METHOD AND APPARATUS FOR DETECTING COMBUSTION PRESSURE IN ENGINE

Abstract: A combustion pressure detecting apparatus for generat­ ing an electrical signal in response to a combustion pressure of an internal combustion engine, wherein an electrical signal based on said combustion pressure is detected (5, 6, 8, 9, 11) at a predetermined angle of rotation of said engine, the detec­ ted value being compared (23) with a predetermined refer­ ence value to derive a difference therebetween, including correcting means for executing correcting operations in dependence on said difference.

Description: FIELD OF THE INVENTION The present invention relates to a method and an apparatus for detecting combustion pressure in an internal combustion engine. More particularly, the present invention concerns a combustion pressure detecting method and apparatus for the engine for the purpose of correcting the detected combustion pressure as detected. DESCRIPTION OF THE PRIOR ART There are various known pressure sensors for detecting combustion pressure in an engine such as those of piezo-electric type, semiconductor type, capacitive type and others. However, every one of these known sensors encounters difficulty in detecting the pressure of combustion with a reasonably high accuracy over an extended period (use life) under adverse environmental conditions such as high voltage, high temperature and high pressure. None of these sensors thus is adapted for installation to motor vehicles. In addition to the aforementioned sensors, there may be mentioned a pressure sensor in which optical fibers are used. By way of example, Japanese Unexamined Patent Publication No. 59-60334 discloses a typical one of the hitherto known pressure sensor in which optical fibers are used. The known pressure sensor employing optical fiber is generally intended for use in process controls. When this sensor is employed as a combustion pressure detecting device in an engine, there arises a problem that temperature of the pressure detecting device itself varies over a very wide range under the influence of combustion temperature in the engine so the device has the disadvantage that it is impossible to accurately measure the combustion pressure. It is known from GB-A-2149535 to provide a method for adjusting the fuel/air ratio supplied to an internal combustion engine by measuring the pressure with a piezoelectric pressure transducer at a predetermined angle of rotation of the engine and to compare the measured pressure with a corresponding standard to thereby effect adjustment of the fuel/air ratio to cause the measure value to approach the standard. However such a piezoelectric pressure transducer has electrical signal noise disadvantages when operating in an internal combustion engine environment and so an optical fiber pressure sensor is preferred. SUMMARY OF THE INVENTION An object of the invention is to provide a combustion pressure detecting apparatus which is capable of detecting the combustion pressure accurately even under the adverse influence of environmental factors such as, inter alia, a wide variation of the ambient temperature. The present invention resides in the provision of correcting means for the combustion pressure detecting apparatus. According to a first aspect of this invention there is provided a method of detecting combustion pressure in an internal combustion engine for generating an electrical signal in response to said pressure, including the steps of producing an electrical signal based on said combustion pressure at a predetermined angle of rotation of said engine by using an optical sensing means including a light emitting means and a light detecting means, obtaining a correcting value between the detected value and a predetermined reference value to derive a difference signal therebetween, and using said difference signal to execute correcting operations to bring said difference toward zero by controlling the output of at least one of said light detecting means and said light emitting means. According to a second aspect of this invention there is provided a combustion chamber pressure detecting apparatus for generating an electrical signal in response to a combustion pressure in an internal combustion engine including means for detecting a predetermined angle of rotation of said engine, means for producing a detected electrical signal based on said combustion pressure at said detected predetermined angle, means for producing a predetermined reference value, comparison means for comparing the detected electrical signal with said predetermined reference value to derive a difference therebetween, and correcting means for executing correcting operations to bring said difference toward zero characterised in that the detecting means is a light sensing means including a light emiting means and a light detecting means whereby said light sensing means outputs said electrical signal to said comparison means and upon detection of said predetermined angle the comparison means compares the detected electrical signal with said reference value and a correcting signal is produced to control the output of at least one of said light detecting means and said light emitting means. In the correcting method deviation of the output of the pressure sensor from a predetermined value at a predetermined angle of rotation of a shaft of the engine may be detected, wherein a correcting value for zeroing the deviation is utilised for varying the operating conditions of the sensor. The correcting value thus generated may be utilised for correcting the sensor output and/or of the detecting means to obtain the corrected value of measurement. By way of example, the correcting value may be utilised for varying the bias of the sensor. Preferably an optical fiber type sensor is used and the correcting value may be utilised for controlling the intensity of light output of light emitting means and/or of the light detecting means. The sensor intended for use in a motor vehicle should preferably exhibit an enhanced insusceptibility to noise and in this connection, an optical fiber type sensor exhibits an excellent noise insusceptibility. Accordingly, in the following description of the preferred embodiments of the invention, it is assumed that an optical fiber type sensor is employed as a typical example. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example in which:- Fig. 1 is a block diagram showing an embodiment of the present invention with one form of learning/correcting circuit; Fig. 2 is a view showing schematically a structure of a detecting portion of a combustion pressure sensor in which optical fibers are employed; Fig. 3 is a view showing the form in which the optical fibers are secured within a pipe of glass; Fig. 4 is a view showing relationship between the quantity of reflected light and a gap; Fig. 5 is a view illustrating relationships between the combustion pressure and light reception sensitivity; Fig. 6 is a view illustrating a waveform of the combustion pressure and a correcting procedure; Fig. 7 is a view for illustrating output characteristics of the combustion pressure sensor; Fig. 8 is a view showing in detail a signal processing circuit; Fig. 9 is a block diagram showing another embodiment of the learning/correcting circuit; Fig. 10 is a flow chart for illustrating arithmetic determination of a correcting factor; and Fig. 11 is a flow chart for illustrating arithmetic determination of the true pressure from the measured value. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a combustion pressure detecting apparatus according to an embodiment of the present invention, and Fig. 2 shows a detail of the detecting portion thereof. The detecting portion (sensor) is composed of a pipe 1 made of boro-silicated glass, optical fibers 2 fixedly secured within the pipe 1 by means of a refractory adhesive and a silicon diaphragm 3 having a recess in the vicinity of the end of the fibers 2 to define a gap therebetween, the diaphragm being anodically bonded to an end face of the glass pipe 1. The form of the optical fibers 2 secured within the glass pipe 1 is shown in Fig. 3. In the Figure 3, a fiber bundle containing light projecting optical fibers 21 and light receiving optical fibers 22 is fixedly secured within the bore of the glass pipe 1 by a refractory adhesive 4 of inorganic material such as pulverized glass. After fixation of the fibers, the end faces of the glass pipe 1, the light projecting optical fibers 22 and the light receiving optical fibers 22 are ground and polished to a mirror quality so that the end faces lie in a single plane. By polishing the end face of the glass pipe 1 to the grade of a mirror surface quality, it is possible to bond the silicon diaphragm 3 with a high bonding strength through the anodic bonding process (i.e. an electrochemical bonding at high voltage and high temperature). Since all the glass pipe 1, the light projecting fibers 21, and the light receiver fibers 22 and the refractory adhesive 4 are made of inorganic materials such as vitreous compositions, these structural components resist thermal distortion even when the ambient temperature changes. Accordingly, any tendency for cracks to be produced in the bonded portions of these structural components is suppressed to a minimum. Further, the end faces of the glass pipe 1, the light projecting optical fibers and the light receiving optical fibers can be maintained at positions lying substantially on one and the same plane regardless of variations in the ambient temperature. The detecting portion or sensor is employed for measuring the combustion pressure by determining the quantity of light reflected at the deformable silicon diaphragm and received by the light receiving optical fibers 22, the light being initially projected from the light projecting optical fibers 21. In the following, a method of measuring the combustion pressure by making use of the amount of refleted light will be described. A relationship between the quantity of reflected light and a gap is illustrated in Fig. 4. When the radius of the optical fiber is represented by r while the gap between the silicon diaphragm and the end face of the optical fiber is represented by ℓ , the quantity of reflected light is such as illustrated in Fig. 4. As will be seen, the quantity of reflected light is at maximum when ℓ/r = 0.35. Experiments conducted by the inventors have shown that the combustion pressure can be detected linearly at regions a and b illustrated in Fig. 4. From the viewpoint of miniaturization of the detecting portion or sensor, the region a is preferred where the quantity of reflected light changes more steeply. Next, an apparatus for detecting the combustion pressure in the engine according to an embodiment of the invention will be described by referring to Fig. 1. A measuring channel is constituted by a detecting portion 5, an optical fiber bundle 6 and a learning/correcting unit 7. The latter, in turn, is composed of a light emitting element 8 such as LED, a light detecting element 9 such as silicon pin photodiode and a signal processing circuit 10 for controlling the quantity of light emitted by the light emitting element 8 in response to a signal produced by a bottom dead point sensor 11. Further, the signal processing circuit 10 produces an output voltage signal E O proportional to the combustion pressure P, which signal is transmitted to an engine controlling microcomputer 12. As described above, the silicon diaphragm of the detecting portion 5 is fixedly bonded to the glass pipe which has a thermal coefficient of expansion substantially equal to that of the diaphragm. Consequently, even when the temperature of the detecting portion 5 varies over a wide range under the influence of combustion temperature in the engine, any change in the gap ℓ set between the end face of the optical fiber bundle and the silicon diaphragm when the combustion pressure