Method and system for controlling a vehicle engine speed

Abstract

Disclosed is a method for controlling a speed of a vehicle combustion engine, the engine including at least one combustion chamber, into which a mixture of air and fuel is injected, and an air box, configured to inject the air into the combustion chamber and having an air flow rate controlled by a regulating butterfly valve, the regulating butterfly valve having a variable angular position, controlled by a predetermined position of an actuator. The method includes the steps of evaluating a so-called “load” resistant torque resulting from a plurality of external loads applied to the engine, determining, from the calculated load resistant torque, a position of the actuator, so as to determine an angular position of the regulating butterfly valve, and controlling the position of the actuator, so as to control the engine speed.

Claims

1. A method for controlling a speed of a vehicle combustion engine configured to operate at a constant speed, said vehicle combustion engine including at least one combustion chamber into which a mixture of air and fuel is injected, and an air box configured to inject the air into said at least one combustion chamber, the air box having an air flow rate controlled by a regulating butterfly valve, said regulating butterfly valve having a variable angular position controlled by a predetermined position of an actuator, said method comprising: evaluating a load-resistant torque resulting from at least one external load applied to said engine to compensate for said load-resistant torque; determining, from said evaluated load-resistant torque, a position of said actuator to determine an angular position of the regulating butterfly valve; controlling the actuator in the position determined from said evaluated load-resistant torque to control said constant engine speed; and predetermining an evolution curve of a theoretical drive torque due to combustion in the combustion chamber during the engine cycle, representing an evolution of a complete engine cycle comprising at least one combustion phase, said evolution curve comprising: a first portion including said at least one combustion phase, representative of a variation in the torque during the at least one combustion phase, to calculate a combustion drive torque, and a second portion that does not include said at least one combustion phase, representative of the load-resistant torque, to evaluate the load-resistant.

2. The method as claimed in claim 1, wherein said vehicle combustion engine includes a crankshaft having an angular position starting from a reference position, said at least one combustion chamber having the combustion phase, the calculating the combustion drive torque comprising: determining a first estimator from said evolution curve of said theoretical drive torque, said first estimator corresponding to a succession of segments connected to one another between an initial point and a final point and comprising a plurality of notable points, each of the segments being representative of a variation in values of the combustion drive torque during the at least one combustion phase, said plurality of notable points comprising the initial point, a plurality of inflection points connecting the segments to one another, and the final point, correlating the initial point, each of the inflection points, and the final point, with the angular position of the crankshaft, measuring a plurality of instants, each of the instants corresponding to the angular position of the crankshaft, and calculating the combustion drive torque from said plurality of measured instants.

3. The method as claimed in claim 2, wherein the engine has a complete engine cycle comprising the at least one combustion phase, said evolution curve of the theoretical drive torque representing the evolution of the complete engine cycle, and the determining the first estimator is carried out for the first portion of said evolution curve of the theoretical drive torque comprising said at least one combustion phase.

4. The method as claimed in claim 3, wherein the first portion of the theoretical drive torque comprises the initial point, four of the inflection points, and the final point, the first estimator depends on six of the instants and calculates the combustion drive torque from the following equation:
TQ_Ind=k*(T6−T5−T4+T3+T2−T1)*N.sup.3 wherein: k is a factor dependent on the inertia of the combustion engine, N corresponds to an engine speed measured by the angular position of the crankshaft during the engine cycle, T1 corresponds to the instant of the initial point of the first estimator, T2 to T5 respectively corresponds to the instants of the four inflection points from the initial point to the final point of the first estimator, and T6 corresponds to the instant of the final point of the first estimator.

5. The method as claimed in claim 3, wherein the friction-resistant torque corresponds to a predetermined torque value.

6. The method as claimed in claim 4, wherein the friction-resistant torque corresponds to a predetermined torque value.

7. The method as claimed in claim 2, wherein the engine has a complete engine cycle comprising the at least one combustion phase, said evolution curve of the theoretical drive torque represents the evolution of the complete engine cycle, and the calculating the load-resistant torque is carried out for the second portion of said evolution curve of the theoretical drive torque not comprising said at least one combustion phase and comprises estimating, from a second estimator, the load-resistant torque based on the notable points of said second portion of the evolution curve of the theoretical drive torque, and determining the position of the actuator as a function of the estimated load-resistant torque and the engine rotation speed.

8. The method as claimed in claim 7, wherein the second curve portion of the theoretical drive comprises the initial point, two of the inflection points, and the final point, said second estimator depends on four of the instants and calculates the load-resistant torque from the following equation:
TQ_Load=k*(T4−T3−T2+T1)*N.sup.3 in which: wherein: k is a factor dependent on the inertia of the combustion engine, N corresponds to an engine speed measured by the angular position of the crankshaft during the engine cycle, T1 corresponds to the instant of the initial point of the second estimator, T2 and T3 respectively correspond to the instants of the two inflection points from the initial point to the final point of the second estimator, and T4 corresponds to the instant of the final point of said second estimator.

