contributes towards reducing control signal dutyv here equal to acceleration output signal dutyv1 derivative branch 24 therefore provides for reducing the value of control signal dutyv as of the instant in which the supercharge pressure derivative exceeds the predetermined threshold i e in advance of the instant in which the desired pressure value is reached the reason for this advanced correction of control signal dutyv is that despite proportional signal u 1 generated by proportional branch 23 eliminating its contribution to control signal dutyv when the desired supercharge pressure value is reached the supercharge pressure continues rising on account of the large amount of residual energy contained in the exhaust gas an important point to note is how deriving block 27 of mixed pd control group 21 operates directly on pressure signal psmis proportional to the measured supercharge pressure as opposed to first error signal err proportional to the pressure error moreover deriving block 27 represents the most suitable performance estimator for faithfully modelling the increase in supercharge pressure of turbodiesel engines with a variable geometry turbine and is therefore the best type of control for generating derivative signal u 2 used for advanced correction of control signal dutyv as soon as the supercharge pressure stops rising or after a predetermined maximum time control is restored to pi control group 22 which operates in the normal manner to ensure the precise desired supercharge pressure value is reached a return to proportional integral control requires that the initial state of the integrator in the integral block be supplied i e that pi control group 22 be supplied with an integral contribution value i plausible with previous control developments more specifically the contribution value.