A Stationary Quirk

[B&J]

I've noticed a quirk in the Energy Monitoring diagram in my Gen 3 Prius.

When stationary, if I keep my foot on the footbrake, I stay stationary, the driver behind gets very annoyed and the diagram shows no energy transfer in any direction.

However, if I apply the foothand brake and remove my foot from the footbrake, I remain stationary, the driver behind is no longer blinded but the energy diagram shows an energy flow from the Battery to the wheels.

Is this just a bug in the energy diagram or is there really some reason why releasing the footbrake but remaining stationary causes an energy flow?

Just asking.

B&J

[Jan&tone]

I think the footbrake disables the power transfer, if you put the parking brake on that facility is not used.

If you are going to use the parking brake you should put it in "P"

Earlier in the year there was some heated discussion on here as to the procedure at traffic lights, roundabouts etc, but basically unless you are stopping for more than 30 seconds you hold it on the footbrake.

[daveybee]

I'm pretty sure that the footbrake acts as a switch and (as J&T said) disconnects the power train. If you engage the handbrake and take your foot off the footbrake, the switch is still engaged, power is channelled to the drivetrain and the car will pull against the brake. This is not an ideal situation and is why you should really engage "P" with the handbrake - leaving the drivetrain in "D" whilst holding the car on the handbrake is potentially dangerous and cannot be recommended (however upset the following driver might be with your brake lights). Dave.

[sjrainsford]

As above. There is slight drive always fed through when the brakes are off to enable an automatic car style creep. Basically with no brakes applied the creep function will make the car drive forwards if in drive.

If you put your foot on the brake this cuts the creep function. However if you have the parking brake enabled creep will still apply so as the car is held when you release the brake.

In the case of the Prius, if you want to apply the parking brake then the car should also generally be put into park or neutral. If at traffic lights, neutral is recommendable as the car can be switched straight to drive without having to push the foot brake plus there will be no potential transmission damage if someone hits you from behind. That is more down to personal opinion though.

[r04drunner1]

If at traffic lights, neutral is recommendable as the car can be switched straight to drive without having to push the foot brake..."

I hadn't thought of that. Brilliant! Saves me from flashing my brake lights at the driver behind, too.

[Cyker]

Yup - If a pause becomes a wait, go neutral+handbrake!

[timberwolf]

Just be aware that the HV Battery can not be charged when in Neutral. The following is from the Gen 3 USA Owner's Manual:

" n When in heavy traffic

If the shift lever is in "N", the hybrid Battery (traction battery) will not be

charged. To help prevent the Battery from discharging, avoid putting the shift

lever in "N" for an extended period of time."

[r04drunner1]

Just be aware that the HV Battery can not be charged when in Neutral. The following is from the Gen 3 USA Owner's Manual:

" n When in heavy traffic

If the shift lever is in "N", the hybrid battery (traction battery) will not be

charged. To help prevent the battery from discharging, avoid putting the shift

lever in "N" for an extended period of time."

Yes, absolutely. But in this particular situation with the ICE off and the car stationary there is no charging being done anyway.

Anyway, that "N" advice from the manual always intrigued me. I take it to mean that having the drive disengaged means that the power from the ICE is not fed to the electric motor so it can't act as a generator. Fair enough. But what about P (Park) mode? If the ICE is on and the car is stationary in P, then does the Battery get charged by the motor/generator? If so, N (Neutral) should not be used in that situation.

[timberwolf]

There are two separate motor/generators. Mechanically, the petrol engine and MG1 and MG2 are always physically connected together by the Power split device.

Neutral is achieved by effectively open circuiting the coils in the motor generators. In Park, everything is electronically connected.

When in Park, the Petrol engine can drive one of the Motor/Generators (I forgot which one) to generate electricity, the other Motor/Generator connected to the wheels has just enough power applied to it to hold it steady so that it doesn't turn the front wheels.

[MSHasan]

Just be aware that the HV Battery can not be charged when in Neutral. The following is from the Gen 3 USA Owner's Manual:

" n When in heavy traffic

If the shift lever is in "N", the hybrid battery (traction battery) will not be

charged. To help prevent the battery from discharging, avoid putting the shift

lever in "N" for an extended period of time."

Yes, absolutely. But in this particular situation with the ICE off and the car stationary there is no charging being done anyway.

