And this is not going to change significantly anytime soon! Meaning that the vast majority of energy used to charge any plug in or electric vehicle is from fossil fuels that produce harmful emissions.

This idea that electric vehicles are “green” is only true if the majority of our energy generation is from renewable or clean energy sources and we utilize an efficient electricity infrastructure to distribute it. The Volt is a step in the right direction, but there are still several more steps that need to be taken.


While we have a good conversation going, would anyone care to speculate as to why the CR-Z "sport hybrid" gets such poor fuel economy? Seriously, that thing should get 45mpg+ easily...


What I really care about is using less gasoline/oil. The Volt technology works for me. My commuting would be all-electric and my occasional longer than 40 mile trip is possible using the gas engine in CS mode with good milage.

The Volt has been well tested and the engineers seem confident its robustness. Further development of the concept and technology continue with future generations and models on the way.

Why all the negativity? So it is not for may be fine for others. So Volt generation 1 is not econobox cheap...there will be more refined and cheaper generations to follow.

With BEV and further Volt and Prius developments to come there is excitement in the air!


Is this the handing off of the Prius’s very illustrious torch?

Only if it proves to be just as reliable.



I've tried to answer that question too: What gives the Prius it's high mileage? Switching off the engine at low speeds? Regenerative braking? Exhaust gas recirculation? The Atkinson engine? The aerodynamics? Driver feedback? Engine downsizing? Low rolling resistance tyres?

The answer is: all of the above. No single component is solely responsible for the Prius' high mileage. When you think it is the Atkinson engine, then think again: why isn't it used in non-hybrid cars? Because the Atkinson engine has low specific power and bad torque at low revs. This would make it unattractive for an ordinary car. In the Prius however, the HSD can mask those drawbacks.

Roger Pham

Well, finally, the truth is told! I figured all along that there is no way that a serial hybrid can optimize efficiency as well as being cost-effective.
This is a clever way for GM to make an electrical planetary CVT without infringing on Toyota's HSD patent. Even though GM uses 3 clutches, vs HSD using no clutch, GM's design is capable of heavy towing, while Toyota's HSD design does not recommend towing trailers.

For those contending that this SINGLE-planetary CVT design is too complicated, please be reminded that in a convention 4-6 speed automatic transmission, there are 3-4 stages of planetary sets with doule to triple the number of wet clutches...and the world is moving on 4 wheels just fine!

To sum up this planetary CVT dynamics, the traction motor 1 is connected to the sun gear, while the drive train is connected to the planetary carrier, while the engine-generator/motor2 is connected to to the ring gear. Clutch 1 is used to stop the ring gear, while clutch 2 is used to disconnect the ring gear from the generator/motor2, while clutch 3 is used to disconnect the engine from the generator/motor2. Very simple!!!

In operation, if the ring gear is stopped, then the traction motor 1 drives the car with a speed reduction of roughly 2:1, based on the diagram. This is used to boost the efficiency of the motor 1 at low speeds. When the vehicle gains speed, the generator/motor 2 will drive the ring gear along, thus cutting back on the speed reduction property of the interaction between the sun gear with the planet gears and the ring gear, all the way until the speed of the ring gear and the sun gear equal each other, at which, you will have a 1:1 drive ratio between the traction motor 1 and the drive train.
In the engine mode, again, this planetary gear CVT will also play its magic. At very low vehicular speed, the ring gear is locked, allowing the engine to drive the generator/motor2 as a serial hybrid. As the vehicle picks up speed, the ring gear will be clutched to the engine-generator/motor2 complex, allowing the engine to drive the vehicle using direct mechanical torque connection, with some supplemental torque from the traction motor. Because the traction motor 1 is capable of 111 kW, while the combination of engine-generator/motor 2 is also ~110 kW, these balance out perfectly, allowing 1:1 drive ratio to the drive train.

Thus, GM's current design, even though more efficient than their announcement of pure serial hybrid earlier, still requires pretty hefty electrical components, with a combined 165-170 kW of installed motor power, and a 16 kWh battery. This is because of the needs to balance out the power of the traction motor 1 to the combined power of the engine-generator/motor 2. In my subsequent posting, I will discuss a much lower-cost and light-weight design that can accomplish the same payload, more seating capacity, and acceleration and speed performance.

Stay tuned!

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