Ionization sensors for internal combustion engine diagnostics K. N. C. Bray and N. Collings

Legislation to ensure acceptable exhaust emission control in automobiles, and pressure for higher performance and better fuel economy all demand more precise understanding of the ignition process. This article reviews the important role of ionization sensors as research tools but questions whether, in the short term, they can be effectively used in on-board control systems.

An ever increasing fraction of the total cost of a new automobile is for the on-board electronics. While some of this is accounted for by entertainment systems, cruise control, and automatic braking systems, a greater part of the increase is due to the equipment which is required to meet the new stringent emissions targets that are being intro- duced, and to meet competitive fuel economy and performance require- ments. A variety of sensors are used to feed information about engine opera- tion to a central electronic controller. These features tend to be invisible to the driver, and it is striking that at a time of massive increases in complexity, the reliability of vehicles is also marked- ly improving.

Sensor requirements for now and the future Emissions are presently, and are likely to remain, a major issue. Although ex- haust catalysts for emission control have been used in virtually all cars in the U.S.A. for nearly two decades, alarm- ing evidence has appeared that, in ser- vice, the majority of catalysts fail before their design life has expired. New leg- islation, initially in California, will re- quire all vehicles to have on-board sys- tems to monitor continuously for the

Nick Callings, Ph.D., 1.Mech.E.

Graduated in mechanical engineering at the Universitv of Bristol. Since 1984 he has been a lecturer at the Engineering Department of the University of Cambridge. Dr Collings’ research interests lie mainly in the field of experimental internal combustion engine investigations, especially those relevant to pollutant forma- tion.

Ken Bray, B.A., M.Sc., Ph.D.

Studied engineering at the Universities of Cam- bridge, Princeton, and Southampton. Now Pro- fessor of Applied Thermodynamics at Cam- bridge, where his reseach interests are primari- ly related to combustion in turbulent flows, with application to internal combustion en- gines.

Endeavour, NW Series, Volume 15, No. 1.1991. 0150-9327/9199.00 +o.oo. Pergamon ROSS pk. Printed in Great Britain.

conditions that may lead to catalyst damage (chiefly misfire), and monitor the efficiency of the catalyst itself. It is inevitable that such legislation will be enacted shortly in all countries in which catalysts are used, which in effect means the whole of the Western World. (The present concern about our environment has meant that emissions legislation has appeared in the EEC countries much more quickly than was thought likely a few years ago.)

Fuel economy has always been an isue, and in fact in the U.S.A. the fuel economy of vehicles is actually pre- scribed by law (CAFE or Corporate Average Fuel Economy legislation). Engine performance is, in itself, an important issue for market competitive- ness, but it needs to also be borne in mind that for the same power require- ment a high specific output engine will also be smaller, lighter, and produce less emissions. For these reasons, it is very desirable to develop engines with the best possible performance trade- offs: in effect, engines in which the combustion event is as precisely deter- mined as possible.

A large range of sensors for use in the cylinder and the exhaust system have been suggested to promote the achieve- ment of the goals outlined above, but in this article a relatively unknown but important type will be introduced - those based on measuring the ionization activity in gases that are burning or have recently been burnt. A commonplace application of this phenomena is to de- tect flame status in automatic light-up central-heating boilers, and they are thus critical components for safe opera- tion. Although there is presently no production utilisation of ionization probes on automobiles, a great deal of ingenious research and development work has been accomplished with them.

The basic physics on which such sen- sors are based is not fully understood, but there is little doubt that the produc- tion of ions during the burning of hydro- carbons is mainly due to the interaction of the CH and 0 radicals to produce positively charged CHO and an elec-

tron. This occurs about once for each million carbon atoms taking part in combustion and, rather strangely, this ‘production rate’ is virtually indepen- dent of the form that the original hydro- carbon was in (propane, benzene, etc.). This fact is exploited in the instrument (ththeoflstame ionization detector) which is

universally used for the measurement of the unburnt hydrocar- bon emissions from vehicles.

In nearly all experimental work on engines which involves the measure- ment of ionization, a small insulated electrode is inserted into the gas, and it is electrically biased in order to attract an ionized species. Although in labora- tory work many fundamental studies have been made on the nature of io- nized gases by using a wide range of configurations, electrical biases, etc., it is fair to generalize that in the engine environment the probe is normally posi- tively biased, with a voltage in the range 10 to 300 volts. In this way electrons are attracted to the electrode, and the re- latively massive ‘inside’ of the engine forms the other electrode which com- pletes the circuit. This arrangement gives relatively high signal levels, as the electrons are much more mobile and more current can be collected per unit area on the electrode attracting them.

