Theory: Use EV mode just before home...

[ZTrekus]

Originally Posted by Rokeby

What to do?
Recognize that high 5 bar, 6 bar SOC is a desirable condition. Don't
use EV mode excessively, ever, for any reason.

Thank you for your very detailed analysis. I think the above contains an error though. A high SOC may be desirable if it was obtained by plugging in the car to an external power supply - but otherwise, a high SOC is obtained because it has burned up your gas supply to get there. At the end of the day - surely it is better to leave the energy in the form of gas in the tank as opposed to bars on the dash representing the SOC in the battery.

What do you say specifically to that proposition?

[ozboy]

Originally Posted by qbee42

The self discharge rate for the NiMh battery is very low - not even a factor over such a short time.

How does lithium ion (Li-Ion) compare?

[CivicQc]

Originally Posted by ZTrekus

A high SOC may be desirable if it was obtained by plugging in the car to an external power supply - but otherwise, a high SOC is obtained because it has burned up your gas supply to get there. At the end of the day - surely it is better to leave the energy in the form of gas in the tank as opposed to bars on the dash representing the SOC in the battery.

Note that a high SOC can also be obtained through regenerative braking...

[bruceha_2000]

At one point in the distant past I thought as the OP does: "I'm going to be charging the battery when the ICE starts in the morning so running down the battery getting into the driveway is a good thing since I'll use less gas getting home."

However, Rokeby's analysis squares 100% with what I have seen. The car will use MUCH more Electric momentum than ICE when the engine is first started. You do yourself no good running the ICE harder in the morning because you have drawn the battery down so far the car won't use it for propulsion or even far enough down that you drop to 2 bars right off the bat. For you newbies (WELCOME!), notice how fast the bars drop on a given 'from home' start when the engine is cold vs already warmed up. It is REALLY noticeable if you start at 4 bars. I would NEVER force it down to 2 unless you are out on the road and facing a LONG downhill, possibly 'B' mode plus brake pedal route that will push to 8 green and force the ICE to run to dump watts out of the battery. I personally never let it go below 4 in EV.

For those living on the hill, while you MIGHT be able to get up the hill in EV, you won't make much distance on the current battery. And remember: The ICE (at least in the Gen II) will run for almost a full minute ANY time you start it even if the engine is already warm enough that the car puts itself into auto-EV for propulsion. So while you might be braking to hold speed down the hill in the morning regenning all the way, the ICE is running and wasting gas. I agree with Rokeby on the use of EV to come down the hill in the morning: Don't let the ICE start until you get off the hill UNLESS the road at the bottom is high speed (ie NOT something the car can handle in normal ICE warm-up with a lot of EV initially). BETTER to let the ICE warm up gently, it is one of the reasons it will last longer than the ICE in a traditional vehicle. Also remember, if you push the battery down to 2 bars, you can't force EV anyway. Better to drive the car normally up the hill and end up with 4-6 bars at night.

[CivicQc]

Thanks, bruceha_2000, for adding this.

For that very purpose then (leaving home in the morning), wouldn't it be nice if we could recharge the battery during the night? I know it is not possible - just a hypothesis. What do you think?

[Rokeby]

Originally Posted by ZTrekus

Thank you for your very detailed analysis. I think the above contains
an error though. A high SOC may be desirable if it was obtained by
plugging in the car to an external power supply - but otherwise, a
high SOC is obtained because it has burned up your gas supply to get
there. At the end of the day - surely it is better to leave the energy in
the form of gas in the tank as opposed to bars on the dash
representing the SOC in the battery.

What do you say specifically to that proposition?

ZTrekus,

You have made a very astute observation.

My answer: If the car is properly managed to achieve high FE/MPGs, a
SOC level of high-5 or 6 bars at the end of the day represents many
little energy savings or scavengings achieved by the HSD and not
inefficient use of fuel by the driver.

Some general stuff to back this up follows. All this applies during the
day before those last few hundred yards as you arrive back at home.
It most definitely determines how much SOC you then have and can
decide to use or save for the next morning.

Fact: Ultimately, all the energy resident in the car comes from having
burned gas from the tank. This would include:
* Kinetic Energy - energy from the vehicle being in motion, =mv^2
(mass times velocity squared)
* Potential Energy - energy possesed by a vehicle as the result as its
being at a height higher (+), or lower (-) than some reference, most
typically where the car is at the beginning of the day. Rolling down a
hill in a "no arrows" glide is using this energy.
* Chemical energy - energy stored in the HV and 12v batteries.

Consequence: You have to burn gasoline to do anything useful with the
car. Tough beans, acccept it, move on. But this isn't the real issue.
The real issue is how can the use the HSD to achieve the highest
system efficiency.

Fact: At any given point in time when the HSD has control either the
ICE or electrical power system may be used inefficiently. But taken
together it is most probable that within the prioritized protections
listed earlier the HSD is operating the combination of the two more
efficiently than the driver can.

Consequence: Whether the driver likes it or not, the HSD is making
many, probably thousands per second, adjustments to the
ICE/electrical power mix. You can see this by running the car on a flat
section of road and setting the Cruis Control, CC -- the speed doesn't
matter. Now watch the screen on the MFD with the arrows (this for the
Gen II. Sorry, I'm keeping up with the minutia of the Gen III displays.)
At what appears to be a steady state, there are flickering yellow
arrows (electricity from the battery) and sometimes the orange
arrows (ICE power either directly or indirectly via generated power
that does not go through the battery). This is the HSD being very
selective in using its available resources in a highly efficient manner.

