Method for calculating a dilution ratio of a fuel in an oil of a spark-ignition internal combustion engine associated with at least one electric machine

Abstract

This method of calculat ing a di lut ion rat io of a fuel in an oi l of a spark-ignit ion internal combust ion engine (2) capable of dr iving at least one wheel of the motor vehic le, said engine (2) being associated with at least one electr ic machine, comprises the steps of: -calculat ion of a variat ion (ΔM) in a mass of fuel di luted in an oi l bath (14) of the engine (2) by a t ime integral of the dif ference between a f irst quantity of fuel (Q_in) which is added to the oil bath and a second quant ity of fuel (Q_out) which is removed from it by evaporat ion, -updat ing a stored value of the diluted fuel mass (M_fuel_oi l) according to the var iat ion (ΔM) calculated, -calculat ion of the dilut ion rat io (Td) as a function of the rat io between the updated mass of diluted fuel (M_fuel_oi l) and a mass of oil (M_oi l) in the oi l bath

Description

[0001] DESCRIPTION

[0002] TITLE: Method for calculating a dilution ratio of a fuel in an oil of a spark-ignition internal combustion engine associated with at least one electric machine

[0003] Technical area

[0004] The invention relates to a method of estimating oil dilution applied to hybrid petrol or equivalent engines in which a heat engine is associated with reversible electric machines. It also concerns a spark-ignition internal combustion engine suitable for implementing such a method.

[0005] Previous techniques

[0006] Many modern engines are equipped with exhaust gas treatment devices, which operate sequentially. Most diesel engines, for example, are fitted with a particulate filter. I n normal operation, these devices trap pollutants but do not treat them. Periodically, when a parameter representative of the quantity of accumulated pollutants reaches a predefined value, a so-called regeneration phase is triggered by an engine computer, during which the engine settings are modified to treat and eliminate the pollutants.

[0007] I n a known way, regeneration is generally achieved by feeding fuel into the system, for example by late post-injections into the engine cylinders. I n this way, quantities of fuel are injected into each cylinder long enough after the cylinder's top dead centre for combustion so that they do not to take part in combustion. These quantities of fuel are evacuated from the engine, during the exhaust stroke of the cylinder in question, to the gas treatment device to regenerate it, i.e. to purge it of accumulated pollutants.

[0008] However, a fraction of the fuel injected during regeneration phases penetrates the oil, particularly through the gaps between the crankcase and the pistons, and is diluted in the oil.

[0009] Dilution, i.e. the amount of fuel contained in the oil, increases each time a regeneration phase is triggered. To avoid problems with engine reliability, such as premature wear of crankcase barrels or crankshaft bearings, a maximum dilution level of around 10% should never be exceeded, as this greatly reduces the viscosity of the oil and therefore its lubricity.

[0010] Diesel engines fitted with a particulate filter are known to use an oil control system (OCS) embedded in the engine computer, which estimates the level of dilution in the oil bath and alerts the driver to the need for an oil change as soon as the maximum dilution threshold is reached. Calibrating such a system is time-consuming and costly, requiring numerous laboratory tests based on samples taken from the oil bath.

[0011] I n the case of non-hybrid petrol engines, although dilution may occur after cold starts and then during the heating phase of the three-way catalysts, it disappears rapidly under the effect of evaporation, as soon as the oil bath is hot, and dilution never reaches the maximum threshold. What's more, when non-hybrid petrol engines are fitted with particulate filters, active regeneration, i.e. induced regeneration of the filter, is rare and does not give rise to high levels of dilution, as driving conditions are sufficient to cause spontaneous combustion of the particles accumulated in the filters.

[0012] However, dilution becomes a problem for the hybrid petrol or equivalent engines covered by the invention, i.e. engines that combine an internal combustion engine running on petrol (or a mixture of petrol and ethanol, or even pure ethanol) with at least one electric machine that can operate in motor mode.

[0013] I n fact, these motorisations often keep the heat engine switched off, as the aim is generally to minimise its use to reduce fuel consumption, in favour of the vehicle being driven forward by the electric machine(s) alone.

