What a locomotive or EMU compressor does
Every locomotive, EMU and metro train carries its own compressor. It charges the main reservoir, and that stored air feeds the brake system and a range of control and auxiliary functions. If the compressor cannot keep the main reservoir up, the train cannot brake reliably and will not be cleared to run.
- Brake air. The main reservoir supplies the brake pipe and brake cylinders through the driver’s brake valve and distributor valves. Brakes are released by pressure and applied by reducing it, so a healthy air supply is a safety matter.
- Control and auxiliary air. Pantograph raising, door operation on multiple units, sanders, horns, wipers, suspension levelling and electro-pneumatic contactors, depending on the vehicle.
- Working pressure. Main reservoir pressure is typically in the range of about 8–10 bar(g) (116–145 psig), with the compressor governor cutting in and out within that band. The exact settings are fixed by the operator’s specification for each class of vehicle and must not be changed on site.
Rolling-stock compressors work hard: frequent starts, vibration, wide temperature swings, dust from the track and brake blocks, and in Telangana and Andhra Pradesh summer under-frame temperatures well above the 45 °C shade temperature.
Compressor types on rolling stock
At a general level, two families of compressor are found on locomotives and multiple units, with smaller auxiliary compressors on some vehicles.
| Type | Where typically found | Service points |
|---|---|---|
| Two- or three-cylinder, two-stage reciprocating (low-pressure and high-pressure cylinders with intercooler) | Many diesel and electric locomotives, older multiple units | Suction and delivery valves, rings, pistons, connecting-rod and main bearings, intercooler, unloader, lubrication |
| Oil-injected rotary screw packages | Newer locomotives, EMUs and metro stock | Airend bearings, oil and separator, minimum-pressure valve, thermostatic valve, cooler, motor and drive coupling |
| Small auxiliary compressor (often single-stage piston) | Used to raise pantographs or prime control air when the main reservoir is empty | Valves, rings, motor brushes or battery supply, non-return valve |
The drive may be from the engine crankshaft, by an electric motor, or through a clutch or coupling, depending on the vehicle. For reciprocating theory in more depth, see two-stage reciprocating compressors and water-cooled reciprocating compressors.
Fault finding: common locomotive compressor problems
The table covers the faults we are most often asked about. Always check against the operator’s maintenance instructions, and isolate and vent the system before opening anything.
| Symptom | Likely causes | Checks |
|---|---|---|
| Low main reservoir pressure | Leaking valves or rings, leaks in pipework and couplings, governor set or failing low, unloader stuck open | Pump-up test (below), leak-down with compressor stopped, governor cut-in and cut-out readings against specification, unloader operation |
| Slow charging | Worn suction or delivery valves, worn rings or liners, blocked intake filter, slipping drive, low motor speed | Intake filter condition, interstage pressure (high interstage points to HP valves, low to LP valves or rings), drive speed |
| Oil carry-over into reservoir and lines | Worn rings or oil-control rings, over-filled crankcase, high crankcase pressure, failed separator on screw units | Oil level, breather condition, ring and liner wear, separator element and scavenge line on screw packages |
| Overheating, high delivery temperature | Fouled or blocked intercooler or aftercooler, leaking delivery valves recompressing hot air, high ambient, low oil | Cooler fins and air path, delivery temperature at each stage, valve condition, oil level and grade |
| Knocking | Worn connecting-rod or main bearings, loose valve or valve cover, excessive piston-to-head clearance change, broken valve plate | Bearing clearances, valve seat and fixing torques, piston bump clearance against drawing |
| Safety valve lifting | Governor not cutting out, unloader not working, blocked line between compressor and reservoir, HP valve fault giving high interstage | Governor and pressure switch settings, unloader, interstage safety valve, line restrictions; safety valve set pressure and reseat |
| Moisture in brake lines | Air dryer not regenerating, failed drain valves, saturated desiccant, aftercooler not cooling | Dryer cycling and purge, drain valve operation, desiccant condition, dew point at dryer outlet |
Valve and piston faults are covered in detail in reciprocating compressor valve engineering, pistons, rings and cylinder condition and unloader systems.
Worked example: charging-time (pump-up) test
The quickest condition check on a rolling-stock compressor is to time how long it takes to raise the main reservoir between two pressures. Operators usually specify a maximum charging time for each vehicle class; this calculation turns the time into delivered free air so it can be compared with the compressor rating. The general method is on our pump-up test page.
Illustrative case:
- Main reservoir volume 1.00 m³ plus pipework and other connected volume 0.05 m³: V = 1.05 m³.
- Pressure rises from 6.0 to 9.0 bar(g): ΔP = 3.0 bar.
- Time 100 seconds = 1.667 minutes.
- Reference atmosphere in Hyderabad about 0.950 bar(a).
