Revenge of the Two-Mode Hybrid

In recent months General Motors has rolled-out a compelling hybrid transmission architecture at the heart of the 2016 Volt, 2016 Malibu hybrid and a possibly related transmission for the 2016 RWD Cadillac CT6 plugin hybrid.

This new hybrid system helps the Volt launch to 30 mph like a Tesla after stops at traffic signals and helps the mid-size Malibu sedan get a stunningly good 47 mpg EPA estimate in combined city and highway driving while pumping out up to 182 horsepower.

How did GM get to where it is today?

A Brief History of Power-Split Hybrid Engineering

The Lohner-Porsche hybrid car, 1900 .

The Lohner-Porsche hybrid car, 1900 .

Thompson Ramo Wooldridge Inc. (TRW)

Although an early hybrid car was designed and sold back in 1900 by Ferdinand Porsche, the modern hybrid era had its stirrings in the late 1960s at an aerospace, credit reporting, and automotive parts conglomerate named TRW. Researchers there invented the modern idea of a power-split hybrid using a planetary gear set, a gasoline engine, and two electric motors. Its 1971 patent, US 3566717, and the closely related US 3732751 issued in 1973 refer to the smaller motor as the “speeder” since it effectively determined the rpm speed of the gas engine and the larger motor is the “torquer” since it added or removed torque going to the wheels of the car.

This was the era of the Apollo moon flights that used spacecraft computers far less capable than today’s Apple wristwatch. Using electric motors and a battery pack to take the place of fixed gears and clutches was an interesting concept but in order to be efficient and useful such a transmission needs to be carefully controlled. A hybrid transmission control computer was not a realistic possibility at that time so its design was not really practical for use in an ordinary consumer vehicle.

Partnership for a New Generation Vehicle

In 1993, shortly after taking office, the PNGV was created by President Bill Clinton and Vice President Al Gore. The program excluded non-U.S. companies and this particularly worried the Japanese car companies like Toyota.

GM, Ford, and Chrysler collaborated on research under PNGV, but each company built its own high mileage prototype car. GM’s car, the Precept, scored the highest EPA mileage estimate at 80 mpg under the more lenient test cycles of that era. It used a 40 kW (54 horsepower) 1.3-liter RWD diesel engine mounted in the back of the car with a 35 kW (47 horsepower) electric motor driving the front wheels using power from a generator hooked off of the engine. They experimented with both NiMH and lithium polymer battery packs. This was a type of approach known as a “through the road” parallel hybrid since the engine powered the rear wheels and the electric motor powered the front wheels.

During this period, GM continued exploring and prototyping other hybrid designs. With the benefit of computer microprocessors, it focused on developing and extending the TRW idea from two decades earlier. It built an awkward prototype and filed a patent in February 1995 titled “One-mode, input-split, parallel, hybrid transmission”. This patent, US 5558595, described many various permutations of how an engine, two motors, and the wheels could be hooked up to one or two planetary gear sets in order to create an Electrically Variable Transmission (EVT or eCVT).

2001 Toyota Prius.

2001 Toyota Prius.

Toyota and the Prius

Meanwhile, in response to the U.S. government’s PNGV effort, Toyota established an internal project code-named G21 that eventually resulted in the design of the Prius. In January, 1995, a team of Toyota engineers began intensive meetings to research and design the transmission for the new car. According to a book titled “The Prius That Shook the World” by Hideshi Itazaki, they carefully studied up to 80 different known approaches. While these meeting were taking place, GM filed its “one-mode” patent which includes as one of its design alternatives the actual approach that Toyota eventually decided on for the Prius. After narrowing the field down to four alternatives, Toyota chose the basic engineering design for the Prius hybrid transmission at a meeting on June 30, 1995. Toyota did not complete the filing of its own Prius hybrid transmission patent, US 5907191, until 1997 although it initiated the filing process on Sept. 24, 1996 which was the day on which the GM one-mode patent was officially issued. The first Prius went on sale in December, 1997 in Japan. A later, modestly updated, version of that model of Prius was first sold in the United States and Europe in the summer of 2000 as a 2001 model.

Attack of the Clone: Is this the 2017 Prius Plug-in Two-Mode Transmission?

No Prius transmission has ever used clutches but it turns out that Toyota now has a freshly minted “Two-Mode” patent derived from the 3rd generation Prius Hybrid Synergy Drive. The patent, published on January 1, 2015, is US 20150005125.

Is this the hybrid system Toyota’s customers are looking for?

The new design uses 2 (or optionally 3) clutches to support the original Prius EVT mode plus a higher-speed EVT mode and a dual motor EV mode intended for a “vehicle such as a plug-in hybrid vehicle, which is frequently placed in an EV drive mode”. The similarities to the Chevrolet Volt transmission cannot go unnoticed.

