Method for determining oil dilution

Abstract

 The invention relates to a method for determining the need for an oil change due to oil dilution of the engine oil in an internal combustion engine, which engine is provided with an electronic control unit. The method involves performing the steps of determining a plurality of engine parameters indicative of soot formation; determining the occurrence of a late fuel injection; determining a fuel volume injected after a threshold value; adding the volume of fuel injected after the threshold value to a counter; and triggering an output signal when an accumulated volume of injected fuel exceeds a predetermined amount.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for determining the need for an oil change due to oil dilution of the engine oil in an internal combustion engine.

BACKGROUND ART

[0002] Changing engine oil is one of the key processes used in extending the life of an internal combustion engine. Generally, the oil in an engine is changed in accordance with a set schedule. The schedule is based on an estimate of the life of the oil under a worst case scenario. Pollution and deterioration of engine oil is often referred to as oil dilution. The cause of oil dilution can be a number of factors, such as soot or non-combusted fuel from the combustion chamber and particles resulting from wear entering the oil. As a rule, particles up to a predetermined size are caught and retained by an oil filter through which the engine oil is constantly passed when the engine is operated. This is a relatively long term problem that can be handled by a scheduled change of the oil filter. Non-combusted fuel present in the relatively hot combustion chamber is likely to evaporate and the major portion of this fuel will pass into the engine exhaust conduit. Hence, a relatively minor amount of fuel leaking past the piston rings will usually be a long term problem that can be avoided by oil changes according to a set schedule. This type of oil dilution is often referred to as fuel dilution. Soot is formed in the combustion chamber during unfavourable combustion condition and may settle on the cylinder walls where it can be drawn into the crankcase by the scraper rings on the piston. The presence of soot in the engine oil is a known cause of oil dilution.

[0003] One example of a known method for estimating the remaining life of the engine oil is described in US 5 750 887. The method includes the steps of measuring a plurality of engine parameters, determining an estimate of the characteristics of the engine oil as a function of the engine parameters, and trending the estimate and responsively determining remaining life of the engine oil. A service message may be triggered when it is estimated that the engine oil is approaching the end of its useful life.

[0004] A problem with this method is that the soot estimate is a polynomial function based on empirical and/or simulation data. The function may be represented by a look-up table. If oil dilution caused by soot occurs at a rate falling outside the empirical and/or simulation data used, then the service message will be triggered too late.

[0005] A further problem is that the method is not adapted to take into consideration driving cycles when a vehicle is operated under conditions when soot formation is likely to occur, such as transient engine operation, exhaust gas recirculation (EGR) operation, low lambda operation and/or regeneration of exhaust purifying devices.

[0006] One object of the invention is to provide an improved method for determining a remaining life of engine oil in an engine in order to solve the above problems. The method according to the invention facilitates detection of excessive oil dilution caused by soot which will require the oil to be changed prematurely.

DISCLOSURE OF INVENTION

[0007] The above problems have been solved by a method according to the appended claims.

[0008] According to a preferred embodiment, the invention relates to a method for determining the need for an oil change due to oil dilution of the engine oil in an internal combustion engine. The method involves using an oil dilution service message function that calculates the oil dilution caused by different engine operating conditions. When at least one predetermined condition is fulfilled a service message for oil change is triggered.

[0009] The method may involve the steps of

• determining a plurality of engine related parameters indicative of soot formation;
• determining the occurrence of a late fuel injection;
• determining a fuel volume injected after a threshold value;
• adding the volume of fuel injected after the threshold value to a counter;
• triggering an output signal when the accumulated volume of injected fuel exceeds a predetermined amount.

[0010] Examples of parameters determined by measurement or calculation in order to perform the method are engine speed, output torque, fuel injection timing and duration. A further parameter may be the currently selected engine map. For instance, if an exhaust gas recirculation (EGR) mode is in operation, an EGR engine map is considered instead of a standard engine map. Similarly, if a regeneration of a catalytic converter or particle filter in the exhaust purification system is being carried out, a regeneration engine map is considered.

[0011] The late fuel injection may sometimes be referred to as a post injection and may be performed after top dead centre (TDC) during the expansion phase. Due to the relatively large number of variable parameters to consider during the combustion cycle, a simplified model has been adapted for the method. When implementing the method it has been assumed that the fuel is injected by a fuel injector into the combustion chamber at a constant rate during the entire energizing period of the injector. It is also assumed that the fuel injected last will also the last fuel to be combusted.

