Summary of some of Volt's SOCs thresholds and kWh (and MM/HOLD mode)
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Thread: Summary of some of Volt's SOCs thresholds and kWh (and MM/HOLD mode)

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    Default Summary of some of Volt's SOCs thresholds and kWh (and MM/HOLD mode)

    Re: Summary of some of Volt's SOCs thresholds and kWh (and MM/HOLD mode)
    via HOLD-Feature-Question thread.

    http://gm-volt.com/forum/showthread....618#post138618

    For those of you that have around for any length of time you know that "When [saghost/Walter] talks, people [should] listen" (80's EF Hutton line). This post was on of his best and is a really nice semi-technical summary of the Volt that I think many new/old owner or lurkers would enjoy ... thus a separate thread.

    What-do-all-the-acronyms-mean (click me)
    Quote Originally Posted by saghost View Post
    The Volt always uses its battery as a power sump, in every mode. In higher speed CS mode (greater than 60ish,) it will nearly zero the battery current and run in a very prius-like CVT mode.

    In CS mode, the Volt strives to maintain a ~22% absolute state of charge - depending on the speed it is going and the power requested of the engine, it may be in any of the four basic operating modes (1 and 2 motor CD/electric, series or parallel hybrid CS mode on the ICE.)

    The car uses a ~400 Wh working window once it hits the target SoC. Once the battery falls out of the bottom of this, the car strives to generate as much power from the engine as you are using, or more (the only time it can't manage this for a sustained period is climbing steep mountains at high speeds. In that case, the SoC will fall to ~15% at which point the car will set PPR (Propulsion Power Reduced) and keep trying to catch up - that's the one case where the car doesn't have the full MG B power available.))

    When the car reaches the top end of this window, if it can't slow the engine down enough to get the power produced under the load required (the slowest it runs is ~1400 rpm, and it never throttles back in normal operation,) it will shut the engine down. Steady state highway travel below 60 or so is like this - it'll run the engine for several miles, charging the battery the whole time, and then hit the top of its cycle window and shut the engine down and run on battery for a mile or two.

    (Note in passing that the car is much more sensitive to power requirements than it is to SoC within the window - if you floor it in this situation, the engine will come on most places in the window, and if you get completely off the gas in L anywhere in this window it shuts the engine off.)

    With this background in the car's normal MO, what mountain mode does is move this target from ~22% to ~45%. Otherwise, the car does everything the same, but this change has a few implications... The obvious one that GM intended is that instead of ~1kWh of reserve for accelerating and little hills, the car now has ~4.5kWh to burn on steep mountain grades - said to be enough to handle any freeway hill in the US at 70 mph. If you put it into mountain mode from the beginning, that's really the only change - the engine comes on earlier, and holds a higher SoC.

    However, there are other options... If you put it in mountain mode with the battery drained, it immediately brings the engine on and leaves it on - that far below the target window it'll keep the engine running during full regeneration. Left sitting stationary, it'll run the engine at ~1700 rpm for ~5 minutes to build the 3.5kWh difference. If you're driving at the time, it will push the engine to higher RPMs to hold onto the rate of charge. There has been a lot of speculation that this can give you the benefits of pushing the car into a more efficient engine window (higher bsfc, - similar to Prius Pulse and Glide but without the annoying speeding up and slowing down,) with a few tests with mixed results. We don't have a bmep/bsfc chart we trust (there's one from ecomodder you'll find a couple places here, but a few folks have had experiences that don't quite match up. More to come as we unwind the story...)

    Finally, with all of that background, your question... With the caveat that we don't have much experience with it because no NA Volt has it and the European cars are only starting to reach customers, what we believe Hold mode does is switch the car immediately into CS mode, and set the battery charge target to whatever the current battery level is (not sure if that's bottom of window, middle, or top of window, but I doubt the 400Wh difference will matter to many.)

    This is different from Mountain mode in two ways... You can enter Hold at anytime and start the CS/engine on phase, including from your driveway with a full battery, and Hold can never command the forced-charging/pulse and glide aspect that Mountain mode potentially can.