[0014] Thus, in critical cases such as a succession of short journeys in town in a cold environment, for example at 0°C, the heat engine generally does not reach the minimum temperature of around 40-50°C required to evaporate the petrol accumulated in the oil bath. I n addition, it is usual in cold ambient conditions to draw calories from the heat engine to heat the passenger compartment, which cools the engine even more. I n these conditions, the level of dilution can increase until it exceeds the maximum acceptable threshold in terms of engine reliability.

[0015] To ensure engine reliability, oil change intervals can be set in advance, or an OCS-type system can be used to estimate the dilution ratio and alert the driver to the need for an oil change.

[0016] However, these solutions have serious drawbacks. For example, prefixed oil change intervals generate additional maintenance costs, as oil changes are carried out even when they are not really necessary. For example, the calibration of OCS-type systems known in the prior art is a long and costly procedure, as indicated above.

[0017] Explanation of the invention

[0018] The invention proposes to remedy the aforementioned drawbacks by targeting a method for calculating a fuel dilution ratio based on a simple, accurate and robust dilution calculation model, applied to hybrid motorizations comprising a spark-ignition internal combustion engine associated with at least one electric machine that can operate in motor mode.

[0019] The invention relates to a method of calculating a dilution ratio of a fuel in an oil of a spark-ignition internal combustion engine capable of driving at least one drive wheel of a motor vehicle, the said engine being associated with at least one electric machine capable of operating in a motor mode in which the said electric machine contributes to the drive torque of the vehicle.

[0020] The process comprises the steps of: calculation of a variation in the mass of fuel diluted in an engine oil bath by a time integral of the difference between a first quantity of fuel added to the oil bath and a second quantity of fuel removed by evaporation, updating a stored value of the mass of fuel diluted in the engine oil bath as a function of the calculated variation, and calculation of the dilution ratio as a function of the ratio between the updated mass of diluted fuel in the engine oil bath and a mass of oil in the engine oil bath.

[0021] Such a process provides precise information about the dilution ratio in the engine, thereby avoiding endangering engine reliability or unnecessarily increasing engine maintenance costs. I n addition, such a process is easy to implement, as it does not require any prior calibration.

[0022] Advantageously, the first quantity of fuel added to the oil bath is obtained by a fuel balance based on a flow of injected fuel and a flow of burnt fuel that ends up in the exhaust. Such a fuel balance represents a simple, accurate and robust method of estimating the first quantity of fuel added to the oil bath.

[0023] For example, the flow of burnt fuel ending up in the exhaust is equal to the product of a richness measured at the exhaust and a ratio between an intake air flow and the value of the combustive power of the engine's fuel.

[0024] Advantageously, the second quantity of fuel which evaporates is obtained from an evaporation model in which the evaporation rate of the fuel is proportional to its instantaneous concentration. Such an evaporation model represents a simple and effective estimation method.

[0025] Preferably, a measurement of the temperature of an engine coolant is used to calculate the second quantity of fuel evaporating. This is because the temperature of the coolant is closer to the temperature of the fuel evaporation surface than the core oil temperature which is conventionally used.

[0026] I n addition, the process also includes a step for issuing an alert message when the calculated dilution ratio exceeds a predetermined threshold value representative of a risk to engine reliability. This step helps to protect the engine.

[0027] For example, the process also includes a preliminary step of initialising the mass of diluted fuel in the engine oil bath, either at a zero value if the oil is new, or at a value previously stored in an engine computer if it is not. Such a process enables continuous monitoring of dilution and helps to protect the engine. According to one characteristic, the time integral is calculated with a time step of predetermined duration.

[0028] According to another aspect, the invention relates to a motor vehicle equipped with a spark-ignition internal combustion engine capable of driving at least one drive wheel of the motor vehicle, the engine being associated with at least one electric machine capable of operating in a motor mode in which the electric machine contributes to the drive torque of the vehicle.

[0029] The engine is equipped with : calculation of a variation in a mass of fuel diluted in an engine oil bath by a time integral of the difference between a first quantity of fuel added to the oil bath and a second quantity of fuel removed from it by evaporation, updating a stored value of the mass of fuel diluted in the engine oil bath as a function of the calculated variation, and calculation of a dilution ratio as a function of the ratio between the updated mass of diluted fuel in the engine oil bath and a mass of oil in the engine oil bath.

