combustion engine

The present invention relates to a combustion engine system and in particular to a method for adaptation of at least one injector for a combustion engine according to the preamble of claim 1. The invention relates also to a system and a vehicle and to a computer programme and a computer programme product which implement the method according to the invention. Background to the invention

Modern combustion engines are usually provided with injectors to supply fuel to the engine's combustion chambers, e.g. one or more cylinders, in order by combustion to generate

propulsive force on an output shaft. These injectors are usually controllable in such a way that a relatively exact and defined amount of fuel can be supplied to the combustion process, and also at a desired time in the combustion cycle.

For it to be possible for desired amounts of fuel to be continually supplied to the combustion process, and for the combustion engine therefore to operate as desired/intended, it is necessary to know the injector's specific characteristics, i.e. how much fuel actually passes through it when it is open, which depends not only upon the pressure at which the fuel is injected and the amount of time for which the injector is open, as below, but also upon the injector's configuration, e.g. the configuration of the hole or holes through which the fuel is injected. The specific configuration may vary from injector to injector, e.g. because of manufacturing

tolerances. The above reasons lead, e.g. during the

manufacture or fitting of the engine, to charting, e.g. in the form of a reference table indicating amounts of fuel actually supplied with reference to specific amounts of opening time and the pressure at which the fuel is supplied when the injector opens. The amount of fuel actually supplied to a combustion chamber when an injector opens is therefore directly affected by the amount of time for which the injector is open as well as the pressure to which the fuel is subjected. The injection pressure may be arranged to be always the same but it may also be arranged to vary, in which case different amounts of fuel may be indicated for the same amount of opening time but for different injection pressures.

The amount of fuel actually to be supplied to a combustion chamber at any given time is usually determined by a control unit in the vehicle's internal control system which determines an amount for injection, e.g. on the basis of the vehicle's prevailing operating conditions, and which then uses said charting to determine amounts of opening time for the

respective injectors. In the operation of a combustion engine it is important that the actual fuel amount supplied to the combustion chambers should also correspond to the intended amount for injection, as above. This is achieved on the basis of said charting whereby the injectors are calibrated individually during their manufacture and/or that of the engine, so that the charting may be specific to each individual injector.

However, the characteristics of an injector may change over time, e.g. because of wear of the hole or holes through which fuel injection takes place, with the result that a certain amount of opening time no longer necessarily causes a desired amount of fuel to be supplied. For this reason, adaptation of the injectors is usually undertaken regularly or as

necessary so that their opening periods are corrected as necessary .

Summary of the invention An object of the present invention is to propose a method for adaptation of at least one injector pertaining to a combustion engine for a vehicle. This object is achieved with a method according to claim 1.

The present invention relates to a method pertaining to adaptation of at least one injector for an engine which has at least combustion chamber into which fuel is injected by using said at least one injector, and a post-treatment system to treat an exhaust flow arising from combustion in said engine. Adaptation comprises a plurality of injections by means of said at least one injector whereby unburnt fuel is supplied to said post-treatment system via said combustion chamber. The method comprises, after a first of said plurality of

injections, the steps of

- estimating an amount of unburnt fuel which has become stored in said post-treatment system, and

- if said estimated fuel stored is less than a first amount, conducting a second injection following upon said first inj ection .

Estimating according to the present invention an amount of unburnt fuel stored in a post-treatment system for adaptation purposes and only conducting a subsequent injection in a sequence of injections as part of an adaptation scheme when the estimated fuel stored is less than a first amount provides assurance that the amount stored is not greater than may be allowed to oxidise or vaporise in the post-treatment system as a result of any possible subsequent temperature rise. A method is thus proposed which may reduce or completely eliminate problems in the adaptation of injectors pertaining to a combustion engine. It may for example decrease the risk of irreversible so-called poisoning, i.e. the risk that situations in which hydrocarbons become attached to active seats of catalysts where the catalyst reaction takes place might occur and cause permanent damage. It also decreases the risk of harmful overheating caused by rapid oxidation of larger amounts of fuel stored in the post-treatment system which are due to injector operation at times when low

temperatures prevail in the post-treatment system.

Further characteristics of the present invention and

advantages thereof are indicated by the detailed description of embodiment examples set out below and the attached

drawings .

Brief description of the drawings

Fig. la depicts a power train in a vehicle in which the

present invention may with advantage be employed.

