Method and device for controlling an internal combustion engine

An internal combustion engine is provided with at least one modulator for adjusting an air mass in a cylinder. It is also provided with an injection valve for metering fuel to which fuel is supplied via a fuel supply device. A maximum fuel quantity which can be metered to the cylinder per working stroke is determined. Depending on the maximum meterable fuel quantity, a maximum producible torque is determined. An air mass flow is determined depending on an air/fuel ratio to be adjusted and the maximum meterable fuel quantity is adjusted by controlling the at least one modulator for adjusting the air mass. The injection valve is controlled in accordance with the maximum meterable fuel quantity when the required torque is greater or equal the maximum producible torque.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2005/053942, filed Aug. 10, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2004 047 622.5 filed Sep. 30, 2004, both of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to a method and a device for controlling an internal combustion engine.

BACKGROUND OF THE INVENTION

The performance and efficiency of internal combustion engines are subject to increasingly stringent requirements. Also the pollutant emissions produced by internal combustion engines have to be kept low due to strict legal provisions. To this end final control elements are provided, which allow a very high level of air delivery to be ensured over wide operating areas of the internal combustion engine. Injection valves are also used, to which fuel is supplied at high pressure, and which then meter said fuel into an intake tract or preferably directly into a cylinder of the internal combustion engine. The high fuel pressure means on the one hand that the fuel can be metered in within a very short time. This for example allows operation with a non-homogenous air-fuel mixture in the cylinder, also referred to as layer operation. On the other hand the high pressure of the fuel allows very fine atomization of the fuel particles, which is favorable for the combustion process, in particular in respect of pollutant emissions.

SUMMARY OF INVENTION

The object of the invention is to create a method and device for controlling an internal combustion engine, which respectively allow user-friendly operation of the internal combustion engine.

The object is achieved by the features of the claims. Advantageous embodiments of the invention are characterized in the subclaims.

The invention is characterized by a method and a corresponding device for controlling an internal combustion engine, with at least one final control element for setting an air mass in a cylinder, with an injection valve for metering in fuel, to which fuel is supplied by way of a fuel supply facility. A maximum fuel mass that can be metered into the cylinder per working cycle is determined, when a required torque is greater than or equal to the maximum torque that can be produced. A maximum torque that can be produced is determined as a function of the maximum fuel mass that can be metered in, when a required torque is greater than or equal to the maximum torque that can be produced. An air mass flow to be set is determined as a function of an air/fuel ratio to be set and the maximum fuel mass that can be metered in, when a required torque is greater than or equal to the maximum torque that can be produced. The air mass flow to be set is set by corresponding activation of the at least one final control element for setting the air mass, also when a required torque is greater than or equal to the maximum torque that can be produced. The required torque here refers to a torque that represents the wish of a driver of a motor vehicle, in which the internal combustion engine can be disposed, or even further torque requirements of functions for controlling the internal combustion engine or further units of the vehicle.

It is thus possible to ensure a good drive response of the internal combustion engine, even when there is an error in the fuel supply facility, resulting in a pressure drop in the fuel pressure. Such an error can result in a very significant pressure drop, particularly in the case of a fuel supply facility, which supplies fuel at very high fuel pressure, for example several hundred bar. By setting the air mass flow into the respective cylinder as a function of the maximum fuel mass that can be metered in, it is possible to produce the maximum torque in the respective operating point of the internal combustion engine in the cylinder or cylinders of the internal combustion engine, thereby ensuring a good drive response on the part of the internal combustion engine.

According to an advantageous embodiment of the invention, the maximum fuel mass that can be metered in is determined as a function of a cylinder segment period and a fuel pressure of the fuel, which is supplied to the injection valve. The fuel pressure is determined in a unit for determining the pressure of the fuel. This can be a suitable fuel sensor for example or can even be embodied to determine the fuel pressure as a function of further measured variables, which are detected by sensors of the internal combustion engine.

A cylinder segment period is the time period required for a working cycle, divided by the number of cylinders of the internal combustion engine. In the case of a four-stroke internal combustion engine with four cylinders for example, the cylinder segment period is obtained from the reciprocal value of half the rotational speed divided by the number of cylinders of the internal combustion engine.