P is substantially equal to the atmospheric pressure when the engine is at bottom dead center remains very insignificant. In other words, this invention provides a combustion pressure detecting apparatus in which a reference value output signal is produced in a combustion chamber when the pressure therein is substantially equal to the atmospheric pressure and said output is not affected by temperature, whereby the measurement of the combustion pressure P can be determined with high accuracy. Further, since the detecting portion 5 is constituted by elements made of similar vitreous materials having substantially equal thermal coefficients of expansion, the detecting portion 5 will not be harmed by the adverse environmental conditions of high temperature and high pressure brought about by combustion, thus ensuring high reliability of measurement of the combustion pressure over an extended period. The detecting portion 5 is mounted on an ignition plug and thus used in the vicinity of the point to which an ignition voltage of several thousand volts is applied. However, since the detecting portion 5 operates by detecting the quantity of reflected light, the detecting portion 5 is unlikely to be subjected to the influence of high voltage. It should be mentioned that the gap ℓ at atmospheric pressure is so selected as to fall within a range of several microns to several tens of Microns. In the following, the learning/correcting system will be described. Relationships between the combustion pressure P and the light reception sensitivity as illustrated in Fig. 5 with the pressure P being taken in terms of the absolute pressure on the assumption that the gap ℓ between the silicon diaphragm and the end face of the optical fiber bundle is set with the region a shown in Fig. 3. The gap ℓ is decreased linearly as a function of increasing combustion pressure P. Consequently the light reception sensitivity of the light receiving element 9 is decreased as the combustion pressure P increases, as shown in Fig. 5. The relation between the light reception sensitivity and the combustion pressure is modified, as indicated by characteristic curves X, Y and Z, due to the light intensity emitted by the light emitting element 8, the light transfer constants of optical couplers (or optical connectors, not shown) disposed, respectively, between the optical fiber bundle 6 and each of the light emitting element 8 and the light receiving element 9, the light transfer constant of the optical fiber bundle, and the reflection factor of the diaphragm surface varying because of external factors such as temperature variation, contamination, vibration and others. It will be seen that all the light reception sensitivities represented by the characteristic curves X, Y and Z change linearly. Fortunately, it has been experimentally established that the characteristics X, Y and Z can be made to coincide perfectly with one another over the whole measuring range of the combustion pressure P by setting the light reception sensitivities (values indicated by circles in Fig. 5) on the characteristic curves at a predetermined pressure value P K equal to one another by predeterminedly varying the light intensity of the light emitting element 8 through the signal processing circuit 10. In particular, the pressure P K corresponds to the combustion pressure at a predetermined engine state, e.g. the state in which the piston within the engine cylinder is positioned at the bottom dead point, as described hereinafter. The possibility that coincidence between the characteristics X, Y and Z may be provided is due to the fact that the zero point drift of the light reception sensitivity with respect to the combustion pressure P, substantially does not take place but only the change in span occurs. This is because the relation between the combustion pressure P and the gap ℓ does not substantially vary even under the external disturbance such as temperature change, as described hereinbefore. More specifically since the detecting portion 5 is constituted by components having approximately the same thermal expansion coefficients and additionally by the anodic bonding between pipe 1 and diaphragm 3 requiring no adhesive, the change in the gap ℓ can remain very small in spite of external disturbance such as temperature change. A waveform representative of change in the combustion pressure is illustrated in Fig. 6. In dependence on the state of combustion in the engine, a so-called knocking phenomenon may take place, resulting in an output waveform of high frequency. In Fig. 6, circles represent the engine state in which the piston assumes the bottom dead point. The peak value PT of the combustion pressure appears periodically and varies in dependence on such factors as air-fuel ratio, ignition timing and others. However, the combustion pressure P K at the bottom dead point is subtantially independent of the state of combustion and assumes a substantially constant value (which is substantially equal to the atmospheric pressure although in the strict sense it differs from the latter). By taking advantage of this property, the learning/correction of the combustion pressure sensor can be realized. A process of the learning/correction will be described in conjunction with the system shown in Fig. 1. On the basis of an output signal of the engine bottom dead point sensor 11 which is adapted to detect the bottom dead point (instead of providing this sensor, the bottom dead point may alternatively be estimated from the output of a crank angle sensor), the light intensity emitted by the light emitting element 8 is controlled by the signal processing circuit 10 (e.g. by controlling the voltage supplied to the light emitting element) so that the light reception sensitivity of the light receiving element 9 remains constant at every bottomdead point. As a result, the output signal of the combustion pressure sensor is automatically and periodically corrected through the