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.