Compression Cycle

The compression cycle in a piston compressor is a four-phase process that repeats continuously.

The Four Phases

1. Intake Stroke (Expansion)

    Intake valve
       OPEN
         ↓
    ┌────▼────┐
    │ ░░░░░░░ │ ← Air entering
    │ ░░░░░░░ │
    │         │
    │         │
    │  ═════  │ ← Piston moving down
    └────┬────┘
         ↓

1. Piston moves downward
2. Pressure in cylinder decreases
3. Intake valve opens due to pressure difference
4. Atmospheric air fills the cylinder

2. Compression

    Valves
    CLOSED

    ┌─────────────┐
    │░░░░░░░░░░░░░│
    │░░░░░░░░░░░░░│ ← Air being compressed
    │░░░░░░░░░░░░░│
    │    ═════    │ ← Piston moving up
    │             │
    └──────┬──────┘
           ↑

1. Piston begins to rise
2. Both valves close
3. Air is compressed
4. Pressure and temperature increase

3. Discharge

    Discharge valve
         OPEN
           ↑
    ┌──────▲──────┐
    │░░░░░░░░░░░░░│ → Air exiting
    │    ═════    │ ← Piston at top
    │             │
    │             │
    │             │
    └──────┬──────┘
           ↑

1. Pressure exceeds system pressure
2. Discharge valve opens
3. Compressed air exits to system
4. Piston reaches top dead center

4. Clearance Volume Expansion

    Valves
    CLOSED

    ┌─────────────┐
    │░░░░░░░░░░░░░│ ← Residual air expanding
    │    ═════    │ ← Piston starting down
    │             │
    │             │
    │             │
    └──────┬──────┘
           ↓

1. Piston begins to descend
2. Residual air (clearance volume) expands
3. Pressure decreases
4. When pressure drops enough, intake valve opens

P-V Diagram (Pressure-Volume)

The compression cycle can be represented on a P-V diagram:

• 1→2: Compression (polytropic)
• 2→3: Discharge at constant pressure
• 3→4: Clearance volume expansion
• 4→1: Intake at atmospheric pressure

Volumetric Efficiency

Volumetric efficiency is the ratio between actual air intake and theoretical piston displacement.

Factors Affecting It

Factor	Effect
Clearance volume	More clearance = lower efficiency
Compression ratio	Higher ratio = lower efficiency
Leakage	Losses through rings and valves
Heating	Hot air is less dense
Flow restrictions	Dirty filters, small valves

Typical Values

Compressor Type	Volumetric Efficiency
Single stage, low pressure	80-90%
Single stage, high pressure	60-75%
Two stage	75-85%

Compression Ratio

The compression ratio is the proportion between discharge pressure and intake pressure:

CR = P_discharge / P_intake

Practical Limits

Stages	Maximum Compression Ratio	Typical Pressure
Single stage	4:1 to 5:1	Up to 100 PSI
Two stage	8:1 to 10:1	Up to 175 PSI
Three stage	15:1 to 20:1	Up to 500 PSI
Multiple	Up to 250:1	Up to 6,000 PSI

Discharge Temperature

Higher compression ratio means higher discharge temperature. That's why high-pressure compressors use multiple stages with intercooling.

Valves

Valves are critical components that control air flow.

Valve Types

Type	Characteristics	Application
Reed	Simple, economical	Small compressors
Disc	Higher flow	Medium compressors
Concentric ring	High efficiency	Large compressors
Plate	Robust	High pressure

Operation

Valves operate automatically by pressure difference:

• Intake valve: Opens when cylinder pressure < atmospheric pressure
• Discharge valve: Opens when cylinder pressure > system pressure

Valve Maintenance

Valves are the most common cause of efficiency loss. Regularly inspect for:

• Wear or deformation
• Carbon deposits
• Weak or broken springs
• Damaged seats