GB2286888A

GB2286888A - Capacitive combustion sensor

Info

Publication number: GB2286888A
Application number: GB9403465A
Authority: GB
Inventors: Miles Edwin Grimshaw Upton; James Digby Yarlet Collier; Richard James Guest
Original assignee: Cambridge Consultants Ltd
Current assignee: Cambridge Consultants Ltd
Priority date: 1994-02-23
Filing date: 1994-02-23
Publication date: 1995-08-30
Also published as: WO1995023288A1; AU1714395A; GB9403465D0

Abstract

In order to sense the onset of combustion e.g. in a diesel type engine, a probe 2 to which an alternating voltage is applied is provided in a combustion space 1 within a cylinder head. At an appropriate point fuel 8 is injected into the combustion space and combusts. At this point the dielectric properties of the fuel/air mixture change because the fuel/air mixture breaks up to form a plasma of ions and radicals. This causes a change in the capacitance of the probe and hence causes the current flowing therein to undergo a change in phase. This change in phase is detected 9, 10, 11 and provides an accurate indication of the start of combustion within the cylinder. Further monitoring of the probe capacitance can provide information concerning the further progress of combustion, the composition of the combustion products, likely emissions and so forth which may be used to control the fuel supply to the engine. <IMAGE>

Description

A SENSOR This invention relates to combustion sensing, and in particular to the development of timing signals that are indicative of the start of combustion in a combustion chamber of an internal combustion engine. The invention also relates to the development of signals indicating the progress of combustion, and the composition of combustion products.

Manufacturers of internal combustion engines would like to obtain accurate information about the exact time of start of combustion (SOC) within a cylinder head. This would enable precise control of injection timing which would increase engine efficiency and reduce emissions.

An accurate and reliable technique for sensing SOC would make other sensors fitted to engines redundant, potentially reducing the cost of the finished engine.

For example, needle lift sensors are currently used to obtain an approximation to start of combustion, but these would become unnecessary.

One known technique for sensing the onset of combustion is pressure sensing. However, the large change in pressure produced by the compression stroke may mask the change in pressure resulting from the combustion, making pressure sensing difficult and expensive. Alternative techniques employ optical or "electrostatic" sensing, as described for example in US 4,463,729. Optical sensors suffer from the build up of soot around the window through which the light from the combustion passes. The "electrostatic" sensor of US 4,463,729 senses the current between two electrodes, effectively measuring the resistance of the combustion mixture. This sensor uses the effect that the combustion mixture, normally nonconductive, becomes a plasma of charged particles (ions) during combustion.The same effect is proposed for use in spark ignition engines in a paper "Flame Arrival Sensing Fast Response Double Closed Loop Engine Management" by Michael G. May, Society of Automotive Engineers (SAE) Paper 840441, 1984. Over time, however, as conductive soot builds up in the cylinder, the resistance change between the electrodes due to combustion ions becomes harder to detect, and hence the accuracy and reliability of the sensor decreases.

The inventors have realised that, rather than using a sensor which effectively measures the direct current resistance between the electrodes of a probe, an alternating current sensor which is capable of detecting changes in the permittivity at an electrode could be used to detect the start of combustion.

The invention provides a method of sensing combustion by detecting variations of permittivity in a combustion space. This may be achieved, for example, by detecting the variation in electrical reactance of a probe provided in the combustion space. Methods in accordance with the invention are less susceptible to the degradation of signals due to sooting up of the transducers, which can be a problem in resistance-based and optical sensors.

Moreover, the measurements of permittivity can yield more information about the quality as well as the timing of combustion.

The invention further provides a method of internal combustion engine control wherein the onset and/or further progress of combustion is monitored by detecting variation in the permittivity of the combustion gases, and control parameters of fuel supply are varied in response to the results of said monitoring. The onset of combustion may be detected, for example, by a sudden change in permittivity and fuel injection timing controlled accordingly. On the other hand, the composition of combustion gases may be monitored during combustion and the timing and the quantity of fuel supply controlled accordingly, for example to reduce smoke emissions. Future diesel injection systems (so called "common-rail" systems) may allow control of injection pressure, in addition to timing and quantity of fuel, to allow increased control of the combustion process.

