Use Of P - Why Bother?

[JonFB]

Don't all shout at once here but...

Assuming you are an ex auto driver like me and therefore never engage the electronic handbrake (unless on v steep hills) you no doubt will simply come to a halt, stick the car in P and switch off and walk away.

My question to you is - why bother even with this. Surely coming to a halt (i.e. parking up) you can just hit the power off button - doesn't that just stick the car in P by default?

And whilst you're about to have a go at me - here's another one...when reversing out of a space and then moving off, why bother using the foot brake, just whizz out in reverse and immediately switch to Drive, the car then automatically "brakes" as it switches direction. Given it's all done by chips - does this do any harm to anything?

Signed....clearly a very lazy Prius driver!

[GreenJuice]

I do the same myself

[timberwolf]

I'm a belt and braces sort of person, so I always apply the Parking brake and have done so on every car with an automatic transmission.

I don't think there is any harm to just powering off the Prius, because as you say it will go into Park before it powers down.

For the reverse to forward, it's probably not what the engineers had in mind, but there maybe similar forces being applied to the electric motors when the car is driving along balancing between petrol and electric motors (I am guessing). There was a PSD simulator link posted recently on (IIRC) the yahoo group, which shows the Prius Gen I, but apparently the Gen II operates the same way. It needs Java to work.

http://www.wind.sannet.ne.jp/m_matsu/prius...tml?Language=en?

I do wonder how an external observer might interpret your drving, seeing the car switch from going in reverse and then forward without seeing the brake lights. If you were to knock over a pedestrian whilst reversing, would you without thinking select forward to brake? Could a prosecution case use the lack of brakes lights as an argument that you weren't in proper control of the car?

[benw123]

Since the transmission is electronically-controlled in the Prius, some of these situations are already handled. For instance, if you're moving forward at more than 1 or 2mph and press P, you get a double-beep warning (and I think the D symbol flashes, could be wrong). I always use it at a set of lights to take pressure off the transmission, rest my right foot and also stop the cars behind me being blinded by the bright red LED brake lights!

Regarding the reverse to forward change, I usually seem to hit it at the right point so that it engages exactly at the moment I reach a stop. Sometimes I've received a jolt where the car has switched to D but I would think as long as this doesn't happen to you every day, there's probably little risk.

Finally, bear in mind that on a car review of the Prius (linked in a recent thread here), a Toyota salesman reported that driveshafts were a weak point on the car above 60K miles. Maybe this reported problem is in someway related to this thread?

[timberwolf]

Since the transmission is electronically-controlled in the Prius, some of these situations are already handled. For instance, if you're moving forward at more than 1 or 2mph and press P, you get a double-beep warning (and I think the D symbol flashes, could be wrong). I always use it at a set of lights to take pressure off the transmission, rest my right foot and also stop the cars behind me being blinded by the bright red LED brake lights!

What is this pressure on the tranmission? If the brake pedal is depressed, the auto like "creep" mode from the electric motor is disabled and the MFD arrows should be black (assuming the ICE (petrol engine) is not running for power generation).

[benw123]

What is this pressure on the tranmission? If the brake pedal is depressed, the auto like "creep" mode from the electric motor is disabled and the MFD arrows should be black (assuming the ICE (petrol engine) is not running for power generation).

If you hold the car still using the brake pedal gently, you'll see the yellow power flow continually to the electric motor. Push it harder, and then it goes out. I'm pretty sure some current is applied because I doubt the creep would be able to start up quickly enough or be so smooth after releasing your foot from the brake.

[GreenJuice]

What is this pressure on the tranmission? If the brake pedal is depressed, the auto like "creep" mode from the electric motor is disabled and the MFD arrows should be black (assuming the ICE (petrol engine) is not running for power generation).

If you hold the car still using the brake pedal gently, you'll see the yellow power flow continually to the electric motor. Push it harder, and then it goes out. I'm pretty sure some current is applied because I doubt the creep would be able to start up quickly enough or be so smooth after releasing your foot from the brake.

That seems to be confirmed using ScanGauge to monitor Battery current. When stationary with light pressure on the brake, it shows about 3 amps from the Battery (and this coincides with the yellow power flow arrow on the MFD screen). Firm pressure on the brake and the Battery current drops slightly to around 2 amps with the yellow arrow going out. Battery current never drops to zero whilst in D, its always positive (electric motor driving) or negative (regenerating).

[timberwolf]

That seems to be confirmed using ScanGauge to monitor Battery current. When stationary with light pressure on the brake, it shows about 3 amps from the battery (and this coincides with the yellow power flow arrow on the MFD screen). Firm pressure on the brake and the battery current drops slightly to around 2 amps with the yellow arrow going out. Battery current never drops to zero whilst in D, its always positive (electric motor driving) or negative (regenerating).

Thanks benw123 & Greenjuice, interesting observations. Is the Battery current that you are monitoring only for the motor? I am wondering how much power is required for example to run the MFD or the A/C and I imagine that is also drawing from the Battery (via the dc-dc converter)?

