The present invention relates to a device in internal combustion engines provided with a supercharger of the rotary screw compressor type and a compressed air assisted fuel injection system, in which the supercharger includes a casing provided with outlet and inlet end walls with inlet port and 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 inter- meshing lobes of the rotor and the casing forming towards the outlet end wall movable chambers with an overpressure of about 1 to 2 bars as the rotors rotate.

In many engine constructions with fuel injection direct into the combustion chamber or alternatively into the intake manifold a pressure source of a relatively high pressure, such as about 5 bars overpressure is required in order to attain an adequate injection. In some cases the injection must take place so fast that the fuel has to be mixed with and injected with the compressed air. For this purpose a suitably driven compressor, e.g. a small reciprocating compressor is provided irrespective of if the engine is supercharged or not. In the case the motor is supercharged with a compressor this means mounting of further a component in connection with the engine with increasing costs as a consequence.

The object of the invention is to attain a corresponding function for the fuel injection of a supercharged engine without the requirement to provide a particular compressor for the fuel injection.

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 from the ordinary outlet port of the compressor after that the ordinary outlet port has been closed, which special outlet port by a pipe is connected to the fuel injec- tion system for pressurizing this with a considerably higher pressure than the outlet pressure of the supercharger. In this way the air volume enclosed between the rotors and the casing at the outlet end after final compression can be withdrawn at the desired pressure level and be used for the fuel injection. In an internal combustion engine with a rotary screw

compressor as supercharger and in which fuel injection with the ^' aid of compressed air is required, an extra compressor or pump for the fuel injection thus.is not necessary _

The invention is explained in detail through the following with reference to the accompany¬ ing drawing, schematically showing two embodiments of devices according to the invention, wherein fig. 1 is a view from above of an; engine with supercharger and fuel injection device, fig. 2 is an end view of a first embodiment of the device according to the invention and fig. 3 is an end view of a second embodiment for achievifl a particularly high pressure ratio and a larger air quantity. _.-

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Fig. 1 shows an internal combustion engine 1 provided with a supercharger 2 of the rotary screw compressor type, driven by a belt transmission 3 from a pulley mounted on the engine crankshaft. The supercharger 2 feeds the intake manifold 4 of the engine through a channel 5 with an ^* overpressure of about 1 bar. The supercharger 2 is provided with a high pressure outlet 6 for compressed air, which will be described in connection with figs. 2 and 3. From the high pressure outlet 6 compressed air with a pressure of about 5 bars is led through a compressed air pipe 7 to a fuel injection device 8, which receives fuel from a pipe 9. The fuel is mixed with the compressed air in the fuel injection device 8 and is supplied to the intake manifold 4 alternatively the cylinder of the engine (at direct fuel injeαion) through injection nozzles.

The supercharger is of a kind, known as such in another context, namely a rotary screw compressor with a special outlet port 11, fig.2, provided in the compressor casing, for empty- ing the air volume enclosed between the rotors and therethrough eliminating the powerful pressure pulses, that can occur after closing of the outlet port. 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 pqrt ,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 corre¬ sponding part of the capacity of the supercharger. This air quantity, however, will be supplied to the engine together with the fuel.

If a particularly high pressure is desired, a recess 20 can be provided in each groove of the female rotor 14F adjacent to the outlet end wall 12 and in that port 21, located such that communication with the recess 20 is established, when the desired pressure has been reached.

In this way normally about 5 % of the capacity of the supercharger can be made use of for compressed air to the fuel injection at an overpressure of about 5 bars. Due to the higher pressure, leakage will increase somewhat and therefore the supercharger should be dimensioned with regard to that.