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is 10% increase on EPA cycle realistic?

Discussion in 'Gen 4 Prius Main Forum' started by cyclopathic, Oct 1, 2015.

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  1. cyclopathic

    cyclopathic Senior Member

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    What we know so far from Japanese staff manual:
    1) there is no weight reduction btw Gen3 and Gen4
    2) there is a battery size reduction from 27kWh to 20kWh
    3) there is a reduction of power from 100kW to 80kW
    4) there is modest 4% efficiency increase of ICE
    5) there are improvements to HSD transaxle similar to ones in C/Aqua

    Published JC08 numbers suggest 10% improvement for core model, and 18% improvement for cheapest Eco. Eco improvements are likely being attributed to different test cycle, applicable to lighter category cars. So on EPA the difference btw Eco and core models will not amount to 8%.
     
    #1 cyclopathic, Oct 1, 2015
  2. wjtracy

    wjtracy Senior Member

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    I am expecting less cold engine MPG hit compared to Gen2/3.
    Reportedly the v is much better about reducing warm up time.
    I don't know if this impacts EPA cycle, but real world and not sure what Consumer Reports crazy MPG test does, but whatever it is, it seems to be a cold engine in the City rating, so this could improve.
     
    #2 wjtracy, Oct 1, 2015
  3. proprius

    proprius Junior Member

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    2) The battery size is neither of those two capacities (kWh). You are mixing it up with the maximum power output of the battery (kW) and I'm not even sure about the 20 kW.
    3) 122 PS (HP) is about 90 kW
    ...
    5) In what way is the transaxle similar? C/Aqua is very similar to Gen3, not Gen4
     
    #3 proprius, Oct 1, 2015
  4. cyclopathic

    cyclopathic Senior Member

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    Agree heat cycle is very likely to be improved, And no this does not have any impact on EPA cycle.

    It is obvious that Gen4 will get at least 4% improvement do to increased efficiency. Perhaps more if sweet spot had been enhanced. Smaller capacity battery is set back, but if the charge/discharge rates improved, then there is a possibility there.

    EPA cycle is split btw 55% city and 45% highway, roughly half and half.

    On highway cycle most of improvements to come from reduced drag and improved ICE efficiency. There could have been 5% increase due to 20% weight reduction, but that is no-go. EV mode is not really utilized in current generations, my 45mi drive last night in C gave 1% EV.

    In the city savings would come from ICE efficiency, reduced weight (look remark above) and better EV management.

    I cannot see much reduction coming from rolling resistance reduction; at least from tires.

    To have 6% MPG increase due to aerodynamics only, you'd need to reduce aero drag by 4*6% = ~25%. This is not likely either.

    As I recall you get ~3% MPG decrease for every 10% increase in power, so perhaps they found needed 6% by reducing power by 20%? If they really did reduce power by 20% that car will be a turtle with 0-60 over 12sec.
     
    #4 cyclopathic, Oct 1, 2015
  5. usbseawolf2000

    usbseawolf2000 HSD PhD

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    I am sure the size was reduced. I think you meant power.
    Gen4 is rated 90kW.

    Yea, I think 10% improvement in EPA cycle is realistic.
     
    #5 usbseawolf2000, Oct 1, 2015
  6. cyclopathic

    cyclopathic Senior Member

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    How?

    As a Prius cheerleader I wanna see at least as much, but as an engineer do not see that happening. They really needed to reduce weight to get there.
     
    #6 cyclopathic, Oct 1, 2015
  7. proprius

    proprius Junior Member

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    Maybe improved efficiency of the electric path (motors, inverter, battery)? There is a lot of power flowing between MG1 and MG2 (in both directions, depending on the driving situation)
     
    #7 proprius, Oct 1, 2015
  8. giora

    giora Senior Member

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    Does EPA have a 'cold start' in its cycle? I think NEDC has one (in its city part) and auto grill shutter may help there.
     
    #8 giora, Oct 1, 2015
  9. cyclopathic

    cyclopathic Senior Member

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    Let's hope but it is not likely for 2 reasons:
    - reduced battery charge/discharge rates.
    - relatively minor impact of EV on MPG.

