Control method for a spark-ignition engine operating on petrol and alcohol

Abstract

On the basis of the determination of a first alcohol rate value (RATE), obtained with an uncertainty of plus or minus x percent, a corrected rate value (RATEcorr) is determined by subtracting x percent from the first value (RATE) and the richness setpoint on the thermal protection zone is adjusted to the value corresponding to the corrected rate, for each point in the zone.

Description

[0001] DESCRIPTION

[0002] Control method for a spark-ignition engine operating on petrol and alcohol

[0003] Technical field of the invention

[0004] The invention relates to a method of controlling the operation of a spark-ignition internal combustion engine. It relates more particularly to the control of engines intended to operate with different types of fuel comprising petrol and alcohol (more precisely: ethanol) in different proportions, which can range in particular from pure petrol to pure alcohol.

[0005] It finds an advantageous application in the form of a method for on-board control of a sparkignition engine in a motor vehicle equipped with a spark-ignition engine whose fuel tank can be filled using the various types of fuel composed of petrol and / or alcohol available at the pump.

[0006] State of the art

[0007] Many modern spark-ignition engines, used in motor vehicles in particular, are designed to run on different types of petrol / alcohol mixtures, depending on the country.

[0008] On the Brazilian market, for example, the tank can often be filled with fuels known as "E25" and "E27", which are fairly similar mixtures of petrol and alcohol, and fuel known as "E100". All three fuels are generally available at the pump. E25 fuel is a mixture of petrol and alcohol comprising twenty-five per cent alcohol (by volume). E27 fuel is a mixture of petrol and alcohol comprising twenty-seven per cent alcohol, and E100 fuel is pure alcohol.

[0009] For example, in the countries of the European Union, three types of fuel available at the pump have been regulated since 12 October 2018. These are: "E5" fuel, which is a mixture of unleaded petrol (SP95 or SP98) comprising a maximum of five per cent alcohol by volume; "E10" fuel, which is a mixture of unleaded petrol (SP95) containing a maximum of ten per cent alcohol by volume; and "E85" fuel, which is a mixture comprising a maximum of eighty-five per cent alcohol by volume, including sixty-five per cent in winter and eighty-five per cent in summer.

[0010] Other types of fuel mixing petrol and alcohol are available depending on the country.

[0011] The control method for such an engine consists of managing the engine using various sensors and actuators associated with a set of control laws or software strategies stored in a computer onboard the vehicle.

[0012] The computer receives information from all the sensors fitted to the vehicle, enabling it to know the state of the engine in real time. On the basis of the physical data received by the computer, it is able to control or adjust all the engine's actuator devices for optimum operation.

[0013] To control a dual-fuel engine capable of running on petrol only, alcohol only, or a mixture of the two, one of the important pieces of information that the computer needs to take into account is the proportion of each of the two fuels contained in the vehicle's fuel tank. This is because engine control parameters such as air mass flow rate, ignition timing and fuel injection pattern (i.e. the duration of fuel injection and its phasing in the combustion cycle) need to be adjusted according to this information. It is important to know as precisely as possible and as quickly as possible what fuel or mixture of fuels is present in the tank in order to adapt these various engine control parameters, to avoid engine damage, minimise pollutant emissions and control engine performance.

[0014] More specifically, there are differences between petrol and alcohol: in net calorific value (NCV); in stoichiometric ratio Ks (ratio between the mass of air and the mass of fuel to obtain complete combustion of the fuel); in richness efficiency; and in advance efficiency.

[0015] For example, a setting adapted to petrol is not suitable when running on alcohol, because for an identical air mass flow, and running at richness 1 , the torque would be a little too high. When the engine is running on alcohol, the air flow rate should be reduced by a few percent if the same torque is to be maintained.

[0016] Conversely, a setting adapted to alcohol is not suitable when running on petrol, because the ignition advance, which is advantageously set at a high optimum value due to the good anti-knock properties of alcohol, can lead to knocking when running on petrol, which has a lower knock clearance. When the engine is running on petrol, the ignition advance should be reduced.

