Control of Selective Catalytic Reduction

The present invention relates to reduction of oxides of nitrogen (collectively known as NO _x ) in vehicle exhaust gases and in particular to controlling the reduction OfNO _x in such a way that exhaust emissions of ammonia (NH ₃ ) as well as NO _x are minimised.

It has been known for many years that NO _x present in vehicle exhaust gases is harmful to the environment and to people's health. There have been increasing moves in recent years to try and encourage reduction of harmful vehicle emissions and various limits have been set on the maximum acceptable level of pollutants such as NO _x which may be emitted by a vehicle. Additionally, consumers are increasingly likely to take environmental factors into account when considering purchasing vehicles. Accordingly, it is desirable to try and limit the amount OfNO _x allowed to escape into the atmosphere from vehicle exhausts.

Conventional methods for reducing NO _x emission usually involve exhaust gas recirculation (EGR). However this recycling of exhaust gases has several drawbacks. These include reduced engine performance, particularly at high loads, and an increase in problematic particulates. Furthermore, there is a limit to the level of NO _x emission reduction possible with this technique.

hi the next few years new legislation on vehicle emissions will come into force in various countries throughout the world that will drastically decrease the levels of NO _x emissions allowed. The limited effectiveness of EGR methods will then become critical as these methods simply will not be able to meet the stringent NO _x emissions limits.

An alternative technique to EGR has been developed which improves the level of NO _x reduction possible. This is a urea based selective catalytic reduction (SCR). hi internal combustion engines that run with a predominantly lean fuel/air mixture, such as direct injection gasoline or diesel engines, SCR is one of the most effective methods for reducing NO _x emissions in the exhaust gas.

The catalyst used in SCR is placed in a vehicle exhaust pipe in such a way that all exhaust gases from the engine pass through the catalyst. An aqueous urea solution is injected in the exhaust gas upstream of the SCR catalyst. The high temperature in the exhaust stream raises the temperature of the catalyst and causes the urea to hydrolyse to ammonia (NH ₃ ), via ammonia related compounds, which reduces NO _x in the catalyst to produce harmless products.

To maximise the NO _x emissions eliminated in this way, it must be ensured that enough ammonia is available to react with all the NO _x . Too little ammonia will result in high NO _x emissions at the exhaust, but conversely, too much ammonia will result in ammonia emission in the exhaust (a phenomenon referred to as ammonia slip).

Emission of ammonia also causes pollution problems and so is also undesirable.

Therefore in an ideal SCR system, the amount of NH ₃ made available to react with the NO _x is exactly the amount necessary to react fully with the available NO _x , without leaving any NH ₃ to be emitted into the atmosphere.

The amount of NH ₃ available to react with NO _x in the catalyst depends on various factors. Depending on conditions in the catalyst, urea may directly decompose in the catalyst to give ammonia or alternatively urea or other NH ₃ related compounds may be stored in the catalyst and will then decompose at a later time. The storage of urea and other ammonia related compounds in the catalyst and the rate of breakdown of urea and ammonia related compounds into ammonia in the catalyst depend on the temperature in the catalyst and on how much urea is already stored in the catalyst. An optimum urea injection rate must take account of these different factors.

Originally SCR was used in steady state systems such as power stations, where it is relatively straightforward to determine the amount of ammonia available for reaction and the urea injection rate needed to obtain this amount of ammonia. Therefore minimising the emission of both NO _x and NH ₃ is not too problematic for such systems. However in dynamic vehicle engine systems with fluctuating temperatures, determining the amount of ammonia available is complicated. As more NH ₃ is

released as catalyst temperature rises and more of the NH ₃ related compounds are stored within the catalyst as catalyst temperature falls, the storage and release of ammonia in the catalyst continuously varies as catalyst temperature fluctuates. It is therefore difficult to achieve the constantly changing appropriate urea injection rate to optimise the performance of vehicle SCR catalysts.

Current SCR methods for reducing pollution from vehicles tend to take the approach of concentrating only on minimising NO _x emission, by attempting to vary the urea injection rate such to ensure that there is always at least enough ammonia available to react with all the NO _x . A certain amount of ammonia slip is considered to be acceptable.

An example of such a system is described in US6959540. This publication discloses an SCR system wherein exhaust temperature is measured. When this temperature is in a range where urea is stored by the catalyst, the urea injection rate is increased so that urea is stored in preparation for when a load on the engine increases NO _x emission and therefore catalyst temperature. This aims to provide quicker ammonia production on, for example, vehicle acceleration than would be the case if the urea injection rate was simply increased when the acceleration started.

