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%.
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?
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?
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
German Prius also has 1:13 compression ratio. Toyota Prius Technische Daten. Die Zukunft atmet auf. Ken@Japan
Thanks for the link. There is a lot of other general interest info there that is semi-readable by English speakers with Google Translate
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
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
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!
