INTERNAL COMBUSTION ENGINE CONTROL

Abstract: A method of monitoring combustion within a cylinder of a reciprocating piston internal combustion engine comprising deriving first and second electrical signals representative of light intensity within the cylinder by means of first and second optical transducers 18, 19 of spaced spectral response, said first and second optical transducers observing the combustion event at a common location, or at closely adjacent locations, and, establishing a signal representing the ratio of said first and second electrical signals said ratio signal being used as, or to obtain, a control parameter for an engine controlled system. The invention also resides in apparatus for monitoring combustion in accordance with the above method.

Description: This invention relates to a method and apparatus for controlling operation of a reciprocating piston internal combustion engine, involving monitoring of combustion in a cylinder of the engine. A primary purpose of monitoring combustion within a cylinder of a reciprocating piston internal combustion engine is to ascertain the crank-shaft position at which combustion of fuel within the cylinder commences. The point in the operation of the engine at which combustion commences is an important parameter in the control of engine operation by means of a closed, or open, loop feedback control system of an engine, and United States patent specification 4381748 discloses engine control systems using the start of combustion as a main control parameter. The start of combustion is not the only parameter which may be used by an engine control or engine management system and other composition related control parameters include the pressure within the cylinder and the combustion temperature. It is known, at least from United States specification 4381748, to monitor the combustion within an engine cylinder by optical means. In US specification 4381748 a phototransistor is disposed at the exterior of a transparent window in the wall of the cylinder and monitors light intensity within the cylinder. Such an arrangement however has significant disadvantages. Firstly, it is recognised that the peak light intensity will vary from cycle to cycle of the engine, and thus in order to determine satisfactorily the point at which a signal representative of for example the start of combustion is passed to the control system it is necessary to establish, over a predetermined number of cycles, an average peak intensity, and then to pass a signal to the control system when the light intensity perceived by the photo-transistor achieves a predetermined percentage of the average peak intensity. The time constant which occurs in the system in order to permit calculation of an average peak intensity slows down the system response and thus the control system cannot respond rapidly to engine operating changes, and may not cope adequately with transient changes. Naturally the window in the cylinder wall will become contaminated during operation of the engine and such contamination will alter the light intensity perceived by the photo transistor. Such changes in perceived intensity can give rise to the control system effecting non-optimum control of the engine operation. British Patent 2098756 also discloses a system which monitors light intensity in a cylinder of an engine but differing from US Patent 4381748 in that the intensity of a plurality of predetermined wavelengths is monitored. It is to be expected therefore that the system of 2098756 will suffer from the disadvantages mentioned above in relation to US Patent 4381748. In order to control operation of a reciprocating piston internal combustion engine so as to minimise pollution by exhaust bourne oxides of nitrogen, it is desirable to monitor the oxides of nitrogen (NOx) content of the exhaust. GB-A-2098756 suggests monitoring of the combustion event within a cylinder of the engine and correlating specific spectral lines of NO₂ and CO in the spectrum of the combustion event, with the infra-red spectrum, or specific areas thereof, and employing the correlation for visual display or automatic control of the fuel/air ratio of the engine. GB-A-2098756 does not disclose detail of how this broad desideratum may be accomplished; it does however contemplate that by employing narrow band filters on the spectral lines of NO and CO, the amount thereof and thus the degree of pollution of the engine emission can be monitored in a simple manner. It is an object of the present invention to provide for control of an internal combustion engine by monitoring the combustion event but without the need for isolation and processing of individual spectral lines of exhaust pollutants. In accordance with a first aspect of the present invention there is provided a method of controlling operation of a reciprocating piston internal combustion engine comprising deriving a signal representative of the oxides of nitrogen (NOx) content of the exhaust of the engine by deriving first and second electrical signals representative of light intensity