The present invention relates to a method for adjusting pressure in a fuel tank and a corresponding fuel tank system.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
It is desirable to reduce the release of hydrocarbons from automobiles with an internal combustion engine into the environment. To this end, automobiles typically have an active charcoal filter which is generally arranged between a tank of the automobile filled with combustible fuel and the environment. This active charcoal filter operates as temporary storage for evaporated hydrocarbons from the automobile. The active charcoal filter is typically regenerated in regular intervals with a flow of fresh air, which takes up the hydrocarbons stored in the active charcoal filter and transports them to the internal combustion engine. These so-called purge phases of the active charcoal filter are intended to ensure that the filter has always sufficient storage capacity for storage of hydrocarbons.
These purge phases cannot be performed very frequently particularly in hybrid vehicles, because the internal combustion engine is only operated intermittently. To ensure that the active charcoal filter has always enough capacity for absorbing hydrocarbons, it has been proposed to reduce the quantity of hydrocarbon vapor generated in the tank by adjusting a predetermined internal tank pressure greater than ambient pressure. To this end, an electrically controllable shutoff valve is arranged between the tank and a connecting line which connects the tank with the active charcoal filter. When the tank is heated, the pressure inside the tank increases to a predetermined, presettable threshold value for the pressure. Hydrocarbons in the fuel then increasingly remain in their liquid phase. The shutoff valve is opened only when this threshold value is exceeded, whereafter hydrocarbon-containing vapors escape from the tank. These vapors are then transferred through the connecting line to the active charcoal filter, where they are absorbed, causing the pressure in the tank to decrease again. As soon as the pressure in the tank is again below the threshold value, the shutoff valve is closed again.
Disadvantageously, the pressure inside the tank cannot be adjusted to be variable. A certain threshold value for the pressure must be preset in advance. A very high pressure is typically selected as the threshold value, allowing vapors to escape from the tank as infrequently as possible. The shutoff valve is used only as an emergency valve in order to prevent damage caused by a very high pressure. The complete tank system therefore requires a rather complex structure to ensure that the tank can withstand regularly occurring high pressures. The manufacturing costs of such tank systems are therefore very high. The absorption capacity of the active charcoal filter is also sometimes not fully utilized.
It is therefore an object of the present invention to provide a method for adjusting pressure in a fuel tank and a fuel tank system, with which the pressure in the fuel tank can be variably adjusted and with which the manufacturing cost of the tank system can be reduced.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved method for adjusting pressure in a fuel tank and a fuel tank system with which the pressure in the fuel tank can be variably adjusted, resulting in a reduction of the manufacturing costs of the tank system.