Abstract:
A fuel injector testing system and method that make accurate determination of the condition of an injector installed in an engine possible even if the injector is hidden under or behind engine components. A waveguide attached to the injector guides stress waves generated when the injector pintle is opened or closed to a location on the engine that is accessible by a technician. A stress-wave sensor attached to the accessible end of the waveguide measures the stress-wave intensity and plots on a display its magnitude vs. time. A technician testing a fuel injector can read from the display the numerically accurate impact intensities and the precise timing of the injector pintle opening and closing movements. The display can also compute automatically the values of the impact intensities and the length of time that the injector valve was open. This allows the technician to quickly detect a faulty injector.

Description:
RELATED PATENT APPLICATIONS 
   The subject patent application expressly claims priority from U.S. Provisional Patent Application Ser. No. 60/950,108 filed on Jul. 16, 2007 under 35 USC § 119(e). The entire contents of U.S. Provisional Patent Application Ser. No. 60/950,108 are herein incorporated by reference. 

   TECHNICAL FIELD 
   This invention relates generally to methods and apparatus for monitoring and/or testing fuel injectors for internal combustion engines. In its most preferred form, the present invention provides a method and apparatus for monitoring one or more fuel injectors to detect a faulty or worn injector based on stress waves that are guided from the tested injectors, through waveguides, to a stress-wave sensor at an accessible location. 
   BACKGROUND OF THE INVENTION 
   There are several methods available for testing the operation of fuel injectors in internal combustion engines. Mechanics often use stethoscopes to listen to the sounds made by fuel injectors. A clicking sound emitted by an injector indicates that the injector pintle is moving. This method will detect injectors that stopped responding altogether, but will miss partially failed injectors. Also, this method cannot be used on injectors that are not accessible by the stethoscope because they are hidden under the intake manifold or under other engine components. 
   U.S. Pat. No. 6,668,633 discloses a battery-operated fuel injector tester with a probe attached to a pistol-shaped handle. When the probe of the tester is in contact with a tested injector on an idling engine, a light emitting diode flashes and an audible sound is emitted each time the pintle within the fuel injector opens. This tester will detect injectors that stopped responding altogether, but will miss partially failed injectors. Also, this method cannot be used on injectors that are not accessible by the probe because they are hidden under the intake manifold or under other engine components. 
   U.S. Pat. No. 4,523,458 discloses a fuel injector tester for injectors used in diesel engines. It uses a transducer comprising a piezoelectric crystal sandwiched between two magnets. The transducer is attached magnetically to a tested injector and displays on a bar graph the intensity of the mechanical impulses it measures. This method cannot separate the injector opening transient from the injector closing transient, it does not provide any information on the length of time when the injector valve was open, and it cannot be used on injectors that are not accessible by the transducer because they are hidden under the intake manifold or under other engine components. 
   U.S. Patent Publication Application No. 2006/0101904 discloses a system where a fuel pressure sensor is installed on the fuel rail and senses fuel pressure fluctuations associated with the operation of the fuel injectors. This method will detect a fuel injector that has failed altogether because the fluctuation expected when that injector was scheduled to open and inject fuel will be missing. However, this method is not accurate enough to reliably detect partially failed fuel injectors. 
   U.S. Pat. No. 5,747,684 discloses a method for determining the opening and closing times for automotive fuel injectors for use by the engine electronic control unit (ECU) to more accurately control an injector stroke, thereby improving engine performance. This method is based on analyzing the energy content of the acceleration of the injector body, measured by an accelerometer attached to the injector body. The main drawback of this method is that injector body vibrations due to the injector opening transient often do not decay by the time the injector closes, making it difficult to distinguish between the opening and the closing transients. This method also requires an accelerometer permanently attached to each injector. 
   The most preferred form of the present invention is based on measuring stress waves that are only generated at the exact moments when the injector valve opens or closes. Therefore, in the most preferred form of the present invention, signals due to these two events do not overlap and the opening and closing times can be determined with high accuracy and with minimal computation. Additionally, the most preferred form of the present invention produces numerically accurate measurements of the intensities of the opening and closing transients of the injector valve and it does so with only one sensor per engine. 
