Playing With The Gear Lever

[Opifex]

My wife recently got a lift in a Gen 3 Prius and noticed that the driver was continually moving the gear lever. My technique is to move the gear lever to D when I set off and leave it alone after that, so I am puzzled what this guy was doing. Has anyone any ideas?

[Duffryn]

No idea, unless he was using the B engine braking to save wear on his pads!

[wass]

Some sort of psychological thing? Was so used to a manual gearbox that he had to move a gear lever a lot even in an automatic car in the same way that cigarette smokers will sometimes"smoke " a pencil?

[ToyotaAurisHybrid]

My money's on using the 'B' too

[geoffatstanpit]

Mine's a Yaris but I think it's the same - B on a downhill approach to a junction seems to put the needle further into the "charge" sector (more amps into the Battery?) than D. Having said that I wonder if the ordinary brakes are getting much use at all. They do seem to be getting a bit rusty!

Also P at traffic lights stops my brake lights blinding the guy behind. Easier than heaving the handbrake on!

[Opifex]

I had wondered whether he might have been moving to N to maintain momentum - a sort of pulse and glide approach.

I remember several discussions on the forum about whether B or N or both actually cut out regenerative braking but can't now remember the details. Has anyone got a better memory than me?

[Chris Dance]

Leave the lever alone except to use B on a steep downhill descent. What do you want to move the gear lever for on an automatic car whilst driving.

[timberwolf]

N the Hybrid system can not recharge the HV Battery either regen or gen from burning fuel.

B to excess can cause the HV Battery to fill up and that means that the system will have to waste energy i.e less mpg not more.

[johalareewi]

B to excess can cause the HV battery to fill up and that means that the system will have to waste energy i.e less mpg not more.

Are you sure?

B mode forces the Prius to engage the petrol engine as a brake (engine braking) so there will less energy going into the HV Battery via regenerative braking. There is a wizard web page all about B mode here.

http://www.techno-fandom.org/~hobbit/cars/b-mode.html

Also, when the HV Battery is full, the Prius will stop using regenerative braking and automatically engage engine braking.

[timberwolf]

How can I be sure when I did not design the Hybrid system and can only observe this complex system from the outside.

As I understand it, "Engine braking" (B mode) can be both regenerative and mechanical (spinning the petrol engine without using fuel). Can "Engine braking" fill up the Battery to full SOC (just over 80% real terms, but displayed on the instruments in the cabin as the full 8 green bars) in B mode?

Yes, I believe it can.

If it does reach full SOC, then when the system has a chance, e.g. you stop at the bottom of long mountain descent, it will need to reduce the energy stored, and it does that by spinning the petrol engine at high RPM with one of the electric motors.

Are there other circumstances in which running B mode will fill the Battery up? I'm not so sure about level ground but I don't see why it shouldn't if the amps going into the Battery is increased. Another point is that extra "engine braking" is going to prevent efficient glides...

[Opifex]

.

If it does reach full SOC, then when the system has a chance, e.g. you stop at the bottom of long mountain descent, it will need to reduce the energy stored, and it does that by spinning the petrol engine at high RPM with one of the electric motors.

I think the understanding is slightly incorrect here. As I remember it going downhill without B will use regenerative braking alone until the Battery reaches 80% (confusingly indicated as full charge) at which point the charging stops and the engine is spun at high speed without fuel to provide the equivalent of conventional engine braking in a low gear. The difference with B is that the engine braking is used all the time not just when the Battery is full.

This photo shows the result of a long descent (and my best ever mpg). The engine wasn't running and the Battery charge state didn't reduce until I started moving.

(12/04/13 - Edited to correct the spelling mistakes)

[timberwolf]

Maybe it is a Gen 2 thing, or maybe it is just that it is harder to spot on the Gen 3 as the car is quieter? I haven't got time to do an exhaustive search at the moment and this thread isn't as conclusive as I would like because engine braking is only mentioned by one poster. It does mention the energy dumping when the car is stopped in a number of the posts.

http://priuschat.com/threads/2006-prius-engine-high-revs-stopped-at-stoplight.88480/

[Opifex]

The problem I have with the "dumping excess energy" theory is that it is a bit illogical to store the energy and then use the engine to dissipate it. Why store the energy in the first place if it is damaging to the Battery and you are later going to dump it anyway?

