Abstract:
A method for operating an internal combustion engine, in particular of a motor vehicle including at least one fuel pump delivers the fuel from a fuel tank into a fuel line. The pressure of the fuel, at least in a region of the fuel line, is increased as a function of an operating state. In order to assure a reliable starting of the engine, the pressure of the fuel, at least In the above-mentioned region of the fuel line is at least intermittently increased when the engine is not running.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 U.S.C. 371 application of PCT/DE 01/04604, filed on Dec. 6, 2001. 
   FIELD OF INVENTION 
   The invention relates to a method for operating an internal combustion engine, in particular of a motor vehicle, in which at least one fuel pump delivers the fuel from a fuel tank into a fuel line and in which the pressure of the fuel, at least in a region of the fuel line, is increased as a function of an operating state. 
   DESCRIPTION OF PRIOR ART 
   A method of the type with which the invention is concerned is known from DE 195 39 885 A1. This reference relates to a fuel supply system of an internal combustion engine. The fuel supply system includes two series connected fuel pumps as well as a number of fuel valves that each inject directly into a combustion chamber. In the known method, a valve device assures that during the starting process of the engine, one of the two fuel pumps delivers the fuel to the fuel valves at an increased pressure. This rinses vapor bubbles out of the fuel line or compresses them in the fuel line, thus permitting the engine to be started in a sufficiently short time. 
   The method proposed in DE 195 39 885 does in fact improve the starting behavior of the engine considerably. However it has turned out that an even greater improvement of the starting behavior and in particular a reduction in the starting time of the engine is sought. The object of the current invention, therefore, is to modify a method of the type mentioned above so that the starting behavior of the engine is improved further. 
   This object is attained with a method of the type mentioned above by virtue of the fact that the pressure of the fuel is at least intermittently increased in the above-mentioned region of the fuel line when the engine is not runninq. 
   BACKGROUND OF THE INVENTION 
   The method according to the invention has the advantage that even when the engine is at rest, i.e. when it is not running, the pressure of the fuel is increased in relation to the normal pressure, which prevents the formation of vapor bubbles from the start. In contrast to the prior art, therefore, the method according to the invention does not rinse already existing gas bubbles out of the fuel lines, but rather prevents them from forming in the first place. This makes it possible to supply fuel to the combustion chambers of the engine even faster during the starting process, which accelerates the starting process itself and improves the starting behavior of the engine. 
   Increasing the fuel pressure in the fuel line only when the engine is not operating also has the advantage over a continuously elevated fuel pressure that the components of the engine are under less stress during normal operation. This applies in particular to the fuel pump, whose service life is extended by the lower normal pressure and also applies to the fuel lines, which are less susceptible to permeation at the lower normal pressure. 
   Advantageous modifications of the invention are disclosed in the dependent claims. 
   A first modification provides that the pressure of the fuel, at least in the above-mentioned region of the fuel line, is increased when the engine is not running if the temperature of the engine is above a limit value. The formation of vapor bubbles is particularly likely when the fuel in the fuel lines is warm. Such a heating of the fuel is in turn to be expected when the engine is switched off after a long period of operation and due to heat dissipation, the hot engine heats the fuel line, the fuel pump, and/or other elements of the fuel system. 
   The modification of the method according to the invention, takes into account this heating of the fuel. On the other hand, the increase of the fuel pressure in the fuel line is also eliminated when the engine is not running if, for example, the engine is started for only a short time, i.e. it has not reached a high operating temperature and therefore no vapor bubble-inducing heating of the fuel in the fuel line is to be expected. 
   The invention also proposes that the pressure of the fuel in the above-mentioned region of the fuel line remain elevated at least during the starting of the engine and preferably during a time interval after the starting of the engine. This accelerates a reliable starting of the engine even more and, with a high degree of reliability, assures a smooth and safe operation of the engine after the starting procedure. 
