Which of the following is an example of a positive displacement air compressor?
Reciprocating piston compressor
Air compressors fall into two broad families based on how they raise pressure:
The distinction between the two families is fundamental:
| Positive displacement | Dynamic (roto-dynamic) |
|---|---|
| Traps a fixed volume of air in a chamber and mechanically reduces that volume to raise pressure | Imparts high velocity (kinetic energy) to the air with an impeller/blades, then converts that velocity into pressure in a diffuser |
| Reciprocating (piston), and rotary types: vane, screw, lobe/Roots | Centrifugal (radial) and axial-flow compressors |
Among the choices, the reciprocating piston compressor is the classic positive-displacement machine: the piston draws air into the cylinder, the valves close to trap a definite volume, and the piston then compresses that trapped charge each stroke. It therefore delivers high pressure ratios in relatively small flow rates.
Why the other choices are wrong: the centrifugal compressor, the radial dynamic compressor, and the axial flow compressor are all dynamic machines. They do not trap and squeeze a fixed pocket of air; instead they accelerate a continuous stream and then decelerate it to build pressure. Hence none of them is a positive-displacement type. (Other positive-displacement machines not listed here include screw, vane and lobe compressors.)
Important Questions from IC Engine Cycles
- The indicated thermal efficiency of an internal combustion engine is defined as:
In a four-stroke cycle, the minimum temperature inside the engine cylinder occurs at the
The working cycle of two stroke engine is completed in _______ revolutions of crank shaft.
The ratio between net work for one cycle in a process and displacement volume is -
The fourth stage of the compression ignition process is:
