METHOD FOR ADJUSTING A FUEL MASS TO BE INJECTED

Abstract

A method for adjusting a fuel mass to be injected into an internal combustion engine. The internal combustion engine including an intake tract, at least one cylinder, and an exhaust tract. In the method, an air mass introduced into the internal combustion engine is ascertained and a fuel mass to be injected into the internal combustion engine is determined. An air-fuel ratio in the exhaust tract of the internal combustion engine is determined which is adjusted in time. Based on the time-adjusted air-fuel ratio and the calculated fuel mass to be injected, a first wall film fuel mass is calculated and the fuel mass to be injected is adjusted based on the first wall film fuel mass.

Claims

1. A method for adjusting a fuel mass to be injected into an internal combustion engine, the internal combustion engine including an intake tract, at least one cylinder, and an exhaust tract, the method comprising the following steps: ascertaining an air mass introduced into the at least one cylinder of the internal combustion engine; determining a fuel mass to be injected into the at least one cylinder of the internal combustion engine; determining an air-fuel ratio in the exhaust tract of the internal combustion engine; temporally adjusting the determined air-fuel ratio to obtain a time-adjusted air-fuel ratio; determining a first wall film fuel mass based on the time-adjusted air-fuel ratio, the ascertained introduced air mass, and the determined fuel mass to be introduced; and adjusting the fuel mass to be injected based on the calculated first wall film fuel mass.

2. The method as recited in claim 1, wherein the temporal adjustment of the determined air-fuel ratio is ascertained based on a model of the exhaust tract.

3. The method as recited in claim 2, wherein the temporal adjustment of the determined air-fuel ratio includes a dead time caused by the exhaust tract and a time delay caused by the determination.

4. The method as recited in claim 3, wherein the dead time is adjusted using predetermined characteristic data and/or the time delay is adjusted using a filter transfer function.

5. The method as recited in claim 1, wherein the adjusting of the fuel mass to be injected based on the calculated first wall film fuel mass includes a determination of an adaptation factor from the first wall film fuel mass and a second wall film fuel mass.

6. The method as recited in claim 5, wherein the second wall film fuel mass is determined from a wall film model.

7. The method as recited in claim 6, wherein a least squares estimator is used for the ascertaining of the adaptation factor.

8. A computing unit configured to adjust a fuel mass to be injected into an internal combustion engine, the internal combustion engine including an intake tract, at least one cylinder, and an exhaust tract, the computing unit configured to: ascertain an air mass introduced into the at least one cylinder of the internal combustion engine; determine a fuel mass to be injected into the at least one cylinder of the internal combustion engine; determine an air-fuel ratio in the exhaust tract of the internal combustion engine; temporally adjust the determined air-fuel ratio to obtain a time-adjusted air-fuel ratio; determine a first wall film fuel mass based on the time-adjusted air-fuel ratio, the ascertained introduced air mass, and the determined fuel mass to be introduced; and adjust the fuel mass to be injected based on the calculated first wall film fuel mass.

9. An internal combustion engine, comprising: an intake tract; at least one cylinder; an exhaust tract; and a computing unit configured to adjust a fuel mass to be injected into the internal combustion engine, the computing unit configured to: ascertain an air mass introduced into the at least one cylinder of the internal combustion engine; determine a fuel mass to be injected into the at least one cylinder of the internal combustion engine; determine an air-fuel ratio in the exhaust tract of the internal combustion engine; temporally adjust the determined air-fuel ratio to obtain a time-adjusted air-fuel ratio; determine a first wall film fuel mass based on the time-adjusted air-fuel ratio, the ascertained introduced air mass, and the determined fuel mass to be introduced; and adjust the fuel mass to be injected based on the calculated first wall film fuel mass.

10. A non-transitory machine-readable storage medium on which is stored a computer program for adjusting a fuel mass to be injected into an internal combustion engine, the internal combustion engine including an intake tract, at least one cylinder, and an exhaust tract, the computer program, when executed by a processor, causing the processor to perform the following steps: ascertaining an air mass introduced into the at least one cylinder of the internal combustion engine; determining a fuel mass to be injected into the at least one cylinder of the internal combustion engine; determining an air-fuel ratio in the exhaust tract of the internal combustion engine; temporally adjusting the determined air-fuel ratio to obtain a time-adjusted air-fuel ratio; determining a first wall film fuel mass based on the time-adjusted air-fuel ratio, the ascertained introduced air mass, and the determined fuel mass to be introduced; and adjusting the fuel mass to be injected based on the calculated first wall film fuel mass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 shows a schematic and sectional view of an internal combustion engine such as may form the basis of a preferred specific embodiment of the present invention.

[0042] FIGS. 2A and 2B schematically show a wall film model and an exhaust tract model according to a preferred specific embodiment of the present invention.

