What am I doing wrong?

<div class='quotetop'>QUOTE(benandersen @ Feb 10 2007, 02:45 AM) [snapback]387940[/snapback]</div>

The best speed for the best mileage I've observed is between 35 and 40 MPH... not city perse, but surface roads with a speed limit around 40 to 45 MPH, where traffic lights or other things to stop you are few and far between. i've had trips with 20 to 30 minute segments of well over 70MPG.

Acceleration isn't always a bad thing. Getting up to cruising speed briskly will help you in the long run rather than babying the throttle all the time. Jackrabbit starts followed by hard braking (say, from one red light to the next) is BAD. Anticipate stops and lay off the gas when you predict you're going to stop! Brake gradually!

evan's guide is really good. After a while, those techniques become second nature.

Don't get discouraged! After a while you'll figure out how to drive in the city better. Even in really obnoxious stop and go traffic in the middle of a big metro area like NYC, I can still hit in the high 40MPGs.

 

In the city, I usually accelerate up to ~ 35 and let off the gas so nothing's running, then lightly accelerate so the electric kicks in...then I feather the accelerator and can drive for 4-5 miles on electric alone (until I have to stop).

My trips in town usually average around 75 mpg if I can do it right.

 

<div class='quotetop'>QUOTE(conversion02 @ Feb 19 2007, 07:30 PM) [snapback]393086[/snapback]</div>

Wow, your trips must be all down hill (or at least quite flat). I can never get more than about 1.5 miles in EV mode. That's with SOC% as I high as I can get it and still get in EV more. Or are the 2007s that much more efficent that the 2005s.

Dave M.

 

I am by no means an expert yet but I do know that the HV battery in the Prius can discharge (provide energy for acceleration, EV mode, etc.) about three times faster than it can accept juice for charging. When you coast (not touching any pedals) it's already doing some regenerative braking. So there's not a huge amount more regen you can do when you go to step on the brake pedal- meaning that when you do step on the brake pedal, you are almost using the friction brakes at least some.

The ideal is not to need to regeneratively brake, though. While it's neat that you can recover that energy instead of wasting it on old-fashioned friction brakes, the process still has lots of inefficiencies. Think about the whole process: the motor produces power for regen braking; the inverter steps the voltage down and converts it to DC power for the battery; the battery stores it. Then later when you need it, the battery give you that power back, the inverter converts it back to AC and steps up the voltage, and the motor uses it to give you propulsion. Even if every one of those steps in itself is better than 90% efficient (and several aren't) with so many steps along the way, you lose a lot.

I don't focus as much on the absolute speed that I drive as some other posters here. But as I get used to my new Prius I am getting much better about anticipating needs to change speed; ideally then you avoid the need to stop so much at all, reduce how much energy you waste on friction braking, and ideally even minimize how much regenerative braking you need. On the other hand if you are going to have to use the brake pedal to slow down, get on it as early as possible and make a loooong, gradual stop. The battery can absorb a lot of power but it can't take it all in one giant burp. If you stop faster, you will end up using more friction braking because the battery can't accept the juice fast enough.

Incidentally when people talk about gliding they are minimizing regenerative braking. When you're at speed in a Prius and lift off the gas pedal, you'll notice that it starts doing some regeneration immediately. If you reapply the accelerator just a little bit, you can get it into glide mode, where there is nothing happening on the Energy screen at all. Even short of pulse-glide, you'll start to find ways to improve your efficiency that draw on those concepts. If you leave some space in front of you, you anticipate your needs farther in advance, and have more ability to give the car more consistent messages about what you want it to do. If you are on and off the accelerator every 5 seconds, even if it's all-electric, you are losing more energy to those losses I described earlier. Instead of regenerating for 5 seconds, EV accelerating for 5 seconds, regenerating for 5 seconds, etc., as the cars in front of you do this and that, that kind of technique gives you the control over your own speed to maybe just glide the whole time.

When you figure out gliding you will discover that you can go a heck of a long way with no energy input at all. The parasitic drags on a regular car (tires, wind resistance, mechanical drag, etc.) are vastly reduced on a Prius so it just doesn't lose speed very much while gliding. Doing so on a downhill is even more dramatic.

