A method for controlling the fuel supply to an engine

Abstract

A method for controlling the fuel supply to a spark-ignition direct injection engine, comprising comparing a current engine temperature with a predetermined engine temperature limit and, if the current engine temperature is below the predetermined engine temperature limit, operating the engine in a transient split injection mode in which at least two fuel injections are provided to each cylinder of the engine, wherein the predetermined engine temperature limit is set to an expected steady-state engine temperature for the engine when operating at the same load and engine speed minus a predetermined temperature difference from the steady-state temperature.

Description

[0001] This invention relates to the control of an internal combustion engine and in particular to the control of the fuel supply of a spark-ignition direct injection engine.

[0002] Raw engine particulate emissions from a spark-ignition direct injection engine during transient operation, in which the temperature of an engine's combustion chamber rises from a relatively low value to a higher value, contribute significantly to total particulate production in most emission driving cycles and under real driving conditions.

[0003] Documents US 6 330 796 B1 and EP 0 982 489 A2 describe methods and systems for controlling the fuel supply to a spark-ignition direct injection engine, wherein the temperature of the exhaust gases is increased to rapidly heat the catalyst by splitting the fuel injection into a partial injection before the ignition point and a partial injection after the compression stroke.

[0004] When operating in a low-load condition, surface temperatures in an internal combustion engine are relatively low, and any significant impact of fuel mist on surfaces causes so-called pool fires (burning puddles of fuel) which generate particulate emissions.

[0005] Transient temperature processes are processes in which the temperature rises from a relatively low value to a higher value, and lead, particularly at the lower temperature end of such a process, to an increase in particle generation due to the impact of fuel mist. Transient temperature processes are therefore characterized by significant particle formation, and it is desirable to minimize impact on surfaces during such transient temperature processes.

[0006] The object of this invention is to provide a method for controlling the fuel supply of a spark-ignition direct injection engine in order to reduce particle emissions during a transient temperature process.

[0007] The aforementioned problem is solved by the features of the independent patent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0008] A method for controlling the fuel supply to a spark-ignition direct injection engine is provided, which includes comparing a current engine temperature with a predetermined engine temperature limit and, if the current engine temperature is below the predetermined engine temperature limit, operating the engine in a transient split injection mode in which at least two fuel injections are provided to each cylinder, wherein the predetermined engine temperature limit is set to an expected steady-state engine temperature for the engine when operating at the same load and engine speed minus a predetermined temperature difference from the steady-state temperature.

[0009] This has the advantage that particle emissions from the engine are reduced during a transient temperature process.

[0010] Preferably, at least two fuel injections can be delivered to each cylinder of the engine during each intake stroke of the engine.

[0011] If the current engine temperature is greater than or equal to the pre-defined engine temperature limit, the procedure may still include operating the engine in a normal single-injection operating mode, in which each cylinder of the engine receives a single fuel injection.

[0012] The single fuel injection can be delivered to each cylinder during each intake stroke of the engine.

[0013] The total volume of fuel injected into a cylinder per intake stroke of the engine when operating in transient split injection mode can be equal to the volume of fuel that would be injected into the respective cylinder when using the normal injection operating mode for the same speed and load conditions.

[0014] When operating in transient split injection mode, each injection can consist of essentially the same volume of fuel.

[0015] If the engine speed is greater than a predefined upper speed limit, then the use of the transient split injection mode can be prevented.

[0016] This has the advantage that transient split injection is not used when it is unlikely to have a significant beneficial effect in terms of reducing particulate emissions.

[0017] If the engine load is lower than a predefined minimum load limit, then the use of the transient split injection mode can be prevented.

[0018] This has the advantage that transient split injection is not used when it is unlikely to have a significant beneficial effect in terms of reducing particulate emissions.

[0019] The procedure may also include generating a value of a current engine temperature through direct temperature measurement or temperature modeling.

[0020] The current engine temperature could be piston base temperature or cylinder wall temperature.

