Hi,
A frustrated automotive engineer once claimed that you had to have a doctorate to understand the Prius transaxle. Learning how it works can lead to a headache but it is understandable. Hopefully, the following charts and data will clear things up.
Power Split Device
The key to understanding is how the planetary gear, power-split device works. The ICE powers the planetary gear carrier and the planetary gears are in a computer controlled dance between MG1, the sun gear, and MG2, the ring gear.
Normal Power Mode
In this mode, MG1 provides the counter torque to the engine, always 28%. This counter torque generate power that electrically passes to MG2. The remaining 72% of torque passes to the ring gear and via the chain drive to the reduction gears and differential.
In order to make the power schematic easier to understand, I have shown MG2 connected to the chain drive pickup. In fact, it is connected to the ring gear in an very compact arrangement. However, this does not change the basic power flow.
Energy Re-Circulation ("heretical" or "overdrive")
Unfortunately, normal mode leads to higher engine power settings and speeds. To put the engine in a lower-speed, highly efficient region, the transaxle can use MG2 as a generator and pass power back to MG1. MG1 then combines this with the ICE power and it 're-circulates' via the chain back towards the wheels.
When some of the earliest Prius owners noticed this it was called "heretical" mode. Toyota engineers published a paper and called it "energy re-circulation" mode. Regardless, it provides a very efficient operating mode.
What happens is MG1 uses some power from MG2 to 'lug' the Prius engine down. The engine opens the throttle plate larger and larger to meet the energy load but MG1 forces the ICE to run slower and slower BUT not so slow as to stall it out. It is a type of 'load managed, engine power control.' The effect is to eliminate pumping losses caused by a partially closed throttle plate.
Transition Between Normal and Re-Circulate Mode
The following data was captured from a Graham miniscanner and shows the transition from 'normal' mode, used for higher power operations. For example, acceleration and hill climbing are high-power, normal modes of operation. But once the target speed is achieved, the car transitions to "energy re-circulate mode."
Optimum System Efficiency
As the Toyota engineers pointed out, optimum system efficiency requires balance and tradeoffs. Sometimes one part has to operate in what appears to be a less efficient mode so another part can perform at peak efficiency. It is the total system performance that is important.
Example of Regeneration
One of the better ways to see regeneration in action is to descend a hill and put the transaxle in "B". Upon entry to the hill at 65
mph, the transaxle was put in "B". The battery was at a nominal 60% charge and quickly reached 80%. Thereafter, no further changing could occur and the rest of the braking was from spinning the engine.
Climbing The Hill
To drain the battery from a nominal 60% down to 40%, drive up a hill at high speed. In this case, there was a small 'leveling' when the vehicle attempted to transit to "re-circulate" mode. Then the hill climb continued.
Hopefully, this makes things a little clearer.
About Efficiency
Dept. of Energy measurements indicate MG1 and MG2 typically have 95% efficiency as either a generator or motor. Switching power supplies typically achieve better than 90% efficiency. So though there are electrical losses, they are fairly modest, especially considering only a fraction of the power passes through this path.
As for mechanical losses, this remains an area of investigation. Some claim that each gear stage is only 95% efficient. This is not something I have direct knowledge of so I'll leave it unchallenged.
Bob Wilson
Introduction to Prius Power
Re: Introduction to Prius Power
Re: Introduction to Prius Power
