Compression ratio
Compression ratio turns up in 2 fields: vacuum, automotive. Here is what it means to the people in each one.
Compression ratio in vacuum
Ratio of a pump's outlet pressure to its inlet pressure for a given gas, measured at zero net flow.
The maximum ratio of exhaust pressure to inlet pressure a pump can sustain for one gas with its inlet closed off. Turbomolecular pumps reach about 10⁹ or more for nitrogen but far less for light gases, often 10³ to 10⁵ for hydrogen, which is why hydrogen dominates the residual gas above a turbo. Roots pumps manage only a few tens, which is why they always need a backing pump.
"Compression ratio for H2 on this little turbo is lousy, so the hydrogen peak on the RGA tracks the foreline pressure."
Compression ratio in automotive
The mathematical ratio of cylinder volume at the bottom of the piston stroke compared to the top.
The compression ratio compares the maximum cylinder volume at bottom dead center to the minimum clearance volume at top dead center. A higher ratio extracts more mechanical energy from the air-fuel mixture, improving thermal efficiency and power output. High-compression engines require higher-octane fuel to prevent premature ignition of the charge.
"We milled the cylinder head to bump the compression ratio up to 11 to 1 for the new race build."