9. The method as claimed in claim 2, wherein the friction-resistant torque corresponds to a predetermined torque value.

10. A method for controlling a speed of a vehicle combustion engine configured to operate at a constant speed, said vehicle combustion engine including at least one combustion chamber into which a mixture of air and fuel is injected, and an air box configured to inject the air into said at least one combustion chamber, the air box having an air flow rate controlled by a regulating butterfly valve, said regulating butterfly valve having a variable angular position controlled by a predetermined position of an actuator, said method comprising: evaluating a load-resistant torque resulting from at least one external load applied to said engine to compensate for said load-resistant torque; determining, from said evaluated load-resistant torque, a position of said actuator to determine an angular position of the regulating butterfly valve; controlling the actuator in the position determined from said evaluated load-resistant torque to control said constant engine speed; predetermining an evolution curve of a theoretical drive torque due to combustion in the combustion chamber during the engine cycle, representing an evolution of a complete engine cycle comprising at least one combustion phase, said evolution curve comprising: a first portion including said at least one combustion phase, representative of a variation in the torque during the at least one combustion phase, to calculate a combustion drive torque, and a second portion that does not include said at least one combustion phase, representative of the load-resistant torque, to evaluate the load-resistant torque; determining a first estimator from said evolution curve of the theoretical drive torque, corresponding to a succession of segments connected by a plurality of inflection points, each of the segments being representative of a variation in values of the theoretical drive torque during the at least one combustion phase in the at least one combustion chamber, and comprising an initial point and a final point, to calculate the combustion drive torque; and determining a second estimator from said evolution curve of the theoretical drive torque, corresponding to at least some of succession of segments connected by two of the inflection points, each of the segments corresponding to the second estimator being situated in the zero or substantially zero torque region of the evolution curve of the theoretical drive torque and comprising an initial point and a final point, to evaluate the load-resistant torque.

11. The method as claimed in claim 10, the evaluating said load-resistant torque comprises: calculating an acceleration drive torque resulting from an acceleration of the vehicle combustion engine, determining a friction-resistant torque resulting from a plurality of frictions in the vehicle combustion engine, calculating said combustion drive torque resulting from the combustion of said mixture of air and fuel in said at least one combustion chamber, and calculating the load-resistant torque from the combustion drive torque, the acceleration drive torque, and the friction-resistant torque.

12. The method as claimed in claim 10, wherein the friction-resistant torque corresponds to a predetermined torque value.

13. The method as claimed in claim 10, wherein said vehicle combustion engine includes a crankshaft having an angular position starting from a reference position, said at least one combustion chamber having the combustion phase, the calculating the combustion drive torque comprising: determining a first estimator from said evolution curve of said theoretical drive torque, said first estimator corresponding to a succession of segments connected to one another between an initial point and a final point and comprising a plurality of notable points, each of the segments being representative of a variation in values of the torque during the at least one combustion phase, said plurality of notable points comprising the initial point, a plurality of inflection points connecting the segments to one another, and the final point, correlating the initial point, each of the inflection points, and the final point, with the angular position of the crankshaft, measuring a plurality of instants, each of the instants corresponding to the angular position of the crankshaft, and calculating the combustion drive torque from said plurality of measured instants.

14. A method for controlling a speed of a vehicle combustion engine configured to operate at a constant speed, said vehicle combustion engine including at least one combustion chamber into which a mixture of air and fuel is injected, and an air box configured to inject the air into said at least one combustion chamber, the air box having an air flow rate controlled by a regulating butterfly valve, said regulating butterfly valve having a variable angular position controlled by a predetermined position of an actuator, said method comprising: evaluating a load-resistant torque resulting from at least one external load applied to said engine to compensate for said load-resistant torque, the evaluating said load-resistant torque comprising: calculating an acceleration drive torque resulting from an acceleration of the vehicle combustion engine, determining a friction-resistant torque resulting from a plurality of frictions in the vehicle combustion engine, calculating said combustion drive torque resulting from the combustion of said mixture of air and fuel in said at least one combustion chamber, and calculating the load-resistant torque from the combustion drive torque, the acceleration drive torque, and the friction-resistant torque; determining, from said evaluated load-resistant torque, a position of said actuator to determine an angular position of the regulating butterfly valve; controlling the actuator in the position determined from said evaluated load-resistant torque to control said constant engine speed; and predetermining an evolution curve of a theoretical drive torque due to combustion in the combustion chamber during the engine cycle, representing an evolution of a complete engine cycle comprising at least one combustion phase, said evolution curve comprising: a first portion including said at least one combustion phase, representative of a variation in the torque during the at least one combustion phase, to calculate a combustion drive torque, and ,a second portion that does not include said at least one combustion phase, representative of the load-resistant torque, to evaluate the load-resistant torque.

