40%-Efficient Engine, Really?

Discussion in 'Prime Technical Discussion' started by mr88cet, Apr 24, 2017.

  1. mr88cet

    mr88cet Senior Member

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    I've seen claims to the effect that the Gen-4 Prius engine is 40% Efficient. Really?

    Since production ICEs historically have rarely had better than 20% efficiency, I find that pretty surprising, and impressive if true.

    I think it's much more likely what they mean is that the entire non-plug-in hybrid system, including times when it can shut off the engine, regenerate, etc., is 40% efficient.


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  2. bwilson4web

    bwilson4web i3 and Prime

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    I've read the SAE papers and it is true with the following:
    • after warm-up - heat engines have to reach an optimum temperature range before peak efficiency
    • operating line - this is where the vehicle control laws seek to keep the engine within the peak efficiency region. The power band for peek efficiency has often been very narrowly defined to specific set of power and rpm ranges. The trick that Toyota figured out is how to tweak the engine and control laws to make that as broad as possible. For example, the earlier hybrids had a maximum 42 mph speed limit so slow and the engine could cycle but faster, the engine ran all the time. This gave the car outstanding City and pretty good highway performance. The Prius Prime has all but eliminated that up to 84 mph and the car always seeks the most efficient engine operating point.
    Toyota also pioneered more effective power metrics for hybrid cars that doesn't use a dyno, selected point but rather the power delivered to the wheels. It means a true HP reading of the car and control laws, not some inflated but impossible to achieve number used to fleece the foolish.

    Bob Wilson
     
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  3. Lee Jay

    Lee Jay Senior Member

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    Some math:

    I've been averaging about 5 miles/kWh (from the wall). At 85% efficiency (assumed), that means the car is consuming about 170Wh/mile from the battery.

    I'm also getting about 69mpg when on gasoline.

    1 gallon of gasoline = 33.7kWh of thermal energy (LHV).

    If the car uses 170Wh/mile, then 69 miles would use 11,730Wh. To get that from 33.7kWh of heat energy, you'd need an efficiency of 11,730/33,700 = 34.8% average.

    Given that the 40% number is peak, and after warm-up as Bob said, 34.8% average definitely implies a peak of around 40%.

    Very impressive.
     
  4. Trollbait

    Trollbait It's a D&D thing

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    I believe modern car ICEs are over 30% thermally efficient. Besides that, any thermal efficient figure for an engine is its peak efficiency.
     
  5. David Beale

    David Beale Senior Member

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    And before you go out on a drive, "just because", keep in mind this 40% peak efficiency implies AT BEST a 60% inefficiency! So go ahead and waste energy. ;)
     
  6. Lee Jay

    Lee Jay Senior Member

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    Don't forget, the thermal power plants that are largely powering the grid are also averaging somewhere in the 40% efficient range. Some are better (CC gas closer to 60%), some are worse (peaking IC plants in the 25% range).
     
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  7. Samprocat

    Samprocat Active Member

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    MB Formula 1 is doing 45% engine efficiency for 2+ year's already and keeping the engine in power mode non stop
    For our Toyota we need electric assist turbocharger and additional hardware to get there easy and efficiently
    And definitely will be blast to drive

    Posted via the PriusChat mobile app.
     
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  8. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    The thing is, the Prius does not rely on the engine in modes that are inefficient. It uses the engine when it is economical to do so. Other cars idle, deaccelerate, stop and go, etc with the engine on.
     
  9. bhtooefr

    bhtooefr Senior Member

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    That's where your assumptions go wrong - modern passenger car gasoline engines are well above 30% peak efficiency, and passenger car diesels have been over 40% for decades now.

    As others have said, the Prius's strength is the ability to stay as close to that 40% peak as possible. Stoichiometric combustion engines tend to lose a lot of efficiency at part throttle, due to pumping losses, and the Prius's hybrid system avoids that.
     
  10. Lee Jay

    Lee Jay Senior Member

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    No. They really meant that the engine has a peak thermal efficiency of 40%.
     
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  11. Samprocat

    Samprocat Active Member

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  12. bwilson4web

    bwilson4web i3 and Prime

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    You have my sympathy. I don't care for stepped transmissions that are trying to keep the ICE in a happy but narrow power/rpm range. More noise and bother than I want.

    Bob Wilson
     
  13. mr88cet

    mr88cet Senior Member

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    Yes, the evolutionary improvement of the Prius drive train seems to me to be largely about raising the power-demand threshold before the ICE kicks in, so that when it does kick in, it is running as efficiently as possible. The longer you can postpone kicking in the ICE, until it's only operating in its optimal-efficiency range, the better mileage you're going to get.

    I personally suspect that the original PiP went pretty much misunderstood. I think it was never really intended to function much as an EV, but simply to use the additional battery power to continue that evolution: Again, having ever greater opportunity to postpone the kick-in of the ICE until it can only do so at the highest possible efficiency. I personally, doubt if it was ever particularly intended for EV operation any substantial amount of time.

    There are other factors, of course, that contribute to Prius mileage, such as aerodynamics and regenerative braking, plus of course the topic here: Simply making the engine more and more efficient.


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    #13 mr88cet, Apr 24, 2017
    Last edited: Apr 24, 2017
  14. fuzzy1

    fuzzy1 Senior Member

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    Don't confuse peak efficiency with average efficiency.

    Note also that this 40% number for Gen4 is only a couple points higher than the Gen3 claim, which in turn was just a small increment above the Gen2 claim. Gen4's efficiency is not a giant leap, but rather a continuation of a pattern of a small increment for each generation.
     
  15. bhtooefr

    bhtooefr Senior Member

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    38.5% for Gen 3, IIRC, and 36.5% for Gen 2, I think?

    Also worth noting that Toyota's already surpassed that, with 41% for the 2018 Camry Hybrid's engine. (Easier to get that higher thermal efficiency on a larger displacement per cylinder engine, though, and the addition of direct injection allows for higher compression.)
     
  16. giora

    giora Senior Member

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    And maintenance...and weight for similar torque rating...
     
  17. Tideland Prius

    Tideland Prius Moderator of the North
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  18. JamesBurke

    JamesBurke Senior Member

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    These are used in some small towns in Denmark to generate electricity and the waste heat used to heat water in huge tanks for distributed residential hot water heating. (Miller cycle) Jenbacher Type 6 Gas Engines | GE Power
     
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  19. bhtooefr

    bhtooefr Senior Member

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    88.1% combined cycle efficiency is... quite impressive.

    For that matter, so is 45.7% electrical efficiency, meaning after electrical conversion losses.
     
  20. Trollbait

    Trollbait It's a D&D thing

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    Other locations have gone larger scale. Co-gen heat with a closed cycle turbine can get over 95% thermal efficiency.
    Not sure where the heat comes from, but buildings in NYC still get outside steam for heating and hot water.
     
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