[0017] It is understood from the above that the engine setting must be adapted in the case of operation with a mixture of petrol and alcohol, depending on the relative proportions of petrol and alcohol. The state of the art is known from publication FR-A1 -2892769, which discloses a method for recognising the level of exotic fuel in a tank. It proposes a method for controlling an internal combustion engine intended to operate either with petrol, or with alcohol, or with a variable proportion of the two fuels, comprising an electronic control unit capable of driving said engine, an intake manifold and an exhaust manifold equipped with a richness probe which is of the on / off type, characterised in that it comprises at least one step consisting in determining the richness of the carburetted mixture entering the combustion cylinders and a step consisting, as a function of the richness of the carburetted mixture, in increasing in stages the duration of fuel injection in order to adapt it to the mixture of fuels present in the engine tank and to obtain a substantially stoichiometric carburetted mixture.

[0018] However, this method is very imprecise, particularly because of the stepwise increase in injection time. It is sufficient for starting the engine but cannot be used to control all the engine's speedload operating points.

[0019] Also known in the art is a method for determining the alcohol rate in a fuel mixing petrol and alcohol, which is based on the proportions of the respective quantities of air admitted to the engine and fuel injected, when the engine is operating in a closed loop at richness 1 , which is the case for at least most of the operating points of a spark-ignition engine. The air flow rate can be determined by a flow meter, or alternatively from the pressure and temperature in a distributor, or intake manifold of the engine, and the fuel flow rate can be calculated in particular from the duration of opening of the fuel injectors. The stoichiometric ratio Ks which is calculated from these flow rates is characteristic of the proportions of the mixture of petrol and alcohol, and therefore of the alcohol rate.

[0020] On the basis of a statistical study taking into account the manufacturing dispersion of a population of engines and proportional oxygen probes used to regulate richness in a closed loop, the applicant has estimated the uncertainty in determining the value of the alcohol rate RATE at approximately + / - 10% using this method.

[0021] Another solution is to fit the engine with an ethanol concentration detection sensor. However, these sensors are bulky and expensive and, although more accurate than the previous method, it is estimated that the sensors available for the automotive industry can lead to an uncertainty of + / - 5% on the measured value of the alcohol rate RATE.

[0022] The uncertainty of plus or minus x per cent, for example ten or five per cent, of these known methods poses a particular problem for engine control at full load.

[0023] In fact, it is known to increase the value of the richness of the air-fuel mixture to a value strictly greater than 1 over a set of engine speed-load operating points close to the engine full load curve, and in particular for high speed values, so as to maintain the temperature of certain components of the engine exhaust circuit, such as the exhaust manifold or the turbine of a turbocharger of the engine if the latter is of the supercharged type, below a temperature limit compatible with the mechanical reliability of these components, for example 950°C or 980°C, depending on the nature of the said components (for example, depending on whether the exhaust manifold is made of cast iron or steel, etc.). This zone of speed-load operating points, where over-enrichment takes place, is known as the "thermal protection zone", so as to limit the engine exhaust temperature to a predetermined maximum value.

[0024] The value of the richness to be applied to keep the temperature within such a maximum temperature limit generally depends on the speed-load operating point considered in the said thermal protection zone, and can reach, for example, richness values R of the order of 1 .20 when the engine is running on pure petrol.

[0025] We can speak of "thermal protection richness" when the said richness value, strictly greater than 1 , is used as a richness setpoint in order to limit the engine's exhaust temperature. A first thermal protection richness map is established for each engine speed-load operating point in the thermal protection zone with pure petrol.

[0026] However, if the engine runs on a mixture of petrol and alcohol, the thermal protection richness can be set at each point to a lower value than in the case of pure petrol, because the combustion of alcohol is cooler than that of petrol. And, in the case of an engine running on pure alcohol, it is even possible that the richness value can be maintained at 1 over a certain number of operating points in the thermal protection zone corresponding to operation on pure petrol.