[0021] According to a second aspect of the invention, an engine system is provided comprising a spark-ignition direct-injection gasoline engine, a fuel supply system for the engine with at least one fuel injector arranged to inject fuel directly into a respective cylinder of the engine, an ignition system with at least one spark plug for igniting the fuel in each cylinder of the engine, and an electronic control unit for controlling the operation of the fuel supply system and the ignition system, wherein the electronic control unit is designed to compare a current engine temperature with a predetermined engine temperature limit and, if the current engine temperature is below the predetermined engine temperature limit, to operate the engine in a transient split-injection mode in which at least two fuel injections are provided to each cylinder of the engine.where the predefined engine temperature limit is set to an expected steady-state engine temperature for the engine when operating at the same load and engine speed minus a predefined temperature difference from the steady-state temperature.

[0022] The at least two fuel injections can be delivered to each cylinder of the engine during each intake stroke of the engine.

[0023] If the current engine temperature is greater than or equal to the pre-set engine temperature limit, the electronic control unit may be configured to operate the engine in a normal single-injection operating mode, in which each cylinder of the engine receives a single fuel injection.

[0024] Preferably, the single fuel injection can be delivered to each cylinder during each intake stroke of the engine.

[0025] The electronic control unit can be used to control the fuel supply system so that the total volume of fuel injected into a cylinder per intake stroke of the engine when operating in transient split injection mode is equal to the volume of fuel that would be injected into the respective cylinder when using the normal single injection operating mode for the same speed and load conditions.

[0026] When operating in transient split injection mode, the electronic control unit can be used to ensure that each injection consists of essentially the same volume of fuel.

[0027] The electronic control unit may be designed to receive a signal indicating a current engine speed, and if the current engine speed is greater than a pre-defined upper speed limit, the electronic control unit may be designed to prevent the use of the transient split injection mode.

[0028] The electronic control unit may be designed to estimate a current engine load, and if the current engine load is less than a pre-defined minimum load limit, the electronic control unit may be designed to prevent the use of the transient split injection mode.

[0029] The electronic control unit may still be designed to generate a value of a current engine temperature by direct temperature measurement using an associated temperature sensor or by modeling using a temperature model stored in the electronic control unit.

[0030] According to a third aspect of the invention, a motor vehicle is provided with an engine system designed according to the second aspect of the invention.

[0031] The invention will now be described by way of example with reference to the accompanying drawing, wherein: Fig. 1 a schematic diagram of a motor vehicle constructed according to a third aspect of the invention with an engine system constructed according to a second aspect of the invention; and Fig. 2 is a flowchart of a method for controlling the fuel supply of a spark-ignition direct injection engine according to a first aspect of the invention.

[0032] With reference to Fig. Figure 1 shows a motor vehicle 5 with an engine system 10. The engine system 10 comprises an engine in the form of a spark-ignition, three-cylinder, direct-injection gasoline engine 11. It is understood that the spark-ignition, direct-injection gasoline engine 11 could have more or fewer than three cylinders, and that the invention is not limited to use with three cylinders, nor is it limited to the use of gasoline as fuel, but that other suitable fuels could be used.

[0033] The engine 11 has an intake manifold 12 through which air enters the cylinders of the engine 11, as shown in Fig. 1 indicated by the arrow 'A', and an exhaust manifold 13, of which as in Fig. 1. Exhaust gases flow out of engine 11, as indicated by the arrow 'E'.

[0034] It is understood that engine 11 could be a naturally aspirated engine or a supercharged engine with a supercharger or turbocharger to increase the pressure of the air flowing to engine 11.

[0035] It is further understood that one or more exhaust aftertreatment devices are normally located downstream of the exhaust manifold 13 to reduce the level of exhaust emissions released into the atmosphere from the engine 11.

[0036] Fuel is supplied to the engine via a fuel supply system with a fuel line 14, which in this example supplies three fuel injectors 15a, 15b, and 15c with fuel at high pressure. Each of the fuel injectors 15a, 15b, and 15c is designed to inject fuel directly into the respective cylinder of the engine 11 to which it is assigned. The operation of the fuel injectors 15a, 15b, and 15c is controlled by an electronic control unit 20. As is well known in this field, the electronic control unit 20 controls the timing of the fuel injection as well as the amount of fuel injected.

