PETROL-ELECTRIC VEHICLES AND MAIN MARINE PROPULSION BY ELECTRICITY
Between the petrol-driven vehicle and the electric automobile there is
an interesting series of links provided by 'petrol-electric' systems.
At one end of the chain, electricity plays an important part in
supplying power to drive the car. At the other end, electrical
apparatus is introduced merely as a form of transmission gear between
the petrol engine and the driving axle. The reason for attempting
the petrol-electric combination will be most readily understood by
considering the latter arrangement first.
The petrol engine is a high-speed engine, capable of working most
satisfactorily when it runs at a uniform rate with a constant load.
On the other hand, the speed of the driving axle of a car varies from
a very much lower speed down to zero. It is therefore necessary, when
driving a vehicle with a petrol engine, to arrange some forms of
variable speed-reducing transmission gear between the engine and the
driving axle. The problem is further complicated by the fact that the
petrol engine is irreversible, has practically no 'starting torque,'
and has a very slight overload capacity. It has to be started running
'light' and then switched on to a low gear which gives sufficient power
to overcome the inertia of the car. As the speed of the car rises,
there have to be successive changes of gear. These difficulties are, of
course, accentuated when dealing with the heavy weight of an omnibus.
[Illustration: Fig. 9. Elevation and plan of a petrol-electric motor
omnibus equipped by W. A. Stevens, Ltd. Directly behind the front
wheels is the petrol engine, driving a dynamo through a flexible
coupling. The dynamo supplies current to the motor directly behind
it; and the motor drives the rear wheels through a cardan shaft. The
transmission of power between the engine and the shaft is electrical
at all speeds.]
Practically all the troubles with petrol motor omnibuses have resided
in the gear; and even the most ardent enthusiast for the all-electric
faith must admit that the motor engineer has overcome these troubles
(in great part if not wholly) with remarkable skill and ingenuity.
But the complications of an adjustable mechanical bridge between a
high-speed engine and a varying low-speed axle are so great that
an electrical bridge was proposed as a substitute. By coupling the
engine direct to a dynamo and by using the current so generated to
drive variable-speed motors geared to the driving axle, the electrical
engineer hoped to get better working results from the petrol motor than
could be obtained with any mechanical transmission gear.
The most conspicuous advantage, apart from the quietness of running
at all speeds, lies in the ease and smoothness with which the
petrol-electric motor can start and gain speed. In this respect the
combination system is practically on the same level as (or even
superior to) the electric tramcar or the electric automobile. There
is an entire absence of the jerks and jarring noises which usually
accompany the starting of a motor omnibus. The same facility of control
is of advantage in adjusting speed to suit the other traffic on the
road, and also in negotiating hills.
In one class of petrol-electric vehicles the electric transmission gear
is continuously used. In another, it is used at all speeds except the
highest, when the engine is coupled directly (by a magnetic clutch)
to a mechanical driving gear. In a third class the arrangement is
more complicated, as it involves the use of storage batteries as an
auxiliary to the power provided directly by the petrol engine. The
Fischer type of petrol-electric vehicle uses electric transmission
solely and has a fairly large battery to supplement the engine-produced
current when steep hills are being negotiated. At ordinary speeds on
level roads the surplus power produced by the engine goes to charge the
battery.
The 'Automixte' type is peculiar in using the mechanical transmission
gear all the time. The dynamo coupled to the engine supplies current to
a small battery when surplus power is available; the same dynamo may
be driven as a motor by current from the battery when such assistance
is wanted at starting or on steep hills. The electric part of the
equipment thus acts first as a generator and then as a motor, the
change taking place automatically.
These different petrol-electric devices are very attractive from the
engineering point of view, but at the present time it is uncertain
whether they will realise the hopes of their inventors. The additional
weight of the electric equipment is against them; and in some cases
there appears to be a lower all-round efficiency. So that the
motor-omnibus world, as a whole, continues to fix its faith upon the
improved forms of mechanical transmission.
The underlying idea of the petrol-electric system has, however, been
suggested for marine propulsion with a somewhat better prospect of
success.
There is a partial analogy between the conditions of motor omnibus
working and of ship propulsion with turbines. The steam turbine
is, like the petrol engine, essentially a high-speed machine. The
screw propeller, on the other hand, works most efficiently at low
speeds. Therefore the marine engineer has to try and find some
common denominator between an engine which runs most efficiently
at high speeds and a propeller which is at its best when revolving
comparatively slowly.