[0012] The formation of soot can at times be very high during the expansion phase of the combustion cycle, but under normal operating conditions most of the soot is oxidized before the exhaust valve opens at the end of the said phase.

[0013] The amount of soot oxidized is mainly of interest from an emission point of view, while the soot remaining in the combustion chamber, in particular the soot deposited on the liner, is dealt with in this example.

[0014] Under most operating conditions it is desirable to monitor soot formation in order to enable a service message and to allow an early oil change, prior to a scheduled oil change, caused by soot oil dilution. An example of unfavourable operating conditions is a transient city cycle, involving numerous starts, stops, accelerations and deceleration. A common type of vehicles operated under such conditions is taxis. Oil dilution is a problem shared by both petrol and diesel engines, although the problem may be more common for the latter. Modern diesel engines are frequently provided with a particle filter in the exhaust conduit. When the particle filter becomes clogged, a regeneration cycle is triggered to burn soot particles in said filter. During regeneration, a different engine map is selected by the engine control system, which map involves late fuel injection. A late fuel injection may, as stated above, cause an increased soot formation on the cylinder liners which in turn may cause oil dilution. As a majority of taxis are typically provided with diesel engines, the combination of a transient driving cycle and a diesel engine with a particle filter is suitable to consider as a worst case scenario.

[0015] Peak in-cylinder soot formation is dependent on a number of engine parameters, such as engine speed, output torque, fuel injection timing and duration. Soot formation is also influenced by the currently selected engine map, for instance, if the EGR is in operation or not and if a regeneration of the exhaust purification system is being carried out. For example, when EGR is in operation the engine map is switched from a standard engine map to an EGR engine map. As the EGR operation introduces an amount of exhaust gas into the air/fuel mixture, the oxygen concentration is lowered and the risk of soot formation is increased. Similarly, when particle filter regeneration is triggered the engine map is switched from a standard engine map to a regeneration engine map. As the particle filter regeneration operation involves late fuel injection the risk of soot formation is increased.

[0016] A concept according to the invention is that the part of the fuel that is combusted after a predetermined percentage of the available fuel has been combusted contributes the most to oil dilution caused by soot. This is particularly relevant when the point where the last of the available fuel has been combusted occurs late in the expansion phase. In the subsequent text, this point is referred to as MBFxx%, where MBF stands for Most Burnt Fuel and "xx" represents the predetermined percentage used for a particular engine type. Consequently, the point MBF90% indicates when 90% of the available fuel has combusted, that is, 90% of the fuel that is available for combustion during the expansion phase. The available fuel may be less than the total injected volume of fuel if, for instance, there is not enough oxygen in the combustion chamber. The reason for the soot contribution is that the late injected fuel spray will be close to or reach the cylinder wall, or liner, under conditions where the oxygen content in the air/fuel mixture in the cylinder is low. In this way, a flame front preceding the combusting fuel may contain a high concentration of soot that is deposited on the liner. Fuel that is deposited directly on the liner and is not evaporated may combust on the liner and leave a layer of soot. In both cases, localized pockets of low oxygen concentration in the air/fuel mixture may contribute to low soot oxidation, which increases the problem with soot deposition. Soot present on the liner may then be drawn into the crankcase by the scraper ring on the reciprocating piston and contribute to oil dilution.

[0017] The simplified model used by the method will assume that the fuel is injected at a constant rate during the time that the injector is energized. A threshold is calibrated so that any fuel injected after the threshold is assumed to be the fuel volume contributing to the undesired soot formation. This fuel volume is assumed to correspond to the fuel being combusted after a late MBF percentage point, such as MBF90% at which point 90% of the available fuel has been combusted. The point when, for instance, MBF90% occurs is determined as a crank angle degree (CAD) that may be calibrated and stored in a map. The MBF point may be determined experimentally or be simulated for the particular model of engine, or engine type, to which the method is applied. The reason for this is that undesirable soot formation at the end of the expansion phase may occur at different crank angle degrees for different engines and even for different engine maps for the same engine. Hence, one engine may use a MBF percentage point set to MBF90%, while a different engine may use MBF80%. When calibrating the threshold value, the method may use an algorithm based on the engine speed, a calculated or measured output torque, the fuel injection timing, the fuel injection duration and the current engine map used. Simulated and/or experimental tests may also be used. Maps containing crank angle degrees for a predetermined MBF percentage point for related speed and torque values over the entire operating range of the engine are established. Separate MBF percentage point maps are provided for normal operation, EGR operation and regeneration operation of the engine. The threshold is determined as a crank angle degree and any fuel injected after the threshold is considered to correspond to the portion of the late injected fuel volume combusted after a predetermined MBF percentage point. The fuel volume injected after the threshold is added to a counter in an electronic control unit to form an accumulated fuel volume. When the accumulated volume of injected fuel exceeds a predetermined amount an output signal is triggered by the electronic control unit and a service message is displayed to the driver.