This system has several shortcomings. Firstly, only the exhaust temperature is measured and this is not related to the actual catalyst temperature, which is the relevant variable. Secondly, the system only changes the urea injection rate based on large scale changes in temperature ranges and does not take into account more detailed variation in temperatures, such as transient fluctuations occurring, for example, at the moment a pedal is depressed. This means that it would never be possible to accurately match the ammonia produced to the NO _x produced as it is never possible to accurately know the rate of engine NO _x emission. Thirdly, the system does not take into account a quantitative measure of urea storage and ammonia release by the catalyst. Fourthly, there is no consideration of the rate of ammonia production beyond attempting to provide an excess of stored urea at low temperatures, leading to a risk of ammonia slip. In particular, because it relies on a simple open loop look-up

table based approach, the disclosed system is inaccurate and inefficient which leads to insufficient ammonia production and therefore NO _x emission from the exhaust in some conditions and over-production of ammonia and associated ammonia slip in other conditions.

A further problem with known systems is the inability of NO _x sensors to distinguish NO _x and NH ₃ .

The present invention is set out in the claims.

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a diagram showing the different mass rates occurring in different parts of an SCR catalyst;

Figure 2a is a graph showing how the catalyst conversion efficiency varies with temperature;

Figure 2b is a graph showing how storage capacity for ammonia related compounds

(including urea and intermediate ammonia related compounds) of the catalyst varies with temperature;

Figure 3 is a conceptual diagram showing the effect of transient engine operation;

Figure 4 is diagrammatic overview of the feed-forward strategy of the present invention;

Figure 5 shows an example model for calculating NO _x emitted from an engine; Figure 6 is a diagrammatic overview of an example system according to the present invention comprising both feed- forward and feedback strategies;

Figure 7 shows an example first feedback mechanism;

Figure 8 is a schematic diagram of an adaptive learning module;

Figure 9 shows an example second feedback mechanism; and Figure 10 shows the signals obtained by a NO _x from NO _x and NH ₃ .

The present invention relates to a method and system for controlling an SCR system to minimise the level of harmful pollutants, that is both NO _x and ammonia (NH ₃ ), that a vehicle containing an SCR catalyst emits into the atmosphere.

A schematic diagram of an SCR catalyst system is shown in figure 1, together with the different component mass rates in different parts of the catalyst 1, which are discussed in more detail below. The system comprises a catalyst 1 which is placed, for example, in vehicle exhaust system 5 (see figure 3), in a position which maximises the amount ofNO _x emitted from the engine 6 that flows through the catalyst 1. A liquid injector (not shown) is arranged such that liquid can be injected upstream of the catalyst 1. A controller (not shown) is connected to the liquid injector to control the rate of liquid injection.

hi operation, a liquid containing an ammonia generating compound is injected at upstream of the catalyst. This is usually an aqueous urea solution but may be any other suitable liquid. NO _x enters the catalyst as shown at 3.

The injected urea may immediately break down to give ammonia, which may be produced via intermediate ammonia related compounds such as biuret. Alternatively, urea or intermediate ammonia related compounds (together termed "ammonia related compounds") may firstly be stored 4 in the catalyst, breaking down to give ammonia at a later time, the respective breakdown/storage rates being temperature dependent. The ammonia reacts with the NO _x at the catalyst in a redox reaction to give harmless products which are then emitted from the exhaust. For example 2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ O.

The efficiency OfNO _x reduction (conversion efficiency) in the catalyst 1 is linked to the rate of production of ammonia in the catalyst from the breakdown of ammonia related compounds. As shown in figure 2a, below 132 ⁰ C almost no ammonia is produced, dependent on the catalyst/SCR temperature. Above 132 ⁰ C the conversion efficiency starts to increase in proportion to temperature. Above about 400 ⁰ C all injected urea immediately decomposes into ammonia. The catalyst 1 stores any

surplus ammonia related compounds not used in converting NO _x . Hence, as shown in figure 2b, the catalyst 1 stores virtually all the injected urea (up to a physical maximum) below 132 ⁰ C and above 400 ⁰ C all injected urea immediately decomposes into NH ₃ . hi between these two temperatures, a mixture of ammonia related compounds is stored, with the particular intermediate ammonia related compounds stored and the overall composition of the mixture, including proportion of urea, varying with varying temperature.

hi the dynamic system of a vehicle engine 6, the temperature of the catalyst 1 is constantly changing, with many transient temperature fluctuations occurring due to actions such as braking or accelerating of the vehicle.

hi overview, as shown in figure 3, the control system of the present invention comprises a feed-forward mechanism 7 with variables related to engine 6 performance as inputs. This controls the rate of urea injection 2 at the catalyst and possibly also the emission NO _x rate from the engine 6. A feedback mechanism 8 may output a correction to the urea injection rate predicted by the feed- forward mechanism, the feedback correction occurring at point 9.

