Abstract:
Fuel injectors ( 10 ) for internal combustion engines are modified and equipped with fiber optic fuel pressure sensors ( 12 ) and fiber optic combustion pressure sensors ( 14 ). The combustion pressure sensors ( 14 ) are located in separate channels ( 26 ) formed in the fuel injectors with the lower portion ( 22 ) of the channels leading to the combustion chambers. Above the combustion pressure sensors ( 14 ) are fiber optic leads ( 24 ). In the preferred embodiments the sensors ( 46 ) are equipped with diaphragms ( 40 ) of novel shape ( 48 ) and employ multiple pairs of fibers ( 86, 88 ), temperature sensitive components ( 72, 74, 126 ) and novel compensation and status monitoring circuits (FIGS.  6, 9, 10, 14, 15, 18 ).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application incorporates by reference application Ser. No. 08/442,218, now U.S. Pat. No. 5,600,125, by the same inventors entitled Compensation and Status Monitoring Devices for Fiber Optic Intensity Modulated Sensors.  
         [0002]     Four stroke direct-injection engines have been under recent intense development for both diesel and gasoline fuel applications due to significant fuel economy improvements and reduced emission levels in comparison to engines with conventional fuel injection. Direct injection diesel engines have a higher baseline thermal efficiency (about 40% peak), 20-35% better fuel efficiency, 10-20% lower CO 2  emissions, near-zero evaporative emissions, and low cold-start emissions. Fuel economy improvements of as much as 35% have been recently reported, combined with a simultaneous increase in engine power and torque of 10%, for a direct gasoline injection engine. Such remarkable performance has been realized through a combination of very lean burn combustion (Air-fuel ratios as high as 40:1) and stratified charge mixing inside each engine combustion chamber.  
       SUMMARY OF THE INVENTION  
       [0003]     A key component that is required for both Direct Diesel Injected (DDI) and Direct Gasoline Injected (DGI) engines is an accurate and cost-effective fuel injector. In diesel engines, the new injection must operate at extremely high pressures (as high as 30,000 psi), provide accurate and repeatable spray patterns, and be precisely timed. In addition, such an injector must operate for as many as 0.5 million miles and be of low cost.  
         [0004]     In DGI applications, in particular, the very poor lubricating nature of gasoline and critical injector specifications make gasoline injectors difficult and expensive to manufacture.  
         [0005]     To provide the required performance, reliability, and low-cost for both DDI and DGI injectors, disclosed below are approaches based on closed loop control of injector operating parameters, where combustion chamber and fuel pressures are used as control parameters. As detailed below a preferred way of obtaining these two pressures is to integrate two miniature fiber optic pressure sensors inside an injector. Such a “smart” injector does not need to be individually calibrated, as currently done, so its price can be significantly lower. Differences caused by manufacturing variability, aging, pressure line fluctuations, or fuel quality can be compensated for by using closed-loop control of fuel injection timing, duration and pressure. The combustion chamber pressure sensor of the smart injector provides, in addition, real-time information about cylinder pressure including peak pressure (PP), indicated mean effective pressure (IMEP), start of combustion (SOC) and location of peak pressure (LPP). When inputs from both fuel and combustion pressure sensors are used to control the injector, simultaneous benefits of reduced emissions, improved fuel economy, increased injector reliability, and reduced cost can be achieved.  
         [0000]     Invention Overview  
         [0006]     The fiber optic sensors utilized in the smart injector are of a novel construction aimed at high accuracy in a very small device exposed to extremely high pressures and temperatures. The sensor tip may be as small as 2.5 mm in diameter or smaller. By using a specially shaped diaphragm in the sensor and two D-shaped optical fibers, high levels of optical modulation can be realized at small diaphragm deflections. Small diaphragm deflections are required to permit high diaphragm yield strength and long fatigue life. Using a two photodiode detection technique, each sensor&#39;s signal interface/conditioner can operate accurately over a temperature range of −50 to 150° C.  
         [0007]     In a preferred configuration two types of sensors are used in the smart injector: (1) a high pressure sensor for monitoring static fuel pressures inside the injector and (2) a sensor for detecting dynamic combustion chamber pressures.  
         [0008]     To compensate for changing sensor response with temperature, a temperature compensation technique utilizes a combination of a thin film deposited on the diaphragm inner surface and a temperature probe mounted in the sensor housing. The thin film reflection coefficient changes with temperature thereby compensating for any intra cycle (short term) diaphragm temperature excursions above its average temperature. Any longer term errors, resulting from increased diaphragm deflection at higher average temperature and other thermal effects on the sensor head, are compensated for by adjusting the pressure sensor&#39;s gain, based on the temperature probe output.  
         [0009]     While most direct injectors require dynamic fuel pressure information, only static pressures must be known in such approaches as fuel rails. Disclosed below is a static pressure sensor utilizing two optical fiber pairs, with one pair acting as a reference device to compensate for errors that may result from temperature effects on opto-electronic components, fiber bending, or other sources of undesirable light intensity fluctuations.  
         [0010]     As above, to compensate for errors arising from intra-cycle diaphragm heating due to the nearby combustion gasses, the compensation technique is similar to the technique used for dynamic sensors. For the static sensor one pair of fibers is exposed to diaphragm deflection and the other pair of fibers is installed in front of a non-deflecting reflector coated with a reflection temperature dependent thin film. A separate temperature probe provides input information for additional compensation for sensor gain and offset changes arising from varying inter-cycle average diaphragm and housing temperatures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic cross-section view of a smart injector with both fuel pressure and combustion pressure sensors;  
         [0012]      FIG. 2  is a schematic cross-section of a preferred fiber pair;  
         [0013]      FIG. 3  is a schematic cross-section of a preferred sensor tip;  
         [0014]      FIG. 4  is a schematic cross-section of the sensor tip of  FIG. 3  under increased pressure;  
         [0015]      FIG. 5  is a graphical comparison of the responses of the preferred sensor tip to a conventional flat diaphragm sensor tip;  
         [0016]      FIG. 6  is a block diagram of the opto-electronic circuity for a dynamic pressure sensor;  
         [0017]      FIG. 7  is a schematic cross-section of a preferred sensor tip including a temperature dependent reflective thin film;  
         [0018]      FIG. 8  is a schematic cross-section of the preferred sensor tip of  FIG. 7  further including a temperature probe;  
         [0019]      FIG. 9  is a block diagram of the opto-electronic circuity for the sensor tip of  FIG. 8  in a dynamic pressure setting;  
         [0020]      FIG. 10  is a block diagram of the opto-electronic circuity for the sensor tip of  FIG. 8  in a static pressure setting;  
         [0021]      FIG. 11  is a schematic cross-section of the preferred sensor tip incorporating separate dual fiber pairs of differing distances from the diaphragm;  
         [0022]      FIG. 12  is a schematic cross-section of the preferred sensor tip with an alternate form of the dual fiber pairs of  FIG. 11 ;  
         [0023]      FIG. 13  is a graphical illustration of two fiber output versus distance from the sensor diaphragm;  
         [0024]      FIG. 14  is a block diagram of the opto-electronic circuity for the sensor tips of  FIGS. 12 and 13 ;  
         [0025]      FIG. 15  is a block diagram of alternative sum/difference error correction opto-electronic circuity for the sensor tips of  FIGS. 12 and 13 ;  
         [0026]      FIG. 16  is a schematic cross-section of the preferred sensor tip having means to compensate for intra-cycle, thermal shock related errors;  
         [0027]      FIG. 17  is a schematic cross-section of the preferred sensor tip further including means for compensation of longer term temperature changes in the sensor tip of  FIG. 16 ; and  
         [0028]      FIG. 18  is a block diagram of the opto-electronic circuity for the sensor tip of  FIG. 17 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     In  FIG. 1  the modified smart injector  10  is fitted -with both a fuel pressure sensor  12  and a combustion pressure sensor  14 . The fuel sensor  12  may be mounted in a modified existing injector opening  16  as shown. The opening  16  comprises a channel communicating with the axial fuel channel  18  of the injector  10 . The sensor  12  may be approximately  5 mm in diameter and threaded into the channel  16  whereby the sensor diaphragm is directly exposed to the fuel pressure in the channel and also the axial fuel channel  18  when the injector plunger  20  is retracted.  
