Atkinson Cycle engine surmise (and question)

Discussion in 'Prime Technical Discussion' started by RonMc5, Oct 3, 2017.

  1. RonMc5

    RonMc5 Member

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    On my bucket list since they used to show the Pike's Peak Hill Climb on the Wide World of Sports, I always pictured me driving one of my muscle cars up to 14K feet. Given my recent trip to the top of Pike's Peak in my Prius Prime I am theorizing that at higher altitudes the intake valves close sooner the keep the cylinder pressure more like sea level (around 150 psi maybe?) My Prime still performed beautifully all the way to 14,115'. I was expecting a pretty serious performance hit. Given it was mostly switchbacks up there I may just not have noticed, but the point is I did NOT notice any performance degradation. Perhaps someone knows the answer for sure, rather than just my surmise (about closing the intake valves sooner at altitude)? I think the ceiling of most naturally aspirated* prop jobs is around 15K feet. *No turbo or supercharger

    My question is basically, do the intake valves always close at the same part of the compression cycle, or is it variable AND is it variable by altitude? My seat of the pants feeling at altitude makes me think so. :)

    Prius Prime Blizzard Pearl
     
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    It is variable, but I am unaware that it is variable by altitude specifically. (my surmise is that since batteries work as well at 15,000 feet as sea level, you suffered less power loss than an all engine powered car)

    [​IMG]
    This chart is for a Gen 2 Prius and a Gen 4 may vary from it. The black arc is the exhaust timing and duration, which is fixed. The white arc is the intake, which has a fixed duration but variable timing. Under heavy load, speed seems to be a factor in the valve timing, as does temperature.
     
  3. Trollbait

    Trollbait It's a D&D thing

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    The gen4 does make use of advances Toyota has made since the gen2, but the variable valve timing isn't the wide range VVT that can switch between Atkinson and Otto cycles, and it is still on the intake only.

    Atkinsonized engines aren't regarded for their performance, and full hybrids have always relied on the traction motor to make up for the ICE's shortcomings in that area.
     
  4. Lee Jay

    Lee Jay Senior Member

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    I agree with that. In fact, I'll add another supposition to it.

    Since MG1 has to react the torque going from the engine to the wheels, it has to act as a generator, pumping its generated power to MG2 to propel the car. This limits how much power can flow from the batteries to MG2 to propel the car from that source.

    If the engine loses power (because of lower barometric pressure, in this case), MG1 has to react less torque and thus provide less power to MG2, leaving a larger amount of MG2's total power for the battery to provide. This might mean that most of the power lost by the engine is made up for by the Prime's larger battery system through supplementing MG2.
     
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  5. RonMc5

    RonMc5 Member

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    Thanks for the reply and the diagrams. I note that it doesn't include altitude or ambient air pressure, but it would be very EZ for them to have added it to the equation. Also, I didn't have any EV in the tank since Las Vegas (where I found a FREE charging station at a local high school. (Son playing in a adult league BB tournament there)). In other words all I had left at the bottom of the mountain (around 7K feet I think) was the HV portion of the battery. Alas the hotel I overnighted at B4 my assault on the mountain did NOT have charging stations. :(
     
  6. fuzzy1

    fuzzy1 Senior Member

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    Changing the valve timing with respect to altitude, implies that it is not using the full available valve adjustment range when down at sea level. I very much doubt that this is the case.

    When climbing at high altitude, I would expect the Prius to simply increase engine RPM as needed to compensate for the reduced air pressure. If you don't compare engine RPM (reported by any of the aftermarket accessories, such as ScanGauge-II) on Pikes Pikes to a similar slope at much lower elevation, and you are not having to completely floor it, then there is nothing else to notice.

    With fixed speed transmissions, the difference will be more obvious as the operator needs to downshift to get the higher RPM needed to compensate for the thinner air. I really noticed it when driving my manual transmission cars high up in Colorado. But with auto-CVT systems, this can easily be hidden from the driver.
     
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  7. RonMc5

    RonMc5 Member

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    If they closed the intake valve earlier at sea level it would diesel (knocking/preignition).
    They leave it open during part of the compression cycle so it doesn't diesel. As you gain altitude cylinder pressure decreases. 9:1 compression (assumed effective value. I am sure it is in the ballpark) so, 14.7 PSI x 9 yields a cylinder pressure of 132 at TDC. If the intake valve was closed a BDC, it would have 14:1 compression. (Atkinson cycle has a long POWER stroke to use up all the power from the gasoline, to make up for that they leave the intake valve open during part of the compression stroke. You can take that to the bank, as it is the definition of the Atkinson vs Otto cycle engines). The only question is the one I originally posed: Is this behavior adjusted for altitude. I know some cars have variable lift and other have variable duration of valve openings. (Via hydraulically actuated loves on the cam lobes. Using this mechanism COULD close the valves earlier by diminishing this lobe on a lobe. The included file makes some reasonable assumptions about compression ratios. If I am off a tad, don't quibble, the relative point remains the same.
     

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  8. fuzzy1

    fuzzy1 Senior Member

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    But the Prius intake valve system can't close at BDC, or anywhere near that. The mechanical system can't adjust that far. See the chart back at post #2.
    And my answer remains the same:

    "Changing the valve timing with respect to altitude, implies that it is not using the full available valve adjustment range when down at sea level. I very much doubt that this is the case."
     
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