The invention further provides a combustion sensor comprising, for example, a probe provided in a combustion space and means responsive to variations in the electrical reactance of the probe. Said means may comprise means responsive to phase changes in an alternating current flowing in the probe relative, for example, to the phase of an alternating voltage applied to the probe.

The invention yet further provides an engine management system and engine management unit incorporating methods and sensors in accordance with the invention as set forth above.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of an engine management system including a sensor for generating a start of combustion signal according to one embodiment of the present invention.

Fig. 2A is a schematic diagram showing a suitable probe arrangement that can be used in the embodiment of Fig.

1.

Fig. 2B shows an equivalent electrical circuit of the arrangement shown in Fig. 2A.

Fig. 3 shows in more detail the elements of the phase monitor and the signal processing units shown in Fig. 1.

Fig. 4 shows in more detail the elements of the phase monitor and the signal processing units in a digital embodiment.

Figs. 5, 6 and 7 show alternative sensor arrangements.

Figure 1 shows a system for controlling the combustion of fuel and air in the combustion space 1 of a compression ignition (diesel type) engine in which a combustion sensing probe 2 is provided. The combustion space 1 is contained above a piston 3 in a cylinder, and a piston 3 is connected to the crankshaft 4 by a connecting rod 5. A fuel injector 6 is driven by a fuel pump 7 to cause the injection of liquid fuel 8 into the combustion space 1.

The combustion sensing probe 2 is connected to a phase monitor 9 which in turn is connected to a signal processing unit 10. This is connected to an engine management unit 11 which controls the operation of the fuel pump 7. A detector 12 coupled with a detector mark 13 on the crankshaft 4 provides positional information to the engine management unit 11.

When the piston 3 is forced up the cylinder it compresses the air in the combustion space 1, which causes an increase in air temperature. When the piston reaches a certain point (usually around top dead centre or "TDC") fuel is injected by the fuel pump 7 and injector 6 into the combustion chamber 1. As the injected fuel disperses, the fuel/air mixture reaches a stoichiometric concentration which results in the fuel combusting due to the heat of the air. During combustion, ions are formed which change the electromagnetic properties of the mixture within the combustion chamber 1, including in particular the electric permittivity (dielectric constant).

Within the combustion chamber 1, there is provided a probe 2 that has an alternating voltage applied to it.

The change in the dielectric properties of the mixture around the probe causes a change in reactance of the probe 2 at the time of combustion, and so causes a change in the phase of an alternating current flowing through the probe 2. The phase of this current is monitored and tracked within the phase monitor unit 9 and the resulting signal is fed into the signal processing unit 10 which calculates the time at which start of combustion occurred and sends this information on to the engine management unit (EMU) 11. With this information, and the information about the load, speed, demand etc, the EMU can improve the control of the fuel injection timing for subsequent cycles.

Figure 2A shows a probe 2 comprising a screw-threaded plug 20 through which passes an electrode 22. The electrode 22 is embedded in an insulating, heat and pressure resistant material 26 which acts as a seal against combustion gases within the cylinder space. The tip of the electrode 22 is open to the combustion space, in a region where combustion is likely to begin. Another electrode 24 of the probe is formed by the inside of the plug 20 and the cylinder head metal, connected for example through the threaded plug 20. An alternating voltage source 28 is connected across electrodes 22 and 24.

Referring to Figure 2A, an alternating voltage vac from the source 28 causes an alternating current iaC to flow in the probe electrodes 22, 24. An equivalent electrical circuit of the probe arrangement is shown in Figure 2B and is a simple variable capacitance C connected in parallel with a variable resistance R connected across the voltage source vac. The electrodes 22 and 24 of the probe act as the plates of a capacitor, and the dielectric between the plates of the capacitor is the fuel/air mixture. At the start of combustion this mixture breaks up to form a plasma of ions and radicals which results in a change in the dielectric properties of the mixture. This change causes a change in the reactance C of the probe and hence causes the current i flowing therein to undergo a change in phase.This change in phase can be detected and hence the time at which combustion occurred can be calculated.