Fig. lb depicts a control unit in a vehicle control system.

Fig. 2 depicts an example of a post-treatment system in a vehicle in which the present invention may with advantage be employed.

Fig. 3 depicts schematically an injection system pertainin to the vehicle depicted in Fig. 1.

Fig. 4 depicts schematically a method according to an

example of an embodiment of the present invention.

Detailed description of preferred embodiments Fig. la depicts schematically a power train of a vehicle 100 according to an embodiment of the present invention. The vehicle depicted in Fig. la has only one axle provided with tractive wheels 113, 114 but the invention is also applicable on vehicles in which more than one axle is provided with tractive wheels, and also on vehicles with one or more further axles, e.g. one or more tag axles. The power train comprises a combustion engine 101 connected in a conventional way, via an output shaft of the engine, usually via a flywheel 102, to a gearbox 103 via a clutch 106.

The engine is controlled by the vehicle's control system via a control unit 115. The clutch 106, which may for example take the form of an automatically controlled clutch, and the gearbox 103 are also controlled by the vehicle's control system by means of one or more suitable control units (not depicted) . The vehicle's power train may of course also be of some other kind, e.g. a type with conventional automatic gearbox etc.

An output shaft 107 from the gearbox 103 then drives the tractive wheels 113, 114 via a final gear 108, e.g. a

conventional differential, and driveshafts 104, 105 which are connected to said final gear.

Combustion engines for vehicles of the kind depicted in Fig. la are often provided with controllable injectors to supply desired fuel amounts to their combustion chambers at desired times, and also at a desired time in the combustion cycle, e.g. at a specific piston position in the case of a piston engine .

Fig. 3 depicts schematically the fuel injection system for the engine 101 exemplified in Fig. la. As below, the injection system takes the form of a so-called common rail system, but the invention is equally applicable on other kinds of injection system. The engine 101 takes the form of a six- cylinder combustion engine with a respective injector for each combustion chamber (cylinder) , schematically indicated as 301- 306. Each injector thus takes care of fuel injection (supply) in a respective combustion chamber. It will be appreciated that the engine may have any desired number of cylinders

(combustion chambers). It is also possible for there to be two or more injectors per combustion chamber. The injectors 301-306 are individually controlled by respective actuators (not depicted) which are associated with respective injectors and which use control signals received as a basis for

controlling the opening/closing of the injectors. These signals for controlling the opening/closing of the injectors by the actuators may be generated by any suitable control unit, in this example the engine control unit 115. The control unit 115 thus determines the fuel amount to be

actually injected at any given time, e.g. on the basis of the vehicle's prevailing operating conditions. Specifically how fuel amounts required are determined is well described in prior art and is therefore not described in more detail here.

The control unit 115 uses charting, e.g. a table as above, to convert a desired fuel amount to a corresponding amount of opening time for the injectors.

The injection system depicted in Fig. 3 further takes the form of a so-called common rail system whereby all of the injectors (and hence all of the combustion chambers) are served by a shared fuel pipe 307 (the common rail) which is replenished by a fuel pump 308 while at the same time the fuel in the pipe is subjected, likewise by the fuel pump, to a certain pressure. The highly pressurised fuel in the common pipe 307 is then injected into the engine's combustion chambers when the respective injectors 301-306 open. More than one opening/closing of a specific injector may take place during a given combustion cycle.

Irrespective of how injection is conducted during a combustion cycle in a combustion chamber, e.g. irrespective of whether it takes the form of one or more consecutive injections etc., it is very important that the actual fuel amount injected into the engine's combustion chambers really corresponds to the intended amount. If the actual amount injected is too small relative to the desired amount, the performance of the engine will be less good than intended and promised, resulting in inferior drivability. Conversely, if the amount injected is too great relative to that intended, the engine's performance may be boosted, e.g. producing more torque/power than

intended. This may cause damage to the engine and/or other components of the vehicle which are not dimensioned for the higher power output.

It is therefore very important that the fuel amount injected should actually correspond to that intended. As above, this leads during the manufacture/fitting of the engine 101 and/or the injectors 301-306 to charting, e.g. in the form of a reference table, whereby fuel amounts desired for supply to a combustion chamber are converted to a certain length of opening time for the respective injector, to cater for

individual differences between injectors. In other words, the injectors are calibrated at individual injector level so that the charting may be specific to each of them.