of the combustion event within a cylinder of the engine by means of first and second optical transducers (18, 19) of spaced spectral response, said first and second optical transducers observing the combustion event at a common location, or at closely spaced locations, establishing a signal representing the ratio of said first and second electrical signals, identifying a minimum in the value of said ratio signal between a first peak value which follows a steep initial rise, and a second peak value, and, utilising the magnitude of said minimum, which is representative of the NOx content of the exhaust of the engine, as a fuel/air ratio control parameter, and adjusting accordingly the fuel/air ratio of the fuel/air mixture supplied to the engine. In accordance with a second aspect of the present invention there is provided apparatus for controlling operation of a reciprocating piston internal combustion engine including light conduction means having one end exposed within the combustion chamber of a cylinder of the internal combustion engine and its opposite end communicating with sensor means for deriving a signal representative of the oxides of nitrogen (NOx) content of the exhaust of the engine and means for adjusting accordingly the fuel/air ratio of the fuel/air mixture supply to the engine, said sensor means comprises first and second optical transducers (18, 19) of spaced spectral response, said first and second optical transducers observing the combustion event, in use, at a common location, or at closely spaced locations, means (23, 24) for establishing a signal representing the ratio of said first and second electrical signals, and means identifying a minimum in the value of said ratio signal between a first peak value which follows a steep initial rise, and a second peak value, the means for adjusting the fuel/air ratio utilising the magnitude of said minimum of said ratio signal, which is representative of the NOx content of the exhaust, as a control parameter for controlling the fuel/air ratio. Preferably said first and second optical transducers (18, 19) exhibit a peak response at 600 nm and 875 nm respectively. It will be recognised that the use of transducers of different spectral response, and the derivation of an output signal from the ratio of the outputs of the two transducers is a totally different concept to that disclosed in United States specification 4381748. Taking somewhat a simplistic view, the disclosure in United States specification 4381748 is of the concept of monitoring the intensity of light emitted during the combustion event whereas the present invention monitors the "colour" of the light emitted during the combustion event. The term "colour" is to be construed broadly, and not to be construed as restricted to radiations in the visible spectrum. One example of the invention is illustrated in the accompanying drawings wherein: Figure 1 is a cross-sectional view of an optical sensor probe, Figure 2 a is a diagrammatic representation of a pair of optical transducers and associated means for deriving a ratio signal, Figure 2 b is a diagram similar to Figure 2 a of a modification, Figure 3 a is a graphic illustration of the outputs of the optical transducer and the ratio of those outputs in a diesel engine, Figure 3 b is a graphic illustration of the use of the modified system of Figure 2 b , Figure 4 is a view similar to Figure 2 of a monitoring system associated with an engine control for a spark ignition internal combustion engine, Figure 5 is a view similar to Figure 3 illustrating petrol engine characteristics, and, Figure 6 is an illustration of the control of air fuel ratio in an engine. Referring first to Figures 1 to 3 of the drawings, apparatus for monitoring combustion within a cylinder of a reciprocating piston internal combustion engine comprises an optical probe 11 connected by means of an optical fibre bundle 12 or the like to an optical transducer assembly 13. It should be noted that the means of transmitting light from the probe 11 to the transducer 13 is not of importance to the present invention, and any of the widely recognised optical conductor arrangements can be utilized. Furthermore, it might prove possible to mount the transducer 13 directly on the probe 11 provided that the temperature of the probe can be maintained at a satisfactory level. The probe extends through the outer wall of the cylinder so as to communicate with the combustion space in the cylinder, and preferably the probe will be mounted in the cylinder head of the engine. The probe 11 comprises an optically conductive rod 14 received within a multi-part metal carrier 15 whereby the probe is sealingly engaged with the wall of a passage extending through the cylinder wall (conveniently a screw threaded passage in the cylinder head). The rod 14 is exposed at one end to the combustion space within the cylinder, and thus light and similar electromagnetic radiation emitted during combustion within the cylinder is conducted by the rod 14 and the fibre bundle 12 if used, to the transducer 