The present invention relates to a device, e.g. a mobile or stationary source of power provided with an internal combustion engine with a supercharger of the rotary screw compressor type as well as a with compressed air plant fed from a compressed air tank, the supercharger including a casing with inlet and outlet end walls provided with an inlet port and an outlet port and two rotors, one being a male rotor and the other one a female rotor, which have helically extending lobes and intermediate grooves, the intermeshing lobes of the rotors and the casing forming towards the outlet end wall movable working chambers as the rotors rotate.

In known devices of this kind a separate compressor driven by the engine usually is connected to the compressed air tank in order to maintain a predetermined pressure. If the tank is completely or partly emptied when the engine is started, it is required that the tank is filled to the predetermined pressure within a short time, which can be within 5 to 10 seconds at the latest, since the compressed air in many cases is used for operation in particular when the source of power is used in a vehicle with pneumatic brakes. The vehicle cannot be driven until the air pressure almost reaches the maximum level. In known devices of this kind a reciprocating compressor with low capacity is used, which means that it takes a long time to fill the tank. Furthermore the compressor is normally engaged all the time and is idling most __ of its running life, if not also a clutch is provided, which engages the compressor only when it is required.

As an example of devices with supercharged internal combustion engines and having a demand for an effective compressed air supply, vehicles with air springs having level adjust¬ ment means, can be mentioned. Also can be mentioned stationary or mobile devices with internal combustion engines having compressed air assisted fiiel injection and motor vinches with pneumatic brake.

The object of the present invention is to attain a device of the kind initially specified in which the above mentioned drawbacks are overcome.

This has according to the invention been achieved in that a rotary screw compressor of a kind known as such is used as the supercharger, which compressor through a special, from the ordinary outlet port separated, outlet port delivers a minor air flow of considerably higher pressure than that delivered from the ordinary outlet port of the compressor after that the ordinary outlet port has been closed, which special outlet port by a pipe provided with a one¬ way valve, herein called high pressure one-way valve, is connected to the compressed air tank. In this way an air flow is made use of, which otherwise by an other design of the outlet port usually is led to the ordinary outlet port in order to avoid the occurrence of very high pressure pulses in the interspace between the rotor lobes adjacent to the outlet port. The air volume can be chosen through the design of the outlet port, through which also the level of the higher pressure can be affected up to e.g. 5 bars overpressure.

In order to attain a faster initial filling of the compressed air tank, a pipe from the outlet of the supercharger to the intake manifold of the engine can, according to a preferred embodi¬ ment, be provided with a branch pipe, which via an one-way valve, herein called charge pressure one-way valve, is connected to the compressed air tank. The surplus capacity of the supercharger when the engine idles, thereby is used for fast filling of the compressed air tank to a pressure level, being equal to the charge pressure of the engine, about 1 to 2 bars over- pressure, which takes about 5 seconds depending on pressure level, tank volume and engine size. Thereafter filling takes place through said high pressure one-way valve. A quick charg¬ ing to a pressure somewhat higher than the charge pressure of the engine, about 3 to 4 bars overpressure, can be attained if a throttle is provided in the inlet manifold of the engine for raising the pressure in the with the charge pressure one-way valve provided branch pipe to the compressor by short partial throttling of the intake air.

When the compressed air tank is full-charged and the high pressure one-way valve has been closed, the high pressure air delivered from the special outlet port advantageously is tempo¬ rary drained to the outlet of the supercharger through a pipe with a normally closed valve, being governed by the pressure in the compressed air tank and arranged to open when the pressure in the compressed air tank has reached the maximal pressure. Alternatively the draining can be done to the inlet of the supercharger in order to avoid that a variation of the air quantity to the engine will occur.

The invention is explained more in detail in the following with reference to the accompanying drawings, schematically showing an embodiment of the invention chosen as an example, wherein fig. 1 illustrates the device during fast charging of the compressed air tank to the charge pressure of the engine, fig. 2 is an end view of the high pressure end wall of the super

charger with an outlet port, fig. 3 illustrates the device during charging of the compressed air tank to high pressure and fig. 4 illustrates the device when the compressed air tank has been charged to high pressure and during draining of the surplus of high pressure air to the intake manifold of the engine.

Fig. 1 shows an internal combustion engine 1 with intake manifold 2. Through a belt trans¬ mission 3 a pulley 4 mounted on the engine crankshaft drives a supercharger 5 of the rotary screw type provided with an inlet pipe 6 with an air filter 7 and an outlet pipe, in the common way connected to the engine intake manifold 2 but also to a compressed air tank KUhrough a branch pipe 9.

The supercharger 5 is of a kind known as such in another context, namely a rotary screw compressor with a special outlet port 11, fig. 2 in the compressor casing for emptying the air volume enclosed between the rotors. The port is here very small and located in a part of the outlet end wall 12 in a position such that the air volume enclosed between the rotors at the outlet end after a final compression will be located to face the port when the desired pressure level has been reached. Fig. 2 also shows the ordinary axial outlet port 13 of the compressor, formed by the axial outlet port 13F and 13M in the outlet end wall. The port thus is formed by two port sections, divided by the end wall part 12 with the flanks of the female rotor 14F and the male rotor 14M nearest to the end wall part 12 indicated by dotted lines. Straight below the port 13F, 13M the radial outlet port 15 of the supercharger is projected with a for a supercharger normal opening edge 16 and an early acting closing edge 17 for the compression of high pressure air. The position of the closing edge 17 is chosen with regard to the desired quantity of high pressure air, which naturally makes use of a corresponding part of the capacity of the supercharger. This air quantity, however, will be supplied to the engine except when it is required to raise the pressure in the air tank.

The special outlet port 11 is connected to the air tank 10 via a spring-loaded one-way valve 18, fig. 1. Also in the branch pipe 9 a spring-loaded one-way valve 19 is provided.

At starting with low or no pressure in the compressed air tank 10, air from the supercharger 5 will flow through the outlet pipe 8 on one hand to the engine and on the other hand to the compressed air tank 10 through the branch pipe 9 and the open one-way valve 19. A minor air quantity will flow from the special outlet port 11 through a high pressure pipe 20 and the open one-way valve 18 to the compressed air tank 10.

When the pressure in the air tank 10 has risen to the charge pressure of the supercharger 5 the air flow to the branch pipe 9 ceases and the valve 19 will close automatically. Thereafter charging of the tank 10 continues through the high pressure pipe 20 until the maximal pressure of the air-tank has been reached. At that moment the air flow to the tank ceases and also the valve 18 will close automatically.

For draining the pipe 20 when this maximal pressure has been reached there is also a draining pipe 21, normally kept closed by a valve 22. The valve 22 is governed by the pressure in the air tank 10 through a pipe 23 to a spring-loaded piston 24, which piston is connected to the valve 22 and opens this when the pressure in the tank 10 has reached the maximal pressure of the air tank.

The above mentioned fast filling of the compressed air tank 10 through the branch pipe 9 to the charge pressure of the engine, about 1 to 2 bars overpressure, can be improved by provid- ing a throttle 25 in the intake manifold 2 of the engine for momentary partial throttling of the intake air, through which a rise of the pressure in the branch pipe 9 to about 3 to 4 bars is possible, before the throttle 25 is opened and the one-way valve 19 again is closed.