It is thus possible to determine the maximum fuel mass that can be metered in particularly simply and by taking the cylinder segment period into account it is also possible in a simple manner to prevent a further pressure drop in the fuel pressure with a high level of probability.

According to a further advantageous embodiment of the invention, the maximum fuel mass that can be metered in is reduced as a function of a gradient of the pressure of the fuel supplied to the injection valve. It is thus possible, if there is an error in the fuel supply facility, to prevent an undesirably large drop in torque in a particularly effective manner, thereby achieving the most constant maximum torque possible.

In a further advantageous embodiment of the invention the at least one final control element is activated to set the air mass in the sense of minimizing a residual gas level in the cylinder, when the required torque is greater than or equal to the maximum torque that can be produced. It is thus possible effectively to prevent the maximum fuel mass to be metered in having to be reduced because the air mass is too small, which would result in a reduction of the torque.

According to a further advantageous embodiment of the invention the method is started, when the fuel pressure is lower by a predetermined threshold value, either absolutely or relative to a fuel pressure to be set, in particular for a predetermined time period. This means that the fuel mass is then only correspondingly limited, when there is an error in the fuel supply facility.

Also the required torque is frequently higher than the maximum torque that can be produced, particularly when there is an error in the fuel supply facility. It is thus still possible to ensure good driveability when subject to the basic conditions of the error.

DETAILED DESCRIPTION OF INVENTION

An internal combustion engine (FIG. 1) has an intake tract1, an engine block2, a cylinder head3and an exhaust gas tract4. The intake tract1preferably has a throttle valve5, also a manifold6and an intake pipe7, which leads to a cylinder Z1via an intake duct into the engine block2. The engine block2also has a crankshaft8, which is coupled via a connecting rod10to the piston11of the cylinder Z1.

The cylinder head3has a valve drive with a gas inlet valve12, a gas outlet valve13and valve drives14,15. The valve drives14,15have or are assigned a camshaft, having cams, which act on the gas inlet valve12and/or the gas outlet valve13. A separate camshaft is preferably assigned respectively to the gas inlet valve12and the gas outlet valve13.

A valve lift adjustment device19can also be provided, to change the lift pattern, allowing a low and high valve lift to be set for example. A phase adjustment device20can also be provided, by means of which a phase angle of the respective camshaft can be adjusted. Phase angle refers to an angle, for example the crankshaft angle between two reference marks, one on the crankshaft and the other on the respective camshaft, in relation in each instance to an absolute position either of the crankshaft or the camshaft.

By varying the phase angle it possible optionally to set a valve overlap, in other words a region, in which both the gas inlet valve12and the gas outlet valve13release the inlet or, respectively, outlet.

The gas inlet valve12, the valve lift adjustment device19and the phase adjustment device20form final control elements to set an air mass in the respective cylinder Z1. Further such final control elements can be provided and are for example formed by the throttle valve5, a switching valve in the intake pipe or manifold, a pulse charging valve or even a turbocharger.

The cylinder head3also has an injection valve, which is disposed in such a manner that it can meter fuel into a combustion chamber of the cylinder1. Alternatively however the injection valve23can also be disposed in the intake pipe7. The cylinder also preferably has a spark plug23.

The internal combustion engine also has a fuel supply facility26. The fuel supply facility26has a fuel tank28, connected by way of a first fuel line to a low-pressure pump30. On the output side the low-pressure pump30is connected to an intake34of a high-pressure pump36. A mechanical regulator32is also provided on the output side of the low-pressure pump30, being connected on the output side to the fuel tank28by way of a further fuel line. The low-pressure pump30, the mechanical regulator32, the fuel line, the further fuel line and the intake34form a low-pressure circuit.

The low-pressure pump30is preferably designed such that it always supplies a sufficiently large quantity of fuel during operation of the internal combustion engine, ensuring that there is no drop to below a predetermined low pressure.