The system of the present invention makes use of a model which takes into account the differing rates of both ammonia related compound storage and ammonia release and how these depend on catalyst temperature to determine the amount OfNH ₃ available to react with NO _x at any particular point in time. This enables the optimum urea injection rate for a desired rate of reduction of NO _x to be determined. The present invention uses a feed-forward approach incorporating this model to continually optimise the urea injection rate for the SCR catalyst 1 so that the amount of ammonia available can always be matched to the amount OfNO _x needing to be reduced. Various forms of feedback may also be used to correct any errors in the feed- forward model. The details of the model used are discussed below.

Turning first to the feed- forward model and referring again to figure 1 , the amount of urea available to reduce NO _x is the sum of injected urea that immediately decomposes into NH ₃ and the ammonia related compounds released by the catalyst 1 :

Mu_avail = X'Mu inj rate + Mu rel rate [1]

where Mu avail = mass rate of urea made available to reduce NO _x

X = fraction of injected urea that immediately decomposes into NH ₃ Mu inj rate = mass rate of injection of urea Mu rel rate = mass rate of release of ammonia related compounds from catalyst

The assumption is made that the rate of release of ammonia related compounds from the catalyst 1 at a given catalyst temperature is proportional to the "State of Fill" of the catalyst:

Mu rel rate = Mu stored / Tau rel [2]

where Mu stored = mass of ammonia related compounds stored in the catalyst ("State of Fill")

Tau rel = time constant for the rate release of ammonia related compounds from the catalyst, which is a function of catalyst temperature

Combining equation [1] and equation [2] gives the urea injection rate as:

Mu_inj_rate = (Mu_avail - Mu_stored/Tau_rel) / X [3]

The optimal urea injection rate is obtained when it results in Mu_avail just sufficient to reduce all the available NO _x :

Mu inj rate opt = (ku*Mnox_rate - Mu_stored/Tau_rel) / X [4]

where Mu inj rate opt = optimal urea mass rate of injection ku = a scaling factor which accounts for conversion from NO _x to urea, including both decomposition of urea to NH ₃ and stoichiometric considerations for reduction of NO _x by NH ₃ Mnox rate = engine-out NO _x emission mass flow rate

Figure 4 shows how equation [4] is used in the feed-forward strategy of the present invention. To obtain the first factor, Mnox rate is obtained from a sensor or modelled as discussed in detail below, "ku" and "X" are obtained from look-up tables 10, 11. These can be dependent on the exhaust 5 temperature, T _ex , and mass air flow rate, MAF. To obtain the second factor, Mu_stored or "state of fill" is obtained by integration as described in more detail below. Tau rel is calculated 12 from a model 13 of catalyst temperature. This model relates catalyst 1 temperature to the vehicle exhaust 5 temperature, T _ex h, the ambient temperature, T _amb and pressure, P _amb and vehicle speed. T _exh is either measured with a temperature sensor or obtained from a look-up table based on engine speed and load. The basis of the model of catalyst temperature is as follows:

Catalyst gains heat from exhaust gas mainly by convection:

Heat Gain = Convection * MAF * (T _eXh - T _cat )

where MAF = mass air flow

T _eat = catalyst temperature

Heat is lost from catalyst to the surroundings by a combination of convection and conduction:

Heat Loss = (Conduction + (Ambient airflow * Convection)) * (T _cat - T _amb ) Ambient airflow = Vehicle_speed * P _amb / T _am b

where T _amb ⁼ ambient air temperature

P _amb - ambient air pressure

Catalyst temperature is obtained by integrating the difference between heat gained and heat lost:

T _eat = T _eat + (Heat Gain - Heat Loss)/Heat Capacity

The mass of ammonia related compounds, Mu_stored, stored in the SCR catalyst is the difference between that added from the current injection of urea, and that released from previously stored amount of ammonia related comopunds. This thus gives the equation for the rate of change of ammonia related compounds stored in the catalyst as:

δ(stored ammonia related compounds) - ammonia related compounds added to storage - ammonia related compounds released from storage i.e. δMu_stored = (1 -X)* Mu_inj_rate*δt - (Mu_stored/Tau_rel)* δt where δt is a short time interval over which the change in stored mass of ammonia related compounds is observed. This expression is formed at points 20a and 20b and integrated 14 to obtain the amount of ammonia related compounds stored at any time.

