A technical look at parallel axis hybrid transaxles

Jan. 1, 2018
A technical look at what’s new and what’s not so new in the latest hybrid transaxles from Chrysler, Toyota, and Ford.

If you are like most people I know, you have not paid much attention to what has been going on with hybrid vehicle powertrains over the years; why would you? Hybrids have been a very small percentage of overall vehicle sales, especially with gasoline prices being “so low.” The trendy vehicle to have today is a truck or an SUV.

While you may have been ignoring hybrids, there has been a quiet battle going on for the most efficient hybrid/plug-in hybrid powertrain design for the last 20 years. The results of this battle (so far) have surprised me, and I want to share with you what has happened. First, we need to define five boring — but necessary to understand — technical terms:

Parallel Hybrids: Parallel hybrids use an internal combustion engine (ICE) and electric motor/generator(s) to simultaneously provide power to the wheels through the vehicle's transmission. In addition to supplementing the power of the engine, the motor/generator can also charge the hybrid battery while the vehicle is in motion. Driving the vehicle with only the motor/generator is also possible. An example is the Honda Integrated Motor Assist (IMA) hybrid system.

Series Hybrids: A series hybrid system uses an electric motor to rotate the wheels through the transmission. The motor can be powered by a hybrid battery and/or an internal combustion engine (ICE) driven generator. The generator is used to provide an electric power source for the motor and/or charge the HV battery. An example is the Chevrolet Volt Plug-in hybrid.

Series-Parallel Hybrids: A Series-Parallel hybrid system has two electric motor/generators. Electrical power can be generated by the first motor/generator using power from the internal combustion engine (ICE). The generated electrical power is used to charge the hybrid battery and also to provide power to the second motor/generator. The second motor/generator directly drives the wheels through the vehicle's transmission (Series operation). The second motor/generator can also receive supplemental motive power from the ICE through the first motor/generator by way of a power-split gear set (parallel operation). Examples are the Toyota, Ford and Chrysler hybrid systems.

Inline Axis: A series parallel hybrid system with two electric motors that share a common axis of rotation. One motor is mounted directly above the other motor. They share a common centerline.

Parallel Axis: A series parallel hybrid system with two electric motors that do not share a common axis of rotation. One motor is mounted to the side (offset) of the other motor. They do not share a common centerline; they each have their own centerline.

Now that we understand the terminology, let’s look at the major contenders in the hybrid powertrain battle.

Contender #1: Honda

This may surprise you, but Honda was first to market a hybrid vehicle, the Insight, in the US for the 2000 model year. The Insight powertrain used an IMA, single electric motor, parallel hybrid system with a 5-speed manual transmission or a belt-driven Continuously Variable Transaxle (CVT). The IMA system used its single electric motor to “assist” the engine’s crankshaft in rotating, for regenerative braking, and it also started the engine after an idle stop event. Honda used the slow-selling IMA system for 17 model years in various vehicles ending with the 2016 CR-Z. I drove a Civic hybrid when I was considering buying a new car, but it was too gutless for my taste. In my opinion, the Honda IMA system could never quite compete with the Toyota hybrid system; it was always rated about 10 mpg lower in fuel economy than the Prius.

The IMA hybrids were plagued with high voltage Ni-MH battery problems as well. They had battery self-discharge problems and too short of a battery life. To make things worse, the 12-volt system is charged from the high voltage system. When the high voltage (HV) battery dies, there is nothing with which to charge the 12-volt battery, so it dies too, and then the engine can die while driving from lack of voltage. This was a huge safety problem for Honda so they extended the warranty on the batteries. The used car market is full of Honda IMA vehicles with dead HV batteries. The cost of the HV battery is higher than the value of the car, so avoid them if possible. As if that was not bad enough, Honda also had problems with failed push-belts in their CVT transaxles that could leave the driver stranded.

The good news for Honda is that they started replacing the IMA system in the 2014 model year with a more efficient two-motor, series-parallel transaxle system called “E-Drive.” The new Honda two-motor hybrid transaxle system is totally unique. No other manufacturer uses this design. It uses one electric motor as a generator and one electric motor as a traction motor, the two motors are not connected together. Unfortunately, the E-Drive system is only available in the hybrid and plug-in hybrid versions of the Honda Accord.

