The present invention relates to balancing methods for engines, in particular internal combustion engines, and to devices for implementing these methods.
More specifically, the invention relates to a balancing method for an engine, in particular a vehicle internal combustion engine, generating a first-order (in other words, at the frequency corresponding to the engine rotation) out-of-balance moment, which out-of-balance moment excites at least one vibration mode.
Methods of this type are known in which the balancing is obtained by adding a balancing shaft, as described for example in document WO-A-02/075134 in the case of internal combustion engines with 3 cylinders.
However^ these known methods have the following disadvantages: first, the balancing shaft is an expensive part, and second, this shaft is a relatively heavy part and energy has to be wasted to set it in motion.
The particular object of the present invention is to alleviate these disadvantages.
To this end, according to the invention, in a method of the type in question the first-order out-of-balance moment is at least partially cancelled by generating at each instant a moment opposed to the out-of-balance moment by means of at least one driven oscillator connected to the engine, in at least one balancing frequency range in the vicinity of the said vibration mode excited by the out-of-balance moment.
By virtue of these arrangements, the engine is at least partially balanced for a reasonable cost, and more

importantly it absorbs very little extra power. This extra power requirement is further reduced by the fact that it is possible to stop the actuation of the driven oscillator when the engine turns at a speed above a certain limiting rotational speed.
The term "driven oscillator" is to be understood as meaning:
an active oscillator with an actuator generating a counterforce acting in opposition to the out-of-balance moment (examples of active oscillators are described for example in patent application FR-0311495 filed on 01/10/2003, and in document US-A-5 718 418, and others),
or a variable-mode oscillator whose mode can be adjusted for example by varying the stiffness or the mass.
In various embodiments of the method according to the invention, recourse may optionally be additionally had to one and/or other of the arrangements below:
at least one active oscillator is used to generate at each instant a moment opposed to the out-of-balance moment;
at least two active oscillators are used and these active oscillators are controlled in phase opposition so as to generate at each instant a moment opposed to the out-of-balance moment;
at least the engine speed is detected (directly or indirectly) and the said oscillator is controlled as a function of this speed according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment;
at least one phase of the operating cycle of the engine is detected (directly or indirectly) and the said oscillator is controlled as a function of this detection according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment;
the engine whose out-of-balance moment is cancelled is an internal combustion engine with two, three or five cylinders;
the engine whose out-of-balance moment is cancelled is a vehicle internal combustion engine connected to the vehicle body by a pendulum suspension;
the balancing frequency range is situated between an engine idle frequency and f©x, where f©x is the frequency of the said vibration mode.
Moreover, another subject of the invention is a device for implementing a method as defined above, this device comprising an engine which, during operation, generates a first-order out-of-balance moment, which out-of-balance moment excites at least one vibration mode, characterized in that it comprises at least one driven oscillator which is connected to the engine and which is controlled by at least one central processing unit so as to at least partially cancel the first-order out-of-balance moment by generating at each instant a moment opposed to the out-of-balance moment by means of the oscillator in at least one balancing frequency range in the vicinity of the said vibration mode excited by the out-of-balance moment.
In various embodiments of the device according to the invention, recourse may optionally be additionally had to one and/or other of the arrangements below:
the said oscillator is an active oscillator;
the device comprises at least two active oscillators controlled by the central processing unit in phase opposition;
the device comprises at least one sensor designed to detect at least the speed of the engine and the central processing unit is designed to control the oscillator as a function of this speed according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment;