13. Clearly the probe may take a number of different forms, but in a convenient form, as illustrated in Figure 1, the rod 14 is resiliently mounted within the carrier 15 and is spring loaded against the inner end of the carrier 15 by means of spring washers 16. The inner-most end region 17 of the carrier 15 is formed from a material resistant to the temperature and atmospheric conditions which arise within the combustion space of the cylinder. Furthermore, the rod 14 is externally arranged to ensure total internal reflection within the rod and thus a high degree of optical conduction between the ends of the rod. Alternatively the rod may comprise a high temperature resistant transparent rod of, for example, fused silica. The transducer assembly 13 comprises first and second photodiodes 18, 19 of spaced spectral response. A suitable commercially available version of the transducer 13 is known as the Sharp PD 150 colour sensor which contains a first photodiode exhibiting a peak response at 600 nm and a second photodiode having a peak response at 875 nm. The photodiodes are thus sensitive to radiation in the infra-red wavelengths as well as the visible wavelengths. When light falls on the colour sensor the photodiodes 18, 19 each produce an output, the outputs being different by virtue of the different spectral responses of the photodiodes. As is evident from Figure 2 a the outputs of the two photodiodes 18, 19 are supplied to respective log amplifiers 21, 22 the resultant signals from which are fed to a subtracting circuit 23. The output of the circuit 23 represents a logarithm of the ratio of the signals derived from the photodiodes 18, 19 and conveniently is processed by a peak-to-peak threshold detector 24 before being passed to an engine control or engine management system. As combustion commences in the cylinder of an engine there is a very rapid temperature increase, and thus there is an equally rapid change of "colour" of the "light" emitted. It is to be understood that references to "colour" and "light" are not to be construed as restricted to emissions in the visible wavelengths. It will be recognised therefore that there is a very rapid rise in the ratio signal output by circuit 23 representative of the start of combustion. The rise in the ratio signal is considerably more rapid than the rise in an intensity signal derived from the same combustion event for example as disclosed by US Patent 4381748, and thus provides much more accurate timing information. In a system as disclosed in United States specification 4381748 it is necessary to average the peak intensity over a predetermined number of engine cycles in order to overcome the relatively wide variation in engine cycle to engine cycle peak combustion light intensity. It has been found that ratio signal issuing from circuit 23 does not exhibit a marked peak to peak variation and therefore it is necessary only to pass the signal through a low pass filter in order to derive a control signal by which the control of, say the point of ignition spark generation within the engine cycle, may be effected. When the output signals of the photodiodes 18, 19 are very small, as may be the case when the engine operates under low load, the output of the respective logrithmic amplifiers 21, 22 may become inaccurate and so give rise to a distorted ratio signal from the circuit 23. In order to minimise this risk the arrangement shown in Figure 2 b is proposed. In Figure 2 b the output of each photodiode is amplified at 18 a . 19 a and is supplied to one input of a respective electronic change-over switch 18 b , 19 b . The other input of each switch 18 b , 19 b is maintained at a predetermined d.c. voltage by a respective d.c. source 18 c , 19 c . The output of each switch 18 b , 19 b is fed via a respective log amplifier 21, 22 to a subtracting circuit 23 as before. The electronic switches 18 b , 19 b are electronically ganged and are controlled by a threshold detector 20 sensitive to the level of the output signal of the amplifier of one of the photodiodes 18, 19 (the photodiode 18 being chosen in Figure 2 b ). In effect the additions to the system of Figure 2 a incorporate a noise gate or "squelch" circuit into the system. When the signal from photodiode 18 falls below a threshold predetermined at detector 20, the switches 18 a , 19 b operate to supply the constant, preset d.c. signals from 18 c , 19 c to the log amplifiers 21, 22 in place of the signals from the photodiodes 18, 19. The d.c. signals from 18 c , 19 c are so chosen as to provide a ratio signal at the output of circuit 23 similar in level to that which should be obtained with low signals from the photodiodes, but of course since the d.c. signals are constant then the output at 23 is more stable than would be obtained from low photodiode signals. When the output signal from photodiode 18 rises above the threshold value the switches 18 b , 19 b change-over so that the photodiode signals are applied to the log amplifiers 21, 22. Thus the effect of the noise gate is to minimise the risk that at