METHOD AND DEVICE FOR DIAGNOSIS OF THE REFRIGERANT QUANTITY IN AN AIR CONDITIONING SYSTEM

Abstract: A method for diagnosis of the refrigerant quantity in an air conditioning system in which an evaporator (10) followed by a liquid-separating accumulator (11), a compressor (12), a condensor (13), and a shut-off device (14) are connected in series after one another in a closed circuit for circulation of a refrigerant. The pressure of the refrigerant is measured between the condensor (13) and the shut-off device (14). The temperature of the refrigerant is measured after the compressor (12) and also before the evaporator (10). The measured values are compared with stored set values, for determining whether the refrigerant quantity is within, or outside, a reference interval.

Description: METHOD AND DEVICE FOR DIAGNOSIS OF THE REFRIGERANT QUANTITY IN AN AIR CONDITIONING SYSTEM. TECHNICAL FIELD The present invention relates to a method and a device for diagnosis of the refrigerant quantity in an air conditio- ning system, in which an evaporator followed by a liquid- separating accumulator, a compressor, a condensor, and a shut-off device are connected in series after one another in a closed circuit for circulation of a refrigerant. BACKGROUND OF THE INVENTION Air conditioning systems in vehicles must normally be able to function in greatly varying conditions, for instance regarding ambient temperature and atmospheric humidity. Also, its function depends on the quantity of refrigerant in the closed system. The most common cause of malfunctions in air conditioning systems is a too high or too low refrigerant quantity. A small leakage in an air conditioning system can be very difficult to discover. If the compressor, over a very long period of time, is forced to function with a too low refrigerant quantity, it might break down. Even a slight shortage of refrigerant can lead to the following negative consequences: low undercooling after the condensor, high overheating after the evaporator, the evaporation pressure is lower than normal, and the output temperature of the compressor is higher than normal. An excess of refrigerant might, on the other hand, lead to the following negative concequences: high undercooling after the condensor, the condensing pressure is higher than normal, - the output temperature of the compressor is lower than normal, the evaporation pressure is higher than normal, and the overheating after the evaporator is low. In practice, a shortage as well as an excess of the refrigerant will force the compressor to work more often and during longer time periods than what otherwise would have been necessary. Up to now, no suitable devices have existed for measuring the refrigerant quantity in a closed system with capillary tubes as a shut-off means, and with a liquid-separating accumulator after the evaporator. For this reason, service and maintenance of cars equipped with air conditioning systems usually mean that the liquid must be evacuated from the system when measuring the quantity of the refrigerant. This evacuation is time consuming, since the system comprises rather long fluid lines with narrow passages. Draining and filling of the refrigerant also lead to risks of discharging substances which are harmful to the environ¬ ment. US 4757693 describes an air conditioning system with an electronic circuit for detection of malfunctions, for example due to an inaccurate refrigerant quantity. This air conditioning system is however equipped with a tank for the refrigerant between the condensor and the expansion valve. US4308725 describes a detector for indicating the level of the refrigerant in such a storage tank. SUMMARY OF THE INVENTION Consequently, a purpose of the present invention is to provide a method and a device for a quick and simple diagnosis of the refrigerant quantity in an air conditio- ning system which does not have a tank for the refrigerant. This object is solved in the method according to the invention in that the pressure of the refrigerant is measured between the condensor and the shut-off device, that the temperature of the refrigerant is measured after the compressor and also before the evaporator, and that the measured values are compared with stored set values for determining whether the refrigerant quantity is within, or outside, a reference interval. The device according to the invention is characterized by pressure sensing means for detecting the refrigerant pressure between the condensor and the shut-off device, temperature sensing means for detecting the refrigerant temperature after the compressor and before the evaporator respectively, and by an analysis unit which is arranged to compare input data from the pressure sensing and the temperature sensing means with stored set values. Advantageous embodiments of the invention will be apparent from the subsequent dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described in greater detail with reference to the accompanying drawings, wherein Fig. 1 schematically shows an air conditioning system with a diagnosis system, according to a first embodiment of the invention, Fig. 2 shows in an analogous way a diagnosis system according to a second embodiment of the invention, Fig. 3 shows a diagnosis system suitable for use in workshops, and Fig. 4 is a diagram which shows the quantity of refrige¬ rant in a system as a function of the ambient temperature and the atmospheric humidity. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The air conditioning system shown in Figs. 1 and 2 compri¬ ses an evaporator 10 with a following liquid-separating accumulator 11, a compressor 12, a condensor 13 and a shut- off device 14 which are connected in series after one another in a closed circuit for the circulation of a refrigerant. According to a first embodiment of the invention which is shown in Fig. 1, a separate monitoring unit 15 is connected to the air conditioning system by means of a control relay 16 for controlling the compressor 12, and also through a pressure-sensing means 17 for detection of the refrigerant pressure between the condensor 13 and the shut-off device 14. Furthermore, the monitoring unit is provided with temperature-sensing means 18, 19 for detection of the refrigerant temperature after the compressor 12 and before the evaporator 10, respectively. The monitoring unit 15 controls the half-speed mode and the full-speed mode of the compartment fan by means of a dual relay 20. In addition to this, the unit 15 receives input signals regarding the operation of the engine via an engine speed sensor 21, regarding the on/off-condition via a sensor 22, and information regarding the compartment fan by a sensor 23. The unit 15 is arranged to compare input data from the pressure-sensing and temperature-sensing means 17-19 with stored set values, whereby deviations from these values will result in a warning indication by means of a warning light 24 on the dashboard of the vehicle. Fig. 2 shows an alternative embodiment of the invention in which a monitoring unit 25 is integrated with an automatic air conditioning system for performing periodical control of the compressor 12 and automatic shut-off at a low refrigerant quantity, in the same manner as the previously described embodiment. Functionally, this embodiment differs from the previous one in that information regarding the ambient temperature and the engine temperature is available and may be used for a timer-controlled start of the compressor as well as for a more accurate diagnosis of the switched-off state. Fig. 3 shows a diagnosis unit 27 intented for use in a workshop, comprising a T-connection 28 which is mounted between the ordinary connection of the vehicle's cabling 29 and the diagnosis unit 15; 25. A temperature sensor 30 is manually fitted at a suitable location between the evapora¬ tor 10 and the accumulator 11. The value of the ambient temperature is entered in the diagnosis instrument 31. The diagnosis is made while idling and with the compartment fan at full speed. From the diagram shown in Fig. 4, it is apparent how the rise in temperature of the compressor in relation to the rise in pressure of the compressor depends on the refrige¬ rant quantity. This relationship can be expressed as a function of the difference between the condensing tempera¬ ture and the evaporator temperature. The Y-axis of the diagram is based on the temperature after the compressor, the temperature before the evaporator and the difference between the saturation pressure after the compressor and the temperature before the evaporator. Four graphs display various operation conditions with respect to the ambient temperature and the atmospheric humidity. Underfilling leads to increasing overheating, causing an increased inlet temperature to the compressor and thereby also an increased output temperature. If overfilled, an overboiling in the evaporator occurs, which leads to drops of liquid entering the compressor. This results in a decrease of the output temperature due to the cooling effect of the liquid. The invention is not limited to the abovementioned em¬ bodiments, but other variations are also possible within the scope of the following claims.

