Theory: Use EV mode just before home...

Self discharge rate of Ni-MH battery is quite low. And this procedure does not make much sense for parking your car over night. Even over weekend.

Ogo

 

That is a very interesting theory - not easy to test though. My main concern would be that the instruments in the Prius are probably not sufficiently precise to provide you with the data that you would need to support it. I believe a battery does not loose much power during a single night. So you would need to find a way to measure very precisely the charge level of a battery.

 

I have several other similar things I want to test myself. For instance: I live on top of a hill. I know in the next morning I will be able to recharge the battery when going back to work (going downhill). The hill is 400 m long, and slope peaks at 18%. I wonder what charge level I should aim for when parking my car in the evening in order to get full charge the next morning at the bottom of the hill (to make sure I dont use brakes), this way optimizing the use of battery.

 

I agree with your theory. I have had similar thoughts recently and started this practice myself. The engine runs and warm up in first start of the day, and will recharge an undercharged battery anyway.

 

Not a good theory, and in fact it will be counter-productive. The self discharge rate for the NiMh battery is very low - not even a factor over such a short time. A much bigger factor is the cost of generating electricity, storing it as chemical energy, converting it back to electricity, and using that to move the car. There are a lot of losses in the conversions, which should be avoided if possible. Your proposal increases the required conversions, which will cost you mileage.

A second disadvantage is that you will be doing the recharge at the ICE's least efficient point: warm up. It is better to start with a higher SOC and let the ICE take it easy during warm up.

One other minor point relates to self discharge rates and how they vary with charged batteries verses discharged batteries. With the Prius, the HV battery SOC swings over a narrow range in the middle of the charge curve. Seldom does it swing very far, and never does it charge up and discharge like you would with a flashlight battery. That would be too damaging to the battery and would severely limit its service life.

Tom

 

The negligible amount of self-discharge may not be a very good reason to use EV.

However, since the ICE needs to warm-up during the next start anyway (and will thus charge the battery), using EV during the last leg may still end up being more efficient.

That's how I see it anyway, although I have only owned my Prius for about a week. Also, my Prius is from 2007, so I don't even have an EV mode right now. However, I may add the Costal eTech solution sometime soon.

Edit: Sorry, I didn't see Tom's post since I opened a bunch of threads together some time ago...

 

I think 100 yards of EV into the garage at the end of the day cannot be bad. Helps reduce emission fumes in the garage too! After all why was the EV mode created in the first place?

 

I am in the exact same situation, almost identical distance and slope. Mine is a little curvy as well. I can't go very fast up the hill, so the speed limit of EV would not be a problem.

So, I wonder if the electric motor, with it's plenty of torque, would be more efficient going up the hill vs the ICE. In fact, I assume the electric motor would be assisting the ICE going up the hill in normal mode.

one of the posts criticizes using regen on startup, but with my hill, breaking is necessary down the hill, so why not get regen going down and therefore, why not use EV going up? I assume that poster was responding to the first post, not your uphill/downhill question.

Edit: come to think of it, some loss of charge would probably happen going up the hill in normal mode, so it is probably a matter of degree. Time and experimentation will tell.

 

hmm... interesting theory.. i see it only working for people around gentle hills who have EV mode.

Otherwise... you're draining "warm up" electricity.. since the car relies heavily on the battery for the 1st half mile or so. (i live on a hill, and i use EV to park the car.. but mileage is not the factor) so.. you drain the battery parking the car.. and now it gets even more depleted because the car is using it to reduce load on the ICE during startup... . so... now you have a potentially low battery and extremely poor mpg for the next couple mins of the warm up process.

 

Have you tried going up the hill in EV mode?

Yes, that is what I assume. I guess he meant regen through the ICE, which is not efficient when the engine is cold.

In your situation however, you would not be able to brake-regen as much energy as you use when going up. Can you tell me how much energy is regenerated when going down your 400 m hill?

 

when your car starts up.. it uses electricity to lighten the load on the ICE.. so you don't fill it back up.. you drain it more.. then you fill.

the golden rule to this car and gas mileage is to NOT use the battery because when you use electricity, you take on the energy losses of having to make more energy.

i used to question things just like you... then i stopped... because it's about efficiency losses.. i'd rather keep my battery level... and have the ICE warm up so the engine can shut off quicker.

if you have stop lights, you don't want to sit there and watch your mpg drop like a bomb because the hybrid system requires more power before shutting off the ICE...

 

My car is still on order. I expect to get it in about 6 weeks. I'll post my experience when I get it.