         [0030]     The combustion pressure sensor  14  is exposed to combustion chamber gases through a short channel  22 . The fiber optic lead  24  for this sensor  14  is located inside a  1  mm diameter cylindrical hole  26  extending from the sensor  14  to the strain relief  28  attached to the injector  10  near the top of the injector.  
         [0031]     Each of the sensors is connected to its own opto-electronic module or connector as further discussed below. The connections are by the fiber optic cables  30  and  32 . The opto-electronic modules contain circuity which in turn is connected to the engine Electronic Control Module (ECM). The ECM controls the injector timing, duration, and fuel line pressure in response to the output of the sensors  12  and  14  as well as other engine sensors and controls.  
         [0032]     The preferred sensor design for the fuel and combustion pressure sensors above utilizes two or more optical fibers. These fibers are modified as illustrated in  FIG. 2 . Rather than using fibers with circular cross-sections, as is conventionally done in fiber optic pressure sensors for other purposes, the fibers are generally D-shaped on the outer surface as shown at  34  where the cladding  36  is substantially reduced in thickness. The multimode fibers are coupled by their flat surfaces  34  to minimize the distance between fiber cores  38 .  
         [0033]     The minimum distance between fiber cores  38  is highly desired for reduced sensor power loss and increased optical modulation, both resulting in improved sensor resolution, signal to noise ratio and output accuracy. In a typical configuration the fibers have core  38  diameters of 140 microns, cladding  36  of 170 microns and flat surfaces  34  separated from the cores by 5-10 microns. For ease of sensor assembly and repeatability, the fibers are semi-permanently bonded together during the fiber manufacturing process by a thin layer of polyamide or other temperature-capable bonding adhesive material.  
         [0034]     In  FIG. 3 a  specially shaped diaphragm  40  sensor tip is shown. The diaphragm  40  is sculptured and is a further improvement on the hat shaped diaphragm first disclosed in application Ser. No. 08,390,970, now U.S. Pat. No. 5,600,070 incorporated herein by reference. The sculptured shape of the diaphragm  40  provides for increased light coupling between the delivering  42  and collecting  44  optical fibers mounted in the sensor body  46 . The result is increased sensor sensitivity to diaphragm  40  deflection. As shown in  FIG. 3  the inner diaphragm surface  48  is concave at the center of the diaphragm  40  to focus light emitted from the delivering fiber  42  as indicated by arrow  50  onto the collecting fiber  44  as indicated by arrow  52 . By design the focus is best when there is no additional external pressure applied to the outside of the diaphragm  40 .  
         [0035]     Under optimized conditions, the curvature of the reflecting surface  48  is ellipsoidal to focus the light from the delivering fiber  42  onto the collecting fiber  44  with no external pressure applied. With the application of external pressure, the diaphragm  40  deflects and the ends of the delivering  42  and collecting  44  fibers are no longer at the foci  54  of the ellipsoidal curved reflecting surface  48  as shown in  FIG. 4 . As a result the collected light is diminished. The diaphragm  40  thickness is substantially retained by curving the outside thereof to generally match the inner curvature and thereby substantially maintain the diaphragm yield strength over the entire diaphragm.  
         [0036]     The ellipsoidal curvature surface  48  and foci  54  locations are optimized for a given fiber size, core separation, and optical fiber numerical aperture. Although the ellipsoidal shape is optimum, more practical to manufacture parabolic or spherical surfaces for surface  48  may be substituted.  FIG. 5  compares the response  56  of the sculptured diaphragm  40  to the response  58  of a substantially flat or regular diaphragm. Clearly the sculptured diaphragm sensor is significantly more sensitive to external pressure changes than a conventional flat diaphragm sensor.  
         [0037]     Another aspect of the present invention comprises a novel design of the sensor electronic and opto-electronic circuity for a dynamic pressure sensor, as shown in  FIG. 6 . The circuity permits an increase in the operating temperature of the sensor and interface conditions module of up to a 150° C. working temperature.  