[0018] In addition to a counter for soot oil dilution caused by late injection and incomplete combustion, the electronic control unit may be provided with one or more additional counters taking into consideration additional sources and/or causes of oil dilution. Examples of such counters may include a timer for engine operation in order to determine of the total operating or running time of the engine, and a counter for an accumulated volume of non-combusted fuel leaking past the piston rings into the crankcase. The values stored in each counter may be added in a common counter where each separate value is weighted according to its relevance to oil dilution. The weighting of the different values is dependent on factors such as the type and model of engine (diesel, gasoline, etc), whether the engine has a particle filter, and the type and quality of oil filter used. When the sum of the accumulated weighted values exceeds a predetermined amount an output signal is triggered by the electronic control unit. In this way an error message may be generated either by the counter for soot oil dilution or by the common counter.

BRIEF DESCRIPTION OF DRAWINGS

[0019] The invention will be described in detail with reference to the attached figures. It is to be understood that the drawings are designed solely for the purpose of illustration and are not intended as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to schematically illustrate the structures and procedures described herein.

Figure 1

shows a schematic flow chart for a method using a soot oil dilution counter according to the invention;

Figure 2

shows a schematic diagram indicating the relationship between the percentage of soot present in the engine oil as a function of when the MBF90% occurs;

Figure 3

shows a schematic diagram indicating a map for MBF90%;

Figure 4

shows a schematic diagram indicating soot formation, rate of heat release and accumulated heat release, as a function of crank angle degrees;

Figure 5

shows a diagram for heat release and injection volume as a function of crank angle degrees; and

Figure 6

shows a schematic flow chart for a method using multiple oil dilution counters.

EMBODIMENTS OF THE INVENTION

[0020] Figure 1 shows a schematic flow chart for a method according to the invention. When the engine is switched on an electronic control unit, or ECU, used for controlling the engine will receive input signals from a number of sensors for engine related parameters. Examples of such parameters are engine speed, fuel injection timing and duration. These input signals allow the ECU to calculate additional parameters, such as the output torque. Further parameters can be stored in the ECU, such as the currently selected engine map. Depending on the current operating conditions, the ECU can select a suitable engine map. For instance, if an exhaust gas recirculation (EGR) mode is in operation, an EGR engine map is selected instead of a standard engine map. Similarly, if a regeneration of a catalytic converter or particle filter in the exhaust purification system is being carried out, a regeneration engine map is selected.

[0021] In this way the ECU can determine a plurality of engine parameters indicative of soot formation. Depending on the currently selected engine speed, output torque and engine map, the ECU can determine the occurrence of a late fuel injection, in this case a fuel injection carried out in the expansion phase of a combustion cycle. The ECU will then select a map containing stored values for crank angle degrees when a predetermined percentage of the available fuel has been combusted, which in this example is selected at 90%. The point where 90% of the available fuel has been combusted is referred to as MBF90%. The MBF90% map is selected depending on the currently selected engine map and determines a fuel volume injected after a threshold value. The threshold is dependent on a number of the engine related parameters and varies with engine speed, output torque and the currently selected stored engine map. A continuously updated value for the threshold is calculated by the ECU using an algorithm based on the said parameters. An example of a MBF90% map is shown in Figure 3. The volume of fuel injected after the threshold value is added to a counter for soot oil dilution. After adding the volume of fuel injected after the threshold value to the counter, the ECU compares the accumulated volume stored in the ECU to a predetermined limit value. When the accumulated volume of injected fuel exceeds a predetermined amount an output signal is triggered and generates a service message, warning the driver that an oil change is required.