Comparing the 2016 Volt transmission with Toyota's new PHEV patent.

Comparing the 2016 Volt transmission with Toyota’s new PHEV patent.

Although the two designs are similar, there are important differences.

Both designs achieve an additive dual motor EV mode by preventing the gas engine from spinning backwards. Toyota does it by engaging a clutch to connect the engine on the first planetary gear set to the planet carrier on the second gear set and then engaging a clutch brake to ground that carrier to the transmission case. GM does this instead with a “one-way” clutch directly on the ring gear of the first planetary gear set in order to ground the engine to the transmission case.

Toyota, like GM, also includes an EVT mode to improve efficiency at higher vehicle speeds. Both designs tie the output of its two planetary gear sets together and to the wheels — Toyota uses the ring gears for that purpose while GM uses the planetary carriers. Both Toyota and GM tie its electric motors to the Sun gears. That leaves the remaining gear on the first planetary gear set for the gas engine. For the remaining gear on the second planetary gear set, both designs include a clutch that can brake that gear to the transmission case and another clutch that can connect that gear to the first planetary set. But to where? Toyota connects it up to the same gear as the gas engine. GM connects it up to the same gear as the smaller electric motor (which it calls MGA).

There are other various details to be pondered. GM’s design gains a single fixed ratio gear but Toyota’s does not. Toyota also says that a one-way clutch might be added in parallel to the clutch brake on the planetary carrier of the second planetary gear set.


The second-generation car, introduced as the 2004 Prius hatchback, used the same basic design but was tweaked with stronger motors, newer battery cells, and an improved gas engine. Toyota later introduced a hybrid version of its small 2006 model year Highlander SUV. The hybrid transmission for the Highlander (and its Lexus twin, the RX 400h) was based on the Prius design but Toyota added an additional planetary gear set after the original power-split gear set. The new gear set acts as a “Motor Speed Reduction Unit” for the larger electric motor. Instead of being directly hooked up at a 1:1 ratio to the output axle it is instead hooked up to the new gear set which acted as a reduction gear allowing the motor to turn approximately 2.5 times for every turn of the output axle. This multiplies the turning force or torque of the motor and allows it to operate more effectively. The same basic design with a slight tweak was also used in the 2006 Camry hybrid and the third generation Prius hatchback when it came out in the 2010 model year.

Ford also began developing a hybrid design based on the TRW ideas and brought out a hybrid version of its Escape small SUV in 2005. Because of the close similarities in the design of the Toyota and Ford hybrids the companies agreed to swap certain hybrid patents.

Paice Back and Forth

Also during the 1990’s, an inventor and engineer named Alex Severinsky developed several hybrid-related designs and patented them beginning with a 1992 filing that was issued in 1994 as US 5343970. Severinsky’s biographical background, where he teaches at the University of Maryland School of Mechanical Engineering, says “Dr. Severinsky is the inventor of the Hyperdrive power-amplified internal combustion engine power train used in the Toyota Prius and other vehicles …”

After the Prius became a commercial success the company Severinsky founded, Paice, filed a patent infringement lawsuit against Toyota in 2004 alleging that the Prius design infringed on this and two other patents.

His patented hybrid design did not use the same kind of TRW-style power-split planetary gear and used one motor rather than two. It used clutches and a type of electrically actuated bevel-geared limited-slip differential the patent refers to as a “controllable torque transfer unit” to allow switching between the use of a single electric motor, a fixed ratio gearing between the gas engine and the wheels, or a combination of the two.

A federal court jury came to the conclusion that Toyota’s Prius design did not literally infringe upon Severinsky’s patents and this was upheld on appeal. However, through a legal concept known as the “doctrine of equivalents”, the jury found Toyota liable for infringing on 2 out of the 40 claims in his first patent.

One claim covered the concept of a hybrid vehicle powertrain with an engine, a single AC motor and inverter, and a battery pack. The remaining claim covered the idea of a battery that “supplies up to about 75 amps”. Elsewhere the patent suggests the pack should be well over 500 volts in a vehicle the size and weight of a Prius and the battery “is capable of delivering between about 30 and about 50 amperes, and possibly up to 75 amperes.” The Prius battery was 201 volts and delivered up to about 100 amperes. Beginning with the 2004 Prius, Toyota’s hybrids contain a DC-DC voltage converter circuit that can step up the battery voltage to above 500 volts for improved efficiency in the electric motors.

