Page 40 - Automotive Engineering

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

In choosing a dynamometer for an engine or range of should be on the input side, otherwise it will be af-
engines, it is essential to superimpose the maximum fected by the torsional vibrations usually experienced by
torque– and power–speed curves on to the dynamometer dynamometer shafts. The motor may be mounted
envelope. See the example in Fig. 2.1-13 which dem- above, below or alongside the dynamometer to save cell
onstrates a typical problem: the hydraulic machine is length.
incapable of developing sufficient torque at the bottom The sizing of the motor must take into account
end of the speed range. the maximum break-away torque expected, usually es-
For best accuracy, it is desirable to choose the smallest timated as twice the average cranking torque, while the
machine that will cope with the largest engine to be normal running speed of the motor should correspond to
tested. Hydraulic dynamometers are generally able to deal the desired cranking speed. The choice of motor and
with a moderate degree of overload and overspeed, but it associated starter must take into account the maximum
is undesirable to run electrical machines beyond their number of starts per hour that may be required, both in
rated limits: this can lead to damage to commutators, normal use and when dealing with a faulty engine. The
overheating and distortion of eddy-current loss plates. running regime of the motor is demanding, involving
Careful attention must also be given to the arrange- repeated bursts at overload, with the intervening time at
ments for coupling engine and dynamometer, see rest, and an independent cooling fan may be necessary.
Chapter 2.1a. Some modern diesel engines, when ‘green’,* require
cranking at more than the normal starting speed, some-
times as high as 1200 rev/min, in order to prime the fuel
2.1.10 Engine starting and cranking
system. In such cases a two-speed or fully variable speed
starter motor may be necessary.
Starting an engine when it is connected to a dynamome- The system must be designed to impose the mini-
ter may present the cell designer and operator with mum parasitic torque when disengaged, since this
a number of problems, and is a factor to be borne in mind torque will not be sensed by the dynamometer mea-
when selecting the dynamometer. If the engine is fitted suring system.
with a starter motor, the cell system must provide the In some cases, to avoid this source of inaccuracy, the
high current d.c. supply and associated switching; in the motor may be mounted directly on the dynamometer
absence of an engine mounted starter a complete system carcase and permanently coupled to the dynamometer
to start and crank the engine must be available which shaft by a belt drive. This imposes an additional load on
compromises neither the torsional characteristics (see the trunnion bearings, which may lead to brinelling, and
Chapter 2.1a) nor the torque measurement accuracy. it also increases the effective moment of inertia of the
dynamometer. However, it has the advantage that
2.1.10.1 Engine cranking, no starter motoring and starting torque may be measured by the
motor dynamometer system.
An alternative solution is to use a standard vehicle
The cell cranking system must be capable of accelerating engine starter motor in conjunction with a gear ring car-
the engine to its normal starting speed and, in most cases, ried by a ‘dummy flywheel’ carried on a shaft with sep-
of disengaging when the engine fires. A four-quadrant arate bearings incorporated in the drive line, but this
dynamometer, suitably controlled, will be capable of may have the disadvantage of complicating the torsional
starting the engine directly. The power available from any behaviour of the system.
four-quadrant machine will always be greater than that
required, therefore excessive starting torque must be 2.1.10.2 Engine-mounted starter
avoided by an alarm system otherwise an engine locked systems
by seizure or fluid in a cylinder may cause damage to the
drive line. If the engine is fitted with its own starter motor on
The preferred method of providing other types of arrival at the test stand, all that must be provided is the
dynamometer with a starting system is to mount an necessary 12 or 24 V supply. The traditional approach
electric motor at the non-engine end of the dynamom- has been to locate a suitable battery as close as possible
eter shaft, driving through an over-running or remotely to the starter motor, with a suitable battery charger
engaged clutch, and generally through a speed-reducing supply. This system is not ideal, as the battery needs to
belt drive. The clutch half containing the mechanism be in a suitably ventilated box, to avoid the risk of

* A green engine is one that has never been run. The rubbing surfaces may be dry, the fuel system may need priming, and there is always the
possibility that it, or its control system, is faulty and incapable of starting.

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