The high-pressure pump is configured such that it delivers the fuel to a fuel storage unit38on the output side. The high-pressure pump36is generally coupled to the camshaft on the drive side and is thus driven by said camshaft and delivers a constant volume of fuel into the fuel storage unit38at a constant rotational speed N of the crankshaft8.

The injection valves22are connected to the fuel storage unit38. The fuel is thus supplied to the injection valves22by way of the fuel storage unit38.

Before or upstream of the high-pressure pump36a volume flow control valve40is provided, which can be used to set the volume flow supplied to the high-pressure pump36. It is possible to ensure, by corresponding activation of the volume flow control valve40, that the required fuel pressure prevails in the fuel storage unit, without an electromagnetic regulator having to be provided on the output side of the fuel storage unit38with a corresponding feedback line into the low-pressure circuit.

Alternatively however the internal combustion engine can also be provided with an electromagnetic regulator on the output side of the fuel storage unit38and with a corresponding feedback line into the low-pressure circuit. Alternatively it is also possible for the volume flow control valve40to be integrated in the high-pressure pump54.

A control device44is provided, to which sensors are assigned, which detect different measured variables and determine the value of the measured variable in each instance. The control device44determines manipulated variables as a function of at least one measured variable, said manipulated variables then being converted to one or more actuating signals to control the final control elements by means of corresponding actuators. The control device44can also be referred to as a device for controlling the internal combustion engine. It has a data and program storage unit and a computation unit, in which programs for controlling the internal combustion engine are processed during operation of the internal combustion engine.

The sensors are a pedal position sensor46, which detects the position of an accelerator pedal48, a throttle valve position sensor52, which detects an opening angle of the throttle valve5, a temperature sensor54, which detects an intake air temperature, a crankshaft angle sensor58, which detects a crankshaft angle, to which a rotational speed N is then assigned. A camshaft angle sensor58is also preferably provided, which detects a camshaft angle. If there are two camshafts present, a specific camshaft angle sensor is preferably assigned to each camshaft. An exhaust gas probe62is also provided, which detects a residual oxygen content of the exhaust gas and the measurement signal of which is characteristic of the air/fuel ratio in the cylinder Z1. A fuel pressure sensor42is also provided, which is used to determine a fuel pressure FUP/AV in the fuel storage unit38.

Any sub-set of the said sensors or even additional sensors can be present, depending on the embodiment of the invention.

Final control elements of the internal combustion engine are for example the throttle valve5, the gas inlet and gas outlet valves12,13, the valve lift adjustment device19, the phase adjustment device20, the injection valve22or the spark plug23.

As well as the cylinder Z1, further cylinders Z2-Z4are also preferably provided, to which corresponding final control elements and optionally corresponding sensors are similarly assigned.

A program for controlling the internal combustion engine is stored in the program storage unit of the control device44and can be processed during operation of the internal combustion engine. The program is started in a step S1(FIG. 2), in which variables are optionally initialized. The start preferably takes place at a time near to the time when the motor is started.

In a step S2it is verified whether a difference between a fuel pressure to be set FUP_SP and a determined fuel pressure FUP_AV is greater than a threshold value FUP_THD, which is predetermined in an appropriate manner. The threshold value FUP_THD is preferably predetermined such that it is representative of a fuel pressure drop indicating an error in the fuel supply facility26. It is thus preferably predetermined as a function of a delivery volume of the high-pressure pump and/or a fuel temperature and/or the rotational speed. Alternatively in step S2a quotient of the fuel pressure to be set FUP_SP and a quotient of the determined fuel pressure FUP_AV can be calculated and compared with the threshold value FUP_THD. Alternatively it can also be verified in step S2whether an integral of the difference between the fuel pressure to be set FUP_SP and the determined fuel pressure FUP_AV is greater than the threshold value FUP_THD, which is then similarly predetermined in an appropriate manner. It can also be verified in step S2whether the determined fuel pressure FUP_AV is below a further threshold value.

If the condition of step S2is not satisfied, processing is continued in a step S4, in which the program is preferably interrupted for a predetermined waiting period or a predetermined crankshaft angle, before processing is resumed in step S2. If however the condition of step S2is satisfied, processing is continued in a step S6. In an alternative embodiment of the program step S2can be dispensed with and processing can be continued directly in step S6.