15. The method as claimed in claim 14, wherein the friction-resistant torque corresponds to a predetermined torque value.

16. The method as claimed in claim 14, wherein said vehicle combustion engine includes a crankshaft having an angular position starting from a reference position, said at least one combustion chamber having the combustion phase, the calculating the combustion drive torque comprising: determining a first estimator from said evolution curve of said theoretical drive torque, said first estimator corresponding to a succession of segments, connected to one another between an initial point and a final point, and comprising a plurality of notable points, each of the segments being representative of a variation in values of the torque during the at least one combustion phase, said plurality of notable points comprising the initial point, a plurality of inflection points connecting the segments to one another, and the final point, correlating the initial point, each of the inflection points, and the final point, with the angular position of the crankshaft, measuring a plurality of instants, each of the instants corresponding to the angular position of the crankshaft, and calculating the combustion drive torque from said plurality of measured instants.

17. A vehicle computer for a vehicle including a combustion engine configured to operate at a constant speed, said combustion engine including at least one combustion chamber into which a mixture of air and fuel is injected, and an air box configured to inject the air into said combustion chamber, the air box having an air flow rate controlled by a regulating butterfly valve, said regulating butterfly valve having a variable angular position that is controlled by a predetermined position of an actuator, wherein said computer is configured to: evaluate a load-resistant torque resulting from a plurality of external loads applied to said engine; determine, from said evaluated load-resistant torque, a position of said actuator to determine an angular position of the regulating butterfly valve; control the actuator in the position determined from said evaluated load-resistant torque to regulate the constant engine speed; and predetermine a curve of a theoretical drive torque due to combustion in the combustion chamber during the engine cycle, representing an evolution of a complete engine cycle comprising at least one combustion phase, said evolution curve comprising: a first portion including said at least one combustion phase, representative of a variation in the torque during the at least one combustion phase, to calculate a combustion drive torque, and a second portion that does not include said at least one combustion phase, representative of the load-resistant torque, to evaluate the load-resistant torque.

18. A vehicle comprising: the vehicle computer as claimed in claim 17; and the combustion engine, having a constant engine speed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will become more clearly apparent from reading the following description. This description is purely illustrative and should be read with reference to the attached drawings, in which:

(2) FIG. 1 schematically illustrates a combustion engine and a regulating butterfly valve of an air box of such a combustion engine.

(3) FIG. 2 is a schematic view of the exchanges of messages and signals between the computer and the engine of the vehicle.

(4) FIG. 3 depicts the evolution of the so-called “theoretical” drive torque in a combustion chamber.

(5) FIG. 4 illustrates a first estimator of the evolution of the drive torque of FIG. 3.

(6) FIG. 5 illustrates a second estimator of the evolution of the load torque of FIG. 3.

(7) FIG. 6 schematically illustrates one embodiment of the method according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The system and the method according to the invention are presented below for the purpose of an implementation in a generator or a lawnmower. However, any implementation in a different context, in particular for any vehicle comprising an engine of which the speed must be constant, is also covered by the invention

(9) As described above, with reference to FIG. 1, a vehicle, of the lawnmower type for example, comprises a combustion engine 1 comprising at least one hollow cylinder 11, in this example a single cylinder 11 delimiting a combustion chamber 11A in which there slides a piston 12 of which the movement is driven by compression and expansion of the gases obtained from the combustion of a mixture of air and fuel introduced into the combustion chamber 11A. The piston 12 is connected to a crankshaft 13, which, rotated by the upstroke and downstroke of the piston 12, allows the engine 1 of the vehicle to be driven.

(10) The speed of rotation of the crankshaft 13 defines the engine speed of the vehicle, that is to say the number of rotations per minute performed by the crankshaft 13 when the engine 1 is operating. In the case of a lawnmower or of a generator for example, it is appropriate for such an engine speed to be constant. Thus, the torque of the engine 1 must be adapted to ensure that the speed remains unchanged whatever the external conditions. Specifically, the torque of the engine 1 corresponds to the force that the engine 1 must provide for example to ensure that the crankshaft 13 turns at the desired speed of rotation, that is to say in this case at the predefined constant speed.

