Patent Application
20180230958
The present invention relates to a system for repositioning a rotor of a combustion engine, for example of a motor vehicle, as well as the associated method.
For reducing the fuel consumption of motor vehicles with heat engines, the use of systems for putting the engine on stand-by automatically is known, so-called “stop-start” systems, which cut out the engine automatically when the vehicle stops, for example at an intersection, at traffic lights or in a traffic jam. The stop-start system restarts the engine when the driver releases a brake pedal, engages the clutch or presses on an accelerator pedal to set off again, so that engine cut-out takes place transparently for the driver.
For vehicles equipped with such systems there will therefore be a large increase in the number of starting cycles, so that the least economy of power during said starting cycles is reflected in a substantial fuel saving.
Heat engines generally comprise a crankshaft forming a rotor, which is rotated relative to a cylinder block forming a stator.
It has been established that, depending on the relative angular position of the crankshaft and cylinder block when the engine is stopped, restarting requires more or less energy, and the difference in energy required may be up to about 30%.
The use of an electric motor, for example an electric starter of the heat engine, for moving the crankshaft to a position close to the position corresponding to a minimum of power required for starting when the engine is stopped, is known.
In the aforementioned devices, crankshaft repositioning is rapid in order to reach the target position for restarting in a short time, which generates inverse reactions of the engine. In particular, when the pistons are moved quickly they compress the air in the cylinders. This compression leads to oscillations of the crankshaft, in particular around the target position, and this is troublesome during precise positioning of the crankshaft at the target position for restarting the engine.
Now, it is found that the optimum position for restarting is an unstable position for the majority of engines. Departure from this optimum position may therefore be reflected in movement of the crankshaft to a stable position, different from said optimum position (generally the stopped position initially adopted).
Furthermore, the electric motors of starters only have one possible direction of rotation, which means that going past the optimum restarting position requires the execution of an additional complete turn of the crankshaft.
It is therefore necessary to find a method for repositioning the crankshaft that allows quick and accurate return to the optimum position for restarting in a repeatable manner.
To solve the aforementioned problem at least partially, the invention relates to a method for rotational positioning of a heat engine rotor that is in a stopped position to a target position, comprising the following steps:
The method thus executed allows quick and efficient repositioning of the rotor of the heat engine.
Said method may have one or more of the following features, used alone or in combination.
The predetermined values of high speed, of rising slope, and of falling slope are stored in an electronic memory of a control unit.
The predetermined values of high speed, of rising slope, and of falling slope are determined as a function of the moment of inertia of the rotor to limit the torque exerted on said rotor to values below a predetermined torque value.
The engine is a car engine and the predetermined torque value is less than or equal to 20 Nm.
The engine is an engine of a heavy goods vehicle, or of civil-engineering or agricultural machinery, and the predetermined torque value is less than or equal to 40 Nm.
It comprises an additional braking step by short-circuiting the electric motor triggered at a predetermined angular distance calculated as a function of the moment of inertia of the rotor, the rotary speed of the rotor, the friction acting on the rotor and the dissipative power of the electric motor in short-circuit to allow complete stoppage of the rotor in a position close to the optimum position for restarting.
The invention also relates to the associated device for rotational positioning of a heat engine rotor that is in a stopped position to a target position, comprising:
Other features and advantages of the invention will become clearer on reading the following description, given as an illustrative, non-limiting example, and the appended drawings, where:
In all the figures, the same references relate to the same elements.
The embodiments described referring to the figures are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features only apply to a single embodiment. Simple features of different embodiments may also be combined to supply other embodiments.
When the engine is stopped, the rotor 3 is set in motion by an electric motor 7, by means of a drive device 9. In a particular embodiment, said electric motor 7 is a starter or starter-generator of the heat engine, and the drive device 9 comprises a drive belt or a set of gears.
Here, starter-generator means an electric motor that functions as a starter when it is supplied with electric current and as an alternator converting a proportion of the kinetic energy of the rotor 3 into electrical energy for recharging a battery, for example the battery of the vehicle when it is not supplied.
The electric motor 7 is controlled by a set-point current ic supplied by a control unit 11. To enable the position of the rotor 3 to be adjusted, the control unit 11 is connected to a position sensor 13, configured to detect the angular position θ of the rotor 3 relative to the stator 5. Said sensor 13 may comprise detecting means that are electromagnetic, capacitive, optical or with electrical contacts.
In particular, the sensor 13 may comprise Hall-effect analogue sensors and a magnetic target, the Hall-effect sensors being arranged on the stator 5 and the magnetic target on the rotor 3.
In a first step 101, the control unit 11 determines the angular difference Δθ between the target position θ0 and the starting position θA measured by the position sensor 13. For this purpose, the control unit 11 interrogates the position detector 13 to obtain a value of the angular position θA at which the heat engine has stopped.
The control unit 11 is connected to or comprises calculating means configured for establishing the difference Δθ between the measured starting position θA at which the heat engine rotor 3 has stopped and the target position for restarting θ0. These calculating means typically comprise a processor and one or more electronic memory units, which are either dedicated, or integrated in a global electronic network of the vehicle.
The following steps 103, 105 and 107 are discussed here with reference to
The angular domain Δθ between the stop position θA and the target position θ0 is divided into three parts corresponding to the next three steps 103, 105, 107 of the method 100.
The method associated with the device 1 as described above is initiated when the heat engine stops, for example at an intersection for an engine equipped with a “stop-start” system.
The rotary speed set-point Ωc(θ) of the rotor 3 of the heat engine is of trapezoidal shape, with a rising slope corresponding to the second step 103, a speed plateau at a high value Ω0 corresponding to the third step 105, and a falling slope corresponding to the fourth step 107. The high value Ω0 of speed may notably be stored in an electronic memory of the control unit 11. In particular, the memory of the control unit 11 may contain a maximum value of high speed Ω0 that is not to be exceeded, the value used in the set-point Ωc(θ) being adjusted as a function of the measured angular difference Δθ and of a target time for execution of the method 100.