[r04drunner1]

I am thinking there is more mileage in the "Pulse and Glide" theory.

Any experts care to comment?

The idea of pulse and glide is:

1: Pulse with the throttle in the position for optimum engine efficiency (roughly at the full portion of the HSI, before the PWR zone). This builds up momentum.

2: Glide with the throttle in the position where power is not flowing to the wheels nor is it flowing from the wheels. Normally the driver achieves this by putting just the right amount of pressure on the throttle pedal to get to that position. Speed will bleed off but more distance will be covered at zero consumption, thus improving the overal MPG reading.

The thing is, it can be pretty tough to achieve and sustain the correct throttle position for a sustained glide. So I am wondering if, by simply putting the driver selector into N (neutral) the same effect can more easily be achieved. And that is perhaps what the Prius driver was doing.

I am aware this is effectively coasting and there is a whole other discussion on

- safety (you can't simply press down on the throttle if you need to accelerate suddenly. You also lose engine braking.)

- legality (there conflicting advice online re the legality of coasting in the UK - the Highway Code does not say it is illegal but some people believe coasting is illegal, although that's a law that would be pretty unenforcable).

[Opifex]

Thanks for bringing us back on topic.

There was a video on the forum a few months ago that showed someone being instructed to use N as a first lesson in hypermiling before going for proper pulse and glide.

[timberwolf]

The problem I have with the "dumping excess energy" theory is that it is a bit illogical to store the energy and then use the engine to dissipate it. Why store the energy in the first place if it is damaging to the battery and you are later going to dump it anyway?

Who said it was damaging the Battery?

If the hv Battery is full, and the driver wants to slow down by pressing on the brake pedal, the regenerative braking can't be used because the electricity generated can not be stored, so the braking system can only make use of the friction brakes.

Thanks for bringing us back on topic.

There was a video on the forum a few months ago that showed someone being instructed to use N as a first lesson in hypermiling before going for proper pulse and glide.

http://www.toyotaownersclub.com/forums/topic/138000-prius-86-mpg-urban/

[Opifex]

The problem I have with the "dumping excess energy" theory is that it is a bit illogical to store the energy and then use the engine to dissipate it. Why store the energy in the first place if it is damaging to the battery and you are later going to dump it anyway?

Who said it was damaging the Battery?

I inferred that it was damaging from your post here:

If it does reach full SOC, then when the system has a chance, e.g. you stop at the bottom of long mountain descent, it will need to reduce the energy stored, and it does that by spinning the petrol engine at high RPM with one of the electric motors.

if it isn't damaging why does it "need to reduce the energy stored"?

[timberwolf]

So that there is room available in the hv Battery in a situation where the driver needs to brake. If there is no room available in the hv Battery then the system can not generate electricity because it has no where to store it - and in those circumstances it has to waste the energy as additional heat via the friction brakes.

Secondly, the car can't see into the future, it doesn't know whether it would be useful to keep the Battery at full charge because a few seconds later the driver is going to demand a lot of power e.g. to drive up a steep hill, or if you are about to go down another long descent and it would be useful to have a nearly empty battery.

[GreenJuice]

My wife recently got a lift in a Gen 3 Prius and noticed that the driver was continually moving the gear lever. Has anyone any ideas?

If you are looking for reasons to play with your gear stick :-), apart from the obvious two of:

i) Shifting to B (decried by many experts and probably used innocently by those not fully understanding what B does)

ii) Shifting to N (not an unreasonable way to start practising P&G, but better to learn how to use the accelerator pedal)

iii).... there is also a little 'ceremony' used by some Japanese hypermilers in the Gen 2 Prius. The aim here is to minimise all current going into and out of the traction Battery (on the basis that all energy conversions are potentially wasteful). A foot on the brake pedal when in D and stationary uses 0.5 Amps. Lifting the foot off from stationary in D draws 2-3 Amps as the car creeps forward.

