High'ish engine-off EV speeds in N.

This is a confused thread. I'm hoping I don't confuse it further...

MG1 is the smaller motor directly connected to the Sun gear on the power-split planetary gear set. The ICE is directly connected to the planetary carrier and the eventual output to the wheels happens on the ring gear.

That ring gear is then connected to the second planetary gear set that acts as the "motor speed reduction unit" for MG2 (the larger motor) in gen 3 cars. MG2 is directly connected to the Sun gear of this gear set and the planetary carrier is physically locked to the transaxle case so it can never turn. The ring gear of this motor speed reduction unit planetary gear set is also connected to the wheels (via a differential etc).

MG2 has a fixed reduction of about 2.6 turns for every turn of the ring gear. The ring gears of the power split planetary gear set and the ring gear of the motor speed gear reduction unit planetary gear set and the wheels are all connected together and so their rpms are always tied to the vehicle speed. Since MG2 is just at a 2.6:1 fixed ratio to its ring gear, it's rpm is also tied to the vehicle speed.

Here's an illustration of how things are hooked together on gen 2 (2004 Prius) versus gen 3 (2010 Prius):


When the ICE is turned off, its reluctance to spin due to friction and the resistance of compressing the air in its cylinders effectively locks it to the transmission case like the planetary carrier on the motor gear reduction unit planetary gear set. Thus, the rpm of MG1 on the power split Sun gear is effectively tied to the vehicle speed when the ICE is off and not spinning.

The gen 3 2010 Prius 13,500 max rpm of MG2 is designed to allow the car to travel at its max speed of about 112 mph. With the ICE not spinning, MG1 reaches its max rpm of 9,800 when the car is in the neighborhood of 83 mph.

In that other thread, pEEf notes that Toyota limited the max EV speed of the gen 2 Prius to about 42 mph but would allow it to go up to about 52 mph downhill in 'N' which is when it is actually getting close to the 6,500 MG1 rpm limit. A similar story presumably applies to the gen 3.

Let's apply the same ratio of max MG1 rpm to the normal ready mode 'D' ICE startup speed. The max rpm of MG1 in the gen 2 Prius was only 6,500 rpm which is why its max speed with the ICE off was 42 mph -- (6500 / 9800) * 63 mph = 42 mph.

So, pEEf was saying on that other thread that in gen 2 in 'N' they make MG1 electrically active and cause it to resist spinning above its max rpm of 6,500. Presumably gen 3 and the PiP do the same thing to avoid MG1 from spinning above its now higher gen 3 max of 9,800 rpm. When MG1 resists spinning and the car travels faster then it causes the ICE to spin instead. They can change the valve settings to minimize compression of air by the pistons in the cylinders.

As an aside, the 2016 Volt transaxle (which has power-split and motor speed reduction planetary gear sets similar to the 2010 Prius) is in an equivalent planetary arrangement to the gen 3 and PiP when the Volt is driving in EV mode and/or it's city speed eCVT mode. It has slightly different gear ratios so the Volt's MG-A (what they call MG1) reaches its max rpm of about 11,000 when the car reaches its max vehicle speed of 98 mph.

 

Do you have a reference to the reduction gear for MG2 being 2.6?
It make sense (using same sun gear and planets both sides) but i don't recall a reference to it.

 

The tooth counts for the power-split planetary and the motor-reduction-gear planetary are different but the Sun-to-ring ratio works out almost identically. I think the power split is 78:30 for a 2.6 ratio and the reduction count is 58:22 so the ratio is 2.636.

Reference is here:
For those who love to calculate, 2010 gear teeth counts | PriusChat

 

As far as I recall, the ring gear is one and common to both sides (and not two rings rigidly connected), if this is true, then different count of planetary teeth (source?) may lead to different count of suns' teeth and different ratio ring/sun.

 

Here is a US government labs tear down of the 2010 Prius. The motor speed reduction planetary tooth counts are described on page 41 of the document and there are pictures of the gears.

http://info.ornl.gov/sites/publications/files/Pub26762.pdf

 

Thanks for the references.

 

And thus the observation that the MG1 and MG2 rpm with the ICE off is very close to identical in the Gen III. But different mechanical designs lead to a higher rpm limit on the MG2.

(Thanks for all the technical data, Jeff.)

Can one conclude from all of the above that the "don't risk damaging MG1 by using N" is a myth, in that both the older Gen II and the newer Gen III will "protect" the MG1 by spinning the ICE without fuel, although the speed limits in the two generations are different: about 52 mph in the Gen II, and about 83 mph in the Gen III?



merged



Speaking of myths, RIP Mythbusters. Season 14 will be the last.

 

It seems likely but it would be good for someone to validate this by gathering OBD-II data on MG1, MG2 and engine rpm in neutral on a mild downhill stretch where you can gather data while the vehicle speed increases.

 

I'd gladly experiment, but I don't plan on exceeding 83 MPH anytime soon. The hills we have aren't steep or long enough to promote coasting at those speeds.

I suppose I could take the vehicle up to 90 MPH with the engine and then put it into N, but I'm not keen on exceeding the rated limits by such a large margin.

Instead I'll just observe max RPM on whatever coasts in N I do and report back here. I'll also note any instance of the engine kicking on, but I doubt it would ever happen; at least not until I approach 83 MPH.

 

I have never driven my prius in N, I am not even sure it's legal here.
Keeping the charge/discharge in the vicinity of zero in downhill D is not a big deal for me, it is done effortlessly.
The vicinity of zero is good enough for me, the currents in and out are so minimal I doubt I am loosing any momentum.

 

Why is the legality of operating the vehicle in N brought up so frequently? Changing lanes without signalling is illegal, and yet less than half of drivers use the device. If there is a serious safety concern, that would be something to discuss, but there is none.

 

Why do so many people mention the ~62 mph limit of the PiP EV mode when that's only the maximum speed (it's actually 100 kph) at which the PiP will shut off the ICE when switching to EV mode? It will travel up to 64 mph in EV mode before turning on the ICE.

 

People mention 62 MPH because that is the actual speed limit at which the gas engine will turn on. The indicated 64 MPH speed at which it turns on is not the ACTUAL speed. For some reason, Toyota has indicated about 2 MPH faster than actual speed, depending on which tires you are using and the wear level.

You might be able to stay in EV mode at an actual 64 MPH if your tires were over-sized.