2000-2016 Honda IMA system stator and rotor

Contender #2:  Toyota

Toyota was second to market a hybrid vehicle, the Prius, in the US for the 2001 model year. The Prius used a twin-electric motor, inline axis, series-parallel, hybrid transaxle. This transaxle design has been part of the Toyota Hybrid System (THS), Toyota Hybrid System II (THS-II) and the Toyota Hybrid Synergy Drive (HSD) for the last 17 model years. This inline axis design uses two motors that share a common centerline or axis, Motor-Generator One MG1 and Motor-Generator Two MG2.

Unlike Honda, Toyota’s hybrid transaxles and HV batteries last a long time. I have been involved in replacing three Prius batteries. All of them were more than 10 years old with more than 150,000 miles on them. Of course, the cost of the HV battery is higher than the value of the car, but the Toyota hybrid powertrain can go for many more miles. A Toyota technician told me that a common complaint with Prius owners is that the odometer stops incrementing at 300,000 miles. The only way to fix it is to replace the instrument cluster.

2001-2016 Toyota inline axis style hybrid transaxle (P410)

The THS hybrid transaxle design made Toyota the overall fuel economy and efficiency champion for many years, but then other contenders (Ford, Chevrolet, Kia, Hyundai) started to catch up and even challenge Toyota’s championship status. Toyota was falling behind in hybrid technology and needed to do something in an attempt to establish dominance again.

To my great surprise, Toyota dropped their inline axis transaxle design and began using a parallel axis transaxle design with the fourth generation Prius for the 2016 model year. This new parallel axis transaxle appears to be a highly refined version of the original 2005 Ford Escape parallel axis hybrid transaxle. The new transaxle, called the P610, is 23lbs (10.4 kg) lighter and 2 inches (51mm) narrower than the previous transaxle, the P410, that it replaced.

2016-2018 Toyota parallel axis style hybrid transaxle (P610/P710)

If you have the new 2017 Prius Prime (or Prius Plug-in outside the US), the P610 transaxle can use both electric motors to propel the vehicle at the same time (under certain conditions) in electric vehicle mode. It uses a one-way sprag clutch to prevent the engine crankshaft from spinning backwards when the MG1 motor is helping the MG2 motor propel the vehicle. The one-way sprag is not a new idea, the 2005 Ford Escape Hybrid used the same method of preventing the engine from spinning backwards, but it is unclear if Ford ever used their generator to help drive the vehicle.

2005 Ford Escape hybrid - One way sprag

Contender #3:  Ford

Ford has used four generations of their two motor, parallel axis, series-parallel hybrid transaxle system since the 2005 model year Ford Escape hybrid. They keep refining the hybrid transaxle design, and it keeps getting better and better. The first two generations of Ford hybrid transaxles were made by Aisin AW, models HD-10 and HD-20. There was nothing serviceable on these transaxles, you have to replace the entire unit if there was a failure. Luckily, they were very reliable and many are still on the road today being used in taxis.

The third- and fourth-generation transaxles made by Ford are the models HF35 and HF45. You would have to look closely to detect any major physical difference inside these four generations of transaxles (besides gear ratios). They all operate almost identically internally.

Externally the Ford versions look quite different than the Aisin versions because Ford separated the inverter assembly (Ford calls it the Transmission Control Module (TCM)) from the transaxle and Aisin did not. The top side of the Aisin transaxles looks like the monster from the Alien movies is hiding under your hood.

2005-2012 Ford Aisin AW "Alien Edition" transaxle (HF-10 and HD-20)

The third- and fourth-generation transaxles are used in the 2013 and above Fusion and C-Max hybrid and Energi Plug-in Hybrids. The Ford hybrids are great and get fantastic fuel economy and in my opinion, the Fusion is a beautiful car. The Fusion hybrid was selling almost as good as the Toyota Prius for quite a while last year. Personally, I think Ford needs to do something to get better batteries in their plug-in hybrids if they want to compete with the other offerings out there. They cost as much as competitors that have twice the electric vehicle (EV) range.

