General information about the pneumatic system

General information about the pneumatic system
Design of the pneumatic system
Design of the pneumatic systemDefinitionsIt is important to have knowledge about the following definitions and concepts re-garding the vehicle’s pneumatic system.

Compressed air tankA compressed air tank is a pressurised tank containing compressed air. Compressed air tanks are used in the brake system and other systems which require compressed air. These tanks are marked so that they can be identified. The marking must not be painted over.

More information on marking is found under the heading “Identification of com-pressed air tanks".

More information is found in the document Modifying the pneumatic system.

More information on marking can be found in the document “Painting chassis-relat-ed components".

APSThe APS unit controls the air pressure in the pneumatic system. APS (Air Processing System) consists of the components which are part of the pneumatic system supply circuit: air dryer, protection valve, pressure sensor and pressure limiting valve. Ve-hicles are equipped with different variants of APS from the factory depending on the specification.

More information on APS is found in the document Auxiliary compressed air.

Scania Truck Bodybuilder 22:10-551 Issue 3 2021-03-16© Scania CV AB 2021, Sweden 1 (9)



Air consumerAn air consumer is a component that uses compressed air, e.g. a brake cylinder.

ABS and EBSABS (Anti-lock Brake System and EBS (Electronic Brake System) are the brakes’ control systems and comprise valves and other components. ABS and EBS distribute the compressed air to the brakes on the wheels.




ank for brake system, parking brake circuit

Tank for air suspension, air suspension circuit

326

308

326

309

Location of compressed air tanksThe compressed air tanks can be located in different positions depending, among oth-er things, on the following:

• Vehicle type• Wheel configuration• Axle distance• Suspension• Type of cab• Transport-mounted

More information on the location of the compressed air tanks can be found in the ve-hicle’s individual chassis drawing (ICD) (Individual Chassis Drawing).

More information on transport mounting of auxiliary compressed air tanks is avail-able in the document “Modifying the pneumatic system".

Identification of compressed air tanksEach compressed air tank is marked with one of the following function symbols:

Brake system tank, front circuit Brake system tank, rear circuit T

326

306

32

6 3

07




2524

26

232221

APS

21

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Schematic diagram of the compressed air supply for leaf spring suspension or for a vehicle with air suspension without a compressed air tank in the air suspension cir-

cuit.

2524

26

232221

APS

21

A

B

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Schematic diagram of the compressed air supply for a vehicle with air suspension with a compressed air tank for the air suspension circuit.

Pneumatic system circuitsThe pneumatic system consists of 6 different circuits with different system pressures:

APS controls the compressor, which in turn supplies the compressed air circuits with compressed air. The compressed air circuits can work directly from the APS or can contain air storage in the form of compressed air tanks.

Component designations, see illustrations:

Circuit Max. pressure (bar)

Connection in the APS

Brakes, rear circuit 12.5 21Brakes, front circuit 12.5 22Parking and trailer 8.5 23Accessories 8.5 24Air suspension 12.5 25Powertrain 8.5 26

Component DesignationAPS Air Processing SystemA Overflow valveB Double check valve




Generally, the greater the number of compressed air tanks a circuit has, the longer it takes to fill the system and the more time the compressor and the APS need to oper-ate. This means that the design of an optimum pneumatic system is dependent on air consumption and driving cycle.

IMPORTANT!

If the air supply is insufficient, the brakes will be affected and the protection system will trigger warning signals.




Air consumptionThe table below shows the highest permitted continuous output of compressed air, 40% DC (duty cycle) of the maximum flow, from each compressor at different en-gine speeds. The table applies to a 13 litre diesel engine at a system pressure of 10 bar/1 MPa.

At lower engine speeds, e.g. at idling, outputs of up to 80% DC of the maximum flow may be permitted. The higher the engine speed, the lower the DC is recommended.

IMPORTANT!

The air taken from the compressor might contain oil particles, formed at oil carry-over, and should therefore not be used in a manner that could cause damage.

IMPORTANT!

For the desiccant in the air dryer to be dehumidified sufficiently, and for the APS to have time to regenerate, the compressor’s connection time must not exceed 50% of the engine’s running time.

Engine speed (rpm) 636 cc (l/min) 720 CC (l/min)500 84 106750 128 1641,000 168 2161,250 204 264


Operating the pneumatic system


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Union for charging compressed air.

Operating the pneumatic systemCharging compressed airExternal filling of compressed air must only be carried out via the union located be-hind the left-hand front wheel mudguard, see illustration. The connection is intended for charging air, not a constant supply from a compressed air unit.

IMPORTANT!

When the vehicle is switched off, the APS air dryer control unit is switched off. If the vehicle is pressurised for an extended period of time when the APS air dryer is switched off, condensation forms in the APS air dryer without being registered by the control unit. This can lead to damage to the pneumatic system when starting the ve-hicle.

If the vehicle requires constant filling of compressed air, the compressed air unit must be designed so that it only pressurises the vehicle for a limited time, max. 5 min-utes per hour.

The maximum filling pressure for the vehicle is 1 MPa (10 bar).

Note:Before charging, check that the external compressed air is free from oil and water.




Assembling and disassembling compressed air pipesDifferent methods are required for assembling and disassembling compressed air pipes depending on whether the insert unions are made of brass or composite mate-rial.

More information on connection and renewal of unions is found in the document Modifying the pneumatic system.

The compressed air pipes must comply with SS-ISO 7628.

It is important that the compressed air pipes are of a high quality. The compressed air pipes and compressed air valves can be purchased from a Scania dealer.

IMPORTANT!

Compressed air pipes in brake circuits must only be joined with Scania approved connector unions made of a composite material. This also applies to trailer brake cir-cuits.More information on compressed air pipes is available in the document “Modifying the pneumatic system".




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1

2

1. Compressed air outlet located at the third crossmember2. Solenoid valves located at the rear end of the chassis.

Connection of air consumersIMPORTANT!

The entire pneumatic system must be depressurised before any part of the pneumatic system is removed. The plug is pushed out if the pneumatic system is pressurised when the plug is removed from the compressed air outlet.

IMPORTANT!

Make sure that the compressed air pipes and the compressed air hoses cannot come in contact with the propeller shaft.

Valves and unions can be purchased from Scania dealers.

Pneumatic diagram can be ordered from a Scania workshop.

The bodybuilder must primarily take air from the compressed air outlets, see illustra-tion.

Information about using higher air pressures and connection can be found in the document “Modifying the pneumatic system".

A prepared compressed air connector (1) for bodybuilders is located at the third crossmember seen from the front, see illustration. Remove the plug from the com-pressed air connector and fit an insert union. Connect a 12 mm diameter compressed air pipe to the insert union.

An air consumer may also be connected to solenoid valves (2) at the rear end of the chassis. Use compressed air pipes with a diameter of at least 8 mm. High-consump-tion air components must not be connected to this connection due to the dimensions of the air pipe and because there are other air consumers at the rear of the truck. The exact location of the solenoid valves depends on the vehicle’s configuration.


Documents

General information about the pneumatic system