The invention relates to a method for detecting the fuel quantity during the refuelling of a motor vehicle.
When a motor vehicle is refuelled at a filling station, fuel is conveyed out of a storage tank through a fuel-volume flowmeter with the aid of a fuel pump and is conducted into the tank of the motor vehicle via a filling hose. The fuel-volume flowmeter (throughflow meter) emits pulses which are displayed, via a filling-column computer, as the volume which is being introduced into the tank of the motor vehicle.
In the prior art, therefore, the fuel quantity is displayed as a volume, even though the fuel would actually have to be sold according to weight (mass), since the volume of a given fuel quantity (mass) is not constant because of the temperature dependency of the density.
It is customary, in the calibration of filling columns, to relate the volume display to a predetermined temperature, as a rule to the average annual temperature of the fuel (which is mostly stored underground) in the respective country or region.
Since the density of hydrocarbons decreases with a rising temperature in the order of magnitude of 0.1% per degree, this being a relatively high temperature dependency, the fuel quantity sold according to the volume display during a refuelling operation may often deviate relatively sharply from the fuel mass actually introduced.
EP 0 772 567 B1 and EP 0 933 619 A1 disclose temperature-compensating fuel discharge devices, by means of which a temperature correction can be carried out on the measured fuel volume. In both instances, however, relatively complicated additional electronic components are required.
The object of the invention is to provide a possibility for the correct detection of the fuel quantity during the refuelling of a motor vehicle which can be implemented without costly additional components.
In the method according to the invention for detecting the fuel quantity during the refuelling of a motor vehicle, the volume of the fuel conducted to the motor vehicle during the refuelling operation is measured, which may take place in the conventional way with the aid of a fuel throughflow meter. Furthermore, the temperature of the fuel is measured, preferably in or on a fuel line which is arranged, for example, inside a filling column. The measured volume is corrected to predetermined temperature conditions by means of the measured temperature.
When the filling station is equipped with a gas recirculation system, as is prescribed in many countries, a monitoring unit for the gas recirculation system is located in a filling column. Monitoring systems of this type for gas recirculation are known, for example, from DE 100 31 813 A1. Within the framework of the method according to the invention, the correction steps are carried in the monitoring unit for the gas recirculation system. The advantage of this is that, in contrast to the previously known methods, there is no need for complicated additional electronic components to be installed in the filling column in order to take into account the temperature dependency of the fuel density, but, instead, the monitoring unit which is present in any case can be utilized in order to correct to predetermined temperature conditions.
When the monitoring unit is connected to a filling-column computer, fuel-volume pulses which are characteristic of the corrected volume are transmitted to the filling-column computer, preferably instead of fuel-volume pulses which are characteristic of the measured volume. These corrected fuel-volume pulses are in this case calculated and provided by the monitoring unit, so that the volume display generated by the filling-column computer corresponds to the corrected volume, without any changes having to be carried out on the filling-column computer.
During correction, preferably, the measured volume is multiplied by the quotient of the density of the fuel at the measured temperature and the density of the fuel at the predetermined temperature. Since the composition of the fuel is at least approximately known, the density may be gathered as a function of the temperature, for example, from a table which is stored in a monitoring unit serving as an evaluation unit. If, therefore, the temperature of the fuel is higher than the predetermined temperature (which, for example, coincides with the customary average annual temperature of the fuel), the density of the fuel is lower and the measured volume higher than corresponds to the mass of the fuel; without correction, therefore, the fuel would be sold too dearly. Opposite proportions prevail at low temperatures. The correction ensures that the volume displayed is a numerical value which is proportional to the actual mass of the fuel delivered. Preferably, during correction, correction is carried out to the temperature conditions on which pricing is based.
In an extension of the method according to the invention, the monitoring unit may additionally be utilized in order to correct the measured volume of the fuel according to gas quantity suction-extracted from the motor vehicle during the refuelling operation. Such a correction does not normally take place. Where hydrocarbons with their relatively high molecular weight are concerned, however, the mass of the fuel vapour which is sucked out of the tank of the motor vehicle by the gas recirculation system during a refuelling operation and is conducted into the storage tank of the filling station is no longer negligible. So that the customer no longer has to give this fuel vapour to the filling station free of charge, the invention affords the possibility of converting the gas volume flow measured during a refuelling operation when the gas recirculation system is in operation into a fuel mass (or continuously into a fuel mass flow) by means of the at least approximately known density of the fuel gas (which, if appropriate, may also be temperature-corrected). The volume display of the filling column can consequently be corrected in a similar way to that explained above in connection with temperature correction.
As already indicated, the correction steps are preferably carried out continuously during the refuelling operation, so that the volume display of a filling column indicates continuously corrected values which are proportional to the fuel quantity (fuel mass) actually introduced. It is also conceivable, however, to carry out the correction calculation only after the termination of the refuelling operation, in which case the volume display can still jump over to a corrected value or only the displayed selling price is changed.
In a device for carrying out the method according to the invention, a monitoring unit for a gas recirculation system is utilized in order to carry out the corrections, as explained. An existing installation can therefore be converted with the aid of a retrofitting kit in such a way that it is suitable for carrying out the method according to the invention. The retrofitting kit merely needs to contain a temperature sensor and an accessory for the monitoring unit. The accessory may require hardware arrangements, for example an interface for connecting the temperature sensor. Depending on the design of the existing monitoring unit, however, it is also conceivable to provide merely an upgrade of the software and/or of the firmware in the accessory. The conversion of an existing installation is therefore cost-effective.