GB2319344A

GB2319344A - Monitoring combustion quality in an internal combustion engine

Info

Publication number: GB2319344A
Application number: GB9623509A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1996-11-12
Filing date: 1996-11-12
Publication date: 1998-05-20
Also published as: GB9623509D0; WO1998021472A1

Abstract

A system is disclosed for monitoring the quality of combustion in a cylinder of a spark ignition internal combustion engine having an ignition coil connected by a high tension lead to the spark plug of the cylinder. The system comprises an isolator 22 arranged in the high tension lead 24 in series with the gap 28 of the spark plug for allowing spark energy to be transmitted to the spark plug from the coil and isolating the spark plug from the coil after a spark has been generated. A circuit 34 monitors the decay of the voltage on the spark plug by leakage of charge through the burnt gases in the combustion chamber after each spark has been extinguished. The isolator 22 comprises a pair of spaced electrodes arranged in a sealed chamber between which electrodes an arc is struck when spark energy is transmitted through the isolator to the spark plug gap.

Description

MONITORING COMBUSTION QUALITY The present invention relates to a system for monitoring the quality of combustion in a cylinder of a spark ignition internal combustion engine having an ignition coil connected by a high tension lead to the spark plug of the cylinder.

It has been proposed during research conducted at Brunel University to monitor the combustion quality in an engine cylinder by isolating the centre electrode of the spark plug from the coil in some way after the combustible charge has been ignited and to monitor the rate of decay of the residual charge on the centre electrode of the spark plug as a result of leakage through the ionised gases in the combustion chamber. Using this technique, it has been found that the better the combustion quality (as determined by pressure measurements) the faster the rate of decay of the residual charge and hence the voltage on the centre electrode.

The research conducted at Brunel University used a high voltage diode as an isolator. The diode conducts naturally to allow spark energy to flow from the coil to the spark plug but it prevents the residual charge on the centre electrode of the spark plug from flowing back towards the coil. While such a construction is adequate to prove the viability of the method of monitoring combustion quality, it is not suitable for use in a motor vehicle engine because of the high cost of high voltage diodes and more seriously because such diodes would not be able to operate reliably for prolonged periods of time.

According to the present invention, there is provided a system for monitoring the quality of combustion in a cylinder of a spark ignition internal combustion engine having an ignition coil connected by a high tension lead to the spark plug of the cylinder, comprising an isolator arranged in the high tension lead in series with the gap of the spark plug for allowing spark energy to be transmitted to the spark plug from the coil and isolating the spark plug from the coil after a spark has been generated at the spark plug gap and means for monitoring the decay of the voltage on the spark plug as a result of leakage of residual charge from the spark plug through the burnt gases in the combustion chamber after the spark has been extinguished, characterised in that the isolator comprises a pair of spaced electrodes arranged in a sealed chamber between which electrodes an arc is struck when spark energy is transmitted through the isolator to the spark plug gap.

The sealed gap isolator of the present invention offers several advantages over a high voltage diode not only of lower cost and improved reliability, but also improved performance. Once an arc has been struck in the isolator, its resistance to current flow is less than that of a diode thus allowing a higher current to flow in the spark plug gap. The total breakdown voltage of the two gaps (i.e. the isolator and the spark plug) in series with one another is increased, allowing a higher HT voltage to be developed before breakdown occurs thus increasing the total energy discharged at the spark plug gap. Once the arc has been extinguished, the spark plug is completely isolated from the coil and the charge decay occurs entirely on account of leakage through the ionised gases in the combustion chamber.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic circuit diagram of a system of the invention.

In the drawing, there is shown an ignition system having a conventional low tension circuit comprising a primary winding 10 of the ignition coil, a DC supply 14, a switch 12 (which may be a mechanical or an electronic switch) and a capacitor 16 connected across the switch 12. While the switch 12 is closed, energy is built up by the gradual increase of current in the primary winding 10 of the ignition coil and when the switch 12 is opened, the sudden breakdown of the current in the primary winding 10 induces a high voltage in the secondary winding 20 of the ignition coil which is transmitted by the HT lead 24 to the centre electrode 26 of the spark plug. The current flowing across the gap 28 of the spark plug to the earth electrode 30 produces a spark which ignites the mixture in the combustion chamber.

The combustion quality monitoring system includes an isolator 22 connected in the HT lead 24 in series with the spark plug gap 28 and a voltage monitoring circuit 34 that is coupled at 32 to the section of the HT lead 24 lying between the isolator 22 and the electrode 26 of the spark plug. The coupling between the voltage monitoring circuit 34 and the centre electrode 26 of the spark plug is preferably capacitative or inductive to avoid direct contact with the high tension and to avoid charge leakage through the monitoring circuit 34.

The isolator 22 comprises two spaced electrodes in a sealed chamber. An arc is struck between the two electrodes once a predetermined breakdown voltage has been applied. In the absence of an arc, the resistance of the isolator is infinite but once an arc has been struck, the resistance of the isolator is negligible and all the energy from the coil is transmitted to the spark plug.