The Mnox rate, or in other words the amount of NO _x needing to be reduced, may be obtained from a model, as shown in figure 5. An estimate of in-cylinder peak temperature is obtained from a dynamic model of in-cylinder combustion. This takes into account various factors, which may be at least some of the following: engine coolant temperature (ECT), intake air temperature (IAT), manifold absolute pressure (MAP), start of injection (SOI), injected fuel mass (IFM), exhaust gas recirculation (EGR) and engine speed (RPM), all of which can be obtained from appropriate sensors or maps as is well know. A steady state NO _x emission rate is obtained from a look-up table based on this in-cylinder temperature estimate and the load (or torque) demand on the engine. A correction is then applied to take into account deviations from the steady state due to, for example as shown in figure 5, transients caused by acceleration being initiated. The Mnox rate obtained is fed into the feed-forward system at point 15 shown in figure 4.

As can be seen from figure 4, the controller combines the different factors as follows: at point 15 the factor "ku" and Mnox rate are multiplied together. At point 16 1/ Tau rel found from the model of catalyst temperature and for example, from Tau rel obtained from a look-up table 17 is multiplied with Mu stored obtained as described above. At point 18 the result from point 16 is subtracted from the result from point 15 and then at point 19 the result from point 18 is multiplied by 1/X to give Mu inj rate opt.

As discussed above, the control system of the present invention may also comprise various feedback mechanisms to remove any errors in the feed-forward strategy. An example system containing two feedback mechanisms is shown in figure 6, discussed in more detail below.

One feedback mechanism, shown in figure 7, removes any discrepancies between the desired and injected quantity of urea. A simple proportional integral (PI) controller may be used, as shown in the figure. This integrates any error over a period of time at 21 to give ki and multiplies the error by a constant to give k _P . A correction to the urea injection rate, k _fb , is then calculated by the controller from ki + k _P . k _\ may also be output to an adaptive learning module 22, shown in figure 6 and outlined in figure 8. This adaptive learning module may compensate for any of the following: flow tolerance of the urea injector, build-up of deposits, and changes in the quality of urea. The effect of urea injector flow tolerance (higher or lower flow at a given drive signal), or effects of ageing on certain system components (e.g. build up of deposits on the urea injector nozzle) will result in a lower or higher reading (offset from the nominal value) by the NOx sensor signal. The integral part (I term) of the PI closed- loop controller is effective in eliminating offsets in the sensor output signal - hence the value of the I term is used to "recalibrate" the model as an adaptive term. When the system is operating correctly then the I term should reduce to zero.

A second feedback mechanism is shown in figure 9. A downstream NO _x sensor 23 and possibly also an upstream sensor 24 sense NO _x to adjust for discrepancies

between the desired and actual amount of emitted NO _x . The desired amount of emitted NO _x is usually zero, as shown in figure 6, but may be non-zero in the situation described below where NO _x engine emission is allowed to increase to prevent ammonia slip.

Current NO _x sensor technology cannot differentiate between NO _x and NH ₃ , as shown in figure 10. Therefore in order to use such a sensor in a feedback control loop, the strategy must be capable of differentiating between NO _x and NH ₃ . This may be done by exploiting the different polarities of the signals as shown in figure 10 and estimating the derivative d(Sensor signal) / d(Mu_inj_rate) based on actual measured values. The polarity of the derivative, calculated at point 25, is used to correct the sensor signal at point 26.

Alternatively, the NO _x sensor signal direction may be used to determine if the model is over or under predicting in a different manner. For example, if the NO _x sensor is reading higher than the model predicts there could be two possible reasons, either there is NO _x slip or NH ₃ slip because the sensor measures both NO _x and NH ₃ . Understanding if the slip is NO _x or NH ₃ may be determined by increasing the urea injection, so that if the slip is NO _x then the NO _x sensor signal reduces but if the slip is NH ₃ then the NO _x sensor signal increases. The converse also holds if the urea injection is reduced - the NO _x sensor signal increases if the slip is NO _x and if the slip is NH ₃ then the NO _x sensor signal decreases. The sensor is able in this way to correct the model up or down depending on if the error is due to high NO _x or high NH ₃ .

A further way in which the NO _x sensor can be used to correct the SCR model is by looking at the SCR catalyst conversion efficiency. The SCR catalyst conversion efficiency is in part a function of the NH ₃ storage level and will become suboptimal at either very low NH ₃ storage levels or very high NH ₃ storage levels, as a function of the maximum storage capacity at the given operating conditions of the SCR catalyst. This information can be used to correct the model and hence the urea dosing level and achieve an improved conversion efficiency.