   It is particularly preferable if the period of time during which the pressure of the fuel remains elevated after the starting of the engine, at least in the above-mentioned region of the fuel line, depends on the temperature of the engine. As explained above, the probability of the formation of vapor bubbles depends on the temperature of the fuel, which in turn depends on the temperature of the engine. During operation of a very hot engine, for example an engine that is started again after a long period of operation followed by a short intermediary stop, the danger of vapor bubbles being produced is particularly pronounced. In this instance, the pressure of the fuel should remain elevated for a particular time interval, which depends on the temperature of the engine. If need be, the pressure can be reduced again when the temperature of the engine has fallen below a limit value. 
   In a particularly preferred modification of the method according to the invention, a high-pressure region and a low-pressure region of the fuel line are connected to each other during the phase with the increased pressure of the fuel, at least in a region of the fuel line, particularly when the engine is not running. Such a fuel line with a high-pressure region and a low-pressure region is used, for example, in internal combustion engines with gasoline direct injection. 
   In a fuel system of this kind, the fuel is first delivered by an electric fuel pump into the low-pressure region of the fuel line and is supplied to a high-pressure pump directly driven by the engine. This high-pressure pump delivers the fuel at very high pressure (up to 120 bar) into a fuel accumulation line, which is also referred to as a “rail”. From this rail, the fuel is supplied directly to the injection valves, which inject the fuel directly into the combustion chambers of the engine. 
   Normally when the engine is not running, the high-pressure region and the low-pressure region of the fuel line are separated from each other. The high-pressure components of a high-pressure region, e.g. the high-pressure pump, the high-pressure injection valves, a quantity control valve, and a pressure control valve, are sometimes subjected to the high pressure in the high-pressure region for a very long period of time. If the high-pressure region is connected to the low-pressure region during the phase with the increased pressure of the fuel, this automatically results in a reduction of the pressure in the high-pressure region to a common pressure value, which is, however, higher than the usual pressure value in the low-pressure region of the fuel line, thus preventing vapor bubbles. 
   The components in the high-pressure region of the fuel line are consequently no longer subjected to the particularly high pressure so that the sealing demands on these components are reduced. This reduces the production costs for the corresponding components and possibly also increases the service life of these components. 
   One possibility for increasing the pressure of the fuel in the above-mentioned region in the above-mentioned manner is comprised in that a device, which sets the pressure of the fuel to a normal level, at least in the above-mentioned region of the fuel line, is switched off during the phase with increased fuel pressure. This variant of the method according to the invention is particularly easy to achieve. 
   It is also possible for the increase of the pressure of the fuel, at least in the above-mentioned region of the fuel line when the engine is not running, to include the activation of at least one fuel pump after the engine is switched off. Such an activation of the fuel pump is very easy to achieve and contributes to the desired result. 
   It is also possible for the increase of the pressure of the fuel, at least in the above-mentioned region of the fuel line when the engine is not running, to take place at least by means of a temperature increase of the fuel in the above-mentioned region of the fuel line. This variant of the method according to the invention takes advantage of the increase in the temperature of the fuel that is expected anyway: heat dissipation from the hot engine can cause such a temperature increase, which due to the closed volume of the fuel in the fuel line, causes the desired pressure increase. In this modification of the method according to the invention, therefore, the pressure increase is produced in a particularly simple manner. 
   The invention also relates to a computer program, which is suitable for executing the above-mentioned method when it is run on a computer. It is particularly preferable if the computer program is stored in a memory, in particular a flash memory. 
   The invention also relates to a control and/or regulating unit for operating an internal combustion engine, in particular of a motor vehicle, in which at least one fuel pump delivers the fuel from a fuel tank into a fuel line and in which the pressure of the fuel, at least in a region of the fuel line, is increased as a function of an operating state. Such a control and/or regulating unit is known from the market. In order to accelerate the starting process of the engine, the invention proposes that the control and/or regulating unit be suitable for controlling and/or regulating the above-mentioned method. It is particularly preferable if the control and/or regulating unit is provided with a computer program of the above-mentioned type. 
