Power flow question

[Highly ImPriused]

I frequently notice that when I am maintaining highway speed (50 - 60 mph with or without cruise) and the SOC is close to full, the display shows the ICE providing power to the wheels as well as to the the electric motor back to the battery. Why would it do this when the battery is already near full (all blue bars or even 1 green bar)? Wouldn't it be more efficient to have the battery helping to provide power or even not involved in these instances? My apologies if this is covered elsewhere, but I didn't seeing anything that directly addresses this after a little searching around.

[Ichabod]

Sometimes the power transfer is minimal even though it shows arrows to/from the battery. If you're cruising along at 55 and you play around with the pedal a little, push it harder, let up a little, you'll notice that it very easily switches between power to and from the battery. It's not a big deal.

Ideally you want to stay off the battery almost all the time, other than accelerating from a stop or coasting/braking to a stop, but in the real world, you'd spend too much time looking at the MFD to achieve that safely.

The best/safest highway driving technique is to accelerate normally to your desired speed, then let off the pedal gently until you slow down a little. Then just keep your foot on the pedal lightly enough to maintain speed. Hills will make that a little challenging, but you can do better than the cruise control at maintaining high FE if you're careful.

[Tideland Prius]

The problem with the arrows is that they don't show how *much* is flowing. So whether it's a trickle or the maximum, it's the same arrow.

[qbee42]

Power from the ICE is transfered to the wheels directly from the ICE and also through MG1 and MG2. Do a search on the PSD (power splitting device) if you want a lot of detail, but essentially what happens is that torque is always split through the PSD at a fixed ratio. To get what amounts to different gears with the PSD, the Prius has to send some of the power through MG1 to MG2, where it is converted back to torque. The MFD only shows one motor, instead of both MG1 and MG2, so you can't see all of the detail.

Tom

[Bill Merchant]

Another, rather major, point is that the ICE MUST run above 42 mph. Please search for details of the PSD. You might look at this thread on Prius Power Flow.

[Highly ImPriused]

<div class='quotetop'>QUOTE(Bill Merchant @ Apr 6 2007, 03:32 AM) [snapback]418672[/snapback]</div>

Another, rather major, point is that the ICE MUST run above 42 mph. Please search for details of the PSD. You might look at this thread on Prius Power Flow.
[/b]

I am well aware that the ICE always runs above 42 mph and I had previously read the thread you linked to prior to posting. I don't see how it directly addresses this question, but perhaps I missed something? I am also aware of the power split device and the different functions of MG1 and MG2. I understand the power flow going from the ICE to the electric motor(s). What I didn't understand is why power would be going back toward the battery when it's already near full; particularly when I can prevent this from happening by slightly altering the pressure on the accelerator pedal. Basically what I'm wondering is why doesn't Prius prevent this on it's own since it seems to be an innefficient way to operate. And I think the best answer given above is that its most likely very little power going that way and nothing to worry about. Thanks all.

[ekpolk]

I’ll take a stab at this one. I’m going to theorize that operating temps in your ICE may be resulting in more ICE emphasis from the ECU than you experience in hotter weather. I just installed a V3 CAN View in my 04 Prius several days ago, and I’ve been using it to investigate for myself the apparent inconsistencies in when the ICE runs and when it does not. I suppose I should not call my observations “inconsistencies” as I’m sure the car is doing exactly what it’s programmed to do. I just don’t understand all the nuances yet.

Anyway, on thing that has leapt out off the CAN screen at me is how quickly the ICE starts cooling off once you drop into stealth mode. Note – I’ve experimented with grille masking, but I’m unmasked for now as I try to learn all the temps as they happen with the car in “stock” configuration. In city traffic (I work in New Orleans and stay here during the week) while commuting, with the ICE running, I’m seeing coolant temps between 180 and 190F. Once I go stealth, or if I’m having only brief ICE runs, the temps quickly drop into the 170s. Once into the low 170s or less, the car is much more reluctant to go stealth.

I theorize that the ECU works hard to keep the parts at proper operating temps, (for both overall economy and emissions reasons) and it probably continues that effort, irrespective of whether the ICE is actually running or not. So, if you’re driving along, even with the ICE on, and you’ve got a large blast of cold air coming in the upper and lower grilles, your ECU may be emphasizing ICE operation to keep everything hot.

Have you tried experimenting with grille masking to see if it makes a difference for you?

[Highly ImPriused]

Thanks for the tips ekpolk. I don't have a problem with the ICE running at highway speeds, I expect that to be the case. I just expected all of its power would be going toward the wheels, both directly and through the electric motors, but not to the already charged battery.

I haven't tried grill blocking yet as I've had the vechile for just 1 week thus far. Perhaps for next winter I'll give it a try. And maybe an EB heater as well.

[bwilson4web]

Hi,

<div class='quotetop'>QUOTE(Highly ImPriused @ Apr 6 2007, 09:44 AM) [snapback]418750[/snapback]</div>

. . . What I didn't understand is why power would be going back toward the battery when it's already near full; particularly when I can prevent this from happening by slightly altering the pressure on the accelerator pedal. Basically what I'm wondering is why doesn't Prius prevent this on it's own since it seems to be an innefficient way to operate. And I think the best answer given above is that its most likely very little power going that way and nothing to worry about. Thanks all.
[/b]

One of the other posts pointed out there are no "units" on the arrows. With my NHW11, a couple of informal spot checks showed that as little as 1 A is enough to 'raise the arrow.' I don't think anyone has really quantified the minimum thresholds for the "ON/OFF" indicators. So in my case, I use a Graham miniscanner. Others are using different scanner or instrumentation techniques. Just treat the MFD energy displays as 'eye candy', pretty to look at but hiding the real details.

Are you interested in some serious instrumentation? I understand Ken has a very nice scanner that works with NHW20s. This would give you engineering units.

Bob Wilson

[Highly ImPriused]

Thanks, it's reasurring to know just how sensitive those power flow arrows are.

<div class='quotetop'>QUOTE(bwilson4web @ Apr 6 2007, 12:53 PM) [snapback]418831[/snapback]</div>

Are you interested in some serious instrumentation? I understand Ken has a very nice scanner that works with NHW20s. This would give you engineering units.
[/b]

Not at this point. I might get a scan gauge sometime this year, but that's about it for now. Most of the more detailed stuff is a bit beyond me without putting some effort into boning up on the basics. Sad to say, seeing as I have an engineering degree, but I haven't looked much at thermodynamics, circuits and the like since I was in school 15 years ago.