(11) The measurement of the speed of rotation of the crankshaft 13 is determined from the angular position of such a crankshaft 13. In order to know such an angular position, still with reference to FIG. 1, the crankshaft 13 comprises a toothed wheel 130 comprising a predetermined number of regularly spaced teeth along with a space free of teeth corresponding to a reference position of the crankshaft 13. Since such a toothed wheel 130 is known per se, it will not be described in more detail here.

(12) A position sensor 16 is mounted facing the toothed wheel 130 so as to allow both the detection of the reference position and the counting of the number of teeth running in front of the position sensor 16 from such a reference position. More specifically, the position sensor 16 delivers a signal representative of the passage of the teeth which allows the computer 30 to determine the angular position from 0° to 360° of the crankshaft 13.

(13) It will be recalled that, in such an engine 1, the air and the fuel are respectively introduced and expelled via intake valves 14A and exhaust valves 14B, connected to a camshaft 15. The camshaft 15, caused to rotate, alternately allows the opening and the closing of the intake valves 14A and exhaust valves 14B, respectively sliding in an intake port 16A and an exhaust port 16B. Each intake port 16A allows the passage of the air from an air intake system up and into the combustion chamber 11A of the cylinder 11.

(14) For that purpose, the air intake system comprises a butterfly valve housing 20 connected to an air box 22. The air box 22 is configured to suck in a stream of air emanating upstream from the exterior of the vehicle and to introduce it into the air intake port 16A connected to the combustion chamber 11A.

(15) In order to regulate the flow rate of the stream of air, still with reference to FIG. 1, the butterfly valve housing 20 comprises a regulating butterfly valve 21, taking the form of a shut-off valve, configured to permit or stop the passage of the air. The invention is described, in this example, for a butterfly valve housing 20 comprising a single regulating butterfly valve 21; however, it goes without saying that the butterfly valve housing 20 could comprise a different number thereof, in particular in the case of an engine 1 comprising a plurality of combustion chambers 11A and therefore a plurality of intake ports 16A.

(16) In order to allow the regulation of the flow rate of the stream of air, the regulating butterfly valve 21 is mounted to rotate about an axis and is configured to change between an open position, in which the air flow rate in the butterfly valve housing 20 is at a maximum, and a closed position, in which such an air flow rate is zero.

(17) The position of the regulating butterfly valve 21 is driven to rotate by an actuator 23 comprising an electric motor controlled by the vehicle computer 30 and connected to a plurality of gears allowing the regulating butterfly valve 21 to be driven to rotate about its axis.

(18) In the case of an engine 1 operating at constant speed, the invention advantageously makes it possible to control, in phase advance, the position of the actuator 23, so as to control the angular position of the regulating butterfly valve 21, with the aim of limiting the fluctuations in the engine speed. Specifically, the invention makes it possible to prevent the fluctuations in the engine speed by anticipating the control of the angular position of the regulating butterfly valve 21.

(19) For that purpose, the vehicle comprises a computer 30 configured to allow the implementation of the method according to the invention.

(20) Specifically, according to a preferred embodiment of the invention, the vehicle computer 30 is configured to evaluate a so-called “load” resistant torque, denoted TQ_Load, resulting from a plurality of external loads applied to said engine 1, with the aim of compensating for such an external load. In the example of the lawnmower, when the latter encounters tall grass, for example, the increase in the height and therefore in the density of the grass to be cut would cause the collapse of the engine speed. The determination of the load resistant torque TQ_Load advantageously makes it possible to anticipate such a collapse by controlling an anticipated angular position of the regulating butterfly valve 21, making it possible to compensate for such a collapse before it occurs.

(21) The computer 30 is then additionally configured to determine, from the evaluated load resistant torque TQ_Load, a position of the actuator 23, so as to determine an angular position of the regulating butterfly valve 21, and to control such a position of the actuator 23, so as to allow the regulation of the engine speed.

(22) The load resistant torque TQ_Load is, according to a preferred embodiment of the invention, evaluated in the following way:
TQ.sub.Load=TQ_Ind−TQ_Fr−TQ_Acc

(23) where: TQ_Ind [N.m]: so-called “combustion” drive torque resulting from the combustion of the mixture of air and fuel in the combustion chamber 11A, TQ_Fr [N.m]: so-called “friction” drive torque resulting from a plurality of frictions acting in the engine 1, TQ_Acc [N.m]: so-called “acceleration” drive torque resulting from an acceleration of the engine 1.

(24) Thus, in order to evaluate the load resistant torque TQ_Load, the computer 30 is configured simultaneously for calculating the acceleration drive torque TQ_Acc, determining the friction resistant torque TQ_Fr and calculating the combustion drive torque TQ_Ind.

