Why does Prius run the ICE at low mpg?

Discussion in 'Gen 2 Prius Technical Discussion' started by ukr2, Feb 14, 2007.

  1. ukr2

    ukr2 Senior Member

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    After Driving in slow traffic with the engine up to temp, I noticed after releasing the brake without pressing the accelerator, the Electric Motor, MG, would initially move the car forward, then when at only 5 mph the ICE would take over at 2 - 20 mpg. WHY??

    Battery was High Blue. Why doesn't the MG power the car whenever the ICE can only achieve low mpg?

    Do we have someone in the Forum that has knowledge of the computer software logic?
    We have many with alot of driving experience, but I need a software guy.
     
  2. efusco

    efusco Moderator Emeritus
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    There are a number of reasons, but if the car 'determines' that you're going to continue to accelerate it will kick in the ICE b/c the ice is ultimately more fuel efficient than the battery. The trick to understanding this is that all the energy in the battery ultimately comes from gas...if you must convert that mechanical energy, to electrical energy, store it in the battery then convert it back to mechanical energy again you will suffer losses.

    So, the car runs the ICE to save gas and energy...as illogical as that seems on the surface.

    Now, this can work against you in conditions where you're not going to continue to accelerate up to cruising speed. That is b/c additional battery energy is required to spin the ICE up (that happens during that first 5mph while you're on battery only), the initial fuel burn is relatively high, then if you slow down immediately again (stop and go traffic type situation). In those cases it's nice to have an EV button to override the default programming and say..."hey, I'm not really going anywhere yet!"
     
  3. ukr2

    ukr2 Senior Member

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    Evan,

    Thanks for the Reply.

    The ICE is more efficient at a higher speed to get better MPG. It would be best to use the MG at the lower speed when there is Blue/Green Battery.

    Without the US-unsupported EV Button, why doesn't the Computer use the MG in this situation until we have a low battery?

    Then there wouldn't be a need for the EV Button !!!
     
  4. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(ukr2 @ Feb 14 2007, 08:59 PM) [snapback]390627[/snapback]</div>
    High speed or low speed the ICE is more efficient and it would NOT be best to use the MG at low speed regardless of the battery SOC....that said, when you're in the green range the system will preferentially use the battery and you'll find you can accelerate more agressively and for a longer time with a green battery than with a blue...not a lot, but noticably more.



    The EV button is US-supported, it's just not equipt with it from the factory. Again, the programmers had to decide if it would make the ICE kick in sooner or later since it can't tell whether you're in stop-and-go traffic or if you're taking off for good. The compromise was for the power and efficiency of running the ICE even at low speed.
     
  5. hobbit

    hobbit Senior Member

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    It's all demand-based. Hopefully this is enough of the software
    picture for you: the hybrid system always seeks a 60% SoC, unless
    it is being called upon to handle other conditions such as electric-
    only propulsion or absorbing energy via regen. Whether or not the
    engine comes on is all calculated via balancing measured SoC versus
    where the driver's foot is. Lower SoC, lower accel-pedal threshold
    for when the engine is started. That, in fact, is one of the simpler
    things that the software does!
    .
    _H*
     
  6. ukr2

    ukr2 Senior Member

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    Eric,

    I'm confused.

    If the EV button is to override the computer to use the MG at low speeds, why do you use it if it's more efficient to use the ICE?

    The Chevy VOLT, concept vehicle at the Auto Show, uses MG to drive the car and ICE only to charge the battery. They are using the ICE at its efficient higher speed when needed.
     
  7. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(ukr2 @ Feb 14 2007, 09:23 PM) [snapback]390648[/snapback]</div>
    It's Evan.

    That is not the only thing that the EV button is for, and that is not how I typically use it. In fact, I only use it in low speed driving conditions such as stop and go traffic...when I know I'm only going to travel 75 feet and stop b/c I won't make it through a light and conditions such as that. In those cases it can be more efficient b/c of the very short distance traveled...you balance it out b/c you don't pay the 'cost' of spinning up the ICE from the battery and thus burn no fuel.