 

I'm not at all positive that I can shed light on this matter. Nonetheless,
I'm foolish enough to try. Here goes...

There are a number of issues, some non-intuitive, mixed up into a
thick gurry here. They include:
* Hybrid Synergy Drive, HSD, Control Algorithms
* ICE and HV battery use at start-up
* Most efficient use of the infamous EV mode/switch
* Short term vs. long term efficiency.

Lest the message get lost in the following flood of words, here it is up-
front:

It is most unlikely that starting a new day with a cold Internal
Combustion Engine, ICE, and only red bars for HV battery State of
Charge, SOC will yield the highest day's end Fuel Efficiency, FE.

Why? Well, because what is know about how the HSD control
algorithms control energy flow to and from the HV battery say,
"Sorry boys, it ain't so. Never has been. Never will be."

OK, some general stuff:

First off, the HSD works with control algorithms prioritized in this
order:
1. Protect the mechanical and electrical systems from damage.
2. Operate the the mechanical and electrical systems to generate the
least possible pollution.
3. Operate the mechanical and electrical systems to maximize fuel
efficiency.

Fact: The HSD is least intrusive when the SOC is at high 5 bar or 6 bar
levels.

Consequence: Whenever SOC is out of this range the HSD will quietly,
ever so sneakily, but inevitably work behind whatever the driver is
trying to do to get SOC to this range.
* At 2 greens bars, the HSD will happily use SOC to spin the ICE to
burn off charge, that is to waste energy. In accordance with its first
priority, it has to do this to have room for more charge if necessary,
* At one or two red bars, the car will refuse access to EV mode, keep
the ICE turning over when otherwise you would expect 0 RPM, and
through the Power Split Device, PSD, divert ICE power to Motor-
Generator 1, MG1, to build up the SOC.

Fact: At start-up, the HSD runs the ICE with a rich fuel mixture to
quickly heat the Catalytic Converter, CC. I does this to get pollution
super low ASAP.

Consequences:
* When cold and running on rich mixtures, the ICE has greatly
reduced torque.
* For the first few minutes after start-up the HSD uses huge amounts
of SOC to accelerate/move the car as requested by the driver via the
go-pedal.

Fact: Using the ICE to burn gasoline to generate power and then
sending that power to the HV battery for storage, and then using that
power to move the car involves huge conversion losses. I've seen
figures here between 58 and 80% losses.

Consequence: The best way to use an EV switch is hardly at all: very
short runs to set up a down hill situation to get the power used back
thru regen, etc.

Fact: Just because the ICE is turning over, it doesn't mean the HSD is
diverting power through the PSD to generate electrical power.

Consequence: At start-up the HSD is taking power out of the HV
battery to help move the car.

Back to the original question: Is it most efficient to end the day with
very low SOC? Answer: Most likely, no.

At start-up the HSD will be using power to move the car. This will
reduce SOC. If you're down in the red bars range, to protect the HV
battery the HSD may not take power from the HV battery and the car
will be very sluggish. Until the ICE and CC get fully warmed up the car
runs at lower FE, somewere ~30-35MPG. When the ICE/CC are fully
warmed up, the HSD will divert power to charge the HV battery vice
move the car. FE will be less than maximum. The ICE may be
operating efficiently, but the overall HSD is not.

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.

Possible exception:
* If you live at the top a hill of sufficient length and declivity to
increase SOC from red bars to green bars during the roll-down,
AND
* You have an EV switch and get into EV mode before the ICE starts,
AND
* Air temps are above 32 degF so you can get into EV mode,
AND
* You limit speeds down the hill to 34 MPH to stay in EV mode and
suppress the inefficient ICE start-up sequence...

Then, and only then, is it possible that starting a day with red bars on
the SOC will yield long term high FE/MPGs.

Hope this helps.

 

Thank you very much, Rokeby, for that very interesting info. I never owned a hybrid, so I dont have any experience and very little knowledge. Your message contains valuable information to me!

 

Oops - just seen it - sorry. Yes, please do not hesitate to post your experience on this thread - I am eager to read it!

 

Rokeby,

I also thank you for your thorough response. It sounds like CivicQc and I just may be in a position to benefit. Time will tell. I will get a baseline and get familiar with my Prius and then experiment.

 

When the Prius warms up the ICE, it does so in a manner that burns gas to make heat but not much power. You can force it to deliver power, but it isn't happy about it and does so in a less efficient manner. On the surface of it, your thoughts make sense, but once you understand the finer details of the entire Prius drive system you realize why it doesn't work that way.

Tom