Also in true stop and go traffic there are a few extras to consider to avoid using the ICE when it really isn't necessary. While granny-acceleration usually isn't particularly helpful in a Prius, true stop and go is one place where it can. As you get to know the car better, you'll get a sense of how fast you can accelerate from a stop on battery alone and how fast you can go before the ICE will kick in. If you're in stop and go traffic, most of the time that you go again will peak at 15, 20, 25 mph before you start slowing down again. In these cases, careful acceleration can keep you on battery alone for a very long time. Anticipation of speeding up and slowing down helps- so instead of 0 mph, 25 mph, 0 mph, maybe you can stroll along at a steady 10 mph and avoid all the hurried stops and starts everyone else feels compelled to make. On the other hand if you really are going to need to accelerate beyond what the battery will readily do, then let the ICE do that. The whole point of Hybrid Synergy Drive is to use electrical aids to allow you to avoid using the ICE whenever it's not able to operate in its efficient range. That means light-load situations are best for electric, but it also means that heavier load situations are most efficient with the ICE.

Finally, you'd be surprised how much it takes to get the Prius warmed up sufficiently that it stops running the ICE purely to produce heat for the climate control system. (To prove this, next time you catch it idling while you're at a stop, you've already been driving for a few minutes, and there's no indication on the screen that it's charging the battery, turn off the climate control system and the ICE will almost always stop immediately.) Idling the ICE solely to make heat is a huge sap on fuel economy, and the Prius is so efficient in slow-moving traffic that even idling the ICE nonstop won't warm it up if you're really cranking the heat. I find I need highway speeds on cold mornings before it is warmed up enough to keep the heat going without the ICE. My commute opens up after a few minutes in nasty traffic, so I usually leave the heat off until I reach the highway section of my trip, where the ICE will be running regardless. If your usage patterns don't suit that kind of approach, you may need to consider blocking off the grille in the winter.

I think a few of us find this odd to do to the Prius, but you will notice that drivers of vehicles with other highly efficient engines (diesel) do the same in cold-climate winters. After all, cabin heating in a standard vehicle is just a way of diverting waste heat from the engine to the passenger compartment. In very efficient vehicles, there is less energy wasted and therefore less waste heat. Recent diesel engine designs, even big ones in trucks and tractor-trailers, have gotten so efficient that they have little waste heat available for cabin heating, especially at idle. Those drivers are covering up their grilles just to get better cabin heating, and I don't see why we'd expect the Prius to behave any differently.

 

<div class='quotetop'>QUOTE(wyounger @ Mar 11 2007, 03:29 PM) [snapback]403855[/snapback]</div>

I thought charging and discharging rates are about the same. Do you still have the source of that information?

I believe the discharge rate in Stealth is at 8C and EV mode at 16C. You are saying the re-gen recharge rate is around 5C?

 

I don't think that the actual discharge and regen currents are different. But what he's saying about it taking 3x as long to charge the battery as to discharge it is basically correct due to the way the battery is utilized. For example, I can take off in EV and run down the battery to nothing in short order while drawing 100 amps or whatever the max is. But the only time I'll ever see that kind of power going back into the battery is under really heavy braking. That almost never happens while I'm behind the wheel. (Actually I use the hybrid system pretty much the way wyounger describes, inherently restricting my use of the battery to times of very low power demand.) This is just as well in my mind, as the resistive energy losses must be pretty big vs. a charge rate in the 5-20A range.

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Mar 11 2007, 06:02 PM) [snapback]403885[/snapback]</div>

Hmm... I wish my brain were organized enough to be able to remember and cite the sources of the trivia it carries around.

I will poke around to see if I can run across it again. I have a link on my office PC that I won't have access again until Tuesday.

 

This isn't where I read it the first time, but NREL has characterized the batteries pretty well. Sounds like my factor three was way off, but the charge rate is still lower than the discharge rate.

Edit: OK, this is the NHW11 battery, but the idea still carries over.

http://www.nrel.gov/vehiclesandfuels/energ...002_01_1962.pdf

 

More info on NHW11- they state the max discharge rate at 80 amps and charge rate at 50 amps.

http://www.cleangreencar.co.nz/page/prius-technical-info

 

I'm just a bounty of links today. See this one:
http://www.vassfamily.net/ToyotaPrius/CAN/brindex.html

This at least reinforces that gentle braking seems to result in more energy recovered than heavier braking.