    In Prius C avg EV usage ~16% (this is from last oil change, 9,500mi), and on hwy trips it is as low as 1% as high as 5-7%. 20% increase in efficiency will only yield 3% improvement in MPG. And 20% is unlikely or they would have screamed about it.

    Reduced battery rates will have negative impact on MPG. I drive Gen3 and "C", and it is much much harder to utilize regenerative braking on "C". The stopping distance is noticeably longer.
     
    #9 cyclopathic, Oct 1, 2015
  10. JimN

    JimN Let the games begin!

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    I believe a 10% increase in the EPA number is possible but I doubt I'd see a 10% increase in real driving.
     
    #10 JimN, Oct 1, 2015
  11. proprius

    proprius Junior Member

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    Sorry if I was not clear: I'm not talking about EV.

    The CVT in a HSD implements its variable transmission by transmitting part of the engine power over the mechanical path and part of it over the electric path. This is especially the case when not in EV mode, but when the engine is running.

    Even when the battery is not touched (no charging or discharging) the motors and the inverter handle a lot of power. Not in all situations, but in most.
     
    #11 proprius, Oct 1, 2015
    Last edited: Oct 1, 2015
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  12. giora

    giora Senior Member

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    Yes, in Gen 3 28% of engine torque is going the electical path.
     
    #12 giora, Oct 1, 2015
  13. proprius

    proprius Junior Member

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    Yes, but be careful here: It's 28% torque. That is not necessarily 28% power. The inverter transmits electric power. You can't directly transmit torque over a wire.

    I know it makes a knot in your brain and is worth an own thread. It took me months to figure out.

    Anyone interested in me opening a thread on how power is split between electrical and mechanical path?
     
    #13 proprius, Oct 1, 2015
  14. bisco

    bisco cookie crumbler

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    sure, if you think you can explain it to the lowest common denominator.:cool:
     
    #14 bisco, Oct 1, 2015
  15. giora

    giora Senior Member

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    How about trying the 'search' tool before making a decision?
     
    #15 giora, Oct 1, 2015
  16. usbseawolf2000

    usbseawolf2000 HSD PhD

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    Perhaps the loss in electric path was reduced.

    It could be done two ways.

    1) Increase efficiency of e-path with better inverter
    2) Minimize e-path power flow. This would increase direct mechanical power flow to the wheels.

    The hint is lower battery power. Acceleration remains the same with lower peak power, for the same weight due to more efficient system.

    They may have done both #1 and #2 to achieve it while cutting HSD cost.
     
    #16 usbseawolf2000, Oct 1, 2015
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  17. Stevevee

    Stevevee Active Member

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    Sounds like there's not much else in the new car to hold people's attention. How many are really thinking, I Can't Wait To Drive This Beauty?
     
    #17 Stevevee, Oct 1, 2015
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  18. cyclopathic

    cyclopathic Senior Member

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    that's a good one, Stevevee!
     
    #18 cyclopathic, Oct 1, 2015
  19. Stevevee

    Stevevee Active Member

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    Well it's true. When you're reduced to talking about glove boxes and whether you can sleep in the back... I remember a statement Steve Jobs made when he got back at Apple.

    "You know why these products are not selling???? There's no sex in these things. "

    of course, he was right about that, and much more. Toyota puts their design team that has a temperature of 98 degrees or more on their regular cars. So they have average appeal. When they find designers with an internal temperature of less than 60 degrees, they ship them off to the Prius team. Could very well be why they've latched onto hydrogen.

    Ba-Da Bing....
     
    #19 Stevevee, Oct 1, 2015
  20. S Keith

    S Keith Senior Member

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    You sure about that? Since you said "aerodynamics only," we'll ignoring the slight drop in SFC. Wouldn't a 6% reduction in drag require 6% less force and use 6% less fuel for a given speed/distance resulting a 6% improvement in mpg?

    Since dynamic pressure is constant for a given density/speed, the drag load varies only by Cd. If the 3rd gen is Cd = 0.25, the 4th gen would have to be 0.94 * 0.25 = 0.235. The improvement from the Gen 2 (0.26) to Gen 3 (0.25) was 3.8%.

    Not saying it's easy, and I cant say there's anything obvious from the pictures that would suggest a significant drag reduction.

    EDIT: Unless it's the antenna... :)
     
    #20 S Keith, Oct 1, 2015
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