(25) For that purpose, with reference to FIG. 2, the computer 30 is configured to receive, from the position sensor 16 of the toothed wheel 130 of the crankshaft 13, a signal representative of the passage of the teeth allowing the computer 30 to determine the angular position from 0° to 360° of the crankshaft 13 from the detection of the reference position. The computer 30 is then configured to determine the speed of rotation of the crankshaft 13 from the evolution of the angular position of said crankshaft 13 for a predetermined duration.

(26) In addition, with the inertia of the engine 1 being predetermined and known, the computer 30 is configured to determine the acceleration drive torque TQ_Acc from the speed of rotation of the crankshaft 13 and from the inertia of the engine 1. According to one exemplary embodiment, the computer 30 is configured to calculate the acceleration drive torque TQ_Acc from the following equation:

(27) ${\mathrm{TQ}}_{\mathrm{Acc}}=J*\frac{d\omega }{\mathrm{dt}}=J_N*N*\left(N_n-N_{n-1}\right)$

(28) where: J: inertia of the engine in kg.m.sup.2 J.sub.N: inertia of the engine in N.m/rpm.sup.2

(29) $\frac{d\omega }{\mathrm{dt}}:$
acceleration of the crankshaft in rad/s.sup.2 N: engine speed (N.sub.n and N.sub.n−1 representing the engine speed at one revolution n and at one revolution n−1) in rpm.

(30) The friction resistant torque TQ_Fr represents the drive torque resulting from a plurality of frictions acting in the engine 1 and corresponds in this example to a predetermined known term. Specifically, the computer 30 is configured for example to store such a value of friction resistant torque TQ_Fr so as to directly incorporate the value in the calculation of the load resistant torque TQ_Load.

(31) Moreover, in order to determine the combustion drive torque TQ_Ind, the computer 30 is configured to: determine a first estimator from an evolution curve of a theoretical drive torque TQ_T, the first estimator corresponding to a succession of segments connected by a plurality of inflection points, each segment being representative of a variation in values of the torque during a combustion phase of the engine cycle, the first estimator additionally comprising an initial point and a final point, perform a correlation between the initial point, each inflection point and the final point and an angular position of the crankshaft 13, measure a plurality of instants, each instant corresponding to an angular position of the crankshaft 13, and calculate the combustion drive torque TQ_Ind from the measured instants.

(32) Specifically, FIG. 3 depicts an example of the theoretical evolution of the drive torque TQ_T due to the combustion of the mixture of air and fuel in the combustion chamber 11. The example depicted in FIG. 3 illustrates such an evolution for an engine 1 comprising two cylinders 11 and therefore two combustion chambers 11A. Thus, the two phases P1, P3 of negative peaks depicted on the curve respectively illustrate the compression of the mixture of air and fuel in the first combustion chamber 11A (P1) and the compression of the mixture of air and fuel in the second combustion chamber 11A (P3), and the two evolution phases P2, P4 of positive peaks depicted on the curve respectively illustrate the combustion of such a mixture in the first combustion chamber 11A (P2) and the combustion of such a mixture in the second combustion chamber 11A (P4).

(33) In this example, an engine cycle CM, that is to say the combustion of the mixture of air and fuel in the two combustion chambers 11A of the engine 1, thus comprises two combustion phases and is carried out for a quarter-turn of the crankshaft 13, that is to say a rotation of 90° of said crankshaft 13.

(34) This evolution curve of the so-called theoretical drive torque TQ_T, depicted in FIG. 3, is known and can be advantageously predetermined or determined in advance. The evolution curve of the drive torque TQ_T can be obtained in a known way, that is to say preferably theoretically from combustion equations, but also, alternatively, by measuring the torque during a precalibration of the engine, for example on the basis of a pressure sensor placed in each combustion chamber of the engine and of a transformation into drive torque, during a complete engine cycle CM. This torque has been termed “theoretical” in the present document for the preferred way of obtaining it by the theoretical route; it is clear that if it is measured, it is no longer “theoretical” in the strict sense of the term but maintains its reference character. The solution of the measurement of this torque is entirely conceivable for an application of the method according to the invention.

(35) The curve of the theoretical drive torque TQ_T, determined in advance as explained above, and depicted in FIG. 3, comprises: a first portion comprising said at least one combustion phase, representative of a variation in the torque during the combustion phase, for calculating the combustion drive torque TQ_Ind, and a second portion not comprising said at least one combustion phase, representative of the load resistant torque TQ_Load, for evaluating the latter.

(36) With reference to FIGS. 4 and 5, the computer 30 is configured to respectively determine a first and a second estimator from such an evolution. According to a preferred embodiment, the first estimator is for example realized on the basis of the zero-mean convolution of the curve representing the evolution of the theoretical torque TQ_T in the combustion chamber 11A. Specifically, the convolution product of the evolution of the torque TQ_T in the combustion chamber 11A is proportional to the combustion drive torque TQ_Ind. Such a convolution product is known per se and will not be described in more detail in this document.

(37) In a preferred manner, two embodiments can be implemented by the computer described above. In the example of the lawnmower, these two embodiments respectively correspond to a lawnmower in which the blades are engaged or clutched (first embodiment), that is to say that external forces are applied to the blade and therefore to the engine, and to a lawnmower in which the blades are free or unclutched (second embodiment), that is to say nonengaged or else that no external force emanating from the cutting blade or blades is applied to the engine.

(38) According to the first embodiment, with reference to FIG. 4, the estimator, denoted first estimator, corresponds to a succession of segments S1, S2, S3, S4, S5 connected by a plurality of inflection points I1, I2, I3, I4, each segment being representative of a variation in values of the torque during a combustion phase in a combustion chamber 11A (that is to say during the phases P1 and P2, for example). Such a first estimator additionally comprises an initial point A and a final point B.

(39) Thus, the first segment S1 represents the estimation of the evolution of the torque TQ_T in the first combustion chamber 11A between the initial point A and the first inflection point I1; the second segment S2 represents the estimation of the evolution of the torque TQ_T between the first inflection point I1 and the second inflection point I2; the third segment S3 represents the estimation of the evolution of the torque TQ_T between the second inflection point I2 and the third inflection point I3; the fourth segment S4 represents the estimation of the evolution of the torque TQ_T between the third inflection point I3 and the fourth inflection point I4; and the fifth segment S5 represents the estimation of the evolution of the torque TQ_T in the first combustion chamber 11A between the fourth inflection point I4 and the final point B.

(40) Each segment, representing a variation in values of the torque, thus has either a negative slope (segment S3) or a positive slope (segments S1 and S5) or a zero slope (segments S2 and S4). According to one exemplary embodiment, since the zero-slope segments do not have a variation in torque values, only the segments in which the slope is not zero are used for determining the combustion drive torque TQ_Ind.

(41) For that purpose, with such a first estimator being realized for a combustion phase of an engine cycle CM, the initial point A, the final point B and each inflection point I1, I2, I3, I4 correspond to a known angular position of the crankshaft 13. Since the speed of rotation of the engine 1 and therefore of the crankshaft 13 are known, each tooth of the toothed wheel 130, that is to say each angular position, corresponds to a given instant from the start of the engine cycle CM. Thus, the computer 30 is configured to record six instants T1, T2, T3, T4, T5 and T6 dependent on the engine 1 and on the engine speed.

(42) By way of example, for a bicylinder engine, in which the two cylinders are offset by a rotation of 90° of the crankshaft 13, the instants T1, T2, T3, T4, T5 and T6 are respectively recorded when the computer 30 detects the following positions of the crankshaft 13: the first instant T1 corresponds to the angular position of the crankshaft 13 at which the piston 12 of the first cylinder 11 passes into a top position, denoted top dead center, and the instant T2 is recorded for a rotation through an angle of 45° starting from the angular position of the crankshaft 13 corresponding to the top dead center of the piston 12 in the first cylinder 11. In a similar manner, the instants T3, T4, T5 and T6 respectively correspond to the instants recorded for a rotation through an angle of 105°, 195°, 255° and 300° starting from the angular position of the crankshaft 13 corresponding to the top dead center position of the piston 12 in the first cylinder 11.

(43) In an advantageous manner, the slopes of the segments defined by the instants T1, T2, T3, T4, T5 and T6 are chosen in such a way that the integral of the curve defined by the succession of segments S1, S2, S3, S4, S5 is zero. That makes it possible to align the positive part of the curve with the positive part of the combustion.

(44) The use of linear segments makes it possible to simplify the calculations by using only additions and subtractions, thereby making it possible in particular to avoid the use of corrective coefficients on the instants T1, T2, T3, T4, T5 and T6.

(45) According to a preferred embodiment of the invention, a clock (not shown) is integrated in the computer 30 so as to allow the recording of the instants T1, T2, T3, T4, T5 and T6 corresponding to each predetermined angular position of the crankshaft 13.

(46) The computer 30 is then configured to calculate three durations d0, d1, d2 corresponding to the three differences of instants relating to the three segments of nonzero slopes, that is to say to the segments S1, S3, S5.