    For myself, I use Mountain Mode on most long trips for two reasons. First, if there's a stop and go portion of the trip, it lets me drop into electric for that part (I like driving on electric much better at low speeds,) and Second, it provides a ~14 mile buffer if anything were to go wrong (from running out of gas to some mechanical fault. The car actually uses the other 7% down to 15% SoC in that situation, too, so the buffer is really ~18 miles.)
    Last edited by scottf200; 04-12-2012 at 01:54 AM.

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    Default What-s-the-most-efficient-highway-speed

    http://gm-volt.com/forum/showthread....653#post128653


    Quote Originally Posted by saghost View Post
    I don't think we have a solid answer for the Volt. With most cars, the most efficient speed is the slowest you can sustain in the top gear. Tesla concluded that the most efficient speed to drive the Roadster at is 17 mph - which seems to be a fairly common range for electrics.

    The Volt changes strategies right around 62. Below this it spends a lot of time in virtual 6th gear - MG B (the larger main electric motor) more or less stopped, with the engine driving the car directly at 3.1:1 through the planetary system (thus virtual 6th gear - it's in the same range as most car's 6th gear) and whatever excess engine power there is (always at WOT, remember) pulled out on MG A and sent to the battery. When it builds up a few hundred watt hours, it shuts the engine down and runs electrically for a mile or two.

    Above this range, it'll run the engine continuously, at lower RPMs (it never runs slower than ~1400 RPM) in a Prius style CVT relationship. I'm not 100% certain, but I think the reason for this change is that at 1400 RPM it is generating too much power for the steady state road loads below 62 mph.

    My original assumption was that the most efficient speed in CS would be the slowest that the car goes parallel, since series is 10-15% less efficient. This may turn out to be the case, but it turns out this isn't "highway speed" - it's about 35 mph.

    Also note that because it's all power traded on the high voltage bus, both A/C and especially Heat have an outsized impact on economy (even after the engine starts, the car will run the electric heater under some circumstances if it's set to comfort.)

    So do a little experimenting, but my overall advice is the best economy is as slow as you are comfortable going...

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    Quote Originally Posted by saghost View Post
    In CS mode, the Volt strives to maintain a ~22% absolute state of charge - depending on the speed it is going and the power requested of the engine, it may be in any of the four basic operating modes (1 and 2 motor CD/electric, series or parallel hybrid CS mode on the ICE.)

    The car uses a ~400 Wh working window once it hits the target SoC. Once the battery falls out of the bottom of this, the car strives to generate as much power from the engine as you are using, or more (the only time it can't manage this for a sustained period is climbing steep mountains at high speeds. In that case, the SoC will fall to ~15% at which point the car will set PPR (Propulsion Power Reduced)...For myself, I use Mountain Mode on most long trips for two reasons. First, if there's a stop and go portion of the trip, it lets me drop into electric for that part (I like driving on electric much better at low speeds,) and Second, it provides a ~14 mile buffer if anything were to go wrong (from running out of gas to some mechanical fault. The car actually uses the other 7% down to 15% SoC in that situation, too, so the buffer is really ~18 miles.)
    [/URL]
    This brings to mind a feature many have brought as long ago as 2007, "Almost home mode". A couple of times already I've had the engine kick in when I was a couple of miles from home. If there is a ~1kwh window reserved for hills, why can't 1/2 of this be tapped for those rare occurrences when we are almost home. I'm sure GM is concerned this will get abused and people that drive outside the CS daily will use it regularly but how is that different from those traveling in hilly country regularly? At least "almost home" would only be used once per charge and only 1/2 the depth. C'mon GM quit protecting us and discriminating against the flatlanders. We want some extra bene's too.
    Koz

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    This brings to mind a feature many have brought as long ago as 2007, "Almost home mode". A couple of times already I've had the engine kick in when I was a couple of miles from home. If there is a ~1kwh window reserved for hills, why can't 1/2 of this be tapped for those rare occurrences when we are almost home.
    Agreed. Like those times (and it has happened more than once) that you are pulling into the garage and the engine starts. Ugghh!

    Excellent information from Walter, as usual. Thanks!
    Last edited by SolarVolt; 04-12-2012 at 09:40 AM.
    Silver Volt VIN: C03305, Nov. 2011

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    Another good tech post.
    Quote Originally Posted by saghost View Post
    PPR sets in two conditions that I know of...