the engine is an internal combustion engine and the device comprises at least one sensor designed to detect at least one particular step of the operating cycle of the engine, and the said oscillator is controlled as a function of this detection according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment;
the engine is an internal combustion engine with two, three or five cylinders;
the device comprises a vehicle having a body, the said engine being connected to the body by a pendulum suspension;
the engine belongs to a propulsion unit which extends longitudinally between two ends and the two oscillators are situated close to the two respective ends of the propulsion unit;
the balancing frequency range is situated between an engine idle frequency and V2.f©x, where f©x is the frequency of the said vibration mode.
Other features and advantages of the invention will become apparent in the course of the description which follows of one of its embodiments, given by way of non-limiting example, with reference to the appended drawings. In the drawings:
Figure 1 is a schematic perspective view of a vehicle provided with an antivibration device according to one embodiment of the invention;
Figures 2 and 3 are schematic front and plan views, respectively, of the propulsion unit of Figure 1;
Figures 4 to 6 are graphs showing, each along one of the three perpendicular axes, the strength of the vibrations in one of the vehicle seats.
The same references have been used in the various figures to denote identical or similar elements.
Figure 1 shows a motor vehicle 1 comprising a propulsion unit 2, which comprises an internal combustion engine 3 (for example an engine with two, three or five cylinders) and a gearbox 4 fastened rigidly to the engine 2. The propulsion unit 2 may, for example, extend in a transverse direction Y of the vehicle and it may optionally comprise, at its two longitudinal ends, two rigid brackets 3a, 3b respectively secured to the engine 3 and to the gearbox 4.
The propulsion unit 2 is connected to the vehicle body 5 by an engine suspension which, in the example represented, is of the pendulum type. As can be seen in more detail in Figures 2 and 3, this pendulum suspension may comprise for example:
two elastic supports 3b, 4b which may, for example, connect the two brackets 3a, 4a to the vehicle body 5,
and a torque-absorbing rod 6 limiting and damping the pivoting movements of the propulsion unit 2 about the transverse axis Y, this rod connecting the propulsion unit 2 to the body 5 and extending, for example, in the plane X, Z.
During operation, the engine 3 generates a first-order out-of-balance moment, first-order meaning at the frequency corresponding to the engine rotation. These effects may be particularly noticeable with an engine 3 with two or three cylinders, particularly at speeds close to the idle speed. For example, if the idle speed is 900 rpm, the corresponding excitation frequency fiN (or first-order excitation) is 15 Hz.
The out-of-balance moment in question may be a turning moment about the longitudinal axis X of the vehicle and/or a turning moment about the vertical axis Z.
This out-of-balance moment excites certain vibration
modes, particularly:
a rotational pitching vibration mode 0x about the axis X, at a frequency fex,
and/or a rotational vibration mode 0z about the axis Z, at a frequency f®z.
This rotational vibration mode or these rotational vibration modes is or are additionally connected to pumping modes, in particular a pumping mode Tz in the vertical direction, as a function of the geometry of the propulsion unit 2.
The excitation of these various vibration modes gives rise to vibrations which are transmitted to the vehicle body 5 and felt with an unpleasant sensation by the users of the vehicle, in particular when the engine is operating at low speed.
According to the invention, the first-order out-of-balance moment or moments is or are at least partially cancelled by generating at each instant at least one moment opposed to the out-of-balance moment or moments by means of at least one driven oscillator connected to the propulsion unit 2, in at least one balancing frequency range in the vicinity of the vibration mode or modes excited by the out-of-balance moment or moments.
The term "driven oscillator" is to be understood as meaning:
either an active oscillator with an actuator generating a counterforce acting in opposition to the out-of-balance moment (examples of active oscillators are described for example in patent application FR-0311495 filed on 01/10/2003, and in document US-A-5 718 418, and others),
or a variable-mode oscillator whose mode can be adjusted for example by varying the stiffness of the elastic return means for the oscillating part or the

mass of the oscillating part.
In the example represented in Figures 1 to 3, the propulsion unit 2 is equipped with two active oscillators 7, 8 fastened, for example, to the aforementioned brackets 3a, 4a in such a way as to maximize the distance e between the oscillators 7, 8 in the direction Y. These active oscillators are controlled by a central processing unit 10 (CPU) as a function of information supplied by a sensor 9 (S) supplying, for example, a pulse signal corresponding to the top dead centre of one of the pistons of the engine 2, which makes it possible to determine both the engine speed and the phase of the operating cycle of the engine.
The active oscillators 7, 8 may be actuated in phase opposition by the central processing unit 10 so as to generate at each instant a moment opposed to the out-of-balance moment. The out-of-balance moment is known at each instant since it is connected in a one-to-one relationship with the information supplied by the sensor 9, with the result that the active oscillators 7, 8 can be actuated by the central processing unit 10 according to a predetermined law designed so that the oscillators at least partially cancel the first-order out-of-balance moment, or equally a second-order out-of-balance moment.
The arrangement represented in the drawings, with the oscillators 7, 8 disposed vertically so as to produce upward and downward vertical forces, essentially makes it possible to cancel the out-of-balance moment about the axis X and therefore to modify the mode 0x. However, it would be possible to dispose the oscillators 7, 8 differently so as to simultaneously cancel the out-of-balance moment about the axis Z and therefore modify the mode 0z as well. In a variant, it would be possible to provide at least one additional
active oscillator, or even two additional oscillators, specifically to cancel the out-of-balance moment about the axis Z and therefore modify the mode 0z.
The aforementioned balancing frequency range may be situated, for example, between the engine idle frequency f1N and . fex, where fex is the frequency of the modified vibration mode excited by the idle frequency.
The invention allows a very significant reduction in the vibrations perceived by the user of the vehicle, as shown by the graphs in Figures 4 to 6 which represent, as a function of the frequency f, the amplitude A of the vibrations generated by the engine 3 at idle, in one of the seats of a particular vehicle 1 equipped with an engine with three cylinders, without the invention (solid-line curve) and with the active oscillators 7, 8 described above (broken-line curve). Figure 4 shows the vibrations measured along the vertical axis Z, Figure 5 shows the vibrations measured along the longitudinal axis X of the vehicle and Figure 6 shows the vibrations measured along the transverse axis Y of the vehicle.

