A utility vehicle typically includes an air compressor which is driven, either directly or indirectly by the (main) engine or (main) motor of the vehicle, a first compressed-air tank which is fed by the compressor and which is pressurised to a selected first pressure range with the aid of control elements, and a plurality of pneumatic power devices, for instance pneumatic piston-cylinder devices, which are used to relieve the load on spring-biased vehicle brakes.

In practice a relatively large quantity of air leaks from the compressed-air system of a utility vehicle, causing the system pressure to fall beneath the lower range limit of the pressure tank within a relatively short period of time, for instance within the space of one hour. In order to be able to drive away the vehicle subsequent to this leakage, it is necessary to start-up the main engine of the vehicle, which is normally a gasoline-driven engine, and to allow the engine to run for a few minutes, so as to enable the compressed-air tank to be charged to the intended air-pressure limit before the compressed-air system can be allowed to operate so as to cause the pneumatic power devices to release the spring- biased brake shoes of the vehicle brakes.

Running of the vehicle engine, necessary in order to charge the compressed-air tank, often gives rise to a noise problem, for instance in the morning before the vehicle shall be driven away from its overnight parking place, or subsequent to a break/lunch. The exhaust emission created by running the engine in order to charge the compressed-air tank also, of course, constitutes a problem.

Accordingly, an object of the present invention is to provide a compressed-air system for an automotive vehicle with which said problems can be avoided, either completely or partially.

A further object of the invention is to provide a novel compressed-air system that can be obtained by modification of conventional compressed-air systems of the aforesaid kind.

These objects are achieved with a compressed-air system according to the present invention.

The invention is defined in the accompanying independent Claim.

Further embodiments of the invention will be apparent from the accompanying dependent Claims.

The invention is based on the concept of providing the system with a pressure booster that is adapted to produce compressed-air of a second pressure level that is higher than the first pressure level applicable to the first, conventional compressed-air tank, a second compressed-air tank which receives compressed-air from the pressure booster, a conduit which extends between the first and the second tank and which includes a valve which is normally closed and which can be controlled, suitably remotely, from the driver's seat and therewith opened instantaneously so as to fill the first tank with compressed-air from the second tank, wherein the volumetric capacity and the set pressure level of the second tank are chosen to enable the first tank to be filled with accumulated air from the second tank to a pressure level in the first tank that corresponds to its first pressure level, therewith enabling the vehicle power devices to be activated instantly and therewith also enabling the vehicle to be driven away immediately after starting its engine, despite the fact that the pressure in the first tank is initially essentially at atmospheric pressure.

The pressure booster may have any appropriate design and may, in principle, comprise a secondary compressor which is driven, either directly or indirectly, from the vehicle drive engine or, for instance, from an electric generator driven by said engine, said generator driving, in turn, an electric motor coupled to said compressor.

However, the pressure booster is preferably a commercially available apparatus which upon receiving compressed-air from the compressor, which charges the first pressure tank, establishes a flow of compressed-air that has a much higher pressure level and delivers this compressed-air flow to the second tank until said tank reaches a selected pressure level, whereafter the store of compressed-air in the second tank is kept in readiness for its transfer to the first pressure tank so as to enable the compressed-air brakes and the like to

be released and enable the vehicle to be driven away immediately after starting the main engine of the utility vehicle.

The invention will now be described with reference to an exemplifying embodiment at present preferred and also with reference to the accompanying drawing.

Fig. 1 is a schematic illustration of a circuit diagram of a compressed-air system for utility vehicles.

Shown in Fig. 1 is an engine or motor 1, which drives a compressor 3 via a controllable, coupling 2. The compressor 3 includes an outlet line 4 that conveys compressed air produced in the compressor 3. The compressed air may have a pressure of 8 bar, for instance. A first branch line 41 leads the compressed air to a first compressed-air tank 5 via a check valve 42. The tank 5 has an outlet line 6, which branches to a plurality of pneumatic piston-cylinder devices 7, which can be controlled individually. At least some of the piston-cylinder devices 7 are included in the brake mechanisms of the utility vehicle, wherewith one such piston-cylinder device 7 can be used to lift a spring-biased brake shoe away from its engagement with a vehicle brake drum, preferably in a conventional manner.