         [0038]     The new design uses one LED source and two photodiode detectors in contrast to previous single detector designs. A single detector design is adequate for a maximum electronics operating temperature of approximately 80° C., however, at a maximum operating temperature of 150° C. the dark current of a PIN photodiode becomes so large as to dominate the total photodiode output.  
         [0039]     At temperatures up to 80° C., the dark current of a typical photodiode is on the order of 1% of the total signal with 99% of the signal being proportional to the light delivered to the detector by the collecting fiber. But at 150° C. the dark current increases more than three orders of magnitude and may represent as much as 500% of the photodiode signal that remains proportional to the light delivered to the detector. At such high levels of dark current the sensor calibration is significantly compromised.  
         [0040]     To combat the dark current effect, the second photodiode is used for the purpose of providing a differentiating dark current input to a differential amplifier. In a preferred design disclosed below, the second diode is packaged inside a common enclosure with both photodiodes formed from the same silicon wafer, so their dark currents change identically with temperature changes. One photodiode is exposed to the light delivered by the collecting fiber while the second diode is covered so its output is only due to the dark current.  
         [0041]     In  FIG. 6  the current outputs from the photodiodes, one of which is the dark current correction, are combined at  60  to correct for the dark current output of the sensor. The voltage output therefrom is applied through the peak and hold circuit  62  and integrator  64  to the LED drive circuit  66  to thereby adjust the photonic output of the LED to the sensor. The voltage output is also passed through an inverting difference amplifier  68  and gain amplifier  70  to provide a suitable voltage range of output to the ECM of the engine.  
         [0042]     One of the most difficult challenges in developing accurate combustion pressure sensors is the need to overcome the effect of temperature changes on the sensor&#39;s diaphragm. Under changing temperature, the diaphragm&#39;s Young&#39;s modulus and Poisson&#39;s number change resulting in increased deflection at elevated temperatures. Two types of temperature errors result: increased deflection due to increased average diaphragm temperature over many cylinder pressure cycles and the intra-cycle diaphragm temperature excursions. Disclosed below are compensation techniques that correct for both of the temperature change errors.  
         [0043]     The short term, intra-cycle, error due to what is sometimes called thermal shock effect is compensated by virtually instantaneously reducing diaphragm reflectivity as the diaphragm temperature changes. In  FIG. 7 a  thin film  72  with a temperature-dependent co-efficient of reflectivity is deposited on the ellipsoidal surface  48  of the diaphragm  40  to cover the area illuminated by the delivery fiber  42 .  
         [0044]     The thin film  72  material is appropriately selected for a decreasing reflection co-efficient as the diaphragm deflection rate increases with temperature increases. The light intensity received by the collecting fiber  44  can be described by 
 
 V   col ( p,T )= I·C·R ( T )· D ( p,T ), 
 
 where I is light intensity emerging from the delivering fiber  42 , C is the collection efficiency dependent on fiber parameters and fiber to diaphragm distance but not dependent on diaphragm characteristics, R(T) is the reflection coefficient of the diaphragm, and D(p,T) is the function describing diaphragm deflection as a function of pressure and temperature. 
 
         [0045]     Through either thin film  72  light transmission or absorption dependence on temperature, the product of the reflection co-efficient R and deflection function D can be maintained independent of temperature if: 
 
 R ( T )= D   −1 ( T ) 
 
 In practice, R(T) may not be exactly an inverse of D but at least should approximate the inverse function. 
 
         [0046]     At least two types of thin films  72  are suitable for temperature compensation as described above. In one type, a dielectric thin film with dichroic mirror-like characteristics is used. In this type reflection decreases with increased temperature due to the thermal expansion of the thin film material. In the alternative type, a semiconductor material such as Silicon or Gallium Arsenide is used. In this type the absorption co-efficient increases with increasing temperature. In either type of thin film  72  the film is deposited over the area illumated by the delivering fiber  42 , however, to minimize internal stresses in the film, the film area should be as small as possible as should the film thickness.  