[0043] FIG. 3 shows a preferred specific embodiment of a method according to the present invention in a block diagram.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0044] FIG. 1 shows a schematic and sectional view of an internal combustion engine such as may form the basis of a preferred specific embodiment of the present invention. The internal combustion engine shown has an intake tract 2, a cylinder 1 and an exhaust tract 9. An inlet valve 5 and an outlet valve 7 are situated in cylinder 1, closing cylinder 1 off relative to intake tract 2 and exhaust tract 9. A throttle valve 6 and an injection valve 4 are arranged in intake tract 2. Injection valve 4 injects fuel before intake valve 5. When the intake valve opens, a portion of the injected fuel enters cylinder 1 directly together with the air mass flow passing through throttle valve 6, while the other portion is deposited on the walls of intake tract 2.

[0045] A lambda sensor 8 is situated in exhaust tract 9 of the internal combustion engine shown, which sensor determines the residual oxygen in the exhaust gas of the engine in order to determine the air-fuel ratio of the combusted mixture.

[0046] In addition, the internal combustion engine includes a computing unit 3, which can for example be the engine control device, which is connected to throttle valve 6, injection valve 4, and lambda sensor 8. Computing unit 3 can receive the signals from the sensors of the combustion engine (e.g. lambda sensor 8) and control the actuators of the combustion engine (e.g. throttle valve 6 and injection valve 4).

[0047] Computing unit 3 can, for example, receive the output signal of the lambda probe 8, calculate the first wall film fuel mass based thereon, and adjust the fuel mass to be injected by injection valve 4 accordingly.

[0048] From FIG. 1, it is clear that there is a storage volume between exhaust valve 7 and lambda sensor 8, which results in a delayed measurement of the air-fuel mixture in cylinder 1.

[0049] FIG. 2a schematically shows a model for the wall film behavior of the fuel injected into intake tract 2 (wall film model). According to this model, the injected fuel mass flow {dot over (m)}.sub.inj is divided into a portion {dot over (m)}.sub.direct, which enters cylinder 1 directly in the current working cycle of the engine, and a portion {dot over (m)}.sub.indirect, which is temporarily stored in a wall film 10 and enters cylinder 1 with a time delay as wall film fuel mass {dot over (m)}.sub.wf. Based on the fuel properties (faster and slower boiling portions), a distinction is made between a portion {dot over (m)}.sub.wf,fast, which evaporates faster from wall film 10 and thus enters cylinder 1 earlier, and a portion {dot over (m)}.sub.wf,slow, which evaporates more slowly and enters cylinder 1 later. The fuel mass flow {dot over (m)}.sub.wf evaporating from wall film 10 or stored in the wall film is added to the fuel mass flow {dot over (m)}.sub.direct, resulting in the total mass flow {dot over (m)}.sub.fuel entering cylinder 1. Depending on the operating point, more or less fuel is evaporated from wall film 10 or is stored in the wall film. By taking into account the wall film fuel mass flow {dot over (m)}.sub.wf in the engine control unit, the injected fuel mass flow {dot over (m)}.sub.inj is corrected accordingly, and the fuel mass flow {dot over (m)}.sub.fuel that results in a desired air-fuel ratio enters cylinder 1.

[0050] The wall film model is standardly adjusted to a limited number of vehicles during the application of the engine controlling. In order to correctly take into account wall film effects over the life of a vehicle and for different vehicles, the advantageous specific embodiment of the present invention described herein includes an adaptation of the wall film fuel mass flow rate calculated using the wall film model {dot over (m)}.sub.wf,2, which is described below in connection with FIG. 3.

[0051] The adaptation makes use of a measured air-fuel ratio λ.sub.sens to determine the real fuel mass entering the cylinder. Because the measured signal of the lambda probe λ.sub.sens reflects the air-fuel ratio λ in the cylinder with a time delay, this time delay has to be taken into account in the adaptation.

[0052] FIG. 2b schematically shows an exhaust tract model in which the stretch between exhaust valve 7 and lambda sensor 8 is modeled as a container in order to represent the storage behavior of exhaust tract 2. The air-fuel ratio λ.sub.sens measured at the lambda sensor 8 has a dead time τ.sub.del and a time delay compared to the air/fuel mixture λ present at the exhaust valve, which is described by a delay function with the time constant τ.sub.exh. The dead time τ.sub.del can, for example, be mapped on the basis of a characteristic curve that is a function of the exhaust gas mass flow of the engine, which can be ascertained for example on an engine test bench. Both model parameters τ.sub.del and τ.sub.exh are a function of the operating point of the engine (e.g. of the engine rotational speed).

[0053] FIG. 3 shows a preferred specific embodiment of the method according to the present invention in a block diagram. Here, function blocks 21 and 22 describe the controlled system, namely the formation and the delayed behavior of the measured air-fuel ratio λ.sub.sens, and function blocks 10 to 16 and 23 and 30 (in the dashed box) describe the feedforward controlling and adaptation of the wall film fuel mass flow {dot over (m)}.sub.wf,2.

