mort
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steep hill vehicle stall

Tue Nov 12, 2019 5:04 pm

Just experienced a new quirk.
Yesterday my neighbor ran out of gas in his smallish KIA on our steep driveway and slid backwards int the ditch.
I told him no problem and pulled out my towing straps and maneuvered my 2018 PHEV into position for what I figured would be a piece of cake tow. Because his car was facing up the hill and our house is located up hill from his car it would be necessary to tow him up the hill until he could back down into his driveway.

I put the PHEV into 4 lock and began to take up the slack. At first his car popped strait out of the ditch but we needed to continue about 30 feet up the steep driveway in order to give him room to back into his driveway. As we continued the PHEV slowly came to a stop, I depressed the throttle further and the ICE came on, still the cars came to a stop. I continued pushing the throttle to the floor and we stayed motionless, despite the PHEV KW meter being in the low teens.

I was only able to move his car up 30 feet by slowly backing down a meter or so putting the PHEV in Fwd and building a little speed each time achieving about 5 feet.
I never witnessed the KW meter move up at all. I preformed this with and without traction control. I never experienced any wheel slippage. The driveway is hard pack mixture of gravel and dirt and at about a 18 degree angle or a 32% grade.
I have found similar but different issues other posters have witnessed while towing a trailer.
Is this a known issue? Is there a procedure I did not do? Is the car really torque stalled and the current meter not accurate? Or is this simply a function of the motor controller not delivering full power at low speeds?


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jaapv
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Re: steep hill vehicle stall

Tue Nov 12, 2019 5:18 pm

You should have disabled traction control. If a wheel spins, the car will apply a bit of brake to stop it. It will do this in succession until all wheels are locked. (This is in the manual) Having said that, pulling a car out of a ditch up a steep incline probably calls for far more power than the electric motors can deliver. There may well be some safety override that prevents them burning out.
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mort
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Re: steep hill vehicle stall

Tue Nov 12, 2019 8:42 pm

Traction control was turned on at first and off on subsequent try’s, no different.
It felt as if the power was being limited. 120 KW=160 HP should be no problem with instant torque at 0 RPM

greendwarf
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Re: steep hill vehicle stall

Wed Nov 13, 2019 2:42 am

Isn't this the same problem identified years ago that the car can't drive over a kerb if stopped - it needs a run at it. Presumably because there is no actual physical connection in an electric motor to deliver the power. If there is insufficient kinetic energy delivered by the initial twist of the armature to provide motion then it just stops and the input energy turns to heat "burning out" the motor unless there is a thermal cut-out. :idea:

As the OP found out, you need to take a run at it in these sort of situations.

ThudnBlundr
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Re: steep hill vehicle stall

Wed Nov 13, 2019 4:52 am

The PHEV has no FWD option at low speed - it's always 4WD. The 4WD button merely attempts to mimic a diff-lock.

I'm not sure that this is the same problem as the kerb issue. In that case, I think that the torque of the motors is limited electronically while the PHEV is stationary to prevent damage to the motors, so it cannot climb a kerb when touching it, but can easily do it with a small run-up. In this case, the PHEV was moving, so that particular torque limit should not have applied.

However I have no idea why this particular problem should have occurred. I guess it's some internal limit in the S-AWC or motor control that is being breached, but I've no idea what or why. Ours has taken us up some very steep roads (25%) with 4 passengers and luggage, and it showed no signs of lacking torque or power, but that's carrying way less than ½ ton. As the OP said, there have been reports of similar behaviour while towing heavier weights. Though the power/torque needed for a fully-loaded car on a steep hill should be similar to towing a 1-ton car up a slight incline.
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jaapv
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Re: steep hill vehicle stall

Wed Nov 13, 2019 5:01 am

mort wrote:Traction control was turned on at first and off on subsequent try’s, no different.
It felt as if the power was being limited. 120 KW=160 HP should be no problem with instant torque at 0 RPM
I would say that 160 HP (although you should be looking at torque, not power ;) ) should be enough to keep it rolling, but probably not enough to start from zero. Don't forget that the PHEV is 1900 kg to start with, the vehicle towed, what? 1300?, add the incline -32% is steep- and you are far beyond the 1500 kg (European) towed weight limit. Somebody might do the maths.
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Tai626
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Re: steep hill vehicle stall

Wed Nov 13, 2019 9:06 am

Maybe towing backwards would work in this case. There is an engine weight as the ballast.
In the US, this PHEV is rated to tow only1500lb (700kg) probably to avoid the liability to be used to tow any car.
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JohanvdH
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Re: steep hill vehicle stall

Wed Nov 13, 2019 11:10 am

See https://youtu.be/bfga7bHUniA
Sometimes the car stalls without any slipping wheels, just like the topicstarter describes.
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mort
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Re: steep hill vehicle stall

Wed Nov 13, 2019 9:57 pm

At 1:45 this is exactly how the car behaved! Also noted the engine does not even race, and you get the inverter whine.

greendwarf
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Re: steep hill vehicle stall

Wed Nov 13, 2019 11:47 pm

Yep, looks like the kerb test - once stopped against the ridge of packed sand the electric motors lack momentum and refuse to turn. The protective cut-out will stop drawing energy so, of course, the ICE doesn't "race".

If you are towing up a driveway, you would have been traveling very slowly, so coming to a stop (even momentarily) without realising is possible. But also there may be a point where the counter forces of gravity & extra mass overcome the low kinetic energy in the slow turning armature and stop the motor. :idea:

This is beyond my A-level physics! Why can't you find an electrical engineer when you want one. :lol:

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