Method and device for monitoring a fuel-metering system

A method and a device for monitoring a fuel-metering system, in particular a common-rail system for a diesel fuel engine. A defect is recognized on the basis of an output signal from a structure-borne noise sensor.

FIELD OF THE INVENTION 
The present invention, a continuation of PCT/DE96/00737 dated Apr. 27, 
1996, relates to a method and a device for monitoring a fuel-metering 
system. 
BACKGROUND INFORMATION 
A method and device of this type are disclosed by U.S. Pat. No. 5,241,933. 
It describes a method and a device for monitoring a high-pressure circuit 
when working with a common-rail system. In the case of the device it 
describes, the pressure prevailing in the rail is regulated. If the 
manipulated variable of the pressure control loop lies outside of a 
specifiable range, the device recognizes the existence of an error. 
In addition, devices are known, where the existence of an error is inferred 
on the basis of the pressure prevailing in the rail. The pressure is 
thereby compared to lower and upper limiting values, and the existence of 
errors is recognized when the pressure lies outside of the specified range 
of values. 
The drawback of these arrangements is that an error is first recognized in 
response to a substantial pressure drop. 
SUMMARY OF THE INVENTION 
Given a device and a method for monitoring a fuel-metering system, the 
object of the present invention is to be able recognize the existence of 
errors in the most reliable and simple manner possible. 
The method and device according to the present invention make it possible 
for errors in the metering system to be recognized reliably and simply. In 
particular, it is possible to reliably verify defective injectors in 
common-rail systems.

DETAILED DESCRIPTION OF THE INVENTION 
The device according to the present invention will now be elucidated based 
on the example of a self-ignition internal combustion engine, in which the 
fuel metering is controlled by means of a solenoid valve. The specific 
embodiment of the present invention shown in FIG. 1 relates to what is 
known as a common-rail system. However, the procedure in accordance with 
the present invention is not limited to these systems. It can be employed 
in all systems where such a fuel metering is possible. 
Element 100 denotes an internal combustion engine, which is supplied with 
fresh air via an intake line 105 and which emits exhaust gas via an 
exhaust pipe 110. 
The illustrated internal combustion engine is a four-cylinder internal 
combustion engine. Assigned to each cylinder of the internal combustion 
engine are injectors 120, 121, 122 and 123. Fuel is metered to the 
injectors via solenoid valves 130, 131, 132 and 133. The fuel arrives from 
what is known as a rail 135, via injectors 120, 121, 122 and 123 in the 
cylinders of the internal combustion engine 100. 
The fuel in rail 135 is pressurized to an adjustable pressure by a 
high-pressure pump 145. The high-pressure pump 145 is connected via a 
solenoid valve 150 to a fuel-supply pump 155. The fuel-supply pump 
communicates with a fuel supply tank 160. 
An electric fuel pump or a mechanical fuel pump can be used as a 
fuel-supply pump. The use of an electric fuel pump requires a preliminary 
filter. Due to the high fuel temperatures, the electric fuel pump is 
preferably arranged in the vicinity of the tank. This results in large 
volumes between the electric fuel pump and the high-pressure pump, and 
substantial and, thus, long switch-off times. A rapid reduction in 
pressure, especially in the event of an error, can only be effected with 
additional outlay. 
These disadvantages are not associated with a mechanical auxiliary supply 
pump arranged near the internal combustion engine. In the case of the 
mechanical auxiliary supply pump, solenoid valve 150 (also referred to as 
a shutoff value) is additionally necessary, which in case of an error 
prevents the fuel from being supplied to the high-pressure pump 145. 
Shutoff valve 150 can be optionally designed as a separate structural 
unit. However, it can also be integrated, on the intake side, in 
high-pressure pump 145 or, on the delivery side, in auxiliary supply pump 
155. 
Valve 150 includes a coil 152. Solenoid valves 130, 131, 132 and 133 
contain coils 140, 141, 142 and 143, which can each receive current by 
means of an output stage 175. Output stage 175 is preferably arranged in a 
control unit 170, which drives coil 152 accordingly. 
Furthermore, a sensor 177 is provided, which detects the pressure 
prevailing in rail 135 and routes a corresponding signal to control unit 
170. Element 180 is a structure-borne noise sensor, which is mounted on 
the engine at a spot that conducts well acoustically. This structure-borne 
noise sensor applies a corresponding signal to the control unit. In place 
of the structure-borne noise sensor, it is likewise possible to use an 
acceleration sensor or a knock sensor. 
One embodiment of the device according to the present invention functions 
as follows. The fuel-supply pump 155 delivers fuel from the supply tank, 
via valve 150, to high-pressure pump 145. High-pressure pump 145 builds up 
a specifiable pressure in rail 135. Usually, pressure values of greater 
than 800 bar are reached in rail 135. 
