This invention relates to an arrangement for supplying fuel to internal combustion engines.
Several different systems have been presented to enable Otto engines to be operated with completely different fuels, such as pure methanol and pure petrol, and to allow a gradual transition, during continuous operation, from pure petrol operation, for example, to pure methanol operation, or a transition to different mixtures thereof. Such a gradual transition is obtained, for example, if the vehicle is stopped after having run the engine on pure petrol and the fuel tank thereafter filled with pure methanol. The fuel present in the main tank will then be a mixture of petrol left in the tank and the newly filled methanol. The problem that arises after filling, and possibly after a prolonged stationary period, is that the engine is started with a new fuel in the main tank which has a completely different fuel value to the fuel with which the engine was stopped. To ensure that the engine is able to be controlled so that the correct quantity of fuel is supplied and the emissions reduced, the use of different types of fuel sensors in the feed lines to the engine has been proposed. This enables the engine control unit to be set to the fuel quality currently supplied to the engine. However, these sensors are relatively expensive and are not very reliable.
Another solution has been proposed in WO 91/04406, in which the lambda sensor installed in the exhaust system can be used as a sensor for switching over the engine control unit. One problem here is that on starting, especially cold starting, the sensor will not reach its required operating temperature until the engine has been running for a few minutes. If, during this starting stage, the engine were to be supplied with a fuel with an entirely different fuel quality to that used before the engine was switched off, there would be a risk that the engine might discharge uncontrolled emissions and might even stop. In WO 91/04406 this problem has been solved by the use of a slave tank in which the fuel gradually undergoes controlled mixing with the fuel mixture present in the main tank, but not until the engine is restarted, and preferably after the lambda sensor has reached the required operating temperature. However, the concept proposed in WO 91/04406 requires two fuel pumps, one between the main tank and slave tank, and one between the slave tank and engine, which involves unnecessarily high costs because fuel pumps which are capable of withstanding both pure methanol and pure petrol are very expensive.
Solutions which are based on the concept proposed in WO 91/04406 have also been presented in DE 4008372, DE 4027947 and DE 4027948.
DE 4008372 differs essentially from WO 91/04406 in that a shutoff valve is installed between the main tank and slave tank. The valve does not open and fuel cannot be drawn from the main tank until the lambda sensor begins to operate. DE 4027947 differs essentially from WO 91/04406 in that a shifting valve shifts the suction side of the pump between the slave tank and the main tank. The slave tank is filled with the fuel returned from the fuel manifold, and as long as the engine is cold or the lambda sensor is not operating, the pump sucks from the slave tank. This solution has one disadvantage in that after the warming up phase the fuel is taken from the main tank and thereafter the slave tank begins to be mixed with the return fuel only. After the warming up phase the valve shifts and suddenly a different fuel quality is supplied to the engine. If the engine is capable of withstanding this changeover, the control unit is set to the fuel quality in the main tank. If the engine is switched off shortly afterwards, the engine will next be started on the slave tank, which may contain a completely different fuel mixture, giving rise to starting difficulties.
In one particular design, DE 4027948 differs essentially from WO 91/04406 in that an ejector technology, known from WO 91/17355, is applied for opening a pressure controlled valve in the slave tank. There is a risk here, however, that the seal of the pressure controlled valve may leak if particles stick to it, which can easily happen because the valve is located close to the bottom of the main tank and loosened deposits may block the valve seat. The spring loaded valve cone shown must be fitted with a relatively weak spring because the ejector pressure must be sufficient to open the valve. Another disadvantage is that the ejector begins to draw fuel from the main tank into the slave tank as soon as the engine is started. In the case of a cold start and at low engine speed this results in most of the fuel supplied from the fuel pump being fed to the ejector, since by this stage the engine is not drawing as much fuel. The consequence of this is rapid mixing in the slave tank. If the engine is only to be run at such short intervals that the lambda sensor does not have time to become operative, starting difficulties arise because a new mixing ratio will prevail in the slave tank, a mixing ratio which the control system has not had time to detect. In a second design in DE 4027948 a duplex pump concept is shown instead of the ejector solution.