[0037] In this example, ignition for engine 11 is provided by three spark plugs 16a, 16b, and 16c, each designed to produce a spark in its respective cylinder of engine 11. The operation of the spark plugs 16a, 16b, and 16c is controlled by the electronic control unit 20. As is well known in the field, the electronic control unit 20 controls the timing of the sparks produced by the spark plugs 16a, 16b, and 16c, i.e., the ignition timing of engine 11.

[0038] It is understood that, in the case of this example, both the fuel injection control and the ignition timing control may be included as part of a single electronic control unit, but in other examples there may be separate electronic control units for controlling fuel injection and ignition, which are linked together to provide the required control of the engine 11.

[0039] A torque request is provided by a user of the motor vehicle 5 by means of an accelerator pedal 17 which has a position sensor 19 which is designed to send a signal indicating the position of the accelerator pedal to the electronic control unit 20.

[0040] Although not shown, the electronic control unit 20 is designed to receive information from a number of other inputs that is necessary for controlling the operation of the engine 11. For example, the mass of air entering the engine can be provided by a MAF sensor, the rotational position of a crankshaft of the engine 11 can be provided by a crankshaft sensor, and the crankshaft sensor can be used to provide a value indicating the engine speed.

[0041] Such submissions are well known in the area and are not described in detail here.

[0042] The operation of the electronic control unit 20, insofar as this invention is concerned, is as follows.

[0043] When engine 11 is operating normally, its control system is conventional, controlling the ignition timing for each cylinder and the timing and quantity of a single fuel injection into each cylinder to produce a desired combination of torque and emissions. That is, in a 'normal injection mode' of operation, the fuel supply to engine 11 is provided by means of a single fuel injection at a predetermined crankshaft rotational position for each cylinder of engine 11.

[0044] However, when the engine 11 operates in a transient temperature condition, in which the temperature rises from a relatively low value to a higher value, the timing of the ignition for each cylinder is essentially the same as in the normal injection operating mode, but instead of providing a single fuel injection into each cylinder, two or more smaller fuel injections are supplied to each cylinder in order to reduce the risk of fuel impact on surfaces during such operations in a mode referred to herein as the 'transient split injection operating mode'.

[0045] A transient temperature condition exists, insofar as this invention is concerned, when components located in a combustion chamber of an engine, such as a combustion chamber wall or a piston crown, operate below an expected steady-state temperature for the current load for more than a predetermined amount, and there has been an increase in the engine load that leads to an increase in the temperature of the engine.

[0046] When a transient temperature condition is detected, the electronic control unit 20 serves to utilize the transient split injection operating mode to supply the engine 11 with the required fuel to meet the current load.

[0047] When operating in the transient split injection mode of the engine fuel supply, the engine 11 is supplied with the same quantity of fuel (a standard injection quantity) as it would normally be supplied by means of a single injection to match the current load, but it is delivered to it in two or more separate injections. That is, if two fuel injections are used, then each injection provides half of the standard injection quantity, and if three fuel injections are used, each injection delivers approximately one third of the standard fuel quantity.

[0048] In this example, the first fuel injection occurs at essentially the same crankshaft position during the intake stroke as a single injection during normal operation. The second and subsequent injections follow later at times determined by both the separation time achievable with the built-in injectors 15a, 15b, and 15c, and the engine speed 11. It is understood that when using transient split injection mode, the timing of the first injection could be adjusted, and more than one fuel injection could be provided.

[0049] For example, and without restriction, in the case of a dual injection arrangement, the first fuel injection can take place at approximately 300°VOT (before top dead center) and the second injection can take place at approximately 260°VOT.

[0050] To achieve the required functionality, the electronic control unit 20 has stored relationships between the engine operating state and the steady-state temperature when the engine operates in this state over a wide range of operating conditions (load / speed). The steady-state temperature relationship for a specific engine is obtained by performing test bench tests on one or more exemplary engines, and the values ​​are stored in the electronic control unit 20 in any suitable manner.

[0051] The electronic control unit 20 is further designed to derive the temperature of the exhaust gas exiting the engine or the temperature of critical components of the engine 11, insofar as particle generation is concerned, such as the temperature of the piston crowns and / or cylinder walls of the engine, either from an exhaust gas temperature model stored in a memory of the electronic control unit 20 or from a direct temperature measurement using one or more temperature sensors. In the case where the derived temperature is that of exhaust gas, it is understood that there is a verifiable relationship between this temperature and the temperature of the critical components of the engine 11.The electronic control unit 20 is therefore constantly supplied with information on the current engine temperature for use in deciding whether to use the normal injection mode or the transient split injection mode for the fuel supply of the engine 11.