   The invention also relates to an internal combustion engine with at least one fuel pump that delivers fuel into a fuel line and with a device that can increase the pressure of the fuel, at least in a region of the fuel line, as a function of an operating state of the engine. An internal combustion engine of this kind is also known from the market. In order to be able to start this engine better, the invention proposes that it be provided with a control and/or regulating unit of the above-mentioned type. 
   In the engine according to the invention, in order to be able to produce the simplest possible pressure increase of the fuel pressure in the fuel line that can be achieved with the control and/or regulating unit, the invention proposes that the engine have a first pressure adjusting device, which can adjust a normal pressure of the fuel, at least in a region of the fuel line. 
   The invention also proposes that the engine have a second pressure adjusting device that can adjust an increased pressure of the fuel, at least in the above-mentioned region of the fuel line; the first and second pressure adjusting device are each connected at least to the above-mentioned region of the fuel line. In addition, the engine should also have a device that can fluidically disconnect the first pressure adjusting device at least from the above-mentioned region of the fuel line when the engine is not running. 
   The assembly of the above-mentioned components is simplified by virtue of the fact that the pressure adjusting devices and the disconnecting device are integrated into a module. 

   
     DESCRIPTION OF DRAWINGS 
     An exemplary embodiment of the invention will be explained in detail below with reference to the accompanying drawings. 
       FIG. 1  shows a schematic block circuit diagram of an internal combustion engine; 
       FIG. 2  shows a flowchart of a first method for operating the internal combustion engine from  FIG. 1 ; and 
       FIG. 3  shows a flowchart of a second method for operating the internal combustion engine from  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIG. 1 , an internal combustion engine is labeled as a whole with the reference numeral  10 . It includes a combustion chamber  12 , which is supplied with air via an intake tube  14 . The exhaust gases are carried away by an exhaust tube  16 . 
   The fuel is supplied to the combustion chamber  12  by means of injection valves  18 , only one of which is shown in  FIG. 1 . The injection valves  18  are connected to a fuel accumulation line  20 , which is commonly referred to as a “rail”. The fuel is delivered to the fuel accumulation line  20  by means of a high-pressure pump  22  and is placed under pressure. A high-pressure fuel line  24  is provided between the high-pressure pump  22  and the fuel accumulation line  20 . The high-pressure pump  22 , the high-pressure fuel line  24 , and the fuel accumulation line  20  constitute a high-pressure region of the fuel system. A low-pressure fuel line  26  leads from the high-pressure pump  22  to a tank  28 . A fuel filter  30  and an electric fuel pump  32  are disposed in the low-pressure fuel line  26 . A branch line  34  branches from the low-pressure fuel line  26  between the fuel filter  30  and high-pressure pump  22  and feeds back into the low-pressure fuel line  26  between the electric fuel pump  32  and the tank  28 . The branch line  34  in turn branches into two parallel branches  34   a  and  34   b . The branch  34   a  of the branch line  34  contains a shutoff valve  36  and a first pressure controller  38 . The first pressure controller  38  is designed so it opens at a pressure of approximately 4 bar in the branch  34   a  of the branch line  34 . 
   The second branch  34   b  of the branch line  34  contains a second pressure controller  40 , which opens at a corresponding pressure of approximately 6 bar. The shutoff valve  36 , the first pressure controller  38 , and the second pressure controller  40  are integrated into a module  42 , which is integrated into the cover of the tank  28  in a manner not shown in detail in  FIG. 1 . This makes it easier to install the pressure controllers  38  and  40  and the valve  36 . A check valve  44 , which closes in the direction of the tank  28 , and a pressure damper  46  are also provided between the high-pressure pump  22  and the fuel filter  30 . 
   An overflow line  38  leads from the high-pressure pump  22  to the tank  28 . Fuel, which overflows from the high-pressure pump  22  due to the high pressure in the fuel accumulation line  20  and the high-pressure fuel line  24 , is conveyed back to the tank  38  via this overflow line  38 . A return line  50  is connected on the one end to the high-pressure fuel line  24  between the high-pressure pump  22  and the fuel accumulation line  20  and is connected at the other end to the low-pressure fuel line  26  between the pressure damper  46  and the high-pressure pump  22 . A quantity control valve  52  is inserted in the return line  50 . 