learning procedure, as described in conjunction with Fig. 5. The output characteristics of the combustion pressure sensor are illustrated in Fig. 7. The output characteristics are obtained by processing the changes in the light reception sensitivity of the light receiving element 9 through the signal processing circuit 10. In the figure, the standardized output characteristic accompanied with no detection error is indicated by a curve y . When the learning/correction according to the instant embodiment is not adopted, the output characteristic is such as indicated by a curve x assuming the light reception sensitivity is increased, while the characteristic curve z is obtained when the light reception sensitivity is reduced. It should be mentioned that by adopting the periodical learning/correcting procedure, the output characteristic always follows the standardized characteristic curve y even when the light reception sensitivity is significantly increased or decreased due to external factors other than the changes in the combustion pressure P. Details of the signal processing circuit 10 of Fig. 1 are shown in Fig. 8. In response to detection of the bottom dead point, the bottom dead point sensor 11 produces an output signal and a switch 20 is closed to cause the light reception sensitivity of the light receiving element 9 to be held by a capacitor 21. The anode voltage of the light emitting element 8 is controlled through a comparator 23 so that the output voltage of a sample and hold circuit 30 constituted by the switch 20, the capacitor 21 and a buffer amplifier 22 is equal to a set voltage V s . As a consequence, the light reception sensitivity at every bottom dead point becomes constant independent of the temperature change and/or other external disturbance. Further, the light reception sensitivity of the light receiving element 9 is transformed into an output voltage E o corresponding to the combustion pressure through a signal converter circuit 24, the output voltage E o being transmitted to the microcomputer 12. In particular, the pulse width of the pulse signal produced by the bottom dead point sensor 11 for opening and closing the switch 20 is selected to be less than several hundred microseconds (µs). Next, another learning/correcting method will be described. This method resides in that the output signal of the combustion pressure sensor is inputted to a digital computer, e.g. microcomputer arranged to be arithmetically corrected. A learning/correcting system based on such a microcomputer is shown in Fig. 9. The light reception sensitivity (the sensitivity shown in Fig. 5) of the light receiving element 9 incorporated in the combustion pressure sensor is converted into a corresponding voltage value which is then transmitted to the microcomputer, wherein the voltage value corresponding to the light reception sensitivity illustrated in Fig. 5 is converted to a digital value through an A/D converter 13, the digital value being inputted to microprocessor unit (MPU) 15 through an input port 14. In this case, the signal processing circuit 10 includes neither the sample and hold circuit 30 nor the comparator 23, wherein the voltage V s (Fig. 8) is applied to the light emitting element 8 as the anode voltage. The output signal is shifted in voltage level and amplified through the signal converter circuit 24 to be subsequently supplied to the input of the microcomputer 12. Figs. 10 and 11 show signal processing flow charts. Upon every bottom dead point, the MPU 15 is interrupted by the output signal of the bottom dead point sensor 11 to execute the processing flow illustrated in Fig. 10. At a step 102, a pressure signal PD is fetched which represents the pressure within the engine cylinder at the bottom dead point of the piston. At a step 104, it is decided whether the pressure signal PD represents a measured value within a predetermined range, to thereby eliminate influence of noise such as ignition noise. At a step 106, the measured value PD is divided by a reference value PR, to give a quotient α which represents a correcting value. At a step 108, this value α is stored in RAM 17. Measurement is normally executed in accordance with the flow chart shown in Fig. 11. The processing flow as illustrated is executed every time the crank reaches a predetermined angle or every time the volume of the combustion chamber changes by a predetermined amount. A system for detecting the combustion pressure upon every predetermined change in volume of the combustion chamber is disclosed in U.S. Patent No. 4,455,980. The output of the signal processing circuit 10 is detected as a pressure signal P at a step 112 and multiplied with the correcting factor α stored in the RAM 17 to arithmetically determine the true pressure PT. In particular, the bottom dead point can be determined on the basis of an output signal from a crank angle sensor. According to the embodiment disclosed above, there can be realized a combustion pressure sensor which resists the influence of temperature and in which the gap between the diaphragm and the optical fiber does not vary with temperature because the pressure detection portion is constituted by components which have substantially equal thermal coefficients of expansion. Further, since the thermal distortion of the pressure detecting portion due to the combustion temperature is insignificant, the pressure detecting portion will not be harmed or damaged under thermal stress. Thus, a combustion pressure sensor having high reliability is realized. Moreover, since the measurement is performed by using light, the detection accuracy will not be degraded even in the high voltage environment of the ignition plug. Further, by virtue of the feature that the output signal of the combustion pressure sensor is periodically subjected to a learning/correction process with the aid of the combustion pressure at the bottom dead point, the pressure of combustion can be detected with high accuracy.

