Abstract:
A method for cooling a fuel injection system and fuel injection systems are proposed that improve the hot-starting performance of the internal combustion engine.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for cooling a fuel injection system and to a fuel injection system for an internal combustion engine. 
     2. Description of the Prior Art 
     Injection systems of internal combustion engines, especially if they are operated with Otto fuel tend to form vapor bubbles under certain conditions. For instance, it can happen that the high-pressure fuel pump and/or the common rail, after the shutoff of the engine, heat up so much that the boiling temperature of the fuel is reached or exceeded. The consequence is vapor bubbles in the fuel injection system, which in a so-called “hot start” of the engine that occurs soon after shutoff impair the starting performance of the engine. 
     From German Patent Disclosure DE 195 39 885 A1, it is known, in order to improve the hot-starting performance of an internal combustion engine, to flush the fuel injection system briefly with relatively cool fuel upon starting and then to raise the pressure in the fuel feed line. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the invention is to furnish a method for cooling a fuel injection system and to furnish a fuel injection system, which further improve the hot-starting performance of the engine. 
     According to the invention, this object is attained by a method for cooling a fuel injection system for internal combustion engines, having a prefeed pump that pumps fuel from a tank via a feed line into a high-pressure region, the high-pressure region including a high-pressure pump, having a return line for removing excess fuel from the high-pressure region, and having a connecting line, connecting the feed line and return line, the flow through the connecting line being controllable by means of a scavenging valve, in which the scavenging valve is closed when upon turn-on of the engine a turn-on criterion has been reached, at least parts of the high-pressure region are scavenged with fuel from the fuel tank, and the scavenging valve is opened as soon as a turn-off criterion is reached. 
     In this method of the invention, the pressure elevation and the scavenging process begin simultaneously, so that the removal of any vapor bubbles that may be present from the high-pressure region and the compression of these bubbles occur simultaneously. The result is improved hot-starting performance of the engine. 
     Variants of the invention provide that a characteristic temperature is used as the turn-on criterion, in particular the temperature of the fuel in the high-pressure region and/or the temperature of components of the fuel injection system and/or the temperature of the air in the surroundings of the fuel injection system. This characteristic temperature can be measured or calculated by means of model calculation. By using a characteristic temperature as the turn-on criterion, it is assured that whenever there is a risk of vapor bubble development, a scavenging operation and compression of the vapor bubbles by an elevation of pressure in the fuel injection system is tripped. On the other hand, unnecessary pressure elevations in the feed line and in the high-pressure region upon engine starting are avoided. 
     Further features of the method of the invention provide that the scavenging valve is opened in time-controlled fashion, and/or that the scavenging valve is opened as a function of the course over time of the characteristic temperature, so that an unnecessary load on the prefeed pump is avoided and nevertheless, the development of vapor bubbles is suppressed immediately after the hot start of the engine. 
     Thus by way of example, the scavenging valve can be opened if the characteristic temperature has undershot a predetermined value. 
     It is also possible for the time during which the scavenging valve remains closed to be defined as a function of the characteristic temperature, and/or for the scavenging valve to be opened as a function of the fuel quantity pumped by the prefeed pump for scavenging purposes, or for the scavenging valve to be opened as a function of the air flow rate aspirated by the engine. In these variants of the method of the invention as well, effective removal and effective compression of the vapor bubbles from the high-pressure region are assured, without overloading the prefeed pump. In addition, a temperature sensor for ascertaining the turn-on criterion can be dispensed with. 
     The hot-starting performance of an internal combustion engine can be further improved if the scavenging valve is closed as soon as voltage is applied to the terminal of the vehicle. As a result, the pressure elevation occurs at the earliest possible instant, which further improves the hot-starting performance. 
     In a further feature of the invention, the turn-on criterion is reduced after the fuel tank has been filled, since “fresh” fuel has a lower boiling point than fuel that has been in the tank for a relatively long time. 
     The object stated at the outset is also attained by a fuel injection system for internal combustion engines, having a control unit, having a prefeed pump that pumps fuel from a tank via a feed line into a high-pressure region, the high-pressure region including at least one high-pressure pump, having a return line for removing fuel from the high-pressure region, the return line having a relief throttle and a low-pressure regulator, in which between the feed line and the return line, a connecting line with a scavenging valve and a mixing line with a check valve are provided; the mixing line discharges into the return line upstream of the connecting line; and the connecting line discharges into the return line upstream of the low-pressure regulator. 
