The major elements of a mobile machine - engine and drives - are a crucial subject of control. Implementation of electric drives in mobile machinery is an emerging field of research and industry. Apart from conventional mechanical drives, they allow for wide control and power management possibilities. This paper considers a tractor-implement as a mobile power grid and addresses the problem of smart engine power management for sustainable operation by using power feedback of electric drives. The main focus lies in providing stable operation of the electrically driven implement. This is achieved by an engine power reserve dynamically adjusted according to power demands and their rate of change. To quantitatively evaluate the power demands, a supervisor approach based on the fuzzy logic theory is suggested. The supervisor has the structure of the Mamdani fuzzy system and computes the amount of the diesel engine power reserve as output. An adaptively changing power reserve providing sustainable operation is demonstrated in a partially stochastic simulation model of an electrified tractor with a rotary swather.