   The art of stress wave measurement is only known to a relatively small community of practitioners as opposed to measurement of vibrations that is well known and widely used. 
   The term vibration refers to motion of a body in a fashion where all or a significant portion of the body&#39;s mass is moving. In an internal combustion engine, for example, there are significant vibrations at the rotational frequency of the crankshaft and at the engine firing frequency. Excitation of engine vibrations requires significant forces and the vibrational motion involves significant energy. 
   Vibrations can be measured with accelerometers that are attached to the vibrating body. A piezoelectric accelerometer  5  is shown schematically in  FIG. 1 . The sensor is enclosed in housing  1 . Piezoelectric crystal  2  is attached to the bottom of housing  1 . Mass  3  is attached to the top of piezoelectric crystal  2 . When housing  1  vibrates in the vertical direction with acceleration a, mass  3  applies force m×a on piezoelectric crystal  2 , where m is the size of mass  3  measured in units of mass. The applied force generates strain in piezoelectric crystal  2  and said crystal generates electrical charge in response to the strain. The charge is proportional to force m×a and, therefore, is also proportional to acceleration a. Electrical leads  4  can be used to connect the charge to electronic processing circuitry, not shown in  FIG. 1 , that converts the charge to voltage proportional to acceleration a. 
   Unlike vibrations, stress waves are elastic waves contained within the solid that comprises the body. These waves are generated by short-duration impacts of the body and they move at the speed of about 5000 m/s through a metallic body. Stress waves in solids can be generated by impacts that involve very low forces and, consequently, the generated waves involve very low amounts of energy as they move through the impacted body. For example, measurable stress waves can be excited in an engine block just by tapping it lightly with a finger. The theory of stress waves generation and propagation is explained in detail in the book  Stress Waves in Solids  by Herbert Kolsky, published by Dover Publications in 1963. 
   Stress waves in solids can be measured with piezoelectric, fiber-optic, MEMS and other stress-wave sensors.  FIG. 2  shows schematically one embodiment of a piezoelectric stress-wave sensor  9  formed in accordance with a preferred embodiment of the invention. The sensor is housed in housing  6 . The sensing element is piezoelectric crystal  2 . Piezoelectric crystal  2  is permanently attached to face plate  7  that is also the bottom of housing  6 . The space inside housing  6  is filled with filler  8  to keep piezoelectric crystal  2  in place and to prevent vibration of the internal components of the sensor. When strain  10  is applied to face plate  7 , it reaches piezoelectric crystal  2  and piezoelectric crystal  2  generates electrical charge proportional to strain  10 . Signal leads  4  are used to connect the generated charge to electronic processing circuitry not shown in  FIG. 2 . Note that  FIG. 2  is only a schematic representation that excludes design details that are required for high gain and low noise measurements of stress waves. 
   Stress-wave sensor  9  in  FIG. 2  incorporates design features that make its response to case acceleration negligible. These features include crystal material selection, shape of the crystal, and the use of filler  8 . Consequently, when sensor-wave sensor  9  undergoes motion that involves acceleration, signal leads  4  do not carry a measurable charge signal due to the acceleration. 
   SUMMARY OF THE INVENTION 
   It is an object of a preferred form of this invention to provide a simple, inexpensive and numerically precise method and apparatus for detecting failures and performance degradation of fuel injectors in internal combustion engines. The method and apparatus of the preferred form of the present invention can be utilized even if the performance degradation of the fuel injector is minor and/or the fuel injectors are hidden under or behind engine components. 
