Patent Grant
7261076
The priority of EP 04 009 028.4 filed Apr. 15, 2004 is claimed and the disclosure thereof is incorporated herein by reference.
The invention relates to a method and a control system for positioning a crankshaft of an internal combustion engine.
In order to start up internal combustion engines, it is necessary to use a starter, e.g. in the form of an electric motor which is directly or indirectly coupled with the crankshaft of the internal combustion engine. The crankshaft is then accelerated by the starter until it reaches the engine speed required to start the internal combustion engine.
The starting torque which the starter initially needs to generate varies according to the position angle of the crankshaft. Particularly when a cylinder is in a compression stroke at that point, a high starting torque is required which has a negative impact on the starting behaviour of the internal combustion engine.
In order to improve the starting behaviour of the internal combustion engine, it is known from publication DE 198 17 497 A1 of the applicant that the crankshaft of the internal combustion engine be brought to a specific starting angle, from which the starting procedure is then carried out. In this way, an unsuitable position of the crankshaft at the beginning of the starting procedure can be avoided, and the starting time of the internal combustion engine or the initial torque to be summoned by the starter during the starting procedure can be reduced. The crankshaft can be positioned after the internal combustion engine has been stopped, or before the internal combustion engine is started.
Normally, electric starting motors are used to start internal combustion engines, which are driven using field-oriented regulation. These electric motors are asynchronous or synchronous motors in particular. Such electric motors are operated using a motor control which requires the rotational speed of the rotor or crankshaft as an input quantity for field-oriented regulation.
For this purpose, the crankshaft is equipped with a position sensor, for example, which determines the position angle of the crankshaft and uses it to calculate the rotational speed of the crankshaft. However, the degree of precision of the calculated rotational speed depends on how precisely the position angle has been determined by the position sensor. Rotational speed sensors are also frequently provided in order to detect the rotational speed directly.
In both cases, the calculated rotational speed is relatively imprecise. Particularly when the crankshaft rotational speed is low, this results in the relative error becoming very large, and field-oriented regulation in order to drive the electric motor is no longer possible. The crankshaft can no longer be actively positioned in a reliable manner using the electric motor, particularly just before the crankshaft comes to a standstill, when the combustion engine is stopped.
A possible solution to this problem would be to use more precise sensors to detect the position angle and/or rotational speed. However, it is desirable to position the crankshaft at a starting angle while maintaining the components of the engine system used to date.
The object of the invention is to provide an improved method and an improved control system for positioning the crankshaft of an internal combustion engine.
An initial aspect of the invention relates to a method for positioning a crankshaft of an internal combustion engine at a starting angle using an electric motor coupled with the crankshaft. The electric motor can be driven according to a closed loop control mode with field-oriented regulation dependent on the rotational speed of the crankshaft. The crankshaft is set to the starting angle after the internal combustion engine is stopped, in order to start the internal combustion angle from this starting angle when the latter is subsequently started, wherein the electric motor is driven below a rotational speed limit in an open loop control mode independently of the rotational speed in an open loop control in order to set the starting angle.
A further aspect relates to a control system for driving an electric motor coupled with a crankshaft of an internal combustion engine, in order to position the crankshaft at a desired starting angle. A detection device for receiving or determining a position angle and/or the rotational speed of the crankshaft is provided for this purpose, together with a control unit which is capable of driving the electric motor in a closed loop control mode with field-oriented regulation dependent on the rotational speed of the crankshaft, and of positioning said crankshaft at the starting angle after the internal combustion engine is stopped, so that in the case of a subsequent start, the internal combustion engine is started from this starting angle. The control unit is structured in such a way as to drive the electric motor according to an open loop control mode below a prespecified rotational speed limit independently of the rotational speed in an open loop control, so that the crankshaft is set to the starting angle.
Further embodiments of the invention are described in the relevant dependant claims.
A switch to an open loop control is proposed when driving the electric motor to position the crankshaft at the starting angle, so that the electric motor is driven independently of the rotational speed. This is advantageous, since with standard engine systems, the detection and evaluation of the rotational speed is usually too imprecise to be able to operate the electric motor with field-oriented regulation when the rotational speed is low. Particularly with very low rotational speeds, such as those which occur when the crankshaft of the internal combustion engine runs down shortly before the crankshaft comes to a standstill, the degree of imprecision for determining the rotational speed may result in a very large relative deviation, so that field-oriented motor regulation is no longer possible without significant running problems occurring while the electric motor is in operation.
