CLIAMS:I claim:
1. A method of operating a common rail system (20) of a vehicle in a limp home mode, said method comprising :
determining an error;
activating a limp home mode when said error is determined;
counting a number of limp home events activated ;
alerting a user of said vehicle when said count of time specific limp home events is more than a predefined value.
2. The method claimed in claim (1), wherein said error can be a rail pressure sensor error or a metering unit error or the like.
3. The method claimed in claim (1), wherein an energizing time of an injector (24) is calculated in dependence of said limp home event.
4. The method claimed in claim (1), wherein said energizing time of said injector (24) is a function of a speed of an engine, a load and a fuel temperature.
5. The method claimed in claim (1), wherein said energizing time of said injector (24) is calculated based on a rail pressure obtained from a rail pressure sensor.
6. The method claimed in claim (1), wherein said limp home event is a time specific limp home event, said time specific limp home event is activated for a predefined time period.
7. An electronic control unit (10) to operate a common rail system (20) of a vehicle in a limp home mode,
said electronic control unit (10) determines an error;
said electronic control unit (10) activates said limp home mode when said error is determined;
said electronic control unit (10) counts a number of limp home events; and
said electronic control unit (10) alerts a user of said vehicle if said count of limp home events are more than a predefined value. ,TagSPECI:Complete specification: The following specification particularly describes the invention and the manner in which it is to be performed.

Field of the invention
[001] This invention relates to a method to operate a common rail system in a limp home mode.
Background of the invention
[002] A common rail system typically includes a source of fuel in the form of a common rail which is charged with fuel at high pressure by means of a high pressure fuel pump. The common rail delivers fuel to a plurality of injectors, each one being arranged to inject fuel into an associated engine cylinder. The common rail is provided with a rail pressure sensor providing an output signal indicative of the pressure of fuel within the common rail and, hence, the pressure of fuel delivered to the injectors. The rail pressure can be controlled by a pressure regulating valve (on the high pressure side) or by a metering unit (on the suction side).In such systems, failure of the rail pressure sensor or metering unit causes engine operation to be halted, leaving the vehicle immobilized until the fault can be corrected.
[003] A US Patent 6578555 A2 discloses a control method to control a rate of flow of fuel delivered from a high pressure fuel pump and a common rail to an injector by means of a metering valve arrangement supplied with a control current. Engine speed is measured and the status of the pressure sensor is monitored to determine a fault condition has occurred. By controlling the fuel system in common rail diesel internal combustion engine to provide a limp home facility in the event of the fault to the rail pressure sensor.
Brief description of the accompanying drawings
[004] An exemplifying embodiment of the invention is disclosed in detail in the description and illustrated in the accompanying drawings:
[005] Figure 1 illustrates a block diagram of an electronic control unit according to one embodiment of the invention;
[006] Figure 2 illustrates a schematic of a common rail system according to one embodiment of the invention; and
[007] Figure 3 illustrates a flowchart of a method of working of an electronic control unit operating a pump in a vehicle in a limited limp home mode according to one embodiment of the invention.

Detailed description of the invention:
[008] Figure 1 illustrates a block diagram of an electronic control unit 10 to operate a common rail system 12 of a vehicle in a limp home mode according to one embodiment of the invention. The electronic control unit 10 determines an error. The electronic control unit 10 activates the limp home mode when the error is determined. The electronic control unit 10 counts a number of limp home events. The electronic control unit 10 alerts a user of the vehicle if the count of the time specific limp home events is more than a predefined value.
[009] Figure 2 illustrates a schematic of a common rail system 20 of a vehicle according to one embodiment of the invention. The common rail system 20 comprises a common rail pump 26 adapted to pump fuel into a common rail 28 from a tank 22. The common rail system 20 also comprises an injector 24 adapted to inject fuel from the common rail 28 to a cylinder (not shown). A common rail 28 stores the fuel at high pressure which is to be supplied to the injector 24. The injector 24 is connected to the common rail 28 via fuel feed lines respectively so that the injector 24 receive the fuel of a pressure equal to a threshold pressure from the common rail 28. The injector 24 comprises control solenoid valve (not shown) which will be opened and closed by control instructions from the electronic control unit 10 periodically, allowing the injection of the high pressure fuel into the cylinder of the engine via the injector 24.