Provided in the tank 5 is a sensor 81 which senses the pressure prevailing in the tank 5 and sends a control signal to a control unit 10, which is adapted to activate and deactivate the coupling 2, so as to maintain the pressure level of the compressed-air in the tank 5 within chosen limits, for example between 7.5-8. 5 bar, by the compressor 3.

The engine 1 of the utility vehicle may be a typical fuel-driven main engine, or an electric motor powered by a generator which, in turn, is driven by the main engine of the vehicle.

The compressor 3 is normally adapted to deliver to the tank 5 compressed air that has a pressure level that corresponds to the desired pressure level P, for the tank 5, and therewith for the pneumatic power devices 7.

A pressure booster 11 is connected to the compressed-air line 4. The pressure booster 11 produces compressed-air that has a much higher pressure level, P2, than the pressure level of the tank 5, and delivers compressed air at this elevated pressure level P2 to a second

compressed-air tank 14. The system may include control means for switching-off the pressure booster 11 when the pressure P2 in tank 14 has exceeded a lower limit value.

These control means are of the same nature as the control means 10 that are responsible for maintaining the first pressure level Pl in the tank 5. Extending between the tanks 14 and 5 is a conduit 20, which includes a valve 21 that can be remotely controlled with the aid of a switch 22, placed in the driver's place in the vehicle.

When the vehicle stops and the main engine, or motor, of the vehicle is switched-off a certain amount of air leaks from the system in practice, for instance through the piston- cylinder devices 7, causing the pressure in the tank 5 to drop well below the lower limit of the desired pressure level Pl. In order to enable the vehicle to be driven away after having started the main engine or main motor of the vehicle, it is, of course, necessary to release the brakes of the vehicle. This is achieved by the driver operating the switch 22 so as to open the valve 21, wherewith compressed air flows from the tank 14 to the tank 5 such that a common pressure will prevail in the tanks 5,14. The volumetric capacity and the pressure level P2 of the tank 14 are chosen so that the tank 5 can reach the pressure level Pl from atmospheric pressure. This enables the brakes of the utility vehicle to be released without needing to run the compressor 3 beforehand, so that the vehicle can be driven away as soon as the main engine (fuel-driven) has been started.

According to one practical embodiment, the pressure booster 11 can establish a pressure of 25 bar, wherein the tank 14 has a volumetric capacity of 4 1, whereas the tank 5 has a volumetric capacity of 8 1 and a pressure level of about 8 bar.

It will be understood that the energy content of the tank 14 may be made slightly greater, so as to provide a safety margin, and that the pressure level P2 may be chosen at a higher level to enable the tank 14 to be given a smaller volume.

In the system illustrated in Fig. 1, the outlet conduit 4 of the compressor 3 includes a first branch 41 which connects to the tank 5 and includes a check valve 42, which opens towards the tank 5. The conduit 4 also has a second branch 43, which connects to the pressure booster 11. The outlet conduit 13 of the pressure booster 11 connects to the second tank 14 and includes a check valve 23 which is adapted to open towards the tank 14.

The piston-cylinder devices 7 can be controlled by means of valves (not shown). At least some of the devices 7 may be connected to, or associated with, vehicle brakes 71 that are spring-biased towards a vehicle braking mode, wherewith the spring force can be nullified, at least partially and gradually, by the associated devices 7. For example, the brakes 71 can include brake shoes that are spring-biased against a brake drum belonging to and braking respective vehicle wheels.

The brakes may be activated in a known manner, for instance by driver actuation of a brake pedal or the like, whereby the brake-controlling piston-cylinder devices are ventilated by said valves, so that the springs can apply the brakes to an extent corresponding to the degree to which the piston-cylinder devices are ventilated and to the extent to which the brake pedal is depressed. The load on the brakes is relieved in a corresponding manner by releasing the brake pedal, therewith resulting in pressurisation of the piston-cylinder devices concerned through corresponding actuation of said valves.