GB2256603A - Operation of an internal combustion engine - Google Patents

Abstract

A method is described of operating an internal combustion engine burning a fuel containing carbon and hydrogen, such as petrol, diesel fuel or alcohol, and having a catalytic converter 10a, 10b arranged in the exhaust system at a distance from the engine and an afterburner 16 and igniter 18 upstream of the catalytic converter to assist in lighting-off the converter. An excess of fuel is introduced to the engine at starting; this ensures the presence of hydrogen in the exhaust. Oxygen is either present in the exhaust or added via pump 30. This inflammable mixture is ignited by means 18 and heats the converter. Afterwards, either the excess fuel or the added air is regulated to give a less flammable mixture which either gives stable combustion or catalytic reaction. <IMAGE>

Description

OPERATION OF AN INTERNAL COMBUSTION ENGINE Field of the invention The present invention relates to a method of operating an internal combustion engine having an afterburner.

Background of the invention As is known, an exhaust catalytic converter only performs its task of reducing the unburnt hydrocarbons, carbon monoxide and oxides of nitrogen content of the exhaust gases after it has reached a critical temperature, termed the light-off temperature, which is between 300 C and 400 C.

Thus during cold starts, it is important to minimise the time taken for the catalyst to reach this temperature, more especially since the emission test drive cycles which are laid down by various legislations all include a cold start.

Various solutions have already been put forward to enable the light-off time to be reduced. The simplest solution is to place the catalyst very near the engine so that it should be heated by the exhaust gases before these have been cooled by flowing through the exhaust system. This method of mounting a catalyst, usually termed close-coupling, creates problems at the other end of the temperature scale when the engine is running under high speed and high load conditions.

Under such conditions, the exhaust gas temperature can exceed 850 C, which is enough to cause permanent damage to the catalyst. It is therefore preferred not to provide a close-coupled catalyst but to use one mounted some distance away from the engine, normally termed an under-body catalyst. Such mounting is safe for high speed and high load operation but exacerbates the warm-up problem because the exhaust gases are cooled before reaching the catalyst during the start-up phase.

To speed up the warming of a catalytic converter, an external heat supply has been proposed, including electric heaters and microwave heaters. These proposals have involved significant additional cost and complexity, more especially when it is appreciated that the power requirement is of the order of 2 to 3 kilowatts, which with a 12 volts supply calls for a current of 166 to 250 amps.

It has also been proposed to use chemical energy to reduce light-off time by injecting fuel into the exhaust pipe and igniting it. The complexity in this case is that petrol/air mixtures do not always ignite reliably in the exhaust pipe at ambient temperature and pressure and if they should fail to do so they aggravate the problem by cooling the catalyst and by dramatically increasing the hydrocarbon emissions in the exhaust. There are further complexities imposed by the need to ensure safety, it being inherently dangerous to provide a fuel line opening into a hot exhaust pipe.

A A still further proposal has been the use of the so-called thermal reactor in which air is injected into the exhaust stream close to the exhaust port to intercept the exhaust gases while they are still hot. If the mixture is set slightly rich, the combustion reaction continues in the exhaust gases, albeit at a reduce rate, and this raises the temperature of the exhaust system to reduce the light-off time of the catalytic converter. Though this proposal works, the benefits one achieves by it are only of limited value. Typically, the light-off time would be reduced to around two minutes, which still falls short of enabling the more exacting permissible emission levels to be met.

A more recent proposal has been to use an afterburner. The engine is once again run with a rich mixture and fresh air is added to the exhaust gas stream but this time the mixture is ignited, for example by a spark to burn within a chamber arranged immediately upstream of the catalytic converter.

It is important to differentiate between the reaction initiated by ignition in an afterburner and the reaction which normally takes place on the surface of a catalytic converter. In an afterburner, there is created a luminous open flame which propagates through the gases and is not confined to a surface. The ignition can be initiated by a spark, a pilot flame or indeed by a heated catalytic element. Once ignited the flame is not confined to the igniter and the gases burn as they would in an unconfined space.

The concept of an afterburner is not in itself new and it has been known since 1967 that under controlled conditions one can re-ignite the fuel in the exhaust mixture. In a report by C. D. Haynes published by the Motor Industry Research Association of Great Britain (MIRA) as report No.

1967/5, there is an early suggestion to use an afterburner as a means of reducing pollution, the heat it produces being merely dissipated in a heat sink. The heat sink can of course be the matrix of a catalytic converter so that afterburner may act to reduce the light-off time of the converter.