Although assurance may thus be provided at the time of

manufacture of a vehicle that the engine will function as intended, the characteristics of an injector may change over time so that a certain amount of opening time (opening period) no longer assuredly causes injection of intended amounts of fuel. The characteristics of the injectors may for example change in such a way that a larger amount of fuel than desired is injected with respect to a given opening period/inj ection pressure condition, with consequent disadvantages as above. For this reason, the injectors are usually subjected regularly or as necessary to adaptation whereby their opening period is corrected so that the amount of fuel actually supplied for the combustion also corresponds to the intended amount.

Adaptation in a system according to Fig. 3 may be conducted as follows. Initially, the pipe (the rail) 307 is pressurised with fuel by means of the fuel pump 308, further supply by which is then shut off. This means that the pipe 307 will contain a certain amount of fuel at a certain pressure.

Opening and closing an injector, e.g. the injector 301, so that it is open for a first period to inject an expected amount of fuel, then makes it possible for the amount actually injected to be determined on the basis of the length of the opening period and a determination of the pressure difference (pressure decrease) which occurs in the pipe 307 when part of the fuel stored in it is supplied to the engine' s combustion chamber by the opening of the injector 301. This pressure difference may for example be determined by means of suitable pressure sensors. The actual amount of fuel injected may then be compared with an expected amount, making it possible for the opening period stored in the vehicle's control system for a certain desired fuel amount to be shortened or lengthened as necessary to ensure that the amount actually injected will continue to correspond to the amount expected. This

determination is conducted individually for each injector 301- 306 and also for any different injection pressures employed in operating the engine.

Adaptation of the engine's injectors thus involves a number of injections which depends on the number of injectors and the number of different opening periods to be tested. Each injector/opening period combination may also be arranged to be tested more than once during an adaptation. In a complete adaptation of the engine's injectors, a large number of injections may thus be conducted. As the pressure difference in the fuel pipe 307 has to be determined for each respective injection, no other injection may take place simultaneously. However, adaptation usually still takes place when the vehicle is in motion, which means that it is usually conducted during so-called "dragging", i.e. in situations where the vehicle is running with the power train closed, i.e. with the engine 101 connected to the tractive wheels 113, 114 but with the fuel supply to the engine shut off. This usually happens when there is a reduced propulsive force requirement, e.g. on downhill runs. The fuel is also injected late enough during the stages of the combustion cycle to ensure that none or only some of it burns in the combustion chambers, with the result that fuel will accompany the exhaust flow to the post- treatment system. The injection may for example take place 30-40 crankshaft degrees, or still later, after top dead centre point. Such injection angles result in the fuel in principle not igniting at all but instead accompanying the exhaust flow in unburnt form. Adaptation thus involves unburnt fuel being supplied to the post-treatment system with which the vehicle is usually provided.

Growing official concern about pollution and air quality, especially in major urban areas, has led to the adoption of emission standards and requirements in many jurisdictions, and endeavouring to meet these requirements has led to the

development of systems for post-treatment (cleaning) of the exhaust gases which arise from combustion in combustion engines . These post-treatment systems often comprise some form of catalytic cleaning process whereby one or more catalysts are used to clean the exhaust gases. Vehicles with diesel engines are often provided with a particle filter to capture the soot particles formed during fuel combustion in the engine's combustion chambers.

Fig. 2 depicts in more detail the post-treatment system 200 for the vehicle depicted in Fig. 1, which is merely one example of such a system. It shows the vehicle's engine 101 from which the exhaust gases (the exhaust flow) generated by the combustion are led to the post-treatment system via a turbo unit 220. The turbo unit may be of different types and the function of various kinds of turbo unit is well-known and is therefore not described in more detail here. The exhaust flow is then led via a pipe 204 (indicated by arrows) to a particle filter (diesel particulate filter, DPF) 202 via an oxidation catalyst (diesel oxidation catalyst, DOC) 205.

The oxidation catalyst is normally used primarily to oxidise remaining hydrocarbons and carbon monoxide in the exhaust flow to carbon dioxide and water. The oxidation of hydrocarbons (i.e. the oxidation of unburnt fuel) results also in the formation of heat which may for example be utilised to raise the temperature of the particle filter at the time of its emptying, its so-called regeneration. Post-treatment systems of the kind here concerned may also comprise other components, e.g. a (single, in the present example) SCR (selective catalytic reduction) catalyst 201 situated downstream of the particle filter 202. SCR catalysts are generally used to reduce the amount of nitrogen oxides NO _x . The post-treatment system may also comprise more components than as exemplified above, or fewer components. It may for example comprise an ASC (ammonia slip catalyst) (not depicted) in addition to, or instead of, said DOC 205.