[0052] In addition to the above, a predefined temperature difference (ΔT) from the steady-state temperature (T) is stored in the electronic control unit 20.

[0053] During operation of the motor 10, the electronic control unit 20 continuously monitors the current motor temperature (t) and compares it with the steady-state temperature (T) for the current operating conditions with respect to motor speed and torque demand or load, in order to assess, based on the steady-state temperature (T) for the current operating conditions and the required temperature difference (ΔT), whether the current temperature (t) is lower than a minimum temperature limit (T0).Lim ) is.

[0054] So, a test like this: Is t <TLim?

[0055] Where: TLim=T−ΔT; t is the current engine temperature; T is the steady-state temperature of the engine; and ΔT is the required predefined temperature difference from the steady-state temperature.

[0056] If the answer to the test is 'Yes', then the electronic control unit 20 is used to activate the operating mode with transient split injection, whereas the electronic control unit 20 is used to use the normal injection operating mode (with single injection) if the answer is 'No'.

[0057] For example, assuming that the steady-state temperature T for the current engine operating conditions is 750°C and ΔT = 200°C, then if the current engine temperature is greater than or equal to 550°C (in this case, the minimum temperature limit T) Lim ), the electronic control unit 20 uses the normal single injection mode, and if the current engine temperature (t) is less than 550°C, the electronic control unit 20 uses the transient split injection mode.

[0058] Therefore, if the engine is operating in a low-load condition that causes its temperature to drop, and a demand is made for more power from the engine, then it is likely that for the desired engine power, the current engine temperature (t) is more than ΔT degrees cooler than the steady-state temperature (T), and therefore the electronic control unit 20 activates a transient split injection until the temperature of the engine 11 reaches the minimum temperature limit (T). Lim ) is reached or exceeded, whereupon the normal fuel supply mode with single injection is activated by the electronic control unit 20.

[0059] In this way, the generation of particulate material by the engine 11 is reduced when the volume of fuel that hits relatively cold engine components such as the top of the pistons (piston crowns) and the cylinder walls is reduced.

[0060] It is understood that there may be physical limitations when using the transient split injection mode, such as engine speed and load.

[0061] For example, it is understood that as engine speed increases, the time per degree of crankshaft rotation decreases, and thus there is a limit at which the fuel injectors cannot precisely perform the desired number of injections within the required time. This depends on the type of fuel injectors used and the required injection times. As a further development of the above, an upper speed limit (N) can therefore be defined. max ) exists, above which the mode with transient split injection is not permitted due to physical limitations of fuel injection. In such a case, logic equation GL1 could be replaced by the following logic equation GL2. Is(t<(T−ΔT)AND N <Nmax?

[0062] Where: N is the current speed of motor 11 and T, T and ΔT have the same meaning as in logic equation GL 1.

[0063] If the answer to test GL2 is 'Yes', then electronic control unit 20 is used to activate the transient split injection operating mode, whereas if the answer is 'No', the electronic control unit is used to operate in the normal single injection mode. Therefore, a normal fuel supply is always used when the engine speed N is greater than N maxEven if the current engine temperature t were to indicate that using the transient split injection mode would reduce particulate emissions, this is because, even when using the transient split injection mode, the inability of the fuel injectors to react quickly or precisely enough would likely result in higher particulate emissions than using a well-timed single injection, as provided by the normal single injection mode.

[0064] In the case of load, it is understood that if the load is below a certain value, it might not be possible to precisely deliver such small fuel volumes from the fuel injectors when the fuel volume is split using multiple injections. Therefore, in some cases, a lower load limit can be set below which the transient split injection mode cannot be used.

[0065] In such a case, the logic equation GL1 could be replaced by the following logic equation GL3. Is (t<(T−ΔT)AND L>Lmin?

[0066] Where: L is the current load, L min the minimum load for which precise fuel metering can be delivered by the fuel injectors, and t, T and ΔT have the same meaning as in logic equation GL1.