[Illustration: Fig. 10. Diagrammatic section of a steamship which
has been 'converted' from the ordinary method of propulsion to the
'Paragon' system of electric main marine propulsion. The reciprocating
engine has been replaced by a steam turbine, coupled direct to an
electric generator which supplies current to a motor attached to the
propeller shaft. The tests carried out with this vessel will indicate
the advantages of the electric method of propulsion even with the
usual long length of shaft. The vessel has a gross tonnage of 1241,
and its speed is 9 knots. The engines replaced ran at 78 revolutions
per minute and gave 500 brake horse power. The turbine now installed
runs at 2500 r.p.m., and develops 630 brake horse power. (Illustration
reproduced by courtesy of _The Electrician_.)]
The gulf between the two has been narrowed by the improved design
of propellers. Some engineers assert that continued improvements will
bridge the gulf completely. Others have sought the solution in the
same way as the motor engineer--by the use of mechanical change-speed
gears. The suggestion has also been made to employ hydraulic gear as
an intermediary; and in some recent vessels reciprocating engines with
comparatively low-speed turbines driven by exhaust steam have been
adopted.
In the electric system the turbine is coupled direct to an electric
generator and may run continuously at the highest economical speed.
The propeller shaft may be quite short and is driven by a slow speed
motor connected by cables to the generator. Various arrangements for
controlling the supply of current to the motor (with appropriate
design of generator and motor) have been devised by Mr Durtnall, Mr
Mavor, and other workers in this field; but whatever the details of
these arrangements may be, they all give a wide range of speed both
ahead and astern. The direct drive with the steam turbine has really
only one speed--full speed ahead; and as the turbine is irreversible,
'astern' turbines have to be installed in addition. These limitations
and complications are removed entirely when electrical transmission is
adopted.
Moreover, the electric system can be so arranged that the control gear
may be operated from the bridge itself. The facility in manoeuvring is,
in fact, so marked that it would recommend electric marine propulsion
even if that system offered no advantages on the score of economy
in weight, space, and steam consumption over the existing systems.
The steam turbine, it may be noted, has been adopted so far only in
high-speed vessels; and it is generally recognised that its extension
to vessels which run at 12 or 16 knots depends upon its adaptation to
slow-speed propellers. Advocates of electric marine propulsion claim
that they hold the most efficient solution of this problem.
It may also be pointed out that a considerable section of marine
engineers look forward to the use of internal combustion engines
(driven by oil or gas) on board ship. For naval purposes especially
it would be a great advantage to do away with funnels and so leave
the decks more free for gun mountings. As internal combustion engines
are irreversible, the electric system offers a means of escape from a
fundamental drawback to their use at sea. Here again the perfection of
manoeuvring power, especially with twin screws (either of which may
be controlled from the bridge through a wide range of speed ahead or
astern), gives the electric system a strong claim for consideration by
the naval authorities.
It is hardly necessary, except as a matter of curiosity, to refer to
the suggestions made, from time to time, of accumulator-driven ocean
steamships. Some wonderful pictures have been published of large
vessels with tons of ballast in the form of storage batteries. They are
likely to remain in this ideal condition, for although the driving of a
large vessel by stored electricity is quite possible, it is also about
the most expensive method which has ever been proposed.
Electric power from storage batteries has been used as an auxiliary
in the propulsion and manoeuvring of submarines. In aerial navigation
electricity has so far been employed to a very limited extent. Small
airships have been designed to carry electric accumulators connected
with various motor-driven propellers for raising, lowering, going
ahead or astern, and steering. The switches which control the passage
of the current to these propellers are connected with a wireless
telegraph receiver, so that each operation may be started or stopped
by a particular ether wave or series of waves. Demonstrations of such
'wireless-controlled' airships have been given in theatres; their
field of usefulness, if any, is in connection with war on land or sea.
Whether they will have any better fate than other devices for dropping
bombs over the enemy's camps or ships remains to be seen.
One inventor has, I believe, suggested a means of direct electrical
propulsion for aeroplanes, the current being derived from a
petrol-driven generator and carried to motors attached to propellers so
arranged as to give certain advantages in stability and manoeuvring.
As yet, however, the probability of electricity being applied to
locomotion in the air as well as on land and on sea is somewhat remote.