A centrifugal compressor

A centrifugal compressor with an impeller (201, fig. 2b), a diffuser passage 401 and a volute 213, wherein the ratio of the impeller's outer diameter to the diffuser's impeller's outer diameter ranges from 0.85 to 0.95. Such ratios aim to provide a dimensionally more compact compressor with a relatively shorter diffuser passage, striking a balance between packaging constraints and degree of pressure recovery by the diffuser. Preferably the ratio is more than 0.88 and less than 0.92, more preferably ranging from 0.89 to 0.91. Preferably the impeller's outer diameter ranges from 17 to 19 cm, more preferably from 17.9 to 18.3 cm. A vehicle with a ventilation system comprising such a compressor and suitable air ducts, drawing air from outside the passenger cabin to the compressor and then discharging it to the passenger cabin, is also claimed.