[0022] In addition to the counter for soot oil dilution caused by late injection and incomplete combustion, the ECU can be provided with one or more additional counters taking into consideration additional sources and/or causes of oil dilution. A second counter can include a timer for engine operation in order to determine of the total operating or running time of the engine. A third counter can be provided for an accumulated volume of non-combusted fuel leaking past the piston rings into the crankcase. The values stored in each counter are added in a common counter where each separate value is weighted according to its relevance to oil dilution. When the sum of the accumulated weighted values exceeds a predetermined amount an output signal is triggered by the ECU. In this way an error message can be generated early by the counter for soot oil dilution or by the common counter weighting the combined effect of all values with a detrimental effect on oil dilution.

[0023] Figure 2 shows a schematic diagram indicating the relationship between the percentage of soot present in the engine oil as a function of when the MBF90% occurs during the expansion phase of an engine operated in a normal mode. Normal mode indicated that no EGR or regeneration takes place. The occurrence of MBF90% is indicated in crank angle degrees (CAD) after top dead centre (TDC) during the expansion phase. During a series of test simulations the engine was operated for 250 hours at a predetermined speed (rpm) and torque (T). The timing of the MBF90% was maintained at a predetermined crank angle degree by selecting a predetermined injection timing and duration for the fuel injections. The fuel injections were performed with an additional delay for each subsequent test, in order to gradually increase the crank angle degree at which the MBF90% occurred. After each test the soot dilution in the engine oil was measured in soot percent and plotted against the timing of the MBF90% measured in crank angle degrees for each test. As indicated by Figure 2 the percentage of soot increases significantly when the timing of the MBF90% exceeds about 45 crank angle degrees after TDC. The diagram shown in Figure 2 is indicative of the particular engine used for the test operated at a predetermined speed. In order to achieve a map that can be used for determining the oil dilution in a method according to the invention, the timing of the MBF90% must be determined for the entire operating range of the engine.

[0024] Figure 3 shows a schematic diagram indicating a map indicating the timing of the MBF90% for the engine used in the tests as described above. In this map, the timing of the MBF90% measured in crank angle degrees (CAD) has been determined at every 250 rpm for output torque intervals of 25 Nm. For reasons of clarity the diagram only indicates a limited number of CAD for the MBF90% points at each intersection of the grid lines within the operation range of the engine (indicated with dash dotted lines). The crank angle degrees are numbered α₁-α_n. From such a diagram it can be determined that a problem area for soot formation for the engine in question is located in a particular torque/engine speed range. In Figure 3 a schematic problem area is shown in the range 1000-2700 rpm, at torques in the range 100-250 Nm (indicated in dashed lines). The relevant crank angle degrees in this area are numbered α_x, α_x+1., etc. In this area, which is typically used in a city cycle, the MBF90% occurs between 50 and 60 crank angle degrees after TDC. From Figure 2 it is obvious that prolonged operation in this area will cause problems with soot dilution. The map in Figure 3 can be determined by calculating when MBF90% occurs for each torque/speed value in the operating range, or by experimental values using an engine test rig.

[0025] Figure 4 shows a schematic diagram indicating soot formation, rate of heat release (RoHR) in Joule per crank angle degree (J/°) and accumulated heat release (HR) in Joule (J), as a function of crank angle degrees (CAD) for the combustion. The CAD corresponding to MBF90% is equal to the CAD where HR reaches 90% of its final value. The example of Figure 4 is intended to show a worst case scenario. As shown in the figure, there is an initial spike of heat release at ignition immediately before TDC at 360 CAD, followed by a lower first peak as the fuel is combusted. This first peak is accompanied by a first soot formation peak at about 385 CAD. Additional fuel is then injected during a post injection at about 390 CAD in the expansion phase. Typically, a late post injection of this type is performed during regeneration of a particle filter in the exhaust system of a diesel engine. Because the post injection is performed during the expansion phase, the resulting heat release from the combustion yields a second peak lower than the first peak, at about 400 CAD. However, the accompanying second soot peak at about 400 CAD is significantly higher than the first soot peak. Also, due to the late injection, the timing of the MBF90% occurs as late as 62° after TDC (indicated by a vertical line). Although some of the soot formed during the combustion will oxidize, a relatively large amount will settle on the liner in the combustion chamber. During operating conditions of this type there will be a marked contribution to the oil dilution caused by soot being drawn into the crankcase by the scraper ring on the piston.

