Downhill: Speed or MPG?

We believe the best strategy for downhill is don't use any fuel.
So...
- do not exceed 42 mph. You'll consume 1 cc of fuel when you exceed 42 mph and ICE starts.
- do not use B mode above 17 mph. You'll consume 1 cc of fuel when you do so and ICE starts.
- do not let the battery goes to full green 8 bars. You'll consume some fuel when you do so and ICE starts.

Ken@Japan

 

<div class='quotetop'>QUOTE(ken1784 @ Jul 31 2007, 07:33 AM) [snapback]487926[/snapback]</div>

I don't understand the last two points. Are you saying engine braking consume some gas? I thought MG1 would spin up the ICE (using just the electricity) and perform the engine braking.

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Jul 31 2007, 11:04 PM) [snapback]487965[/snapback]</div>

I'm saying Prius uses 1 cc of fuel just to start ICE.

Ken@Japan

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Jul 31 2007, 09:04 AM) [snapback]487965[/snapback]</div>

I'm still unclear as to why having a full green battery is bad -- even if you do burn some small amount of fuel.

 

<div class='quotetop'>QUOTE(Washington1788 @ Jul 31 2007, 09:51 AM) [snapback]488002[/snapback]</div>

I suppose it depends on what you mean by "bad". If we agree that unnecessarily burning gasoline is "bad" (bad for oil independence, bad for the environment, bad for your pocketbook). Then depending on the situation having a full green battery is likely to be bad for 2 reasons:

1) Acquiring full green battery in many situations requires the unnecessary burning of extra gasoline to generate the extra energy that the battery is now holding. There are situations where you obtain full green battery without the unnecessary burning of extra gasoline, and in these cases this reason doesn't apply, but if it was avoidable it is a waste and by way of the previously stated understanding of "bad", it is bad.

2) Once you have full green battery, if you need to slow/stop the car, there is no way to recapture any of the kinetic energy for re-use later. All that energy goes entirely to waste to generate heat to be dissipated into the atmosphere without doing any further work. Therefore you will burn additional gasoline in the future that wouldn't have needed to be burned if you had the recaptured energy available.

<div class='quotetop'>QUOTE(ken1784 @ Jul 31 2007, 09:30 AM) [snapback]487984[/snapback]</div>

You learn something new every day!

<div class='quotetop'>QUOTE(ken1784 @ Jul 31 2007, 09:30 AM) [snapback]487984[/snapback]</div>

I didn't realize that the Prius "starts the ICE" in these 2 situations.

I'm not sure why it would either. That seems so counter-intuitive. Why would the car produce additional energy when it is trying to dissipate energy. It seems it would be better to keep the gas and spark out of the ICE and just let the wheels turn it to slow down the car.

 

<div class='quotetop'>QUOTE(Washington1788 @ Jul 31 2007, 10:51 AM) [snapback]488002[/snapback]</div>

I'm sure he's not suggesting it's "bad," just that it results in fuel being burned when it might not have to. When the battery is "fully" charged ("fully" in quotes because it's still at only ~80% with 8 green bars), it tends to force B mode instead of allowing more regeneration. B mode runs the ICE.

Having said that, avoiding 8 green bars on a long downhill may be difficult or impossible. On my recent trip to Hybridfest, on several occasions coming through the mountains of VA and WV I had 8 green bars. Avoiding them would have required shifting into B anyway (instead of braking) or coasting. But then if I had tried coasting on some of those roads, I might not have survived the trip.

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 31 2007, 10:08 AM) [snapback]488015[/snapback]</div>

Ok, that makes sense. However, the only time when I get a fully green battery is coming down from the mountains (hills here in the east) and usually can coast all the way down without even hitting the brakes -- perfect glide slope!

That car operates even better with a fully green battery...I love it in the rare instances that it happens!

 

<div class='quotetop'>QUOTE(Washington1788 @ Jul 31 2007, 10:14 AM) [snapback]488023[/snapback]</div>

Yes, mountains act like a great big battery added to the car. You burn lots of fuel to store energy in a great big "gravitational potential energy bottle" as your car climbs the mountain. Then later when the car comes down the mountain it converts all that energy into kinetic energy, accelerating the car with much less (zero?) use of gasoline. Unfortunately you end up with way more energy stored up than you can safely use over the distance involved on the way down. Therefore, once you reach your desired velocity, the car starts trying to store up the rest of that gravitational potential energy in the battery. When that is full, the rest of the energy you stored up on the way up all gets thrown into the atmosphere as heat.

If going up and down a mountain is necessary, then the wasted energy is simply the cost of accomplishing your task. On the other hand, if a person can choose between 2 routes to their destination, one that involves climbing and descending a mountain, and another that doesn't. The route that doesn't climb/descend the mountain is far more likely to provide better efficiency per mile (better long term MPG).

