GB2131177A

GB2131177A - Flame front sensor

Info

Publication number: GB2131177A
Application number: GB08233376A
Authority: GB
Inventors: Ma Thomas Tsoi-Hei
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1982-11-23
Filing date: 1982-11-23
Publication date: 1984-06-13
Also published as: GB2131177B

Abstract

A flame front sensor in cylinder 10 of an I.C. engine comprises two probes 14a and 14b staggered in the direction of flame propagation. The difference between the output signals of the two probes is taken as indicative of the arrival of the flame front when this difference exceeds a predetermined level. Because the difference is taken, the output signal background level is substantially independent of the level of ionisation within the combustion chamber prior to the arrival of the flame front at the sensor. <IMAGE>

Description

SPECIFICATION Flame front sensor The present invention relates to a flame front sensor.

lonisation probes have been used for some time in combustion research where the combustion of gaseous mixtures is to be detected and quantified.

This has been done by applying a voltage across a small gap located within the combustion space and measuring the change in ionisation current through the gap as the ionisation in the gas increases with combustion.

Figure 1 shows a piston 12 in a cylinder 10 with an ionisation probe 14 in the combustion chamber. The probe 14 is connected in series with a resistor 16 and a voltage source 20 which may be the vehicle battery but which may alternatively be a source of higher voltage, e.g. 400V.

Prior to the arrival of the flame front, there is only a small current across the gap but on arrival of the flame front the current increases rapidly causing a sharp voltage drop. By setting a suitable triggering threshold, it is possible to determine the time of arrival of the flame front by the instant at which the threshold is exceeded. Thus the ionisation signal can provide information on the time of arrival of the flame front and can also indicate the level of ionisation after the flame front has passed the probe at which time the probe is permanently in the burnt gas zone. The arrival time measured from spark indicates the flame propagation speed while the level of ionisation indicates the intensity of combustion.

Various systems have been proposed in the past in which the combustion characteristics inside the combustion chamber have been monitored and used as input data in a closed loop feed back control system to vary spark time and air-fuel ratio to maintain the engine running continuously at optimum conditions. The present applicant in particular has proposed a system using an in-cylinder ionisation probe to detect flame arrival time as the input parameter. This has the advantage over other systems that the ionisation probe is used merely to identify the time of flame arrival and not used to quantify the flame intensity. Because of this, the probe does not need to be calibrated for ionisation current, such calibration being difficult to achieve as the current is affected by several factors including probe design, temperature and pressure as well as chemical combustion effects.

Figure 2 shows the signal under good operating conditions when the signal to noise ratio is high for a well defined flame front. Figure 3 shows a worse case where the signal to noise ratio is reduced due to poorflame ionisation and high background leakage.

In practice the signal to noise ratio is poor under light engine loads, at weak mixtures and in conditions where a high background leakage current prevails. This clearly affects the sensitivity of the detecting system and failure to maintain a high and consistent detection sensitivity over a wide range of engine operating conditions prevents successful development of an incylinder sensor suitable for feedback engine control applications.

With a view to mitigating the foregoing disadvantages, the present invention provides a flame front sensor comprising two ionisation probes staggered from one another in the direction of flame propagation and means for determining the difference between the output signals of the two probes to produce an output signal indicative of the arrival of the flame front.

It is important to ensure that the flame front does not arrive simultaneously at both probes. When only two probes are provided in a plug designed to be inserted into a cylinder head in the same fashion as a spark plug, one cannot be certain of the orientation of the two probes when the plug is fully screwed home. In order to mitigate this problem, it is possible to provide three probes in the same plug and to use either the two probes furthest staggered apart in the direction of movement of the flame front, the latter being assumed to propagate from the spark plug, or the outputs from all three probes may be suitably combined electronically such that the nett output is substantially independent of probe orientation.For example, if the signals from the respective probes are designated A, B and C with the signal A being from the first probe nearest the spark plug, the two different signals A - B and A - C may be evaluated and summed to produce a signal having a sharp leading edge indicating the time of arrival at the first probe in a manner which is substantially independent of the orientation of the plug.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is, as previously described, a schematic diagram of a conventional flame sensor, Figures 2 and 3 show the output voltage of the flame sensor and at different operating conditions, Figure 4 shows schematically an arrangement in accordance with the invention, Figure 5 shows the two output voltages at the points A & B in Figure 4, and Figure 6 shows the plot of the different signal used to detect the arrival of the flame front.

In Figure 4, each of the probes 14a and 14b generates an output signal generally similar to that produced by a conventional probe. However, because the probes are staggered with respect to one another in the direction of flame propagation, the flame front arrives at the two probes at slightly different times so that the two signals A & B derived from the two probes have a phase delay. These two signals are subtracted from one another in a differential amplifier 18 the output signal from which is shown in Figure 6. Because of the common mode rejection, regardless of the general level of ionisation, the difference between the two signals A & B prior to arrival of the flame front would be generally low.When the flame front is at the first probe but not at the second, then there will be a sharp difference between the two signals, this signal exceeding the trigger threshold. When however the flame reaches the second probe then once again the output from the two probes would be generally matched and will tend to cancel one another out. Consequently, the output of the differential amplifier is, as shown in Figure 6, a sharp pulse coinciding with the arrival of the flame front but the background noise level is substantially independent of the level of ionisation prior to and after the arrival of the flame front.

It is necessary, in order to achieve the time delay, for the two electrodes to be staggered relative to one other in the direction of the flame propagation. It is envisagedthattwo probes will be mounted in a common plug the body of which is to be screwed into the cylinder head. In order to ensure that the plug will contain two probes staggered in the direction of flame propagation when the plug is fully screwed home, it is possible to provide three probes in each plug and to select from amongst these three the two most staggered in the direction of flame propagation. Alternatively, the outputs from all three probes may be suitably combined in an electronic circuit.

Claims

1. A flame front sensor comprising two ionisation probes staggered from one another in the direction offlame propagation and means for determining the difference between the output signals of the two probes to produce an output signal indicative of the arrival of the flame front.

2. A flame front sensor as claimed in Claim 1, wherein the ionisation probes are mounted within a common plug.

3. A flame front sensor as claimed in Claim 2, wherein the plug comprises three probes disposed in such a manner that at least one pair of probes is staggered in the direction of flame propagation regardless of orientation of the plug relative to the cylinder head.

4. A flame front sensor constructed, arranged and adapted to operate as hereinbefore described, with reference to and as illustrated in Figure 4 of the accompanying drawings.