Figure 3 shows in more detail the phase monitor unit 9 and signal processing unit 10 of the system of Figure 1, according to a first embodiment. The probe 2 is shown at the left-hand side, connected to the phase monitor 9.

An output of the phase monitor 9 is connected to the input of a high pass filter 30. The output of the high pass filter 30 is connected through a buffer amplifier 32 to an input of a threshold detector comprising a comparator 34 and a voltage reference source. The output of the threshold detector is connected to the engine management unit 11, shown in Figure 1.

Within the phase monitor 9 in Figure 3, the phase of the current flowing through a known impedance Z, for example a known resistance, is compared with the phase of the voltage source 28 by a phase detector 29. The resulting signal is a voltage dependent on the phase difference between the current in the probe lac and the voltage applied vac. This signal from the phase detector 29 is then applied to the signal processing unit 10 where it is high-pass filtered to remove the slowly varying changes in the capacitance of the probe. These may be caused, for example, by the motion of the metal pistons in proximity to the probe electrodes. The filtered signal is then amplified at 32 and applied to a threshold detector formed by the comparator 34 having a predetermined reference voltage Vref at the other input.

As shown at the right hand side in Figure 3, the resulting output from this comparator will be a pulse that starts at the time of onset of combustion (Tsoc).

The engine management unit 11 is thus able to control the timing and quantity of fuel injection to improve the performance of the engine. The control mechanisms for this purpose are known to those skilled in the art, for example as described in US 4463729, and the references cited therein. In addition to indicating Tsoc, the pulse may, by its duration (TD in Figure 3), also provide a useful indication of the progress of combustion, for example to monitor the composition of the combustion products.

Those skilled in the art will also appreciate that various types of fuel supply arrangements may be used in diesel engines. The supply means may for example comprise a separate pump and injector 6 for each cylnder as shown, but combined pump and injector devices are also known. Furthermore, "common rail" systems are also proposed, in which a single high pressure pump is provided, which supplies fuel via a common high pressure pipe to a number of individual injectors. The exact type used is of course not significant for an understanding of the present invention.

Fig. 4 shows an alternative "digital" embodiment, in which, after the phase signal has been filtered, it is converted into a digital signal by means of an A/D convertor 40. In this embodiment, subsequent signal processing is carried out within the EMU, which may also include the A/D convertor.

Having the phase signal waveform sampled and available in digital form within the EMU allows a greater variety of calculations to be performed. These may include a thresholding operation equivalent to that performed in the analogue domain in the circuit of Fig. 3, but may also include more detailed analysis of the pulse shape.

For example, changes in the gradient of the reactance curve may indicate the likely smoke emissions.

Such analysis, concerning the amplitudes, slopes and durations of various parts of the signal, can be used to obtain indications of the composition of the combustion products, since different ions and radicals have different effects on the dielectric properties of the gases.

For example, as the dielectric constant depends on ionic mass, measurements of reactance allow the mass of ions in the combustion chamber to be estimated. This has application in predicting smoke emissions which are associated with the presence of heavier ions in the combustion chamber, due to incomplete combustion of fuel.

Also in alternative embodiments, rather than monitor the phase signal for the entire duration of the combustion cycle, only the relevant portion of the combustion cycle could be monitored. This can easily be effected by using the signal which initiates the fuel injection to start the monitoring process for a set time, thus effectively "windowing" the phase signal or, alternatively, using the signal from the position sensor 12. In such an embodiment the sensor, phase detector and/or the comparator can be switched on only in the required period. Alternatively in the "digital" embodiment of Fig. 4 the A/D convertor output can be ignored for most of the time, and read only during this "window" period.

This has the particular advantage that the amount of processing required by the EMU is reduced.

The sensor in the above embodiments took the voltage across a known impedance Z and compared its phase with the phase of the voltage source. This is not the only way to produce a signal that is indicative of a change in permittivity of the combustion gases at the onset of combustion. Figures 5 to 7 show three other circuits that could be used to generate an appropriate signal.

Note that by including a thin insulating layer around the probe it is possible to minimise changes in resistance of the probe due to sooting during combustion. This does not detract from ability to measure reactive effects, and may simplify the detection circuitry. Preferably the material of the layer should be selected to have a low dielectric onstant, and the probe and layer design should be such as to minimise soot build-up.