[0030] Brief description of the drawings

[0031] Other aims, characteristics and advantages of the invention will become apparent from the following description, which is given solely by way of non-limiting example, and is made with reference to the appended drawings in which :

[0032] [Fig. 1 ] is a schematic plan view of a petrol hybrid motorisation device capable of implementing a process according to the invention;

[0033] [Fig 2] schematically illustrates a heat engine of the hybrid motorisation device of [Fig 1 ]; and

[0034] [Fig. 3] is a flowchart of a process for calculating a dilution ratio of a fuel in an oil in the engine of [Fig. 2] according to one embodiment of the invention.

[0035] Detailed descri ption of at least one embodiment Figure 1 schematically illustrates a hybrid motorisation device 1 known per se, fitted in particular to a motor vehicle.

[0036] The hybrid motorization device 1 comprises a spark-ignition internal combustion engine 2, combined with at least one electrical machine, in this case a first electrical machine 3 and a second electrical machine 4.

[0037] The heat engine 2 produces engine torque, which results from the combustion of a mixture of fresh air and fuel. The fuel is petrol, a mixture of petrol and ethanol, or even pure ethanol.

[0038] The electrical machines 3, 4 are able to operate in "generator" mode under the supervision of a control box not shown. I n addition, at least the first electrical machine 3 is able to operate in "motor" mode. I n other words, the first electrical machine is reversible. However, it is not essential for the implementation of the method according to the invention that the second electrical machine 4 be able to operate in "motor" mode.

[0039] I n "generator" mode, an electrical machine 3, 4 is an alternator which supplies an electrical current for storage in a storage battery not shown.

[0040] I n "motor" mode, the first electrical machine 3 is powered by current previously stored in the storage battery and provides a driving torque that can be transmitted to the vehicle wheels, in addition to or instead of the torque provided by the heat engine 2.

[0041] The motorisation device 1 is associated with a transmission system 5 enabling the torque supplied by the motorisation device 1 to be transmitted to the wheels 6. The motorisation device 1 is adjusted so that the torque it supplies reaches a set torque value, or required torque, corresponding to the driver's "desire to accelerate", materialised, for example, by a value for the depression of an accelerator pedal of the vehicle or the pressure exerted on said pedal. The transmission system 5 comprises a gearbox 7, a differential axle 8 and a drive shaft 9. The gearbox is connected to the heat engine 2 and the electrical machines 3, 4 on the one hand, and to the wheels 6 via the differential 8 and the drive shaft 9 on the other. The engine 2 comprises an engine control computer 10 configured to control the various components of the heat engine 2 on the basis of data collected by sensors at different locations on the engine 2. Figure 2 is a simplified illustration of the heat engine 2 of Figure 1 , which is suitable for implementing a method for calculating a fuel dilution ratio in an oil bath of the engine according to the invention. The heat engine 2 is an in-line three-cylinder engine with one injector 1 1 per cylinder. The engine comprises a top engine 12 comprising a cylinder head and a crankcase, the bottom of which is closed by an oil sump 13. The oil sump 13 contains an oil bath 14 for lubricating the engine 2 and which also collects crankcase gases, also known as "blow-by" gases.

[0042] During engine operation, a portion Q_in of the fuel Q_inj injected by the injectors 1 1 passes from the top of the engine into the oil sump 13, thus increasing the mass of fuel M_fuel_oil present in the oil bath 14 of the engine 2.

[0043] Conversely, a portion Q_out of the fuel present in the oil bath 14 is removed by evaporation through a degassing circuit 15 in the oil sump 13 opening into an air intake circuit 16, downstream of an intake flap or throttle body 17.

[0044] The air intake circuit 16 comprises, upstream of the throttle body 17, a flow meter 18 configured to measure the intake air flow rate Q_air. Alternatively, the flow rate can be estimated from the pressure and temperature in an engine intake manifold, not shown, and from a filling model.