For this purpose, the afterburner has been the most effective proposal to date. When the engine is run with a moderately rich mixture and fresh air is added to the exhaust gases while they are still hot, it is possible to re-ignite the mixture because a so-called cool flame reaction is still taking place in the exhaust system when the gases are not entirely cold. This allows the warm-up time to be reduced to less than one minute but one must wait some time after the engine has started before the gases become ignitable at the afterburner.This is because when the engine and the exhaust system are cold, the mixture arriving at the afterburner will have lost most of its heat to the exhaust system and any cool flame reaction taking place in the gases as it left the engine will have been quenched during passage through the cold exhaust manifold and downpipe and on being diluted by the additional cold air injected into the exhaust gas stream. In the absence of the cool flame reaction which is known to assist ignition, the exhaust/air mixture will not be ignitable on its own. One must therefore wait until the exhaust pipe has warmed to a temperature which enables the cool flame reaction to be sustained until the exhaust gases reach the afterburner.

Once the afterburner is fired, the catalytic converter begins to warm up very rapidly but nevertheless during the initial phase, when the emission level is at its highest, the gases are discharged to atmosphere without being cleaned by the afterburner nor by the catalyst.

Object of the invention The inventors therefore set out to provide a system for firing up an afterburner to heat a catalytic converter within a short time after starting of an engine so as to mitigate the foregoing problems of the prior art.

Summary of the invention According to the invention, a method of operating an internal combustion engine burning a fuel containing carbon and hydrogen, such as petrol, diesel fuel or alcohol, and having a catalytic converter arranged in the exhaust system at a distance from the engine and an afterburner upstream of the catalytic converter to assist in lighting-off the converter, which method comprises the steps of introducing an excess of fuel into the combustible charge at an early stage in starting the engine to undergo excessively rich combustion in the engine and thereby create a proportion of unburnt hydrogen in the exhaust gas stream, ensuring the presence in the exhaust gas stream of additional air from an external source or unused air from the engine to mix with the hydrogen to achieve an exhaust mixture having sufficiently high concentrations of both oxygen and hydrogen to be ignitable under ambient temperature and pressure, igniting the exhaust mixture so as to burn as a flame in the afterburner, and, following ignition of the exhaust mixture, modifying at least one of (i) the quantity of excess fuel in the combustible charge and (ii) the quantity of air introduced into the exhaust gas stream to maintain in the afterburner a less flammable mixture capable of sustaining stable combustion in the afterburner or catalytic reaction in the converter.

Whereas the afterburner systems of the prior art rely on a moderately rich mixture being supplied to the engine throughout the warm-up phase, the present invention recognises that such a mixture will not be ignitable for some time after the engine has started while it is still cold and proposes instead a much richer mixture than one would normally contemplate in conventional engine calibration, even for a cold start.

An engine with a three way catalyst is normally calibrated to run with an equivalence ratio of 1.0, i.e. a stoichiometric ratio. To assist starting at ambient temperature, for example 20 C, one may increase the fuel to air equivalence ratio to 1.3 - 1.5, this also being the general range proposed for prior art afterburner systems. Initially, not all the extra fuel appears in the cylinders for combustion, as much of it will serve only to wet the surfaces of the inlet manifold and cylinder walls and the equivalence ratio in the combustible charge still approximates to 1.0. During this time, the afterburner cannot possibly fire as there is insufficient surplus fuel in the exhaust gases.After starting of the engine and a steady state of wall wetting has been achieved, if the rich mixture is maintained the extra fuel appears as an increased hydrocarbons and carbon monoxide level in the exhaust but the exhaust mixture, which is diluted by the addition of cold fresh air, is still not ignitable. Only when eventually the mixture at the igniter is sufficiently hot can afterburning commence.

The present invention is predicated upon a deeper understanding of the gases in the exhaust/air mixture arriving at the afterburner. During starting with a rich mixture, the exhaust gases contain combustible constituents including carbon monoxide, unburnt hydrocarbons and hydrogen, and diluent gases including carbon dioxide, nitrogen and water.

Each of the combustible constituents has a threshold concentration (flammability limit) below which it cannot form an ignitable mixture when cold, regardless of the amount of oxygen added. The oxygen present in the mixture must also reach a threshold concentration which is unique for each fuel constituent to make it ignitable. It should be born in mind that as fresh air is added to the exhaust gas stream, the concentration of the combustible constituents in the mixture is lowered and the concentration of oxygen in the mixture is shared between the air and the exhaust gases.

