The compression cycle, stroke by stroke
Every piston compressor, from a small workshop unit to a large two-stage industrial machine, repeats the same four-part cycle on each revolution of the crankshaft. The crankshaft and connecting rod turn rotary motion from the electric motor, diesel engine or tractor PTO into the straight-line movement of the piston.
- Suction. The piston moves away from the valve plate. Cylinder pressure falls slightly below the inlet pressure, the spring-loaded inlet valve lifts off its seat and atmospheric air flows in through the intake filter.
- Compression. The piston reverses. The inlet valve closes, the discharge valve is still held shut by line pressure and its spring, and the trapped air is compressed as the volume shrinks. Its temperature rises sharply.
- Discharge. When cylinder pressure rises just above the pressure in the discharge line, the discharge valve opens and hot compressed air is pushed out towards the aftercooler, receiver or next stage.
- Re-expansion. The piston cannot touch the valve plate, so a small clearance volume of compressed air stays behind. As the piston starts to retreat, this air expands first, and fresh air only enters once cylinder pressure falls below inlet pressure again.
The valves are self-acting: they open and close purely because of the pressure difference across them, not because of a camshaft. That simplicity is why piston compressors are robust and easy to repair, and also why valve condition matters so much to their output.
The main components and what each one does
| Component | Job in the machine | What wear or failure looks like |
|---|---|---|
| Cylinder and liner | Forms the compression space; cooled by fins (air-cooled) or a water jacket | Scoring, ovality, oil carry-over, loss of output |
| Piston and rings | Compression rings seal the air; oil-control rings limit oil reaching the cylinder head | Blow-by into the crankcase, rising oil consumption, slower pump-up |
| Inlet and discharge valves | Plate, reed or ring-type valves that control the air path | Hot cylinder head, knocking, low delivery, air pulsing back through the intake |
| Crankshaft, connecting rods and bearings | Convert drive rotation into piston stroke | Knocking, vibration, oil pressure loss on pressure-fed machines |
| Crankcase and lubrication system | Splash or pump-fed oil to bearings, pins and cylinder walls | Low or milky oil, sludge, breather oil mist |
| Intercooler and aftercooler | Remove heat between stages and after the last stage | High stage temperatures, more water downstream |
| Unloader and pressure switch or pilot | Start, stop or unload the compressor between set pressures | Hard starting under load, short cycling, pressure overshoot |
| Safety valve | Relieves pressure if controls fail | Weeping, sticking; must be tested, never plugged |
| Air receiver | Stores air, smooths pulsation, lets condensate settle | Water carry-over, corrosion, frequent cycling if undersized |
On a belt-driven machine you also have the flywheel, which carries cooling fan blades, the belts and the motor pulley. On a direct-coupled or engine-driven machine, coupling and alignment take the place of belts.
Why the air gets hot, and why stages and cooling matter
Compressing air does work on it, and most of that work turns into heat. In a single cylinder the discharge temperature climbs steeply as the pressure ratio rises. A single-stage piston compressor is therefore normally used for working pressures up to about 7–10 bar (101.5–145 psig). Above that, the discharge temperature, the load on valves and oil, and the loss of volumetric efficiency all get worse.
A two-stage compressor splits the job. A large low-pressure cylinder compresses air part of the way, an intercooler removes much of the heat, and a smaller high-pressure cylinder completes the compression. In a well-designed machine each stage takes roughly the same pressure ratio, close to the square root of the overall ratio. The benefits are standard engineering:
- Lower discharge temperature from each cylinder, which is kinder to valves, rings and oil.
- Less total power for the same delivered air, because cooled air takes less work to compress further.
- Better volumetric efficiency, because each stage has a lower pressure ratio across its clearance volume.
- Practical working pressures typically in the 10–14 bar (145–203.1 psig) class, with three or more stages used for higher-pressure duties such as PET blowing or high-pressure testing.
The aftercooler then cools the final discharge so that most of the water vapour condenses before the air reaches the plant. Hot, wet air passed straight into pipework is the most common cause of water problems at the point of use.
Displacement versus free air delivery
Piston compressors are often sold on piston displacement: the swept volume of the cylinders multiplied by speed. That figure is a theoretical maximum. The air actually delivered, the free air delivery (FAD) measured at the discharge and expressed at inlet conditions, is always lower because of:
- Re-expansion of the air trapped in the clearance volume.
- Pressure drop across the intake filter and inlet valve.
- Heating of incoming air by hot cylinder walls, which reduces its density.
- Leakage past rings and valves, which grows with wear.
On small and medium piston units the gap between displacement and FAD can be large, and it widens with higher discharge pressure. When you compare machines, ask for FAD at your working pressure under a stated reference basis such as ISO 1217, not displacement. For the reasoning behind this, see our page on airflow, FAD and reference conditions.
Lubrication: oil-lubricated and oil-free designs
Most industrial piston compressors are oil-lubricated. Smaller machines usually rely on splash lubrication, where a dipper on the connecting rod throws oil around the crankcase. Larger machines often have a gear pump that feeds oil under pressure to the main and big-end bearings. Either way, a little oil reaches the cylinder wall, so some oil leaves with the compressed air as aerosol and vapour. The amount increases as rings and liners wear.
Oil-free piston compressors use non-lubricated rings, often PTFE-based, and either a sealed crankcase with distance pieces or permanently lubricated bearings. They suit applications where oil contamination is unacceptable, but they still need filtration and drying, and ring life must be watched closely. Read more on oil-lubricated piston compressors and oil-free piston compressors.