Because of the inherent capacitance of the centre electrode 26 of the spark plug and the length of HT lead 24 connected to it a residual charge is retained on the centre electrode after the spark has been extinguished. The rate of decay of the residual charge depends on and is indicative of the quality of combustion. If the gases in the combustion chamber are left highly ionised because of good combustion, the decay will be very rapid whereas as combustion quality deteriorates, the decay time is prolonged. The waveform of the signal received by the voltage monitoring circuit 34 can therefore be analysed to determine combustion quality. The result of the analysis may be used purely to provide fault indication or it may be used in the control of the engine to avoid combustion instability for whatever reason, for example excessively lean or rich mixture strength, excessive exhaust gas recirculation, excessive spark timing setting or excessive valve timing setting. Such closed loop control can take into account operating parameters that are in themselves difficult to measure and to allow for, such as fuel grade and engine wear. Thus when attempting to use fuel recovered from a vapour trap, it is possible to maintain combustion quality by closed loop control despite the variability of the quantity of vapour stored in the trap and the rate at which it can be purged.

It is preferred that the polarity of the windings of the ignition coil be such that a positive high voltage is applied to fire the spark plug. In this way, the residual charge retained in the high tension lead is positive and decays at a rate dependent upon the concentration of negative ions in the gases within the combustion chamber.

As explained above, the rate of decay of the residual charge immediately after firing of an ignition spark for igniting the mixture during an engine cycle is indicative of the ignition quality at the start of the engine cycle, the slower the rate of decay the poorer the ignition quality.

One can therefore set a lower threshold rate of decay in order to indicate misfires.

It is possible additionally to fire a second monitoring spark at a preset time after the first ignition spark during each engine cycle. In this case, the rate of decay of the residual charge immediately after firing of the second spark is indicative of the average combustion quality throughout the engine cycle. Once again, the slower the rate of decay, the poorer the combustion quality and one can set a lower threshold rate of decay in order to indicate partial burn cycles.

The voltage monitoring circuit may, if desired, compute the standard deviation of the rates of decay of the residual charge for a population of engine cycles from each engine cylinder and in this way derive a measure of the combustion instability of each individual engine cylinder.

If the monitoring system of the invention is used in the control of a multi-cylinder engine then the data gathered from one cylinder may either be used exclusively in the control of that individual cylinder or for several or all of the cylinders of the engine. A further possibility is to derive data from several cylinders and to combine the data in order to arrive at a single the optimum setting for all the engine cylinders. The data may either be used as an error signal in a closed loop control system or to update stored calibrations in an open loop control system.

The invention lends itself particularly to engines in which a separate ignition coil is provided for each spark plug and in which the coil and the HT lead are built into the connector for the spark plug, thereby dispensing with the need for a separate high tension lead. In this case, the isolator and the monitoring circuit can be built integrally into the "coil-on-plug" module. If there should not be sufficient natural capacitance at the centre electrode of the spark plug because of the small size of the module, then it is possible to build a capacitor into the module to achieve a readily measurable rate of decay.

Claims

1. A system for monitoring the quality of combustion in a cylinder of a spark ignition internal combustion engine having an ignition coil connected by a high tension lead to the spark plug of the cylinder, comprising an isolator arranged in the high tension lead in series with the gap of the spark plug for allowing spark energy to be transmitted to the spark plug from the coil and isolating the spark plug from the coil after a spark has been generated at the spark plug gap and means for monitoring the decay of the voltage on the spark plug as a result of leakage of residual charge from the spark plug through the burnt gases in the combustion chamber after the spark has been extinguished, characterised in that the isolator comprises a pair of spaced electrodes arranged in a sealed chamber between which electrodes an arc is struck when spark energy is transmitted through the isolator to the spark plug gap.

2. A system as claimed in claim 1, wherein the spark plug has a centre electrode connected to the ignition coil and an earth electrode and wherein the polarity of the ignition coil windings is such as to apply a positive voltage to the centre electrode of the spark plug relative to the earth electrode.

3. A system as claimed in claim 1 or 2, wherein the means for monitoring the decay of the voltage on the spark plug is coupled to the spark plug by means of a capacitative or an inductive coupling.

4. A system as claimed in any preceding claim, wherein, during engine operation, two sparks are fired during each engine cycle, the first timed to ignite the mixture and the second timed to occur during the bulk combustion period, the voltage monitoring circuit serving to monitor the charge decay after each of the sparks has been extinguished.

5. A system as claimed in any preceding claim, wherein the voltage monitoring circuit is operative to compute the standard deviation of the rates of decay of the residual charge for a population of engine cycles from each engine cylinder in order to derive a measure of the combustion instability of each individual engine cylinder.

6. A system for monitoring the quality of combustion in a cylinder of a spark ignition internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.