The appropriate solenoid valves 130 through 133 are driven by conducting 
current through coils 140 through 143. The drive signals for the coils 
thereby establish the beginning of injection and the end of injection of 
the fuel through injectors 120 through 123. The drive signals are 
established by the control unit in dependence upon various operating 
conditions, such as the driver's desire, speed, and other variables. 
When working with a common-rail system, such a sustained injection of an 
injector can not be easily recognized with certainty, given a balancing of 
masses in the rail. This can lead to an unwanted increase in torque at one 
cylinder and even cause destruction of the engine when the peak cylinder 
pressures or the permissible temperatures are exceeded. 
With the aid of the structure-borne noise sensor or by means of an 
acceleration sensor, in accordance with the present invention, the 
vibrations emanating from the combustion chamber are detected and 
reprocessed by means of an evaluation circuit. 
If the vibration of one individual cylinder deviates significantly from the 
remaining cylinders or from the expected value, then the inference is made 
that an error exists in the corresponding injector. 
The output signal from the structure-borne noise sensor is plotted in FIGS. 
2a-2c over the arc of crankshaft rotation. The output signal from the 
structure-borne noise sensor when all injectors are experiencing a faulty 
operation is plotted in FIG. 2a over the arc of crankshaft rotation. The 
metering into the first cylinder takes place within the range of the top 
dead center, i.e., at 0.degree. arc of crankshaft rotation of the first 
cylinder. This leads during metering or during combustion to a significant 
signal from the structure-borne noise sensor. A corresponding signal 
occurs in response to combustion in the second cylinder at 180.degree. arc 
of crankshaft rotation, in response to combustion in the third cylinder at 
360.degree. arc of crankshaft rotation, and in response to combustion in 
the fourth cylinder at 540.degree. arc of crankshaft rotation. 
FIG. 2b illustrates the corresponding signal given a faulty injector of the 
second cylinder. The sound emission during the combustion in the second 
cylinder is noticeably prolonged. This indicates that the injector of the 
second cylinder is not working properly. This injector is in its open 
state for longer than intended. 
In FIG. 2c, no fuel is injected into the second cylinder, which means the 
injector allocated to the second cylinder does not enable any fuel 
metering. 
The evaluation process for recognizing the error is illustrated by way of 
example in FIG. 3. In step 301, the output signal from the structure-borne 
noise sensor is detected when fuel is metered into the first cylinder Z1. 
Correspondingly, in step 300, the structure-borne noise sensor signal is 
detected during combustion in the second cylinder Z2. In steps 302 and 
303, the structure-borne noise sensor signal is detected for cylinders Z3 
and Z4. In step 310, the amplitudes of the four signals are summed and 
divided by four. This yields the average value M of the four 
structure-borne noise sensor signals. 
In step 320, a counter I is set to 0 and increased by 1 in subsequent step 
330. Query 340 checks whether the difference between the amplitude Zi of 
the I-th cylinder and the average value M is greater than a threshold 
value S. If this is not the case, query 350 checks whether I is greater 
than or equal to four. If this is not the case, then step 330 follows 
again, or when I is greater than four, step 300 follows. 
If query 340 recognizes that the amount of the difference between the 
amplitude of the I-th cylinder Zi and the average value M is greater than 
the threshold value S, then the existence of errors is recognized in step 
360 and appropriate measures are introduced. 
The method delineated here was described based on the example of a 
four-cylinder internal combustion engine. By properly choosing the 
parameters, in particular that of I, the method according to one 
embodiment of the present invention can also include internal combustion 
engines having different numbers of cylinders. 
Optionally, not the amplitude of the signal, but rather the time duration 
of the signal can also be evaluated for recognizing errors. 
Another advantageous embodiment of the present invention is illustrated in 
FIGS. 4-6e. Schematically illustrated in FIG. 4 is a four-cylinder diesel 
fuel engine having two structure-borne noise sensors 410 and 411, which 
are mounted so as to be acoustically conductive on the engine. Element 415 
denotes a needle-motion sensor and 420 a cylinder-pressure sensor. Element 
105 denotes the fresh-air pipes, and 110 the exhaust pipes. 
In FIG. 5, the signal evaluation for the two knock sensors 410 and 411 is 
illustrated as a block diagram. The output signal from the first knock 
sensor 410 arrives via a propagation-delay correction 201 at a cylinder 
selection 220. Accordingly, the output signal from the second knock sensor 
411 arrives via a second propagation-delay correction 202 at cylinder 
selection 220. 
