Method for controlling a spark-ignition engine running on gasoline and alcohol

Abstract

Method for controlling a spark-ignition engine running on gasoline and alcohol. The invention relates to a method for adjusting a gasoline and alcohol engine within a thermal protection zone close to full load, for which the fuel mixture is set to a target value strictly greater than 1. This target value is determined for each operating speed-load point within this zone as a function of the alcohol content (AB) to limit the exhaust temperature to a predetermined maximum value. Starting from the determination of a first AB value (AB), obtained with an uncertainty of plus or minus x percent, a corrected AB value (AB) is determined by subtracting x percent from the first AB value. The fuel mixture target is then set within the thermal protection zone to the value corresponding to the corrected AB for each point within the zone. (Short figure: Fig. 2)

Description

Title of the invention: Method for controlling a spark-ignition engine running on gasoline and alcohol. Technical field of the invention

[0001] The invention relates to a method for controlling the operation of a spark-ignition internal combustion engine. More particularly, it relates to the control of engines intended to operate with different types of fuel comprising gasoline and alcohol (more precisely: ethanol) in different proportions, which may range in particular from pure gasoline to pure alcohol.

[0002] It finds an advantageous application in the form of an on-board control method for a spark-ignition engine in a motor vehicle equipped with a spark-ignition engine whose fuel tank can be filled using the different kinds of fuels composed of gasoline and / or alcohol available at the pump. State of the art

[0003] Many modern spark-ignition engines, particularly those used in motor vehicles, are designed to be able to operate with different types of gasoline and alcohol mixtures, depending on the country.

[0004] For example, in the Brazilian market, one can often fill up with so-called "E25" and "E27" fuels, which are fairly similar blends of gasoline and alcohol, and with so-called "E100" fuel. All three of these fuels are generally available at the pump. E25 fuel is a blend of gasoline and alcohol containing 25 percent alcohol (by volume). E27 fuel is a blend of gasoline and alcohol containing 27 percent alcohol, and E100 fuel is pure alcohol.

[0005] For example, in the countries of the European Union, three types of fuels available at the pump have been regulated since October 12, 2018. These include: "E5" fuel, which is a mixture of unleaded gasoline (SP95 or SP98) containing a maximum of five percent alcohol by volume; "E10" fuel, which is a mixture of unleaded gasoline (SP95) containing a maximum of ten percent alcohol by volume; and "E85" fuel, which is a mixture containing a maximum of eighty-five percent alcohol by volume, including sixty-five percent in winter and eighty-five percent in summer.

[0006] Other types of fuel varieties mixing gasoline and alcohol are available depending on the country.

[0007] The method of controlling 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 on board the vehicle.

[0008] The computer receives information from all the sensors with which the vehicle is equipped, allowing it to know the engine's status in real time. Based on the physical quantities received by the computer, it is able to control or adjust all the engine's actuator devices for optimal operation.

[0009] To control a dual-fuel engine capable of running on gasoline only, alcohol only, or a mixture of the two, one of the important pieces of information that the engine control unit (ECU) must take into account is the proportion of each of the two fuels contained in the vehicle's fuel tank. Indeed, engine control parameters, such as mass air flow, ignition timing, and fuel injection pattern (i.e., the duration of fuel injection and its timing in the combustion cycle), must be adjusted according to this information. It is important to know as precisely and as quickly as possible which fuel or fuel mixture is present in the tank in order to adapt these various engine control parameters, prevent damage, minimize pollutant emissions, and optimize performance.

[0010] More specifically, differences are observed between gasoline and alcohol: in lower heating value LHV; in stoichiometric ratio Ks (ratio between the mass of air and the mass of fuel to obtain complete combustion of the fuel); in fuel-air ratio efficiency; and in ignition advance efficiency.

[0011] Thus, a setting adapted for gasoline is not suitable for operation with alcohol, because for the same mass air flow rate, and operating at a 1:1 air-fuel ratio, the torque would be slightly 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.

[0012] Conversely, a setting adapted for alcohol is not suitable for gasoline operation, because the ignition timing, which is advantageously set to a high optimum value due to the good anti-knock properties of alcohol, can cause knocking when running on gasoline, which has less knock clearance. When the engine is running on gasoline, the ignition timing should be reduced.

