In the Fell locomotive, however, the four main drive engines received their charge from Roots blowers driven by two further auxiliary engines which were governed such that when the traction power demand was more than minimal, they operated at essentially a constant speed. Since a Roots blower is a positive-displacement device, this meant that the mass flow rate at which charge was delivered to the main engines depended not on the speed of the main engines but on that of the auxiliary engines, so the power output of the main engines was essentially defined by the speed of the auxiliary engines.