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 an engine running on gasoline and alcohol in a thermal protection zone close to full load, for which the richness is set to a setpoint value strictly greater than 1, determined for each speed-load operating point of this zone as a function of the alcohol content (TAUX) to limit the exhaust temperature to a predetermined maximum value. From the determination of a first alcohol content value (TAUX), obtained with an uncertainty of plus or minus x percent, a corrected value of the content (TAUXcorr) is determined by subtracting x percent from the first value (TAUX) and the richness setpoint is set on the thermal protection zone to the value corresponding to the corrected content, for each point in the zone. Abstract 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. It relates more particularly 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 types 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 run on different types of gasoline and alcohol mixtures, depending on the country.

[0004] For example, in the Brazilian market, one can often fill the tank with so-called "E25" and "E27" fuels, which are fairly similar mixtures 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 mixture of gasoline and alcohol comprising twenty-five percent alcohol (by volume). E27 fuel is a mixture of gasoline and alcohol comprising twenty-seven percent alcohol, and E100 fuel is pure alcohol.

[0005] For example, in the countries of the European Union, three types of fuel available at the pump have been regulated since October 12, 2018. We can find: so-called "E5" fuel, which is a mixture of unleaded gasoline (SP95 or SP98) comprising a maximum of five percent alcohol by volume; so-called "E10" fuel, which is a mixture of unleaded gasoline (SP95) comprising a maximum of ten percent alcohol by volume; and, so-called "E85" fuel, which is a mixture comprising a maximum of eighty-five percent alcohol by volume, in particular 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 for controlling such an engine consists of managing the engine using different 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, so as to allow the computer to know the state of the engine in real time. From the physical quantities received by the computer, it is capable of controlling or adjusting all the actuator devices of the engine 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 computer must take into account is the proportion of each of the two fuels contained in the vehicle's fuel tank. Indeed, the engine control parameters, such as the air mass flow rate, the ignition advance, and the fuel injection pattern (i.e.: the duration of fuel injection and its phasing in the combustion cycle) must be adjusted according to this information. It is important to know as precisely as possible and as quickly as possible which fuel or which mixture of fuels is present in the tank in order to adapt these different engine control parameters, to avoid its deterioration, minimize pollutant emissions, and control its performance.

[0010] More precisely, we observe between gasoline and alcohol, differences: in lower calorific value PCI; 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.

[0011] Thus, a setting adapted to gasoline is not suitable in the case of operation with alcohol, because for an identical air mass flow, and operating at richness 1, the torque would be a little too high. When the engine operates on alcohol, it is advisable to reduce the air flow by a few percent if the same torque is to be maintained.

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

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

[0014] Publication FR-A1-2892769 is known from the state of the art, which discloses a method for recognizing exotic fuel levels 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 richness sensor which is of the all-or-nothing 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, as a function of 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 obtain a substantially stoichiometric fuel mixture.

[0015] However, this method is very imprecise, in particular due to the stepwise increase in the injection duration. It is sufficient to be used for starting the engine but cannot be used to control it over all of its speed-load operating points.

[0016] Also known from the state of the art is a method for determining the alcohol content in a fuel mixing gasoline and alcohol, which is based on the proportions of the respective quantities of air admitted into the engine and of fuel injected, when the engine operates in a closed loop at richness 1, which is the case for at least 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 intake manifold of the engine, and the fuel flow rate can be calculated from in particular 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 gasoline and alcohol, therefore of the alcohol content.

[0018] From a statistical study taking into account the manufacturing dispersion of a population of engines and proportional oxygen sensors used to adjust the richness in a closed loop, the applicant estimated the uncertainty in determining the alcohol level 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 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 level TAUX.

[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 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 full load curve of the engine, 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 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 is carried out, is called a "thermal protection zone" so as to limit the temperature at the engine exhaust to a predetermined maximum value.

[0022] The value of the richness to be applied to contain 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 operates on pure gasoline.