Claims: A method of detecting combustion pressure in an internal combustion engine for generating an electrical signal in response to said pressure, including the steps of producing an electric signal based on said combustion pressure at a predetermined angle of rotation of said engine by using an optical sensing means (5, 6, 8, 9) including a light emitting means (8) and a light detecting means (9), obtaining a correcting value between the detected value and a predetermined reference value to derive a difference signal therebetween, and using said difference signal to execute correcting operations to bring said difference toward zero by controlling the output of at least one of said light detecting means and said light emitting means. A method as claimed in claim 1 characterised in that the correcting value corresponding to the difference signal between said detected value and said reference value is arithmetically determined and stored to be utilized to bring said difference signal toward zero. A method as claimed in claim 1 or 2 wherein said predetermined angle of rotation of said engine is bottom dead centre of said crank shaft. A method as claimed in any preceding claim wherein the light sensing means further includes a diaphragm (3) located in the combustion chamber of said engine, optical fibers (2) extending from said diaphragm to said light emitting means (8) and said light detecting means (9), said diaphragm being deformable under the combustion pressure in said engine, said optical fibers introducing light from said light emitting means to a light reflecting surface of said diaphragm and introducing light from said reflecting surface to the light detecting means, the reflected light being dependent upon the relative position of said diaphragm to said light detecting means, comparing the output of the light detecting means produced at bottom dead centre of said crank shaft with said reference signal value and a correcting means arranged to vary the output of said light emitting means to bring said difference value toward zero. A method as claimed in any preceding claim wherein the intensity of light output of the light emitting means (8) and/or the light detecting means (9) is/are controlled at said predetermined angle of rotation of said engine to be substantially constant. A combustion chamber pressure detecting apparatus for generating an electrical signal in response to a combustion pressure in an internal combustion engine including means (11) for detecting a predetermined angle of rotation of said engine, means (8, 9, 30) for producing a detected electric signal based on said combustion pressure at said detected predetermined angle, means for producing a predetermined reference value, comparison means (23) for comparing the detected electric signal with said predetermined reference value to derive a difference therebetween, and correcting means for executing correcting operations to bring said difference toward zero characterised in that the detecting means is a light sensing means (5, 6, 8, 9) including a light emitting means (8) and a light detecting means (9) whereby said light sensing means outputs said electric signal to said comparison means (23) and upon detection of said predetermined angle the comparison means compares the detected electric signal with said reference value and a correcting signal is produced to control the output of at least one of said light detecting means (9) and said light emitting means (8). A combustion chamber pressure detecting apparatus as claimed in claim 6, wherein arithmetic and storing means (13-17) are provided for producing a correcting value corresponding to the difference between said detected electric signal and said reference value and for utilisation to correct the detected electrical signal. A combustion chamber pressure detecting apparatus as claimed in claim 7, wherein said arithmetic and storing means includes a microprocessor (15) and a storing means (16, 17). A combustion chamber pressure detecting apparatus as claimed in claim 6 wherein the intensity of light output of the light emitting means (8) and/or the light detecting means (9) is/are controlled at said predetermined angle of rotation of said engine to be substantially constant. A combustion chamber pressure detecting apparatus as claimed in claim 6 wherein both the light detecting means and light emitting means outputs are varied by said correcting signal. A combustion pressure detecting apparatus as claimed in claim 6 wherein the light sensing means comprises the light emitting means (8), the light detecting means (9), a diaphragm (3) with a light reflecting surface which is deformable under the combustion pressure in the engine, optical fibers (2) for introducing light from said, light emitting means to said light reflecting surface and for introducing light reflected from said light reflecting surface to the light detecting means, and signal processing means (10) for detecting said combustion pressure from the output of said light detecting means, said signal processing means including said comparison means (23) and said correcting means. A combustion chamber pressure detecting apparatus as claimed in any of claims 6 to 11 wherein the means for detecting a predetermined angle of rotation of said engine is a crankshaft angle detector (11) which is adapted to produce an output signal upon detection of bottom dead center of the crankshaft.

Inventors: AMANO, MATSUO, MIKI, MASAYUKI, SASAYAMA, TAKAO, SUZUKI, SEIKOO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: G01L23
IPC codes: G01L9, G01L23, G01L11