Fact: At least on the Gen II, the arrows on the Power Screen are only
an approximation of what the HSD is doing with power flows. Some
folks call the Power screen a "mimic," because it doesn't tell the whole
story.

Consequence: There are two electric motors/generators (MG) not one.
There is no way with only one motor depicted to show electrical flow
between them. Also, it is known that all electrical power flows are not
shown, such as the small flow from the battery to the wheels when in
a "no arrows glide."

Fact: The demi-intelligence in the HSD knows nothing about the real
world that the car is operates in. What little it knows is limited to
outside air temp, air density, -- from which can be derived the
atmospheric pressure (altitude is a factor here) -- and the car's
velocity. It has not a clue about what is about to happen;
climb/descend a hill, slow/come to a stop, shut down for the day,
whatever. So the HSD has to maintain its resources in a very
conservative condition to be able to react to anything that might
happen.

Consequence: The driver is the interface/intermediary between the
HSD operating in its near perfect world and the constantly changing
demands of the real world. Fortunately, the algoriths used by the HSD
are not go-no go; they have some fiddle room for the driver to play
with.
As an example, let's examine accelerating up a gentle hill. You just
step on the gas, and up you go, but with a yellow arrow showing
power going into the battery with attendant conversion losses. If you
push down on the on the accelerator ever so little, the yellow arrow
goes away. This would be a more efficient way to use the gas that you
must burn.
Likewise, if you saw a orange arrow going to the front wheels, this
represents power sent by the Power Split Device, PSD, first to MG1 to
be turned into electricty and then sent by the HSD to MG2 to drive the
wheels. Here too there are conversion losses. If you ease off the
accelerator ever so little, the arrow disappears. This to is a more
efficient way to burn your fuel.

Question:What does the accelerator, the "go-pedal" really do?

Answer: When the driver responds to conditions in the real world,
he/she makes an acceleration. Speeding up is an positive acceleration,
slowing down is a negative acceleration. Changing direction, turning, is
also an acceleration. In the Prius all these changes are communicated
over wires. No acceleration control -- accelerator or "go-pedal," brake
pedal, or steering wheel is mechaniclly connected to the thing it
controls. The car is "fly by wire;" all command signals go over wires
to a control computer.

As I understand it, pressing on the go-pedal sends an electronic
request to the appropriate computer to increase torque to the front
wheels. Not very exciting is it? In turn that torque will accelerate the
car to speed up, or hold a contant velocity up a hill, or overcome drag
forces that are acting on the vehicle -- there are many, including; in
the ICE, in the MGs, where the rubber meets the road, and those
created by the air as the car passes through it.

Now, back to the original question:

"a high SOC is obtained because it has burned up your gas supply
to get there. At the end of the day - surely it is better to leave the
energy in the form of gas in the tank as opposed to bars on the dash
representing the SOC in the battery."

As the driver you can most definitely affect how much SOC you have
at the end of the day. I use "affect" intentionally. Because the HSD is
busily working away in the background no matter what you do, you
cannot fully determine what the SOC is. The HSD is going to give and
take as it sees fit.

That said, you can intentionally send little power to the HV battery,
and take little from it. In reality due inevitable regen from necessary
braking on hills, mis-timed traffic signals, interactions with traffic, you
should always put more power into the the battery than you
intentionally use. This is no real problem as if the SOC level gets too
high -- high-7 or 8 green bars -- the HSD will first try to use it in
moving the car, or as a last resort dump it by spinning the ICE without
fuel.

For your convenience, here again is my answer:

If the car is properly managed to achieve high FE/MPGs, a SOC
level of high-5 or 6 bars at the end of the day represents many little
energy savings or scavengings achieved by the HSD and not inefficient
use of fuel by the driver.

[Philosophe]

Just a quick note on something that I haven't seen while reading through the thread:

The reason the Prius favours electric use during warm up is for pollution control, not for efficiency reasons.

That said, even after reading the thread, I remain to be convinced whether the OP suggestion would be good or bad, from some empirical evidence.

[FireEngineer]

The archives of the Gen II forum have threads about the same do or don't. Some owners actually did the experiment and found the charge depletion at night made for less overall fuel economy. But if someone wants to repeat history.....

Wayne

[jburns]

Originally Posted by ZTrekus

There is another advantage of course.

They say most accidents occur on the way and close to home (the carelessness factor creeping in).

That's one of the all time bogus stats. I think the deep voiced announcer said, "Remember, most accidents happen within 25 miles of home."

DUH! MOST DRIVING OCCURS WITHIN 25 MILES OF HOME!

[lamontcranston]

First, let me thank all who participated in this thread. So far it's been the most useful one I've read here.

I have been switching into EV mode about 1/2 mile before home and it does get my MPG's up as I head into the garage. However, I have also noticed that MPGs drop down to the same level they were at before EV mode, when I start it up the next day.

I'm going to stop the practice and just take it slow coming into the neighborhood, and I'll see if it makes any observable difference.