[0023] We can speak of "thermal protection richness" when said richness value, strictly greater than 1, serves as a richness setpoint in order to limit the temperature at the engine exhaust. A first thermal protection richness map is established for each engine speed-load operating point in the thermal protection zone with pure gasoline.

[0024] However, if the engine is operated with a mixture of gasoline and alcohol, the thermal protection richness can be set at each point to a lower value than in the case of pure gasoline, because the combustion of alcohol is cooler than that of gasoline. And, in the case where the engine is operated with pure alcohol, it is even possible that the richness value can be maintained at 1 at a certain number of operating points of the thermal protection zone corresponding to operation with 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 allowed for reliability is a decreasing function of the alcohol level TAUX in the fuel. Each value can be determined based on the exact alcohol level by preliminary tests on the engine bench using fuels mixed in perfectly known proportions (known volume of pure gasoline added to a known volume of pure alcohol), in the same way as the first and second maps are established using pure gasoline and pure alcohol respectively. In a simplified variant, each thermal protection richness value 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 considered in the thermal protection zone.

[0027] It is then possible to either embed in an engine computer a thermal protection richness map to be applied as a function of the alcohol level TAUX determined in the vehicle's 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 of calculation by interpolation of the thermal protection richness as a function of a determined alcohol level TAUX in the fuel.

[0028] However, it is understood from the above that the uncertainty of plus or minus x percent relating to the value of the alcohol content TAUX determined by a method of the state of the art as mentioned above has a disadvantage: in the case where the value of the alcohol content TAUX is overestimated compared to the real value, due to the imprecision of the method, there is in reality less alcohol in the mixture, and the combustion is hotter than imagined, so that the value of thermal protection richness chosen, for example determined by interpolation as indicated above, is insufficient. There is then a risk of an increase in the temperature of the components of the engine exhaust beyond their reliability threshold, and damage to the engine. Presentation of the invention

[0029] The invention aims to remedy the lack of precision of known methods for determining the alcohol content by limiting their negative impact on the reliability of the components of the engine exhaust circuit. To this end, it proposes a method for controlling an internal combustion engine operating either with gasoline, or with alcohol, or with a variable rate of the two in a mixture, comprising a computer capable of controlling said engine, said computer comprising means for determining, as a function of the alcohol content in the mixture, a richness setpoint 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 temperature at the engine exhaust to a predetermined maximum value, said method 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 characteristic of the method according to the invention is that it further comprises a second step in which:

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

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

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

[0034] The invention will be better understood by reading a non-limiting embodiment thereof, with the support of the appended figures among which:

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

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

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

[0038] This device comprises an internal combustion engine 2, which is here for example in 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, or alcohol (ethanol), or with a mixture of gasoline and alcohol. In a non-limiting manner, it may be naturally aspirated or supercharged. It may also have other features without detracting from the generality of the invention, for example one or more circuits for partial recirculation of the exhaust gases at the engine intake.

[0039] The engine 2 may be of the direct injection type, as in the example illustrated by [Fig.l]. Each cylinder of the engine is individually supplied with fuel by an injector 3 via a common feed 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 becomes empty. More precisely, it can be filled with at least a first type of fuel composed of a mixture of gasoline and alcohol having a first proportion of zero or relatively low alcohol, and a second type of fuel composed of a mixture of gasoline and alcohol having a second alcohol proportion relatively high or equal to one hundred percent. By the expressions "relatively low" and "relatively high" is meant that the second alcohol proportion is greater than the first alcohol proportion.

[0040] For example, in the Brazilian market, the tank can be filled with so-called "E25", "E27" or "E100" fuel. E25 and E27 fuels are mixtures of gasoline and alcohol comprising respectively twenty-five and twenty-seven percent alcohol by volume, 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 over long periods depending on the purchase price, with the authorities playing on fuel taxation to favor either the two fuels E25 or E27 with the lowest alcohol content, or E100 fuel, depending on the level of alcohol production in the country.

