- With reference to a reciprocating air compression:
(a) define the term Volumetric Efficiency;(4)
(b) explain how the operation of the suction and delivery valves affect the volumetric efficiency of the compressor.(6)
a) Definition of Volumetric Efficiency (4 marks)
- Volumetric Efficiency is the ratio of the actual volume of air drawn into the cylinder during the suction stroke to the swept volume (displacement volume) of the piston.
- It is usually expressed as a percentage:
- In practical terms, it indicates how effectively the compressor fills its cylinder with fresh air during each suction stroke.
- Volumetric efficiency is always less than 100% due to clearance volume, heating of air, and valve losses.
(Definition + formula + explanation + note = 4 marks)
(b) Effect of Suction and Delivery Valve Operation on Volumetric Efficiency (6 marks)
The performance of the suction and delivery valves directly affects how much air enters and leaves the cylinder — therefore influencing the volumetric efficiency:
- Late opening of the suction valve
- If the suction valve opens late, part of the suction stroke is lost before air begins to enter.
- This reduces the amount of air drawn in and therefore lowers volumetric efficiency.
- Early closing of the suction valve
- If the valve closes before the piston reaches the end of the suction stroke, air entry stops prematurely.
- Again, less air is drawn in, reducing efficiency.
- Restricted or sluggish suction valve movement
- Caused by weak springs, fouling, carbon deposits, or sticky seats.
- Results in poor air flow and pressure drop across the valve, decreasing the volume of air taken in.
- Leakage through delivery valve during suction stroke
- If the delivery valve is not seating properly, compressed air may leak back into the cylinder during suction.
- This raises the clearance pressure and reduces the effective suction volume, lowering efficiency.
- Excessive valve inertia or heavy spring tension
- Valves that are too heavy or stiff respond slowly to pressure changes, delaying opening or closing.
- This restricts air passage and reduces the effective volumetric filling.
- Valve cleanliness and condition
- Clean, free-moving valves with correct spring tension ensure timely operation and minimal leakage, improving volumetric efficiency.