What Exactly does the Added Clutch Do?

Advancement of Toyota Hybrid System II (THS-II) | TOYOTA Global Newsroom talks about it.

Looks like it's on the engine, so that MG1 isn't turning the engine backwards when it's helping out.

 

Seems like the term Sprag clutch is a misnomer; if the Prius uses clutches like that shown in the article. Sprag clutches use bearings; not ratcheting pawls like a bicycle freewheel.

 

However, what'll happen is that MG1 will apply torque to try to accelerate itself to higher negative RPMs, which would turn the engine in reverse. The one-way clutch prevents that, so MG1's torque instead spins the ring gear faster.

Play with Toyota Prius - Power Split Device some... it doesn't show torques, only RPMs, but realize that MG1 increasing torque to increase vehicle speed would be in the direction of more negative RPMs.

 

It's a sprag clutch.

"A sprag clutch is a one-way freewheel clutch."

 

It’s a sprag clutch used in industrial power transmission for decades. Highly reliable.

 

Can't believe we went four years without clearing that up.

 

The press release says it's for 2.5L engines, which means it's not for the Prime?

 

From my new-car features manual, the one-way clutch is connected to the planet-gear carrier, the latter of which is also connected to the ICE. MG1 is connected to the sun gear, and when the ICE is on, it turns in the same direction as the ICE, which charges the HV battery as well as providing power to MG2. When the ICE is off, MG1 functions as a motor and turns in the opposite direction as the ICE but with minimal torque to only keep the ICE from spinning, not really providing power to the wheels. However, in the EV mode under high acceleration, MG1 turns in the opposite direction with high torque and functions as a motor supplementing the larger MG2. The one-way clutch locks to the transmission case when that happens and applies a stationary torque on the planet-gear carrier, which holds the ICE stationary and prevents it from turning in the reverse direction, and it allows MG1 in this way to serve as a motor instead of a generator. In summary, the one-way clutch is free from the transmission case to rotate freely when MG1 is working as a generator, and it is locked into the transmission case and applies a stationary torque on the ICE and planet-gear carrier to keep them from rotating when MG1 is working as a motor.

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