Before discussing some of the work that has actually been done, it is impor- tant to distinguish the three major cate- gories of ionization measurement. These are:

1. In-cylinder measurement of flame arrival and pre-ignition 2. In-cylinder measurement of post- flame ionization levels 3. Exhaust manifold measurement of ionization levels.

The ionization level in the flame front is many orders of magnitude greater than that in the post-flame gases, since the ions produced in the combustion itself rapidly recombine. Thus very different current levels may be expected from flame front measurements, though the actual differences are not as large as

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Pre-ignition reactions

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1 2 3 4 5 6 7

Time (ms)

Figure 1 Flame arrival probes. Normal and pre-ignition signals.

might be implied by ionization density differences.

It is unfortunate, but a uniform result from experience, that the absolute level of the ionization current from an en- gine-mounted probe is usually not of great significance. The reason for this is that the level is affected by a whole range of parameters, one of which - time between production and measure- ment - has already been noted. In addi- tion, air: fuel ratio, fuel additives, temperature and pressure (engine load) all affect the signal to a greater or lesser extent. No serious suggestion has yet been made that the signal level from such sensors could be used in a feed- back engine control loop in the way that, for example, in-cylinder pressure sensors could.

In-cylinder flame arrival and pre- ignition probes Easily the most famous use of probes for flame arrival was by S. Curry [l] who built an experimental engine in order to try and to resolve the conflict then (as now!) active, as to whether engine knock (detonation) was a result of the spontaneous explosion of an end gas region, or if it was due to the sudden acceleration of the flame front to very high velocities. Ionization probes are ideally suited to this application, as they have an extremely high frequency re- sponse .

Curry mounted no less than 49 prob- es, both on the cylinder head and the piston, and found that for his engine arrangement, knock was associated with a sudden acceleration of the flame front. It is worth noting that the alterna- tive hypothesis has also been found to be valid in other situations. Current thinking is that one or the other may occur, depending on only marginal changes in operating conditions.

The other major use of in-cylinder

probes is for the measurement of nor- mal flame development. In 1984 M. G. May [2] developed the idea that the time of arrival of the flame at a point remote from the spark plug could be used as the basis for a feedback control system. The concept relies on the proven fact that, at a given operating condition, the ‘best’ operating condition will be related to a particular flame arrival time, on average. For example, at a fixed air:fuel ratio, it would merely be necessary to vary the spark timing until the flame arrival time was that desired, and one would then be con- fident that the engine had been ‘tuned’. The flame arrival probes worked well, although there was some question about the electrode becoming a possible site for pre-ignition. The idea was not pur- sued for a variety of reasons, some to do

with the fact that a long average of many individual arrival times was re- quired in order to make confident calibration changes, and this required impracticably long steady-state running conditions. Also the requirements for engine design associated with emissions legislation somewhat overtook many of these concepts.

Pre-ignition occurs when the mixture in the cylinder starts to burn before the spark is fired. (It is also used to describe reactions occurring before the arrival of the flame, but these reactions should be carefully distinguished from knock which is related to an abrupt and very large rate of combustion.) It can be caused by overheating of the spark plug or other internal surfaces and by incor- rect ignition timing. It presents particu- lar problems for engines using some of the alternative fuels that are presently being promoted (e.g. methanol).

If there is a pre-ignition problem in an engine, then one would expect, and indeed finds, that an ionization signal occurs before flame arrival, and in fact it may be before the spark itself. Figure 1 shows an example of such an event. The pre-ignition is in the regions remote from the spark plug, and a few seconds after this signal was recorded, the en- gine failed (with damage to the piston).

In-cylinder post-flame ionization sensors Immediately after their generation in the flame, the original ions undergo a complex process of charge exchange and recombination which leads to a host of differing ionic species [3]. The longest-lived species appears to be the hydronium ion HsO+, and its hydrates, and COs- and its hydrates. At the

Current flow from plug I From

distributor or QIS High value 200 V isolated

power supply

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Figure 2 Ionization current at the spark plug.

11

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RPM

Engine speed

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Exh’aust ionization current

t 1 Cvlinder oressure

2 3 4 5 6 7 8 9 10 11

0 2000 4000 6000 8000

Cold start Crank angle (degrees)

Figure 3 Three signal types generated by sensors mounted in engine exhaust system.

higher cylinder temperatures, equilib- rium considerations suggest that elec- trons will also be present.

