Accelerating from a red light

Discussion in 'Gen 3 Prius Fuel Economy' started by markabele, Jun 6, 2012.

  1. markabele

    markabele owner of PiP, then Leaf, then Model 3

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    Let's say the light just turns green and you have a long stretch of road in front of you with a speed limit of 40.

    Now some people (including Wayne) say to use just ev power to get up to about 15-18 mph then use the ICE after that point.

    I'm sure it is probably right, but I just need it properly explained to me. Because to me it seems like you always want to use the ICE whenever there is much of a load at all on the vehicle. Accelerating from stop definitely seems like it would be quite a load on the battery.

    Thanks in advance for the help and good discussion. ;)
     
  2. fuzzy1

    fuzzy1 Senior Member

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    I don't understand the issues well enough to give a full answer, or any crossover speeds, but can offer some insight.

    Energy from the battery involves more conversion losses than energy directly from the ICE, so for sustained high power loads we generally prefer to the ICE. However, the fixed overhead power loss of operating the ICE is much higher than that of electric propulsion, so for low power loads, electric may be more efficient.

    Looking at a BSFC (brake specific fuel consumption) chart for the Prius engine, it shows that the ICE does not reach its efficient operating zone until it gets past the vertical load load on the left, and into the flat load line on top. This efficient zone starts somewhere in the vicinity of 10 horsepower (from memory, I don't have the chart in front of me at the moment). (Traditional nonhybrid engines are much worse for this than Prius and Honda hybrid engines.)

    Also, energy transfer from the engine to the wheels is split between two paths, one mechanical and one electrical. The power in the mechanical path is really just torque times RPM of the shaft coming out of the PSD (NOT the ICE shaft), which is proportional to the wheel speed. At low speed, this means little power goes through this path, so most of the ICE power therefore is going through the less efficient electrical path. The overall PSD efficiency gets better at higher speeds when more of the power takes the mechanical path, and peaks when MG1 RPM is zero.

    The real mechanical power for acceleration is proportional to speed. Thus, from a standing start, this power is fairly low at low speeds and climbs with the speedometer.

    So I believe that, while battery propulsion always involves significant conversion losses, the initial acceleration takes so little power that it isn't worthwhile to turn on the ICE and its relatively large and unavoidable overhead loss just to make it spin.

    However, I don't believe startup leaving a stop light is the first thing to work on, this can be left as a slightly more advanced topic. The approach to and stop at the stop light offers more opportunity for budding hypermilers to save energy.
     
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  3. ProximalSuns

    ProximalSuns Senior Member

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    Make sure I'm on PWR, smoke the BMW in back of me up to 45 and then watch him play catch up and tail gate me for 10 miles. Pulse and glide at it's best. I do pretty good too...average 54 mpg.
     
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  4. edwardob

    edwardob Member

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    thanks for that reminder
    i use amps on my scanguage..and i notice more "conversion "is taking place at lower engine/mph speeds and as we go faster nearly all mechanical is taking place [state of charge battery changes this slightly]

    so your saying electrical motor is more efficient to use ,,so it is best to use it at the lower speeds until we get up to speed

    i do remember reading somewhere some hypers wait till 15 mph to change over to mechanical power??

    most studies i seen have been on the ice and not on electrical motor
    i fact use of electrical motor has been discouraged

    but the conversion loss may still be more efficient than the ice under certain circumstances

    now if i could just figure out how to use the electrical motor the correct way????
    any one can add to this please???
     
  5. john1701a

    john1701a Prius Guru

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    Engine only is inefficient, hence the hybrid approach. The addition of an electric motor allows the engine to run in a less wasteful manor. That's what you want.

    Using the electric motor exclusively consumes your limited supply of electricity quickly. On the Eco-Meter, you can see that. The energy-bar is much larger for acceleration than it is for simply maintaining a steady speed. That's why you want to share the burden with the engine during times of high demand.

    Also, keep in mind that there's a second electric motor. While the engine is running to provide thrust, it will use that to generate electricity at the same time. This allows the engine to run at a more consistent RPM, which is more efficient. This recharges the battery-pack along the way too.