low photodiode output levels the output of the circuit 23 will be a misleading output derived from system noise rather than low level but genuine photodiode signals. The amplifiers 18 a , 19 a are transimpedance amplifiers conveniently based upon National Semiconductor L.F. 355 operational amplifiers. The d.c. sources 18 c , 19 c may be fixed or variable resistors reducing the voltage from a system supply rail to a predetermined acceptable voltage. Conveniently the threshold detector 20 is based upon a National Semiconductors L.M. 339 comparator, the threshold value of which is adjustable by a potentiometer. Suitable electronic switches 18 b , 19 b are available as Siliconix D.G. 211. Figure 3 b illustrates graphically the effect of the noise gate. The upper curve of Figure 3 b illustrates the signal at the output of the circuit 23 in a system not equipped with the noise gate, while the lower curve illustrates the signal at the output of circuit 23 in a system having a noise gate and under the same input conditions. It can be seen that the start of combustion (x on the curves) is considerably more clearly defined in the lower curve. Inherently therefore the response of a system utilizing the ratio of the outputs of a pair of spectrally spaced detectors is significantly more rapid than that of the system of the kind disclosed in United States specification 4381748. Moreover, a system utilizing the ratio of the signals from a pair of detectors of spaced spectral response is considerably less sensitive to light attenuation arising from contamination of the "window" through which the combustion event is viewed. Although "window contamination" does not constitute a true neutral density filter, it can, in many circumstances approximate to a neutral density filter, and this of course will not affect the ratio derived from the circuit 23 whereas it would affect the intensity measurement produced by the system of United States specification 4381748. It follows therefore that the sharp onset of the "colour signal" perceived by the transducer assembly 13 and its associated circuitry provides an ideal control signal for passing the start of combustion parameter to an engine control system, the control signal being one which does not require knowledge of previous combustion events to provide a threshold level at which the signal is passed to the engine control system. In some applications for example where signal strength is low in relation to noise on the signal it is preferable to use the peak-to-peak threshold detector 24 in order to derive a control signal to be passed to the engine control system. As an alternative a comparator, for example Schmdt trigger arrangement, could be used to observe the signal and to produce and output when the value of the signal exceeds a predetermined reference value. As yet further alternative the signal could be differentiated in themicroprocessor control of the system which then looks for a predetermined value of gradient and uses its existence to signify the start of combustion. While in the above example the signal representing the ratio is in the form of the logarithm of the ratio it will be appreciated that this is simply for convenience of electrical processing. When used in association with a self igniting reciprocating piston internal combustion engine (for example a diesel engine) the derived control signal may be used by the engine control system to determine the point in the rotation of the crank-shaft of the engine at which fuel is injected into the respective cylinder of the engine. If desired each cylinder of a multi-cylinder engine could be similarly monitored, although more probably the parameters monitored in one cylinder will be used to produce a control signal for all cylinders. Figure 3 a illustrates graphically the signals produced by the photodiodes 18, 19 (curves 1 and 2 respectively) when "viewing" a single combustion event. The corresponding ratio signal produced by the circuit 23 is represented by curve 3. It can be seen that curve 3 exhibits an extremely steep leading edge and a marked peak and thus provides an ideal waveform for determining the start of combustion. Vertical line A in Figure 3 a represents a datum point within the rotation of the crank-shaft and in so far as the engine control system is concerned a corresponding datum signal may be supplied from a crank-shaft angle transducer of known form. Vertical line B represents the point at which fuel is injected into the cylinder, and the timing between A and B is controlled by the engine control system in accordance with the control signal representing start of combustion. Vertical line C represents the start of combustion signal which in practice is determined as mentioned above. The spacing between line B and line C reflects the delay between injection of fuel into the cylinder and combustion reaching a point at which the start of combustion signal is generated. Having determined the point in crank-shaft rotation, relative to the datum, at which combustion has commenced in one cycle, the