speed until various limiting factors become significant. 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. Since the speed of the auxiliary engines was held constant, the main engines had a power curve which was constant with rotational speed; since power is the product of torque and rotational speed, the main engines were endowed with a torque curve inversely proportional to speed, producing maximum torque at a low speed and reducing as the speed increased. Thus the necessary increased low-speed torque output for starting and hill climbing was provided. Withdrawal In July 1952, 10100's gearbox was severely damaged after a loose bolt fell through the geartrain, and the locomotive was out of service for over a year. British Railways subsequently lost interest in the project, and an improved version of the locomotive under development was abandoned. 10100 remained in service until 16 October 1958, when its steam heating boiler caught fire at . It was returned to Derby Works, where it was slowly stripped of parts before being scrapped in July 1960. See also Paxman Hi-Dyne engine References Further reading External links Photo of 'The Fell Locomotive - 10100' Richard Carr's Paxman History Pages Photo of Lt. Col. L.F.R. Fell with Sir Harry Ricardo Technical details of the locomotive 10800 Experimental locomotives Individual locomotives of Great Britain 4-8-4 locomotives Railway locomotives introduced in 1952 Supercharging Scrapped locomotives Unique locomotives Standard gauge locomotives of Great Britain 2′D2′ locomotives