It is found that with respect to the unburnt hydrocarbons and carbon monoxide concentrations and the oxygen concentration in the exhaust/air mixture, the engine cannot produce, even with an extremely rich fuel calibration, enough quantities of these constituents in the exhaust gases mixed with the additional air to reach the threshold concentrations simultaneously to form an ignitable mixture under ambient temperature and pressure.

However, the invention is based on the discovery that by supplying a very rich mixture to the engine, i.e. a fuel to air equivalence ratio in excess of 1.5, a sufficient quantity of hydrogen is produced in the exhaust which when mixed with additional air can simultaneously achieve oxygen and hydrogen concentrations which are within the flammability limit for hydrogen at ambient temperature. Thus, by running the engine at a very high equivalence ratio of between 1.7 2.1, the exhaust/air mixture arriving at the afterburner will be well inside the flammability zone for hydrogen and can be ignited reliably as soon as the engine has started.

Once the hydrogen has been ignited, the carbon monoxide and hydrocarbons can also burn in the flame even though they are below their respective flammability thresholds.

Burning mixtures of this very high equivalence ratio in a combustion chamber is unacceptable for any length of time as the engine runs unevenly, the power output is very low and heavy deposits will build up inside the combustion chamber.

During deceleration such a mixture strength may occur unintentionally but under normal circumstances one would never calibrate an engine to burn with this large quantity of excess fuel. This should not be confused with the fuel enrichment used during a conventional cold start in which the amount of excess fuel actually present in the combustible charge is far less as most of the fuel enrichment is introduced to compensate for wall wetting.

Furthermore, in the present invention, the calorific value of this excess fuel when it is burnt in the exhaust system is so great that damage would be done to the catalytic converter if it were maintained for more than a few seconds.

In this respect it should be pointed out that the afterburner of the invention would typically be equivalent to a 7 KW heater for an engine of 1.8 litre capacity, as compared with typically 3 KW for a prior art afterburner operating with a lower degree of excess fuel. For these reasons, it is important in the present invention to return to a more acceptable fuel to air equivalence ratio for the engine at an early opportunity once the afterburner has been fired.

If the equivalence ratio is reduced immediately after firing the afterburner, there is a risk that the flame in the afterburner will be extinguished by the cold gases arriving from the exhaust system which is still cold and the catalytic converter would immediately be cooled. This may not occur during idling but if the vehicle is driven away immediately after starting, then the volume of cold exhaust gases produced at relatively high engine speed may have the effect of cooling the catalyst and extinguishing any flame that is present. It is therefore desirable to effect a controlled reduction of the mixture strength in the exhaust system back to acceptable levels consistent with the afterburner remaining alight and the temperature of the catalytic converter continuing to rise.Such control may take the form of gradual reduction of the equivalence ratio supplied to the engine, a reduction of the amount of additional air or both or by intermittent switching between extra rich and acceptable settings to maintain the temperature of the converter within a preset range.

Thus in the present invention, the excess fuel quantity is initially set very high to enable the exhaust mixture to be ignited reliably in the afterburner immediately after starting. Ignition may typically occur in around two seconds from start taking into account the transport time for the exhaust mixture to travel from the engine to the afterburner. This condition may be maintained for typically five seconds and no longer than twenty seconds after which the excess fuel is reduced to provide a mixture which would not be ignitable when cold but is sufficiently rich to maintain stable combustion once the afterburner has fired.

It is stressed that the values of fuel to air equivalence ratio given above are to assist in comparing the extra rich range used in the present invention with the less enriched range used in the prior art at the same ambient temperature.

These values are not however limiting as they will depend on the ambient temperature on starting. The criterion to be met is the hydrogen concentration in the afterburner rather than the absolute value of the equivalence ratio required to achieve it.

The afterburner may be operated simultaneously with the cranking of the engine as the invention is capable of achieving ignition of the afterburner immediately after the engine fires. It is not however essential to set an excessively rich mixture strength before cranking and this may be carried out after the engine has fired. This may be required if the extra rich mixture interferes with the starting of the engine.

In a homogeneous-charge spark-ignited internal combustion engine, the presence of hydrogen in the exhaust can be ensured by supplying an excessively rich mixture to the engine.

The method of implementing the invention may be slightly different when applied to a stratified-charge engine.

