Powered by the vehicle’s engine, the air compressor draws in air at normal pressure and forces it into a much smaller space, causing the pressure of the air to increase. The Air pressure is measured in pounds per square inch (psi). All components used to attach and power the air compressor must be kept in good condition to ensure a constant supply of compressed air.
The air compressor is typically cooled by the engine coolant system and lubricated by the engine oil supply. The compressor's
unloader valve and governor control the
air brake system pressure between a preset maximum and minimum pressure level by monitoring the pressure in the service (or “supply”) reservoir.
Normaly between 80 and 135 psi.. When the air pressure becomes greater than that of the preset “cut-out”, the governor controls the unloader mechanism of the
compressor to stop the compressor from building air and also causes the
air dryer to purge.
Test Air Compressor
As the service reservoir air pressure drops to the “cut-in” setting of the governor, the governor returns the compressor back to building air and the air dryer to air drying mode.
As the atmospheric air is compressed,
all the water vapor originally in the air is carried along into the air system, as well as a small amount of the lubricating oil as vapor. The duty cycle is the ratio of time the compressor spends building air to the total engine running time. Air compressors are designed to build air (run “loaded”) up to 25% of the
time. Higher duty cycles cause conditions that affect air brake charging system performance which may require additional maintenance.
Factors that add to the duty cycle are:
Air suspension, additional air accessories, use of an undersized compressor, frequent stops, excessive leakage from fittings, connections, lines,
chambers or valves, etc.
The discharge line allows the air, water-vapor and oil-vapor mixture to cool between the compressor and air dryer. The typical size of a vehicle's discharge
assumes a compressor with a normal duty cycle, operating in a temperate climate.
When the temperature of the compressed air that enters the air dryer is within the normal range, the air dryer can remove most of the charging system oil. If the temperature of the compressed air is above the normal range, oil as
oil-vapor is able to pass through the air dryer and into the air system.