The use of the open loop control mode for the electric motor also has the advantage that the electric motor is operated independently of the rotational speed detected for the crankshaft. This is made possible by operating the electric motor with a specific drive frequency and with a specific voltage or current in order to reach a prespecified torque. The electric motor can thus be operated in such a way as to allow the position angle to be set. The current or voltage and frequency are selected so as to ensure that the torque that is reached is sufficient to prevent the internal combustion engine from reversing. This has the advantage of also enabling position/rotational speed sensors which do not recognize left/right rotation to determine the precise position of the internal combustion engine.
Further characteristics of the invention can be taken from the objects and methods revealed, or can be seen by persons having the ordinary skill in the art from the following detailed description of the embodiments and the appended drawings.
Embodiments of the invention will now be described by way of examples and with reference to the attached drawing, in which:
In this exemplary embodiment, an electric motor 4, which acts as a starter/generator, is positioned directly on the crankshaft 2. The electric motor 4 comprises a rotor (not shown) which is firmly connected to the crankshaft 2 and a stator (not shown) which rests e.g. on the housing of the internal combustion engine 1. In the exemplary embodiment shown, the electric motor 4 is an asynchronous motor 4, but it can also be a synchronous motor or similar.
An electric motor 4 of this type has a high torque for operating as a starter. In other embodiments of the invention (not shown), the electric motor 4 is coupled with the crankshaft 2 via a transmission gear, if necessary via single-track gears which are connected in series. The electric motor 4 is designed in such a way that it can reach the necessary torque in the direction of rotation of the internal combustion engine to set the required crankshaft angle position, as well as achieve the starting power required when starting to directly drive the crankshaft 2 to the necessary starting rotational speed.
In the exemplary embodiment according to
The control unit 5 receives the current position angle of the crankshaft from a position sensor 7, which is attached to the crankshaft in the exemplary embodiment shown. According to a further embodiment of the invention, the position sensor can also be integrated in the electric motor 4, and can be e.g. a Hall effect torque-synchro sensor, in order to measure the angle of the rotor. Due to the direct coupling of the rotor with the crankshaft 2, the rotor angle measured corresponds to the position angle.
The starting procedure for the internal combustion engine 1 is prepared in a particular way. After the engine operation has ended, e.g. when, or just after, the ignition of the motor vehicle is switched off, the control unit 5 drives the electric motor 4 via the drive unit 6 in such a way that the crankshaft 2 is brought into a crankshaft angle position suitable for the subsequent start: the starting angle. Here, the electric motor 4 controls the crankshaft 2 of the internal combustion engine 1 which is in the process of running down, in order to set the desired starting angle.
The position sensor 7 is used primarily to detect the position angle. However, it is also used to detect the rotational speed for a wide range of functions within the engine system, and in particular to control the injection of the internal combustion engine. The position sensors which are commonly used, however, generally only have a degree of precision sufficient for the standard functions. A signal edge of the position sensor 7 can indicate that the starting angle has been reached. In particular, a CAN signal can indicate that the starting angle has been reached.
The electric motor 4 is usually operated in a closed loop control mode, i.e. the drive signals are generated by the drive unit 6 dependent on e.g. the rotational speed and the desired set rotational speed. However, if the rotational speed of the crankshaft 2 or the current position angle are only detected with a low degree of precision, this leads to significant relative errors, above all when the rotational speed of the crankshaft 2 is low, which prevent the electric motor 4 from being operated in a precise manner in a closed loop control mode. Low rotational speeds occur when the crankshaft is running down just before it comes to a standstill, however. Here, it is just when the position angle is being set while the crankshaft is running down after the engine operation has ended that a particularly precise operation of the electric motor 4 is required in order to set the starting angle.
The control unit 5 is therefore designed in such a way as to operate the electric motor 4 below a rotational speed limit in an open loop control mode while the internal combustion engine is running down. This means that the electric motor 4 is no longer operated using field-oriented regulation, which takes into account the current rotor speed when generating the drive signals, but is now operated independently of the current rotational speed with specified values such as the drive frequency, and specified current and voltage curves, in order to turn the crank angle slightly further to the starting angle with a torque determined by these values. A signal flank from the position sensor 7 can indicate that the starting angle has been reached. The control unit 5 then immediately stops driving the electric motor 4.