    The Chevy volt is a different situation b/c it's primary energy source is not the ICE...it's electricity from the Grid. The MG is very efficient...the where you lose efficiency is when you have to start converting from ICE to battery and back to MG again. In those cases the Volt will be quite inefficient as you'll essentially be powering the car via a 1liter ICE and all the inherent losses as the power goes through all the conversions.
     
  8. Bill Merchant

    Bill Merchant absit invidia

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    One other small point, ukr2, is that the ICE always runs as efficiently as possible. The HSD let's the ICE run at an appropriate RPM; energy not needed for propulsion, either directly or by spinning one motor as a generator to provide electicity for the other motor, is sent to the HV battery. The ICE never lugs at low speed as it might in a conventional car.

    And sometimes the ICE will start just to warm itself and the catalytic converter. You might want to think of the Prius as first and foremost a low emissions car. It accomplishes this by producing the fuel efficiency most people associate with the Prius.
     
  9. donee

    donee New Member

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    Hi All,

    In the summer, at low speeds, the Prius will use the battery in the slow stop-and-go traffic to a much accelleration. In the winter, while the engine may be warmed up, that does not mean the battery is. In the recent heavy snow commuting, I found the car would hang in electric mode readily toward the end of the trip. Technically, I think this is due to the available power from the battery versus temp. The battery ECU may report to the hybrid ECU the available battery power, estimated from battery temp and SOC. If demand exceeds battery power, then the engine is assigned the bulk of the power duty. Of course, at lower temps, the power required to flex the cold rubber in the tires is allot more too. The hybrid system can directly measure the needed wheel torque by the MG2 current. So, if the MG2 current exceeds what the battery can deliver, the engine comes on, supplying the missing current, and or reducing the amount of current required.

    Additionally, in cold weather there is the 35 mph coast engine shut-off routine. Once the engine is running, it wants to keep running until the car is at low power greater than 35 mph operation. Then it will shut off, and small accelleration will bring the battery on-line. One can access this, by lifting throttle for a few seconds when one is above 35 mph, and then bringing the throttle back up slowly. At which point one can cruise along in electric down at speeds below 35 mph if traffic conditions dictate. Once the temp is up above 50 F (?) or so, this changes too, and the car will drop the engine quickly. It might be sensing engine coolant temp to do this, or just enviormental temp. Not sure.
     
  10. Earthling

    Earthling New Member

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    I have noticed recently that my Prius is far more likely to shut the ICE down at stops if I have the climate control turned off. I live in town, and when I make short trips, I will keep the climate control off as long as it isn't brutally cold in the car.

    Harry
     
  11. D1CK1E

    D1CK1E New Member

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    So... how do I get one of these EV buttons? =)
     
  12. ukr2

    ukr2 Senior Member

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    Dickie,

    Allannde today posted this on the Prius Modifications Forum -
    ----------------------------------------------------------------------------

    I installed mine in less than an hour using these instructions at a cost of less than $5.00.

    http://www.calcars.org/prius-evbutton-install.pdf

    It is not rocket science. It is important not to be sloppy and to follow the instructions.

    There are several types. You need to pick one whether you do it or have it done.

    There is the one from Coastal which is activated by the Cruise Control lever
    There is the High Beam Flasher type which is a DIY job (see darelldd's site) (I think it is evnut.com)
    There is the Momentary Switch type which is also DIY and involves no touching of the wiring of the car.
    There is the OEM (factory) switch type which is available from Sigma which looks stock but is harder. This is also the most expensive

    Do a search on the site. There is lots of information.
    -----------------------------------------------------

    Most posters mention these options.
     