[0013] It is understood from the above that the engine setting must be adapted in the case of operation with a mixture of gasoline and alcohol, according to the relative proportions of gasoline and alcohol.

[0014] Prior art is known from publication FR-A1-2892769, which discloses a method for recognizing the percentage of exotic fuel in a tank. It proposes a method for controlling an internal combustion engine intended to operate either with gasoline, or with alcohol, or with a variable proportion of the two fuels, comprising an electronic control unit capable of controlling said engine, an intake duct and an exhaust duct equipped with a mixture sensor of the on / off type, characterized in that it comprises at least one step consisting of determining the richness of the fuel mixture entering the combustion cylinders and a step consisting, depending on the richness of the fuel mixture, of increasing in stages the fuel injection duration to adapt it to the fuel mixture present in the engine tank and to obtain a substantially stoichiometric fuel mixture.

[0015] However, this method is very imprecise, particularly due to the stepwise increase in injection duration. It is sufficient for starting the engine but cannot be used to control it across all its operating points under varying engine speeds and loads.

[0016] A method for determining the alcohol content in a fuel mixture of gasoline and alcohol is also known from the prior art, which is based on the proportions of the respective quantities of air admitted into the engine and fuel injected, when the engine operates in closed loop at richness 1, which is the case at least at most of the operating points of a spark-ignition engine.

[0017] The air flow rate can be determined by a flow meter, or alternatively from the pressure and temperature in a distributor or engine intake manifold, and the fuel flow rate can be calculated, in particular, from the opening time of the fuel injectors. The stoichiometric ratio Ks, which is calculated from these flow rates, is characteristic of the proportions of the gasoline and alcohol mixture, and therefore of the alcohol content.

[0018] Based on a statistical study taking into account the manufacturing dispersion of a population of engines and proportional oxygen probes used to regulate the closed-loop mixture, the applicant estimated the uncertainty of the determination of the alcohol content value TAUX at approximately + / - 10% by this method.

[0019] Another solution is to equip the engine with an ethanol concentration detection sensor. However, these sensors are bulky and expensive, and, although more precise 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 content.

[0020] The uncertainty of plus or minus x percent, for example ten or five percent, of these known methods, poses a particular problem for the control of the engine at full load.

[0021] Indeed, it is known to increase the air-fuel mixture richness to a value strictly greater than 1 at a set of engine operating points (speed-load) close to the engine's full-load curve, and particularly at high engine speeds, in order to maintain the temperature of certain components of the engine's exhaust system, such as the exhaust manifold or the turbine of a turbocharger (if the engine is of the turbocharged 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 said components (for example, depending on whether the exhaust manifold is made of cast iron or steel, etc.). This range of operating points (speed-load), where the mixture is enriched, is called the "thermal protection zone" in order to limit the engine exhaust temperature to a predetermined maximum value.

[0022] The value of the richness to be applied to keep the temperature within such a maximum temperature limit generally depends on the operating point speed-load considered in 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 gasoline.

[0023] The term "thermal protection richness" can be used when the said richness value, strictly greater than 1, serves as a richness setpoint to limit the engine exhaust temperature. A first thermal protection richness map is established for each engine speed-load operating point within the thermal protection zone using pure gasoline.

[0024] However, if the engine is running on a mixture of gasoline and alcohol, the thermal protection mixture can be set at each point to a lower value than in the case of pure gasoline, because alcohol burns at a lower temperature than gasoline. Furthermore, if the engine is running on pure alcohol, it is even possible to maintain the mixture value at 1 at several operating points within the thermal protection zone corresponding to operation on pure gasoline.

[0025] Also, a second thermal protection richness map is established for each engine speed-load operating point in the thermal protection zone with pure alcohol.

[0026] Generally speaking, the thermal protection richness value to be applied to maintain the engine exhaust temperature within the limit set for reliability is a decreasing function of the alcohol content in the fuel. Each value can be determined based on the exact alcohol content through preliminary engine bench tests using fuels blended in precisely known proportions (a known volume of pure gasoline plus a known volume of pure alcohol), in the same way that the first and second maps are established using pure gasoline and pure alcohol, respectively. In a simplified version, each thermal protection fuel-air ratio value corresponding to a given ratio can be deduced by interpolating between the value of the first map and the value of the second map corresponding to the considered operating point within the thermal protection zone.