         [0047]     The temperature compensation technique described above does not correct for increased diaphragm deflection resulting from an increased average temperature of the diaphragm-sensor assembly over many pressure cycles. To correct for this type of error a compensation technique which relies on the electronic control of sensor gain is based on the output of a temperature probe mounted inside the sensor housing as shown schematically in  FIG. 8 . A temperature probe  74 , such as a thermocouple, is mounted inside the sensor assembly in the sensor body  46  preferably as close as possible to the sensor diaphragm  40 . A pair of wires  76  lead from the thermocouple  74  along side the optical fibers  42  and  44 .  
         [0048]     A schematic block diagram of the temperature-compensating opto-electronic circuitry for the sensor of  FIG. 8  is shown in  FIG. 9 . Compared to the circuitry of a temperature uncompensated sensor, a thermocouple amplifier  78  receives input from the thermocouple  74  and outputs to the photodiode amplifier  80  to adjust the gain value of the photodiode amplifier. This is in contrast to a fixed gain photodiode amplifier used for an uncompensated sensor. The calibration coefficients relating the sensor-gain to temperature change are obtained during an initial calibration and are uniquely assigned to each sensor when it is calibrated.  
         [0049]     A number of innovative designs for a high accuracy static pressure sensor suitable for fuel pressure detection in a smart injector are disclosed below. The first design utilizes a two-fiber construction similar to what is shown in  FIG. 3 . The processing electronics differentiate between the dynamic and static sensors.  
         [0050]     The difference between the sensor electronics is in the LED current control. While in a dynamic pressure sensor version the current is continuously adjusted based on the differential output of a minimum-maximum detector, in the static pressure sensor version the current is adjusted only during discrete periods of time when the injector is filled with fuel and the sensor is only exposed to atmospheric pressure. Suitable electronic circuitry is shown in  FIG. 10 , wherein when the LED current is adjusted the nominal light intensity is restored and any potential offset and gain drifts of the sensor are corrected. The timing trigger  82  for the current reset is provided by the engine ECM to the LED level control at  84 .  
         [0051]     The second design of a static sensor uses three or four optic fibers, two detectors and one LED. One pair of optic fibers is used to detect diaphragm deflection as above and the other pair serves as a compensation device to correct for potential errors due to fiber bending, coupling efficiency changes between the LED and fibers, the temperature dependence of LED intensity or photodiode sensitivity, as well as diaphragm deflection temperature dependence.  
         [0052]     In a temperature non-compensated version of the second design as shown in  FIG. 11 , both fiber pairs  86  and  88  are located with tips  90  and  92  in optical view of the reflecting surface  48  of the diaphragm  40 . In  FIG. 11  each pair  86  or  88  is embedded in its own ferrule  94  or  96 . The ferrules  94  and  96  are spaced apart so that the tips  90  and  92  are approximately separated by 500 microns.  
         [0053]     Alternatively, as shown in  FIG. 12  both fiber pairs  86  and  88  are embedded in one, larger ferrule  98  and the separation between the tips  90  and  92  is formed by the center of the ferrule  100  and polishing the ferrule tip at an angle to provide a separation distance between the fiber pairs  86  and  88  of approximately 500 microns.  
         [0054]     In both  FIG. 11  and  FIG. 12 , the tip  90  of fiber pair  86  is spaced further from the reflecting surface  48  than the tip  92  of fiber pair  88 . With this geometry the sensors of  FIG. 11  and  FIG. 12  make use of both slopes  104  and  106  of the two fiber sensor response curve of  FIG. 13  where one fiber pair operates on the ascending part of the curve while the other fiber pair uses the descending part of the curve.  
         [0055]     By appropriately locating the pairs  86  and  88  with the tips  90  and  92  at different distances from the diaphragm  40 , the pairs are positioned to straddle the peak intensity  102 . The maximum slope or sensor sensitivity occurs if the fiber tip  92  is positioned inside of the peak intensity  102  at about 250 microns. With the fiber pair tip  92  positioned on the first slope  104 , the distance between the diaphragm  40  and the fiber pair tip decreases as pressure increases and the sensor output is caused to decrease.  