[0027] A second thermal protection richness map is also established for each engine speed-load operating point in the thermal protection zone with pure alcohol.

[0028] Generally speaking, the value of thermal protection richness to be applied to maintain the engine exhaust temperature within the limit required for reliability is a decreasing function of the alcohol rate RATE in the fuel. Each value can be determined as a function of the exact alcohol rate by prior tests on the engine test bench using fuels mixed in perfectly known proportions (known volume of pure petrol added to a known volume of pure alcohol), in the same way as the first and second maps are established using pure petrol and pure alcohol respectively. In a simplified variant, each value of thermal protection richness corresponding to a given rate RATE can be deduced by interpolation between the value of the first map and the value of the second map corresponding to the operating point in question of the thermal protection zone.

[0029] It is then possible either to embed in an engine computer a thermal protection richness map to be applied as a function of the alcohol rate RATE determined in the vehicle tank in order to control the engine in the zone close to full load, or to embed the first map and the second map and means for calculating by interpolation the thermal protection richness as a function of a determined alcohol rate RATE in the fuel.

[0030] However, it is understood from the above that the uncertainty of plus or minus x per cent in the value of the alcohol rate RATE determined by a prior art method as mentioned above has a disadvantage: in the event that the value of the alcohol rate RATE is overestimated in relation to the actual value, due to the imprecision of the method, there is in reality less alcohol in the mixture, and combustion is hotter than imagined, so that the chosen value of thermal protection richness, for example determined by interpolation as indicated above, is insufficient. There is then a risk that the temperature of the engine exhaust components will rise above their reliability threshold, and that the engine will be damaged.

[0031] Presentation of the invention

[0032] The invention aims to remedy the lack of precision of known methods of determining the alcohol rate by limiting their negative impact on the reliability of the components of the engine exhaust circuit. To this end, a method is proposed for controlling an internal combustion engine operating either with petrol, or with alcohol, or with a variable ratio of the two in the mixture, comprising a computer capable of controlling said engine, said computer comprising means for determining, as a function of the alcohol rate in the mixture of a richness reference value strictly greater than 1 to be applied for each engine speed-load operating point of a thermal protection zone so as to limit the engine exhaust temperature to a predetermined maximum value, said method comprising a first step of predetermining an alcohol rate in the mixture with an estimated uncertainty of plus or minus jgoer cent.

[0033] The main feature of the method according to the invention is that it further comprises a second step in which :

[0034] -it is tested whether the engine speed-load operating point is within the said thermal protection zone;

[0035] -if this is the case, a corrected value for the alcohol rate in the mixture is determined, calculated by subtracting the value of the uncertainty x_from the value of the rate in the predetermination step; and,

[0036] -the engine is set to the thermal protection richness value corresponding to the corrected alcohol rate.

[0037] Presentation of figures

[0038] The invention will be better understood on reading a non-limiting embodiment thereof, in support of the appended figures among which :

[0039] [Fig. 1] Figure 1 is a schematic representation of an example of a motorisation device suitable for implementing the method according to the invention.

[0040] [Fig. 2] Figure 2 is a flow chart representing the various steps of the method according to the invention, according to one embodiment.

[0041] Detailed description

[0042] Figure 1 shows a motorisation device 1 suitable for implementing the method according to the invention.

[0043] This device comprises an internal combustion engine 2, which here takes the form, for example, of a four-cylinder in-line engine. This engine, for example a motor vehicle engine, is a sparkignition internal combustion engine, running on petrol, or alcohol (ethanol), or with a mixture of petrol and alcohol. In a non-limiting way, it can be naturally aspirated or supercharged. It may also have other special features without affecting the generality of the invention, for example one or more circuits for partial recirculation of exhaust gases at the engine intake.

