CLIAMS:I claim:
1. A method for diagnosing a metering unit (12) of a common rail fuel injection system, said method comprising the following steps,
increasing the duty cycle of the metering unit (12) by increasing the current supplied to the metering unit (12) by a current control unit (16), (S0);
detecting whether the engine is running or stalled on increasing of the duty cycle (S1);
decreasing the duty cycle of the metering unit (12) by reducing the current supplied to the metering unit by a current control unit (16), (S1); and
detecting whether a pressure release valve of the common rail is open or closed on reducing of the duty cycle (S2);
determining whether the operation of the metering unit (12) is satisfactory based on the engine running condition and the status of the pressure release valve (S3) by means of a control unit (16).
2. A method as claimed in claim (1) wherein increasing the duty cycle of the metering unit (12) is carried at low idle, medium idle and high idle operating conditions of the engine.
3. A method as claimed in claim (1) wherein decreasing the duty cycle of the metering unit (12) is at low idle or at a predefined idle speed condition.
4. A method as claimed in claim (1) wherein said stalling of engine when duty cycle is increased indicates that the metering unit (12) closes progressively with increase in the current.
5. A method as claimed in claim (1) wherein said running of engine when duty cycle is increased indicates that the metering unit (12) does not close progressively with increase in the current and may be stuck at particular position.
6. A method as claimed in claim (1) wherein said opening of the pressure release valve when duty cycle is decreased indicates that the metering unit (12) opens progressively with decrease in the current.
7. A method as claimed in claim (1) wherein said not opening of the pressure release valve when duty cycle is decreased indicates that the metering unit (12) does not open progressively with increase in the current and may be stuck at particular position.
8. A method as claimed in claim (1) wherein indicating satisfactory operation of the metering unit only if the engine is stalled when duty cycle of the metering unit (12) is increased and the pressure release valve opens when the duty cycle of the metering unit (12) is decreased.
9. A device (10) for diagnosing a metering unit (12) of a common rail fuel injection system, said device (10) comprising
a current control means (14) adapted to vary the current supplied to the metering unit (12); and
a control unit (16) adapted to communicate with an engine ECU (18) and receive the engine speed and status of the pressure release valve of common rail, said control unit (16) further adapted to determine whether metering unit (12) operation is satisfactory based on said received inputs.
10. A device as claimed in claim (9) wherein an indicating means is provided for indicating if the metering unit operation is satisfactory.
11. A device as claimed in claim (9) wherein an engine idling control means is provided to control the engine idling speed to perform the diagnosis at different idling conditions.
12. A device as claimed in claim (9) wherein said device receives the actual rail pressure value from the rail pressure sensor to perform the diagnostic operations on the metering unit (12).
,TagSPECI:Field of the invention:
[001] The invention relates to engine diagnostic devices and method. Particularly the invention relates to a method and device for diagnosis of a metering unit of a common rail fuel injection system.
Background of the invention:
[002] A common rail fuel injection system supplies high pressure fuel to the injectors of an engine. The high pressure of the fuel in the common rail enables continuous supply of fuel which can be delivered in precise quantities. Further the fuel also gets atomized resulting in efficient engine operation. It is hence important to maintain the required fuel pressure in the rail. One way of controlling the rail pressure is to control fuel delivery on the suction side. A metering unit is connected to a high-pressure pump supplying fuel to the fuel rail. The metering unit makes sure that the pump delivers exactly the right quantity of fuel to the fuel rail in order to maintain the injection pressure required by the system. In a fault situation, the pressure-relief valve on the rail prevents rail pressure from exceeding a maximum safe limit and allows the vehicle drive in limp home mode.
[003] There are different techniques known in the present state of art which are used for diagnosing faults in the fuel supply devices. Typically the detection of these faults is limited to the faults in the high pressure pump. DE102011079519 discloses one such method for diagnosing a fuel supply device used in a motor vehicle. In the method disclosed the flow rate for fuel pump is varied by means of a metering unit. Further geometrical volume of the pump is compared with the flow rate of the fuel supply.
[004] The present invention discloses a quick check method for diagnosing the metering unit sticky which supplies fuel to the high pressure pump.

Brief description of the drawings:
[005] The present invention is described with reference to the following accompanying drawing,
[006] Fig. 1 illustrates a flowchart corresponding to the method for diagnosing a metering unit of a common rail system in accordance with one embodiment of the invention; and
[007] Fig. 2 illustrates a device for diagnosing a metering unit of a common rail system in connection with a metering unit and an ECU, in accordance with one embodiment of the invention.
