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Discussion Starter · #1 ·
I find it a lot easier and time saving to use the engine as a brake when I change directions. For example when I back out of a spot to get pointed in the right direction, I drop it into drive without using the brakes, and with a little gas the Prius stops, changes direction, and I'm off.

Does anyone else do this, and do you think it's hard on the engine or drive train? I'm talking about maybe three or four miles an hour in reverse before changing direction.

My feeling is that it shouldn't be hard on the car at all, but what do others think?
 

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I do that every morning as I back into the street. I am usually at 0-2 MPH and it seems no different than when I did the same thing using a clutch on my HCH or the tranny on the Venture van.

I certainly HOPE the gearbox is able to withstand this activity...

Now you have me wondering......
 

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Discussion Starter · #3 ·
Using a clutch, it would defintitely cause more wear than if you used the brakes to stop first... although probaly not enough wear to worry about.

Sorry if I caused you to wonder about it for the Prius. I really don't think it would be hard on it at all. It's the kind of question someone might have a difinitive answer to. My guess is that Toyota would not allow the vehicle to go into gear if it was going too fast, and could cause damage.

I have put it into Park while still moving very slowly, (unintentionally), and it is a little jarring. I believe it will not go into Park at a higher speed.

On that score, a friend of mine was in a terrible accident because the Volvo in front of her was inadvertantly put in Park at driving speed, and since no lights came on at the rear, she had no warning the car was going to suddenly stop. So, I guess some cars can go into Park at higher speeds, but I believe the Prius protects against that.
 

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I wouldn't be so sure. With the Prius, it's a bit smarter but it won't shift > 8mph so under that, there's be a hard jerk (depending on how fast you go). I wouldn't make it a habit. My friend used to do that. He'd shift to drive while the car was still moving backwards "to save time". Ended up grinding the gears. His transmission had to be replaced and the car's barely 5 years old.
 

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That is/was called a 'J'; back in the days of 455 CID V-8 motors and 4-speed Hydromatics, we would drive in reverse full-throttle and jam it in to drive....leaving white smoke and rubber marks in the shape of a 'J' on the pavement... :)

Ahh youth, and $0.19 per gallon gasoline... :wink: :wink: :wink:
 

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Discussion Starter · #6 ·
wstander said:
That is/was called a 'J'; back in the days of 455 CID V-8 motors and 4-speed Hydromatics, we would drive in reverse full-throttle and jam it in to drive....leaving white smoke and rubber marks in the shape of a 'J' on the pavement... :)

Ahh youth, and $0.19 per gallon gasoline... :wink: :wink: :wink:
I guess this would be a good way to confuse the Prius. It would have all that torque due to the high-torque electric motor, and would try to grab the pavement and shoot you forward, but then the anti-spin program would kick in, and it would cut the power... so your backward momentum would keep you going backward... maybe the Prius computer would go schizoid, and start behaving like a muscle car... bellowing and spewing white smoke...
 

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When changing direction, I stop completely before shifting. It's much easier on the driveline. Brakes are much cheaper to replace than transmission parts.
 

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Discussion Starter · #8 ·
Technogeek said:
When changing direction, I stop completely before shifting. It's much easier on the driveline. Brakes are much cheaper to replace than transmission parts.
What you say seems to be intuitively obvious. But just because it seems like it is probably true is the same for me as it is for you. I want to know if if really is true.

Driving a car is obviously a lot harder on it than not driving it, but this doesn't mean we should leave our cars in our garages.

So... does anyone know for sure if it really causes undue harm to the drivetrain of the Prius to start accelerating forward from -3 mph than from 0 mph? Actually, I doubt that it does.
 

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There does not appear to be any mechanism in the Prius drivetrain that would be damaged shifting into D while rolling backward at low speed. Shifting into P at any speed greater than 0 will cause substantial stress to the parking pawl, however. So don't do that.
 

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shift to drive while in reverse

Well, I did it backing out of my garage, and the big warning light came on (The '!' in a triangle). And nothing I tried got it to go away until I turned off the car and started again.

Thank goodness I live on a cul-de-sac! Guess I won't be doing that again in a hurry.
 

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Discussion Starter · #11 ·
Re: shift to drive while in reverse

Cattmatt said:
Well, I did it backing out of my garage, and the big warning light came on (The '!' in a triangle). And nothing I tried got it to go away until I turned off the car and started again.

Thank goodness I live on a cul-de-sac! Guess I won't be doing that again in a hurry.
I've never had anything like that... how fast were you going in reverse?
 

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Re: shift to drive while in reverse

Cattmatt said:
Well, I did it backing out of my garage, and the big warning light came on (The '!' in a triangle). And nothing I tried got it to go away until I turned off the car and started again.

Thank goodness I live on a cul-de-sac! Guess I won't be doing that again in a hurry.
Did you happen to have the driver door open? That will produce the triangle, with a misleading message on the MFD to not leave the car while in neutral, due to the risk of discharging the HV batteries.

As for stress, at low speeds I am sure it is negligible, but still more stressful than starting out at 0. Essentially, you are having MG2 switch directions while it is still moving. This is the same as changing the direction of a reversable fan before it has stopped.
 

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Re: shift to drive while in reverse

Canadian Prius said:
Cattmatt said:
Well, I did it backing out of my garage, and the big warning light came on (The '!' in a triangle). And nothing I tried got it to go away until I turned off the car and started again.

Thank goodness I live on a cul-de-sac! Guess I won't be doing that again in a hurry.
I've never had anything like that... how fast were you going in reverse?
Not too fast, but I was rolling a bit. Don't think my door was open, but I do remember seeing that warning about neutral too, as I tried shifting a couple of times before rebooting.
 

