Page 41 - Automotive Engineering

CH AP TER 2 .1 Measurement of torque, power, speed and fuel consumption

accidental shorting, and will take up valuable space. a compressor and a receiver of capacity at least as large as
Special transformer/rectifier units designed to replace that recommended for the engine in service.
batteries for this duty are on the market. They will Compressed air or hydraulic motors are sometimes
include an ‘electrical services box’ to provide power in used instead of electric motors to provide cranking power
addition for ignition systems and diesel glow plugs. In but have no obvious advantages over a d.c. electric motor,
large integrated systems there may be a bus bar system apart from a marginally reduced fire risk in the case of
for the d.c. supplies. compressed air, provided the supply is shut off auto-
The engine starter will be presented with a situation matically in the case of fire.
not encountered in normal service: it will be required to In Chapter 2.1a, attention is drawn to the possibility
accelerate the whole dynamometer system in addition to of overloading flexible couplings in the drive line during
the engine while a ‘green’ engine may exhibit a very high the starting process, and particularly when the engine
breakaway torque and require prolonged cranking at high first fires. This should not be overlooked.
speed to prime the fuel system before it fires.

2.1.10.3 Non-electrical starting systems 2.1.11 Choice of dynamometer

Diesel engines larger than the automotive range are Table 2.1-3 lists the various types of dynamometer and
usually started by means of compressed air, admitted to indicates their applicability for various classes of engine
the cylinders by way of starting valves. In some cases it is being tested in steady or mild transient states.
necessary to move the crankshaft to the correct starting In most cases, several choices are available and it will
position, either by barring or using an engine-mounted be necessary to consider the special features of each
inching motor. The test facility should include type of dynamometer and to evaluate the relative

Table 2.1-3 Dynamometers: advantages and disadvantages of available types

Dynamometer type Advantages Disadvantages
Froude sluice plate Obsolete, but many cheap and reconditioned Slow response to change in load. Manual control not
models in use worldwide, robust easy to automate
Variable fill water brakes Capable of medium speed load change, ‘Open’ water system required. Can suffer from
automated control, robust and tolerant of cavitation or corrosion damage
overload. Available for largest prime-movers
‘Bolt-on’ variable fill Cheap and simple installation. Up to 1000 kW Lower accuracy of measurement and control than fixed
water brakes machines

Disc type hydraulic Suitable for high speeds Poor low speed performance
Hydrostatic For special applications, provides four quadrant Mechanically complex, noisy and expensive. System
performance contains large volumes of high pressure oil
d.c. electrical motor Mature technology. Four quadrant performance High inertia, commutator may be fire and maintenance
risk
asychronous motor (a.c.) Lower inertia than d.c. Four quadrant Expensive. Large drive cabinet needs suitable housing
performance
Permanent magnet motor Lowest inertia, most dynamic four quadrant Expensive. Large drive cabinet needs suitable housing
performance. Small size in cell
Eddy current Low inertia (disc type air gap). Well adapted to Vulnerable to poor cooling supply. Not suitable for
computer control. Mechanically simple sustained rapid changes in power (thermal cycling)
Friction brake Special purpose applications for very high Limited speed range
torques at low speed
Air brake Cheap. Very little support services needed Noisy. Limited control accuracy
Hybrid Possible cost advantage over sole electrical Complexity of construction and control
machine

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