Paice attempted to block all imports of certain Toyota hybrid cars in 2009. Toyota (and Ford with its similar hybrid system) eventually agreed on a financial settlement. Toyota capitulated shortly before an import ban on the Prius might have been issued and ultimately licensed 23 of Paice’s patents. At the same time, Ford also reached a settlement with Paice and may have licensed fewer of Paice’s patents or only the single patent that was found to be indirectly infringed in the 2004 lawsuit against Toyota. In 2010, Paice sued Hyundai and Kia after they entered the hybrid car market and they subsequently settled. In 2014, Paice again sued Ford for infringing its hybrid patents. That case is continuing although the U.S. patent office recently ruled on September 28, 2015 that 2 of the patents in contention in that case are invalid because of “prior art”.

GM has never been sued by Paice for infringing on its hybrid patents.

The Two-Mode Hybrid Transmission

A San Francisco bus using the Allison Transmission hybrid system.

A San Francisco bus using the Allison Transmission hybrid system.

After filing its patent on the “one-mode” design in February, 1995, GM went on to innovate and patent a variety of hybrid designs using two, three, and even four planetary gear sets. These designs used typical automatic transmission clutches to allow the power flowing between the gas engine, the electric motors, and the wheels to be hooked together in different ways while the vehicle was driving in order to optimize for changing speed and torque requirements. From many design variations patented over the following years, GM ultimately created its initial products from two of these extended designs.

GM’s first power-split hybrid product was a transmission for transit buses first deployed for testing in 2001 not long after the first Prius arrived in the US. This design, now sold through a GM spinoff company known as Allison Transmission, has been the largest selling hybrid transit bus transmission, and is one of the two major hybrid bus systems sold today. It is sometimes known as the Two Mode EVT design. The two transmissions based on this design are the H 40 EP, and the somewhat heavier-duty H 50 EP which handles up to 330 horsepower from a matching Cummins diesel engine. The design is protected by US 5931757 issued in 1999. It describes 3 planetary gear sets and 2 clutches that allow for two different modes of operation — an EVT mode used at lower (city) speeds and an alternate EVT mode used at higher (highway) speeds for better efficiency — thus the name “two-mode”.

The BMW Active Hybrid X6 using the Two-Mode hybrid transmission.

The BMW Active Hybrid X6 using the Two-Mode hybrid transmission.

Global Hybrid Cooperation

GM later modified that design for use in SUVs and pickup trucks by adding an extra 2 clutches which effectively created four different fixed gears along with the original 2 EVT modes. This design is sometimes known as the AHS-2 design. It used two 60 kW (80 horsepower) electric motors and was matched with engines rated up to 300 kW (402 horsepower) and 358 kW (479 horsepower) combined system output and up to 6,200 pounds of towing capability.

In 2005 GM formed a consortium of companies known as the Global Hybrid Cooperation that included Chrysler, BMW, and Mercedes Benz. Vehicles based on this design were sold between the 2008-2013 model years. These companies eventually cancelled this hybrid version of their vehicles due to slow sales. The vehicles based on this transmission architecture were designed as “power hybrids” that improved vehicle performance while also improving efficiency as opposed to “efficiency hybrids” that included engine downsizing and control software designed to optimize primarily for the highest mpg. Most hybrid customers have focused on maximizing fuel economy and sales of power hybrids from various makers have generally been less successful, even for Toyota and its Lexus brand.

GM’s “New” 2016 Hybrid Transmission Design

For its newest hybrid cars coming out under the 2016 model year we are told that GM’s engineers evaluated many different approaches using sophisticated computer simulations to choose its new hybrid architecture. But however it got there, GM’s “new” hybrid architecture is actually a “Two-Mode” design that was conceived and patented some 15 years ago but never used. GM has now dusted off this old design and put a new shine on it.

2016 Chevrolet Volt.

2016 Chevrolet Volt.

The original patent filing titled “Hybrid Electric Powertrain Including a Two-Mode Electrically Variable Transmission” is US patent US 6478705 and was filed on July 19, 2001 and issued on Nov., 12, 2002. The patent describes its design as providing “a hybrid electric powertrain that exhibits the advantages of the … [transit bus] powertrain, but with reduced mechanical complexity and improved ease of control.”

A newer patent filing titled “Multimode Electrically Variable Transmission Having a Ferrite Magnet Motor and Method of Operating the Same” is US 8602938 and was filed on July 6, 2012 and was issued on December 10, 2013. From an engineering perspective, this patent is largely the same as the earlier one.

GM has seemingly gone out of its way to avoid mentioning any similarity between the new 2016 hybrids and its previous “Two-Mode” hybrid transmission era. For instance, the 12 page SAE technical paper on the 2016 Volt hybrid design never mentions “Two-Mode”. Neither does the 2013 patent filing that largely restates the transmission design of the 2002 patent.

In the language used 15 years ago, Two-Mode has “input-split” and “compound-split” EVT modes optimized for city and highway driving but the newer transmission patent calls these “Low Extended Range” and “High Extended Range”. The newer patent’s “Multimode” name is more precise since the “Two-Mode” designs have always had one or more fixed ratio modes in addition to its EVT modes.