A yet further method of correcting the SCR model by the NO _x sensor is as follows. The urea injection can be stopped until the stored NH ₃ is fully used up. This will have the effect of causing a NO _x breakthrough at the tailpipe which can be measured by the NO _x sensor. The point of breakthrough can be used to zero the NH ₃ storage level of the SCR model and correct for errors.

The system shown in Figure 6 comprises a controller 27 which comprises a feedforward module 28 as shown in figure 4. An estimate of engine NO _x emission is input 32 into the feed-forward module 28. This may be an estimate obtained as shown in figure 5 or may be obtained from a second NO _x sensor 24. At point 29 the output of the feed-forward module 28, Mu inj rate opt, is combined with the output of a feedback module 29 shown in figure 9. This feedback module 29 has an input at point 30 ofNO _x levels in the exhaust 5 from the NO _x sensor 23. A second input at point 30 is an NO _x setpoint 31 of the desired NO _x level in the exhaust 5. The value at point 29 may give the urea injection rate or this value may be further corrected at point 30 in accordance with the feedback mechanism and adaptive learning discussed in relation to figures 7 and 8. A urea injector 31 is then controlled to produce a urea injection rate according to the value at point 31. The sensor 23 then senses the resultant NO _x exhaust emission and feeds back this information into the feedback module 29 as discussed above.

The present invention further preferably controls the state of fill of the catalyst, Mu stored. The main purpose of this aspect of control is to maintain the Mu stored at a pre-determined target level to provide a buffer for deviations in the rate of production OfNO _x by the engine and the rate of NH ₃ production in the catalyst from the estimates provided by the models. The aim is to ensure that any imbalance in the NO _x and NH ₃ rates does not result in NO _x emission.

This control may include, under certain conditions, increasing the NO _x produced by the engine, for example for load increases where the exhaust temperature is predicted to increase. If it is predicted that ammonia will be generated at a higher rate than the estimated rate of NO _x emission, increased NO _x emission from the engine mops up the

ammonia to avoid ammonia slip. In this way both NO _x and NH ₃ emission into the atmosphere are minimised.

This control uses EGR to control emission OfNO _x from the engine and exhaust gas temperature. As discussed already, these factors influence catalyst temperature, catalyst storage capacity and optimum liquid injection rate. By a combination of the above effects, Mu stored is controlled to a target level. This also improves fuel economy as the engine is allowed to run at higher thermal efficiency conditions to increase the NO _x emitted by the engine.

This strategy anticipates an acceleration to follow during a deceleration. Instead of reducing the rate of urea dosing under such conditions, as would be conventionally expected, the amount injected is increased, corresponding to the increased storage capacity as the estimated catalyst temperature reduces. When operating under nominal "steady state" conditions, EGR is increased when the stored mass of ammonia related compounds begins to approach a target value. This reduces exhaust gas temperature which in turn cools the catalyst and hence increases the storage capacity for urea. The rate of urea injection is then increased to the new temperature defined capacity.

Conversely, when a condition where engine-out NO _x would be far less than the NH ₃ released by the catalyst (typically this will happen when catalyst temperature is gradually rising while engine load is reducing - it is a transient phenomoenon), the EGR is reduced until the NO _x level is increased to a level sufficient to react with the extra NH ₃ .

The present invention may be used with any internal combustion engine which runs with a predominantly lean fuel/air mixture. For example this may be a diesel engine or a direct injection petrol engine and may be a two-stroke or a four-stroke engine or any other engine type. The engine may be a non-vehicle engine. The SCR catalyst may be a conventional SCR catalyst and may be, for example, made from metallic or ceramic substrate (typically titania or zeolite) coated with catalytic material (typically iron, vanadium or copper). The liquid injector is a conventional liquid injector.

Although an aqueous urea solution has been discussed above as being injected, the injected liquid may be any other suitable liquid containing an ammonia generating compound. The controller may be implemented in hardware or software and may be, for example, an engine control unit (ECU). The look-up tables may be any conventional look-up tables. For example, the tables may be obtained from test calibration runs of the engine. Any sensors, such as a temperature sensor, may be conventional sensors.

The present invention represents a significant improvement over state of the art vehicle SCR systems as the detailed modelling of the variation of catalyst behaviour with temperature in terms of the catalyst's ability to store and release ammonia generating compounds means that the levels of ammonia can be far more accurately matched with the levels of NO _x at the catalyst. Ammonia slip can therefore be minimised at the same time as maximising NO _x reduction. A conventional NO _x sensor may be used in order to provide feedback to the system of the present invention by distinguishing a sensor signal due to NO _x from a sensor signal due to NH ₃ by exploiting the different polarity of the sensor signal in each case.