[0044] The engine 2 can be of the direct injection type, as in the example shown in Figure 1 . Each cylinder of the engine is individually supplied with fuel by an injector 3 via a common supply rail 4, from a fuel tank (not shown), which can be completely or partially filled with fuel of at least two distinct types when it is empty. More specifically, it can be filled with at least a first type of fuel composed of a mixture of petrol and alcohol having a first proportion of alcohol of zero or relatively low, and a second type of fuel composed of a mixture of petrol and alcohol having a second proportion of alcohol relatively high or equal to one hundred percent. The terms "relatively low" and "relatively high" mean that the second alcohol proportion is higher than the first alcohol proportion.

[0045] On the Brazilian market, for example, the tank can be filled with fuel known as "E25", "E27" or "E100". E25 and E27 fuels are mixtures of petrol and alcohol comprising twenty-five and twentyseven per cent alcohol by volume respectively, while E100 fuel is pure alcohol. These three types of fuel are generally available simultaneously at the pump, and depending on the region, consumers prefer to use one or the other for long periods depending on the purchase price, with the authorities playing on fuel taxation to favour either the two lower-alcohol E25 or E27 fuels, or the E100 fuel, depending on the level of alcohol production in the country.

[0046] For example, in the countries of the European Union, three types of fuel available at the pump have been regulated since 12 October 2018. These are: "E5" fuel, which is a mixture of SP95 or SP98 unleaded petrol comprising a maximum of five per cent alcohol by volume; "E10" fuel, which is a mixture of SP95 unleaded petrol comprising a maximum of five per cent alcohol by volume; and "E85" fuel, which is a mixture comprising a maximum of eighty-five per cent alcohol by volume. It should be noted that the exact proportion of alcohol in E85 fuel is neither fixed nor known with certainty, but it is known to be between sixty-five per cent in winter and eighty-five per cent in summer.

[0047] In addition, the engine is associated with an air intake circuit 5, enabling fresh air taken from the outside atmosphere to be supplied to the engine's cylinders, and with an exhaust gas circuit 6, enabling combustion gases from the cylinders to be discharged into the outside atmosphere.

[0048] The air intake circuit 5 comprises a gas intake valve 7, or throttle body 7, the degree of opening of which makes it possible to adjust the air mass flow Qair admitted to the engine, and an intake manifold 8, or distributor 8. The intake manifold 8 is equipped with a pressure sensor 9, which continuously measures the manifold pressure Pcoll, i.e. the pressure within it, downstream of the throttle body 7 (in the direction of air flow). The intake manifold 8 is also equipped with a temperature sensor 10, capable of continuously measuring the temperature Tcoll of the intake air, i.e. the temperature of the air entering the engine. In a manner known per se, the air mass flow Qair can be determined at each instant from the said pressure Pcoll and temperature Tcoll values and from a filling model.

[0049] The exhaust circuit 6 comprises, from upstream to downstream in the direction of flow of the burnt gases, an exhaust manifold 11 , an engine combustion gas depollution catalyst 12, and an exhaust pipe 13. As this is a spark-ignition engine in this motorisation system, the catalyst 12 is a "three- way" type catalyst, capable of post-treating the pollutants emitted in the engine combustion gases: nitrogen oxides (NOX), unburnt hydrocarbons (HC) and carbon oxides (CO). The catalyst 12 is associated with at least one proportional-type oxygen probe 14 whose output signal, generally a voltage, is used to determine a value for the residual oxygen richness in the combustion gases from the engine and consequently the richness of the carburetted mixture, and which is used in a manner known per se to regulate the richness in a closed loop at most of the engine's operating points, by adjusting the duration of fuel injection.

[0050] The motorisation device 1 can also be associated with other sensors and actuators not shown, in particular means for determining the engine speed N, generally in the form of a sensor for the number of rotations of a target mounted at the end of the engine crankshaft.

[0051] An electronic control system, or ECU computer (not shown), is used to control the motorization device 1 so that the engine supplies the torque required to drive the vehicle.

