The present invention relates to a fuel supply system for combustion engines which can be driven by different kinds of fuel and which system comprises a fuel tank into which different fuels can be filled, and a control system for among other things the adjustment of the fuel-air mixture to the engine, that allows a relatively slow change from one type of fuel to another.

The object of the invention is to bring about a fuel system which permits a change from one type of fuel to another type of fuel and that this can be performed with the engine running. The control system of the engine, therefore, shall always be able to have the engine adjusted such that it can be started up independently of if the tank is filled up with a completely new fuel compared to the fuel used when the engine was stopped.Therefore, the idea of the inven- tion is among other things, that when the engine has been stopped by some reason and put to rest during a shorter or longer time period, the renewed start or starting up of the engine should be performed with the ^' same type of fuel as used when the engine was stopped. This implies for instance that, after driving on 100 % petrol until the tank is empty and a following filling up with 100 % alcohol, the engine can be started and the change to driving by 100 % alcohol can be done gradually.

In order to achieve the above-mentioned object the invention has obtained the characteristic features set forth in the following claims.

A presumed embodiment of the invention will now be described with reference to the accom¬ panying drawing. This shows schematically a combustion engine with the new fuel supply system.

Referring to the drawing there consequently is a fuel tank 1 which accordingly consists of a normal fuel tank in the vehicle. From the fuel tank the fuel is fed to a slave storage tank 2 and the feeding can be done by a pump 3. In the slave storage tank 2 there is a liquid level control 4 and its purpose is to secure that a certain minimal liquid level is maintained in the slave sto¬ rage tank. Accordingly, when the engine consumes fuel from the slave storage tank so that the level is lowered beneath the predetermined liquid level, the liquid level control 4 makes the fuel pump 3 to feed more fuel.

From the slave storage tank the fuel is led in a pipe to the combustion engine which has a not shown injection system for supplying the fuel to the engine's combustion space. Redundant fiiel which is not used in the injection system is returned to the slave storage tank 2.

The engine injection system is controlled by an electronic control device 6 of a kind known per se and the signals to the electronic control device concerning the fuel's mixture ratio are received from a lambda sond 5 placed at the engine's exhaust pipe 8. The wide range lambda sond 5 accordingly indicates the composition of the combustion gases and delivers a signal to the electronic control device 6 which then regulates the ratio for the fuel-air mixture, that is fed to the engine by the injection system. With this system the electronic system manages to adjust the amount of fuel in order to always maintain the correct fuel-air ratio. This also means that the need of a special fuel sond is eliminated, which makes the system less expen¬ sive and more reliable than what is the case up till now. There is also possible to arrange the electronic control device in a way so that a special cold start system is controlled. Further- more, in the main tank, a liquid level indicator 7 is placed in order to give the driver an indi¬ cation that the fuel tank is empty, which prevents a totally emptying of the fuel system.

As said before the invention consequently offers the possibility of the engine being driven by a certain type of fuel which is in the fuel tank for a certain period of time under optimal ad- justment of the injection system. With the assistance by the electronic control device 6, and after that the fuel tank is filled with another type of fuel so that the total mixture in the fuel tank becomes different than what the electronic control device is adjusted to, the slave storage tank ensures that when you again start the engine you can drive the engine with the adjust¬ ment of the injection system which the electronic control device made before the tank was filled up. Consequently by this, a reliable starting up is ensured and also is ensured a reliable driving condition for a certain time until the electronic control device manages to respond to the new fuel mixture and to adjust the injection system of the engine.

In the showed embodiment the slave storage tank is accomplished in the shape of a separate tank. As an alternative the slave storage tank can be arranged inside the fuel tank by e.g; compartmentilization with or without a controlled natural slope from the fuel tank to the slave storage tank. Above it has been emphasized that a certain smallest volume of fuel always shall be in the slave storage tank. In practice this may mean that the slave storage tank during operation is completely filled.