u03b4n featured in the table may easily be calculated so that the decrease in reference point speed follows any predetermined profile for example a parabolic hyperbolic or exponential one the correction u03b4n obviously decreases with time so that it cancels itself out at the moment t when the reference point speed n meets the final low idle speed n this is what is expressed in the table of fig5 by taking the absolute value u03b4t of the time elapsed between the moments t and t into account the graph of fig4 is read somewhat 34 backward 34 as the u03b4t axis represented in this figure indicates the value of the reference point speed n t could also be expressed by making direct use of a function of time representing the values to be obtained by way of example the following function makes it possible to obtain values similar to those given in the table of fig5 equ1 where u03b4n represents the discrepancy between the engine speed at the moment t and the final reference point speed n it will be understood that the decrease in reference point speed is initialized at the moment t so as to change in accordance with the time law n t fixed by the table regardless of any previous changes see graph curves a or b fig4 in the actual engine speed from the moment t onward the reference point speed enters a transition phase during which the computer uniformly decrements the value of u03b4n in accordance with the table of fig5 until the moment t for which u03b4t 0 and n n 700 rpm for example the means.