140 m.p.h. Without Moving
Page 62
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rxA VEHICLE test dynamometer having a maximum absorption capacity approaching 400 h.p. has been installed by Heenan and Fronde, Ltd., at the engine laboratory of the Associated Ethyl Co., Ltd., Bletchley, Bucks. Its purpose is to enable much of the vehicle test work normally carried out on roads to be conducted under laboratory-controlled conditions.
The primary use of the facilities offered by the plant will be for investigation into the matching of gasoline fuels and engines in the search for greater economy without loss of power by increasing compression ratios and modifying combustion chamber shape and so on. Speeds of up to 140 m.p.h. can be accommodated with the maximum power absorption, which is also available down to the equivalent of some 10 m.p.h.
By means of a control panel, the eddycurrent dynamometer characteristic can be changed from that of constant speed to propeller or cube law to represent the effect of vehicle mass when accelerating. To increase this effect, disc weights can be added to the dynamometer shaft, the total maximum inertia -effect obtainable from the two combined being equivalent to a vehicle weight of some 7,000 lb.
The functional part of the dynamometer is located below floor level. The only parts protruding above are the duct for cooling air, which can reach a maximum velocity of 80 m.p.h., and the 4-ft.diameter rear-wheel power-take-off drums, carried on an Eaton two-speed axle.
This axle is freely located in trunnion bearings and restrained by a torque arm which transmits movement to a balance in the control cubicle. This enables the power being absorbed to be measured.
What is normally the input shaft of the axle is used to drive the 300-h.p. eddycurrent dynamometer at speeds of up to 6,000 r.p.m. The ratio used in the axle and the speed of the shaft are determined by the vehicle weight and power. The maximum imposed load on the axle can be 8 tons.
The free end of the dynamometer shaft is connected to a similar two-speed axle, which, like the power-take-off axle, has its differential locked, but is used as a driving member for the wind-producing fans. The larger of these absorbs up to 90 hp. and is used for most tests, but a smaller fan absorbing only 74 h.p. can be employed for tests with small vehicles.
In addition to the uses mentioned, the equipment enables transmission friction losses and engine pumping losses to be determined by switching over rapidly from power absorption to motoring at a given speed.
So far it has been found that Michelin X tyres give the closest co-relation between road-test performance results and those measured on the dynamometer, because the number of revolutions per mile is identical both on the road and the machine.