Patent Application
20110277542
This application claims the benefit of DE 10 2010 020 755.1 filed May 17, 2010, which is incorporated by reference herein.
The invention relates to a method and to a control device for determining a characteristic viscosity variable of engine oil in an internal combustion engine with hydraulic control of the gas exchange valves.
In modern motor vehicles, the so-called gas exchange valves, that is to say the inlet and/or outlet valves for the motor vehicle engine, are nowadays controlled in a load-dependent manner for the purposes of minimizing pollutants and reducing consumption. Various systems are used in the process. All systems share the common feature that the closing and/or opening times of the gas exchange valves are changed in a load-dependent manner in relation to the crankshaft position (rotational angle). Hydraulic control systems in which a change in the closing and/or opening times of the gas exchange valve is induced with the aid of a hydraulic fluid, specifically the engine oil, are also used for this purpose.
EP 1 544 419 A1, for example, discloses a first hydraulic control system, specifically a hydraulic camshaft adjuster. In a system of this kind, the phase angle between the crankshaft and the camshaft of a motor vehicle engine is changed with the aid of the hydraulic fluid. This is achieved by variably filling pressure chambers of an adjusting apparatus. Control valves, which are in the form of solenoid valves in particular, are usually provided for actuating and filling and emptying the pressure chambers.
The article “Elektrohydraulische Ventilsteuerung mit dem “Multi Air”-Verfahren [Electrohydraulic valve control using the “MultiAir” method]” in the German automotive engineering journal MTZ, 12/2009, discloses an alternative hydraulic control system for actuating the gas exchange valves. In the case of this electrohydraulic valve control arrangement, provision is made for the movement of the camshaft to be transmitted to a respective gas exchange valve via the hydraulic liquid. A control or switching valve, which is in the form of a solenoid valve in particular, is provided for control purposes. In the closed state, the camshaft is connected to the respective gas exchange valve via a so-called hydraulic linkage, and therefore the gas exchange valve necessarily follows a cam of the camshaft. By also partially opening the switching valve, the hydraulic fluid can pass into a compensation or pressure chamber, and therefore the gas exchange valve is decoupled from the movement of the cam. As a result, it is possible to vary the opening time point, the closing time point and the stroke of the gas exchange valve within an envelope curve which is predefined by the movement of the cam. This variation can be performed in a cylinder-selective manner.
Exact actuation of the gas exchange valves is of critical importance with regard to the high level of efficiency together with low emission of pollutants as is required in modern internal combustion engines. In hydraulic systems, the type of hydraulic liquid used, that is to say, in particular, the quality of the engine oil used, has a significant effect on operation. In particular, the effectiveness of the hydraulic control means is sensitive to fluctuations in the viscosity of the oil used. Such fluctuations occur for operational reasons due to different oil temperatures. As disclosed in the article “Elektrohydraulische Ventilsteuerung mit dem “MultiAir”-Verfahren [Electrohydraulic valve control using the “MultiAir” method],” the temperature-dependent fluctuations in oil viscosity have to date been taken into consideration in a model-based control algorithm which takes into consideration the measurement values from an oil temperature sensor.
This temperature-dependent and therefore indirect determination operation has disadvantages and does not take into consideration, for example, aging of the engine oil or wear of the hydraulic components.
Proceeding from this, the invention is based on the problem of specifying an improved method and an improved control device for directly determining a characteristic viscosity variable for an engine oil, it being possible, in particular, to take aging and wear effects into consideration too.
According to the invention, the problem is solved by a method and a control device for determining a characteristic viscosity variable of an engine oil in an internal combustion engine with hydraulic control of the gas exchange valves, in particular with electrohydraulic control of the gas exchange valves, as disclosed, for example, in the article “Elektrohydraulische Ventilsteuerung mit dem “MultiAir”-Verfahren [Electrohydraulic valve control using the “MultiAir” method]” in the German automotive engineering journal MTZ 12/2009. In the method, provision is made for a comparison variable, which is correlated with a measured oxygen concentration in an exhaust gas from the internal combustion engine, to be used as a measure of the characteristic viscosity variable, in particular as a measure of the viscosity itself. Therefore, in principle, a measure of the viscosity is derived or established from the measured oxygen concentration.