     This fuel injection system of the invention is very simple in design, since to improve the hot-starting performance, only one connecting line with a scavenging valve and one mixing line with a check valve have to be provided. The pressure control valve presents a flow resistance upon starting of the engine, so that a pressure elevation in the feed line and the high-pressure region ensues as soon as the prefeed pump is in operation and the scavenging valve has closed. To improve the hot-starting performance, all that is needed is to monitor a turn-on criterion and to close the scavenging valve if necessary. Because of the simultaneity of the scavenging and compression of the vapor bubbles, the engine turns over after only a very short time even in a hot start. 
     In a variant of the invention, a relief throttle is provided between the discharge point of the mixing line and connecting line into the return line, and that the check valve is loaded by a spring. The opening pressure Δ p  of the check valve, which can also be embodied as a spring-loaded check valve, causes a pressure difference to occur at the relief throttle as soon as the scavenging valve is closed. By a suitable adaptation of the opening pressure Δ p  of the check valve, the pumping capacity of the prefeed pump, and the opening pressure Δ p  of the check valve, the scavenging flow in the high-pressure region of the fuel injection system can be adjusted. Because the check valve operates as a function of pressure and need not be triggered by the control unit, the effort: and expense for laying a signal line is omitted, and there is less of a load on the control unit. The control unit can be relieved still further if the scavenging valve is opened when without current. 
     Further features of the invention provide that the check valve is spring-loaded, so that the opening pressure Δ p  of the check valve is easily adjustable. 
     In another variant of the invention, the scavenging valve is open when without current, so that the scavenging valve has to be triggered only in the case of a hot start of the engine. 
     The above-stated object is also attained by a fuel injection system for internal combustion engines, having a control unit, having a prefeed pump that pumps fuel from a tank via a feed line into a high-pressure region, the high-pressure region including at least one high-pressure pump, having a return line for removing fuel from the high-pressure region, wherein the return line has a check valve and discharges into the feed line, and having a leakage line for removing fuel from the high-pressure region into the tank, in which a check valve is provided in the return line, and a low-pressure regulator is provided in the leakage line. 
     In this fuel injection system of the invention, the high-pressure pump is permanently cooled, so that both during a hot start and in the heating mode, the formation of vapor bubbles is effectively suppressed. By a suitable adaptation of the pumping capacity of the prefeed pump and the pressure established by the low-pressure regulator in the leakage line, the scavenging quantity can be adjusted such that on the one hand the tank is not unnecessarily heated up, and on the other a more-reliable hot start and hot operation of the engine are assured. 
     In a further feature of the invention, it is provided that the high-pressure region includes a common rail and a pressure regulating valve, and that the pressure regulating valve regulates the pressure in the common rail by diverting fuel out of the common rail into the return line, so that the common rail can be scavenged as well. 
     In a further feature of the invention, it is provided that at least one temperature sensor is present, to ascertain the turn-on criterion or criteria or the turn-off criterion or criteria, so that the pressure elevation upon hot starting is effected only if there is a risk of vapor bubble formation, and on the other hand, the pressure elevation is not maintained any longer than necessary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which: 
     FIG. 1 schematically shows a first exemplary embodiment of a fuel injection system of the invention; 
     FIG. 2 schematically shows a second exemplary embodiment of a fuel injection system of the invention; 
     FIG. 3 is a flowchart for one exemplary embodiment of the method of the invention; and 
     FIG. 4 shows alternative turn-on criteria. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a first exemplary embodiment of a fuel injection system of the invention is shown. An electric prefeed pump  1  pumps fuel out of a tank  3  via a feed line  5  to a high-pressure region  7  of the fuel injection system. A filter  9  is built into the feed line  5 . 
     In the exemplary embodiment of FIG. 1, the high-pressure region  7  comprises a high-pressure pump  11 , a common rail  13  with injectors  15  connected to it, and a pressure regulating valve  17 . In normal operation, the pressure in the common rail  13  is measured by a pressure sensor, not shown. The measured pressure is evaluated by a control unit, also not shown, and taking additional parameters into account, triggers the pressure control valve  17  in such a way that the desired set-point pressure in the common rail  13  is maintained. 