Adaptation will thus cause a supply of unburnt fuel to the post-treatment system. Although this is desired in certain situations, e.g. during certain kinds of regeneration of the particle filter, such supply during adaptation has undesired consequences. If the prevailing heat in the post-treatment system is high during adaptation, the unburnt fuel may oxidise (ignite) and thus further raise the temperature of the whole or parts of the post-treatment system. If this temperature rise is too great, there will be risk of damage to components of the post-treatment system. If conversely the prevailing temperature in the post-treatment system is low and the unburnt fuel therefore does not oxidise, hydrocarbons will for example become attached to the active sites of catalysts where the catalyst reaction takes place and will thereby prevent the actual substances intended to react in the catalyst from reaching the catalytic location. The catalyst will thus be "poisoned" by the unburnt fuel. Heating the catalyst to such a temperature that the stored fuel oxidises or vaporises may reverse this harmful poisoning, but too large amounts of fuel stored in the catalyst may cause irreversible, i.e. permanent, poisoning, with the result that the catalyst function may thus be somewhat further impaired each time adaptation takes place. In situations where large amounts of fuel have become stored in the catalyst it is possible that, when the catalyst is subsequently warmed and a large amount of fuel "slips" and at the same time oxidises, the catalyst temperature may at least locally rise to very high levels, with the result that the so- called "wash coat" which serves as a support for catalytic material and is used to increase the catalyst' s effective cross-section becomes overheated, in which case surfaces may sinter together and permanently reduce the catalyst's active cross-section. The overall effect is that hydrocarbons (i.e. unburnt fuel) readily become attached to various parts of the post-treatment system, with consequent risk of damage due to poisoning and/or overheating.

Supplying unburnt fuel to the post-treatment system may thus cause undesirable and potentially harmful storage of unburnt fuel. The present invention proposes a method and a system for reducing the risk that adverse effects in the post- treatment system might arise from adaptation of the engine' s injectors. A method example 400 according to the present invention is depicted in Fig. 4. The invention may be

implemented in any suitable control unit, e.g. the engine control unit 115 depicted, but equally well in a control unit dedicated to the present invention, or wholly or partly in one or more other control units with which the vehicle is already provided. Control systems in modern vehicles generally comprise a communication bus system consisting of one or more communication buses for connecting together a number of electronic control units (ECUs), e.g. the control unit or controller 115, and various components on board the vehicle. Such a control system may comprise a large number of control units and taking care of a specific function may be shared by two or more of them. For the sake of simplicity, only the control unit 115 appears in Fig. la.

Control units of the type here concerned are normally arranged to receive sensor signals from various parts of the vehicle. The function of the control unit 115 (or the control unit or units in which the present invention is implemented) will for example probably depend on information such as signals received from various sensors pertaining to the engine and from other control units, e.g. the control unit which takes care of temperature requirements in the post-treatment system, and/or signals from temperature sensors in the post-treatment system. Such control units are also usually adapted to delivering control signals to various parts and components of the vehicle, e.g. the control unit 115 will for example deliver signals to the injectors' actuators. Control is often governed by programmed instructions, typically in the form of a computer programme which, when executed in a computer or control unit, causes the computer/control unit to effect desired forms of control action, e.g. method steps according to the present invention. The computer programme usually forms part of a computer programme product which comprises a digital storage medium (see Fig. lb) with the computer

programme 109 stored on it. Said digital storage medium 121 may for example take the form of any from among ROM (read-only memory) , PROM (programmable read-only memory) , EPROM (erasable PROM) , flash memory, EEPROM (electrically erasable PROM) , a hard disc unit etc., and be situated in or in communication with the control unit, in which case the computer programme is executed by the control unit. The vehicle's behaviour in a specific situation may thus be modified by altering the computer programme's instructions.