Free air delivered = V × ΔP ÷ (Patm × t) = 1.05 × 3.0 ÷ (0.950 × 1.667) = 3.15 ÷ 1.583 = 1.99 m³/min (about 70 cfm).
If the compressor is rated at 2.2 m³/min, it is delivering about 90% of rating. That usually calls for a valve inspection and leak check rather than an immediate overhaul. Points to note:
- Using sea-level 1.01325 bar(a) instead of 0.950 gives 1.87 m³/min, so state which reference you use and keep it the same from test to test.
- Do a leak-down first with the compressor stopped. If the reservoir falls 0.4 bar in one minute, leakage = 1.05 × 0.4 ÷ 0.950 = 0.44 m³/min of free air, which is lost during the test and must be added back or, better, the leaks fixed.
- Test at a normal working temperature and keep the test short; a hot compressor and cooling reservoir both distort the result.
Overhaul scope and inspection measurements
A locomotive compressor overhaul is done to the operator’s schedule and drawings, with limits taken from the maker’s or operator’s documents. The table shows a typical scope for a two-stage reciprocating unit and the measurements we record. We do not substitute our own limits for the specified ones.
| Component | Typical work | Inspection and measurement |
|---|---|---|
| Suction and delivery valves | Dismantle, clean, lap or replace seats, replace plates and springs | Plate flatness and wear, spring free length, seat condition, leak test of assembled valve |
| Piston rings | Replace compression and oil-control rings | Ring end gap in bore, side clearance in groove |
| Pistons and cylinders or liners | Clean, inspect, hone or replace liners as specified | Bore diameter at several heights and angles (ovality and taper), piston diameter, piston-to-bore clearance, bump clearance |
| Bearings and crankshaft | Replace main and connecting-rod bearings, check journals | Journal diameter and ovality, bearing clearance, crankshaft end float, run-out |
| Intercooler and aftercooler | Clean externally and internally, pressure test | Leak test at specified pressure, fin condition, intercooler safety valve setting |
| Unloader and governor | Overhaul diaphragms, seals, springs | Operating pressures, response, leak tightness |
| Coupling and drive | Replace flexible elements, check alignment | Angular and parallel alignment, element wear, key and keyway condition |
| Assembled compressor | Run-in and test | Charging time or FAD, interstage and delivery pressures and temperatures, oil pressure, vibration, oil carry-over |
For our general approach to overhaul, see compressor overhauling and refurbishment and reciprocating service and reliability.
Air drying for brake systems
Compressed air leaving the compressor is saturated with water vapour. As it cools in the reservoir and brake pipe, water condenses. In brake equipment that water washes out lubricant, corrodes valve bodies, swells rubber seals and, on cold mornings or in high-altitude routes, can freeze in small ports. It also carries dust and oil into distributor valves, causing sticky brakes and slow release.
- Dew point is what matters. Air must be dried so that its pressure dew point stays well below the lowest temperature the pipes will see. Rolling stock commonly uses twin-tower heatless desiccant dryers that regenerate with a share of the dried air.
- Monsoon load. At 35 °C and high humidity in July–September, the water entering a compressor is several times that of a dry winter day. Drain valves and dryers must cope with that peak, not an average.
- Check, do not assume. A dryer can cycle normally and still pass wet air if its desiccant is oil-fouled or crushed. Measure outlet dew point or check reservoir drains for water.
Depot dryers and the theory behind them are covered in desiccant adsorption air dryers.
Depot and workshop plant air: illustrative sizing
Depot plant air is ordinary industrial compressed air, and here we can supply new, used or refurbished compressors, dryers and receivers as well as service them. The design principles for depots are on compressed air for rail and metro maintenance depots; the example below shows the sizing arithmetic for a single maintenance shed.
| User (illustrative) | Number | Air each (m³/min) | Use factor | Average demand (m³/min) |
|---|---|---|---|---|
| 1/2 inch impact wrenches | 8 | 0.35 | 0.30 | 0.84 |
| Grinders | 4 | 0.90 | 0.25 | 0.90 |
| Blow guns | 6 | 0.25 | 0.15 | 0.23 |
| Brake test rigs | 2 | 0.60 | 0.50 | 0.60 |
| Under-frame blowing lances | 2 | 1.50 | 0.30 | 0.90 |
| Air jacks and hoists | 2 | 0.50 | 0.10 | 0.10 |
| Total | Connected 13.1 | 3.57 |
- Average demand 3.565 m³/min + 10% leakage allowance (0.357) = 3.92 m³/min.
- Plus 20% for growth: 3.92 × 1.2 = 4.71 m³/min (166 cfm). A duty compressor of about 5 m³/min (177 cfm) at about 8.5 bar(g), to give 7–8 bar at the brake rigs after pressure drop, with an identical standby (N+1).