   There is provided, in accordance with a preferred form of the invention, a method for monitoring the stress waves generated by impacts of the pintle of the fuel injector when the injector is activated and deactivated, and determining the condition of the injector by comparing the stress-wave intensity signals during activation and deactivation to those of other injectors in the engine, or to documented characteristics of an injector that is known to be in good operational condition, or to signals from the same injector that were collected and stored during past inspections. Additionally, the preferred method can be used to accurately measure the time during which the injector pintle valve was open. Preferably, the stress waves generated by a tested injector that is hidden under or behind engine components are guided through waveguides to a location that is accessible by a stress-wave sensor, allowing the testing of fuel injectors that are hidden under or behind engine components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a piezoelectric accelerometer. 
       FIG. 2  is a sectional view of a piezoelectric stress-wave sensor. 
       FIG. 3  is a sectional view of a conventional electromagnetically-actuated fuel injector for internal combustion engines. 
       FIG. 4  is a sectional view of a fuel injector with a modified body and equipped with a stress-wave waveguide in accordance with a preferred embodiment of the present invention. 
       FIG. 5  is a sectional view of a fuel injector with an unmodified body but with an adapter for attaching to the injector body a stress-wave waveguide in accordance with a preferred embodiment of the present invention. 
       FIG. 6  shows the setup for inspecting a fuel injector equipped with a stress-wave waveguide in accordance with a preferred embodiment of the present invention. 
       FIG. 7  shows a plot of the stress waves generated by a fuel injector and measured in accordance with a preferred embodiment of the present invention. 
       FIG. 8  shows the setup for inspecting multiple fuel injectors with multiple stress-wave waveguides in accordance with a preferred embodiment of the present invention. 
       FIG. 9  shows the setup for inspecting multiple fuel injectors with a single stress-wave waveguide and a single stress-wave sensor in accordance with a preferred embodiment of the present invention. 
       FIG. 10  shows the setup for inspecting multiple fuel injectors with the fuel rail serving as a stress-wave waveguide and a single stress-wave sensor in accordance with a preferred embodiment of the present invention. 
       FIG. 11  shows the setup for inspecting multiple fuel injectors with a stress-wave waveguide integrated into an electrical wire harness and a single stress-wave sensor in accordance with a preferred embodiment of the present invention. 
       FIG. 12  shows the setup for inspecting a fuel injector with a removable stress-wave waveguide in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The preferred forms of the invention will now be described with reference to the accompanying drawings. The appended claims are not limited to the preferred forms and no term and/or phrase used herein is to be given a meaning other than its ordinary meaning unless it is expressly stated otherwise. 
     FIG. 3  presents a conventional fuel injector  11 . Injector body  12  houses axially movable injector pintle  14  and solenoid coil  16  that is fixed to the injector body  12 . Solenoid armature  18  is attached to injector pintle  14 . When injector  11  is activated by applying voltage across the solenoid contacts  20  and  22 , magnetic flux generated in the solenoid coil  16  pulls the solenoid armature  18  toward the center of the solenoid coil  16 . The location of the injector pintle  14  when the injector  11  is activated is determined by the pintle stop  24  that comes in contact with the injector body stop  26  on injector body  12 . 
     FIG. 3  shows the conventional fuel injector  11  in the activated state. The pintle sealing surface  28  is away from the orifice  30  so that fuel  32  can be sprayed through the orifice  30 . Fuel  32  is being supplied pressurized through the injector inlet  34  and through internal passages in injector body  12  that are not shown in  FIG. 3 . Injector inlet  34  is connected to a fuel pump through a fuel rail that is not shown in  FIG. 3 . Seal  36  provides sealing between the injector body  12  and the fuel rail. Seal  38  provides sealing between injector body  12  and the internal combustion engine, which is not shown in  FIG. 3 . 
   When injector  11  is deactivated by disconnecting the voltage applied across solenoid contacts  20  and  22 , spring  40  moves the injector pintle  14  toward the orifice  30 , and valve sealing surface  28  closes the inlet to orifice  30 . In the deactivated state of the injector  11 , fuel  32  is not sprayed through orifice  30 . 
