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A technical look at parallel axis hybrid transaxles

Monday, January 1, 2018 - 09:00
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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.

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