2003-2016 Ford parallel axis style hybrid transaxle (HF35/HF45)

Contender #4:  GM

General Motors has produced a variety of hybrid powertrain designs over the last 14 years, but none of them were very impressive, efficient or cost effective until the Chevrolet Volt appeared for the 2011 model year. I actually traded in my 2010 Prius to get a 2012 Volt! The dealership I bought the Volt from had never had anyone trade in a Prius for any Chevrolet before; they did not know what to do at first.

The Volt powertrain, a series hybrid powertrain design (in most driving conditions), was a game changer for plug-in hybrid vehicles. Its transaxle, the 4ET50, used an inline axis, two motor, system with hydraulic clutch packs to connect or disconnect the motors from the final drive and from each other and even the engine crankshaft on rare occasions.

The Volt transaxle is an amazing design, but it is heavy, complex, and expensive. The 2017 Chevrolet Malibu Hybrid uses the second-generation Volt transaxle, the 5ET50, which is lighter and more efficient than the previous design. The Malibu hybrid is rated at 49 mpg city! Too bad nobody wants a car anymore. This would be a great pick.

2014 Chevrolet Volt inline axis transaxle (4ET50)

Contender #5:  FCA

FCA’s 2017 Chrysler Pacifica Plug-in Hybrid Mini Van is new to the battle and Chrysler’s first mass produced hybrid vehicle. Their new two motor, series-parallel, parallel axis transaxle, is called the “Single Input Electronic Variable Transaxle” (Si-EVT). It is new to FCA, but it is hardly a new design. I have to chuckle to myself at times when I am talking to Chrysler trainers and technicians because everything hybrid is new to them and they act like it is new to everyone.

I had one trainer tell me about the unique design that Chrysler had developed for the Pacifica hybrid transaxle. I told him it was a modified copy of a 2005 Ford Escape transaxle and took him back to my storage room and showed him. Obviously, he was a little surprised to see and learn this. He said, “But the engineers told us they designed it in-house.” I chuckled a little more inside. The Pacifica transaxle is old school brute force hybrid technology with a few refinements. The cool thing about the Pacifica plug-in hybrid is that it is currently the only plug-in minivan on the market in the US. There are a few others outside the US.

2017-2018 Chrysler Pacifica parallel axis style hybrid transaxle (Si-EVT)

The design trend

Are you seeing the hybrid transaxle trend now? Parallel axis hybrid transaxles in a series-parallel hybrid vehicle seem to be the configuration of choice for efficiency, space utilization, weight reduction, simplicity, and configurability. Let’s look at what is common to all of three transaxles from Ford, Toyota and Chrysler.

They all have a large traction motor that has only two functions: 1. it simply propels the vehicle with or without the assistance of the engine; and 2. it is used as a generator when decelerating to provide regenerative braking when possible. Toyota calls the large motor “MG2,” Ford calls it the “Motor,” Chrysler calls it “Motor B.”

They all have a small motor/generator that has four functions: 1. it starts the engine; 2. it acts as a generator to charge the high voltage battery; 3. it acts as a generator to provide electrical power to the traction motor; and 4. it, along with a power-split planetary gear set, is used to vary how much torque the engine can contribute to traction motor to help propel the vehicle as you drive. Toyota calls the large motor “MG1,” Ford calls it the “Generator,” Chrysler calls it “Motor A.”

Now for some mild humor

China recently announced that they will require 10 percent of all vehicles sold in China (28 million sold last year) by 2019 be plug-in hybrids (PHEV) or Battery Electric Vehicles (BEV). According to what I have read, PHEV and BEV production last year was around 1 million vehicles. About 2.8 million will be needed in China just one year from now.

The required percentage of these vehicles will go up every year until 100 percent is reached. Not many days after China’s announcement, almost every vehicle manufacturer in the world announced that they will produce and sell an all-electric or plug-in lineup by 2020-2025 (yes, that is the humorous part). The auto market in China is almost double what the market is here in the US. What does this mean for you? Expect to see a lot more PHEVs and BEVs available in the U.S.A in the next few years. I expect to see the powertrain types of most PHEVs and BEVs to converge on one or two really efficient designs. The world’s automotive market is changing again, get ready for it by furthering your electrical and hybrid education.

About the Author

John Kelly | Contributing Editor

John D. Kelly is a professor of automotive technology at Weber State University in Ogden, Utah, and a former technician. He specializes in automatic and manual drivetrain and NVH diagnosis and hybrid and electric vehicle technology.

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