The working principle of a reciprocating piston CNG compressor relies on the back-and-forth motion of a piston within a cylinder to compress the gas. The process typically involves the following stages:  

1. Intake Stroke:
   • The piston moves downwards, increasing the volume within the cylinder.
   • This creates a vacuum, causing the intake valve to open and allowing low-pressure CNG to enter the cylinder.
2. Compression Stroke:
   • The piston moves upwards, reducing the volume within the cylinder.  
   • This compresses the trapped CNG, increasing its pressure.  
   • Both the intake and discharge valves remain closed during this stage.
3. Discharge Stroke:
   • As the piston continues its upward motion, the pressure within the cylinder exceeds the discharge pressure.
   • This causes the discharge valve to open, and the high-pressure CNG is expelled from the cylinder.  
4. Expansion/Clearance Stroke:
   • Once the discharge stroke is completed, a small amount of gas remains in the cylinder. This amount of gas is called the clearance gas.  
   • The piston then begins its downward motion, allowing this gas to re-expand.
   • This prepares the cylinder for the next intake stroke.

Key Components:

Reciprocating piston CNG compressors consist of several essential components that contribute to their operation:  

• Cylinder:
  • The cylindrical chamber where the piston moves, facilitating gas compression.  
• Piston:
  • A cylindrical component that moves back and forth within the cylinder, compressing the gas.  
• Piston Rings:
  • Seals between the piston and cylinder wall, preventing gas leakage and maintaining compression efficiency.  
• Crankshaft:
  • Converts rotary motion from the motor into reciprocating motion for the piston.  
• Connecting Rod:
  • Connects the piston to the crankshaft, transmitting the reciprocating motion.  
• Intake Valve:
  • Allows low-pressure CNG to enter the cylinder during the intake stroke.
• Discharge Valve:
  • Allows high-pressure CNG to exit the cylinder during the discharge stroke.  
• Cooling System:
  • CNG compression generates heat, so cooling systems (air or water) are essential to maintain optimal operating temperatures.
• Lubrication System:
  • Lubricates moving parts to reduce friction and wear.
• Control System:
  • Monitors and controls compressor operation, including pressure, temperature, and flow rate.
• Frame/Casing:
  • Provides structural support for all the internal components.
• Motor:
  • Provides the power to the crankshaft.

Key Considerations:

• Reciprocating compressors are known for their ability to achieve high pressures.  
• Multi-stage compression is often used to achieve very high pressures and improve efficiency.
• Proper maintenance and lubrication are crucial for reliable and efficient operation.