An example of a control unit (the control unit 115) depicted schematically in Fig. lb may itself comprise a calculation unit 120 which may for example take the form of any suitable kind of processor or microcomputer, e.g. a circuit for digital signal processing (Digital Signal Processor, DSP), or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC) . The calculation unit 120 is connected to a memory unit 121 which provides it with, for example, the stored programme code 109 and/or the stored data which the calculation unit needs for it to be able to perform calculations. The calculation unit is also arranged to store partial or final results of calculations in the memory unit 121.

The control unit is further provided with respective devices 122, 123, 124, 125 for receiving and sending input and output signals. These signals may comprise waveforms, pulses or other attributes which the input signal receiving devices 122, 125 can detect as information for processing by the

calculation unit 120. The output signal sending devices 123, 124 are arranged to convert calculation results from the calculation unit to output signals for conveying to other parts of the vehicle's control system and/or the

component/components for which the signals are intended. Each of the connections to the respective devices for receiving and sending input and output signals may take the form of one or more from among a cable, a data bus, e.g. a CAN (Controller Area Network) bus, a MOST (Media Oriented Systems Transport) bus or some other bus configuration, or a wireless connection.

Reverting to Fig. 4, step 401 determines whether adaptation of injectors should commence. If such is the case, the method moves on to step 402 while at the same time a counter i is set to 1.

Moving on from step 401 to step 402 may be subject to various conditions. It may for example be decided that adaptation should take place because a certain time has passed since a previous adaptation and/or for some other reason. Prior art should generally be referred to for suitable conditions for commencement of adaptation. It may also be necessary as above that the vehicle be run with the fuel supply shut off, as when dragging, so it may also be necessary for this criterion to be fulfilled to enable the method to move on from step 401 to step 402. However, the invention is inherently appropriate in all cases where unburnt fuel is supplied during adaptation, even when it takes place with continuing fuel supply to generate propulsive force. The invention is nevertheless exemplified below for cases where adaptation takes place during dragging. As mentioned above, the number of injections during a complete adaptation procedure may be quite large, so there is no certainty of being able to complete an adaptation during the time when the vehicle is dragging. A total adaptation of all of the injectors for all of the injection periods may

therefore be spread over a smaller or larger number of

consecutive occasions where the vehicle is dragging. In addition to the criterion that there must be dragging there are also criteria described below which according to the present invention have to be fulfilled for adaptation to take place. The counter i represents injection no. i, where i may for example represent adaptation of a certain injector and a certain opening period. The adaptation may be arranged to cover a number of injectors/injection period combinations i as above whereby the injectors are adapted one at a time and for different opening periods and possibly different injection pressures .

At step 402, injection number i in an adaptation scheme takes place, in this case injection number 1. In cases where adaptation has previously been halted, e.g. because the vehicle changed from dragging to an operating condition in which torque is required and fuel is therefore injected to propel the vehicle, it is possible when moving on from step 401 to step 402 for i to be set instead to the next injection which has not yet taken place, so that adaptation may be resumed at the point where it was previously halted. The injection at step 402 takes place as above with the fuel supply to the fuel pipe 307 shut off. The pressure in the fuel pipe in such systems may for example be any suitable pressure within the range 1000-2000 bar.

When thereafter injection i has taken place at step 402, the method moves on to step 403 at which an amount Mi of fuel injected is determined for injection i. This determination may as above be based on the injector's opening period and the pressure/pressure change to which the fuel pipe 307 is

subjected at the time of injection i. The fuel amount may be determined in any suitable form, e.g. by volume and/or by weight. The method then moves on to step 404 or 405,

depending on a prevailing temperature T in the post-treatment system. If this temperature is below a temperature T ₀ , the method moves on to step 404 at which estimated fuel M _est stored in the post-treatment system is accumulated as a previous estimated amount M _est plus the amount Mi determined at step 403. If conversely the temperature T in the post-treatment system is above T ₀ , the method moves on instead to step 405 at which the estimated fuel M _est stored in the post-treatment system is determined as a function of time since previous injection i and prevailing temperature T. The temperature T may be arranged to be measured at suitable locations in the post- treatment system, e.g. at a particle filter and/or the