WE Claims
Balancing method for an engine (3) generating a first-order out-of-balance moment, which out-of-balance moment excites at least one vibration mode, characterized in that the first-order out-of-balance moment is at least partially cancelled by generating at each instant a moment opposed to the out-of-balance moment by means of at least one driven oscillator (7, 8) connected to the engine (3), in at least one balancing frequency range in the vicinity of the said vibration mode excited by the out-of-balance moment.
Method according to Claim 1, in which at least one active oscillator (7, 8) is used to generate at each instant a moment opposed to the out-of-balance moment.
Method according to Claim 2, in which at least two active oscillators (7, 8) are used and these active oscillators are controlled in phase opposition so as to generate at each instant a moment opposed to the out-of-balance moment.
Method according to any one of the preceding claims, in which at least the engine speed is detected and the said oscillator (7, 8) is controlled as a function of this speed according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment.
Method according to any one of the preceding claims, in which at least one phase of the operating cycle of the engine (3) is detected and the said oscillator (7, 8) is controlled as a function of this detection according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment.
Method according to any one of the preceding

claims, in which the engine (3) whose out-of-balance moment is cancelled is an internal combustion engine with two, three or five cylinders.
Method according to any one of the preceding claims, in which the engine (3) whose out-of-balance moment is cancelled is a vehicle internal combustion engine connected to the vehicle body (5) by a pendulum suspension.
Method according to any one of the preceding claims, in which the balancing frequency range is situated between an engine idle frequency and •f0x/ where fex is the frequency of the said vibration mode.
Device for implementing a method according to any one of the preceding claims, this device comprising an engine (3) which, during operation, generates a first-order out-of-balance moment, which out-of-balance moment excites at least one vibration mode, characterized in that it comprises at least one driven oscillator (7, 8) which is connected to the engine (3) and which is controlled by at least one central processing unit (10) so as to at least partially cancel the first-order out-of-balance moment by generating at each instant a moment opposed to the out-of-balance moment by means of the oscillator (7, 8) in at least one balancing frequency range in the vicinity of the said vibration mode excited by the out-of-balance moment.
Device according to Claim 9, in which the said oscillator (7, 8) is an active oscillator.
Device according to Claim 10, comprising at least two active oscillators (7, 8) controlled by the central processing unit (10) in phase opposition.
Device according to any one of Claims 9 to 11,

comprising at least one sensor (9) designed to detect at least the speed of the engine (3) , the central processing unit being designed to control the oscillator (7, 8) as a function of this speed according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment.
Device according to any one of Claims 9 to 12, in which the engine (3) is an internal combustion engine and the device comprises at least one sensor designed to detect at least one particular step of the operating cycle of the engine, and the said oscillator is controlled as a function of this detection according to a predetermined law designed so that the oscillator at least partially cancels the first-order out-of-balance moment.
Device according to any one of Claims 9 to 13, in which the engine (3) is an internal combustion engine with two, three or five cylinders.
Device according to any one of Claims 9 to 14, in which the device comprises a vehicle (1) having a body
(5), the said engine (3) being connected to the body (5) by a pendulum suspension (4a, 4b, 6).
Device according to any one of Claims 9 to 15, in which the engine (3) belongs to a propulsion unit (2) which extends longitudinally between two ends and the two oscillators (7, 8) are situated close to the two respective ends of the propulsion unit.
Device according to any one of Claims 9 to 16, in which the balancing frequency range is situated between an engine idle frequency and V2.f0X, where fex is the frequency of the said vibration mode.
Balancing method for an engine generating a first-order out-of-balance moment substantially as herein described with reference to foregoing examples and accompanying drawings;
Device for implementing a balancing method for an engine generating a first-order out-of-balance moment substantially as herein described with reference to foregoing examples and accompanying drawings;