Behavior of the PSD in "neutral"

Hi PriusChatters,

I've been lurking here since I found out how the Power Split Device transaxle in the Prius works. It's pretty cool, fascinating technology with few moving parts. Personally, I don't drive a Prius since I'm clinging to my love of the manual transmission, but from an engineering standpoint the PSD in the Prius is very much of interest. My dad recently purchased a 2010 Prius and likes it - although he has moved up from a 1989 car and has commented that the Prius is occasionally "confusing" although he certainly seems to like it .

I searched through the archives but haven't found a detailed discussion about what is going on when the PSD is placed into neutral. I know it is not a true neutral since it does not result in disengagement from the wheels because the PSD is always fully engaged. This means if the vehicle is in neutral at n MPH the ICE and MG1 speeds are necessarily related. So what exactly happens when you put the transaxle in neutral? As I understand it, the computer will do anything with the pedal input, and the ICE is cut if it is running.

What happens if the ICE physical, mechanical throttle is stuck open? ie, gummy stuck throttle body. In the worst case scenario the ICE throttle is wide open under heavy acceleration at around 60 mph and it sticks there. This would put the ICE near the top of its RPM range, according to the simulator at aehart (sorry, apparently I can't put the whole link in since this is my first posting here...). So what if neutral is then engaged, and the vehicle slowed? What is going on inside the PSD?

Based on the simulator, it appears that this situation would result in a severe overspeed of the MG2's 10,000 RPM limit - nearly 50% overspeed as the car finally reaches 0 MPH or until the butterfly valve becomes unstuck. My questions are, how is power and torque flowing during this scenario? How does this scenario (simulated "neutral" with engagement) differ from the case where the vehicle remains engaged in Drive and the vehicle is stopped using the brakes alone? In "neutral" is the hybrid ECU running MG1 somehow?

I suppose that by running MG1 as a generator the hybrid ECU might be able to reduce the ICE speed by loading it down. But eventually, the battery pack will become full and then where can it dump the excess energy?

I understand that in the scenario where the pedal itself is sending an "uncommanded" signal to accelerate (due to electrical fault, floor mats, or driver idiocy) the ECU can will stop the ICE. But what if the ICE cannot be controlled properly due to a fault?

Thanks!

 

So in that case, the MG1 will overspeed (15,000 rpm) but while producing no power?

 

OK - So, if you shift to neutral above ~40 mph the hybrid ECU will apply power to the MG1 in order to spin the ICE and prevent MG1 from backwards overspeed?

The situation I was envisioning had the ICE under power (ie, ICE runaway from mechanical cause) and at high RPM (near ICE redline). In this case, the MG1 heads towards positive overspeed regime as the vehicle slow down with the ICE speed constant. Would this lead to a MG1 overspeeding? Maybe the ECU just cuts spark?

 

It would seem that if the vehicle was braked to a stop during a "runaway" episode as opposed to selecting "N" then this overspeed could occur, since the ECU would not cut spark to the engine as it does when selecting neutral. This would indicate that selecting "N" in the unlikely event of a runaway is probably better for the transaxle/MG1 health since it is less likely to lead to an MG1 overspeed.