The signal from a probe immersed in the post-flame gases (using the spark plug itself for convenience) shows con- siderable detail (figure 2). Three dis- tinct regions can be identified: the spark itself; the high ionization levels associ- ated with the flame; and then a much longer region which is a combination of an ion level decay process superposed on an ionization level closely associated with the cylinder pressure. In fact the cylinder pressure and temperature are of course closely related, and it is thought that the ionization current fol- lows the temperature due to its effect on the electron concentration.

It is simple to use such a sensor as a knock detector, as the pressure fluctua- tions typically associated with knock are also seen on the ionization signal. It is an intriguing fact that the signal-to- noise ratio can in certain circumstances actually be better than that obtained from a pressure transducer, though this is not always the case, and other dif- ficulties beset the application of the concept as a routine technique. These include:

1. The fact that electrical interfer- ence from the spark can overwhelm the signal.

2. The ‘coil on’ event (the re- energisation of the spark coil, in prepa- ration for the next spark) can occur at the same time as the knock detection window, and cause severe interference.

3. The fact that DIS (distributorless ignition) systems, which are now be- coming commonplace, have sparks of both polarities, and the ionization knock system can then be used only if

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the system is doubled up, and the nega- tive ends either grounded, or attached

4. Fuel additives can affect the signal level dramatically.

to dummy plugs.

The use of this type of system was first suggested by R. B. Blauhut et al. [4]. A typical signal looks exactly like that in figure 2, except that a ‘ripple’ appears in the portion of the signal marked ‘post flame ionization’. What is happening is that knocking is associated with strong pressure waves traversing the cylinder, and these pressure waves modulate the conductivity of the io- nized gases within the cylinder. A con- trol strategy was suggested in which automatic spark retardation (leading to reduced knock) was applied in response to a certain signal level from the knock sensor. It is worth noting that in fact a knock problem often occurs due to im- precise air:fuel ration control, though it is more difficult to control an individual cylinder’s air:fuel ratio.

Exhaust sensors Sensors mounted in the exhaust system can give valuable data. The signal types recorded can be grouped into three main types:

1. No signal, indicating a complete misfire.

2. A small but finite signal, indicating the presence of historical ions from the combustion event in-cylinder.

3. A very large signal, indicating that burning mixture is passing the exhaust valve. This happens when the burn within the cylinder is very late (due, for example, to over-lean conditions; this is why engines which run too lean burn out exhaust valves).

Figure 3 shows all of these signal types, in a signal taken from the exhaust of an engine during a start [4]. The pressure within the .cylinder of the en- gine is recorded, along with the exhaust ionization level. The individual cycles are numbered 1, 2, etc, and all of the burn types are illustrated. Cycle 1 is very late burning, as seen from the pressure, and the ionization signal illus- trates that there was a great deal of burning gas in the exhaust. Cycle 2 is a good burn, as indicated by the very much higher cylinder pressure, and the fact that the ionization signal is small but finite. The next cycle is a complete misfire: the pressure diagram just shows the gas being compressed and expanded again and the ionization signal is com- pletely missing. There then follow two very late burns, after which the combus- tion settles down.

The future To sum up, we have seen how the various sensor types can help to under- stand the behaviour of engines, in a way that is helpful in developing better en- gines.

It seems certain that ionization prob- es of one type or another will continue to be very useful research tools. Whether they will ever be used in pro- duction vehicles is not easy to predict. It should be remembered that any new sensor represents an enormous invest- ment for a manufacturer, and can only be considered if a cost-effective argu- ment can be made for their introduc- tion.

The present authors feel that with current engines and fuels, it is unlikely that an application will be forthcoming, but that in a rapidly changing environ- ment, it is quite possible that a suitable function will be best tackled using ionization sensors.

References 111

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Curry, S. ‘A Three Dimensional Study of Flame Propagation in a Spark Ignition Engine’, SAN Trans., Vol. 71, No. 3, 1963. May, M. G. ‘Flame Arrival Sensing Fast Response Double Closed Loop Engine Management’, SAN Trans. Vol. 92, 1984. Calcote, H. F. ‘Studies of Ionization in Flames by Means of Langmuir Probes’, 5th Annual Symposium on Combustion, Combustion Institute, 1955. Blauhut, R. B., Horton, M. J. and Wil- kinson, A. C. N. ‘A Knock Detection System Using the Spark Plug Ionization Current’, 4th International Conference on Automotive Electronics, IEE, Lon- don, 1983. Callings, N. and Willey, J. ‘Cyclically resolved HC emissions from a spark igni- tion engine’, SAE paper No. 871691, 1987.