[0041] For example, in the countries of the European Union, three types of fuel available at the pump have been regulated since October 12, 2018. We can find: so-called "E5" fuel, which is a mixture of unleaded SP95 or SP98 gasoline comprising a maximum of five percent alcohol by volume; so-called "E10" fuel, which is a mixture of unleaded SP95 gasoline comprising a maximum of five percent alcohol by volume; and, so-called "E85" fuel, which is a mixture comprising 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, making it possible to supply the engine cylinders with fresh air taken from the outside atmosphere, and with a gas exhaust circuit 6, making it possible to evacuate the combustion gases from the cylinders to the outside atmosphere.

[0043] 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 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 makes it possible to continuously measure the manifold pressure Pcoll, that is to say the pressure which prevails within it, downstream of the throttle body 7 (in the direction of air circulation). The intake manifold 8 is also equipped with a temperature sensor 10, capable of continuously measuring the temperature Tcoll of the intake air, that is to say the temperature of the air which enters the engine. In a manner known per se, the air mass flow rate Qair can be determined at any time from said pressure Pcoll and temperature Tcoll values ​​and a filling model.

[0044] The exhaust circuit 6 comprises, from upstream to downstream in the direction of circulation of the burnt gases, an exhaust manifold 11, a catalyst for depolluting the combustion gases of the engine 12, and an exhaust pipe 13. In this device for powering a spark-ignition engine, the catalyst 12 is here a catalyst of the “three-way” type, capable of carrying out the post-treatment of the polluting substances 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 type oxygen sensor 14 whose output signal, generally a voltage, makes it possible to determine a value of residual oxygen richness in the combustion gases from the engine and consequently the richness of the fuel mixture, and which is used in a manner known per se for closed-loop richness regulation at most operating points of the engine, 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 speed N of the engine, generally in the form of a sensor for the number of rotations of a target mounted at the end of the engine crankshaft.

[0046] An electronic control system, or ECU (not shown), makes it possible to control the motorization device 1 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 wishes, corresponding to a given degree of depression of the vehicle's accelerator pedal by the driver of the vehicle, and for a given speed N, the computer determines an air mass flow rate Qair to be admitted into the engine as a function of an advance efficiency and a richness efficiency. It further determines an ignition advance value AA, preferably the optimal advance value, which makes it possible to maximize the advance efficiency, and a fuel flow rate Qcarb to obtain a given mixture richness, which is generally adjusted in a 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 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 opening duration of the injectors 3 and at least one opening duration of the injectors 3. minus one instant of opening of said injectors relative to the combustion cycle (generally counted in degrees of crankshaft angle relative to top dead center). The computer also regulates the ignition advance, which is expressed by the instant of spark discharge at 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 comprises said computer means for determining, as a function of the alcohol level in the mixture, a richness setpoint 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.

[0052] In one embodiment of these means, the computer comprises a first richness setpoint map to be applied as a function of the speed-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 alcohol RATE determined in the fuel, by interpolation between the setpoint values ​​of the first and second maps.

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

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

[0055] [Fig.2] illustrates the different steps of the method according to the invention, in a preferred embodiment thereof.

[0056] The method comprises a step 100 of determining an alcohol level value RATE in the fuel, with an uncertainty of plus or minus x percent, and of adjusting the engine corresponding to this level.

[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 RATE is deduced from the stoichiometric ratio Ks, which is calculated from the air and fuel flow rates entering the engine when the latter is set in a 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 then being estimated at + / - 5%.

[0062] To regulate the engine, the proportional oxygen sensor 14 is used. At 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 as a function of the characteristics of the fuel having the alcohol content RATE considered (lower calorific value PCI; stoichiometric ratio Ks; richness efficiency; advance efficiency, etc.), according to the state of the art.

[0063] According to the invention, the method then comprises a second step 200 during which, successively, the following sub-steps are carried out:

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

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

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

[0067] This third sub-step is justified by the fact that the determination of the rate includes 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 in the event of overestimation of the rate. Subtracting the margin of uncertainty of x% makes it possible to eliminate this risk.