point at which fuel is injected in the next cycle can be adjusted to the optimum point in relation to the crank-shaft datum. It would be recognised that the delay period between injection and start of combustion will depend upon fuel quality and other factors such as the fuel spray pattern, the cylinder temperature, and the presence of residual gases either through inefficient purging of the cylinder, or through deliberate recirculation of exhaust gases. The optimum point of start of combustion will be known, and by comparing the actual start of combustion with the known optimum, the subsequent injection of fuel in the next cycle can be timed such that start of combustion occurs at the optimum point. Measurement of the peak height of curve 3 may be considered as a measurement of peak temperature, and this parameter could be used in the control system of a diesel engine. Similar monitoring of the combustion event can be applied to a spark ignition reciprocating piston internal combustion engine (a petrol engine) where the combustion monitoring can provide information for the control of many more parameters than would normally be concerned in the control of a diesel engine. Basically the engine management system of a petrol engine will control the spark timing and the fuel/air mixture ratio. It has been previously proposed to use a start of combustion signal in a petrol engine as a control parameter for determining spark timing. Previous proposals have suggested monitoring cylinder pressure or cylinder temperature to achieve a start of combustion signal, but as described above in relation to a diesel engine a control signal derived from the ratio of the outputs of a pair of spectrally spaced optical detectors has significant advantages. It has previously been proposed to control fuel/air mixture on the basis of exhaust gas analysis, but of course this is a relatively slow response system, and necessitates the use of complex and therefore expensive exhaust gas analysis equipment. It has been determined however, that the colour ratio signal derived from a colour sensor as described above exhibits a well defined envelope as shown in Figure 5, comprising an initial rise to a first peak A, followed by a falling characteristic reaching a minimum B, before rising again to a further, less well defined, peak C. The magnitude of a ratio at point B is found to correlate very closely with the proportion of oxides of nitrogen (NOx) in the exhaust emission of the engine. This correlation is particularly useful in the so called "lean-burn" operating conditions of a petrol engine and thus the ratio signal derived from the outputs of the colour sensor can be considered as a measure of the NOx emissions of the engine, and thus can be used as a control parameter in a closed loop feed-back system controlling the fuel/air mixture supplied to the engine. It will be appreciated that the two upper curves of Figure 5 are each the outputs from the associated photodiodes 18,19 respectively. Figure 4 illustrates in diagrammatic form a convenient monitoring system for supplying control parameters to a petrol engine management system. The photodiodes 18, 19 produce individual output signals which are amplified by the respective log amplifiers 21, 22 and which are passed to a subtraction circuit 23. Subtraction circuit 23 is fed from a threshold circuit 30 which responds to the existence of a real light intensity signal (in this case photodiode 18). This signal serves to gate circuit 23 to a peak detector circuit 31, which determines the minimum value equivalent to B of Figure 5. It will be appreciated that the envelope in Figure 5 is a true ratio, while the output of circuit 23 is the logarithm of the ratio and so is a compressed version. The magnitude of the signal from peak detector 31 is representative of the amount of NOx emitted from the engine exhaust. Figure 6 provides an illustration of the control of Air Fuel Ratio (A.F.R.) in an internal combustion engine. The internal combustion engine is illustrated in Figure 6 as item 46 and it incorporates a plurality of sensors 45 which provide output signals representative of rotational speed, crank shaft angle, and engine load. The combustion process is indicated in Figure 6 by item 53 and the light output of the combustion process is supplied to a measurement circuit in the manner described above. The measurement circuit is indicated at 41 and actually comprises parts of the circuit described above with reference to Figure 4. We have described above how a signal representative of NOx is derived, and this signal is output by the measurement circuit 41 as signal KTact. A summing point 43 subtracts KTact from a reference signal KT derived from a three dimensional map 42 stored in the memory of the system, to produce an error signal KC. Predetermined locations of the map 42 are referenced by speed and load signals applied thereto from the sensors 45. The output signal from the map 42, dependent upon the input speed and load signals, is supplied by way of a roughness limit circuit 44 to