Talk:Specific fuel consumption

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This page incorrectly confuses compression ratio (used for positive displacement engines) and pressure ratio, used for turbines. See the table below for conversion. From this it is easy to see that a diesel engine operating at a CR of 25:1 has a much higher pressure ratio than an advanced aircraft turbine engine.

CR 1:1 3:1 5:1 10:1 15:1 20:1 25:1 35:1
PR 1:1 2:1 10:1 22:1 40:1 56:1 75:1 110:1


I partly agree with Mr. Anonymous here. The compression ratio is defined via the volume reduction,
CR=\frac{V_1}{V_2},
but the pressure ratio is defined as the pressure increase
PR=\frac{P_2}{P_1}.
Using the combined gas law we get:
\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2} \Rightarrow \frac{V_1}{V_2}=\frac{T_1}{T_2} \frac{P_2}{P_1} \Leftrightarrow CR=\frac{T_1}{T_2} PR
Since T2 is much higher than T1 (compressing gases puts work into them, i.e. heats them up), CR is much lower than PR. - Alureiter 17:01, 22 October 2005 (UTC)

[edit] SFC table

I expanded the table. I think it's easier to reference existing examples than blurry values, so I use it. The table is nearly fully referenced by wikipedia articles. For the MAN thermal efficiency system, I asked an employee there to confirm, but it's not as good as it could be. It is an interesting value, though: the maximum efficiency of a reciprocating engine as of today.--Marc Lacoste 23:34, 25 October 2006 (UTC)

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