Examples of such engines are those in which fuel is injected directly into the combustion chambers, such as the FORD PROCO four stroke engine, the ORBITAL two stroke engine and diesel engines.

The effect of charge stratification is to create within the combustion chamber regions of rich and weak mixture strengths. The rich regions are responsible for creating the hydrogen and the weak regions contribute to the presence in the exhaust system of the oxygen required to mix with the hydrogen to form an ignitable mixture. In such an engine, it may not prove necessary to enrich the mixture nor indeed to introduce additional air into the exhaust system. It may however be necessary to throttle the intake in order to reduce the air content in the combustion chamber.

In an in-cylinder injected two-stroke engine, delayed injection timing results in fuel entering the exhaust system directly and this technique may be used to augment the amount of heat released after the afterburner has been fired.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically an engine together with its intake and exhaust system for implementing the invention, and Figure 2 consists of four juxtaposed graphs which will be used to explain the constituents of the exhaust gases under different operating conditions.

Detailed description of the preferred embodiment Figure 1 shows an engine 12 to which air is supplied through an air flow meter 22, the air supply being regulated by a butterfly throttle 24. Fuel is introduced into the air stream by an injector 20. The exhaust gases from the engine are conducted by a pipe 14 to a catalytic converter made up of two bricks 10a and lOb which is preceded by an afterburner 16 having a spark igniter 18. Air is added to the exhaust gas stream in the exhaust pipe 14 by a pump 30, the additional air flow being regulated by a valve 32.

During normal operation, the engine runs with a stoichiometric fuel to air ratio and no additional air is added to the exhaust stream. The afterburner 16 is ineffective and the catalytic converter will work satisfactorily to clean the exhaust gases, causing the unburnt fuel in the exhaust to react with unused oxygen to complete their combustion reaction within the catalytic converter. Once chemical reaction within the converter has been started by the converter reaching the light-off temperature, the temperature of the exhaust gases assisted by the exothermic reaction taking place within the converter serves to maintain the converter at a suitably high temperature for it to operate correctly without assistance from the afterburner 16.

The purpose of the afterburner 16 is to reduce the light-off time of the catalytic converter 10a, lOb. During starting the engine is run rich by introducing excess fuel through the injector 20 to ensure that the exhaust gas stream contains combustible constituents, additional air is introduced by the pump 30 to mix with these constituents to form a combustible mixture and the spark igniter 18 in the afterburner 16 ignites the mixture to produce a flame which heats up the converter brick 10a. The invention is concerned with the control of the excess fuel and additional air to ensure that the mixture in the afterburner 16 is readily ignitable.

In Figure 2, the negative side of the X-axis has plotted along it the fuel to air equivalence ratio supplied to the engine. At the origin, the charge mixture is stoichiometric and the values plotted increase in mixture strength with distance from the origin. Further along the axis, the value of 2 indicates that the total amount of fuel added to the engine mixture charge is twice that at stoichiometry. On the negative side of the Y-axis there is plotted the ratio of the total gas flow in the afterburner to the amount of exhaust gas flow from the engine. Hence at the origin, the value of 1 indicates that no additional air is introduced into the exhaust gas stream prior to entry into the afterburner. Lower down the same axis, the value of 2 indicates that the amount of additional air is equal to the volume of the exhaust stream from the engine.The line 10 drawn at 450 to these two axes is the locus of all points at which the exhaust/air mixture is stoichiometric, in other words just enough air is added to react completely with the unburnt fuel in the exhaust stream to minimise pollution.

Even though the exhaust mixture in the afterburner 16 is stoichiometric, it does not mean that it is ignitable be cause it is heavily diluted by inert gases, namely carbon dioxide and nitrogen. As is documented in the literature, combustible gases and oxygen must be present between certain limits of concentration for a mixture to be ignitable, the limits being themselves dependent upon temperature and pressure. If the mixture is cold and non-reacting, the lower flammability limit of the constituents are well defined in published data and are significantly higher than the same limits when the gases are hot and a cool flame reaction is already in progress.It is this flammability limit which must be exceeded initially for the afterburner 16 to come into effect at ambient temperature but once the afterburner has been ignited, the combustion would continue even if the oxygen and flammable gas concentrations drop below the flammability limits.