The control unit 5 therefore drives the electric motor 4 below the rotational speed limit in an open loop control mode in order to set the starting angle. The rotational speed limit is set at a rotational speed at which standard rotational speed detection is no longer precise enough to enable the electric motor to be operated using field-oriented regulation without problems arising. This rotational speed limit can be between 5 and 50 RPM, for example, but may also be below or above this value. The current or voltage and frequency are selected so as to ensure that the torque that is reached is sufficient to prevent the internal combustion engine from reversing. This enables position/rotational speed sensors which do not recognize left/right rotation to determine the precise position of the internal combustion engine.
For the standard rotational speed sensors commonly used in engine systems, or when position sensors are used, the direction of rotation is not detected, since the internal combustion engine is only operated in one rotational direction. For this reason, it is necessary for the control unit 5 to drive the electric motor 4 in the open loop control mode in such a way that the crankshaft 2 continues to be turned in the usual direction of rotation of the crankshaft, in order to set the starting angle. The current or voltage and frequency are selected so as to ensure that the torque that is reached is sufficient to prevent the internal combustion engine from reversing. This has the advantage of also enabling position/rotational speed sensors which do not recognize left/right rotation to determine the precise position of the internal combustion engine.
When the internal combustion engine is stopped, the control unit 5 drives the internal combustion engine 1 in such a way that the fuel supply to the internal combustion engine is shut down below an additional rotational speed limit which is larger than the rotational speed limit. The rotational speed limit should preferably be within a range below which no independent engine operation of the internal combustion engine 1, and no rotating engine operation, is possible. The additional rotational speed limit in particular is approx. 800 RPM, but can also have higher or lower values. Furthermore, the electric motor 4 can continue to be driven dependent on the rotational speed with field-oriented regulation below the additional rotational speed limit according to the closed loop control mode, in order to cushion against vibrations of the crankshaft which occur when the internal combustion engine is stopped. For this purpose, the running down of the crankshaft 2 is controlled with the aid of the electric motor 4, i.e. the electric motor 4 limits the vibrations emitted when the rotational speed is reduced while the internal combustion engine 1 is running down by means of an additional torque.
When the rotational speed falls below the rotational speed limit, a switch is made from field-oriented regulation in the closed loop control mode to open loop control of the open loop control mode, in order to set the starting angle to the highest possible degree of precision at the end of the running down procedure. The electric motor 4 is driven in such a way that it sets the starting angle with a prespecified torque, i.e. a prespecified rotational speed, or with a prespecified torque progression.
The “optimum” position angle for starting an internal combustion engine—in other words, the starting angle—depends on different factors, such as the engine type, number of cylinders, firing sequence etc., as well as on the desired starting behaviour, for example whether a low starting torque at the beginning of the starting procedure for the subsequent start, a reduced starting period, or at least a reproducible starting procedure with consistently uniform starting conditions are required. For a four-cylinder, four-stroke internal combustion engine 1 such as the one shown in
The advantage of this set starting angle is that at the beginning of the subsequent starting procedure, the initial break away torque to be reached by the starter machine 4 is significantly lower than with commonly used starter systems. If the internal combustion engine 1 is started from this set crank angle position, the electric machine 4 is countered by a relatively low, predominantly friction related torque by at least the two external cylinders of the internal combustion engine. Up to the following compression stroke (of the two inner cylinders), the electric motor 4 is able to supply the system with sufficient (starting) power to surmount the compression.
Alternatively, a suitable starting angle may be shortly before the inner dead center position when the main aim is to achieve a reproducible starting behaviour with consistently uniform starting conditions, since this starting angle position is more stable against any vehicle movements which may arise between the point in time when the internal combustion engine is switched off and the subsequent starting procedure.
The flow chart in
If the current rotational speed of the crankshaft 2 falls below the additional rotational speed limit (step S2), then the closed loop control for the electric motor 4 is set or put into operation (step S3). The electric motor 4, driven by the control unit 5, then guides the crankshaft 2 (step S4) in order to cushion against or damp any vibrations which occur when the internal combustion engine 1 runs down. The electric motor is thereby operated using field-oriented regulation, and can therefore be optimally driven by the drive unit 6. If the rotational speed falls below the rotational speed limit (step S5), the value of which indicates that the detection of the rotational speed is now too imprecise to operate the electric motor in the closed loop control, then a switch is made in step S6 to the open loop control of the electric motor 4. In the open loop control, the starting angle can then be set.
| Number | Date | Country | Kind |
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| 04009028 | Apr 2004 | EP | regional |
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| Number | Date | Country | |
|---|---|---|---|
| 20050229889 A1 | Oct 2005 | US |
