Page 25 - Automotive Engineering Powertrain Chassis System and Vehicle Body

2.1
Chapter 2.1

Measurement of torque, power,

speed and fuel consumption;

acceptance and type tests,

accuracy of the measurements

A.J. Martyr and M.A. Plint

2.1.1 Introduction almost invariably of the strain gauge type, together with
an appropriate bridge circuit and amplifier. The strain
gauge transducer or ‘load cell’ has the advantage of being
The torque produced by a prime mover under test is
resisted and measured by the dynamometer to which it is extremely stiff, so that no positional adjustment is nec-
connected. The accuracy with which a dynamometer essary, but the disadvantage of a finite fatigue life after
measures both torque and speed is fundamental to all the a (very large) number of load applications. The backlash
other derived measurements made in the test cell. and ‘stiction’-free mounting of the transducer between
In this chapter the principles of torque measurement carcase and base is absolutely critical.
are reviewed and then the types of dynamometer are The trunnion bearings are either a combination of
reviewed in order to assist the purchaser in the selection a ball bearing (for axial location) and a roller bearing
of the most appropriate machine. or hydrostatic type. These bearings operate under

2.1.2 Measurement of torque:
trunnion-mounted (cradle)
machines

The essential feature of trunnion-mounted or cradled
dynamometers is that the power absorbing element of
the machine is mounted on bearings coaxial with the
machine shaft and the torque is restrained and measured
T
by some kind of transducer acting tangentially at a known
radius from the machine axis.
Until the beginning of the present century, the great
majority of new and existing dynamometers used this F
method of torque measurement. In traditional machines R
the torque measurement was achieved by physically
balancing a combination of dead weights and a spring
balance against the torque absorbed (Fig. 2.1-1). As the
stiffness of the balance was limited, it was necessary to
adjust its position depending on the torque, to ensure W
that the force measured was accurately tangential. Fig. 2.1-1 Diagram of Froude type, trunnion-mounted, sluice-
Modern trunnion-mounted machines, shown di- gate dynamometer measuring torque with dead weights and
agrammatically in Fig. 2.1-2, use a force transducer, spring balance.
Engine Testing, 3rd edn; ISBN: 9780750684392
Copyright Ó 2007 Elsevier Ltd. All rights of reproduction, in any form, reserved.