How capacity is controlled
A piston compressor delivers a fixed volume per stroke, so capacity is controlled by stopping, starting or unloading the machine.
| Control method | How it works | Where it suits |
|---|---|---|
| Start/stop with pressure switch | Motor stops at cut-out pressure and restarts at cut-in pressure | Small and intermittent duty with a well-sized receiver |
| Constant-speed unloading | Inlet valves are held open or the inlet is closed, so the machine runs without compressing | Larger machines where motor starts per hour must be limited |
| Dual control | Runs unloaded for a set time, then stops if demand stays low | Variable demand through the shift |
| Step unloading | Some cylinders or cylinder ends are unloaded to give part capacity | Larger multi-cylinder industrial machines |
Too many starts per hour overheat motors and starters. A small receiver makes that worse, which is why receiver sizing belongs in the selection, not as an afterthought.
Duty cycle and where piston compressors fit
Many smaller piston compressors are designed for intermittent duty: they need rest periods to shed heat. Running one at full load all day in a 45 °C Hyderabad summer is a quick way to burn valves and carbonise oil. Heavy-duty industrial machines, usually slower-running and often two-stage, are built for much longer loaded running, but the manufacturer’s duty rating still applies.
Piston compressors are a strong choice for:
- Workshops, garages and tyre shops with intermittent demand.
- Higher pressures for small flows, such as testing, PET blowing and boosting.
- Rural and site work, including PTO-driven tractor-mounted units of about 100–140 cfm at 7–10 bar (101.5–145 psig) for breakers and rock drills.
- Plants that value simple, locally repairable machines.
For steady, continuous plant air of larger volumes, a rotary screw compressor is often the better fit. Our reciprocating compressors hub compares the options.
Common faults explained by the working principle
Because you know the cycle, most faults make sense:
- Slow pump-up or low pressure: leaking valves, worn rings, a choked intake filter, slipping belts or simply plant leaks greater than the compressor’s FAD.
- Very hot cylinder head: a leaking discharge valve lets hot air flow back and get compressed again.
- Air puffing from the intake: a broken or leaking inlet valve.
- Oil in the air lines: worn rings or liners, wrong oil grade, overfilled crankcase or a blocked breather.
- Knocking: loose flywheel or pulley, worn bearings, carbon build-up, or a loose valve.
- Safety valve lifting: pressure switch or unloader fault; stop and investigate before running again.
For a structured approach, see our breakdown diagnostics page.
How Reddy Compressors helps
Reddy Compressors has worked with compressed air since 2002 from our office at Tarnaka, Secunderabad. We sell new and used or refurbished piston compressors, rent compressors from our own fleet, and service and repair reciprocating machines of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Kirloskar and other makes. We are an independent multi-brand specialist, not an authorised distributor of any brand. We have tie-ups with all major and reputed brands.
Our team of over 20 service engineers and technicians carries out valve and ring work, overhauls, pressure switch and unloader repairs, and receiver and safety valve checks across Telangana and Andhra Pradesh. Spare parts are identified from the nameplate, serial number or the old part, and fit is confirmed before we quote. Machines and spares are supplied across India. Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.
What to send us
- Make, model and serial number from the nameplate (a photo is ideal)
- Working pressure you need at the tools or process, in bar or psig
- Single-stage or two-stage, drive type (motor, engine or PTO) and motor kW or HP
- Running hours per day and how long it runs loaded
- The symptom, when it started and any recent repairs
- Photos of the worn part if you need spares
Frequently asked questions
How does a piston air compressor work in simple terms?
A piston moving down a cylinder draws air in through an inlet valve. Moving back up, it squeezes that air into a smaller space, raising its pressure. When the pressure is high enough, a discharge valve opens and the air goes into the tank. The crankshaft repeats this many times a minute, and a pressure switch or unloader stops compression when the tank is full.
What is the difference between single-stage and two-stage piston compressors?
A single-stage compressor compresses air to final pressure in one cylinder stroke and is typically used up to about 7–10 bar (101.5–145 psig). A two-stage machine compresses partly in a large cylinder, cools the air in an intercooler, and finishes in a smaller cylinder. It runs cooler, uses less power for the same air and typically suits 10–14 bar (145–203.1 psig).
Why does my piston compressor get so hot?
Compression always generates heat, but excessive heat usually points to a leaking discharge valve, blocked cooling fins, a slack or wrong belt, low oil, a dirty intake filter or running beyond the machine’s duty rating. In Indian summer heat, poor ventilation around the compressor is a frequent cause. Check the room airflow first, then valves and oil.
Why is the FAD of my compressor lower than the displacement?
Displacement is the swept volume of the pistons and is a theoretical figure. Free air delivery is lower because the air trapped in the clearance volume re-expands, the intake and valves cause pressure losses, hot cylinders heat the incoming air and some air leaks past rings. Always compare compressors on FAD at your working pressure, not on displacement.
How long does a reciprocating compressor last?
A well-maintained industrial piston compressor can give many years of service, because its wearing parts such as valves, rings, liners and bearings can be replaced. Life depends on duty, cleanliness of intake air, oil quality, cooling and whether faults are repaired early. Running small intermittent-duty machines continuously shortens their life considerably.
Is a piston compressor or a screw compressor better for my factory?
Piston compressors suit intermittent demand, smaller flows and higher pressures, and are simple to repair. Screw compressors suit continuous, steady demand and larger volumes with lower maintenance per hour of running. The right choice depends on your flow, pressure, running hours and air-quality needs, so it is best to measure demand before deciding.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.