This refinement is based on the consideration that, particularly in the case of electrohydraulic actuation of the gas exchange valves, the oxygen concentration contained in the exhaust gas, with given operating parameters of the internal combustion engine, depends on the viscosity of the engine oil used. In the case of an electrohydraulic control system for actuating the gas exchange valves, deliberate actuation of a control or switching valve decouples the movement of the gas exchange valve from the movement of the camshaft. With the switching valve open, the hydraulic connection between the cam of the camshaft and the gas exchange valve, which connection is otherwise firm, is usually broken. The gas exchange valve usually moves to its starting state, the closed state, in a spring-operated manner. This automatic closing operation of the gas exchange valve, which closing operation is decoupled from the movement of the cam, is also called the ballistic phase in the text which follows since, in this region, the gas exchange valve is no longer forcibly guided. During the ballistic phase, the spring force operates against a frictional force which is definitively established by the viscosity of the engine oil. This means that the profile of the ballistic phase and therefore the time until the gas exchange valve is closed depends on the viscosity of the engine oil. At the same time, the closing time point also establishes the quantity of air drawn in for the respective combustion process. If, for example, the oil has a very high viscosity, the closing process of the gas exchange valve slows down and the closing time point shifts backward, and therefore a greater quantity of air is drawn into the cylinder overall in comparison to an engine oil with a lower viscosity. This is evidenced since the quantity of injected fuel (petrol/diesel) is predefined by an injection control arrangement in different oxygen concentrations in the exhaust gas.
On account of these relationships, the oxygen concentration contained in the exhaust gas is therefore at least an indirect measure of the current viscosity of the engine oil. Therefore, with the described method, the profile of the ballistic phase is, in principle, evaluated particularly in correlation with an envelope curve which is predefined by the movement of the cam.
The oxygen concentration is used for so-called λ adjustment in motor vehicles in order to set the ratio of fuel to oxygen in a desired manner. A stoichiometric ratio is set for the value λ=1, the engine is operated with an excess quantity of fuel (rich mix) when λ<1, and the internal combustion engine is operated with an excess quantity of air (lean mix) when λ>1. A so-called λ probe is used for λ adjustment, said λ probe determining the residual oxygen content in the exhaust gas, in particular by comparison with the oxygen content in the ambient air. The present method therefore uses pre-existing installations and information in the case of a motor vehicle. Therefore, no additional components are required to establish the characteristic viscosity variable of the engine oil. The viscosity is therefore determined, proceeding from an existing λ adjustment operation, solely in terms of control and therefore in a software-based manner.
The comparison variable which is used as a measure of the characteristic viscosity variable is, in one variant, preferably the measured oxygen concentration itself. As an alternative to this, the comparison variable is a variable which is derived from said measured oxygen concentration or the profile of a variable which is derived from said measured oxygen concentration, for example a control signal which is output by the λ adjustment means and is monitored for characteristic changes.
The characteristic viscosity variable is preferably the viscosity itself. However, as an alternative, it is also possible to use the comparison variable as an indirect measure of the viscosity variable for further evaluation operations.
According to an expedient development, the λ adjustment is evaluated in order to determine the characteristic viscosity variable. Therefore, in the case of this concept, the absolute value of the oxygen concentration is not evaluated, but rather the control behavior of the λ adjustment means. This is based on the consideration that, for example as a result of aging phenomena, the viscosity of the engine oil rises and that—in comparison to the preceding state—a faulty setting is made by means of the λ adjustment means and the oxygen content measured in the exhaust gas deviates from the oxygen content expected—if the oil property remains unchanged. This “fault” is corrected by the λ adjustment means. This correction, which is ultimately a correction of the oxygen concentration in the exhaust gas, is preferably used to determine the characteristic viscosity variable. The λ adjustment is therefore monitored, in particular, for typical deviations which can be induced by changes in the viscosity of the engine oil.
According to an expedient development, a correlation between the comparison variable and the characteristic viscosity variable is stored in a control device, for example as part of a family of characteristic curves. In this case, the correlation was determined, in particular, in advance in series of tests and experiments, so that only comparison of the detected comparison variable with the stored values is required to establish the characteristic viscosity variable during operation.
On account of the extremely wide variety of conditions under which the internal combustion engine can be operated, this correlation is, or the families of characteristic curves are, stored as a function of at least one operating parameter of the internal combustion engine, preferably as a function of a large number of operating parameters. Such operating parameters of the internal combustion engine are, in particular, the temperature of said internal combustion engine (and therefore the temperature of the engine oil), the speed of said internal combustion engine, the injection quantity selected on the basis of the current load requirement etc.
Provision is also preferably made for the correlation to be preferably additionally stored as a function of a characteristic variable of the engine oil. Such a characteristic variable is, in particular, the viscosity class, for example according to the SAE classification. This classification characterizes the warm-running properties at comparatively high ambient temperatures (summer oil) and the cold-running properties at comparatively cold ambient temperatures (winter oil). Therefore, families of characteristic curves are preferably stored for different engine oil classifications, again as a function of the operating parameters of the internal combustion engine.
