Method and arrangement for checking the operability of a tank-venting system

DETAILED DESCRIPTION The method according to the invention for checking the operability of a tank-venting system of the kind mentioned above has the following steps: closing the shut-off valve; opening the tank-venting valve; determining the build-up gradient of the underpressure building up in the tank; closing the tank-venting valve; determining the decay gradient of the decaying underpressure in the tank; mathematically combining the build-up and decay gradients in such a manner that the influence of the fill level has as little effect as possible on the evaluation variable formed by the combination; comparing the value of the evaluation variable to a threshold value and evaluating the system as non-operative if the value of the evaluation variable and the threshold value fulfill a pregiven relationship.
The arrangement according to the invention has a sequence controller for driving the shut-off valve and the tank-venting valve; a gradient determination device for determining the above-mentioned gradients; an evaluation-variable formation device for forming the above-mentioned quotient and a comparison/evaluation device for carrying out the above-mentioned comparison and the corresponding evaluation.
It is noted that when reference is made below to gradients of the underpressure build up or decay, almost always positive (absolute) values (in terms of magnitude) are meant.
Only FIGS.
2a and 2b relate to these gradients with reference to the sign.
It has been shown that the method according to the invention provides evaluation results which are hardly influenced by the fill level of the tank.
If the tank is almost full, both gradients are relatively high, while, in the case of an almost empty tank, they are both relatively low.
The relative changes in both gradients as a function of the fill level of the tank depend essentially, in the same way, on the fill level so that quotient formation essentially eliminates the effects exerted on the gradients by the fill level.
In a preferred embodiment, the quotient of the decay gradient and the build-up gradient is formed and the system is determined to be non-operative if the quotient is greater than the mentioned threshold value