Detailed description of the embodiments:
[008] Fig. 1 illustrates a flowchart of the method for diagnosing a metering unit (12) of a common rail system in accordance with one embodiment of the invention.
[009] The invention proposes a quick and easy method for diagnosing a metering unit (12) of a common rail fuel injection system. The metering unit (12) is normally open when no current is supplied to it and closes progressively as the current supplied to it increases. In accordance with one embodiment of the method the duty cycle of the metering unit (12) is increased by increasing the current supplied to the metering unit (12) by a current control unit (16), (S0). As the duty cycle is increased the fuel supply to the engine gradually comes to a stop as the metering unit (12) closes when the current increases. By detecting whether the engine is running or stalled on increasing of the duty cycle (S1) it can be determined if the metering unit is closing satisfactorily. The duty cycle of the metering unit (12) is reduced in a further step by reducing the current supplied to the metering unit by a current control unit (16), (S1). As the current supplied to the metering unit decreases, the metering unit opens progressively thereby increasing the fuel supplied to the common rail. When the metering unit is fully open the fuel rail is continuously supplied with fuel and the rail pressure increases. When the rail pressure crosses the maximum limit for the pressure, a pressure release valve of the common rail opens releasing the excess pressure. By detecting whether the pressure release valve of the common rail is open or closed when the duty cycle is reduced it can be determined whether the metering unit is opening satisfactorily (S2). Thus, it is determined whether the operation of the metering unit (12) is satisfactory and it is not stuck in a particular position. based on the engine running condition and the status of the pressure release valve (S3) by means of a control unit (16).
[0010] In accordance with one embodiment if the invention, increasing the duty cycle of the metering unit (12) is carried at different speeds, low idle, medium idle and high idle operating conditions of the engine. By increasing the current supplied to the metering unit (12) to at least three engine idling speeds, it is determined if the metering unit has the required response for closing at different engine operation conditions. The opening of the metering unit needs to be checked only when the engine idling speed is low or predefined based on the requirement. Hence decreasing the duty cycle of the metering unit (12) for diagnosis is performed at lower engine idle condition.
[0011] The stalling of the engine when duty cycle is increased indicates that the metering unit (12) closes progressively with increase in the current whereas if the engine continues to run when duty cycle is increased indicates that the metering unit (12) does not close progressively with increase in the current and may be stuck at particular position.
[0012] The opening of the pressure release valve of the common rail when duty cycle is decreased indicates that the metering unit (12) opens progressively with decrease in the current whereas not opening of the pressure release valve when duty cycle is decreased indicates that the metering unit (12) does not open completely with increase in the current and may be stuck at particular position.
[0013] Only if both the above mentioned steps of the diagnosis indicate satisfactory operation of the metering unit, then a corresponding indication may be given on the device for the same. In case if one of the above steps indicates that the metering unit is stuck in an intermediate position then a warning is given regarding failure of the metering unit.
[0014] Fig. 2 illustrates a device for diagnosing a metering unit of a common rail system in connection with a metering unit and a engine ECU, in accordance with one embodiment of the invention.
[0015] The device (10) for diagnosing a metering unit (12) of a common rail fuel injection system comprises a current control means (14) adapted to vary the current supplied to the metering unit (12) and a control unit (16). The control unit (16) is adapted to communicate with an engine ECU (18) and receive the engine speed and status of the pressure release valve of common rail. The control unit (16) is further adapted to determine whether the metering unit (12) operation is satisfactory based on inputs received for engine speed and the status of the pressure release valve. An indicating means maybe provided on the device for indicating if the metering unit (12) operation is satisfactory. An engine idling control means may be provided to control the engine idling speed to perform the diagnosis at different engine idling conditions.
[0016] In accordance with one embodiment the present device is implemented as a part of the vehicle onboard diagnostic test equipment.
[0017] In accordance with another embodiment of the invention, the device is adapted to receive the actual rail pressure value from the rail pressure sensor to perform the diagnostic operations on the metering unit (12). Correlating the rail pressure value with the corresponding position of the metering unit (12) can be further used for performing the diagnosis of the metering unit (12).
[0018] It should be understood that the embodiments explained in the above description for diagnosing the metering unit are only for illustrative purpose and do not limit the scope of the invention. Many such embodiments and modifications in the embodiments can be envisaged. The scope of the invention is only limited by the scope of the claims.