[0067] If the answer to the GL3 test is 'Yes', then the electronic control unit 20 is used to activate the operating mode with transient split injection, whereas the electronic control unit 20 is used to use the normal single injection mode if the answer is 'No'.

[0068] It is understood that the logic equations GL2 and GL3 could be combined to provide a fourth logic equation GL4: Is(t<(T−ΔT)AND N<Nmax UND L> Lmin?

[0069] If the answer to the GL4 test is 'Yes', then the electronic control unit 20 is used to activate the operating mode with transient split injection, whereas the electronic control unit 20 is used to use the normal single injection mode if the answer is 'No'.

[0070] However, it is understood that the primary controlling factor is the engine temperature and that the other factors are limitations due to the physical capabilities of the fuel supply system and, in particular, the fuel injectors.

[0071] With reference to Fig. Figure 2 shows an overview flow diagram of a method (100) for controlling the fuel supply of a spark-ignition direct injection gasoline engine, such as engine 11.

[0072] Procedure 100 begins in box 110, which is a key-on operation or an equivalent operation that results in the starting of an engine of a motor vehicle, such as engine 11 of motor vehicle 5.

[0073] The procedure moves from box 110 to box 120, where the engine is running, and then on to box 130, where it checks whether the conditions for a transient split injection mode are met.

[0074] The primary condition for using the transient split injection mode is that the current engine temperature under the current operating conditions (load / speed) is lower than the expected steady-state temperature by more than a predetermined amount. Engine temperature is the temperature of a component or region known to have a real effect on particulate matter generation when operating at a relatively low temperature. Non-restrictive examples of such components are the engine's cylinder walls and the tops of the pistons (piston crown temperature). However, it is understood that the temperature of these components can be inferred from, for example, the temperature of the exhaust gas exiting the engine.

[0075] It is understood that when the engine's current temperature (t) reaches the expected steady-state temperature (T), the reduction in particulate matter achieved by using multiple split injections rapidly diminishes, and in some cases, the use of multiple split injections can have a negative impact on overall raw engine emissions. Therefore, to maximize the gains in reduced particulate emissions while minimizing adverse effects on other exhaust emissions, the use of multiple split injections is terminated before the engine's current temperature reaches the expected steady-state temperature.

[0076] That is, a minimum temperature limit (T) Lim The temperature for using multiple split injections is set below the steady-state temperature (T). The predefined temperature difference between the temperature limit (T) LimThe difference between the operating temperature (T) and the steady-state temperature (T) is denoted as Delta T (ΔT). In this example, it is a predetermined fixed value, but it could be a variable value based on other engine operating factors such as load or the magnitude of the steady-state temperature. The value of Delta T (ΔT) is set based on experimental data from an engine of the same type, from which it can be deduced when the advantage gained by using the transient split injection mode is minimal and potential disadvantages begin to become significant.

[0077] Referring again to box 130, if the current temperature (t) of the engine is less than (T Lim If ) is the case, then the transient split injection mode, which uses multiple fuel injections, is activated and the procedure advances to box 140; otherwise, it is not activated and the procedure advances to box 150.

[0078] Referring first to box 140, the engine is supplied with two or more fuel injections, the first of these injections occurring at substantially the same rotational position of the engine's crankshaft as when using the normal single-injection mode of fuel supply.

[0079] The subsequent fuel injection(s) occurs later than the first injection in the combustion cycle, with all injections taking place during a single intake stroke of the engine. It is understood that the first injection could occur at the same time used for the normal single-injection mode, or could be adjusted when using the transient split injection mode.

[0080] For example, and without restriction, in the case of a dual injection arrangement, if the primary time for a single fuel injection is at 320°VOT, then a subsequent injection could be scheduled for 240°VOT.

[0081] The total volume of fuel injected during the two injections is equal to the volume that would be injected using the normal single-injection mode to produce the desired engine power. In this example, the injected fuel quantity is divided equally among all the multiple injections. Therefore, in the case of two injections, each injection delivers approximately 50% of the total fuel supplied during the intake stroke of a single cylinder. However, it is understood that other ratios could be used.