   Injector  11  is shown in  FIG. 3  with electromagnetic valve actuation means. However, one skilled in the art would recognize that the invention applies to injectors with other means of actuation, including piezoelectric, magnetostrictive, pneumatic, mechanical, and actuation by fuel pressure. Furthermore, injector  11  is shown in  FIG. 3  with one type of orifice  30  and one type of pintle sealing surface  28 . However, one skilled in the art would recognize that the invention applies to injectors with any other type of orifice and sealing surfaces, such as a spherical pintle sealing surface  28 , a flat pintle sealing surface  28 , and a design with a conical orifice  30  and a conical sealing surface  28 . 
     FIG. 4  shows fuel injector  60  according to a preferred form of the present invention. A stress-wave waveguide  62 , made of metal, plastics or other suitable material, is attached to the modified injector body  13  by means of plug  64 . Plug  64  presses the waveguide flange  66  into modified injector body  13  so that stress waves generated at the instant when pintle stop  24  impacts the injector body stop  26  when the injector  60  is activated, or when pintle sealing surface  28  impacts orifice  30  when the injector  60  is deactivated, can propagate into waveguide  62 . 
   Waveguide  62  is protected from stress waves that do not originate in injector body  13  by sleeve  68  that is made of substantially soft and heat-resistant material, such as silicone foam rubber. At the end of waveguide  62  is sensor attachment surface  70 . A stress-wave sensor attached to sensor attachment surface  70  can, therefore, measure the stress waves generated when injector  60  is activated or deactivated and generates stress waves that propagate along waveguide  62  into sensor attachment surface  70 . 
   One skilled in the art would recognize that the invention applies to any other type of attachment of a stress-wave waveguide to a fuel injector body, such as a threaded waveguide end, a press fit, a clamp, and attachment by adhesives such as epoxy. A particularly important alternative method of attaching a stress-wave waveguide to a fuel injector is by means of an adapter that fits on a standard, unmodified injector. Thus, a fuel injector according to a preferred form of the present invention can be realized by installing an additional part on a standard injector.  FIG. 5  shows fuel injector  61  according to a preferred form of the present invention and with such alternative waveguide attachment method. Adapter  42  is installed tightly onto injector body  12  by means of a press fit, one or more screws, or any other means. Waveguide  62  is attached to the adapter  42  by means of plug  64 . Plug  64  presses the waveguide flange  66  into the adapter  42 . Since the interfaces between injector body  12  and adapter  42 , and between adapter  42  and waveguide flange  66  are tight, stress waves originating in injector body  12  can propagate into waveguide  62  without significant intensity loss. This alternative method of attaching a stress-wave waveguide to a fuel injector can be applied to injectors that were originally not designed for condition monitoring through stress-wave measurement according to a preferred form of the present invention. 
   Fuel injector  60  shown in  FIG. 4  or fuel injector  61  shown in  FIG. 5  can be located under the engine air intake manifold or be hidden under or behind other engine components. However, as long as sensor attachment surface  70  is accessible, fuel injectors  60  or  61  can be easily and accurately inspected by a technician.  FIG. 6  shows the setup for testing an injector according to the present invention. Injector  63  is mounted on engine  90 . Engine component  100 , which represents the air intake manifold or other component, is obstructing access to injector  63 . Fuel rail  94  supplies pressurized fuel to injector  63  and other injectors on the engine, and electrical wire harness  96  carries electrical current that is controlled by the engine fuel injection control unit and actuates injector  63 . Waveguide  62  is long enough so that sensor attachment surface  70  is out of the area obstructed by engine component  100 . Waveguide  62  can be short, such as 10 cm, or long, such as 1 meter, depending on the size of the obstructing engine component  100 . Said waveguide  62  can be bent to whatever shape is required to reach from the obstructed location where injector  63  is located to an accessible location. It is so because stress waves propagate well through waveguides of any shape. 