    Intentionally using the engine is counter-intuative. New owners tend to fight it, not realizing they are preventing the system from doing what it designed to do... causing lower MPG. Enjoy electric-only when not much power is needed. The electricity is better used that way. When you do need power, don't be afraid to let the engine start. It shuts off quickly afterward anyway.
     
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  6. edwardob

    edwardob Member

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    when not much power needed..means when it is inefficent to use ice????
     
  7. john1701a

    john1701a Prius Guru

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    Correct. And interestingly, that applies to the plug-in model as well. You have more kW & kWh available, but there's still just a finite amount. So, you want to consider when to consume it.

    Typically, when lots of power is needed, take advantage of the engine.
     
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  8. edwardob

    edwardob Member

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    in practical terms when we get too near midpoint on hsi say ...to right 1/4 inch or so
    change over to electric
    and on the electric side when say 1/4 inch near to midpoint change to ice
     
  9. markabele

    markabele owner of PiP, then Leaf, then Model 3

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    Thanks to all who have contributed so far...great stuff ITT.
     
  10. catgic

    catgic Mastr & Commandr Hybrid Guru

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    Markabele - If you are running your 2012 with only Factory OEM Instrumentation, the best fuel economy technique strategy for acceleration is to use the "½" Value Rule-Of-Thumb For Acceleration. That is, if you are accelerating from a dead stop to your stated desired 40 MPH steady state speed, do so by depressing the foot throttle to have iMPG = iMPH/2 displayed on the MID.
     
  11. edwardob

    edwardob Member

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    you mean ie..... impg=50 then speed should be 100mph....impg=20 then speed should be 40mph

    not making sense..or have i picked that up wrong?????
     
  12. Delux

    Delux Junior Member

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    I'm so confused by all this haha
    I need to go over to the glossary post and figure out what all abbreviations mean
     
  13. ProximalSuns

    ProximalSuns Senior Member

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    MPG = miles per gallon
    PWR button = the Power Mode button on the dash. "EV" "ECO" and "PWR" modes of operation.
    Smoke = get away from the light twice as fast as the guy next to you or in back of you.
     
  14. Delux

    Delux Junior Member

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    thanks haha
    I had to look up ICE, iMPH (not in the glossary), PSD & MG1
     
  15. MJFrog

    MJFrog Active Member

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    Yes, you got it right, but you're doing it backwards

    MPH = 40, then iMPG target is >= 20; MPH = 50 then iMPG is >= 25... until you finish accelerating.
     
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  16. edwardob

    edwardob Member

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    should work the same with kilometers??right
     
  17. kalome

    kalome Member

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    quote="Delux, post: 1573768, member: 92363"]I'm so confused by all this haha
    I need to go over to the glossary post and figure out what all abbreviations mean[/quote]

    I'm confused too.
    He said " If you are running your 2012 with only Factory OEM Instrumentation.....etc"
    I assume he means instant miles per gallon. I have a 2012 and it does not have this feature.
    If it does, can someone tell me how to get to the instant miles per gallon on my 2012?
     
  18. Delux

    Delux Junior Member

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    I'm confused too.
    He said " If you are running your 2012 with only Factory OEM Instrumentation.....etc"
    I assume he means instant miles per gallon. I have a 2012 and it does not have this feature.
    If it does, can someone tell me how to get to the instant miles per gallon on my 2012?[/quote]

    A noob answering a noob's question, but I think that meter is on your display by the MPH right side??
     
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  19. kalome

    kalome Member

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    A noob answering a noob's question, but I think that meter is on your display by the MPH right side??[/quote]

    Edit comment -
    Ah i forgot about that...yeah that meter that goes up and down. I'm use to seeing the digital iMPG on other vehicles.
    I wish the Prius had a digital iMPG read out.
     
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  20. markabele

    markabele owner of PiP, then Leaf, then Model 3

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    That would make for a pretty quick acceleration. It seems like Wayne, Bob, and a few others recommend a slow acceleration before you let off to glide.
     
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