Operation

Compressor charging (Fig. 12.11(a)) When the compressor starts to charge, air will flow to the reservoir by way of the horizontal passage between the inlet and outlet ports.

The chamber above the relay piston is vented to the atmosphere via the open outlet pilot valve so that the return spring below the relay piston is able to keep the exhaust valve closed, thus permitting the reservoir to become charged.

Compressor unloaded (Fig. 12.11(b)) As the reservoir pressure acting on the right hand end face of the pilot piston reaches a maximum (cut-out setting), the pilot piston pushes away from its inlet seat. A larger piston area is immediately exposed to the air pressure, causing the pilot piston to rapidly move over to its outlet seat, thereby sealing the upper relay piston chamber atmospheric vent. Air will now flow along the space made between the pilot piston and its sleeve to act on the upper face of the relay piston. Consequently, the air pressure on both sides of the relay piston will be equalized momentarily. Air pressure acting down on the exhaust valve overcomes the relay piston return spring force and opens the compressor's discharge to the atmosphere. The exhaust valve will then be held fully open by the air pressure acting on the upper face of the relay piston. Compressed air from the compressor will be pumped directly to the atmosphere and so the higher pressure on the reservoir side of the non-return valve forces it to close, thereby preventing the stored air in the reservoir escaping.

Compressor commences charging (Fig. 12.11(a and b)) As the air pressure in the reservoir is discharged and lost to the atmosphere during brake applications the reservoir pressure drops. When the pressure has been reduced by approximately one bar below the cut-out setting (maximum pressure), the control spring overcomes the air pressure acting on the right hand face of the pilot piston, making it shift towards its inlet seat. The pilot piston outlet

i> Compressor charging

Fig. 12.11 (a and b) Unloader valve (piston type)

i> Compressor charging

Fig. 12.11 (a and b) Unloader valve (piston type)

valve opens, causing the air pressure above the relay piston to escape to the atmosphere which allows the relay piston return spring to close the exhaust valve. The discharged air from the compressor will now be redirected to recharge the reservoir.

The difference between the cut-out and cut-in pressures is roughly one bar and it is not adjustable, but the maximum (cut-out) pressure can be varied over a wide pressure range by altering the adjustment screw setting.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment