Abstract: The starting unit comprises start-up means for the engine, comprising: engagement means capable of being displaced in translation for engagement in complementary receiving means linked to the engine; actuating means for displacing the engagement means in translation; and drive means capable of driving the start-up means.

Abstract: In accordance with the invention, an iterative search operation for the optimum value .theta..sub.opt of the angle of advance is carried out by adding periodically an angle of advance increment .DELTA..theta. to the instantaneous angle of advance .theta..sub.o, and by detecting the instantaneous speeds N.sub.1t and N.sub.2t which correspond to the angles of advance .theta..sub.o and .theta..sub.o +.DELTA..theta., and by determining the direction of variation .DELTA.N of the engine speed so as to correct the angle of advance .theta..sub.o such that the variation .DELTA.N is as low as possible, or even zero, such that the angle of advance .theta..sub.o is kept in the vicinity of its optimum value .theta..sub.opt. In the case of transitory states, a "floating cartography" is superposed and takes into account a curve of the advance as a function of the engine speed. The invention may be used for controlling the ignition advance of the engines of motor vehicles.