         [0056]     If the second fiber pair tip  90  is positioned on the second slope  106  at approximately 600 microns the sensor output is caused to increase with pressure applied to the diaphragm  40 . In what is sometimes called a “push-pull” effect, the sensor output is essentially a ratio of the two outputs from the pairs of fibers  86  and  88  resulting in increased sensor sensitivity to deflection compared to a sensor operated on one slope only with a single fiber optic pair.  
         [0057]     In the processing scheme of  FIG. 14  the light intensity detected by the reference pair tip  90  is maintained at a constant level by changing a LED driving current as illustrated in this block diaphragm. The sensor  46  output is proportioned to the light intensity detected by the measurement detector  108 . Similar to the two fiber dynamic sensor above, the LED  110  light intensity is continuously adjusted by changing the current level to keep the reference photodiode  112  output at a constant level. As the light intensity is restored in the reference fiber pair  86 , so is the light intensity in the measurement fiber pair  88  thereby compensating for any offset or gain errors of the sensor  46 .  
         [0058]     Alternatively, as shown in  FIG. 15  the LED  110  current level is based on the sum  114  of the measurement detector  108  output and reference detector  112  output. In this configuration the zero relative pressure outputs of the two fiber pairs  86  and  88  are made equal by providing gain  116  to one output and summing  114  to provide current level control to the LED  110 . If an environmental condition causes a light intensity change in the fiber pairs  86  and  88 , both light intensities decrease or increase in the same direction and approximately the same amount. Thus, this light intensity change is directly corrected by changing the LED  110  light intensity.  
         [0059]     By contrast, a relative pressure change applied to the sensor  46  will cause one output to increase and the other output to decrease having a net result of no change in the summing  114  of the outputs. The difference  118  of the two outputs passes through offset, filtering and gain  120  to provide the corrected sensor output. With the use of a sensor  46  also having a thermocouple therein, in addition to the two fiber pairs  86  and  88 , correction  122  may be made for the average temperature of the sensor  46  and applied to the offset, filtering and gain  120  of the corrected sensor output.  
         [0060]     In  FIGS. 16 and 17  two additional versions of temperature-compensated static pressure sensors are described. The version in  FIG. 16  is intended to compensate for intra-cycle, thermal shock related errors only. As above, one fiber pair  88  is exposed at its tip  124  for reflection of light off the thin film  72  to measure diaphragm  40  deflection. The other fiber pair  86  or reference pair is terminated by a non-deflecting reflector  126  with the surface covered by a temperature dependent thin film. The processing circuitry may be the same as disclosed in  FIG. 14  above with the reference fiber pair  86  connected to photodiode  112 . Depending on the application, both temperature dependent thin films  72  and on reflector  126  may be used or only on reflector  126 .  
         [0061]     In  FIG. 17  the sensor  46  incorporates compensation for both short- and long-time constant temperature errors. The sensor  46  incorporates a combination of the temperature dependent reflectivity of the thin film  72  on the diaphragm  40  and the thin film on reflector  126  with the thermocouple temperature probe  74  all inside the sensor with the measurement fiber pair  88 . As noted above, the thin films are used to compensate for the short time constant temperature change induced diaphragm deflection errors and the temperature probe is used to compensate for long time constant sensor average temperature errors. While the thin film correction is inherent in the temperature change induced reflectivity changes, the temperature probe correction is achieved through adjustment of the sensor opto-electronic control circuitry gain and offset as illustrated in  FIG. 18 .  
         [0062]     The opto-electronic control circuitry of  FIG. 18  is similar to the circuitry of  FIG. 14 , however, the thermocouple amplifier  78  and an Eraseable Programmable Read Only Memory (EPROM)  128  are added. In addition to correcting for short- and long-time constant temperature changes, differing responses among fiber pairs arising from temperature changes can be countered. This non-identical response among fiber pairs may result in both sensor offset and sensor gain errors. The resistance of certain components in the circuitry may be adjusted to calibrate an individual sensor and thereby compensate for the inherent differences among the fiber pairs. Alternatively, the EPROM  128  may be used to store and provide the electronic calibration values.