     The pressure control valve  17  can lower the pressure in the common rail  13  by opening slightly, thus enabling the outflow of fuel from the common rail  13  into a return line  19 . 
     The return line  19  has a relief throttle  21  and a low-pressure regulator  23 . The relief throttle  21  divides the return line  19  into two portions  19   a  and  19   b . The relief throttle  21  has the task of limiting the return quantity (scavenging quantity) into the tank. The low-pressure regulator  23  has the task of establishing a virtually constant pressure of 3 to 4 bar, for instance, in the return line  19 . 
     Between the return line and the feed line, upstream of the relief throttle  21 , there is a mixing line  25  with a check valve  27 . The check valve can also be spring-loaded. 
     Upstream of the low-pressure regulator  23 , there is a connecting line  29  with a scavenging valve  31 . The scavenging valve  31  is a 2/2-way valve that is open when without current. 
     In normal operation of the internal combustion engine, not shown, the scavenging valve  31  is open, so that the fuel pumped by the prefeed pump  1  that is not used by the high-pressure region  7  is pumped into the return line  19  via the connecting line  29 . Accordingly, in normal operation the prefeed pump  1  pumps counter to the pressure established by the low-pressure regulator  23 . 
     If in a hot start there is the risk of vapor bubble formation in the high-pressure region  7 , then the scavenging valve  31  is closed, so that the pressure in the feed line  5  and in the high-pressure region  7  is independent of the pressure in the portion  19   b  of the return line  19 . 
     The prefeed pump  1  has a pumping level of 5 to 8 bar, for instance. When the feed line  5  and the high-pressure region  7  are acted upon by that pressure, any vapor bubbles present are compressed, so that the pumping of fuel into the injectors  15  takes place without delay. The prefeed pump  1  is designed such that during normal operation, it always pumps a certain scavenging quantity, which flows back into the tank  3  via the throttle  21 . Additionally, when the scavenging valve  31  is closed, scavenging of the high-pressure region  7  also takes place, so that any vapor bubbles present there are flushed out of the high-pressure region  7 . 
     The mixing line  25  and the check valve  27 , in conjunction with the relief throttle  21 , have the effect that in the portion of the return line  19  located upstream of the relief throttle  21 , a higher pressure than the pressure established by the low-pressure regulator  23  prevails during the scavenging operation. At the onset of pumping of fuel by the prefeed pump  1  into the high-pressure region  7 , with the scavenging valve  31  closed, the pressure control valve  17  does not yet regulate the pressure in the common rail  13 , since the pressure in the common rail  13  is not high enough. In this operating state, the pressure control valve  17  presents a flow resistance, which contributes to the pressure elevation in the feed line  5  and in the high-pressure region  7 . 
     This means that the pressure furnished by the prefeed pump  1  prevails in both the high-pressure region  7  and the portion  19   a  of the return line  19  located upstream of the relief throttle  21 . This pressure is limited, when the scavenging valve  31  is closed, by means of a pressure limiting valve, not shown separately but integrated with the prefeed pump  1 . Since the check valve  27  in the mixing line  25  does not open until a certain pressure difference Δ P  prevails between the portion  19   a  of the return line  19  and the feed line, the fuel during the scavenging operation flows out solely through the relief throttle  21 . Since the relief throttle  21  causes a pressure reduction, and the pressure in the portion  19   b  of the return line is regulated to approximately 3 to 4 bar by the low-pressure regulator  23 , the pressure in the portion  19   a  of the return line is higher than in the portion  19   b . By the appropriate adaptation of the relief throttle  21 , the opening pressure Δ p  of the check valve  27 , and the pumping capacity of the prefeed pump  1 , the desired scavenging flow can be established. The scavenging flow also has a cooling effect, since the high-pressure region  7  is flushed with cool fuel from the tank  3  and is thus cooled as well. 
