increase the pressure by a factor of 4. A similar single stage axial compressors can only increase the pressure by a factor of 1.2 but axial compressors have an advantage over centrifugal compressors because of their ability to have multiple stages. In the multi-staged compressors, the pressure is multiplied from row to row which can increase the pressure by a factor 40. It is much more difficult to produce an efficient multistage centrifugal compressor because the flow has to be ducted back to the axis at each stage therefore most high-compression jet engines incorporate multi-staged axial compressors. But if only a moderate amount of compression is required a centrifugal compressor is much simpler and efficient to use. Mixed-flow compressor In the future, mixed flow compressors may feature at the small end of the market, particularly in turbofans, where the relatively large diameter across the diffuser of a centrifugal compressor is a significant disadvantage. Staged compression Two-stage At high overall pressure ratios, the compression system is usually split into two units; a low-pressure (LP) compressor mounted on one shaft followed by a high-pressure (HP) compressor mounted on the HP shaft and driven by its own (HP) turbine. On civil turbofans, the first stage of the LP compressor is often a single-stage fan. If the HP compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry (i.e. variable stators) on the first few stages, to make the surge line on the compressor map more shallow, to accommodate the shallow working line. Three-stage On their large civil turbofans, Rolls-Royce split the compression system into three: a fan, an intermediate-pressure (IP) compressor and an HP compressor, each unit being driven by its own turbine unit. The RB199 military turbofan also has this arrangement. Free-piston gas generator A free-piston gas generator is a free-piston engine whose exhaust is used to power a gas turbine. It combines the functions of compressor and combustion chamber in one unit. These machines were quite widely used in the period 1930–1960 but then fell out of favour. References Jet engines Gas compressors Gas turbine technology