   This quantity control valve  52  is a 2/2-port directional-control valve, which in its extreme position, completely closes the return line  50  and in the other extreme position, completely opens the return line  50 . The quantity control valve  52  is actuated by a magnetic actuator  54 . When it is without power, the quantity control valve  52  is pressed into its completely open extreme position by a spring  56 . 
   The fuel accumulation line  20  is connected to a pressure control valve  58 , which in turn is fluidically connected to the low-pressure fuel line  26  at a point between the pressure damper  46  and the filter  30 . The pressure control valve  58  is a spring-loaded ball valve with an opening pressure of approximately 125 bar. 
   The pressure in the fuel accumulation line  20  is detected by a pressure sensor  60 , which sends corresponding signals to a control and regulating unit  62 . This unit also receives signals from a temperature sensor  64  that measures the temperature of the engine  10 , e.g. the temperature of cooling water (not shown). On the input side, the control and regulating unit  62  is also connected to a position sensor  66  of an ignition lock (not shown). On the output side, the control and regulating unit  62  is connected to the magnetic actuator  54  of the quantity control valve  52 , the injection valves  18 , the electric fuel pump  32 , and the shutoff valve  36 . 
   During normal operation of the engine  10 , the control and regulating unit  62  triggers the shutoff valve  36  so that it is open and the branch  34   a  of the branch line  34  is open. The fuel that the electric fuel pump  32  delivers from the tank  28  into the low-pressure fuel line  26  is therefore set by the pressure controller  38  to a pressure of approximately 4 bar. The pressure controller  40  in the second branch  34   b  of the branch line  34  is not active because it only opens with a pressure of approximately 6 bar in the branch line  34  (naturally, the pressure in the branch line  34  and in the sections  34   a  and  34   b  is equal to the pressure in the region of the low-pressure fuel line  26 , which is disposed between the fuel pump  32  and the high-pressure pump  22 ). 
   The branch line  34  and the components  36 ,  38 , and  40  contained therein, the low-pressure fuel line  26 , and the electric fuel pump  32  thus constitute a low-pressure region of the fuel line. This fuel, which is “precompressed” to 4 bar, is compressed to a pressure of approximately 125 bar by the high-pressure pump  22  and is conveyed into the high-pressure fuel line  24  in the direction of the fuel accumulation line  20 . The rate of flow is controlled by the quantity control valve  52 . 
   In order to be able to start the engine  10  as rapidly as possible, when the engine  10  is not running, a process is executed, which will now be explained in detail with reference to  FIG. 2 . A non-running state is understood to be one in which the engine  10  is switched off, i.e. the crankshaft (not shown) is not rotating and, for example, the ignition is also switched off. The process shown in  FIG. 2  is stored in the form of a computer program in the control and regulating unit  62 . 
   After a start block of  68 , in block  70 , the program checks whether, based on the position of the position sensor  66  of the ignition lock or based on a movement of this position sensor  66 , a shutdown sequence of the engine  10  has been initiated. If so, in block  72 , the program checks whether the temperature T of the engine  10  detected by the temperature sensor  64  (for example the temperature of the cooling water of the engine  10 ) is greater than a limit value TG. If this is also true, then in block  74 , the control and regulating unit  62  triggers the shutoff valve  36  so that it closes. In block  75 , the electric fuel pump  32  is switched on and after a certain time interval has elapsed (block  76 ), the electric fuel pump  32  is switched back off in block  78 . In block  80 , a flag is set. The program ends in the end block  82 . The process also leaps to block  82  if the answers to the queries in blocks  70  or  72  are negative. 
