An air-compressor governor switches the compressor between loaded and unloaded states as receiver pressure passes lower and upper setpoints. The compressor can remain mechanically driven while unloaded. This schematic shows the pressure signal, sensing piston and spring, and unloader connection. Use the calculator to compare assumed steady air supply and demand with receiver refill time; the inputs are not recommended vehicle or equipment settings.
Air Compressor Governor Interactive Calculator
Compare assumed compressor delivery, steady demand, receiver volume and pressure band to estimate loaded duty and refill time. The animation separately shows the complete fill-and-drain cycle.
Equation Used
For a steady repeating cycle, loaded duty fraction is demand divided by compressor delivery. Fill time uses net supply after demand, receiver volume and pressure band. It excludes the unloaded drain interval. Gallons are converted to cubic feet and minutes to seconds; all flow rates must use the same free-air reference.
- Compressor and demand are free-air flow rates in CFM.
- Reservoir volume is converted from gallons to cubic feet.
- Atmospheric reference pressure is 14.7 psi.
- DeltaP equals cut-out pressure minus cut-in pressure.
Fill time is the loaded refill interval, not a complete pressure cycle. The full cycle also includes receiver drain-down while unloaded. Fill-only rate is 3600 divided by fill time, not actual cycles per hour.
Loading and unloading the compressor
Receiver pressure acts on a sensing element opposed by a spring. At cut-out, the governor changes the unloader signal so the compressor stops delivering air to the receiver. When demand lowers pressure to cut-in, the signal changes back and delivery resumes.
In the illustrated pneumatic arrangement, unloading pressure acts on the compressor’s unloader mechanism and is vented when loading resumes. The actual unloader action depends on the compressor. A motor pressure switch may instead stop and restart an electric drive, so the two control arrangements are not universally interchangeable.
Parts in the schematic
- Receiver connection: supplies the sensed pressure.
- Piston and spring: respond to pressure and the governor’s mechanical setting.
- Signal passage: communicates the required state to the unloader.
- Exhaust: vents the signal in the appropriate state.
Cut-in and cut-out form a hysteresis band. They are not independent adjustments on every governor: some models have a fixed differential or non-adjustable settings. The independent sliders explore a mathematical band and do not describe an adjustment procedure for a particular product.
Pressure control in an air system
Governor-controlled unloading is used where compressor operation must respond to stored-air pressure. Complete systems can include dryers, check valves, reservoir protection and controls appropriate to the drive and application.
Vehicle brake systems require compatible components and the vehicle manufacturer’s pressure settings. A calculated refill time does not establish brake-system performance, compliance or an acceptable replacement governor. It also does not diagnose leaks or justify changing the band to conceal a fault.
Fill time is only part of the cycle
Let Qc be loaded compressor delivery and Qd steady demand in CFM on the same free-air basis. For 0 < Qd < Qc and a positive pressure band, average loaded duty fraction is DC = Qd/Qc. The displayed percentage is 100DC.
The receiver volume is V = gallons × 0.133681 in ft³. With ΔP = cut-out − cut-in in psi and reference pressure 14.7 psi, the equivalent stored free-air change is C = VΔP/14.7 in ft³. The model assumes constant temperature, fixed flows and matching reference conditions.
Loaded refill time is 60C/(Qc − Qd) seconds. While unloaded, receiver drain time is 60C/Qd seconds. A full cycle is the sum of fill and drain times. The Fill-only rate card is 3,600 divided by fill time; it is not actual cycles per hour. Actual cycles per hour would be 3,600 divided by the complete cycle duration.
At zero demand, the receiver fills once and then holds unloaded in this idealization; there is no repeating cycle. When demand equals or exceeds supply, pressure cannot refill. A reported demand-to-supply percentage above 100% indicates inadequate supply, not a realizable loaded-time fraction.
Cut-out must exceed cut-in and receiver volume must be positive for a physical cycle. A zero-volume input is only a degenerate arithmetic limit. Real flow varies with pressure and drive speed, and thermal effects can change measured timing. This calculation determines neither permissible duty nor receiver or relief settings.
Worked example: refill versus full cycle
Assume Qc = 15.8 CFM, Qd = 4.0 CFM, receiver volume 60 US gallons and a 20 psi pressure band. These are hypothetical inputs rather than settings for a named vehicle.
Loaded duty is 4.0/15.8 ≈ 25.316%. Receiver volume is 8.02086 ft³ and the stored free-air change across the band is approximately 10.9127 ft³.
Net fill is 15.8 − 4.0 = 11.8 CFM, giving fill time approximately 55.49 seconds. Drain time is approximately 163.69 seconds. The full repeating cycle is therefore about 219.18 seconds, or 16.42 cycles per hour.
The Fill-only rate card shows approximately 64.88 per hour because it divides 3,600 by 55.49; it does not include drain time. Widening the hypothetical band by 50% lengthens both intervals by 50% under the same assumptions, while average duty remains unchanged. It does not establish that the wider pressure band is acceptable for real equipment.
Different pressure-control functions
| Device | Function | Important distinction |
|---|---|---|
| Pneumatic governor | Signals a compressor unloader from receiver pressure. | The compressor drive can keep turning. |
| Motor pressure control | Commands an electric drive to start or stop, with appropriate switching equipment. | Compatibility depends on the motor, starter and compressor. |
| Downstream regulator | Controls pressure supplied to a downstream branch. | It does not replace receiver pressure control or protective relief. |
| Electronic air management | Coordinates sensors and controlled components. | Functions and limits depend on the particular system. |
Setpoints, differential, port arrangement and control compatibility must match the installation. Service life and accuracy are product-specific, not universal properties of these device names.
Governor calculation questions
Fill time includes only the loaded interval from cut-in to cut-out. A repeating cycle also includes the unloaded period while demand lowers receiver pressure.
Not in this constant-flow idealization. A larger receiver lengthens fill and drain intervals in proportion, while Qd/Qc remains the same.
No. It is the reciprocal of fill time expressed per hour. Actual repeating cycles require both fill and drain time.
Net fill is zero, so pressure cannot recover from cut-in to cut-out under the stated constant-flow assumptions. There is no finite refill interval.
The model fills the receiver once and then holds at cut-out while unloaded. The illustrative demonstration repeats, but a real zero-demand idealization has no continuing pressure cycle.
No. They explore assumed setpoints. Follow the vehicle and component specifications; some governor differentials or complete settings are not adjustable.
No. Demand, leaks, receiver volume, control behavior and measurement conditions can all affect timing. The calculator is not a diagnostic procedure.
No. A regulator controls downstream pressure and does not by itself control compressor loading or protect the receiver from overpressure.
References and further reading
U.S. Department of Energy: Improving Compressed Air System Performance explains receiver storage and supply-demand analysis. Bendix commercial-vehicle systems catalog lists governor variants and differing pressure ranges, including non-adjustable versions. Use the applicable service documentation and original-equipment specifications for a particular installation.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
