Compressor charging (Fig. 12.9(a)) During the charging phase, air from the compressor enters the reservoir, builds up pressure and then passes to the braking system (Fig. 12.9(a)). A small sample of air from the reservoir is also piped to the underside of the governor piston via the governor inlet port.

When the pressure in the reservoir is low, the piston will be in its lowest position so that there is a gap between the plunger's annular end face and the exhaust disc valve. Thus air above the unloader plunger situated in the compressor's cylinder head is able to escape into the atmosphere via the governor plunger tube central passage.

Compressor unloaded (Fig. 12.9(b)) As the reservoir pressure rises the control spring is compressed lifting the governor piston until the exhaust disc valve contacts the plunger tube, thereby closing the exhaust valve. A further air pressure increase from the reservoir will lift the piston seat clear of the inlet disc valve. Air from the reservoir now flows around the inlet disc valve and plunger tube. It then passes though passages to the unloader plunger upper chamber. This forces the unloader plunger down, thus permanently opening the inlet disc valve situated in the compressor's cylinder head (Fig. 12.9(b)). Under these conditions the compressor will draw in and discharge air from the cylinder head inlet port, thereby preventing the compressor pumping and charging the reservoir any further. At the same time, air pressure acts on the annular passage area around the governor plunger stem. This increases the force pushing the piston upwards with the result that the inlet disc valve opens fully. When the brakes are used, the reservoir pressure falls and, when this pressure reduction reaches 1 bar, the control spring downward force will be sufficient to push down the governor piston and to close the inlet disc valve initially.

Instantly the reduced effective area acting on the underside of the piston allows the control spring to move the piston down even further until the control exhaust valve (tube/disc) opens. Compressed air above the unloader plunger will flow back to the governor unit, enter the open governor plunger tube and exhaust into the atmosphere. The unloader plunger return spring now lifts the plunger clear of the cylinder head inlet disc, permitting the compressor to commence charging the reservoir.

The compressor will continue to charge the system until the cut-out pressure is reached and once again the cycle will be repeated.

Fig. 12.9 Compressor mounted unloader with separate governor

12.3.4 Unloader valve (diaphragm type)

Compressor charging (Fig. 12.10(a)) When air is initially pumped from the compressor to the reservoir, the unloader valve unit non-return valve opens and air passes from the inlet to the outlet port. At the same time, air flows between the neck of the exhaust valve and the shoulder of the relay valve piston, but since they both have the same cross-sectional area, the force in each direction is equalized. Therefore, the relay piston return spring is able to keep the exhaust valve closed. Air will also move through a passage on the reservoir side of the non-return valve to the chamber on the plunger side of the diaphragm.

Compressor unloaded (Fig. 12.10(b)) As the reservoir pressure rises, the diaphragm will move against the control spring until the governor plunger has shifted sufficiently for the exhaust valve to close (Fig. 12.10(b)). Further pressure build-up moves the diaphragm against the control spring so that the end of the plunger enters its bore and opens the inlet valve. The annular end face of the plunger will also be exposed to the air pressure, so that the additional force produced fully opens the inlet valve. Air now passes through the centre of the plunger and is directed via a passage to the head of the relay piston.

Eventually a predetermined maximum cut-out pressure is reached, at which point the air pressure acting on the relay piston crown overcomes the relay return spring, causing the relay exhaust valve to open, expelling the compressed air into the atmosphere. This enables the compressor to operate under no-load conditions while the reservoir and braking system is sufficiently charged.

Compressor commences charging (Fig. 12.10 (a and b)) As the stored air is consumed during a braking cycle, the pressure falls until the cut-in point (minimum safe working pressure) is reached. At this point the control spring force equals and exceeds the opposing air pressure force acting on the diaphragm on the plunger side. The diaphragm and plunger will therefore tend to move away from the control spring until the plunger stem closes the inlet valve. Further plunger movement pushes the exhaust valve open so that trapped air in the relay

(a> Compressor charging

Fig. 12.10(a and b) Unloader valve (diaphragm type)

(a> Compressor charging

Fig. 12.10(a and b) Unloader valve (diaphragm type)

piston crown chamber is able to escape to the atmosphere. The relay piston return spring closes the relay exhaust valve instantly so that compression of air again commences, permitting the reservoir to recharge to the pressure cut-out setting.

12.3.5 Unloader valve (piston type)

Purpose The unloader valve enables the compressor to operate under no-load conditions, once the reservoir is fully charged, by automatically discharging the compressor's output into the atmosphere, and to reconnect the compressor output to the reservoir once the air pressure in the system drops to some minimum safe working value.

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