   A stress-wave sensor  80  is shown attached to sensor attachment surface  70 . Sensor  80  is attached to sensor attachment surface  70  temporarily with a magnet, a spring or other means by the technician who is testing injector  63 . The sensor, preferably a piezoelectric device that generates electrical charge when mechanically stressed, is designed with a natural frequency that is much higher than any forced or natural vibration frequency of engine  90 , all its components, and fuel injector  63 . Sensor  80  may take the form of piezoelectric sensor  9  illustrated in  FIG. 2 . Sensor  80  measures two types of signals. Signals of the first type are stress waves due to forced and natural vibrations of engine  90 , all its components, and injector  63 . These signals have relatively low frequency content. Signal of the second type is a stress wave that passes through waveguide  62  at the instants when injector  63  is activated or deactivated. When the stress wave generated by injector  63  reaches stress-wave sensor  80 , it acts as an impulse excitation of very short duration applied to sensor  80 . An impulse of very short duration has very high frequency content and it excites high frequency response of sensor  80 . One skilled in the art will realize that sensor  80  can be based on principles other than piezoelectricity as long as it can measure high-frequency stress waves. 
   Cable  82  carries the two types of signals measured by sensor  80  to filter module  84 . Module  84  first high-pass filters the arriving signals with the filter corner frequency set above the highest engine vibration frequencies. This filtering process filters out all signals of the first type, i.e., stress waves due to forced and natural vibrations of engine  90 , all its components, and injector  63 . The only signals left after the high-pass filtering stage are those generated by impulse excitations of sensor  80  due to stress waves that are generated by activation or deactivation of fuel injector  63 . Module  84  then amplifies the high-pass filtered signal, rectifies it and extracts the envelope of the rectified signal, so that only the low-frequency envelope of the rectified high-frequency response to the impulse excitations remains. The envelope extraction is accomplished with a low-pass filter. The low-frequency signal leaving module  84  is fed through cable  86  into a display  88  that can be an oscilloscope or a digital device equipped with an analog-to-digital converter. Display  88  in  FIG. 6  shows a typical injector signal  89 . 
   An expanded view of the injector signal  89  from display  88  is shown in  FIG. 7 . It consists of two peaks separated by time T. The first peak is due to the activation of fuel injector  63  and its intensity is P 1 . The second peak is due to the deactivation of fuel injector  63  and its intensity is P 2 . The spacing time between the two said peaks, T, is the length of time that injector  63  was open and injected fuel. In a typical idling automobile engine, T is several milliseconds. 
   The three parameters readable from injector signal  89  shown in  FIG. 7 , P 1 , P 2  and T, are indicators that carry information on the health condition of injector  63 . These three indicators can be compared to nominal values that correspond to an injector in good operational condition. Furthermore, when more than one injector in an engine is tested, a technician can compare the three indicators among all the tested injectors. In a steady idling condition, all injectors that are in good condition have substantially similar stress wave signals and substantially similar indicators computed from said signals. If an engine is misfiring and one injector&#39;s indicators deviate from the indicators of the other injector, the technician can determine with high degree of certainty that that injector is not operating properly. For example, a faulty solenoid coil and contamination can cause the impact indicators P 1  and P 2  to be lower, and can cause the opening time T to be either shorter or longer than in an injector in good operating condition. A faulty electrical circuit that supplies current to the solenoid coil can cause impact indicators P 1  and P 2  to be lower. 
   The three injector indicators readable from display  88  in  FIG. 6  and shown in  FIG. 7 , P 1 , P 2  and T, can be also determined automatically if display  88  is a device with computing capability. The computational algorithm for determining automatically the three indicators from a signal like the one shown in  FIG. 7 , consisting of steps a-g, follows.
