2013 Prius Flat Towing

Do it! The manual says don’t tow, jump start other vehicles, use cruise control at low speeds and stay neutral for a long time. And everybody does it, the manual is just another mans suggestion.

 

Now you're admitting the traction battery (which has nominal voltage of 201.6V, NOT 220V) "provides 12v DC power to run all the vehicles systems in ready mode," which is completely contrary to your earlier denial that that's the case. The traction battery is not disconnected in "N," as you claimed. The motor-generators are, which means they won't change mechanical power to electrical or vice versa.

As long as you believe "if the car is in neutral the battery circuit will be disconnected entirely," try leaving your car in "N" for a few hours and watch what happens to the traction battery state of charge. It won't matter whether the wheels are turning or not.

 

I agree with many of your posts, but you've got a few inaccuracies there.
In "N," there's no "torque input on the wheels," other than friction, windage, etc.
In "N" with the engine stopped, the speed of MG1 is very predictable, based on numbers of teeth on relevant gears and simple arithmetic. At 38 mph and zero engine RPM, MG1 spins "backward" at 4477 RPM (assuming tires that rotate 832 times per mile)---a long way from 10krpm.
The engine doesn't choose when "to be deadhead-dragged up to a higher RPM" by the wheels. Programming in a computer makes that decision (for reasons ChapmanF kindly explained). With the engine "dragged" at 1000 RPM (typical in that mode, coasting downhill in "D"), a forward speed of about 69 mph would result in the same -4477 RPM MG1 speed.

Hypothetical question: What bad things would happen if he flat tows in "D," besides the wasted extra gas you mentioned, and the traction battery getting charged to its highest level allowed by programming?

 

To be clear, the 12v battery turns the relays and computers on, which turns the hybrid battery pack on, which is a high voltage system that ranges from 201v to 240v. DC.. (You can nominal it all you want even if that's irrelevent). Point is this pack sends a charge to the 12v, which runs all the car's systems.

And I do agree if you shift into "N" the motor generators are disconnected from the system and that doesn't mean the Hybrid pack is shut down, it means its no longer doing high voltage input and output, but it is still sending a charge to the 12v system via the battery. And perhaps others could clarify that the high voltage pack is still available to power HVAC systems, but not clear on the specifics of that (aka: decision tree that allows or prevents that)

 

Good questions! PriusCamper was very confused.

 

So is your objective in your participation to actually help people by explaining how things work, or simply complain/insult people who are attempting to do so? Seems like you get more joy out of hair splitting to prove people wrong than actually teaching and learning for the benefit of everyone?

 

Or more specifically what would happen if you towed with all 4 wheels on the ground with transmission in neutral and vehicle turned off vs. turned on?

Also, why are you the only person on here who thinks someone wants to try to tow a Prius with car in "D." How does that even make sense? Or maybe you're as you say: "very confused" ???

 

First challenge: getting the car to stay in neutral when you turn it off.

Remaining challenges are the same as when it's turned on (except without drain on the battery).

 

That error wasn't hair splitting, it was major.

He did that first, then pointed out major errors later.

 

I wonder if you can get these racks in Prius size?

 

I'm speaking only of the torque created by forcing the Prius' drive wheels to roll while under flat towing circumstances.

I thought about that and deliberately chose to characterize it as unpredictable because the hybrid system is no longer under active control when being flat-towed in READY mode with N selected. I did not want to assume that the non-linear acceleration provided by the tow vehicle would be easy to predict. Your point about the overall hard gear ratio making things easier to predict is a good one, I just didn't feel like that detail was needed in my response to OP.

I never attempted to explain what would happen if the flat tow were attempted in D mode. In D mode, the computer would be able to do everything you and ChapmanF explained, but the case as I wrote it was for what could happen when the tow is attempted in READY with neutral selected- when the computer has no authority to do anything. I'll admit I didn't account for the difference that might result if N mode had been selected while the engine was running, but again- a detail that I judged to be probably not necessary for the OP.

To follow your hypothetical? I would expect a Prius flat-towed in D mode to act like it was descending a hill of infinite length. First it would charge the traction battery, then it would employ more and more engine braking. I can't predict what response would come after that, but I doubt it would be anything positive for the car.

 

The computer promises not to do anything with neutral selected, but it has a couple fingers crossed behind its back. In the Gen 2 New Car Features manual, on page TH-46 under "Shut Down Control", it explains that the ECU can veto your selection of "neutral" in three special circumstances, and picking up speed beyond the safe range is one of them. So it can then decide "neutral" wasn't quite what you meant, at least long enough to crank up the engine and keep the speeds under control. I'm betting later generations do the same.

Again, this is in READY mode.

 

Something I love about Prius-as-a-hobby... there's always another layer to learn. Thank you!

 

I agree about the "hill of infinite [or unnaturally long] length," and the battery charging, but why should it resort to engine braking? Other than the slight engine braking that normally occurs during coasting in "D" above ~45 mph with accelerator released, engine braking only happens, as far as I know, when "B" is selected, or if the brake pedal is pressed while the battery is "full." Those are circumstances in which the computer has reason to believe safety requires some form of braking.