Furthermore, provision is made, in a preferred development, for engine oils which are individualized according to manufacturer or brand to be stored in the families of characteristic curves. In this case, the families of characteristic curves represent, for example, the dependence of the residual oxygen concentration in the exhaust gas in relation to a control signal from the λ adjustment means or else directly in relation to the viscosity for predefined operating parameters and characteristic variables of the engine oil.
In a preferred development, the characteristic viscosity variable is determined in a cylinder-specific manner. This means that the comparison variable is determined for each cylinder of the internal combustion engine or at least for groups of cylinders of the internal combustion engine, and the characteristic viscosity variable is derived from said comparison variable. This is used, in particular, in motor vehicles of the kind which provide cylinder-specific λ adjustment or oxygen measurement. Therefore, the evaluation and analysis of the oxygen content in the exhaust gas and therefore the determination of the characteristic viscosity variable is more robust and more accurate and less susceptible to faults.
In preferred refinements, the determined characteristic viscosity variable is, as an alternative or in combination, used for a variety of derived measures and evaluation operations. In addition to evaluating the currently determined characteristic viscosity variable, the evaluation of the change in the characteristic viscosity variable can also be used in this case.
According to a preferred first alternative, a measure of the oil quality used is determined, as is, in particular, the SAE classification met by the engine oil. A profile of the comparison variable, for example as a function of the temperature etc., can be determined from the various measurement points by measuring or determining the comparison variable at various operating points of the internal combustion engine, that is to say at specific temperatures, rotation speeds etc. The kind of engine oil being used can be established from this profile by comparison with a stored family of characteristic curves. In the same way, engine oil which has been impermissibly used is also identified in a preferred refinement. Therefore, damage to the engine can be prevented by outputting a corresponding warning signal.
According to a preferred further evaluation, provision is made for the characteristic viscosity variable to be monitored for an abrupt change in order to identify, for example, that the engine oil has been changed. In this case, the control device preferably sets an identifier or marker which designates and identifies the change in engine oil. This identifier is preferably also used to establish the oil change interval. In a further preferred refinement, the characteristic viscosity variable is used, in principle, to establish the oil change interval. Specifically, deterioration in the engine oil is identified in good time on account of the continuous measurement. A deterioration of this kind compared to an originally calculated oil change interval can occur, for example, in the case of short journeys.
Finally, in a particularly expedient refinement, the valve control is influenced on the basis of the determined characteristic viscosity variable. Firstly, for example when aging of the engine oil is detected, that is to say when an increase in the viscosity is detected, the time points for actuating the switching valve are varied and matched to the new conditions. As a result, gas valve control which is as exact as possible is ensured in accordance with the respective requirements. Therefore, this measure has the result that the ballistic phase is terminated at the desired time point and the gas exchange valve is back in its closed position. The entire electrohydraulic control operation is preferably performed on the basis of the determined characteristic variable for the viscosity and not, as was previously customary, using the current oil temperature. Therefore, the entire control operation of the gas exchange valves is based on the actual state of the oil.
Furthermore, provision is made in a preferred refinement for a warning signal to be output when the determined oil viscosity does not permit disturbance-free operation of the engine. Therefore, even starting of the engine can be suppressed, for example in the case of extremely low external temperatures and an excessively high viscosity and an associated lack of lubrication of the engine. Specifically, a minimum external temperature which must not be undershot can be established on the basis of the oil state determined in a preceding operating cycle.
The characteristic viscosity variable is preferably determined continuously or at intervals during operation of the internal combustion engine and therefore more or less continuously. In this case, provision may be made for the characteristic viscosity variable to be determined only once or a few times during one operating cycle of the internal combustion engine (that is to say between starting and stopping of the engine).
An exemplary embodiment of the invention will be explained in greater detail below with reference to the figures, in which:
Identically acting parts are provided with the same reference symbols in the figures.
An electrohydraulic control system for hydraulically controlling a gas exchange valve 2 in a motor vehicle 4 which is indicated by a dashed border in
The oil can pass into the pressure chamber 14 by opening of the solenoid valve 8, and therefore the movement of the gas exchange valve 2 is decoupled from the movement of the cam 6.
The solenoid valve 8 is connected to a control or evaluation unit 20. The control unit 20 is integrated, for example, in the engine control means. The solenoid valve 8 is supplied with a control signal by means of the control unit 20. In the exemplary embodiment, this control signal is a field current I for a magnet coil of the solenoid valve 8.