    The most common one is when climbing a steep hill at speed in normal mode. This often requires more power than the the engine can provide, and the car responds by digging into its ~1kWh buffer (22% to 15% SoC) that it uses to streamline CS Mode. If the load continues, the car sets PPR when it hits 15% SoC, and reduces the power output to hold the battery floor.

    The other time it sets PPR is when it decides the engine is unavailable, whether out of gas or faulted. This one will occur at any battery SoC.

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    Thx for posting this Scott and (Walter),
    Every time I think I should buy a dashdaq I find out that walter has already figured it out.
    I will study these threads tomorrow more in detail.
    2012 Silver Ice Volt w/ leather and polished aluminum wheels
    lease over in May 2015
    new vehicle TBD

    2012 Gray Model S

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    Walter quote:
    "When the car reaches the top end of this window, if it can't slow the engine down enough to get the power produced under the load required (the slowest it runs is ~1400 rpm, and it never throttles back in normal operation,) it will shut the engine down. Steady state highway travel below 60 or so is like this - it'll run the engine for several miles, charging the battery the whole time, and then hit the top of its cycle window and shut the engine down and run on battery for a mile or two."
    Lets assume we are in this mode you are talking about here with the engine cycling on and off at 50 -55MPH in CS mode.

    Question:
    How many KW is the generator putting out when it is charging the battery back up prior to shutting down for a few miles?

    Thx,
    GSB
    Last edited by scottf200; 06-23-2012 at 02:03 PM. Reason: quote clean up
    2012 Silver Ice Volt w/ leather and polished aluminum wheels
    lease over in May 2015
    new vehicle TBD

    2012 Gray Model S

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    Quote Originally Posted by George S. Bower View Post
    Lets assume we are in this mode you are talking about here with the engine cycling on and off at 50 -55MPH in CS mode.

    Question:
    How many KW is the generator putting out when it is charging the battery back up prior to shutting down for a few miles?

    Thx,
    GSB
    I don't have a log of those conditions handy, and I haven't made a run to specifically collect your data. I'm also not 100% confident in my calculated engine kW display (reported engine torque times reported RPM divided by the appropriate constant, but it often seems slightly out of whack with the battery and MG kW reports - might be off by 10-15%, most likely low if it is off.) With those caveats, this is roughly what it looks like in steady state from my recollection...

    As soon as the car starts the (already warm) engine, it goes into series and straight to virtual 6th gear (so ~22-2400 rpm in your speed range, with MG B more or less stopped - less than 100 rpm forward or backwards,) with the engine putting out 20-25 kW, and somewhere around 10-15 of it pulled out on MG A.

    It'll continue this way for something like 2/3 of the charge window (250wh @ 10 kW is about a minute and a half,) and then slow the engine in HSD fashion, floating gradually down to the 1400 rpm base speed and 12-14 kW for the remainder of the window (significant power use on MG B now, but MG A is still pulling more than MG B is using. In the 55 range, the battery is probably gaining around 2kW now.) Aside from virtual 6th gear and 1400 rpm, the speed I see most often is around 1700 rpm, it'll probably spend some time there on the way down.

    After it spends some time at 1400 rpm (maybe 30 seconds?) and the battery floats over the top of the window, it'll go electric in the manner described in the earlier posts for one and a half to two miles in this speed range, then repeat the cycle.
    Walter
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    These are great discussions. I've asked ChrisC to add them to his FAQ.

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    Quote Originally Posted by saghost View Post

    As soon as the car starts the (already warm) engine, it goes into series and straight to virtual 6th gear (so ~22-2400 rpm in your speed range, with MG B more or less stopped - less than 100 rpm forward or backwards,) with the engine putting out 20-25 kW, and somewhere around 10-15 of it pulled out on MG A.



    .
    Thx Walter,

    Using the road loads, PG set eqns and engine maps I have for the Volt I get this solution. It seems to jive up pretty well with what you said.
    Let me know if you think this is a reasonable resemblance to what you saw on Dashdaq
    2012 Silver Ice Volt w/ leather and polished aluminum wheels
    lease over in May 2015
    new vehicle TBD

    2012 Gray Model S

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