[0027] It is then possible either to load into an engine computer a thermal protection richness map to be applied according to the alcohol content determined in the vehicle's tank in order to control the engine in the area close to full load, or to load the first map and the second map and means of calculation by interpolation of the thermal protection richness according to a determined alcohol content determined in the fuel.

[0028] However, it is clear from the foregoing that the uncertainty of plus or minus x percent in the alcohol content value determined by a prior art method as mentioned above presents a drawback: if the alcohol content value is overestimated compared to the actual value due to the inaccuracy of the method, there is actually less alcohol in the mixture, and the combustion is hotter than expected, so that the chosen heat protection enrichment value, for example determined by interpolation as indicated above, is insufficient. This risks an increase in the temperature of the engine exhaust components beyond their reliability threshold and engine damage. Presentation of the invention

[0029] The invention aims to overcome the lack of precision in known methods for determining the alcohol content by limiting their negative impact on the reliability of the engine's exhaust system components. To this end, it proposes a method for controlling an internal combustion engine operating on either gasoline, alcohol, or a variable mixture of the two, comprising a computer capable of controlling said engine. This computer includes means for determining, based on the alcohol content in the mixture, a fuel mixture setpoint strictly greater than 1 to be applied at each engine operating speed and load point. It also includes a thermal protection zone to limit the engine exhaust temperature to a predetermined maximum value. process comprising a first step of predetermining an alcohol level in the mixture with an estimated uncertainty of plus or minus x percent.

[0030] The main feature of the method according to the invention is that it further comprises a second step in which:

[0031] -we test whether the operating point of the engine speed-load is located in said thermal protection zone;

[0032] -if this is the case, a corrected value for the alcohol content in the mixture is determined, calculated by subtracting the value of the uncertainty x from the value of the content in the predetermination step; and,

[0033] -the engine is set to the thermal protection richness value corresponding to the corrected value of the alcohol content. Presentation of the figures

[0034] The invention will be better understood upon reading a non-limiting embodiment thereof, with reference to the accompanying figures, among which:

[0035] [Fig-1] Fig. 1 is a schematic representation of an example of a device motorization suitable for implementing the process according to the invention.

[0036] [Fig.2] The [Fig.2] is a flowchart which represents the different steps of the process according to the invention, according to one embodiment. Detailed description

[0037] In [Fig.1], a motorization device 1 suitable for implementing the process according to the invention is shown.

[0038] This device comprises an internal combustion engine 2, which here, for example, takes the form of a four-cylinder in-line engine. This engine, for example, a motor vehicle engine, is a spark-ignition internal combustion engine running on gasoline, alcohol (ethanol), or a mixture of gasoline and alcohol. Without limitation, it may be naturally aspirated or turbocharged. It may also have other features without affecting the generality of the invention, for example, one or more partial exhaust gas recirculation circuits at the engine intake.

[0039] The engine 2 can be of the direct injection type, as in the example illustrated in [Fig. 1]. Each cylinder of the engine is individually supplied with fuel by an injector 3 via a common rail 4, from a fuel tank (not shown), which can be filled totally or partially with at least two distinct types of fuel when it is emptying. More specifically, it can be filled with at least a first type of fuel consisting of a mixture of gasoline and alcohol having a first proportion of zero or relatively low alcohol, and a second type of fuel consisting of a mixture of gasoline and alcohol having a second proportion of alcohol that is relatively high or equal to one hundred percent. By the expressions "relatively low" and "relatively high", it is understood that the second proportion of alcohol is greater than the first proportion of alcohol.

[0040] For example, in the Brazilian market, one can fill up with so-called "E25," "E27," or "E100" fuel. E25 and E27 fuels are mixtures of gasoline and alcohol containing 25% and 27% 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 favor the use of one or the other over long periods based on the purchase price. Authorities use fuel taxes to favor either the two lower-alcohol fuels, E25 or E27, or E100 fuel, depending on the level of alcohol production in the country.

[0041] For example, in European Union countries, three types of fuel available at the pump have been regulated since October 12, 2018. These include: so-called "E5" fuel, which is a blend of unleaded SP95 or SP98 gasoline containing a maximum of five percent alcohol by volume; so-called "E10" fuel, which is a blend of unleaded SP95 gasoline containing a maximum of five percent alcohol by volume; and so-called "E85" fuel, which is a blend containing a maximum of eighty-five percent 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 percent in winter and eighty-five percent in summer.

[0042] Furthermore, the engine is associated with an air intake circuit 5, allowing the engine cylinders to be supplied with fresh air taken from the outside atmosphere, and with an exhaust gas circuit 6, allowing the combustion gases from the cylinders to be evacuated to the outside atmosphere.

[0043] The air intake circuit 5 includes a gas intake valve 7, or throttle body 7, the opening of which allows adjustment of the mass flow rate of air Qair admitted into the engine, and an intake manifold 8, or distributor 8. The intake manifold 8 is equipped with a pressure sensor 9, which allows continuous measurement of the manifold pressure Pcoll, i.e., the pressure within it, downstream of the throttle body 7 (in the direction of airflow). The intake manifold 8 is also equipped with a temperature sensor 10, capable of continuously measuring the intake air temperature Tcoll, i.e., the temperature of the air entering the engine. In a manner known per se, the mass flow rate of air Qair can be determined at any instant from the said values ​​of pressure Pcoll and temperature Tcoll and a filling model.

[0044] The exhaust circuit 6 comprises, from upstream to downstream in the direction of flow of the burnt gases, an exhaust manifold 11, a catalyst for the pollution control of the combustion gases of the engine 12, and an exhaust muffler 13. As this engine system is a spark-ignition engine, the catalyst 12 is here a “three-way” type catalyst, capable of carrying out the after-treatment of pollutants emitted in the combustion gases of the engine: nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon oxides (CO).The catalyst 12 is associated with at least one proportional oxygen sensor 14 whose output signal, generally a voltage, allows the determination of a residual oxygen content in the combustion gases from the engine and consequently the richness of the fuel mixture, and which serves in a manner known per se for closed-loop richness regulation at most engine operating points, by adjusting the fuel injection duration.

[0045] The motorization 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 on the end of the engine crankshaft.

[0046] An electronic control system, or ECU (not shown), allows the motorization device 1 to be controlled in such a way that the motor provides the torque necessary to drive the vehicle.

[0047] For example, for a torque setpoint C reflecting the driver's intent, corresponding to a given degree of depressment of the vehicle's accelerator pedal, and for a given engine speed N, the computer determines a mass air flow rate Qair to be admitted into the engine based on an advance efficiency and a fuel-air ratio efficiency. It further determines an ignition advance value AA, preferably the optimal advance value, which maximizes the advance efficiency, and a fuel flow rate Qcarb to obtain a given fuel-air ratio, which is generally set in closed loop to a setpoint value.

[0048] This setpoint value is generally a richness setpoint equal to 1, at most engine operating points, which allows the three-way catalyst 12 to operate optimally.

[0049] 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 of the exhaust circuit.

[0050] To obtain the desired mass air flow rate Qair, the computer adjusts the degree of opening of the throttle body 6, and to obtain the desired fuel flow rate Qcarb, the computer adjusts at least a duration of injector opening 3 and at minus one moment of injector opening relative to the combustion cycle (generally measured in degrees of crankshaft angle relative to top dead center). The computer also adjusts the ignition timing, which is the moment a spark jumps across the terminals of a spark plug (not shown) present in each cylinder of the engine.

[0051] In order to control the engine in the thermal protection zone, the computer further includes said computer means for determining, as a function of the alcohol content in the mixture, a setpoint value for the mixture strictly greater than 1 to be applied for each operating point of the engine speed-load in a thermal protection zone so as to limit the temperature at the exhaust of the engine to a predetermined maximum value.

[0052] In one embodiment of these means, the computer includes a first richness setpoint map to be applied according to the operating point engine speed-load and a second richness setpoint map to be applied according to the operating point engine speed-load, and means for determining the richness setpoint to be applied according to the operating point engine speed-load and according to the determined alcohol content in the fuel, by interpolation between the setpoint values ​​of the first and second maps.

[0053] In another embodiment of the means, the computer can carry as many richness maps as a function of the engine speed-load point as there are possible alcohol levels (for example, a map per unit of rate between 25% and 100% for Brazil).

