Prius' MG's and neutral
 

So here's a question for the techies. As I understand it, one of the MG's in the Prius transaxle stays connected to the wheels while in drive, and if you go over a certain speed without the ICE running... BOOM.

The question is, and specific to the 3rd generation: Does shifting into neutral disconnect the MG's, allowing freewheeling at higher speeds, or no?

Actually, Toyota claimed MG1 would over-rev and many assumed this meant a catastrophic failure. However, Hobbit with the NHW20 and I with the NHW11 have done the 'forbidden' experiment: coasting down hill above 42 mph with the engine off, and survived. Our consensus opinion is there is more of a risk to the inverter because the voltage generated by MG1 at higher rpms might exceed the electrical characteristics of the power IBGs.

As background, MG1 handles the variable part of the CVT transmission and spins at speeds defined by the engine RPM and wheels. MG2 is the larger, traction motor generator and is always driven by the wheels. Short of a Pike's Peak descent, there is no real risk of over-speeding MG2 but MG1 could generate such high voltages that 'the smoke escapes.'

With the engine running, the vehicle in neutral, and picking up speed on a downgrade, I've seen the engine RPM increase as the vehicle speed increases above 42 mph. The car control computers adjust the engine RPM to make sure MG1 does not over-speed. In neutral, the inverter power functions are disabled.

In the NHW11 and NHW20, the critical threshold speed was 42 mph (no I don't know if they were fans of Douglas Adams.) In the ZVW30 it is 46 mph but other than internal gearing and higher voltages, it is the same architecture:

• NHW11 - runs 272 V. pack without voltage doubling
• NHW20 - runs ~200 V. pack but uses voltage doubling
• ZVW30 - runs ~200 V. pack and uses voltage boosting higher than NHW20

What happened is the higher voltages allows MG1 and MG2 to spin faster and use less current. This reduces the amount of copper needed and size of the transaxle housing MG1 and MG2 . . . a very clever solution.

Bob Wilson

Just got a 2005 Prius, a few days ago, what is the verdict on using the B lever in city driving?

As I don't have a Prius, I'm unfamiliar with the term... What's the B lever?

Were voltage measurements taken to see if the inverter was receiving higher than standard voltages?

This has me very curious now.

Also, is that a definite no on the MG1 physically disconnecting when the transaxle is shifted to neutral?

That is a definite no. Everything is always connected.

Hi,

A couple of quick notes:

• Dept. of Energy, UT Battelle - there are a number of Prius studies done in 2002-2006 that cover key Prius systems and the transaxle. Google 'em up and you'll find the charts showing Voltage as a function of RPM.
• "B" - uses the engine as a brake when descending hills. This is a good thing because charging the traction battery, fully charging the traction battery, heats it up ... a bad thing. Use "B" for long and steep grades.

Bob Wilson

That's helpful to know. I didn't know the Prius had that mode.

Just a little more detail...

Since the armatures of MG1 and MG2 hold the PM magnets and no copper wires like a regular motor it's not the centrifugal forces that are the main worry. Who knows what the maximum rpm is. I've had my NHW11 up to 60mph with no problem.

What I don't understand is that if it was so bad to leave the engine in neutral ICE off over 42 then why didn't Toyota explicitly warn against it or install a warning/error/failsafe?

Where does the prohibition come from?

never had an issue but could be