The second step 103 corresponds to a phase of acceleration of the rotor 3, according to a predetermined rising slope a, in this case fixed. The rising slope is limited in absolute value to avoid exerting an excessive torque on the rotor 3 by the electric motor 7. In particular, the slope a remains low enough for the torque exerted to stay below 20 Nm for an engine of small or usual size, typically for a car, and below 30 to 40 Nm for engines of larger vehicles (bus, lorry, agricultural or civil-engineering machinery, boat).
Limitation of the slope a thus allows the engine to evacuate the air contained in the cylinders owing to the gradual nature of the acceleration. The resultant limitation of the accelerator torque exerted additionally reduces the noise and vibrations of the engine and of the device 1. As a result, the method 100 may then be executed without the user feeling vibrations or hearing noise which would then be perceived as parasitic since the heat engine is switched off.
In the third step 105, the speed is kept constant, in particular less than or equal to a high value Ω0. Limitation in terms of maximum speed to the value Ω0 again allows the heat engine to evacuate the air contained in the cylinders during return of the pistons to top dead centre.
In the fourth step 107, the rotor 3 is slowed down, once again gradually, with a falling slope less than or equal in absolute value to a predetermined value b. Limitation in absolute value of the falling slope b corresponds to a limitation of the braking torque, once again making it possible to limit noise and vibrations.
During its motion the rotor 3 is subject to solid friction, due to friction with the stator 5. When the electric motor 7 is cut out, the rotor 3 continues to turn owing to its moment of inertia, but its movement is braked and then stopped by said friction.
The values of rising slope a and falling slope b are determined for a given heat engine taking into account the moment of inertia of the rotor 3 to limit the torque to be exerted on said rotor 3. Maximum values of slope a and b may in particular be stored in an electronic memory of the control unit 11, the value used in the method 100 being adjusted as a function of a target execution time and the measured angular difference Δθ.
The rotor 3 is set in rotation by actuation of the electric motor 7. The control unit 11 then modulates the supply current ic with optional feedback taking into account the measurement of the position θ in real time obtained by the position sensor 13 of the rotor 3.
For accurate stopping in the optimum position for restarting θ0, the speed set-point Ωc(θ) is established taking into account the moment of inertia of the rotor 3, and the value of the torque generated by the solid friction.
The curve of speed as a function of time Ω(t) may be divided into three portions that correspond to the three steps 103, 105, 107 of acceleration, of speed plateau at the value Ω0 and of deceleration.
In the acceleration step 103, the speed Ω(t) increases according to a rising parabola. In the speed plateau step 105, the speed Ω(t) is constant and has the value Ω0. In the deceleration step 107, the speed Ω(t) decreases according to a falling parabola.
The rising and falling parabolas of steps 103, 107 of acceleration and of deceleration result from the change of variable between the angular position θ and the time t on the rectilinear rising and falling slopes in
The time interval shown in
In the first interval corresponding to the acceleration step 103, the angular position θ gradually increases, which corresponds to gradual acceleration at the start of the method 100.
In the second interval, the angular position θ increases linearly, with Ω0 as the direction parameter. In the constant speed step 105 this interval corresponds to the value Ω0.
In the third interval corresponding to the deceleration step 107, the angular position θ increases more and more slowly and stabilizes at the value θ0 corresponding to the required target position.
An alternative embodiment of method 100 is illustrated in
The embodiment in
During this step 109, short-circuiting of the electric motor 7 generates a braking torque by dissipation of magnetic flux. Short-circuiting is in particular triggered when the rotor 3 is at a predetermined angular distance δθ from the optimum position for restarting θ0.
The predetermined angular distance δθ is in particular calculated as a function of the moment of inertia of the rotor 3, of its speed, of the friction to which it is subjected and the dissipative power of the electric motor 7 in short-circuit to allow complete stoppage in a position as close as possible to the optimum position for restarting θ0.
The next step 111 is a step of maintaining the short-circuit, for at least some hundredths to some tenths of a second, during which the control unit 11 verifies in particular that the angular position θ remains constant. This maintaining of the short-circuit, and therefore of the rotational braking, makes it possible to ensure that no reaction, possibly delayed, of the pistons or of the engine 1 alters the final position of the rotor 3, near the target position θ0.
A delayed reaction of this kind may notably result from the air escaping from the cylinders at a low flow rate. Maintaining the braking 111 also allows the forces of static friction, which are higher than the forces of dynamic friction, to come into effect.
The fact that the electric motor 7 is maintained in short-circuit also makes it possible to increase the energy required for the rotor 3 to travel through an angular opening in question. The rotor 3 is thus less likely to overshoot the optimum restarting position θ0 and then adopt a more stable position spontaneously (top dead centre or bottom dead centre of the pistons). This avoids a large overshoot of the target position θ0 by the rotor 3, making it possible to use electric motors 7 with a single sense of rotation, as implemented in most starters and starter-generators.
The speed set-point Ωc(θ) may in particular be calculated additionally taking into account the pulley ratio between the electric motor 7 and the heat engine and the tensioning of the belt or pulley of the drive device 9.
The method according to the invention therefore allows accurate and repeatable positioning of the rotor 3 of a heat engine in a position θ0 allowing restarting of the heat engine with less energy. Moreover, the method according to the invention essentially uses devices that are already present in most vehicles (position sensor 13 of the rotor 3, starter-generator as electric motor 7) and may therefore easily be implemented in the majority of vehicles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1751168 | Feb 2017 | FR | national |