The control unit 20 is also connected to a so-called λ probe 22. The λ probe 22 is arranged in an exhaust gas line (not illustrated in any detail here), usually in an exhaust gas manifold, and measures the (residual) oxygen content KO in the exhaust gas from the internal combustion engine of the motor vehicle 4 in a manner which is known per se. The λ probe 22 outputs a corresponding oxygen measurement signal S(KO) to the control unit 20 and is used there for so-called λ adjustment. The combustion parameters such as injection quantity, opening and closing time points of the gas exchange valves 2, possibly ignition time points etc., are adjusted by means of the λ adjustment means in a manner which is known per se as a function of the current (load) requirements. In this case, the adjustment variable is the measured oxygen content KO.
The solenoid valve 8 is usually open in the inactivated state, and therefore the hydraulic line 10 is free in the direction of the pressure chamber 14. In the activated state, that is to say when the solenoid valve 8 is supplied with an adequate field current I, the solenoid valve 8 is in its closed position. In this case, the solenoid valve 8 has a typical design which is known per se. An armature is operated by the magnet, which is formed by an electrical coil, in the closing and opening directions. A closure element for closing the hydraulic line 10 is arranged on this armature. The magnetic force usually acts against a spring force of a spring which is mounted in the solenoid valve 8 and pushes the solenoid valve 8 into its starting position, in particular its open position, in the inactive state.
The field current I exhibits a typical profile, as illustrated in
In addition to the field current I, the profile of the stroke H of the gas exchange valve 2, which profile is associated with said field current, is also plotted against time in
The stroke movement of the gas exchange valve 2 deviates from the envelope curve h by the field current I being switched off at time point t4. The gas exchange valve 2 closes at an earlier time point. The actual profile of the stroke movement of the gas exchange valve 2 in the case of the profile of the field current I which is illustrated in
The area under the curve for the stroke movement of the gas exchange valve 2 is correlated with the quantity of air drawn in for a combustion stroke and therefore determines, given a defined injection quantity of the injected fuel, the mixing ratio between fuel and air. Therefore, the oxygen content KO in the exhaust gas is also influenced at the same time.
Experiments have now shown that the profile of the ballistic phase b1, b2 is dependent on the viscosity η of the engine oil used. At a relatively high viscosity, the ballistic phase b1, b2 shifts to the right, that is to say the gas exchange valve 2 closes more slowly. The reason for this can be found in the higher frictional force created by the higher viscosity.
In this respect,
A correction factor fλ which shows a deviation (ratio) of the actual λ value in relation to the setpoint λ value is plotted on the y axis. When fλ=1, the actual λ value corresponds to the setpoint λ value. In general, combustion is too lean when the λ value is too high and combustion is too rich when the λ value is too low. The λ value is measured by means of the λ probe in a manner which is known per se and is a measure of the residual oxygen concentration KO in the exhaust gas. A desired λ value is set using the usual λ adjustment means as a function of the current operating requirements. In this case, the adjustment parameter is usually the injected quantity of fuel.
In the experiments, the phase angle Φ was successively varied at different predefined engine parameter values and different oil quantities. The critical parameters in this case were the speed of the engine, the position of the accelerator pedal and the engine oil temperature.
As also disclosed in
The experiments showed that the correction factor fλ varies as a function of the oil used, that is to say the deviation from the setpoint λ value is oil-dependent. The measured residual oxygen content KO therefore deviates from the setpoint oxygen content in the manner in which it is intended to be set by means of the λ adjustment means. In general, the deviation from the setpoint λ value increases as the deviation of the actual viscosity from a stored nominal viscosity curve increases. A decreasing correction factor fλ means an increase in the λ value in the lean direction, and vice versa.
Proceeding from this basic information, provision is therefore made to integrate an analysis unit in the control device 20, said analysis unit drawing conclusions about the current viscosity η of the engine oil used on the basis of the oxygen concentration KO measured by means of the λ sensor during operation. In this case, the measured oxygen concentration KO is selectively directly evaluated or else the control signal from the λ adjustment means is evaluated and analyzed. As shown specifically by
To this end, a large number of families of characteristic curves are preferably stored for various sets of parameters. Such families of characteristic curves indicate, for example, similarly to what is illustrated in
During the analysis, conclusions are then preferably drawn about the viscosity and quality of the oil used by simply comparing the measured values with the stored families of characteristic curves. This information is then preferably used selectively or in combination to:
In comparison to the previous control operation, as a function of the currently measured oil temperature, the last-mentioned point has the critical advantage that the currently actual properties of the engine oil are used for the control operation, and therefore aging effects, for example, are automatically taken into consideration.
A particular advantage of this method is the fact that additional hardware components besides the components already present in the motor vehicle, for example the λ probe, are not required and not provided either. Evaluation and analysis are performed solely using measurement data which is available in any case.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 020 755.1 | May 2010 | DE | national |