     In FIG. 2, a second exemplary embodiment of a fuel injection system of the invention is shown. Identical components are identified by the same reference numerals, and what has been said for the first exemplary embodiment applies accordingly. In this exemplary embodiment, the return line  19  discharges into the feed line  5  upstream of the high-pressure pump  11 . In the return line  19 , a check valve  33  is provided, which prevents fuel from the feed line  5  from flowing directly into the return line  19 . A leakage line  35  with a low-pressure regulator  23  branches off at the high-pressure pump  11  and discharges into the tank  3 . As in the first exemplary embodiment, the low-pressure regulator  23  has the task of regulating a counterpressure. In this exemplary embodiment, continuous flushing of the high-pressure pump  11  with the full pumping quantity of the prefeed pump  1  takes place. The scavenging quantity in this operating state is not limited by a throttle. That is, each time the engine is started, the prefeed pump  1  is switched on, which pumps fuel into the feed line  5  and builds up the maximum pumping pressure level. This pumping level compresses any vapor bubbles that may be present. The excess fuel that is not used by the high-pressure region  7  flows back into the tank  3  via the leakage line  35 , so that permanent scavenging and cooling of the high-pressure pump  11  is achieved in the simplest possible way. 
     In the exemplary embodiments shown, the high-pressure region  7  has a common rail  13 , injectors  15 , and a pressure control valve  17 . However, the invention is not limited to common rail fuel injection systems but instead can be employed usefully in other fuel injection systems as well. 
     In FIG. 3, a block circuit diagram of one exemplary embodiment of a method of the invention for scavenging and cooling a fuel injection system is shown, in accordance with the first exemplary embodiment (FIG.  1 ). As soon as voltage is applied to the terminal of the vehicle, in a first interrogation operation it is checked whether the motor vehicle has just been refueled, or whether the fuel has been in the tank for a relatively long time. In the normal case, the fuel has already been in the tank for a relatively long time, so that the answer is then “No”(N). If this branch is then taken, a turn-on criterion is then checked, in the form of a temperature condition T threshold 2 . 
     If the temperature condition T threshold 2  is not met (branch N), then the risk of vapor bubbles in the high-pressure region does not exist, and a shift to normal operation of the fuel injection system is made. 
     If the temperature condition T threshold 2  is met, then the scavenging valve (or SV for short)  31  is triggered (branch Y); that is, the scavenging valve  31  is closed. In a subsequent interrogation operation, the triggering of a timer (timer  2 ) is checked. If the answer to the check is negative, then the fuel injection system shifts to normal operation. Otherwise, the scavenging valve  31  is kept closed. 
     In FIG. 4, alternative turn-on criteria for triggering the timer are shown. In FIG. 4 a , the triggering of the timer can be triggered as a function of a temperature condition. In FIG. 4 b , the temperature condition can be such that the scavenging valve  31  continues to be triggered, if a measured or calculated temperature is greater than a temperature T threshold . 
     In FIG. 4 c , the scavenging valve can continue to be triggered if the integral of the pumped fuel is less than a threshold M threshold . 
     In FIG. 4 d , the scavenging valve can continue to be triggered if the integral of the air flow throughput is less than a threshold M threshold . 
     In the case where the vehicle has just been refueled, then the left branch of the flow chart in FIG. 3 is taken. The essential distinction from the right branch in FIG. 3 is that the temperature condition T threshold 1 , which causes the closure of the scavenging valve  31 , is different. The temperature condition T threshold 1  is lower for a motor vehicle whose tank has just been filled, since the boiling temperature of fresh fuel is lower than that of fuel that has already been in the tank for some time. 
     It is understood that in a simplified embodiment of the invention, the question “Vehicle just refueled?” is dispensed with, and the temperature condition T threshold 2  is always used instead. 
     The term “turn-on criterion” is understood in conjunction with the invention to mean a limit value that must be reached or exceeded in order to trip a scavenging operation. Measured or calculated temperatures, but also calculated variables, which result from the load state of the engine before it is turned off, or time intervals are all suitable as a turn-on criterion. The term “turn-off criterion” should be understood accordingly. As soon as a turn-off criterion is reached or exceeded, for instance if the temperature of the high-pressure pump drops below a certain limit value that for the sake of generalization is called a turn-off criterion, then the scavenging operation is terminated. 
     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.