A basic planetary gear set.

A basic planetary gear set.

As illustrated above, a simple planetary gear set consists of 3 geared components meshed together which have a fixed ratio to one another. A so-called Sun gear is in the middle and planetary gears connected to a common carrier plate rotate around it while also meshing with an outer ring with inward facing gear teeth.

In the interest in simplifying things and focusing on the higher level concepts that is the last time you will see actual gears in this article. Instead, we will show groups of three colored boxes representing these gears — a red box containing an “R” for the Ring, a blue box containing a “C” for the planet Carrier, and a green box containing an “S” for the Sun. A vertical group of three boxes represents one planetary gear set like the one pictured above but the ordering does not represent the physical arrangement of the gears.

And now let’s look at some power-split architectures.

GM's 2002 patent (upper) versus its 2013 patent (lower).

GM’s 2002 patent (upper) versus its 2013 patent (lower).

The upper illustration shows the hybrid design taken from the 2002 patent and below it is the equivalent illustration used in the new 2013 patent filing. They use different stylistic conventions and superficially look very different but to a mechanical engineer they illustrate the same basic design showing how to hook up a gas engine, two electric motors, and a final drive output via two planetary gear sets with three clutches.

GM's 2002 hybrid patent versus its 2013 patent.

GM’s 2002 hybrid patent versus its 2013 patent.

By using a simpler diagram style it is readily apparent that the old and new patents describe the same basic hybrid architecture. The newer patent describes the potential use of ferrite (iron) instead of so-called rare earth metals in the permanent magnets of the smaller of the two motors. It also describes specific possible variations in the style of clutch used between the engine and the ring gear of the first planetary gear set. The original patent shows an engine clutch but does not describe its use or implementation. The newer patent shows that the Internal Combustion Engine (ICE) can be “grounded” or locked to the transmission case.

In actual vehicles, the 2016 Chevrolet Malibu does not have an engine clutch and it uses rare earth metals in the smaller motor. The 2016 Volt uses a one-way engine clutch which enables its “dual motor” EV mode that uses both motors together. Without the one-way clutch, the Volt’s smaller motor would spin the gas engine backwards if it were energized during EV driving. Having the clutch allows the smaller motor to push against the locked up ring gear so all of its spin is transferred additively to the second planetary gear set together with its larger motor. The details regarding the RWD hybrid transmission in the 2016 Cadillac CT6 are not yet available.

Toyota Camry hybrid versus the Chevrolet Malibu hybrid.

Toyota Camry hybrid versus the Chevrolet Malibu hybrid.

As the illustration above shows, today’s Toyota, Ford, and GM hybrid designs have converged on the same basic approach. The GM hybrid transmission is shown illustrating the power flow when the clutches are in the Low Extended Range EVT position which is also used for EV driving when the gas engine is off. The Volt’s one-way clutch, used for dual motor EV mode, is not shown. The Ford and Toyota transmissions do not have clutches and are always in this configuration.

Ford and Toyota have also chosen to have the gas engine power come in on the planetary carrier and go out to the wheels on the ring gear. GM swaps that around and chooses to have the gas engine come in on the ring gear and have the output to the wheels go out on the planetary carrier.

For the record, Ford’s newest hybrid transmission and the recently announced fourth generation Toyota hybrid system for the 2016 Prius do not use a second planetary gear set to perform the gear reduction from the larger motor to the output axle. They perform the equivalent large motor gear reduction using a different “parallel” gearing style.

Unlike the Toyota and Ford transmissions, GM uses a multimode design that can also set its clutches into a fixed gear ratio mode and into a High Extended Range mode used mostly to gain improved efficiency at highway speeds.

The first generation 2011-2015 Volt and the 2014 and 2016 Cadillac ELR share a uniquely different design. The ELR and older Volts have a series hybrid mode where there is no mechanical path to the wheels which they use where the new Volt uses its Low Extended Range power-split mode. The ELR and older Volts also have a highway speed power-split mode that is technically different but similar to the new Volt’s High Extended Range. The ELR and older Volts do not have a fixed ratio mode.

Several other car makers like Hyundai, Volkswagen, and previously Honda have used a parallel hybrid approach that uses a single electric motor connected with the engine and then combine that with a conventional transmission. This is easier to design but typically is somewhat less efficient in city driving. It also retains the greater mechanical complexity of traditional transmissions. Honda is now shifting to a unique new hybrid architecture with the Honda Accord hybrid that uses a combination of a series mode under about 43 mph and a fixed ratio gear above that speed when the gas engine is running.

For the full details on how both the first generation and new generation Volt (and Malibu) transmissions work see: Second-Generation Volt Transmission Operation Explained.