the summing 43. The error signal KC is fed to an A.F.R. correction map 47. As with the map 42, the map 47 is stored in memory and locations thereof are referenced by speed and load signals from the sensors 45, applied by way of an input line 47a. The resultant output from the A.F.R. correction map 47 is summed, at a summing point 48, with a reference value derived from an A.F.R. map 49 again stored in memory. As with the other maps, locations of the A.F.R. map 49 are referenced by speed and load signals from the sensors 45 and the output of the map 49 is processed by a roughness limit circuit 51 before being fed to the summing point 48. The output from the summing point 48 is a corrected A.F.R. signal which is then applied to the fuelling control arrangement 52 of the engine 46. While the sensors 45 supply speed and load signals to the maps 42 and 49, they supply signals representative of speed and crank shaft angle to the measurement circuit 41. The "target" maps, that is to say the map 49 of air fuel ratio, and the map 42 of NOx are established in advance by the engine designer. The system illustrated in Figure 6 provides a means by which the target value of NOx can be maintained in the presence of engine to engine and day to day variability. It is recognised that a very substantial cycle to cycle variation in NOx production occurs, and thus to obtain a representative control signal the system, which is a microprocessor system, will filter the output of circuit 31 in Figure 4. It will be recognised that NOx control is achieved without the need for exhaust gas analysis. Within a lean burn control system of an internal combustion engine it is necessary to control the air fuel ratio to a value close to the lean misfire limit. The outputs of the "roughness" detection circuit 38 in Figure 4 and the abnormal combustion circuit 39 also shown in Figure 4, are used to alter the output values of A.F.R. and KT applied to the summing points 48 and 43 respectively in the direction of enrichment. Such enrichment is proportional to the magnitude of the "roughness" or "abnormal combustion" signals. The "roughness" signal is employed by noting the statistical spread of the signal since this is representative of the cyclic variability in the combustion process. As the engine fuelling approaches the lean misfire limit the standard deviation of the point of peak intensity within the combustion cycle (the output of circuit 38) increases.

Claims: A method of controlling operation of a reciprocating piston internal combustion engine comprising deriving a signal representative of the oxides of nitrogen (NOx) content of the exhaust of the engine by deriving first and second electrical signals representative of light intensity of the combustion event within a cylinder of the engine by means of first and second optical transducers (18, 19) of spaced spectral response, said first and second optical transducers observing the combustion event at a common location, or at closely spaced locations, establishing a signal representing the ratio of said first and second electrical signals, identifying a minimum in the value of said ratio signal between a first peak value which follows a steep initial rise, and a second peak value, and, utilising the magnitude of said minimum, which is representative of the NOx content of the exhaust of the engine, as a fuel/air ratio control parameter, and adjusting accordingly the fuel/air ratio of the fuel/air mixture supplied to the engine. Apparatus for controlling operation of a reciprocating piston internal combustion engine including light conduction means having one end exposed within the combustion chamber of a cylinder of the internal combustion engine and its opposite end communicating with sensor means for deriving a signal representative of the oxides of nitrogen (NOx) content of the exhaust of the engine and means for adjusting accordingly the fuel/air ratio of the fuel/air mixture supply to the engine, said sensor means comprises first and second optical transducers (18, 19) of spaced spectral response, said first and second optical transducers observing the combustion event, in use, at a common location, or at closely spaced locations, means (23, 24) for establishing a signal representing the ratio of said first and second electrical signals, and means identifying a minimum in the value of said ratio signal between a first peak value which follows a steep initial rise, and a second peak value, the means for adjusting the fuel/air ratio utilising the magnitude of said minimum of said ratio signal, which is representative of the NOx content of the exhaust, as a control parameter for controlling the fuel/air ratio. A method as claimed in Claim 1 or an apparatus as claimed in Claim 2 wherein said first and second optical transducers (18, 19) exhibit a peak response at 600 nm and 875 nm respectively.

Inventors: EXTANCE, PHILIP, HAZELDEN, ROGER JOHN, NUTTON, DAVID
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: F02D35
IPC codes: G01J1, F02B1, G01N21, G01L23, G01M15, F02D41, F02D35