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Piston Compressors

Piston compressors are a common sight on many types of mobile equipment. For example, they are used to supply pressurized air in a pneumatic brake circuit.
Compressor
A pneumatic brake circuit
But what exactly are they doing? What is going on inside one of these things?
In this lesson we will first examine the basic structure of a piston-style air compressor. Then we will discuss the compression cycle, which is how the compressor transforms boring ol' regular air into fancy, usable, pressurized air.

The Structure

The piston compressor is made up of two main sections. The top section is known as the cylinder head and the lower section is the crankcase.
Cylinder Head
Crankcase
Air Discharge
Air Inlet
Coolant
Coolant
Governor Port
Side
Front

The cylinder head contains the porting needed draw in air and discharge it out again.

In addition there may be a mounting port used to attach a governor (unloading valve) and additional ports used to circulate coolant through the compressor.

Cylinder heads come with many different porting configurations, depending on the needs of the specific application.

Inlet Port
Inlet Valve
Discharge
Port
Discharge
Valve
Pistons
Crankshaft

The crankcase houses the pistons and crankshaft that do the actual pumping.

Crankcases also come in different configurations. These depend on the mounting and drive connections needed for the specific application.

The Compression Cycle

The compression cycle is the series of events that occur as the crankshaft completes one revolution.

The cycle starts with the piston moving down.

As the piston moves down there is a slight vacuum created in the space above the piston.

This vacuum creates a differential between the pressure in the cylinder bore and the pressure in the inlet port.

The pressure differential in turn causes the inlet valve to open...

...creating a path for air to enter the compressor.

As the piston begins its upstroke, it is no longer drawing air into the cylinder bore.

That lack of a pressure differential now allows the spring-loaded inlet valve to close again.

The air is compressed until the air pressure in the cylinder bore is able to overcome the spring setting of the discharge valve.

At which point, as the piston continues its upward stroke, the compressed air is pushed out through the discharge port.

Keep in mind that there are two pistons inside the air compressor. They are positioned to pump in opposition; when one piston is on a downstroke, the other is on an upstroke, and vice versa.

Once the pressure of the compressed air reaches the cut-out setting of the governor (more on that in the governor lesson) the compressed air is used to hold the inlet valve open.

Governor

This allows air to pass in and out of the inlet port without it being compressed.

When the pressure drops below the cut-in setting (more in the governor lesson) the air holding the inlet valve open is allowed to escape through the governor and the compressor begins the compression cycle once more.

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