Towing a Prius with 4 wheels down ..... question

We're not talking about scratching the paint. "Not recommended" here means, "if you do this you can destroy the car". Somewhere above 42 MPH the transmission will spin too fast; the damage will be mechanical, not electrical. When that happens your RV will suddenly be trying to pull a 3,000 pound brick. Tow it with the front wheels on a dolly, or drive it.

 

The other problem is that the batteries will drain while you're towing it ( it wont generate any power in "N"). If the car is in "ready" mode then the 12V will stay charged from the HV battery via the DC-DC converter, but it too will drain dangerously low if left like this for hours on end.

Regarding the speed. It's not recommended but people sometimes do allow their prius to coast downhill in "N", so I guess this would have the same impact on the transmission as towing. Theoretically you shouldn't exceed 42 MPH . The reason for this is that if the engine is not turning than the motor generator (MG1) spins relatively fast. At 42 MPH for example the MG1 speed reaches approx 6500 RPM which is the "soft" limit that the prius normally maintains. Some people claim to coast in "N" at higher speeds but it's obviously at their own risk.

So in summary, if you want to tow with all four wheels on the ground then it had better be at low speed (preferably <42 MPH) and for a short time duration.

 

I believe it would be ok to tow if the car was made ready and put in "D" the car would regen until battery is full the ICE would start and stop when required and the car would use very little fuel. I agree however the best way would be with the wheels off the road.

 

I was thinking that, but it would be kind of inefficient, like towing with the brakes dragging or something.

 

I agree but the Prius glides very easily and providing the heating or aircon are turned off the engine would not be required to make heat, it would probably spin some of the time without fuel injection.
Reversing might prove to be the difficult problem however, but on a steep hill my car will roll back even in drive and this does not seem to upset the ECU's so taking it steady in reverse should be ok.

 

Wouldn't pulling the Prius with wheels on the ground be the same as the car going down a hill? You have your foot off of the accelerator but the car becomes an overhauling load. The car will regen pretty hard depending on the slope of the hill.

The faster you tried to pull the car the more it will regen until battery is full and then it will fire the ICE and become an air pump. Like uart said it would be like pulling with your foot on the brakes.

Maybe if someone sat in the car and feathered the pedal to keep it out of regen then it will glide easily. But then as said you would have to be careful not to overspeed MG1.

 

Providing the car is in "D" MG1 will not over speed the ICE will run or be turned by MG1 in the same way it does if you go down a long steep hill.

 

But wouldn't it be like pulling with the brakes on?

 

Below 42mph the Prius will probably be assisting slightly. With a full battery and warm ICE the ECU's will try to dump energy but when the car is regening and spinning the ICE the valves are held open and this does not slow the car very much. I go down a long steep hill every day and shortly after I get all green bars on the display I feel the car surge slightly as the regen stops, and the car rolls more freely. When I reach the bottom of the hill there is a stop sign, and when I stop the car is always using the disc brakes "no regen" and the ICE is spinning without injecting fuel.

 

This is a case where I'd definitely stick with Toyota's recommendation, and just don't do it.

But hypothetically...

Towing in Neutral would be bad, I'm pretty sure you're right, the ICE might never spin, and MG1 could over-rev. The battery might not be kept charged either (we know it won't charge in Neutral while stopped; I'd guess this also applies while rolling).

Towing in Drive has its problems too. I think the car would react just as it does if you coast, no foot on either pedal. In this case, the ICE will spin at higher speeds to protect MG1. Whenever coasting, the car does regen braking very lightly, simulating the drag of a conventional engine and automatic transmission (but only lightly.. the car still picks up speed on downhills). This may lead to a full SOC on the battery, and the braking energy dissipated in some other way. Another bad thing happens in Drive, though, the car "creeps", also simulating the tendency of automatic transmissions to do this. So, while stopped, the car will be continually trying to push you along. I don't think you'd want this

 

If you really need to tow wheels down, you need to install some sort of locking front hubs, like the free running hubs used on 4WD trucks. That way the front wheels could spin without spinning the driveline components.

Tom

 

Nahh, on second thought, in the holy names of empiricism and engineering science he should save the tens of dollars it would cost to rent a dolly, tow the Prius on all fours, and let us know what happens. With pictures. Empiricism Rocks!

 

Towing in "ready" and "D" would be ok. It would be the same as constantly going downhill. Once the battery is "fully" charged (about 80%) there would be very little drag. The engine would be spun by the MGs to dump the excess charge but little or no fuel would be used. You would have to use the type of tow device that forces the steering to match the tow vehicle ("A" frame tow bar).

That being said, -I- wouldn't want to do this.
Toyota says don't. That's one reason (you have to believe their engineers know -something- about the car).
The car would be on and doing things behind you. That would make me uncomfortable. That's another reason.

Now, I have descended very long mountain pass' in ready and "D" or "B" without incident (perhaps 10-20 miles), so it would -probably- be just fine.

It would certainly be an interesting experiment.