oxidation catalyst. Temperature T ₀ means that the temperature T in the post-treatment system is high enough for the injected fuel supplied in unburnt form to the post-treatment system to begin to oxidise and therefore not to the same extent result in undesired storage in the post-treatment system. T ₀ may for example be within the range 200-250°, but might also be higher or lower. The temperature indications are meant as examples which actual values may deviate from. The way in which the temperatures are determined/calculated may for example affect the temperature limits. With regard to the example of a post- treatment system depicted in Fig. 2, the temperature T may for example be determined upstream and/or downstream of the oxidation catalyst 205 and/or upstream and/or downstream of the particle filter 202. The temperature T ₀ may for example also be determined as a weighted value based on two or more temperature sensors. Some other suitable temperature sensor may also be used, e.g. in conjunction with a model of the post-treatment system and/or, for example, the exhaust flow at the time, to calculate a temperature T of the post-treatment system. At lower temperatures such that T<T ₀ , substantially no

oxidation takes place and fuel supplied is substantially stored. So long as the temperature T in the post-treatment system is below T ₀ , the injected fuel amounts Mi are thus accumulated at step 404. When the temperature T in the post- treatment system is above T ₀ , the fuel injected will wholly or partly oxidise, so the estimated stored fuel M _es t accumulated at step 405 takes this into account by using the time between the injections and prevailing temperatures T in the post- treatment system as a basis for subtracting an estimated amount of fuel oxidised from the estimated cumulative stored fuel Mest - The cumulative stored fuel M _es t may thus be

increased by a smaller amount than that determined at step 403 or may even, depending on prevailing temperature T, decrease despite injections having taken place. The method then moves on to step 406 for determination of whether the estimated stored fuel M _{e s} t is greater than or equal to a set limit M _L . This limit M _L may be set at any suitable amount of fuel such as, but absolutely not limited to, any desired number of grams of fuel within the range 1-50 grams, e.g. of the order of 10 grams. It may be set on the basis of the prevailing configuration of the particular post-treatment system and may also be arranged to vary with prevailing operating parameters of the vehicle. So long as it is

determined at step 406 that amounts of fuel injected are below the limit M _L , the injection counter i will increment by 1 and the method move on to step 413 to determine whether the adaptation has been completed, in which case the method ends at step 402. Otherwise it goes back to step 412 for a next injection i. It should be noted that the method illustrated may be subordinate to an overarching method whereby adaptation is halted if the vehicle changes from dragging to some other operating mode as above. The total amount of fuel

supplied/injected in an individual injection may for example be any suitable number of milligrams within the range 1-500 mg, so the limit M _L may be an amount which corresponds to two or more injections. If it is determined at step 406 that the estimated cumulative amount of fuel stored M _est is greater than the set limit M _L the method moves on to step 407 for determination of whether a prevailing temperature T in the vehicle's post-treatment system is within a range Τ ₀ <Τ<ΊΊ, with T ₀ defined as above.

The upper temperature limit T _x may be set at an upper level beyond which continued injection of unburnt fuel should not/is not allowed to take place when further temperature rise might cause risk of damage to components of the post-treatment system. So long as the temperature T at step 407 is within said range, the method goes back to step 402, via step 413 to determine whether the adaptation has been completed, while at the same time the injection counter i increments by 1.

If conversely it is found at step 407 that the temperature T is higher than the upper limit Ti, the method moves on to step 408 while at the same time a time counter ti is set to 0. Step 408 determines whether the temperature T continues to be higher than Ti, and so long as such is the case the method will stay at step 408 so that no further injections take place, to avoid risk of undesired/harmful temperature rise. When

thereafter the temperature T in the post-treatment system has dropped to ΚΤχ, the method goes back to step 402 while at the same time the injection counter i is set to i +1, but only after step 409 to determine whether the adaptation has been completed. If it has, the method ends at step 412.

As the high temperature will have caused any accumulated unburnt fuel M _est in the post-treatment system to at least partly oxidise, the cumulative estimated amount of fuel injected M _es t when the method moves on to step 402 is reduced in any suitable way, e.g. as a function of the amount of time ti which the method has spent at step 408 and/or the post- treatment system' s prevailing temperature T while the method has been at step 408.

Depending on the length of time t _x the method has stayed at step 408, the cumulative amount of fuel M _est may thus have been reduced to a greater or lesser extent, and if the method has been at step 408 long enough, this cumulative amount will have been reduced to zero, but might therefore, depending on time/temperature, assume some value between 0 and the

cumulative amount M _es t-

Instead of determining whether the temperature T is higher than Ti, it is instead possible for step 408 to compare whether the temperature T is higher than some temperature which is lower than Ti. In other words, a hysteresis function may be employed, since it may for example be inappropriate at step 408 to continue adaptation if the temperature T is below T _x by only one or part of a degree, in which case Ti might well be quickly exceeded again, with risk of the temperature T

oscillating about Ti, resulting in a slow adaptation procedure. It is generally the case that a similar hysteresis function may be employed on the other temperature limits applied with reference to Fig. 4.