 

I've got a hill near me that I traffic regularly. It is not long enough to bother with B mode, so I always descend it in D mode. The speed limit is 45 and I usually drive close to that. I can usually hold that speed with no pedals pressed. The result is always the same: I get to the bottom of the hill at about 4k RPM, usually a few mph over limit. I let it coast down on the level ground at the bottom and the RPM subsides very quickly.

This leads me to think that the car's natural reaction to a tow in D mode would be to charge up the battery with that slight bit of drag- no driver in the car to press pedals, right? And then after the battery is full, the drag duty gets shifted to the engine. Thinking about it more, the car could probably handle doing that all day long... it just wouldn't be great for the tow vehicle fuel economy.

 

Interesting! That's very different from my experience descending long downhills. Was it doing that before the charge indicator showed "full" (which isn't really), or only after it did?

If that grade near you is only steep enough to maintain about 45 mph with no pedals pressed before the battery is fully charged, then the level of regenerative braking ordinarily associated with 4k RPM would bring speed down fairly rapidly, unless the hill becomes steeper about that time. Perhaps only part of the power to spin your engine that fast is coming from the wheels, and the remainder is coming from the battery. Relatedly, I've read of (but never experienced) Prii engines spinning to burn off excess battery charge even after the car has come to a complete stop after a big downhill. Of course, that puzzles people who don't expect such behavior.

 

There is more ...

A long time ago, coasting without the foot brake on a shallow but long descent through a national park at 30-ish mph while we were sightseeing and watching for elk, the mild D-mode 'engine drag' was initially pure regeneration because the car was moving slow enough for the ICE to safely stop spinning. I left it in D because that was sufficient for the mild grade, and not feathering the gas pedal to eliminate the synthetic drag, because a bit was actually needed.

A ways down, I was startled when the ICE suddenly spun up. It turned out that the earlier very light regeneration had filled up the traction battery, so it needed to spin the ICE for true engine braking. It was still low RPM, as not much braking was needed, but higher than idle RPM.

Since then, I have infrequently experienced this in D-mode when the battery filled from just coasting regeneration, both at sub-45 mph when the ICE was initially at 0 RPM, and above 45 mph when the ICE was initially at idle speed.

Though the vast majority of the time I fill the battery, it is with B-mode.

 

To be honest I haven't paid much attention to that detail. 99% of the time we have our screen configured to show the "simple" data display, which is just time of day, fuel level and outside air temp.

While I like data in general, I'm not a huge fan of watching it while I'm driving.

EDIT: adding detail

When I have used the detailed screens on the dash, I've noticed that the battery stays close to full most of the time unless we are using heavy air conditioning at low traffic speeds. So driving 40-50mph through hilly Pennsylvania secondary and tertiary roads in winter seems to result in a boring SOC chart for me anyway.

 

I don't know any way to configure my 2011 3rd-generation display NOT to show battery charge level. That ultra-low resolution bar-graph display shows 6 of its 8 segments most of the time. Too much braking within a short period, or getting caught in congestion with a cold engine will force it up to 7 or 8 (i.e., "full").
My ScanGauge supposedly can be configured to show battery charge with greater resolution, but I haven't bothered.

 

I did a test run down a hill last week at similar speeds and engine RPM, with the details in this post about halfway down (the part responding to @fuzzy1).

My hill was steeper, so simply no-pedals-pressed in D would not be enough to hold speed, but what was almost enough was no-pedals-pressed in D with cruise control set for a lower speed. I often use that combination in areas with lots of alternating up and down hills; I can leave cruise set at a minimal speed and just go faster with my foot when appropriate, and get a relatively strong "one-pedal driving" brake effect when I back off. B mode, I would use on a longer sustained descent, to go more gently on the battery.

So the brake effect in D, no pedals pressed, cruise targeting a lower speed, started off as around 13 kW of pure regen. When the state of charge reached 77.2% (actual, i.e., the OBD-II PID, not the dash display), the car began some low-rpm engine braking, and did a gradual shift of the balance between regen and engine until exactly 80.0% SoC, when it was no regen and all engine. At that point, it was doing about 12.6 kW of braking, with the engine turning 3936 RPM and road speed of 36 MPH. The power was delivered to the engine as about 3.3 kW on the transmission's mechanical path and 9.3 kW on the electrical.

The road speed of 36 MPH and engine RPM of 3936 made for a CVT effective ratio just then about the same as 2nd gear in my last stick-shift car, which is probably about the gear I'd have picked on that hill, so the engine braking would have sounded and felt about the same.

As far as I know, the slight resistance you get by just having no foot on the go pedal, with the cruise disengaged, is just a smaller number programmed into the firmware, but otherwise not that different from the cruise actively seeking a lower speed. So I would expect the same behavior where the light regen starts to go to engine around 77.2% SoC and is completely engine by 80.0%.

Another interesting detail from that test run is that the power management control ECU does have PIDs for the amount of "Regenerative Brake Torq" being requested and applied, but they stay zero when the braking (whether regen or engine) is being applied internally by that ECU (as by the cruise control, which is implemented there). Apparently those PIDs only show values when the request was received from the brake ECU.