         a. Find three adjacent candidate peaks P i  that have n 1  signal points immediately to the left of P i  that are lower than P i , and n 1  signal points immediately to the right of P i  that are lower than P i . Parameter n 1  is set so that n 1 ×Δt is about 0.3 milliseconds, where Δt is the sampling period of the stress wave signal.   b. For each candidate peak P i , compute the average of n 2  signal points to the left of the n 1  signal points that are before the peak, and call the computed average g 1 . Parameter n 2  is set so that n 2 ×Δt is about 0.3 milliseconds.   c. For each candidate peak P i , compute the average of n 2  signal points to the right of the n 1  signal points that are after the peak, and call the computed average g r .   d. If r×g 1 &lt;P i  and r×g r &lt;P i , candidate peak P i  is a valid peak. Parameter r is set to about 4 and it assures that peak P i  is significantly higher than the points that surround it.   e. If less than three peaks are valid peaks, continue inspecting peaks till three valid adjacent peaks are found.   f. Select the two peaks that are closest to each other out of the three found valid peaks. These two peaks, called P 1  and P 2 , are the opening and closing transients of the injector.   g. P 1 , P 2  and T=t(P 2 )−t(P 1 ) are the three injector indicators, where t(P i ) represents the time of peak P i .       

   One skilled in the art would recognize that there are other similar forms of this algorithm that still express the same essential algorithm for determining injector indicators P 1 , P 2  and T. 
     FIG. 8  shows a preferred embodiment of the present invention where three fuel injectors  91 ,  92  and  93  are equipped with dedicated stress-wave waveguides  101 ,  102  and  103 . Each waveguide ends with a sensor attachment surface that is not obstructed by obstructing engine component  100 . In this embodiment, these three injectors can represent the three inaccessible injectors in a V6 engine, or three injectors out of any number of inaccessible injectors in any engine configuration.  FIG. 8  shows the testing of fuel injector  91  with stress-wave sensor  80  that is attached to sensor attachment surface  106  of waveguide  101 . One sensor can be used for testing of all the fuel injectors in an engine by moving it to other sensor attachment surfaces. For clarity,  FIG. 8  does not show the injector fuel rail or the injector electrical wire harness. 
     FIG. 9  shows an alternative embodiment of the present invention wherein three fuel injectors  91 ,  92  and  93  are mounted on engine  90 . In this embodiment, these three injectors can represent the three inaccessible injectors in a V6 engine, or three injectors out of any number of inaccessible injectors in any engine configuration. For clarity,  FIG. 9  does not show the injector fuel rail or the injector electrical wire harness. All three injectors  91 ,  92  and  93  in  FIG. 9  are coupled to one waveguide  74  which has one sensor attachment surface  76 . Consider the engine depicted in  FIG. 9  to be of the Sequential Multi-Port Fuel Injection type. In this type of engine, the injectors are activated sequentially (one after the other) so that when the engine is idling, significant time passes between the deactivation of one injector and the activation of the next one. Sensor  80 , when attached to sensor attachment surface  76  by a technician, will pick up the activation and deactivation impacts of all three injectors  91 ,  92  and  93 . The impacts will be separated in time because the injectors are activated sequentially. If one of the injectors is not in good condition, the technician will see on the display that its signature differs from the signatures of the other two injectors. However, without additional information, the technician will not know which one of the three injectors produced the signature that indicated faulty operation. 
   To resolve this injector identification problem, one embodiment of the present invention utilizes an engine fuel injector control unit  95  that produces a selectable injector-specific triggering signal  98 . Injector selector  97  allows the technician to select the injector he wants to display by means of a manual switch or other means. In the example in  FIG. 9 , the injector selector  97  is shown in position  2  that corresponds to injector  92 . The engine fuel injector control unit  95  then outputs the selected injector-specific triggering signal  98  a precise period of time, such as 1 millisecond, before it sends activation current to the injector selected by the technician through injector selector  97 . Display  99  accepts through cable  86  the processed sensor signal that includes activation and deactivation impacts of all three injectors  91 ,  92  and  93 . Display  99  also accepts the injector-specific triggering signal  98 . Upon arrival of the injector-specific triggering signal  98 , display  99  captures and displays a short segment, such as 20 milliseconds, of signal arriving via cable  86 . Since cylinders in the engine do not fire at the same time, display  99  will capture and display only the activation and the deactivation impacts of the one selected injector  92 . By changing the setting of the injector selector  97 , the technician can display signals from the three injectors  91 ,  92  and  93  one at a time and determine if any of them is not in good operational condition. 