[Mkuk0]

Um....maybe I'm the daft one here, but I thought you had to have your foot on the brake pedal to engage R or D anyway? Certainly from rest I can't make the car move unless my foot is on the brake pedal while I select a gear.

[Cyker]

Um....maybe I'm the daft one here, but I thought you had to have your foot on the brake pedal to engage R or D anyway? Certainly from rest I can't make the car move unless my foot is on the brake pedal while I select a gear.

Maybe it's different in the Prius?

The few 'real' auto cars I've driven have all required me to be holding the foot brake before I can select different gears/drive modes/whatever, which always got me because I'm one of the few manual drivers who goes to neutral and handbrake when waiting at traffic lights rather than sitting on the foot brake!

[GreenJuice]

Is the battery current that you are monitoring only for the motor? I am wondering how much power is required for example to run the MFD or the A/C and I imagine that is also drawing from the battery (via the dc-dc converter)?

I use the Xgauge BTA (and will need a little time to research exactly what it measures!).

I had a lot of start-stops on my commute home this evening and did some experimenting...

If I hold the car still in D with a light foot on the brake so that the yellow arrow is still on, it reads around 2 - 3 amps.

A firm pressure on the brake, drops its down to around 1.5 amps, as the yellow arrow clears. This reading stays the same regardless of whether I am in D, N or P.

Lift off the brake in D and the reading rises to 2 -3 amps as the car creeps forward.

Lift off the brake in P and the reading falls to 0.9 amp.

I guess this last figure represents the background or baseline energy use you were wondering about. Also, the application of the brake itself appears to consume energy, so there does seem to be some (minimal) advantage in applying ben123's technique of using P when stationary at lights, etc.!

[GreenJuice]

...I thought you had to have your foot on the brake pedal to engage R or D anyway?

There's a post suggesting you can change from D to R (to D) when travelling up to around 6 - 8 mph, although I haven't had the guts to try it out myself! :D

[benw123]

I guess this last figure represents the background or baseline energy use you were wondering about. Also, the application of the brake itself appears to consume energy, so there does seem to be some (minimal) advantage in applying ben123's technique of using P when stationary at lights, etc.!

Apart from anything it saves your right foot from aching! Nice research though and a good write-up.

[timberwolf]

I use the Xgauge BTA (and will need a little time to research exactly what it measures!).

I had a lot of start-stops on my commute home this evening and did some experimenting...

If I hold the car still in D with a light foot on the brake so that the yellow arrow is still on, it reads around 2 - 3 amps.

A firm pressure on the brake, drops its down to around 1.5 amps, as the yellow arrow clears. This reading stays the same regardless of whether I am in D, N or P.

Lift off the brake in D and the reading rises to 2 -3 amps as the car creeps forward.

Lift off the brake in P and the reading falls to 0.9 amp.

I guess this last figure represents the background or baseline energy use you were wondering about. Also, the application of the brake itself appears to consume energy, so there does seem to be some (minimal) advantage in applying ben123's technique of using P when stationary at lights, etc.!

Thanks, those readings would appear to show that when the brake pedal is pressed down firmly that the transmission is not being driven, as the creep mode demands an extra 1-2 Amps (at 201v).

When the brake pedal is being pressed the rear brake lights would be lit and the Brake Actuator activated, and there was roughly 25 Amps being demanded by the 12v systems.

The baseline consumption of the various 12v systems was roughly 15 Amps. So there was 10 Amps being demanded by the 12v systems when the brake pedal was pressed, I would guess the LED lights and Brake Actuator combined would be sufficient to account for that additional current demand.

[GreenJuice]

Thanks, those readings would appear to show that when the brake pedal is pressed down firmly that the transmission is not being driven.

... I would guess the LED lights and Brake Actuator combined would be sufficient to account for that additional current demand.

Yes, and from what I can see, it takes about a second or so of firm pressure on the brake when stationary, for the drive to the transmission to switch off. You were correct when you said before, that there should be no 'pressure on the transmission', about a second after stopping. Thanks for giving that explanation for the additional current demand!

I posted these readings to an existing thread on Priuschat for comments. I was worried about the possibility of being shunted from behind while in P at lights, and wrecking the transmission. I received a reply from famous Ken from Japan which I thought I should share here. He (quaintly) described a technique of Japanese hypermilers as their 'ceremony' when approaching a stop:

1. As the car drops below 7 mph, shift into N.

Below this speed the car will have switched to friction brakes, so there are no more gains from regenerative braking. Leaving D will cut any chance of energy loss from the Battery driving the transmission, when the foot is light on the brake (i.e. creep).

2. Once the car comes to a stop, apply the parking brake and release the foot brake. This cuts the losses from brake application that you detailed.

3. To set off again, release the parking brake and shift to D (I noticed I don't need to have my foot on the foot brake to shift to D). The car creeps as soon as it enters D, so its a left hand, left foot movement and I'm off!