From cylinder selection 220, the signal arrives at a first band pass 210 
and at a second band pass 215. The output signals from the band passes 
arrive at a signal processing 230, which in turn applies signals to a 
valve-timing unit 240. Furthermore, output signals from band passes 210 
and 215 arrive directly at engine timing 240. Furthermore, signal 
processing 230 processes signals from various sensors 235. 
This device functions as follows: the propagation delay of the diverse 
signals from a signal source to the different knock sensors 410 and 411 
varies. This propagation delay is compensated by the propagation delay 
corrections 201 and 202. On the basis of the signal height, which in turn 
is a function of the distance between the signal source and the sensor, 
the cylinder recognition assigns the signal to a specific sensor. This 
enables an allocation to be performed between the detected signal and the 
corresponding cylinder. 
In principle, the procedure described in the following can also be carried 
out with a structure-borne noise sensor. The signal quality can be 
substantially improved when of two or more structure-borne noise sensors 
are used. It is especially beneficial for the structure-borne noise 
sensors to be arranged at spatially different installation sites on the 
engine. By summing the signals that have been corrected for propagation 
delay, the useful signal can be substantially increased in comparison to 
spurious signals. 
The present invention provides for the first band pass to have break 
frequencies of 10 kHz and 30 kHz. The second band pass 215 has break 
frequencies of 500 Hz and 4 kHz. These frequency values merely represent 
recommended values, and they can vary depending on the type of internal 
combustion engine. 
The band passes filter the output signals from knock sensors 410 or 411. On 
the basis of the filtered signals, the signal processing defines different 
variables which characterize the injection or combustion. The thus 
obtained signals are used by the engine timing for the open and 
closed-loop control of the internal combustion engine. 
Plotted over time in FIG. 6a is the cylinder pressure, in FIG. 6b the 
output signal from the needle-motion sensor; in FIG. 6c the output signal 
from one of the knock sensors; in FIG. 6d the output signal from the first 
band pass; and in FIG. 6e the output signal from the second band pass. In 
response to the small quantities for the preliminary injection, the valve 
needle generally does not open up to the top limit stop. 
In the case of the preliminary injection, one can merely perceive the 
needle hitting against the lower limit stop at the end of the injection 
process. At this instant, the amplitude of the output signal from the 
knock sensor rises. Also at this instant, the high-frequency components of 
the output signal from the knock sensor increase. This instant is 
designated VE. 
At the beginning and end of the main injection, the needle of the 
needle-motion sensor moves to the lower or to the upper limit stop. At 
these instants, the amplitude of the output signal from the knock sensor 
and, in this case, in particular the high-frequency components rise. This 
instant is designated as HE. 
The beginning and end of the main injection are recognized when the needle 
of injectors 120 through 123 moves during opening operation up to the 
upper limit stop and during closing operation to the lower limit stop. 
These instants are recognized on the basis of the output signal's rise 
from the first band pass over a first threshold value. If the injector 
needle's hitting is not recognized, or if it is not recognized when the 
injector is closing, then this is evidence of a sustained injection. 
Based on these signals, the decision is made during every injection whether 
a sustained injection is taking place or not. The monitoring preferably 
follows individually for each cylinder. When a specifiable number of 
sustained injections is recognized for one cylinder, this is evidence of a 
defect. 
If the fuel-supply pump is designed as a mechanical auxiliary supply pump, 
for example as a gear pump, then there is no actual way to interrupt the 
delivery of fuel by means of the auxiliary supply pump, since it is driven 
directly by the engine. Therefore, the present invention provides for the 
fuel delivery from auxiliary supply pump 155 to high-pressure pump 145 to 
be interrupted by means of the electrical shutoff valve 150 between 
auxiliary supply pump 155 and high-pressure pump 145. 
When an error is recognized, valve 150 interrupts the supply of fuel to 
high-pressure pump 145. An error can be recognized in this case, for 
example, using the described procedure. However, other methods for 
recognizing errors are feasible. 
If valve 150 is designed as a 2/2 valve, i.e., it blocks the flow between 
auxiliary supply pump 155 and high-pressure pump 145, then a pressure 
builds up upstream from the valve when the valve is closed. Appropriate 
measures are provided to avoid this pressure build-up. For example, a 
relief valve can be integrated in the auxiliary supply pump. 
Alternatively, the shutoff valve can be designed as a 3/2 valve. In such a 
case, when valve 150 is driven, the fuel arrives via a line, drawn in with 
a dotted line, from auxiliary supply pump 155 directly back in fuel supply 
tank 160. The need has been eliminated in this alternative embodiment of 
the present invention for a relief valve in auxiliary supply pump 155.