[0082] From box 140, the procedure moves to box 160 to check if a key-off operation is present, and if a key-off operation is present, the procedure moves to box 190 where it ends; otherwise, the procedure returns from box 160 to box 130, where it checks again if the conditions for using the transient split injection mode are met.

[0083] The procedure then continues through boxes 130, 140 and 160 until a key-off operation occurs or until the conditions for using the transient split injection mode no longer exist.

[0084] If, with further reference to box 130, the conditions for using the transient split injection mode are not met when checking in box 130, the procedure proceeds to box 150, where the engine is operated normally using the normal single injection mode to inject fuel during the intake stroke of each cylinder in order to meet an actual torque demand for the engine.

[0085] From box 150, the procedure moves to box 160 to check if a key-off operation is present, and if a key-off operation is present, the procedure moves to box 190 where it ends; otherwise, the procedure returns from box 160 to box 130, where it checks again whether the conditions for using the transient split injection mode are met.

[0086] The procedure then continues through boxes 130, 150 and 160 until a key-off operation occurs or until the conditions for using the transient split fuel supply are met, at which point it advances to box 140 as described above.

[0087] The decision as to whether to use the transient split injection mode or the normal single injection mode is therefore primarily based on the temperature of potential particle-generating engine components, referred to as 'engine temperature', and in particular on whether the engine temperature is lower than an expected steady-state temperature for the engine when operating in such a state of load and speed.

[0088] Whenever the engine is cooler than an expected steady-state temperature for these load and speed conditions by more than a predetermined amount, the transient split injection mode, which uses multiple fuel injections, is advantageously used to reduce particulate emissions from the engine.

[0089] It is understood, however, that the physical limitations of the fuel delivery system must be taken into account, and for this reason the test in box 130 may also include restrictions on when the transient split injection mode can be used. For example, at higher engine speeds, the time available for fuel injection is shorter because the time it takes the crankshaft to rotate 360° BOT to the last available position at which fuel can be injected is shorter, and there is a minimum interval between injections based on injector functionality. That is, there is a maximum engine speed N max , which offers no real benefit when using multiple injections, partly due to the physical capabilities of the fuel injectors and the control system used, and partly due to combustion dynamics.

[0090] Similarly, there is a limitation based on engine load. Engine load can be estimated in various ways, including but not limited to the mass airflow into the engine, the torque delivered by the engine, manifold pressure, and the engine's volumetric efficiency based on the ratio of the volume of air entering a cylinder compared to the actual volume of the cylinder.

[0091] It is understood that at very low engine loads, the required amount of fuel is also low, and there is a load value below which the fuel injectors cannot precisely deliver the required amount of fuel when multiple injections are used. This means that the use of multiple injections is only possible above a predetermined load threshold L. min be used.

[0092] In practice, the test in box 130 may therefore require a number of conditions to be met before testing for the use of transient split fuel supply can be carried out.

[0093] For example, the test in box 130 could include three tests: (a) If L (the current engine load) is greater than L min (the minimum load for which precise fuel metering by the fuel injectors can be provided); AND (b) is N (the current motor speed) less than N max (the maximum engine speed that provides sufficient time to effect multiple fuel injections); AND c / If (a) and (b) are passed, is t < (T-ΔT)?

[0094] Only if all three of these tests are passed will the transient split fuel supply mode be used, and if one of the tests is not passed, then the normal single injection mode will be used.

[0095] Experimental tests using a motor where a value of 0.6 of the maximum load for L min The value of N was used max Setting the engine speed to 4000 rpm and using ΔT values ​​in the range of 250 to 200°C resulted in usable reductions in engine particulate emissions compared to using a single injection mode.

[0096] Whenever an engine runs cooler than an expected steady-state temperature by more than a predetermined amount (ΔT) under the same load and engine speed, a transient split injection mode is used to reduce the generation of particulate matter, provided the functionality of the fuel supply system allows such use.

[0097] Although the invention has been described with regard to embodiments in which the fuel injections take place in an intake stroke of the engine, it is understood that in a certain case the final injection of fuel could begin in the intake stroke and end in the early part of a compression stroke of the engine.

[0098] It is further understood by the person skilled in the art that although the invention has been described by way of example with reference to one or more embodiments, it is not limited to the disclosed embodiments, and that other embodiments could be developed without deviating from the scope of protection of the invention defined by the appended claims.