Anyway, that "N" advice from the manual always intrigued me. I take it to mean that having the drive disengaged means that the power from the ICE is not fed to the electric motor so it can't act as a generator. Fair enough. But what about P (Park) mode? If the ICE is on and the car is stationary in P, then does the Battery get charged by the motor/generator? If so, N (Neutral) should not be used in that situation.

Yes of course the Battery can charged by the ICE when in P.

What the manual is referring to is that, say for example, its cold, you have the heater on and you're in slow moving traffic, your HV Battery will run down sooner rather than later - in this situation if the car is in N then the engine will not be able to start up and recharge the battery as it is (I believe) physically impossible to do so. The above was just an example, it does not have to be a cold day etc.. your HV battery could be on low charge anyway say down to the last bar or two, it won't take long for it to become depleted and require energy from the engine. Hence, if you will be using N, you should keep an eye on the state of charge whilst in N so as to move back to D or P if you think the charge is getting too low. As you can see, this can become quite tedious and so most people would recommend to either just use the foot brake or change to P and not risk hundreds of pounds of HV battery damage!

Hasan

[Chris Dance]

Dealer recommended putting the car in P for long stationary waits and foot/hand brake brake on. This is what I do and I have had no problems at all. Short stops or slow moving traffic I just hold it on the foot brake.

[GreenJuice]

If at traffic lights, neutral is recommendable as the car can be switched straight to drive without having to push the foot brake..."

I hadn't thought of that. Brilliant! Saves me from flashing my brake lights at the driver behind, too.

This is a technique Japanese hypermilers used to apply with the Gen 2 because there were minimal but measurable energy savings. It was described as a 'ceremony' in a post responding to one of mine about traction Battery current readings on Scangauge when holding the car at stoplights:

http://priuschat.com/forums/gen-ii-prius-technical-discussion/21854-how-many-amps-drawn-when-stopped-light-foot-brake-2.html#post853143

This technique is less important with the Gen 3 as the tendency to 'creep' or 'pull against the brake' when in D was refined. It does take a second or so for this to happen and Scangauge can show this more clearly then the car's energy monitor. I posted a table comparing the Gen 2 and Gen 3 behaviour recently:

http://priuschat.com/forums/gen-iii-2010-prius-technical-discussion/100559-observations-battery-current-bta-amp-gen-2-gen-3-compared.html

[r04drunner1]

Fascinating... thanks Greenjuice, good find. And thanks to the other repliers.

[timberwolf]

All interesting stuff.

I think the key points are that for Toyota's HSD, that when in Neutral the HV Battery is not charged, and that it is the exception to the rule. As for a reason, without getting technical, I believe it was to keep the design simple.

[B&J]

I just thought I was asking a simple question!

We have some traffic lights around here that literally take minutes to change if pedestrians push the crossing button. I've tried putting it into N and applying the handbrake, today. It works well for me. I really don't like sitting at lights with the brake lights illuminated, especially at night. So I'm going to use this technique for long waits. For short waits, it's going to be the footbrake.

Thanks to everyone who's contributed to this thread.

B&J

[korat102]

I tend to keep my foot on the brake until someone pulls up behind me then release it. My philosophy is that it's better to show people that you're braking/stationary with nice bright lights. Once I'm sure they've noticed I'll take my foot off the brake.

[Sagitar]

I tend to keep my foot on the brake until someone pulls up behind me then release it. My philosophy is that it's better to show people that you're braking/stationary with nice bright lights. Once I'm sure they've noticed I'll take my foot off the brake.

Then you are doing it right. Here is the relevant sentence from paragraph 114 of the Highway Code.

"In stationary queues of traffic, drivers should apply the parking brake and, once the following traffic has stopped, take their foot off the footbrake to deactivate the vehicle brake lights. This will minimise glare to road users behind until the traffic moves again".

Note that this instruction is a "should" (i.e. advisory) and not a "must" (i.e. a legal requirement).

[r04drunner1]

I just thought I was asking a simple question!

B&J

No such thing around here, mate. HSD geeks love to talk techie!

I tend to keep my foot on the brake until someone pulls up behind me then release it. My philosophy is that it's better to show people that you're braking/stationary with nice bright lights. Once I'm sure they've noticed I'll take my foot off the brake.

Excellent - yep, I do the same, while watching the rear view mirror like a hawk in case somebody is coming up too fast behind and not paying attention. If so, I can try pumping the brakes to flash my lights - another reason not to have the car in D... Oh, and checking the mirror also allows you to spot bright blue flashing lights from way back and plan how to get out of their way...