   When the engine  10  is switched off by turning the ignition key in the ignition lock, and when the determination is made that the temperature T of the engine is higher than a limit value TG, the method shown in  FIG. 2  causes the pressure controller  38  to be deactivated by the closed shutoff valve  36 . If the fuel pump  32  is then switched on in block  75 , the pressure control in the low-pressure fuel line  26  is set by the second pressure controller  40  in branch  34   b  of the branch line  34 , i.e. the pressure is set to a higher pressure, namely 6 bar in this instance. This increased pressure in the low-pressure fuel line  26  causes already existing vapor bubbles to be compressed and reliably prevents new vapor bubbles from being produced. In an exemplary embodiment that is not shown, the pressure increase is produced additionally or exclusively by means of the heating of the fuel due to the heat dissipation from the hot engine. 
   Since the 2/2-port quantity control valve  52 , when it is without current when the engine  20  is not running, is pressed into its completely open position by the spring  56 , the low-pressure fuel line  26  is fluidically connected to the high-pressure fuel line  24 . Therefore, the same pressure prevails in both of the fuel lines  24  and  26  and in the fuel accumulation line  20 , namely the pressure of  6  bar that is mentioned above. This pressure is considerably lower than the pressure otherwise present in the high-pressure region of the fuel system. This reduced pressure in the high-pressure region of the fuel system when the engine is not running considerably reduces the sealing demands on the components in the high-pressure region, for example the injection valves  18 , so that they can be more simply and inexpensively designed. 
   When the engine  10  is started, the following procedures are executed (the corresponding method shown in  FIG. 3  is likewise stored in the form of a computer program in the control and regulating unit  62 ): 
   After a start block  84 , in block  86 , the program executes a query as to whether—for example due to a corresponding movement of the key in the ignition lock, which is detected by the position sensor  66 —an ignition sequence of the engine  10  is in progress. If so, then in block  88 , the program executes a query as to whether the flag has been set. If this is also the case, which indicates that during the preceding non-operation of the engine  10 , an increased fuel pressure was set in the low-pressure fuel line  26 , then in block  90 , the program queries the temperature T of the engine  10  detected by the temperature sensor  64  and compares it to a limit value TG. 
   If the actual temperature T of the engine  10  is lower than the limit value TG, then in block  92 , the shutoff valve  36  is opened, which reactivates the pressure controller  38  and sets the pressure in the low-pressure fuel line  26  to a lower pressure, 4 bar in this instance, (the quantity control valve  52  was previously closed so that the low-pressure fuel line  26  and the high-pressure fuel line  24  are fluidically decoupled from each other). Then in block  94 , the engine  10  is started and in block  96 , the flag is deleted. The method finally ends in block  98 . This takes into account the fact that if the temperature of the engine is so low that the formation of vapor bubbles in the low-pressure fuel line  26  is not to be expected, then it is not necessary to start the engine  10  with increased pressure of the fuel in the low-pressure fuel line  26 . 
   However, if the actual temperature T of the engine is greater than the limit value TG, i.e. if a so-called “hot start” is being executed, then in block  100 , the engine  10  is started. The starting of the engine  10  in this case therefore takes place with the increased pressure of the fuel in the low-pressure fuel line  26  set by the pressure controller  40 . After a time interval t, which is determined in block  102 , the control and regulating unit  62  triggers the shutoff valve  36  so that it opens (block  103 ). This reactivates the first pressure controller  38  in the branch line  34   a , as a result of which the fuel in the low-pressure fuel line  26  is set to a normal pressure of approximately 4 bar. 
   Since the pressure in the low-pressure fuel line  26  is only reduced after the passage of a particular time interval after the starting of the engine  10 , this assures that an elevated fuel pressure prevails during the entire starting process of the engine  10  and even during a period of time that is long enough for the engine  10  to cool down, and this prevents vapor bubbles from being produced in the low-pressure fuel line  26 . In block  104 , the flag is deleted. 
   In an exemplary embodiment that is not shown, the time interval t in block  102  can depend on the temperature T of the engine  10 . This assures that the pressure in the low-pressure fuel line  26  is only reduced to a normal level if the temperature of the engine  10  has fallen to a point that a formation of vapor bubbles in the low-pressure fuel line  26  is no longer to be expected. 
   The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.