I tried this out on my commute with the most start/stops this morning (30 lights in 4 miles - yes, really!). Right foot, left hand, left foot, right foot, left foot, left hand, right foot :(

I started to see why they call it a ceremony! [n.b. and to remember, I thought that having Scangauge was distracting!]

To my surprise, I ended up at my destination with 2% more charge (60%) and highest ever mpg for that particular journey (73 mpg, from cold start). I thought any gains could be so minimal as to be unnoticeable, but with so many stops I guess it all adds up!

[timberwolf]

Um....maybe I'm the daft one here, but I thought you had to have your foot on the brake pedal to engage R or D anyway? Certainly from rest I can't make the car move unless my foot is on the brake pedal while I select a gear.

Yes, the brake pedal is needed to move from Park to R or D, you'll get a warning beep without the brake pedal. Moving from Park seems to be the main special case, a feature that you probably wouldn't find too useful when shifting from B to D, or N to D (or in a conventional automatic from 1,2 or 3 to D).

I tried going from R to D the other day in a supermarket car park, and although I conciously decided to try it out, I found that I'd automatically pressed the brake pedal anyway! :o

[Mkuk0]

Lol yeah since 1st reading this post I've been trying it as well, it does work, but, like you, I unconsciously go for the brake pedal anyway!!

[timberwolf]

It's not something I intend to do in my normal use of my Prius... so I'm not concerned that I didn't manage to break my autonomic like response.

[Fujisan]

I posted these readings to an existing thread on Priuschat for comments. I was worried about the possibility of being shunted from behind while in P at lights, and wrecking the transmission. I received a reply from famous Ken from Japan which I thought I should share here. He (quaintly) described a technique of Japanese hypermilers as their 'ceremony' when approaching a stop:

1. As the car drops below 7 mph, shift into N.

Below this speed the car will have switched to friction brakes, so there are no more gains from regenerative braking. Leaving D will cut any chance of energy loss from the battery driving the transmission, when the foot is light on the brake (i.e. creep).

2. Once the car comes to a stop, apply the parking brake and release the foot brake. This cuts the losses from brake application that you detailed.

3. To set off again, release the parking brake and shift to D (I noticed I don't need to have my foot on the foot brake to shift to D). The car creeps as soon as it enters D, so its a left hand, left foot movement and I'm off!

I tried this out on my commute with the most start/stops this morning (30 lights in 4 miles - yes, really!). Right foot, left hand, left foot, right foot, left foot, left hand, right foot :(

So let me see if I've got this right, at traffic lights etc., basically the way to go is put it in to neutral and set the parking brake, rather than leave it Drive with your foot on the brake. And there is no need to touch the transmission P button.

OK, I can handle that, that is what I used to do with my last auto anyway, if I thought I would be stationary for more than a few seconds. Sorry to say for very short stops the guys behind had to break out the sunglasses. B)

[timberwolf]

On every automatic that I've driven, if I am waiting at the lights then I hold the car on the brake pedal.

On the Prius, I won't use Neutral because when in Neutral the HV Battery does not get charged up. It is unlikely that you could drain the HV Battery sufficiently whilst waiting at a traffic light, but I don't want to get into the habit because I might forget and use Neutral when in a very long traffic jam.

[wooski]

I use the Xgauge BTA (and will need a little time to research exactly what it measures!).

I had a lot of start-stops on my commute home this evening and did some experimenting...

If I hold the car still in D with a light foot on the brake so that the yellow arrow is still on, it reads around 2 - 3 amps.

A firm pressure on the brake, drops its down to around 1.5 amps, as the yellow arrow clears. This reading stays the same regardless of whether I am in D, N or P.

Lift off the brake in D and the reading rises to 2 -3 amps as the car creeps forward.

Lift off the brake in P and the reading falls to 0.9 amp.

I guess this last figure represents the background or baseline energy use you were wondering about. Also, the application of the brake itself appears to consume energy, so there does seem to be some (minimal) advantage in applying ben123's technique of using P when stationary at lights, etc.!

Thanks, those readings would appear to show that when the brake pedal is pressed down firmly that the transmission is not being driven, as the creep mode demands an extra 1-2 Amps (at 201v).

When the brake pedal is being pressed the rear brake lights would be lit and the Brake Actuator activated, and there was roughly 25 Amps being demanded by the 12v systems.

The baseline consumption of the various 12v systems was roughly 15 Amps. So there was 10 Amps being demanded by the 12v systems when the brake pedal was pressed, I would guess the LED lights and Brake Actuator combined would be sufficient to account for that additional current demand.

Hi,

When I took my test drive of the Prius my dealer taught me to do this at traffic lights. He said most people waste energy with their Prius by not pressing the brake pedal firmly enough to disable the creep mode. He reckons with a lot of traffic lights and waiting this can make quite a difference in fuel consumption. He said the way to learn was to watch the energy monitor to see when energy stops flowing from the Battery. I thought it was a good tip.

[timberwolf]

Agreed, a good tip.

The only one that I can recall being shown, was that the front defogger/demister causes the ICE (petrol engine) to run more often. It may also be another thing that denies EV mode?