Claims

[1] A method for controlling the fuel supply to a spark-ignition direct injection engine, comprising comparing an actual engine temperature with a predetermined engine temperature limit and, if the actual engine temperature is below the predetermined engine temperature limit, operating the engine in a transient split injection mode in which at least two fuel injections are provided to each cylinder of the engine, wherein the predetermined engine temperature limit is set to an expected steady-state engine temperature for the engine when operating at the same load and engine speed minus a predetermined temperature difference from the steady-state temperature. [2] Method according to claim 1, wherein the at least two fuel injections are supplied to each cylinder of the engine during each intake stroke of the engine. [3] Method according to claim 1 or claim 2, wherein, if the current engine temperature is greater than or equal to the predetermined engine temperature limit, the method further comprises operating the engine in a normal single-injection operating mode, in which each cylinder of the engine is supplied with a single fuel injection. [4] Method according to any one of claims 1 to 3, wherein the total volume of fuel injected into a cylinder per intake stroke of the engine when operating in the transient split injection mode is equal to the volume of fuel that would be injected into the respective cylinder when using the normal injection operating mode for the same speed and load conditions. [5] Method according to claim 4, wherein when working in the transient split injection mode, each injection consists of substantially the same fuel volume. [6] Method according to any one of claims 1 to 5, wherein if the engine speed is greater than a predetermined upper speed limit, then the use of the transient split injection mode is prevented. [7] Method according to any one of claims 1 to 6, wherein if the engine load is less than a predetermined minimum load limit, then the use of the transient split injection mode is prevented. [8] Method according to any one of claims 1 to 7, wherein the method further comprises generating a value of a current engine temperature by direct temperature measurement or temperature modeling. [9] Method according to claim 8, wherein the current engine temperature is a piston bottom temperature or cylinder wall temperature. [10] Engine system comprising a spark-ignition direct-injection gasoline engine, a fuel supply system for the engine with at least one fuel injector designed to inject fuel directly into each cylinder of the engine, an ignition system with at least one spark plug for igniting the fuel in each cylinder of the engine, and an electronic control unit for controlling the operation of the fuel supply system and the ignition system, wherein the electronic control unit is designed to compare a current engine temperature with a predetermined engine temperature limit and, if the current engine temperature is below the predetermined engine temperature limit, to operate the engine in a transient split-injection mode in which at least two fuel injections are provided to each cylinder of the engine,where the predefined engine temperature limit is set to an expected steady-state engine temperature for the engine when operating at the same load and engine speed minus a predefined temperature difference from the steady-state temperature. [11] System according to claim 10, wherein the at least two fuel injections are supplied to each cylinder of the engine during each intake stroke of the engine. [12] System according to claim 10 or claim 11, wherein, when the current engine temperature is greater than or equal to the predetermined engine temperature limit, the electronic control unit is designed to operate the engine in a normal single-injection operating mode in which each cylinder of the engine receives a single fuel injection. [13] System according to one of claims 10 to 12, wherein the electronic control unit serves to control the fuel supply system such that the total volume of fuel injected into a cylinder per intake stroke of the engine when operating in the transient split injection mode is equal to the volume of fuel that would be injected into the respective cylinder when using the normal single injection operating mode for the same speed and load conditions. [14] System according to claim 13, wherein when operating in the transient split injection mode, the electronic control unit serves to ensure that each injection consists of substantially the same volume of fuel. [15] System according to any one of claims 10 to 14, wherein the electronic control unit is designed to receive a signal indicating a current engine speed, and wherein, if the current engine speed is greater than a predetermined upper speed limit, the electronic control unit is designed to prevent the use of the transient split injection mode. [16] System according to any one of claims 10 to 15, wherein the electronic control unit is designed to estimate a current engine load, and wherein, if the current engine load is less than a predetermined minimum load limit, the electronic control unit is designed to prevent the use of the transient split injection mode. [17] System according to any one of claims 10 to 16, wherein the electronic control unit is further designed to generate a value of a current engine temperature by direct temperature measurement using an associated temperature sensor or by modeling using a temperature model stored in the electronic control unit. [18] Motor vehicle with an engine system according to any one of claims 10 to 17.

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