2013 Prius Flat Towing

Things I've learned from this thread, and bits that stood out as worth repeating.

• It should be known that the O.P. was hoping to use a tried-and-true strategy for flat-towing cars behind RVs. Many imported FWD cars of a certain vintage can be towed engine-on with the automatic transmission in N to keep the transmission lubricated. A more modern solution is to install an electrically driven ATF pump to circulate oil in the towed car using electricity supplied by the RV.
• Those solutions don't apply to any Prius because the fundamental problem with flat towing a Prius is not lack of lubrication- it is the lack of a true mechanical disconnect between the engine and the wheels that disqualifies the Prius family from flat towing.
• There actually is an engine-driven ATF pump in the Prius transaxle. I'm going with the idea that it only feeds the planet carrier bearings since they deal with high mechanical loading and wouldn't need lube when still.
• Neutral mode in the Prius is not only faked electrically, it is apparently also shy and may revert to D if the computer feels like it.

 

I'm not sure that it reverts to D in any sustained sense, only that if you cross the speed threshold where the engine oughtta be spinning, it'll go back on its "I won't do anything electrical" promise just long enough to give the engine a kick, and then look all innocent and say it was in neutral the whole time.

Same deal if ABS detects a wheel slipping.

And if you select N while braking regeneratively, it will say "yes, sir, I'll be in neutral as soon as I finish transitioning to friction brakes here."

If it weren't for those exceptions to the rules, I wouldn't necessarily say the neutral is "faked electrically", unless I would also say another transmission fakes neutral by moving gears out of mesh. Either way achieves "turning this shaft doesn't turn that shaft", which is pretty much neutral, all right.

But having it be neutral-with-possible-momentary-exceptions, that is kind of weird.

As is the sort of unavoidable production of volts by permanent magnets.

I just got done watching the P610 (Gen 4) deep dive by Prof. Kelly, and the engine-driven oil pump there is responsible for circulating the oil through the external cooler (Gen 4's inverter coolant doesn't pass through channels in the transaxle anymore, only through a little exchanger on the outside with the hot oil being pumped through). The first place the pumped oil goes after cooling is over the MG1 and MG2 windings.

In the Gen 4 Prime, they add an electric oil pump that can do that when the engine isn't running. In the non-Prime, I guess the firmware just has one more reason it might start the engine when you're not expecting it, based on those winding temperatures.

 

From which we can infer that your engine's friction torque was about 30.6 Nm, or about 23% of its output at that speed.

 

Well, in that other thread where more of the details were, there had also been a long discussion of exactly where most of the work goes in engine braking. We can infer that the total resulting torque was about 30.6 Nm, but just how much of that's due to engine friction, how much to pumping air across a closed throttle, how much to pumping air into the exhaust system, how much to compression (but it bounces back!) and so on, doesn't seem to be easily agreed on.

 

Why would winding temperatures rise high enough to require cooling when the car is coasting or being towed in "N" and winding current should be near zero?

 

True, but in my old I.C. Engines textbook, all those losses are rolled together under the FHP (friction horsepower) term for simplicity. Thus, BHP = IHP-FHP.

 

In that comment, I was just thinking aloud about how they might have needed to add that condition in the firmware when they built a tranny with no cooling except an external pumped oil cooler and the non-Prime version has no independent oil pump.

I wasn't necessarily tying it to this thread and whether the condition would be triggered in flat towing.