- Peak check: if both blowing lances run together for 3 minutes, demand = 3.565 − 0.90 + 3.00 + 0.357 = 6.02 m³/min, which is 1.02 m³/min above one machine.
- Receiver to ride through that peak with a 1.0 bar allowed drop: volume = 1.02 × 3 × 0.950 ÷ 1.0 = 2.9 m³, so a 3 m³ receiver, or let the standby cut in on a lower pressure setting.
Brake test rigs should have their own point-of-use filter and, if the main dryer is refrigerated, a local desiccant dryer to give a lower dew point at the rig.
How Reddy Compressors helps
- Service, overhaul support and spare parts for railway and locomotive air compressors, both reciprocating and screw types; we do not sell rolling-stock compressors.
- Work done to the operator’s own specifications, drawings and limits, with measurements recorded for the operator’s records.
- Supply, service and AMC of depot and workshop plant compressors, air dryers and filters of all major makes.
- Rental fleet of 50–2,000 cfm at 2–30 bar for interim air while a depot compressor is overhauled.
- 20+ service engineers and technicians, based in Secunderabad since 2002, covering Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad; project-based work elsewhere in India.
What to send us
- Photos of the compressor make and model plate, and of the unit in place.
- Drawings, parts lists or the operator’s maintenance specification for the compressor, if available.
- Failure history: symptoms, charging times, interstage and delivery pressures and temperatures, and last overhaul date.
- Scope required: inspection, repair, overhaul, spares only, or testing.
- For depot air: list of users, shift pattern, existing compressors and dryers, and power supply.
Frequently asked questions
Do you service railway compressors in Hyderabad?
Yes. We provide service, repair, overhaul support and spare parts for railway and locomotive air compressors in Hyderabad, Secunderabad and across Telangana and Andhra Pradesh, and on a project basis elsewhere in India. We work to the operator’s specifications and drawings. We do not sell rolling-stock compressors, but we do supply and service depot and workshop plant compressors, dryers and filters. Send the compressor plate details and a description of the fault.
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Short explainers on this topic
Slide-by-slide engineering explainers from Reddy Compressors. More in the video library.
What pressure does a locomotive air compressor work at?
Main reservoir pressure on locomotives and multiple units is typically in the range of about 8–10 bar(g), or about 116–145 psig. The compressor governor cuts in at the lower setting and out at the upper one, and a safety valve protects the reservoir above that. The exact settings are fixed for each vehicle class by the operator’s specification and should only be checked and adjusted to those values.
Can you supply locomotive compressor spares?
We supply spares such as valve assemblies and plates, springs, piston rings, gaskets and seals, bearings, filters, unloader and governor parts, and couplings, matched from the make and model plate, part numbers and drawings. Where the operator specifies approved sources or particular part numbers, those requirements govern what can be fitted. Send photos of the plate and the old part, and any drawing or part number you have.
What is included in a loco compressor overhaul?
A typical overhaul of a two-stage reciprocating unit covers valves, rings, pistons and cylinders or liners, main and connecting-rod bearings, intercooler cleaning and pressure test, unloader and governor, and coupling, with measurements of bores, clearances and ring gaps against the specified limits. The overhauled compressor is then run in and tested for charging time or delivered air, pressures, temperatures and oil carry-over. The scope and limits come from the operator’s schedule.
Why is my locomotive compressor taking a long time to charge?
The usual causes are worn or broken suction and delivery valves, worn rings or liners, a blocked intake filter, an unloader stuck open, a slipping drive or leaks in the reservoir system. Do a leak-down with the compressor stopped, then a timed charge between two pressures and convert it to delivered free air. Interstage pressure helps locate the fault: high interstage suggests the high-pressure stage, low interstage the low-pressure stage.
Why is there oil and water in the brake pipe?
Oil usually comes from worn rings, high crankcase pressure or overfilling on piston compressors, or a failed separator on screw units. Water means the air dryer is not removing moisture: saturated or oil-fouled desiccant, a failed purge or drain valve, or an aftercooler not cooling. Both damage brake valve seals. Fix the oil source first, as oil ruins desiccant.
What size compressor does a rail depot workshop need?
Size it from the users and how often each runs, not by adding up nameplates. In our illustrative shed with impact wrenches, grinders, blow guns, two brake test rigs and blowing lances, average demand with leakage and growth was about 4.7 m³/min, so one 5 m³/min duty machine plus an identical standby and a 3 m³ receiver covers the cleaning peak. We can supply and service depot plant compressors of all major makes.
Related pages
- Compressed air for rail and metro maintenance depots
- Two-stage reciprocating compressors
- Reciprocating compressor valve engineering
- Pistons, rings and cylinder condition
- Reciprocating compressor unloader systems
- Compressor overhauling and refurbishment
- Compressor pump-up test and capacity check
- Compressor spare parts in Hyderabad
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.