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Re: shift to drive while in reverse

DanMan32 said:
As for stress, at low speeds I am sure it is negligible, but still more stressful than starting out at 0. Essentially, you are having MG2 switch directions while it is still moving. This is the same as changing the direction of a reversable fan before it has stopped.
MG2 doesn't switch directions, at least not instantaneously. It does, however, switch the direction of its torque, just like it does when you take your foot off the gas after accelerating to your desired speed.

MG2 does not mind having the direction of electron flow change (it happens hundreds of times a second even with steady force). MG2 does not mind having the magnitude or direction of its torque change (that's its job: to apply torque at the driver's whim). MG2 does mind instantaneous change in angular velocity, but that does not occur in this situation. That occurs when you're slipping on ice and suddenly catch pavement (which is why MG2 shuts down when you start to slip). It also happens when you jam on the brakes, but ABS reduces the stress and hopefully you don't do that too often.
 

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Discussion Starter · #15 ·
Re: shift to drive while in reverse

RSnyder said:
MG2 does mind instantaneous change in angular velocity, but that does not occur in this situation.
That's the kind of explanation I hoped for, and what I intuitively felt was probably the case.

Can you elaborate on what happens when you apply forward power from reverse at 3 mph or so? Like DanMan, I thought it was something like changing the direction of a reversable fan before it stops, but you say not.

I guess it is graphically illustrated on Graham's page,

http://home.earthlink.net/~graham1/MyTo ... IDrive.htm

Without studying it as long as I need to to know with certainty, it does seem to show that MG2 changes direction from reverse to drive. But, as you say, MG2 is constantly changing direction anyway, so that in itself wouldn't seem to cause any problems.
 

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Actually, even the fan would not change direction instantaneously, but would change torque, and you can hear the strain. The electrical strain is longer under a reverse than from a standstill. But unlike a fan motor, MG2 is probably better designed to handle such a situation.

As for ABS, that I have to disagree. MG2 torque would certainly be disengaged or at least reduced during ABS, as it would be counter-productive to try and keep the car moving when you are trying to stop. Now you COULD have regen braking from MG2 while ABS is operating.
 

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Ok, I'll try a couple explainations.

First, what happens to MG2 when you shift to D and step on the "gas" while drifting backward:
While drifting, there is very little electric current being sent through the motor, it's just turning backward. When you shift to D and push the accelerator, the computers determine the current spin in reverse and take steps to slow that spin to zero then spin forward. They do this by measuring the current position of the motor every millisecond or so, and adjusting electric current through the motor windings to position a magnetic field ahead of the rotor poles that pushes back on the rotor poles and a magnetic field behind the rotor poles that pull on the rotor poles. This slows the motor to zero in a second or two. The computers continue to monitor position and place magnetic fields, but this time, as the motor starts rotating forward, the pushing field is behind the poles (still in the same relative position, but since direction is reversed, what was ahead is now behind). So in effect, the motor doesn't really see any difference as far as torque is concerned, between slowing down reverse spin and speeding up forward spin.

DanMan, I think you misread my post about ABS. I said ABS helped protect the motor when you jam on the brakes, but you seem to have read it as potentially hurting the motor. So let me try that explanation again also.

When you jam on the brakes, if there were no ABS, the wheels would almost instantly stop spinning, even though the car continues to travel forward. This causes the wheels to skid. But it also causes the drive train to stop spinning almost instantly. MG2 is part of the drive train, so it would be stopped almost instantly. But it's got a lot of mass, especially rotational mass, so its inertia is going to resist stopping. That means the drive train is subject to incredible torques between the stopped wheels and the motor that doesn't want to stop quite so fast. MG2 may still be contributing magnetic torque one way or the other (if the computers haven't reacted quite yet), but it's much smaller than the inertial torque in this case. Fortunately, we do have ABS and it comes to the rescue. It notices that the wheels have suddenly stopped and briefly releases/applies the brakes in pulses to let the wheels spin a bit further. This relaxes the torque on the drive train. Basically, it allows MG2 a bit more time to slow down to a stop. So the rotational energy of MG2 is removed more slowly and the drive train incurs less stress torque.

Hope these notes help.
 

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Oh yeah, I forgot to mention: There's also a difference with electric fans. Most of them don't have computerized commutation to control the strength and position of electric currents and hence magnetic fields. They usually rely on the 60 Hz household AC to do their commutation for them. But this is not well suited to slow or reverse spinning of the motor and can develop overly large currents during the transition. This means overheating for a longer period than when just starting from a stop (add that time to the time to stop the reverse spin). If the fan isn't designed to handle the extra heat, or if the current exceeds the circuit breaker capacity, "bad things" (TM) happen.
 

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Discussion Starter · #19 ·
RSnyder said:
When you jam on the brakes, if there were no ABS, the wheels would almost instantly stop spinning, even though the car continues to travel forward. This causes the wheels to skid. But it also causes the drive train to stop spinning almost instantly
Thanks for your explanations.

I'm wondering now if the impulse from the brakes being applied might also be communicated to the computer controlling both MG1 and MG2. i.e. due to the regenerative braking, applying brakes is actually not a mechanical operation but an electronic one. As you noted, conditions are monitored by the computers on a millisecond by millisecond basis. So, if the brakes are applied, (harshly, if that needs to be known), there would be plenty of milliseconds to send a message to the computers to release all magnetic force within the drivetrain, so that it would now be completely free wheeling. Therefore, the sudden requirement for the ring gear to be stopped would only express itself in overcoming the inertia of the ring gear itself, none of the rest of the drive train: motor and generator would be free to spin, expending any built up momentum. Does this make sense?:
 

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In panic stops, the car would most certainly stop providing kinetic power, though it may choose to apply regen braking as a supplement to friction braking controlled at each individual wheel, but it more likely disengages regen braking.
 
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