[0068] In addition, 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 RATE determined at step 100 is thirty percent with an uncertainty of + / - 10%, the corrected value TAUXcorr 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 TAUXcorr, but advantageously with the exception of the ignition advance AA. More precisely, for the production of an engine torque C, the air flow Qair is chosen according to the characteristics (PCI, Ks, ...) of the fuel having the corrected alcohol rate, but it may be preferable to apply the ignition advance AA corresponding to the fuel having the uncorrected rate, that is to say the first rate TAUX of step 100, so as to increase the performance of the engine or to limit fuel consumption. This ignition advance AA being higher for a more alcoholic fuel, it is advantageous to use an anti-knock advance correction method, that is to say, a method in which the presence of knock is detected using a knock sensor, and an advance is removed to make it disappear, if necessary. Such procedures are known per se and will not be detailed further..

[0070] Advantageously, in an improved embodiment of the method according to the invention, the method also comprises a third step 300 of canceling the rate correction carried out in step 200 for the most alcoholic fuel if the rate thereof 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 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 most alcoholic fuel E85 does not define a fuel with a fixed alcohol level, but only a maximum alcohol level, which is also variable according to the seasons.

[0072] This third step 300 is based on the fact that the correction of the rate of 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 in fact overestimated.

[0073] This third step 300 aims to acquire a sufficient presumption that the tank does indeed contain only, or almost only, the most alcoholic fuel and to consider that the determined RATE rate is no longer affected 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, especially the most alcoholic fuel, which is the case in Brazil during periods when E100 fuel is tax-favored.

[0075] Step 300 consists of verifying whether, after a succession of several consecutive fill-ups, i.e. after a succession of total or partial fill-ups of the tank whose total volume exceeds a predetermined multiple of the total capacity of the tank, the first alcohol level value RATE of step 100 remains, after each fill-up, close to the maximum level, i.e. in a range between, on the one hand, the maximum alcohol level RATE reduced by the uncertainty x of step 100 and, on the other hand, the maximum level.

[0076] If after each significant fill-up, the rate remains between these limits, it can be concluded that the fill-up was carried out with the most alcoholic fuel, because filling with an identical volume of significantly less alcoholic fuel would have caused the RATE value 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 present 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), the gasoline being present at most only in trace amounts.

[0078] In this case, the rate correction carried out in step 200 will be cancelled and the value of the thermal protection richness will be determined as a function of the rate TAUX of the first step 100 and not of the corrected rate TAUXcorr of the second step. This makes it possible to avoid a risk of overconsumption of fuel 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 level, and a second fuel, having a relatively high and perfectly known alcohol level, the difference in alcohol level between the two being at least greater than the uncertainty x of the determination of the level RATE of the fuel.

Claims

Claims

1. Method for controlling an internal combustion engine (2) operating either with gasoline, 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 alcohol rate in the mixture, a richness setpoint 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 (100) an alcohol rate (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 engine speed-load operating point is in said thermal protection zone;-if this is the case, a corrected value (TAUXcorr) of the alcohol level in the mixture is determined, calculated by subtracting the value of the uncertainty x from the value of the level (TAUX) of the predetermination step (100); and, -the engine is set to the thermal protection richness value corresponding to the corrected value (TAUXcorr) of the alcohol level.;

2. Method according to claim 1, characterized in that the predetermined alcohol level (TAUX) 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 adjusted in a closed loop to the value 1, and in that the corrected alcohol level (TAUXcorr) is obtained by reducing said level (TAUX) by ten percent.

3. 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. Method according to 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 the fuels available at the pump.

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

6. Method according to 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 rate of the most alcoholic fuel available at the pump being a fixed and known maximum rate, the first fuel rate remains close to said maximum rate after a succession of fillings of the tank whose total volume exceeds a predetermined multiple of the volume of the tank.

7. 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 reduced by the uncertainty x of the method for determining the alcohol rate (RATE) of the first step.

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