Of course if the non-mountain route is significantly longer, it may still burn more gasoline than the mountain route even though it gets better MPG.

 

<div class='quotetop'>QUOTE(Vadercar @ Jul 31 2007, 06:40 AM) [snapback]487921[/snapback]</div>

Everything written below assumes permissive traffic conditions and is a compilation of suggestions I read here and personal experience. I'm in my 3rd consecutive 60MPG tank and my trip to work on similar terrain to work gets me about 75-83mpg... In addition to what's written below I have inflated my tires to 44/44 psi, do not use A/C unless on the highway, take an extra 5 minutes for my route, drive at non rush-hour times and have learned to time traffic lights so as to keep as much MOMENTUM as possible.

The answer is complicated but the most simple rule to follow would be to keep your MPH at or below 40MPH WITHOUT the engine spinning.

To force the engine off while gliding EARLY in your drive you have to be driving below 25mph

To force the engine off at around 5 mins after you started driving, stop at a red light, put your left foot hard on the brake and press the accelerator pedal until the engine starts. Then disengage and allow the engine to shut off. This will help your engine stop while you glide.

Avoid all energy transfer screens (any arrows) unless you need to stop at a red light ahead; if so stop progressively to use the regenerating system the best way.

Don't hesitate to use warp stealth (yellow arrows only) between 30-41mph IF and only IF your further route permits for regeneration of that charge before you reach your destination. Do not use warp stealth at low speeds unless you have a very aggressive downhill ahead of you.

The Prius seems to be aggressively wasting energy when the battery is too charged (between 7.5-8 bars) and will force start the engine if you the charge is too low.

 

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<div class='quotetop'>QUOTE(catgic @ Aug 2 2007, 07:46 PM) [snapback]489149[/snapback]</div>

Thank you for your comments.
Also, thank you for your visiting my site.
http://priusdiy.fc2web.com/
Maybe, the babelfish or google translation site will help you.

Yes, the “1 cc fuel slurping†is very important.
Actually, we, among my SuperMID M-1 mileage meter users, are playing a game, which is "How far we can drive with zero cc of fuel".
http://priusfan.ddo.jp/c-board/c-board.cgi...e=1;id=tinkirok
(sorry in Japanese only)
The current record holder is myself at 26.35 km (16.4 miles) drive with no fuel used.

Ken@Japan

 

<div class='quotetop'>QUOTE(Vadercar @ Jul 31 2007, 06:40 AM) [snapback]487921[/snapback]</div>

I have a feeling your basic question is not being answered. Alot of the former replys give complicated guidance concerning long, steep, downhill driving, ie..Mountains.

If your drive to work includes several basic hills, the technique depends on how steep the hill is and the predominent speed that road should be traveled.

But in any case the emphasis would be on MPG.
If the predominant speed will be less than 41 MPH, do the no arrow "glide"...Max MPG and no ICE.

If the hill is steep enough (forcing you to gain speed) but that lower speed is still a factor, the speed can be modulated with "regen" coasting or even braking... No ICE is running and the battery gets charged and your at max MPG.

If speed is not a factor, and the hill is steep enough, "regen" coasting, is allowing you to build speed, battery charge, and MPG... Best of all worlds until the batter is fully charged. Which shouldn't be happening on just basic hills.
A moderate hill that requires you to maintain a higher speed ( above ) 41 MPH, could be accomphished with "warp stealth " if momentum is to be maintained.
If these technique terms are unfamiliar, read up on them. Its important in getting the most out of the Prius.

 

<div class='quotetop'>QUOTE(ystasino @ Aug 1 2007, 03:41 PM) [snapback]488779[/snapback]</div>

ystasino - thank for the interesting post ... those are impressive numbers to drive to work. Just out of interest what do you get for the return trip and do you use the same route?
Thanks!

 

<div class='quotetop'>QUOTE(Vadercar @ Jul 31 2007, 03:40 AM) [snapback]487921[/snapback]</div>

you guys are killing me!! I loooove and admire your hypermiling obssession. one little nagging thought; there are so many Priuses here in my 7X7 mile city (SF, California) that without an air traffic controller-type oversight, if all the Prius drivers were hypermiling and watching their display screens as vivgilantly as you guys seem to be, the resulting Prius collision gridlock would bring our streets to a standstill!

d

 

<div class='quotetop'>QUOTE(efusco @ Jul 31 2007, 11:23 AM) [snapback]488029[/snapback]</div>

IIRC, At HybridFest a couple of weeks ago, Hobbit mentioned something about MG1 generating a lot of resistance by spinning so fast that it seemed to self-regulate the speed.