[0052] For example, for a torque setting C reflecting the driver's wishes, corresponding to a given degree of depression of the vehicle accelerator pedal by the vehicle driver, and for a given speed N, the computer determines a mass flow rate of air Qair to be admitted to the engine as a function of an advance efficiency and a richness efficiency. It also determines an ignition advance value AA, preferably the optimum advance value, which enables the advance efficiency to be maximised, and a fuel flow rate Qcarb to obtain a given mixture richness, which is generally set in closed loop to a set-point value.

[0053] This set-point value is generally a richness set-point equal to 1 , at most of the engine's operating points, which enables the three-way catalyst 12 to operate optimally.

[0054] However, in a so-called thermal protection zone, the setpoint is set to a richness value strictly greater than 1 in order to contain the temperature of certain components in the exhaust system. To obtain the desired air mass flow Qair, the computer adjusts the degree of opening of the throttle body 6, and to obtain the desired fuel flow Qcarb, the computer adjusts at least one duration of opening of the injectors 3 and at least one time of opening of the said injectors in relation to the combustion cycle (generally counted in degrees of crankshaft angle in relation to top dead centre). The computer also adjusts the ignition advance, which is expressed by the time at which a spark is thrown across the terminals of a spark plug (not shown) present in each cylinder of the engine. In order to control the engine in the thermal protection zone, the computer also comprises said computer means for determining, as a function of the alcohol rate in the mixture, a setpoint richness value strictly greater than 1 to be applied for each engine speed-load operating point in a thermal protection zone so as to limit the engine exhaust temperature to a predetermined maximum value.

[0055] In one embodiment of these means, the computer comprises a first richness setpoint map to be applied as a function of the load operating point and a second richness setpoint map to be applied as a function of the speed-load operating point, and means for determining the richness setpoint to be applied as a function of the speed-load operating point and as a function of the rate RATE of alcohol determined in the fuel, by interpolation between the setpoint values of the first and second maps.

[0056] In another embodiment of the means, the computer can include as many richness maps as a function of the speed-charge point as there are possible alcohol rates (for example, one map per rate unit between 25% and 100% for Brazil).

[0057] The term "thermal protection zone" refers to a set of engine speed-load operating points close to full load, particularly at high engine speeds, at which the engine exhaust temperature is maintained at a limit temperature for exhaust reliability by over-enriching the mixture.

[0058] Figure 2 illustrates the various steps of the method according to the invention, in a preferred embodiment thereof.

[0059] The method comprises a step of determining 100 a value for the alcohol rate RATE in the fuel, with an uncertainty of plus or minus x per cent, and adjusting the engine according to this rate. The rate RATE thus determined is a centred value. In other words, the actual rate of alcohol in the fuel, as could be accurately measured by metrological means by mixing known volumes of pure petrol and pure alcohol, is between, on the one hand, this centred rate RATE minus x per cent and, on the other hand, this centred rate RATE plus x per cent.

[0060] In one embodiment, the rate RATE is deduced from the stoichiometric ratio Ks, which is calculated from the air and fuel flow rates entering the engine when it is set in closed loop to richness 1 . Remember that the stoichiometric coefficient Ks of pure alcohol is approximately 8.4 and that of pure petrol is approximately 14.7, with mixtures taking intermediate values, each corresponding to a specific rate.

[0061] The uncertainty x of the determination by this method is estimated to be + / - 10%.

[0062] In another embodiment of the first step 100, the rate RATE is determined from the measurement of an alcohol concentration sensor, the uncertainty x then being estimated at + / - 5%.

[0063] Proportional oxygen probe 14 is used to regulate the engine. At engine operating points outside the thermal protection zone, the air flow rate Qair and the ignition advance AA are selected to produce the required engine torque C as a function of the characteristics of the fuel with the alcohol rate RATE in question (lower calorific value LCV; stoichiometric ratio Ks; richness efficiency; advance efficiency, etc.), according to the state of the art.

