The object of supercharging is to raise the density of the charge in the cylinder at the end of the suction stroke, i.e. a greater weight of mixture or air is trapped in the cylinder. Supercharging can be used to maintain sea-level performance at altitude and has been extensively developed in aero engines both to enhance output at sea level and then to maintain or even increase it with altitude. The larger diesel engines are super-charged by means of an exhaust driven turbo-blower.
Types of Blowers:
- Used particularly on large low-speed engines but is being replaced by the exhaust turbo-blower.
- Consisting of two rotors each having two or three lobes phased together by gears so that the rotors do not touch. Though very suitable for low pressures this type becomes increasingly inefficient as the pressure is raised. It has no Internal compression.
- Having internal compression, typical examples are the Lysholm compressor using two specially shaped rotors, Centric and Zoller eccentric vane types. These may be used at somewhat higher pressures than the Roots type with adiabatic efficiencies as high as 65%.
- has been used extensively running at 20,000 - 30,000 r.p.m. in aero engines but is later mainly used on oil engines coupled to an exhaust turbine forming a turbo-compressor unit which, in the smaller sizes, runs at considerably higher speeds. Pressure ratios of 2 to 1 can readily be obtained with adiabatic efficiencies of 70-75%.