Claims: CLAIMS 1. Method for diagnosis of the refrigerant quantity in an air conditioning system in which an evaporator (10) followed by a liquid-separating accumulator (11) , a compressor (12) , a condensor (13) and a shut-off device (14) are connected in series after one another in a closed circuit for circulation of a refrigerant, c h a r a c ¬ t e r i z e d i n that the pressure of the refrigerant is measured between the condensor (13) and the shut-off device (14) , that the temperature of the refrigerant is measured after the compressor (12) and also before the evaporator (10) , and that the measured values are compared with stored set values for determining whether the refrigerant quantity is within, or outside, a reference interval. 2. Method according to claim 1, c h a r a c t e r i ¬ z e d i n that the reference refrigerant quantity is derived through computation of the difference between the temperature of the refrigerant after the compressor (12) and the temperature before the evaporator (10) , divided by the difference between the temperature of the refrigerant corresponding to the refrigerant pressure between the condensor (13) and the shut-off device (14) minus the temperature before the evaporator (10) . 3. Device for diagnosis of the refrigerant quantity in an air conditioning system in which an evaporator (10) followed by a liquid-separating accumulator (11) , a compressor (12) , a condensor (13) , and a shut-off device (14) are connected in series after one another in a closed circuit for circulation of a refrigerant, c h a r a c ¬ t e r i z e d b y pressure-sensing means (17) for detecting the refrigerant pressure between the condensor (13) and the shut-off device (14) , temperature-sensing means (18, 19) for detecting the refrigerant temperature after the compressor (12) and before the evaporator (10) respectively, and by an analysis unit (15; 25; 27) which is arranged to compare input data from the pressure-sensing and the temperature-sensing means (17-19) with stored set values. 4. Device according to claim 3, c h a r a c t e r i z e d i n that the analysis unit 25 is integrated with an automatic air conditioning system (26) .

Inventors: SARAKINIS GEORGIOS, SARAKINIS, GEORGIOS
Applicants: SARAKINIS GEORGIOS, VOLVO AB, AB VOLVO
Application year: 1993
Main classification: B60H1
IPC codes: B60H1, F25B49