A cylinder segment period T_SEG is determined in step S6. The cylinder segment period can be determined simply as a function of the rotational speed N and the number of cylinders Z1-Z4. In the case of a two-stroke internal combustion engine with four cylinders, it can be determined from a quotient of a reciprocal value of half the rotational speed N and the number of cylinders.

In a subsequent step S8a maximum fuel mass MFF_MAX that can be metered into the respective cylinder Z1-Z4per working cycle is calculated as a function of the cylinder segment period T_SEG and the determined fuel pressure FUP_AV. This can be done for example by means of a previously determined set of characteristics or even by means of an analytical relationship. The link between the maximum fuel mass MFF_MAX that can be metered in and the cylinder segment period T_SEG and the determined fuel pressure FUP_AV is preferably determined beforehand by tests on an engine test bed or even by simulations.

It can be ensured by means of the dependency on the cylinder segment period T_SEG that a maximum period required to meter in the maximum fuel mass MFF_MAX that can be metered in does not in any case exceed the cylinder segment period T_SEG. It is thus possible in a simple manner to reduce significantly the probability of the fuel pressure, in other words the determined fuel pressure FUP_AV, dropping in an undesirable manner.

In a step S10a maximum torque TQ_MAX that can be produced is then determined as a function of the maximum fuel mass MFF_MAX that can be metered in and an air/fuel radio LAM_SP to be set. The air fuel ratio to be set can for example be predetermined in a fixed manner but is preferably determined by a function for controlling the internal combustion engine or by a further function for controlling the internal combustion engine during operation of the internal combustion engine. Alternatively, when determining the maximum torque that can be produced, it is also possible to take into account a value of a manipulated variable of a lambda controller that is optionally present. It is also possible to take further influencing variables into account in this process.

In a step S12a required torque TQ_REQ is then read in, which is determined in a further function of the internal combustion engine, preferably for example as a function of the position of the accelerator pedal48and optionally further torque requirements, for example from units, such as a transmission.

In a step S14it is verified whether the required torque TQ_REQ is greater than the maximum torque TQ_MAX that can be produced.

If this is not the case, in a step S16an air mass flow MAF_CYL to be set in the respective cylinder Z1-Z4is determined as a function of the required torque TQ_REQ. The air mass flow MAF_CYL to be set in the respective cylinder corresponds to the air mass flowing into the respective cylinder Z1-Z4per working cycle.

In a step S18an actuating signal S_IM is determined for at least one of the final control elements for setting the air mass, as a function of the air mass flow MAF_CYL to be set. Also in step S18an actuating signal S_INJ for activating the injection valve22is determined, as a function of the air mass flow MAF_CYL into the cylinder to be set and the air/fuel ratio LAM_SP in the cylinder to be set, optionally taking into account the value of the manipulated variable of the lambda controller.

Processing is then continued in step S4.

If however the condition of step S14is satisfied, in a step S20the air mass flow MAF_CYL to be set is determined as a function of the maximum fuel mass MAF_MAX that can be metered into the respective cylinder Z1-Z4per working cycle and the air/fuel ratio to be set.

In a step S22at least one actuating signal S_IM for the at least one final control element for setting the air mass is determined as a function of the air mass flow MAF_CYL to be set. In this context the determination of the actuating signal(s) S_IM for the final control elements for setting the air mass preferably takes place in such a manner that the residual gas level in the cylinder before combustion of the air/fuel mixture is minimized, in order to be able to ensure that the highest possible torque is produced. The actuating signal S_INJ for activating the injection valve22is also determined, as a function of the maximum fuel mass MFF_MAX that can be metered into the cylinder per working cycle. The program is then continued in step S4.

It is particularly advantageous if, as an alternative to step S8, a step24is carried out, in which the maximum fuel mass MFF_MAX that can be metered in is determined as a function of the cylinder segment period T_SEG, the determined pressure FUP_AV and also as a function of a gradient FUP_GRD of the fuel pressure. It is thus possible to prevent a further undesirable pressure drop in the fuel pressure in a simple manner.