Returning to Figure 2, the top left hand quadrant and bottom right hand quadrant represent look-up tables to allow the concentrations of the combustible gases and oxygen, respectively to be read off for any operating point on the line 10. The lines 12, 14 and 16 are drawn through points computed from published data indicating the percentage by volume of hydrocarbons, hydrogen and carbon monoxide respectively present in the exhaust mixture at the afterburner when an engine is run with excess fuel and the exhaust gas is diluted with additional air. The curve 18 is similarly drawn through points indicating the percentage by volume of oxygen in the exhaust mixture at the afterburner, allowance having been made for the dilution of the additional air by the exhaust gases. Therefore by drawing lines as represented by the dotted lines 20a, 20b, 20c, 20d, 20e and 20f one can map each point along the line 10 in the lower left hand quadrant onto points in the top right hand quadrant giving the concentrations of oxygen and, as the case may be, hydrocarbons, hydrogen or carbon monoxide in the afterburner 16. In the top right hand quadrant, there have also been drawn shaded areas which show the lower flammability limits for the various gases in the exhaust stream at ambient temperature and pressure. These limits are once again derived from published literature and can be found in text books on fuels. It can be seen from Figure 2 that the carbon monoxide and hydrocarbons flammability limits cannot be reached at any point along the line 10.In the case of carbon monoxide, a sufficiently high concentration of the combustible gas cannot be achieved and in the case of the hydrocarbons, a sufficient concentration of oxygen cannot be achieved.

The present invention is based on the realisation that both the hydrogen and oxygen concentrations can be brought into the flammability range if a very rich mixture is burnt in the engine and a substantial amount of air is added.

As can be seen from Figure 2, the flammability limit is only just reached when the engine is run with an equivalence ratio of 1.5 and for reliable ignition, it is preferred to operate in the region of 1.7 to 2.1. This is outside the range of mixture strengths ever contemplated in the past and indeed in this context it should be mentioned that the data shown in the top left hand quadrant of Figure 2 for mixture strengths richer than 1.7 could not be found in the prior art or in published literature and was obtained by extrapolation. The reason for this is that the engine does not in fact operate satisfactorily with such mixture settings.

Plug fouling, uneven burning, low power output, high fuel consumption and high emissions are just some of the reasons for not operating an engine in this range. It is not therefore practicable to run the engine in this manner for any length of time. However, unless one does run the engine in this manner, the exhaust and air mixture will not be ignitable at ambient temperature and operation of the afterburner will only commence if the engine has been running for so long that the gases in the afterburner are sufficiently hot to support a cool flame reaction, in other words only shortly before the catalytic converter would have reached its light-off temperature without assistance.

The invention therefore proposes operating the engine at or immediately after starting with an excessively rich mixture (above 1.7 equivalence ratio) but only for enough time to ensure that the afterburner is ignited at ambient temperature and that on returning to the usual mixture strengths employed for starting and idling, that is to say less than 1.5 equivalence ratio, the flame in the afterburner will not be extinguished. The change to a normally rich mixture can be gradual or sudden and may be controlled by feedback from a a sensor in the exhaust system. The sensor may be an optical or infra-red sensor or an ionisation sensor acting to ensure that the flame in the afterburner remains ignited or that the front face of the catalytic converter is above light-off temperature.

It should be mentioned that in referring to the mixture strength in the combustion charge, the invention is not solely concerned with the quantity of fuel metered but with the fuel density in the part of the charge in which combustion is initiated in the engine cylinders. Thus due allowance must be made, for example for the effect of wall wetting which has the effect in a normal engine of weakening the charge during cold starting. One must allow for the fact that in the combustion chamber the mixture strength may not be uniform, charge stratification being common in diesel engines and two stroke engine with direct in-cylinder injection.In the present invention, it is sufficient for the richest regions of the combustion charge to have excessive quantities of fuel and it is of less significance that the overall average mixture strength may be below the threshold for entering the cold exhaust gas ignition regime.

The invention is distinguished from the prior proposals to use an afterburner by the following important points: 1. It has not previously been possible to ignite an afterburner at ambient temperature immediately after a cold start.

2. The prior art fails to identify the existence of a cold exhaust gas ignition regime or recognise its significance applied to the engine afterburner to ignite the exhaust gases from cold.

3. The prior art fails to appreciate the importance of hydrogen in the reaction taking place in the afterburner, this constituent alone being capable of cold ignition. Other combustible constituents in the exhaust gases cannot ignite by their own at ambient temperature unless a sufficient concentration of hydrogen is present.