[0026] Figure 5 shows a diagram for heat release and injection volume as a function of crank angle degrees (CAD). The figure illustrates a simplified model used by a method according to the invention. When performing the method it is assumed that the fuel is injected at a constant rate during the time that the injector is energized. In the figure, the start of the fuel injection is indicated by ϕ and the energizing time of the fuel injector is indicated by the arrow ET. The total injected volume V_T is indicated by a rectangle created by the start and the end of the fuel injection, and the constant rate of injection I. A threshold value T_LIM is calibrated so that any fuel injected after the threshold is assumed to be the late injected fuel volume V_S (indicated by hatched lines) contributing to the undesired soot formation. This fuel volume V_S is assumed to correspond to the fuel being combusted after a late MBF90%, at which point 90% of the fuel in the combustion chamber has been combusted. When calibrating the threshold value T_LIM, the method will use an algorithm based on the engine speed, the fuel injection timing, the fuel injection duration and a MBF90% map related to the currently used engine map. The fuel volume V_S is added to a counter in an electronic control unit to form an accumulated volume V_ACC. When the accumulated volume of injected fuel exceeds a predetermined amount an output signal is triggered by the electronic control unit.

[0027] Figure 6 shows schematic flow chart for a method according to the invention using multiple counters for monitoring oil dilution. When the engine is switched on a number of counters are started for accumulation values relating to oil dilution. In addition to a counter for soot oil dilution caused by late injection and incomplete combustion, the electronic control unit for controlling the engine is provided with a timer for engine operation in order to determine of the total operating or running time of the engine. The electronic control unit is further provided with a fuel dilution counter for an accumulated volume of non-combusted fuel leaking past the piston rings into the crankcase. The values accumulated and stored in each counter are added in a common counter where each separate value is weighted according to its relevance to oil dilution. For instance, soot dilution is weighted higher than both the fuel dilution counter and the engine-on counter. The reason for this is that soot accumulated in the engine oil will create considerably more wear than fuel in the oil or wear caused by normal contact between moving engine components. In addition, the fuel dilution counter is weighted higher than the engine-on counter. When the sum of the accumulated weighted values exceeds a predetermined amount an output signal in the form of an oil change service message is triggered by the electronic control unit. In this way an error message can be generated by the counter for soot oil dilution, as shown in Figure 1, or by the common counter, as shown in Figure 6.

[0028] The invention is not limited to the above examples, but may be varied freely within the scope of the appended claims.

Claims

1. A method for determining the need for an oil change due to oil dilution of the engine oil in an internal combustion engine, which engine is provided with an electronic control unit, characterized in

• determining a plurality of engine parameters indicative of soot formation;

• determining the occurrence of a late fuel injection;

• determining a fuel volume injected after a threshold value;

• adding the volume of fuel injected after the threshold value to a counter; and

• triggering an output signal when an accumulated volume of injected fuel exceeds a predetermined amount.

2. Method according to claim 1, characterized in determining that the late fuel injection is a fuel injection performed during the expansion phase after top dead centre (TDC).

3. Method according to claim 1 or 2, characterized in determining the threshold value based on the fuel volume that is combusted after a predetermined percentage of the available fuel has been combusted.

4. Method according to any one of claims 1-3, characterized in determining the engine speed and output torque in order to determine the threshold value.

5. Method according to any one of claims 1-4, characterized in determining the fuel injection timing and duration in order to determine the threshold value.

6. Method according to any one of claims 1-5, characterized in determining a current engine operating condition and selecting an engine map for the said condition to determine the threshold value.

7. Method according to claim 6, characterized in determining if an exhaust gas recirculation (EGR) mode is in operation, and using an EGR engine map to determine the threshold value.

8. Method according to claim 6, characterized in determining if a regeneration of a catalytic converter or particle filter in the exhaust purification system is being carried out, and using a regeneration engine map to determine the threshold value.

9. Method according to claim 1, characterized in weighting accumulated values from at least one counter and generating an oil change message if the weighted accumulated values exceeds a predetermined amount.

10. Method according to claim 9, characterized in using a timer to determine of the total operating time of the engine, and accumulating the operating time in a counter.

11. Method according to claim 9 or 10, characterized in determining an accumulated volume of non-combusted fuel leaking into the crankcase, and accumulating the volume of non-combusted fuel in a counter.

Drawing