(47) From these durations and therefore from these instants, the combustion drive torque TQ_Ind is calculated in this example according to the following equation:
TQ_Ind=k*(d0−d1+d2)*N.sup.3

(48) where: k: known factor dependent on the inertia of the engine, d0: duration [ms] of the first segment S1 having a positive slope, that is to say duration between the instants T1 and T2, d1: duration [ms] of the third segment S3 having a negative slope, that is to say duration between the instants T3 and T4, d2: duration [ms] of the fifth segment S5 having a positive slope, that is to say duration between the instants T5 and T6, and N: engine speed [rpm] measured by means of the position sensor 16 of the toothed wheel 130.

(49) Thus, such an equation can also be written in the following way:
TQ_Ind=k*[(T2−T1)−(T4−T3)+(T6−T5)]*N.sup.3 that is to say: TQ_Ind=k*(T6−T5−T4+T3+T2−T1)*N.sup.3

(50) The computer 30 thus makes it possible, as described above, to evaluate the combustion drive torque TQ_Ind.

(51) In the second embodiment, with reference to FIG. 5, the estimator, denoted second estimator, corresponds to a succession of segments S1, S2, S3 connected by two inflection points I1, I2, all of the segments being situated in the part of the theoretical drive torque TQ_T with zero or substantially zero value. Such a second estimator additionally comprises an initial point A and a final point B. Such a second estimator then makes it possible to directly determine the load resistant torque TQ_Load.

(52) Thus, the first segment S1 represents the estimation of the evolution of the torque TQ_T between the initial point A and the first inflection point I1; the second segment S2 represents the estimation of the evolution of the torque TQ_T between the first inflection point I1 and the second inflection point I2; and the third segment S3 represents the estimation of the evolution of the torque TQ_T between the second inflection point I2 and the final point B.

(53) Since the three segments are situated in the region of zero theoretical torque, the result of the convolution product is insensitive to the combustion torque and is sensitive only to the load resistant torque.

(54) Each segment, representing a variation in values of the torque, then has either a negative slope (segment S3) or a positive slope (segment S1) or a zero slope (segment S2). Since the zero-slope segments do not have a variation in torque values, only the segments in which the slope is not zero are, in this example, used to determine the load resistant torque TQ_Load.

(55) For that purpose, in a similar manner to the first estimator, with a second estimator being realized during an engine cycle, the initial point A, the final point B and each inflection point I1, I2 correspond to a known position of the crankshaft 13, that is to say correspond to a precise tooth of the toothed wheel 130 of the crankshaft 13. Since the speed of rotation of the engine 1 and therefore of the crankshaft 13 are known, each tooth of the toothed wheel 130 corresponds to a given instant from the starting of the engine cycle CM. Thus, the computer 30 is configured to record four instants T1, T2, T3, T4 dependent on the engine 1 and on the engine speed.

(56) By way of example, for a bicylinder engine, in which the two cylinders are offset by a rotation of 90° of the crankshaft 13, the instants T1, T2, T3 and T4 are respectively recorded when the computer 30 detects the following positions of the crankshaft 13: the first instant T1 is recorded for a rotation through an angle of 270° starting from the angular position of the crankshaft 13 corresponding to the top dead center of the piston 12 in the second cylinder 11; the instant T2 is recorded for a rotation through an angle of 315° starting from the angular position of the crankshaft 13 corresponding to the top dead center of the piston 12 in the second cylinder 11; the instant T3 is recorded for a rotation through an angle of 390° starting from the angular position of the crankshaft 13 corresponding to the top dead center of the piston 12 in the second cylinder 11; and the instant T4 is recorded for a rotation through an angle of 435° starting from the angular position of the crankshaft 13 corresponding to the top dead center of the piston 12 in the second cylinder 11.

(57) According to a preferred refinement of this second embodiment of the invention, a clock (not shown) is integrated in the computer 30 so as to allow the recording of the instants T1, T2, T3, T4 corresponding to each predetermined angular position of the crankshaft 13.

(58) The computer 30 is then configured to calculate two durations d0, d1 corresponding to the two differences of instants relating to the two segments of nonzero slopes, that is to say to the segments S1, S3.

(59) From these durations and therefore from these instants, the load resistant torque TQ_Load is calculated in this example according to the following equation:
TQ_Load=k*(d0−d1)*N.sup.3

(60) where: k: known factor dependent on the inertia of the engine, d0: duration in milliseconds of the first segment S1 having a positive slope, that is to say duration between the instants T1 and T2, d1: duration in milliseconds of the third segment S3 having a negative slope, that is to say duration between the instants T3 and T4, and N: engine speed in rpm (revolutions per minute) measured by means of the position sensor 16 of the toothed wheel 130.

(61) Thus, such an equation can also be written in the following way:
TQ.sub.Load=k*(T4−T3−T2+T1)*N.sup.3

(62) The computer 30 thus makes it possible, as described above, to directly evaluate the load resistant torque TQ_Load.