4. Prior art afterburners have operated in a fuel to air equivalence ratio range between 1.3 - 1.5 which is outside the cold exhaust gas ignition regime. This exhaust/air mixture is not ignitable when the exhaust system is cold.

5. No engine has been run intentionally, for any length of time with an excessively rich mixture, having an equivalence ratio more than 1.7. Even scientific study into this very rich region is scarce and little information can be found in the published literature because there has been no useful application for it. This very rich domain is uncomfortably close to the rich misfire limit for the engine (equivalence ratio 2.5 - 3.5).

6. Having identified the need to run the engine beyond a threshold fuel to air equivalence ratio of 1.7 in order to enter the cold exhaust gas ignition regime, to allow the afterburner to fire, the invention also recognises the hazards of operating the engine in this manner and proposes switching to a more acceptable mixture setting at the earliest opportunity.

7. In view of the excessively rich mixture, the invention also calls for unprecedent amounts of additional air to be introduced into the exhaust system. For example, in the present invention an air blower capable of delivering some 400 litres/min is required for a 1.8 litre capacity engine as compared with a prior art afterburner using less fuel enrichment which would only typically require an air blower capable of supplying some 200 litre/min.

Claims (13)

1. A method of operating an internal combustion engine burning a fuel containing carbon and hydrogen, such as petrol, diesel fuel or alcohol, and having a catalytic converter arranged in the exhaust system at a distance from the engine and an afterburner upstream of the catalytic converter to assist in lighting-off the converter, which method comprises the steps of introducing an excess of fuel into the combustible charge at an early stage in starting the engine to undergo excessively rich combustion in the engine and thereby create a proportion of unburnt hydrogen in the exhaust gas stream, ensuring the presence in the exhaust gas stream of additional air from an external source or unused air from the engine to mix with the hydrogen to achieve an exhaust mixture having sufficiently high concentrations of both oxygen and hydrogen to be ignitable under ambient temperature and pressure, igniting the exhaust mixture so as to burn as a flame in the afterburner, and, following ignition of the exhaust mixture, modifying at least one of (i) the quantity of excess fuel in the combustible charge and (ii) the quantity of air introduced into the exhaust gas stream to maintain in the afterburner a less flammable mixture capable of sustaining stable combustion in the afterburner or catalytic reaction in the converter.

2. A method as in claim 1 wherein the fuel to air equivalence ratio of the mixture charge supplied to the engine is sufficiently high to permit ignition of the exhaust gas and additional air mixture in the afterburner in less than 15 seconds after the engine has started.

3. A method as in claim 1 wherein the fuel to air equivalence ratio of the mixture charge supplied to the engine is sufficiently high to permit ignition of the exhaust gas and additional air mixture in the afterburner in less than 5 seconds after the engine has started.

4. A method as claimed in any preceding claim, in which the excessively rich mixture is supplied to the engine and air is added to the exhaust gases during cranking.

5. A method as claimed in any of claims 1 to 3, in which the excessively rich mixture is supplied to the engine and air is added to the exhaust gases only after cranking and starting of the engine.

6. A method as claimed in any preceding claim, in which the fuel to air equivalence ratio of the mixture supplied to the combustible charge of the engine to permit early ignition of the afterburner lies in the range from 1.

7 to 2.1 7. A method as claimed in any preceding claim, wherein the engine is a homogeneous-charge spark-ignited internal combustion engine and the presence of hydrogen in the exhaust is ensured by supplying an excessively rich homogeneous mixture to the engine, air being added directly into the exhaust gas stream.

8. A method as claimed in any of claims 1 to 6, wherein the engine is a stratified-charge engine, the charge stratification being operative to create within the combustion chamber regions of excessively rich and less rich mixture strengths, the former being responsible for creating the hydrogen and the latter at least contributing to the presence in the exhaust system of the oxygen required to mix with the hydrogen to form an ignitable mixture.

9. A method as claimed in claim 8 , wherein charge stratification is achieved by injecting fuel directly into the combustion chamber.

10. A method as claimed in any preceding claim, wherein the engine is a spark ignited engine.

11. A method as claimed in any of claims 1 to 9, wherein the engine is a diesel engine.

12. A method as claimed in any preceding claim, wherein the engine is a two-stroke engine.

13. A method as claimed in any preceding claim, further comprising the step of controlling the afterburning by means of a closed loop which further comprises means for sensing the presence of a flame in the afterburner or the temperature of the catalytic converter.