[0045] The engine 2 comprises an exhaust circuit 19 for burnt gases equipped with a probe 20 adapted to measure the richness Ri_exh at the exhaust. For example, measuring the richness Ri_exh enables the computer 10 to adjust the richness of the air-fuel mixture in a closed loop to a set value, generally a unit value, by adjusting the quantity of fuel injected. The engine 2 may have other specific features depending on the configuration, without going beyond the scope of the invention. For example, the engine may be of the supercharged type. For example, the engine may be equipped with one or more circuits for partial recirculation of exhaust gases to the intake. Figure 3 illustrates a flowchart of a process for calculating a dilution ratio Td of fuel in the oil of engine 2 according to an example of implementation of the invention.

[0046] The process begins with a preliminary step 21 of initialising the mass of fuel in the engine oil M_fuel_oil either at a zero value if the oil is new, or at a value previously stored in the computer 10 of the engine 2 in the opposite case.

[0047] The process continues with a step 22 of calculating a variation AM of a mass of diluted fuel in the oil bath 14 of the engine 2. The calculator 10 calculates the variation AM by a time integral of the difference between a first quantity of fuel Q_in which is added to the oil bath 14 and a second quantity of fuel Q_out which is subtracted therefrom by evaporation.

[0048] Preferably, the time integral is calculated with a time step dt of predetermined duration, for example of the order of 100 milliseconds. The first quantity of fuel Q_in is obtained by a fuel balance from a flow rate of injected fuel Q_inj and a flow rate of burnt fuel which ends up in the exhaust Q_exh. I n other words, the flow of fuel introduced into oil 14 is the difference between the flow of fuel recovered at the exhaust and the flow of fuel injected.

[0049] Preferably, the value of the injected fuel flow Q_inj is directly available in the computer 10.

[0050] The value of the flow of burnt fuel that ends up in the exhaust Q_exh is equal to the product between the richness Ri_exh measured at the exhaust by the probe 20 and a ratio between the intake air flow Q_air and the value of the combustive power Ks of the fuel in the engine 2. The combustive power of a fuel is the ratio between the quantity of air and the corresponding mass of fuel for complete combustion. This ratio is also known as the stoichiometric mass ratio.

[0051] The second quantity of fuel Q_out is obtained from an evaporation model in which the evaporation rate of the fuel is proportional to its instantaneous concentration.

[0052] For example, fuel evaporation is modelled by a first-order kinetic law, in which the evaporation rate of the fuel is equal to the opposite of the product of a rate parameter k of the evaporation reaction and the instantaneous value of the dilution. I n a known way, the rate parameter k of such a reaction depends on the evaporation temperature T and the activation energy Ea of the reaction, i.e. the enthalpy of vaporisation of the fuel according to the following equation :

[0053] -Eg k = kQe RT (1 )

[0054] I n which : ko is a predetermined speed constant, R is the perfect gas constant, and T is a value representative of the evaporation temperature of the fuel.

[0055] The activation energy or enthalpy of evaporation Eais a function that depends on the composition of the fuel, more specifically the proportion of heavy hydrocarbons contained in the fuel. This proportion increases over time, with the lightest hydrocarbons evaporating first.

[0056] It should be noted that the time step dt for calculating the integral is chosen to be sufficiently small for the enthalpy of evaporation Ea to be considered constant. For example, the variation in the enthalpy of evaporation Ea may be considered negligible over a period of a few minutes.

[0057] Advantageously, the representative value of the fuel evaporation temperature T is not the core oil temperature as conventionally measured or estimated on engines, but rather the temperature of the cooling liquid, known as the water temperature, circulating in the engine 2. This temperature is closer to the temperature of the fuel evaporation surface. It can be measured by a sensor, not shown in the figures.