[0064] According to the invention, the method then comprises a second step 200 during which, successively, the following sub-steps are carried out:

[0065] In a first sub-step, it is tested whether the engine speed-load operating point is within the thermal protection zone; if this is not the case, the method resumes at step 100 and the engine is set to richness 1 .

[0066] If this is not the case, the method continues with a second sub-step in which a corrected value of alcohol rate RATEcorr is determined by subtracting the uncertainty x% of determination from the alcohol rate value RATE determined in the first step 100.

[0067] Then, in a third sub-step, the richness setting is adjusted to the thermal protection richness value corresponding to this corrected rate RATEcorr, using the first map, the second map and the interpolation means previously stored in the engine computer, and the engine is controlled using this setting. This third sub-step is justified by the fact that determining the rate involves a margin of error of plus or minus x percent, for example + / - 10% or + / - 5% according to the examples cited in step 100, which could excessively increase the exhaust temperature if the rate is overestimated. Subtracting the x% uncertainty margin eliminates this risk.

[0068] In addition, the corrected value is saturated with the value of the lowest alcohol rate present in the fuels available at the pump. For example, in Brazil where the fuel with the lowest alcohol content is E25, if the rate RATE value determined in step 100 is thirty percent with an uncertainty of + / - 10%, the corrected RATEcorr value will be twenty-five percent and not twenty percent. Alternatively, it is also possible to saturate at a lower value, for example 22% corresponding to a homologation fuel, used in particular for tests.

[0069] The engine is adjusted on the basis of this corrected rate RATEcorr , but advantageously with the exception of the ignition advance AA. More precisely, to produce an engine torque C, the air flow rate Qair is chosen as a function of the characteristics (PCI, Ks, ...) of the fuel with the corrected alcohol rate, but it may be preferable to apply the ignition advance AA corresponding to the fuel with the uncorrected rate, i.e. the first rate RATE of step 100, so as to increase the engine performance or limit fuel consumption. As this ignition advance AA is higher for a more alcoholic fuel, it is advantageous to use an anti-knock advance correction method, i.e. a method in which the presence of knock is detected by means of a knock sensor, and advance is withdrawn to eliminate it, if necessary. Such methods are known per se and will not be further detailed.

[0070] Advantageously, in an improved embodiment of the method according to the invention, the method also comprises a third step 300 of cancelling the rate correction carried out in step 200 for the most alcoholic fuel if the rate of the latter is perfectly known and the other fuel or fuels have alcohol rates much lower than that of the most alcoholic fuel, in particular a difference greater than the uncertainty x in determining the rate in step 100. This is particularly the case in Brazil where the most alcoholic fuel E100 is pure alcohol and where the other two fuels have alcohol rates which differ by more than seventy percent from the maximum rate.

[0071] This is not the case in the European Union, where the highest-alcohol fuel E85 does not define a fuel with a fixed alcohol rate, but only a maximum alcohol rate, which also varies according to the season.

[0072] This third step 300 is based on the fact that correcting the rate in step 200 may lead to over- consumption of fuel, due to the choice of the thermal protection richness value, in the case where the ethanol rate is not in fact overestimated.

[0073] The aim of this third step 300 is to acquire a sufficient presumption that the tank contains only, or almost only, the most alcoholic fuel and to consider that the rate RATE determined is no longer subject to uncertainty x. This step is made possible when the motorist systematically fills the vehicle's tank with a single type of fuel over a long period of time, in particular the most alcoholic fuel, which is the case in Brazil at times when E100 fuel is fiscally favoured.

[0074] Step 300 consists in checking whether, after a succession of several consecutive fill-ups, i.e. after a succession of total or partial fill-ups of the tank, the total volume of which exceeds a predetermined multiple of the total capacity of the tank, the first alcohol rate value RATE of step 100 remains close to the maximum level after each filling-up, i.e. in a range between, on the one hand, the maximum alcohol rate RATE reduced by the uncertainty x of step 100 and, on the other hand, the maximum level.