Moreover, if at step 407 it is instead found that the

temperature T in the post-treatment system is below T ₀ , the method moves on to step 410 and stays there so long as such is the case. The reason for this is that so long as the

temperature T in the post-treatment system is below T ₀ no fuel or substantially no fuel will oxidise therein, which means that unburnt fuel supplied will wholly or at least largely be stored in the post-treatment system, with potential consequent damage as above. When the method has thus reached step 410 because the estimated cumulative amount stored M _es t is equal to or higher than the limit M _L , continued adaptation will thus result in the amount of unburnt fuel accumulated continuing to rise to still higher levels beyond M _L .

Thus when the estimated cumulative fuel amount M _est has reached M _L , the cumulative fuel amount M _es t will also have reached the limit for the amount of unburnt fuel accumulated in the post- treatment system which is permissible without substantial risk of components of the system being damaged by subsequent temperature rise. By this procedure it is possible to ensure during the injector adaptation that the amount of unburnt fuel stored in the post-treatment system is never more than can be allowed to oxidise, with associated further heat increase, without risk of damage due to subsequent temperature rise in the post-treatment system.

The method thus stays at step 410 so long as the temperature T in the post-treatment system is below T ₀ , since in this case substantially no oxidation of stored fuel will occur, nor will the amount of fuel stored decrease. When thereafter the temperature increases, the method moves on, provided that T is still below Ti, to step 411 while at the same time a timer t ₂ is started. If it is determined at step 410 that Τ>ΊΊ, the method moves on instead to step 408 as above. The temperature rise may for example be due, when adaptation has been halted, to the vehicle having travelled in conditions in which the engine worked actively and generated a hotter exhaust flow, resulting in associated temperature rise in the post-treatment system.

Step 411 determines first whether the adaptation has been completed, i.e. whether all of the injections involved have taken place. If such is the case, the adaptation ends at step 412. If the adaptation has not been completed, the method stays at step 411 until the timer t ₂ reaches a set time tT2. This value denotes a period of time during which, because the temperature T in the post-treatment system is higher than To, fuel stored in it will oxidise and the stored amount M _est will therefore decrease. The timer t ₂ may for example be set to a value which causes a decrease in the estimated fuel stored M _es t by an amount corresponding to one or more coming injections i. Alternatively, the period may be set to correspond to expected oxidation of x% of stored fuel, e.g. 10%, 50% or some other suitable percentage within the range 0-100%. The timer value may for example be arranged to depend on current temperatures in the post-treatment system. The more they exceed T ₀ , the quicker the oxidation of stored fuel will be and hence also the quicker the decrease in fuel stored.

Depending on how long a time tT2 the timer t ₂ has been set to, it may also be advantageous at step 411 to monitor the

temperature T. If for example t ₂ is set to a relatively long time, the temperature T may change during that time in such a way that it will, e.g. because of changing running conditions, exceed Ti, in which case the method may switch to step 408. Conversely, if T goes below T ₀ , the method may be arranged to go back to step 410.

In one embodiment no timer at all is set when the method moves on to step 411, when the temperature T ₀ may for example be set at such a level that fuel injected will certainly oxidise and there will be no increase in fuel stored so long as the temperature in the post-treatment system is above To-

The present invention thus proposes a method which by

monitoring the amount of unburnt fuel in the post-treatment system may reduce or completely eliminate problems such as poisoning and/or heating caused by fuel storage in the post- treatment system. It will be appreciated that the method depicted in Fig. 4 is merely one example of how the present invention might be applied. Moreover, the present invention is exemplified above in relation to vehicles. The invention is nevertheless also applicable to any other means of transport /processes in which post-treatment systems as above are applicable, e.g.

watercraft or aircraft with combustion processes as above. Further embodiments of the method and system according to the invention are referred to in the attached claims. It should also be noted that the system may be modified according to different embodiments of the method according to the invention (and vice versa) and that the present invention is in no way restricted to the embodiments described above of the method according to the invention, but relates to and comprises all embodiments within the protective scope of the attached independent claims .