(63) With reference to FIG. 6, there will now be presented a method for controlling the speed of an engine, operating at constant speed, according to the first embodiment described above, in which the estimator determined for calculating the combustion drive torque TQ_Ind corresponds to the first estimator described above (blades engaged or clutched). In this first embodiment, the estimator is thus realized during a combustion phase of an engine cycle CM.

(64) First of all, in a step E0, the computer 30 evaluates whether the blades of the lawnmower are engaged or not, for example by means of a clutch sensor, then calculates, in a step E1, the load resistant torque TQ_Load, determines, in a step E2, from said calculated load resistant torque TQ_Load, a position of the actuator 23, so as to determine an angular position of the regulating butterfly valve 21, and controls, in a step E3, the actuator 23 in said position so as to control said engine speed.

(65) If the computer detects, in step E0, that the blades are engaged/clutched, the load resistant torque TQ_Load is evaluated simultaneously from the acceleration drive torque TQ_Acc, calculated from the speed of rotation of the crankshaft 13 and inertia of the engine 1, from the friction resistant torque TQ_Fr corresponding to a predetermined value, a function of the engine, and from the combustion drive torque TQ_Ind.

(66) In this case, in a preferred manner, in step E1, the method comprises a first substep F1 of calculating the acceleration drive torque TQ_Acc, followed by a second substep F2 of determining the friction resistant torque TQ_Fr.

(67) Step E1 then comprises a substep F3 of determining a first torque estimator characterized by an initial point A, a final point B and one or more inflection points I1, I2, I3, I4 occurring at a plurality of instants, in this example six instants T1, T2, T3, T4, T5, T6.

(68) The computer 30 correlates, in a substep F4, the initial point A, the final point B and each inflection point I1, I2, I3, I4 with an angular position of the crankshaft 13, and therefore with a given instant T1, T2, T3, T4, T5, T6.

(69) The computer 30 measures, in a substep F5, each instant T1, T2, T3, T4, T5, T6 by means of a clock. For example, in practice, the clock transmits each instant to the computer 30 when one of the predetermined angular positions of the crankshaft 13 is detected by means of the position sensor 16.

(70) The computer then calculates, in a substep F6, the combustion drive torque TQ_Ind from the measured instants T1, T2, T3, T4, T5 and T6, as described above.

(71) Next, in step E2, the computer 30 determines, from the calculated load resistant torque TQ_Load, a position of the actuator 23, so as to determine an angular position of the regulating butterfly valve 21.

(72) The computer 30 then controls, in step E3, the actuator 23 in the determined position so as to control the engine speed and anticipate a runaway or a collapse.

(73) By way of example, the angular position of the regulating butterfly valve 21 can be determined from a double entry table dependent on the engine speed and on the load resistant torque TQ_Load. Specifically, such a table can, according to one exemplary embodiment, be created experimentally or theoretically and stored in the computer 30 of the vehicle. Once the engine speed is known and the load resistant torque TQ_Load has been calculated, the computer 30 can be configured to read the value of the angular position of the regulating butterfly 21 directly from the table and apply such an angular position, via a position of the actuator 23.

(74) If the computer 30 detects, in step E0, that the blades are not engaged, the computer 30 uses, in step E1, a second estimator for evaluating the load resistant torque TQ_Load.

(75) In this case, in a preferred manner, in step E1, the method comprises an estimation of the load torque based on the taking into account of the notable instants T1, T2, T3, T4 of the second curve portion. More precisely, a second estimator is determined and a plurality of instants, in this example four instants T1, T2, T3, T4, are recorded by the computer 30 by correlating the initial point A, the final point B and each inflection point I1, I2 with an angular position of the crankshaft 13, and therefore with a given instant. The instants T1, T2, T3, T4 are measured by means of a clock which transmits each instant to the computer 30 when one of the predetermined angular positions of the crankshaft 13 is detected by means of the position sensor 16.

(76) Next, in step E2, the computer 30 determines, from the estimated load resistant torque TQ_Load and from the engine rotation speed, a position of the actuator 23, so as to determine an angular position of the regulating butterfly valve 21.

(77) The computer 30 then controls, in step E3, the actuator 23 in the determined position so as to control the engine speed and anticipate a runaway or a collapse.

(78) Such a method advantageously allows a rapid and reactive adaptation of the engine speed, making it possible to anticipate for example a collapse of the engine speed, without waiting for the variation in such an engine speed in order to compensate for it. The method according to the invention thus makes it possible to limit the fluctuations in the engine speed, making it possible to limit the risks of damage to such an engine and, where appropriate, to equipment powered by the engine.