Therefore, a Gen 2 hypermiler might do the following when coming to a stop at a red light:

1. shift to N below 7mph (no more regen expected below this speed; also prevents the 'creep' in D)

2. apply parking brake when stopped, then release foot brake (to save 0.5 amp)

3. shift to D and release parking brake when ready to move on

Read more: http://priuschat.com/threads/how-many-amps-drawn-when-stopped-with-light-foot-on-brake.21854/#ixzz2QNswEtVr

I monitor BTA on Scangauge and have found that the 'creep' in D in the Gen 3 is so low as to be negligible, so this is technique is probably not worth doing when in a Gen 3 Prius.

(Unless you really like playing with your stick, in which case this one is really for your own pleasure and no one else's!)

[johalareewi]

I monitor BTA on Scangauge and have found that the 'creep' in D in the Gen 3 is so low as to be negligible, so this is technique is probably not worth doing when in a Gen 3 Prius.

A lot of the gen2 hypermiling tricks seem to be plumbed in or not relevant to the gen3. So there isn't really any need to play with the stick in a gen3.

[timberwolf]

I'd prefer to call them quirks in the design, they still exist (in the Gen 2 at least) whether they are exploited by a labelled group or not. :-)

[PeteB]

This may have changed for the Gen 3, but some members of the Yahoo Prius-UK group put meters on the Gen 1 & 2 and found that B mode reduced the amount of energy sent to the Battery compared to using the service brake. We concluded it's main purpose was to assist control of the car when descending long steep hills, particularly if the HV Battery is as full as the software allows.

That said, it was useful in the Gen 1 (Classic) in conjunction with Cruise Control when on hilly roads, as it helped keep the speed from increasing whilst going downhill. The Gen 2 would not allow B mode and Cruise Control at the same time, but the Gen 3 seems to provide much more regenerative braking in Cruise Control whilst descending hills.

Regards, PeteB

[timberwolf]

Yes, I think that fits with my understanding unless you say that 2. below generates less than 1. with a low SOC in the HV Battery?

1. In "D", lift foot off accelerator, the most glide/coast and least regeneration lowest Amps.

2. In "B", lift foot off accelerator, more retardation and also more regeneration than 1. i.e. more Amps generated than 1. but much less than 3.

Use of the petrol engine without fuel as an "Engine" brake. At this point, it is not clear when these two methods to slow the car down operate.

3. In "D" or "B", brake pedal most regeneration much higher Amps depending on how hard the pedal is pressed, but there must be an upper threshold because as I understand it friction brakes stop the fastest.

[PeteB]

"3. In "D" or "B", brake pedal most regeneration much higher Amps depending on how hard the pedal is pressed, but there must be an upper threshold because as I understand it friction brakes stop the fastest."

Yes, on the Gen 1 & 2 regen braking worked as follows:

1 only above 7 mph
2 in D or B
3 if HV Battery below artificial 'full' (about 80% in reality)
4 below a certain pressure on the brake pedal *

* below this pressure, the pads make fine contact with the discs, just enough to heat them (and drums on some US models) if necessary.

In N, or very heavy braking, or if ABS cuts in, the friction brakes assist or take over.

On the Gen 1, a US firm called Coastal tech sold an accessory that showed a red and green LED alongside the Multi Function Display: red came on if the brake pedal was pressed hard enough to activate the friction brakes, green when the ICE was running.

The Gen 3 seems to do some regen below 7 mph, and may have modified some of the other behaviours.

Selecting N and pressing the brake pedal can be useful to clear rust off the discs if they become noisy.

Regards, PeteB

[Sooty]

"Selecting N and pressing the brake pedal can be useful to clear rust off the discs if they become noisy"

I tend to do this on a weekly or 2 weekly basis on a slight slope if the road is quiet, just to use the friction brakes and stop the discs from rusting up.