[0054] In function block 21, the air-fuel ratio λ prevailing in cylinder 1 is calculated based on the input variables air mass flow {dot over (m)}.sub.air, fuel mass flow {dot over (m)}.sub.fuel and stoichiometric air requirement L.sub.st. The fuel mass flow {dot over (m)}.sub.fuel entering cylinder 1 is here made up of the fuel mass flow {dot over (m)}.sub.direct, which enters the cylinder from the current injection, and the wall film fuel mass flow {dot over (m)}.sub.wf,2.

[0055] The delayed behavior of the air-fuel ratio λ.fwdarw.λ.sub.sens is mapped here by a PT1 element with the time constant τ.sub.exh in function block 22. Because the dead time is considered separately (via a simple shift of the values on the time scale) and is not included in the transfer function 23 for the adaptation model 30, it is not shown in the present block diagram.

[0056] Function blocks 10 to 14 show the wall film model for dynamic feedforward control of the wall film fuel mass flow {dot over (m)}.sub.wf,1. The fuel mass situated in wall film 10 is preferably stored in the engine control unit in a corresponding map 10 as a function of the engine rotational speed n.sub.eng and the engine temperature t.sub.mot. The input variables of map 10 are not limited to the variables shown; additional or other boundary conditions, such as pressure and/or temperature in intake tract 2 of the engine, can be taken into account.

[0057] Function block 11 describes a filter transfer function with a filter time constant π.sub.wf, that calculates the wall film fuel mass flow using the time derivative of the wall film fuel mass. The wall film fuel mass flow calculated in this way is subsequently divided into a portion that flows into the cylinder quickly and a portion that flows into the cylinder with a significant delay. This is realized by two function blocks 12 and 13 connected in parallel, which have filter transfer functions with a slow time constant τ.sub.slow and a fast time constant τ.sub.fast.

[0058] Because, in addition to the above-described functional dependence of the wall film behavior on the engine rotational speed and the engine temperature, there may be other dependencies in the fuel mass flow (e.g. a directional dependence of the mass flow entering or exiting the wall film), the fuel mass flow resulting from function blocks 12 and 13 is advantageously multiplied again here by a correction factor f.sub.corr, by a multiplier 14.

[0059] This block 10 to 14 is usually not individually parameterized for the specific engine, so that the determined second wall film fuel mass flow {dot over (m)}.sub.wf,2 is not (always) optimal.

[0060] Therefore, advantageously, the determined second wall film fuel mass flow {dot over (m)}.sub.wf,2 is now subsequently multiplied by the adaptation factor f.sub.corr,adp, which is ascertained in function block 30 from, inter alia, the measured and time-adjusted air-fuel ratio λ.sub.corr. This multiplication at the multiplier 15 results in the first (positive or negative) wall film fuel mass flow {dot over (m)}.sub.wf,1, which is added to the fuel mass flow {dot over (m)}.sub.direct at the addition point 20.

[0061] In function block 30, the calculation of the adaptation factor f.sub.corr,adp, inter alia, is carried out also using the first wall film fuel mass flow rate {dot over (m)}.sub.wf,1, which however is calculated differently than in 15. The first wall film mass flow rate {dot over (m)}.sub.wf,1 can be calculated for example according to equation (2), and the adaptation factor f.sub.corr,adp can be calculated therefrom, for example according to equation (4). The adaptation factor f.sub.corr,adp can preferably also be determined using a recursive least squares estimator, which increases the numerical stability of the calculation.

[0062] In order to perform the calculation steps according to equations (2) and (4), function block 30 receives the measured and time-adjusted air ratio λ.sub.corr, the air mass flow {dot over (m)}.sub.air, the fuel mass flow {dot over (m)}.sub.direct, and the filtered wall film mass flow {dot over (m)}.sub.wf,2flt, ascertained from the wall film model, as input variables.

[0063] For the temporal adjustment of the measured air-fuel ratio λ.sub.sens to the air ratio λ present in the cylinder, the filter transfer function shown in function block 23 and described in equation (3) is used. This produces the time-adjusted air ratio λ.sub.corr, which is used in function block 30 to calculate the first wall film fuel mass flow {dot over (m)}.sub.wf,1 according to equation (2).

[0064] From a comparison of function blocks 22 and 23, it can be seen that the filter transfer function shown in function block 23 is the inverse of the time delay of the measured air-fuel ratio λ.sub.sens shown in function block 22. In addition, the transfer function 23 has in the denominator a further filter function with the predetermined time constant τ.sub.flt.

[0065] The time constant τ.sub.flt is used in the same way to filter the first wall film fuel mass flow {dot over (m)}.sub.wf,2 in function block 16, so that the input variables {dot over (m)}.sub.wf,2flt and λ.sub.corr enter function block 30 synchronously in time, which block has the adaptation factor f.sub.corr,adp as output variable.

[0066] With the help of the adaptation factor calculated in function block 30, for example on the basis of equations (2) and (4), the wall film fuel mass flow can be continuously adjusted to the real engine conditions during vehicle operation. In this way, the stoichiometric air-fuel ratio can be reliably maintained even during load change processes, and transient mixture deviations can thus be reliably minimized.