[0075] If, after each significant refuelling, the rate remains within these limits, it can be concluded that the refuelling was carried out with the most alcoholic fuel, as refuelling with an identical volume of fuel with significantly less alcohol would have caused the RATE value to fall below the minimum value in the range. However, no other type of fuel is available.

[0076] If this is the case, it can be deduced that the tank has been filled with E100 fuel on each occasion, and that the fuel mixture present in the tank has been sufficiently diluted to contain only, or practically only, E100 fuel when the total volume of fuel is large enough (for example several times the capacity of the tank), with petrol present at most only in traces.

[0077] In this case, the rate correction carried out in step 200 is cancelled and the value of the thermal protection richness is determined as a function of the rate RATE of the first step 100 and not the corrected rate RATEcorr of the second step. This avoids the risk of over-consumption of fuel while eliminating the thermal risk.

[0078] This third step can be generalised to any situation in which only very different types of fuel are available: a first fuel, with a relatively low alcohol rate, and a second fuel, with a relatively high and perfectly known alcohol rate, the difference in alcohol rate between the two being at least greater than the uncertainty x in determining the rate RATE of the fuel.

Claims

CLAIMS1. Method for controlling an internal combustion engine (2) operating either with petrol, or with alcohol, or with a variable rate of the two in a mixture, comprising a computer (ECU) capable of controlling said engine (2), said computer comprising means for determining, as a function of the rate of alcohol in the mixture, a setpoint for richness strictly greater than 1 to be applied for each engine speed-load operating point of a thermal protection zone so as to limit the engine exhaust temperature to a predetermined maximum value, said method comprising a first step (100) of predetermining an alcohol rate (RATE) in the mixture with an estimated uncertainty of plus or minus >^per cent, CHARACTERISED IN THAT it further comprises a second step (200) in which:-it is tested whether the engine speed-load operating point is within the said thermal protection zone;-if this is the case, a corrected value (RATEcorr) of alcohol rate in the mixture is determined, calculated by subtracting the value of the uncertainty x from the value of the rate (RATE) of the predetermination step (100); and,-the engine is set to the thermal protection richness value corresponding to the corrected value (RATEcorr) of the alcohol rate.

2. Method according to claim 1 , characterised in that the predetermined alcohol rate (RATE) is deduced from the stoichiometric ratio (Ks) of the fuel, said ratio being obtained from the air and fuel flow rates determined at engine operating points where the richness is set in closed loop to the value 1 , and in that the corrected alcohol rate (RATEcorr) is obtained by reducing said rate (RATE) by ten percent.

3. Method according to claim 1 , characterised in that the predetermined alcohol rate (RATE) is obtained from an ethanol concentration sensor, and in that the corrected alcohol rate (RATEcorr) is obtained by decreasing the said rate (RATE) by five percent.

4. Method according to one of the preceding claims, characterised in that the minimum value of the corrected value (RATEcorr) of the alcohol rate is saturated at the lowest value of alcohol rate present in the fuels available at the pump.

5. Method according to one of the preceding claims, characterised in that during step 200, the engine is set with operating parameters (Qair, Qcarb) determined according to the corrected value (RATEcorr) of the alcohol rate, with the exception of the ignition advance (AA) which is determined according to the value of the alcohol rate as predetermined (RATE).

6. Method according to one of the preceding claims, characterised in that it also comprises a third step (300) of cancelling the rate correction of the second step (200) if, the alcohol rate of the most alcoholic fuel available at the pump being a fixed and knownmaximum rate, the first fuel rate remains close to the said maximum rate after a succession of fillings of the tank, the total volume of which exceeds a predetermined multiple of the volume of the tank.

7. Method according to claim 6, characterised in that it is determined that the fuel rate remains close to the maximum rate when it is greater than the maximum rate reduced by the uncertainty > of the method for determining the alcohol rate (RATE) of the first stage.

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