1232 Reciprocating air compressors

The source of air pressure energy for an air brake system is provided by a reciprocating compressor driven by the engine by either belt, gear or shaft-drive at half engine speed. The compressor is usually base- or flange-mounted to the engine.

To prevent an excessively high air working temperature, the cast iron cylinder barrel is normally air cooled via the enlarged external surface area provided by the integrally cast fins surrounding the upper region of the cylinder barrel. For low to moderate duty, the cylinder head may also be air cooled, but for moderate to heavy-duty high speed applications, liquid coolant is circulated through the internal passages cast in the aluminium alloy cylinder head. The heat absorbed by the coolant is then dissipated via a hose to the engine's own cooling system. The air delivery temperature should not exceed 220° C.

Lubrication of the crankshaft plain main and bigend bearings is through drillings in the crankshaft, the pressurized oil supply being provided by the engine's lubrication system, whereas the piston and rings and other internal surfaces are lubricated by splash and oil mist. Surplus oil is permitted to drain via the compressor's crankcase back to the engine's sump. The total cylinder swept volume capacity needed for an air brake system with possibly auxiliary equipment for light, medium and heavy commercial vehicles ranges from about 150 cm3 to 500 cm3, which is provided by either single or twin cylinder reciprocating compressor. The maximum crankshaft speed of these compressors is anything from 1500 to 3000rev/min depending upon maximum delivery air pressure and application. The maximum air pressure a compressor can discharge continuously varies from 7 to 11 bar. A more typical maximum pressure value would be 9 bar.

The quantity of air which can be delivered at maximum speed by these compressors ranges from 150 L/min to 500 L/min for a small to large size compressor. This corresponds to a power loss of something like 1.5 kW to 6kW respectively.

Compressor operation When the crankshaft rotates, the piston is displaced up and down causing air to be drawn through the inlet port into the cylinder on the down stroke and the same air to be pushed out on the upward stroke through the delivery port. The unidirectional flow of the air supply is provided by the inlet and delivery valves. The suction and delivery action of the compressor may be controlled by either spring loaded disc valves (Fig. 12.9) or leaf spring (reed) valves (Fig. 12.8). For high speed compressors the reed type valve arrangements tend to be more efficient.

On the downward piston stroke the delivery valve leaf flattens and closes, thus preventing the discharged air flow reversing back into the cylinder (Fig. 12.8). At the same time the inlet valve is drawn away from its seat so that fresh air flows through the valve passage in its endeavour to fill the expanding cylinder space.

On the upward piston stroke the inlet valve leaf is pushed up against the inlet passage exit closing the valve. Consequently the trapped pressurized air is forced to open the delivery valve so that the air charge is expelled through the delivery port to the reservoir.

The sequence of events is continuous with a corresponding increase in the quantity of air delivered and the pressure generated.

The working pressure range of a compressor may be regulated by either an air delivery line mounted unloader valve (Figs 12.10 and 12.11) or an integral compressor unloader mechanism controlled by an external governor valve (Fig. 12.9). A further feature which is offered for some applications is a multiplate clutch drive which reduces pumping and frictional losses when the compressor is running light (Fig. 12.8).

Clutch operation (Fig. 12.8) With the combined clutch drive compressor unit, the compressor's crankshaft can be disconnected from the engine drive once the primary reservoir has reached its maximum working pressure and the compressor is running light to reduce the wear of the rotary bearings and reciprocating piston and rings and to eliminate the power consumed in driving the compressor.

The clutch operates by compressed air and is automatically controlled by a governor valve similar to that shown in Fig. 12.9.

Release piston

, 12.8 Single cylinder air compressor with clutch drive

The multiplate clutch consists of four internally splined sintered bronze drive plates sandwiched between a pressure plate and four externally splined steel driven plates (Fig. 12.8). The driven plates fit over the enlarged end of the splined input shaft, whereas the driven plates are located inside the internally splined clutch outer hub thrust plate. The friction plate pack is clamped together by twelve circumferentially evenly spaced compression springs which react between the pressure plate and the outer hub thrust plate. Situated between the air release piston and the outer hub thrust plate are a pair of friction thrust washers which slip when the clutch is initially disengaged.

When the compressor air delivery has charged the primary reservoir to its preset maximum, the governor valve sends a pressure signal to the clutch air release piston chamber. Immediately the friction thrust washers push the clutch outer hub thrust plate outwards, causing the springs to become compressed so that the clamping pressure between the drive and driven plates is relaxed. As a result, the grip between the plates is removed. This then enables the crankshaft, pressure plate, outer hub thrust plate and the driven plates to rapidly come to a standstill.

As the air is consumed and exhausted by brake or air equipment application, the primary reservoir pressure drops to its lower limit. At this point the governor exhausts the air from the clutch release piston chamber and consequently the pressure springs are free to expand, enabling the drive and driven plates once again to be squeezed together. By these means the engagement and disengagement of the compressor's crankshaft drive is automatically achieved.

12.3.3 Compressor mounted unloader with separate governor (Fig. 12.9(a and b))

Purpose The governor valve unit and the unloader plunger mechanism control the compressed air output which is transferred to the reservoir by causing the compressor pumping action to 'cut-out' when the predetermined maximum working pressure is attained. Conversely, as the stored air is consumed, the reduction in pressure is sensed by the governor which automatically causes the compressor to 'cutin', thus restarting the delivery of compressed air to the reservoir and braking system again.

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