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1:13 vs 1:14 compression ratio and MPG loss

Discussion in 'Gen III 2010+ Prius Fuel Economy' started by cyclopathic, May 16, 2011.

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  1. cyclopathic

    cyclopathic Senior Member

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    US Prii have 1:13 compression ratio and Japanese/European have 1:14.. this is due to different minimal gasoline octane rating 85-87 (R+M) in US and 95 R in other markets.

    theoretically the loss of efficiency ~7.7%, in reality probably ~3-5%.
  2. Insight-I Owner

    Insight-I Owner 2006 Insight-I MT + 2011 Prius

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    Are these the geometric CR's or the effective CR's? If they are geometric CR's, the effective CR's are less due to the late intake valve closing of Atkinson cycle engines like the Prius'. From Wikipedia on Atkinson cycle engines: "Note that the compression ratios shown above reflect the expansion ratio, which is the ratio of the combustion chamber volumes when the piston is at bottom dead centre and top dead centre. The effective compression ratio of the air-fuel mixture in an Atkinson cycle engine, with respect to atmospheric pressure, is lower due to the aforementioned delay in closing the intake valve."

    And is the theoretical CR-vs-efficiency comparison you are using valid for Atkinson cycle engines?

    And what sort of "efficiency" is meant: is it max HP or something more closely related to mpg?
  3. GrumpyCabbie

    GrumpyCabbie Senior Member

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    So why have different compression ratios for different markets? Why not just leave it the same - surely that would make it easier for parts etc?
  4. ken1784

    ken1784 SuperMID designer

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    I don't know where you get the numbers, but JP Prius has 1:13 compression ratio.
    JP Prius spec (Japanese language only)

    Ken@Japan
  5. ken1784

    ken1784 SuperMID designer

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  6. xs650

    xs650 Senior Member

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    Thanks for the link. There is a lot of other general interest info there that is semi-readable by English speakers with Google Translate
  7. qbee42

    qbee42 My other car is a boat

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    Perhaps his manual has a compressed font.

    ;)

    Tom
  8. usbseawolf2000

    usbseawolf2000 HSD PhD

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    Maybe he has info on the secret Gen4 Japanese model Prius. ;-)
  9. Lantec

    Lantec New Member

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    Actually, the octane between US and Europe/Asia is the same. But the rating system is different. It's 95 RON vs 87 (R+M). Like MPH vs KMH
  10. xs650

    xs650 Senior Member

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    The difference in rating systems is smaller than that.

    US 87 (R+M)/2 Octane is approximately European 91-92 R Octane.

    US 91 Octane is approximately European 95-96 Octane
    1 person likes this.
  11. GrumpyCabbie

    GrumpyCabbie Senior Member

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    Except that in Europe very few countries (if any) still offer US87/EU92 petrol.

    US91/EU95 Premium is the lowest we get and then US95?/EU98 Super is reserved for high performance cars - and it costs a small fortune!
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