[0058] The process continues with a step 23 of updating a value stored in a memory of the computer 10 of the mass of diluted fuel M_fuel_oil in the oil bath 14 of the engine 2 as a function of the variation AM calculated. The computer 10 then calculates the dilution ratio Td as a function of the ratio between the updated mass M_fuel_oil of diluted fuel in the engine oil bath 14 and the mass of oil M_oil in the oil bath 14 of the engine 2 (step 24) . Preferably, the oil mass M_oil of the oil bath 14 of the engine 2 is available in the computer 10. For example, the engine 2 is fitted with means for measuring the oil mass M_oil of the oil bath 14 adapted to transmit the value of M_oil to the computer 10. The method also comprises a step 25 of emitting an alert message when the calculated dilution ratio Td exceeds a predetermined threshold value representative of a risk to the reliability of the engine 2. For example, an alert may be issued on the motor vehicle dashboard to warn the driver that the threshold value has been exceeded.

Claims

CLAI MS1 . Method of calculating a dilution ratio (Td) of a fuel in an oil of a spark-ignition internal combustion engine (2) suitable for driving at least one drive wheel (6) of a motor vehicle, said engine (2) being associated with at least one electric machine (3) capable of operating in a motor mode in which said electric machine (3) participates in the drive torque of the vehicle, characterised in that the method comprises the steps of :-calculation of a variation (AM) in a mass of fuel diluted in an oil bath (14) of the engine (2) by a time integral of the difference between a first quantity of fuel (Q_in) which is added to the oil bath (14) and a second quantity of fuel (Q_out) which is subtracted therefrom by evaporation, -updating a stored value of the mass of diluted fuel (M_fuel_oil) in the oil bath (14) of the engine (2) as a function of the variation (AM) calculated, and-calculation of the dilution ratio (Td) as a function of the ratio between the updated mass of diluted fuel (M_fuel_oil) in the oil bath (14) of the engine and a mass of oil (M_oil) in the oil bath (14) of the engine (2).

2. Method according to claim 1 , in which the first quantity of fuel (Q_in) which is added to the oil bath (14) is obtained by a fuel balance from a flow of injected fuel (Q_inj) and a flow of burnt fuel which ends up in the exhaust (Q_exh).

3. Method according to claim 2, in which the flow rate of burnt fuel which ends up in the exhaust (Q_exh) is equal to the product between a richness (Ri_exh) measured at the exhaust and a ratio between a flow rate of intake air (Q_air) and the value of the combustive power (Ks) of the fuel of the engine (2) .

4. Method according to any one of claims 1 to 3, wherein the second quantity of fuel (Q_out) which evaporates is obtained from an evaporation model in which the evaporation rate of the fuel is proportional to its instantaneous concentration.

5. Method according to any one of claims 1 to 4, in which a measurement of the temperature of a coolant of the engine (2) is used to calculate the second quantity of fuel (Q_out) which evaporates.

6. Method according to any one of claims 1 to 5, further comprising a step of emitting an alert message when the calculated dilution ratio (Td) exceeds a predetermined threshold value representative of a risk to the reliability of the engine (2) .

7. Method according to any one of claims 1 to 6, further comprising a prior step of initialising the mass of diluted fuel (M_fuel_oil) in the oil bath (14) of the engine (2) either at a zero value if the oil is new, or at a value previously stored in a computer (10) of the engine (2) in the opposite case.

8. Method according to any one of claims 1 to 7, in which the time integral is calculated with a time step (dt) of predetermined duration.

9. Motor vehicle equipped with a spark-ignition internal combustion engine (2) capable of driving at least one driving wheel (6) of the said motor vehicle, the said engine (2) being associated with at least one electric machine (3) capable of operating according to a motor mode in which the said electric machine (3) participates in the driving torque of the vehicle, characterised in that the said engine (2) is equipped with means of : calculation of a variation (AM) in a mass of diluted fuel (M_fuel_oil) in an oil bath (14) of the engine (2) by a time integral of the difference between a first quantity (Q_in) of fuel which is added to the oil bath (14) and a second quantity of fuel (Q_out) which is removed therefrom by evaporation, updating of a stored value of the mass of diluted fuel (M_fuel_oil) in the oil bath (14) of the engine (2) as a function of the variation (AM) calculated, and calculation of a dilution ratio (Td) as a function of the ratio between the updated mass of diluted fuel (M_fuel_oil) in the oil bath (14) of the engine (2) and a mass of oil (M_oil) in the oil bath (14) of the engine

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