Patent Publication Number: US-8113039-B2

Title: Method for testing the operation of a pressure sensing unit of an injection system of an internal combustion engine

Description:
The invention relates to a method for testing the operation of a pressure sensing unit of an injection system of an internal combustion engine according to the features of the preamble of main claim  1 . 
     Fuel injection apparatuses for operating an internal combustion engine have generally been known for some years. With a so-called common rail injection system, the fuel is supplied to the respective combustion chamber of the internal combustion engine through injectors. A high injection pressure is advantageous here, since a high specific output of the internal combustion engine on the one hand and a low emission of pollutants on the other hand can be achieved. When using a high pressure pump and an accumulator (rail) for the fuel, injection pressures of 1600 to 1800 bar can be achieved. As a result of the high injection pressures, high demands are thus also placed on the safety engineering and the components thereof. The operation of a pressure sensing unit provided for an internal engine is usually tested by the internal combustion engine being switched off and the rail pressure continually reducing with the switch-off process being considered. As both injection systems with a high pressure control valve and also without a high pressure control valve are used, different operation testing methods must therefore be used in each instance. 
     Methods are known from the prior art, in which the injection system has a high pressure control valve. The operational capability of a pressure sensing unit with a switched-off internal combustion engine takes place by monitoring the rail pressure. The pressure in an injection system with a high pressure control valve is released by opening the high pressure valve, after the internal combustion engine has been switched off. The pressure drop takes place very quickly here so that it is possible after a very brief predeterminable time to determine whether the pressure sensing unit functions efficiently. To this end, account is taken into whether the pressure value determined after the pressure drop is above a predetermined limit value after a predetermined time. 
     With injection systems in which no high pressure control valve is used, the rail pressure drop by contrast takes place only very slowly (e.g. by means of leakage losses). The rail pressure drop can even last longer in this case than the control device is powered. As the operation testing method of the pressure sensing unit is however a fundamental element of the control device, no statement as to the operational capability of a pressure sensing unit can be made after cutting off the power to the control device. 
     The object underlying the present invention now consists in providing a method, which, during the use of an injection system without a high pressure control valve, allows the operational state of a pressure sensing unit to be determined. 
     This object is achieved according to the invention by the features of claim  1 . Advantageous configurations of the invention are identified in the subclaims. 
     The advantages achieved with the invention consist in particular in a method being available for injection systems without a high pressure valve for determining the operational capability of a pressure sensing unit. In particular, the method determines the operational capability of the pressure sensing unit in the case of a slow rail pressure loss. 
    
    
     
       Particulars of the invention are described in more detail with reference to the drawing, in which: 
         FIG. 1  shows a flow chart to determine the operational state of the pressure sensing unit 
         FIG. 2  shows the temporal rail pressure curve for the case that no failure is identified in the pressure sensing unit, 
         FIG. 3  show the temporal rail pressure curve for the case that a failure is identified in the pressure sensing unit. 
     
    
    
       FIG. 1  shows a flow chart to determine the operational state of the pressure sensing unit. The internal combustion engine is switched off in step S 1 . Pressure values are then continuously determined in step S 10  and the respective pressure gradients are determined based hereupon with the aid of two consecutive pressure values. The time span between determining these two pressure values, which are needed to determine the pressure gradients, is predefinable. It is determined in step S 20  whether the predetermined pressure gradient exceeds a stored pressure gradient limit value. For the case that the determined pressure gradient exceeds this pressure gradient limit value, it is assumed that the pressure normally drops via a leakage. The operational test of the pressure sensing unit is then not pursued further. If the pressure gradient does not achieve the stored pressure gradient limit value, step S 30  checks whether the pressure value determined at the second instant is greater than a stored pressure limit value. If the pressure value determined at the second instant is greater than the stored pressure limit value, corresponding measures are introduced in step S 40 . It has proven particularly advantageous for the pressure value determined at the second instant to correspond to the pressure value determined last. 
     The operational test of the pressure sensing unit can also take place prior to the next engine start. This is particularly necessary if the operational test is subsequently not active in some circumstances, because the condition that the pressure gradient value is smaller than a stored pressure gradient limit value is not identified before the control device switches off. As the operational test is an integral part of the control device, this can also only function when the control device is being powered. Subsequently is understood to mean that the engine control device also implements functions for a certain period of time after the internal combustion engine has been cut off. 
     With a longer idle time of the internal combustion engine, the rail pressure is dissipated completely. The pressure gradient value is then automatically smaller than a stored pressure gradient limit value, so that the fault diagnosis can be implemented immediately when the internal combustion engine is switched on and the last determined pressure can thus be compared with the stored second limit value. 
       FIG. 2  shows the temporal rail pressure curve  1  for the case that no failure is identified in the case of the pressure sensing unit. The dashed line corresponds here to the temporal curve of the stored pressure limit value P 2  for determining the operational capability of the pressure sensing unit. The internal combustion engine is cut off at instant t 1 . From this time, the rail pressure curve P 1  drops. Until instant t 2 , the pressure gradient curve is above a stored pressure gradient limit value, with both curves not being shown in the diagram. Only after instant t 2  does the pressure gradient curve drop below the stored pressure gradient limit value and a value comparison is carried out between the rail pressure P 1  determined at the second instant and the stored pressure limit value P 2 . The result shows that the rail pressure P 1  at instant t 2  is below the stored pressure limit value P 2  and the pressure sensing unit is not faulty. 
       FIG. 3  shows the temporal rail pressure curve P 10  for the case that a failure is identified in the pressure sensing unit. The dashed line corresponds here to the temporal curve of the stored pressure limit value P 20  for determining the operational capability of the pressure sensing unit. The internal combustion engine is cut off at instant t 1 ′. The rail pressure curve P 10  drops after this time. The pressure gradient curve is above a stored pressure gradient limit value up until instant t 2 ′, with both curves not being shown in the diagram. Only after instant t 2  does the pressure gradient curve drop below the stored pressure gradient limit value and a value comparison is carried out between the last determined rail pressure P 10  and the stored pressure limit value P 20 . As the rail pressure P 10  is above the stored pressure limit value P 20 , an error is identified in the pressure sensing unit.