[0054] By thermal protection zone, we mean a set of engine operating speed-load points close to full load, particularly at high speed, for which the engine exhaust temperature is maintained at a limit temperature for exhaust reliability through over-enrichment of the mixture.

[0055] Figure 2 illustrates the different stages of the process according to the invention, in a preferred embodiment thereof.

[0056] The method includes a step of determining 100 an alcohol content value in the fuel, with an uncertainty of plus or minus x percent, and adjusting the engine accordingly.

[0057] The rate RATE thus determined is a centered value. In other words, the actual rate of alcohol in the fuel, as it could be measured precisely by metrological means by mixing known volumes of pure gasoline and pure alcohol, is between, on the one hand, this centered rate RATE minus x percent, and on the other hand, this centered rate RATE plus x percent.

[0058] In one embodiment, the rate TAUX 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 at richness 1.

[0059] It is recalled that the stoichiometric coefficient Ks of pure alcohol is approximately 8.4 and that of pure gasoline is approximately 14.7, the mixtures taking intermediate values ​​each corresponding to a precise rate.

[0060] The uncertainty x of the determination by this method is estimated to be + / - 10%

[0061] In another embodiment of the first step 100, the rate RATE is determined from the measurement of an alcohol concentration sensor, the uncertainty x is then estimated at + / - 5%.

[0062] To adjust the engine, the proportional oxygen probe 14 is used. On the operating points of the engine which are not part of the thermal protection zone, the air flow Qair and the ignition advance AA are chosen to produce the required engine torque C according to the characteristics of the fuel having the alcohol content TAUX considered (lower heating value LHV; stoichiometric ratio Ks; richness efficiency; advance efficiency...), according to the prior art.

[0063] According to the invention, the process then comprises a second step 200 during which the following substeps are carried out successively:

[0064] In a first sub-step, it is tested whether the engine operating speed-load point is in the thermal protection zone; if not, the process resumes at step 100 and the engine is set to richness 1.

[0065] Otherwise, the process continues with a second sub-step in which a corrected value TAUXcorr of alcohol content is determined, by subtracting from the alcohol content value TAUX, determined during the first step 100, its uncertainty x % of determination.

[0066] Then, in a third sub-step, the richness setpoint is set to the thermal protection richness value corresponding to this corrected rate TAUXcorr, using the first map, the second map, and the interpolation means stored beforehand in the engine computer, and the engine is controlled using this setpoint.

[0067] This third substep 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 margin of uncertainty of x% eliminates this risk.

[0068] Furthermore, the corrected value is saturated at the value of the lowest alcohol content present in the fuels available at the pump. For example, in Brazil where the lowest alcohol content fuel is E25, if the value of the determined rate is Step 100 is thirty percent with an uncertainty of + / - 10%; the corrected value TAUXcorr will be twenty-five percent, not twenty percent. Alternatively, one can also saturate at a lower value, for example 22%, corresponding to a homologation fuel used particularly for testing.

[0069] The engine is tuned based on this corrected ratio TAUXcorr, but advantageously with the exception of the ignition advance AA. More precisely, for the production of engine torque C, the air flow rate Qair is chosen according to the characteristics (LHV, Ks, ...) of the fuel with the corrected alcohol content, but it may be preferable to apply the ignition advance AA corresponding to the fuel with the uncorrected ratio, i.e., the first TAUX ratio of step 100, in order to increase engine performance or limit fuel consumption. Since this ignition advance AA is higher for a fuel with a higher alcohol content, it may advantageously be possible 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 described in further detail..

[0070] Advantageously, in an improved embodiment of the process according to the invention, the process further includes a third step 300 of canceling the rate correction made in step 200 for the most alcoholic fuel if the rate of the latter is perfectly known and the other fuel(s) have alcohol rates much lower than that of the most alcoholic fuel, in particular a difference greater than the uncertainty x of determination of the rate of step 100. This is in particular the case in Brazil where the most alcoholic fuel E100 is pure alcohol and where the two other fuels have alcohol rates which differ by more than seventy percent of the maximum rate.

[0071] This is not the case in the European Union, where the most alcoholic fuel E85 does not define a fuel with a fixed alcohol content, but only a maximum alcohol content, which is also variable according to the seasons.