   Alternatively, it is also possible to provide injector selection without the dedicated injector selector  97  shown in  FIG. 9 . Triggering signal  98  can be provided by a clamp current probe that the technician attaches to a wire that carries current to the injector he wants to monitor. The current probe then generates the triggering signal  98  according to the injector wire to which the probe is attached. Alternatively, triggering signal  98  can be generated by any other means of sensing current or voltage in a wire leading to an injector. 
   Yet another method for resolving the injector identification problem without the dedicated injector selector  97  is for fuel injection control unit  95  to modulate signal  98  with an injector identification code whenever any of the injectors is activated. For example, signal  98  could be the number of the activated injector transmitted over a serial digital line. Alternatively, signal  98  could be an analog signal that has a voltage level that is indicative to the number of the activated injector, or signal  98  could include the injector number using any other encoding scheme. In these cases, display  99  would include an interface for reading, processing and displaying the injector identification code from signal  98 . In one embodiment, display  99  could decode signal  98  and numerically display the number of the injector that produced an injector activation impact peak near the peak shown on the display. One skilled in the art would recognize that the invention applies to other possible methods, either digital or analog, that allow fuel injection control unit  95  to communicate the number of the activated injector to display  99 . 
   The setup of  FIG. 9  can also be used to measure the speed of response of injectors. Display  99  can be programmed to display both a time mark corresponding to the instant when current is sent to the injector, and signal  89 . The time difference between the said time mark and peak P 1  is the injector activation time delay d 1 . It can be compared to a maximum allowed delay, or compared to time delays of the other injectors. An injector in good condition has a time delay that is shorter than a maximum allowed delay. Similarly, one can also measure the injector deactivation delay d 2 , defined as the time delay between when the current to the injector is stopped and time of peak P 2 . Let these two time delays be called d 1  and d 2 , respectively. They can be added to the three previously defined injector performance indicators P 1 , P 2  and T. Thus, the condition of an injector can be summarized by the five indicators P 1 , P 2 , T, d 1  and d 2 . 
   Furthermore, display  99 , when implemented digitally, can provide functionality that helps the technician in comparing injectors to each other, or to a standard. For example, display  99  can include eight or more screen-storage function keys, for examining engines with up to eight cylinders or more. When the technician captures the signal from the injector for engine cylinder No. 1, for example, he can press key No. 1 and store the displayed signal. Similarly, he can store signals from injectors for all the other cylinders in the engine. Using a recall function key on display  99 , he can then display simultaneously any number of injector signals, each in different color or different line type. He can also display a standard signal corresponding to an injector in good condition. A scroll key on display  99  can allow the technician to scroll the displayed signals horizontally, to align them in time. This way, the technician can easily detect an injector that is malfunctioning because its signal differs from the signals generated by the other injectors or it differs from the standard signal. 
   Display  99  can also include data storage means that can store injector signature data collected at different times, allowing performance trending over time. For example, the signatures of all the injectors in an engine can be stored each time a scheduled maintenance is performed. If an engine develops a performance problem, such as misfiring of cylinders, signatures of all the injectors can be acquired and compared to their respective signatures from the most recent scheduled maintenance, when the engine was not misfiring. This will immediately pinpoint a failing injector if it is the cause of the problem. The database of past injectors&#39; signatures can reside on the display  99 , or it can be implemented on a central computer in the maintenance facility to which all instruments are networked. 
   In another preferred embodiment of the present invention, the waveguide function in  FIG. 9  can be performed by the fuel rail. Fuel rail is usually made of material that transmits stress waves well, and it interconnects multiple injectors in internal combustion engines. Fuel rail  114 , shown in  FIG. 10 , interconnects injectors  111  and  112 . Injectors  111  and  112  and fuel rail  114  are designed to provide tight interfaces that facilitate good propagation of stress waves from the injectors to the fuel rail. Sensor attachment surface  116  is attached to fuel rail  114  to facilitate attachment of sensor  117  to said fuel rail. Thus, the functions of waveguide  74  in  FIG. 9  can be performed by fuel rail  114  shown in  FIG. 10 , eliminating the need for a separate waveguide and the need for injectors with waveguide attachment means. For clarity,  FIG. 10  does not show the electrical wire harness that interconnects the injectors. 
