Page 42 - Automotive Engineering

Measurement of torque, power, speed and fuel consumption CHAPTER 2.1

importance of these in the particular case. These fea- a separate starter to the dynamometer shaft. This
tures are listed in Table 2.1-3 and other special factors represents an additional maintenance commitment
are considered later. and may increase inertia.
7. Is there an adequate supply of cooling water of
2.1.12 Some additional satisfactory quality? Hard water will result in
considerations blocked cooling passages and some water treat-
ments can give rise to corrosion. This may be a good
reason for choosing d.c. or a.c. dynamometers,
The final choice of dynamometer for a given application
despite extra cost.
may be influenced by some of the following factors:
8. Is the pressure of the water supply subject to
1. The speed of response required by the test
sequences being run: steady state, transient, sudden variations? Sudden pressure changes or
dynamic or high dynamic. This will determine the regular pulsations will affect the stability of
technology and probably the number of quadrants control of hydraulic dynamometers. Eddy-current
of operation required. and indirectly cooled machines are unaffected
providing inlet flow does not fall below emergency
2. Load factor. If the machine is to spend long periods trip levels.
out of use, the possibilities of corrosion must be
considered, particularly in the case of hydraulic or 9. Is the electrical supply voltage liable to vary as
wet gap eddy-current machines. Can the machine the result of other loads on the same circuit?
be drained readily? Should the use of corrosion With the exception of air brakes and manually
inhibitors be considered? controlled hydraulic machines, all dynamometers
are affected by electrical interference and voltage
3. Overloads. If it may be necessary to consider changes.
occasional overloading of the machine a hydraulic
machine may be preferable, in view of its greater 10. Is it proposed to use a shaft docking system for
tolerance of such conditions. Check that the torque coupling engine and dynamometer? Are there
measuring system has adequate capacity. any special features or heavy overhung or axial
loads associated with the coupling system?
4. Large and frequent changes in load. This can give Such features should be discussed with the
rise to problems with eddy- current machines, dynamometer manufacturer before making
due to expansion and contraction with possible a decision. Some machines, notably the Schenck
distortion of the loss plates. flexure plate mounting system, are not suited to
5. Wide range of engine sizes to be tested. It may be taking axial loads.
difficult to achieve good control and adequate The supplier of any new dynamometer should offer
accuracy when testing the smallest engines, while an acceptance test and basic training in operation, cali-
the minimum dynamometer torque may also be bration and safety of the new machine. A careful check
inconveniently high. should be made on the level of technical support, in-
6. How are engines to be started? If a non-motoring cluding availability of calibration services, spares and
dynamometer is favoured it may be necessary to fit local service facilities, offered by the manufacturer.

means a simple matter. Incorrect choice or faulty system
2.1a. Coupling the engine design may give rise to a number of problems:
to the dynamometer
torsional oscillations;
vibration of engine or dynamometer;
whirling of coupling shaft;
2.1a.1 Introduction damage to engine or dynamometer bearings;
excessive wear of shaft line components;
The selection of suitable couplings and shaft for the catastrophic failure of coupling shafts;
connection of the engine to the dynamometer is by no engine starting problems.

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