[0010] The common rail pump 26 is shown in the figure 2 .The pressure is measured on a high pressure side in the common rail 28. A Metering unit opens a slit to allow low or high fuel flow. Due to this fuel delivery to the common rail pump 26, the rail pressure is adjusted. The electronic control unit 10 activates a limp home mode when an error is detected. The error can be can be a rail pressure sensor error or a metering unit error or the like. An injector back flow pressure will exceed the limitation during an over pressure condition (i.e., when the error is detected).
[0011] Figure 3 illustrates a flowchart of a method operating a common rail system of a vehicle in a limp home mode according to one embodiment of the invention. In step S1, an error is determined. A limp home mode is activated when the error is determined in step S2. In step S3, a number of limp home events activated are counted. A user of the vehicle is alerted when the count of limp home events is more than a predefined value in step S4.
[0012] The electronic control unit 10 can be chosen from a group of electronic control unit like a microcontroller, a microprocessor, an integrated circuit or the like. The limp home mode is a mode in which the engine will be running with less power or a reduced speed for a time period.
[0013] The method of operating a common rail system of a vehicle in a limp home mode, the electronic control unit 10 determines the error occurred either in a metering unit 11 or in a rail pressure sensor. For instance, if the error occurred is in rail pressure sensor, the electronic control unit 10 compares the pressure value with a threshold pressure value. The threshold pressure value is stored in the electronic control unit 10. The electronic control unit 10 activates the limp home mode when the above error is detected, i.e.., the electronic control unit 10 activates the limp home mode if the determined pressure value is more than the threshold pressure value. The electronic control unit 10 counts a number of time specific limp home events activated. The limp home event which is time specific is an event during which the limp home mode is activated for a specific time period. For an instance, the time period of the first limp home event when activated can be a minute. The limp home mode is activated for a particular time period. The electronic control unit 10 counts the number of activated limp home events when the error is occurred. The electronic control unit 10 alerts the user of the vehicle when the number of limp home events When the limp home mode is activated, the electronic control unit 10 calculates an energizing time of an injector 24 which is in dependence with the specific time period of the limp home event. The energizing time of the injector 24 is a time period for which the injector 24 is energized to inject a fuel into a cylinder (not shown).The energizing time of the injector 24 can be a function of a speed of an engine and a load in the vehicle. The energizing time of the injector 24 can also be calculated based on a rail pressure obtained from a rail pressure sensor(not shown).The energizing time of the injector 24 is controlled by the electronic control unit 10.
[0014] The electronic control unit 10 counts the number of the limp home events occurred when the error is determined. The number of the limp home events is stored in a memory (not shown). Based on the number of the limp home events activated, the electronic control unit 10 provides an alert to the user of the vehicle. For example, if the number of limp home events activated is more than a predefined number, the electronic control unit 10 provides an alert to change a component in the vehicle or to take the vehicle to a service center. For instance, when an error is occurred in the vehicle, the electronic control unit 10 activates the limp home event for a particular time period, say, for one minute. In this duration, the vehicle will be running with less speed and with minimum number of components working. After one minute, the vehicle is made to stop and when the user starts operating the vehicle, the electronic control unit 10 checks for the functionality of the components. If again the error is detected, the electronic control unit 10 activates the limp home mode for a specific time period, say 45 seconds and the vehicle runs with minimum speed and with minimum number of components working. The electronic control unit 10 checks for the number of limp mode events activated and if the number of limp home event activated is more than the predefined value, the electronic control unit 10 provides an alert to the user of the vehicle.
[0015] With the above method of operating a common rail system of the vehicle in a limp home mode, stalling of the engine can be avoided. With the above method disclosed, the vehicle can be taken to the service center with less effort.
[0016] It must be understood that the examples and embodiments of the components explained in the detailed description are only illustrative and do not limit the scope of the invention. The scope of this invention is only limited by the scope of the claims