   Alternatively, the waveguide function in  FIG. 9  can be performed by the electrical wire harness that includes the electrical wires that carry injector activation currents. The wire harness interconnects multiple injectors in most internal combustion engines.  FIG. 11  shows electrical wire harness  124  interconnecting injectors  121  and  122 . Flexible waveguide  125  is integrated into wire harness  124  is and it also interconnects injectors  121  and  122 . Tight contacts between waveguide  125  and injectors  121  and  122  are provided by harness connectors  128  and  129 . Sensor attachment surface  126  is connected to end of waveguide  125  to facilitate attachment of sensor  127  to said waveguide. Thus, the functions of waveguide  74  in  FIG. 9  can be performed by waveguide  125  that is integrated into electrical wire harness  124  as shown in  FIG. 11 . For clarity,  FIG. 11  does not show the fuel rail. 
   As another alternative, the waveguide function in  FIG. 9  can be performed by the intake manifold or other engine part into which the injectors are inserted. Preferably, the stress waves are guided from the injectors to a sensor attachment surface on the manifold by ribs forged into the manifold body, or by waveguides embedded into the walls of the manifold, or by waveguides permanently attached to the surface of the manifold. 
   In yet another preferred embodiment of the present invention, the waveguide  62  seen in  FIG. 4  is not attached permanently to injector body  13 . In this embodiment, shown in  FIG. 12 , insertion guide  132  is permanently attached (i.e., attached during normal engine use and testing) to any suitable engine component or vehicle body component in such a way that one of its ends is at an accessible location and the other end is close to and pointing at injector  131 . Any suitable attachment means may be used.  FIG. 12  shows attachment of insertion guide  132  by means of guide holders  133  and  134 . Removable waveguide  135  is flexible and sufficiently long so that when inserted into the accessible end of insertion guide  132  its end can pass through insertion guide  132  and touch injector  131 . When the end of waveguide  135  is pressed into injector  131 , stress waves generated inside injector  131  will propagate into waveguide  135  and can be measured with sensor  137  that is attached to sensor attachment surface  136  that is at the accessible end of waveguide  135 . A user inserts waveguide  135  into insertion guide  132  only when injector  131  is being tested.  FIG. 12  shows removable waveguide  135  when it is inserted into insertion guide  132  and it contacts injector  131 . For clarity,  FIG. 12  does not show the fuel rail or the electrical wire harness. 
   A typical use of the preferred forms of the present invention is testing of fuel injectors in an idling engine. However, there are other uses. For example, a technician can use an instrument based on the present invention to acquire the activation and deactivation impacts from all the injectors at a specific operating condition of the engine, such as an automotive engine at a specific driving speed. The acquired signals can be examined once the automobile is back in the maintenance facility. Alternatively, an engine control computer can monitor all the injectors automatically and continuously whenever the engine is running, and detect incipient injector failures before they affect the performance of the engine. This continuous monitoring function can be part of an On-Board Diagnostic system, such as OBD-II that is used in today&#39;s automobiles. 
   Yet another use of the preferred forms of the present invention is to monitor automatically and continuously all the injectors whenever the engine is running, and use the derived information to fine-tune in real time the control laws that govern the activation and deactivation timing of the injectors. 
   While this invention has been described as having a preferred design, it is understood that the preferred design can be further modified or adapted following in general the principles of the invention and including but not limited to such departures from the present invention as come within the known or customary practice in the art to which the invention pertains. The claims are not limited to the preferred embodiment and have been written to preclude such a narrow construction using the principles of claim differentiation.