Mobile Dynamometer for M.I.R.A.
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TO augment a conventional trailer-type vehicle dynamometer in use since 1949, the Motor Industry Research Association, Lindley, have put into service a large, self-propelled unit for testing heavy vehicles. The dynamometer is based on a Scammell Constructor 6 x 4 chassis powered by the Leyland 11.1-litre sixcylindered oil engine.
Twelve forward speeds are provided for the vehicle by the use of a six-speed main gearbox and a two-speed transfer gearbox, which also distributes the drive to the two rear axles. In addition, a power take-off gearbox is situated between the main gearbox and the clutch.
From the power take-off two generators are driven through chains. Each generator is capable of absorbing 100-125 tt.p. and their output is dissipated by four large resistors. A small aircooled oil engine is used to drive an auxiliary generator which supplies the current for the field ,windings of the main generators.
When in use as a dynamometer the driving clutch of the vehicle is kept permanently disengaged and the power take-off is brought into action. Therefore, the generators are driven direct from the road wheels through the transmission, which provides 12 speeds in this reverse condition also. Thus it is possible to adjust power absorption in two ways: by changing speed in the transmission and by alternating the field strength of the generators. The dynamometer is capable of absorbing 250 h.p. at road speeds from 3-30 m.p.h.
The drawbar fitted can be varied in height and incorporates strain gauges to measure pull on the dynamometer. A Wheatstone bridge and galvanometer system gives calibrated load readings, the maximum value for which is in the order of 20 tons. The readings arc presented
on instruments in the cab of the Scammell and telephone intercommunication is provided between the towed and the towing vehicles.
Concrete blocks are fitted as ballast to preserve tyre adhesion and the small auxiliary engine drives a compressor to retain brake power when the main engine is not running.
On test the dynamometer was towed by a special Thornycroft Mighty Antar with an up-rated engine, this vehicle being among the largest and most powerful likely to be encountered. It was found that the dynamometer could not halt the Thornycroft, although the Mighty Antar was unable to accelerate from a walking pace. The contest was declared a draw and a convincing demonstration of the new unit's ability to provide additional test facilities.