[0072] This third step 300 is based on the fact that the correction of the rate in step 200 can lead to overconsumption of fuel, due to the choice of the thermal protection richness value, in the case where the ethanol rate is not actually overestimated.

[0073] This third step 300 aims to acquire a sufficient presumption that the tank contains only, or almost only, the most alcoholic fuel and to consider that the determined rate is no longer tainted by an uncertainty x.

[0074] This step is made possible when the motorist systematically fills the vehicle's tank with a single type of fuel for a long period of time, particularly the most alcoholic fuel, which is the case in Brazil during periods when E100 fuel is given tax advantages.

[0075] Step 300 consists of verifying whether after a succession of several consecutive refills, that is to say after a succession of total or partial refills of the tank whose total volume exceeds a predetermined multiple of the total capacity of the tank, the first value of alcohol level RATE from step 100 remains after each refill close to the maximum level, that is to say in a range between on the one hand, the maximum alcohol level RATE less the uncertainty x of step 100 and, on the other hand, the maximum level.

[0076] If, after each significant refueling, the rate remains within these limits, it can be concluded that the refueling was carried out with the fuel with the highest alcohol content, because refueling with an identical volume of fuel with a significantly lower alcohol content would have caused the RATE to fall below the minimum value of the range. However, no other type of fuel is available.

[0077] If this is the case, it can be deduced that the tank was filled each time with E100 fuel, and that the fuel mixture in the tank was 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 gasoline being present at most only in trace amounts.

[0078] In this case, the rate correction performed in step 200 will be canceled, and the thermal protection mixture value will be determined based on the rate TAUX from the first step 100, and not on the corrected rate TAUXcorr from the second step. This avoids the risk of excessive fuel consumption while eliminating the thermal risk.

[0079] This third step can be generalized to any situation in which only very different types of fuel are available: a first fuel, having a relatively low alcohol content, and a second fuel, having a relatively high and perfectly known alcohol content, the difference in alcohol content between the two being at least greater than the uncertainty x of the determination of the fuel content.

Claims

Demands

1. A method for controlling an internal combustion engine (2) operating with either gasoline, or alcohol, or a variable ratio 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 alcohol ratio in the mixture, a setpoint value for the mixture strictly greater than 1 to be applied for each operating point speed-load of the engine within a thermal protection zone so as to limit the temperature at the exhaust of the engine to a predetermined maximum value, said method comprising a first step of predetermining an alcohol ratio (RATE) in the mixture with an estimated uncertainty of plus or minus x percent, CHARACTERIZED IN THAT it further comprises a second step (200) in which: -it is tested whether the operating point speed-load of the engine is within said thermal protection zone;-if this is the case, a corrected value (TAUXcorr) of the alcohol content in the mixture is determined, calculated by subtracting the value of the uncertainty x from the value of the rate (TAUX) of the predetermination step (100); and, -the engine is adjusted to the thermal protection richness value corresponding to the corrected value (TAUXcorr) of the alcohol content.;

2. A method according to claim 1, characterized in that the predetermined alcohol content (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 mixture is set in closed loop to the value 1, and in that the corrected alcohol content (RATEcorr) is obtained by decreasing said content (RATE) by ten percent.

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

4. A method according to any one of the preceding claims, characterized in that the minimum value of the corrected value (TAUXcorr) of the rate alcohol is saturated at the lowest alcohol content value present in fuels available at the pump.

5. A method according to any one of the preceding claims, characterized in that during step 200, the engine is set with operating parameters (Qair, Qcarb) determined according to the corrected value (TAUXcorr) of the alcohol content, with the exception of the ignition advance (AA) which is determined according to the value of the alcohol content as predetermined (TAUX).

6. A method according to any one of the preceding claims, characterized in that it further comprises a third step (300) of canceling the rate correction of the second step (200) if, the alcohol level of the most alcoholic fuel available at the pump being a fixed and known maximum level, the first fuel level remains close to said maximum level after a succession of tank fillings whose total volume exceeds a predetermined multiple of the tank volume.

7. A method according to claim 6, characterized in that it is determined that the fuel rate remains close to the maximum rate when it is greater than the maximum rate less the uncertainty x of the method for determining the rate (RATE) of alcohol in the first step.

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