BACKGROUND Fuel systems for internal combustion engines are usually designed in such a way that exchanging a fuel filter involves emptying the fuel line between the outlet of the filter housing and the cylinder head. In this connection, the environmentally harmful substance should be collected to avoid spillage. Exchanging a fuel filter is a common service action, especially on heavy diesel engines which are used a great deal. It is usual for fuel systems for diesel engines to be provided with a bleed nipple and a hand pump which can be used when air bleeding and refilling of fuel is carried out. Before the engine is started, a large number of strokes of the hand pump are required, and it is necessary to open one or more bleed nipples in order to remove the quantity of air present in the new filter. This involves expensive workshop time and also a risk of fuel spillage because the abovementioned nipples are generally not closed before the fuel flows.

There are fuel systems with devices for automating the procedure for removing air (see, for example, US 5534161). This patent describes a pump which can be driven in two directions by means of a microprocessor

and can be used for removing water from However, this device can only facilitate the procedure for bleeding air from the fuel system, and further actions are required in order to remove air from the high-pressure side of the system. This device therefore means that the number of components in the fuel system increases without fully solving the problem of facilitating the procedure for bleeding.

SUMMARY OF THE INVENTION One object of the invention is therefore to produce a fuel system which makes possible automated removal of air after filter exchange without the system being complicated or expensive.

The fuel system designed for this purpose comprises a fuel tank, a fuel pump and a fuel filter located in a flow duct between the fuel pump and the fuel consumers of the engine, the flow duct comprising a valve chamber, located downstream of the fuel filter, with a valve cone arranged movably in said chamber between an upper valve seat and a lower valve seat, and, according to the invention, is characterized in that the upper part of the valve chamber has an outlet for bleeding to the fuel tank, and in that the valve cone is provided with an inner passage with a valve seat which, under normal fuel pressure, interacts with a bleed cone which is movable in the passage. On the one hand, this design of the fuel system ensures that fuel spillage during filter exchange is minimized, and, on the other hand, the subsequent bleeding of the fuel system can take place automatically.

Advantageous illustrative embodiments of the invention emerge from the dependent claims which follow.

BRIEF DESCRIPTION OF FIGURES The invention will be described in greater detail below with reference to illustrative embodiments shown in the accompanying drawings, in which FIG. 1 shows diagrammatically an internal combustion engine with a fuel system according to the invention, FIG. 2 shows on enlarged scale a filter holder, forming part of the fuel system in Fig. 1, with associated filter, and FIG. 3-5 show diagrammatically on further enlarged scale three different functional states of a valve chamber forming part of the fuel system according to the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The internal combustion engine 10 shown diagrammatically in Fig. 1 is a 6-cylinder diesel engine with a corresponding number of injectors 11 of standard type. This engine can be used in order, for example, to drive a heavy truck.

Diesel fuel is fed by a pump 12 driven by the engine from a tank 13 via a common feeder line 14 to the injectors 11. A combination valve 15 for pressure control and bleeding is mounted downstream of the injectors 11. A return line 16 for uncombusted fuel is connected to the combination valve. A bleed line 17 is also connected to the combination valve, which bleed line communicates with the fuel tank 13.

The fuel system also comprises a filter holder 18 with a fuel filter 19. The filter holder is shown in greater detail in Fig. 2 and is provided with a cylindrical valve chamber 20 which is located downstream of the fuel filter and is shown in greater detail in Figs 3-5.

The lower end wall 20a of the chamber is provided with a central inlet 21 from the fuel filter 19. Via a central outlet 22, the upper end wall 20b communicates

fuel tank 13 via the bleed line 17. The also communicates with the feeder line 14 via a laterally directed outlet 23 arranged in the lateral surface 20c.

The valve chamber 20 accommodates a valve cone 24 which is provided with an inner passage 25 with a valve seat 25a and a bleed cone 26 which is movable in the passage. This bleed cone is arranged to interact with the lower valve seat 27 of the valve chamber at low fuel pressure (see Fig. 3). As the fuel pressure increases, the bleed cone, which is considerably more freely moving than the valve cone 24, can remove itself from the valve seat 27, air bubbles then being able to move past the bleed cone and out via the bleed port 22 (see Fig. 4). When the fuel pressure is sufficiently high, the bleed cone 26 interacts with the valve seat 25a, the valve cone 24 then being moved from the valve seat 27 so that fuel can pass out through the outlet 23 to the feeder line 14.

When the engine is started after exchange of the fuel filter 19, the fuel pump 12 sucks fuel from the tank 13 and fills the filter 19. When the pressure in the filter has become sufficiently great, the bleed cone 26 is lifted from the valve seat 27 and air can pass through the passage 25 past the bleed cone and on through the outlet 22. When the fuel reaches the valve chamber 20, the bleed cone will close the valve seat 25a, the valve cone 24 then being displaced upward and air-free fuel starting to be pushed to the consumers 11 of the engine via the outlet 23 to the feeder line 14.

The weight of the bleed cone 26 is adapted in such a way that air bubbles in the fuel easily cause it to lift from the lower valve seat 27. The weight of the valve cone 24 is in turn adapted in such a way that only the fuel pressure is capable of causing it to lift from the lower valve seat 27.

Since the bleed cone 26 serves as a non- and can rapidly close the inlet 21, a large part of the positive pressure in the feeder line 14 can be retained during filter exchange. This positive pressure constitutes a prerequisite for the engine starting and continuing to run for the time it takes before all the air has passed out through the bleed port 22 and the pressure can rise to the normal level which is maintained by the combination valve 15. The engine therefore runs even during the bleeding process because fuel still remains around the injectors 11 and in the fuel line 14 leading to these. It should be pointed out that this assumes that the engine is running under low load, that is to say with low fuel consumption, which makes it possible for the pump to replace the air quantity present in the filter with fuel. Moreover, this bleeding process takes only about 10 seconds.

The invention is not to be regarded as being limited to the illustrative embodiments described above, but a number of further variants and modifications are conceivable within the scope of the following patent claims. It is possible, for example, to use spring means which act in the closing direction as a supplement to the own weight of the valve cone 24 and/or the bleed cone 26.