Figure 5 shows a circuit which is similar to the one used in Figures 3 and 4 except that the probe in this case acts as an unknown impedance in a bridge circuit.

In Figure 6, a reference capacitor CREF is charged from the variable capacitance of the probe 2. As the permittivity changes within the combustion chamber, the capacitance of the probe relative to the reference capacitor changes and consequently the output voltage changes.

Figure 7 shows an alternative technique where a change in the permittivity of the combustion gases causes a change in the output frequency of an oscillator. This output signal is then compared with a reference frequency using a phase detector to produce a voltage signal dependent on the permittivity of the combustion gases.

Frequency changes could also be detected by other, more direct means.

Although the invention has been described in the context of a compression ignition type of engine, such as a diesel engine, it will be appreciated that accurate and reliable combustion sensing can also be of benefit in other situations. The paper by Michael May referred to above, for example, illustrates the value of such a sensor in a spark ignition engine management system.

These and other variations will be apparent to the skilled reader, and it should be understood that the invention is in no way limited to the particular embodiments described herein.

Claims

1. Sensing combustion by detecting variations in permittivity in a combustion space.

2. Sensing combustion by detecting variations in electrical reactance of a probe provided in the combustion space.

3. A method according to Claims 1 or 2 applied in the combustion space of an internal combustion engine.

4. A method of internal combustion engine control wherein the onset and/or further progress of combustion is monitored by detecting variations in the electrical reactance of a probe provided in the combustion space, and control parameters of fuel supply are varied in response to the results of said monitoring.

5. A method according to Claim 4, wherein onset of combustion is detected by a change in said permittivity, and fuel injection timing is controlled accordingly.

6. A method according to Claim 4 or 5 wherein the composition of combustion gases is monitored during combustion and the timing and/or quantity of fuel supply is controlled accordingly.

7. A method according to Claim 6 wherein said control is effected to reduce smoke emissions indicated by said monitored composition.

8. A combustion sensor responsive to variations in permittivity in a combustion space.

9. A sensor according to Claim 8 comprising a probe provided in said space and means responsive to variations in the electrical reactance of the probe.

10. A sensor according to claim 9 comprising means responsive to phase changes in an alternating current flowing in the probe.

11. A sensor according to Claim 9 or 10 comprising means for applying an alternating voltage to the probe and means responsive to phase differences between the applied voltage and a current flowing in the probe.

12. A sensor according to any of Claims 9 to 11 further comprising means for indicating a start of combustion in response to a change in reactance.

13. A sensor according to any of Claims 8 to 12 including means for discriminating the presence of different combustion products by reference to said variations.

14. An engine emissions monitor including a sensor according to any of Claims 8 to 13.

15. An engine management system for controlling the quantity and/or timing of fuel supply in an internal combustion engine, the system including a combustion sensor according to any of Claims 8 to 13.

16. An engine management system wherein control parameters of the engine are varied in response to combustion signals monitoring combustion in a space within the engine, and wherein a combustion sensor generates said combustion signals in response to changes in permittivity in said space.

17. A system according to Claim 16 wherein said combustion sensor detects changes in the electrical reactance of a probe provided in said space.

18. An engine management system as claimed in Claim 16 or 17 including means for indicating the onset of combustion in response to said changes.

19. An engine management system according to any of Claims 16 to 18 wherein the control parameters include timing and/or quantity of fuel supply.

20. An engine management system as claimed in any of Claims 16 to 19 including means for indicating a composition of combustion products in response to said changes.

21. An engine management system according to Claim 21 including means for reducing emissions by controlling quantity and/or timing of fuel supply in response to the indicated composition.

22. An engine management unit including signal processing means for receiving signals reporting changes in the electrical reactance of a probe provided in a combustion space, and for deriving from said signals indications of the onset and/or further progress of combustion in said space.

23. A method of sensing combustion substantially as described herein with reference to the accompanying drawings.

24. A combustion sensor substantially as described herein with reference to the accompanying drawings.

25. An engine management system substantially as described herein with reference to the accompanying drawings.

26. An emissions monitor substantially as described herein with reference to the accompanying drawings.