  13. darelldd

    darelldd Prius is our Gas Guzzler

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    <div class='quotetop'>QUOTE(efusco @ Feb 14 2007, 07:04 PM) [snapback]390631[/snapback]</div>
    Evan -

    I know that you know this - I just wanted to point out to others that it is only in this situation that the ICE is more efficient than using the battery power (only in the situation where the ICE has to recharge the batteries). Taking this one line out of context can give folks the wrong impression. An electric motor is quite a bit more efficient than any gasoline ICE can ever hope to be. The problem here, of course, is that the PRIUS requires gasoline to run the EV part of the program. :(
     
  14. ukr2

    ukr2 Senior Member

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    Darell,

    My normal commute includes leaving the house and going through town under 30 mph for a mile (when I could use EV), then 50 to 65 mph traffic where I get a lot of Regen, then 30 mph for the last mile (when I could use EV again).

    Do you or anyone feel that if the EV can be used, to use it as long as Regen during and after High Speeds will recharge the batteries?
     
  15. ken1784

    ken1784 SuperMID designer

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  16. darelldd

    darelldd Prius is our Gas Guzzler

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    ukr2-

    Every driving situation is unique. If you can use battery power for traction, and then recharge that battery mostly through regen, then you're doing GREAT. If, in stead, the battery is rechared through extra load on the ICE, then you are losing ground.

    The best solution is to toss the gas engine altogether and get into a real EV... You know - once they're for sale again. :)
     
  17. kevinwhite

    kevinwhite Active Member

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    <div class='quotetop'>QUOTE(Bill Merchant @ Feb 14 2007, 10:48 PM) [snapback]390729[/snapback]</div>


    Actually the Prius does its best to "lug" the engine at low speed.

    It even uses the energy recirculate mode (described in another post by Bob Wilson) to intentionally lower the engine speed when the engine is producing too much power for a given road-speed. The Prius normally runs the ICE at almost full throttle whenever it is above 1200RPM and controls engine output by changing the engine speed - not by changing the throttle position.

    kevin
     
  18. bwilson4web

    bwilson4web BMW i3 and Model 3

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    <div class='quotetop'>QUOTE(kevin17 @ Mar 5 2007, 07:56 PM) [snapback]400633[/snapback]</div>
    This is a brilliant way to describe ICE power management. Ordinarily, this approach to engine power management would be unstable. As the ICE rpm increased, the power would increase and the rpm increase even faster. But the ability to moderate MG1 torque to keep the rpm at a 'commanded speed' prevents power run-away, speed up or stalling from too little rpm.

    Thanks,
    Bob Wilson
     
  19. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    actually the EV switch is handy for a lot of things. case in point (doesnt apply on the shift ive had for nearly two years but in the old days...

    i work at a place that employees about 400 people in a complex that employees 3,000 people for a dozen different agencies. we have 75 parking spaces in the lot next to the building and two overflow parking areas that are either 150 or about 500 feet away. now i am into exercise, walking and all that crap...but in Western Wa where rain is frequent, not to mention an average of 3 trips a day out to my car for something or another, parking next to the building is a highly sought after position. considering the 50 different shifts and start times, the parking lot constantly has traffic coming and going.

    i used to work later in the morning and was forced to park in the overflow lot. i would move next to the building at my earliest convenience and EV mode was perfect for the 500 ft trip.
     
  20. alexstarfire

    alexstarfire New Member

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    It's too bad that I can't get the EV button on my '02 Prius. There are plenty of times where my ICE is running and it's not really doing anything useful, ie running but not using any power. It'd be a real help during the winter when I could just shut off the ICE at stoplights. It would still help during the summer, but not as much since the warm-up period is shorter.

    BTW, since blocking the grill helps decrease the warm-up period, why don't they offer some type of feature like that on every car? Something along the lines of blocking the grill, or radiator, until everything warms up enough, then press a button, or have it done automatically, that removes the blockage so everything runs like it should. Wouldn't that, in effect, increase the MPG on pretty much every car that is driven today?