A DIESEL OIL-
IN recent years the steam locomotive has experienced increasing competition, especially in connexion with the smaller trains and coach units, from the Diesel-
The oil engine is particularly suited for railway work abroad, in places where coal supplies are difficult to obtain and where oil fuel is more readily available. Apart from this advantage, however, there are more important reasons why the modern oil engine is superior to one of the steam type for the purposes previously mentioned.
In the first place, the type of oil engine used -
Despite this, the oil engine scores heavily in so far as fuel cost is concerned. And it has been the regular experience of users of Diesel-
Secondly, the Diesel engine for the same power output weighs only a fraction of the weight of a steam locomotive, for it does not require the relatively large boiler, firebox, and tender of the steam locomotive. For the same reasons it occupies only a small proportion of the space required by the steam locomotive; it can be stowed away in a comparatively small compartment or even installed underneath the coach of a rail-
The Diesel engine, on account of its lower weight and fewer units, is much cheaper in initial cost; indeed, it has been stated by one well-
Further, the Diesel type locomotive possesses the marked advantage of being able to start from the cold in a matter of seconds, whereas the steam-
Those of our readers with experience of petrol engines, such as motor-
Fourthly, the Diesel type locomotive requires much less room for the storage of its fuel than the steam type. Apart from the fact that it is not necessary to carry anything approaching the weight of coal of a similar output steam-
Despite its numerous advantages, the Diesel locomotive is inferior to the steam type in one important respect-
A further advantage of the steam-
EXTERNAL VIEW of a 130hp, six-
It is therefore necessary to transpose between the engine and the driving wheels some form of gearing or transmission which will reduce the speed but at the same time increase the driving effort -
There is one factor, however, where the steam engine will probably always score, and that is in its greater length of useful life and lower maintenance costs. Sufficient experience has not yet been acquired to show what is the useful life of a modern Diesel locomotive, or to give reliable maintenance costs, but it must be evident that this high-
Engineers are overcoming this wear problem in a satisfactory manner by arranging for the Diesel engine to be mounted in its cab or compartment in such a manner that it can readily and quickly be taken out, and another new or reconditioned engine substituted. The worn engine can then be dismantled and overhauled at leisure; in this way the locomotive need only be put out of service for a few hours while the engines are changed. Further, it is well known to Diesel engineers that the most serious items of engine wear are the valves, valve-
The troubles which were experienced in the early days of high-
The engineer and metallurgist have already made very big strides with the problem of lengthening the life of the Diesel engine; the results of their improvements have been to lower the maintenance costs and to reduce the risk of breakdown while in service.
There are two further points concerning the Diesel engine which have been the subject of criticism in the past; the objectionable exhaust fumes and the annoying “thump” or knock of this type of engine.
It is true that as recently as 1933 high-
SECTIONAL VIEW showing the interior of a 130-
Fortunately, intensive scientific investigation has led to the causes of both of these troubles being discovered. As a result, new designs of combustion chamber have more recently been evolved which entirely overcome these drawbacks. This statement will be appreciated when it is mentioned that at present there are several hundred motor-
A MODERN TYPE of British Diesel Locomotive, which was built at Leeds. The engine weighs 19 tons, is of 120 h.p., and was made for the standard gauge.
Although this type of engine owes its inception to the inventive genius of an Englishman, Herbert Ackroyd-
The Diesel engine differs from the steam engine-
Whatever type of engine is used, the higher the working temperatures and the average pressures in the cylinders or chambers of the engine, the greater will be the proportion of the fuel's heat available for useful work in the form of horse power. It is for this reason that modern steam engines of the reciprocating and turbine types endeavour to use high temperature -
The fuel used for modern Diesel engines is a moderately viscous heavy oil derived from the distillation of petroleum, known as Diesel oil. This fuel is heavier and has more “body” than either petrol or paraffin, yet will readily flow through small pipes.
The principle of the Diesel engine consists in compressing a charge of pure air to a very high pressure. This process of compression causes the air to become extremely hot; its temperature reaching 500 to 600 degrees Centigrade; these temperatures will be appreciated when it is stated that zinc melts at 419 degrees Centigrade, and aluminium at 659 degrees Centigrade. The fuel oil is sprayed into this highly-
The piston is at the bottom of its stroke, the cylinder being filled with air previously “sucked” in from the outside atmosphere. Next, the piston is made to ascend so as to compress this air into a very small volume; in Diesel practice the air at the end of the compression stroke occupies about one-
Usually, during the combustion process the pressure rises to 800 or 1,000 lb per square inch. This high pressure forces down the piston, while the highly-
THE IMPOSING EXTERIOR of a Beardmore high-
The next upward stroke of the piston is used for clearing the burnt gases out of the cylinder through an opening known as the exhaust port. Normally this is closed by the exhaust valve, but during the gas-
As the piston moves downwards once more the suction effect created causes the air from outside to flow into the cylinder, until the latter is completely filled with air at the same pressure as that outside, namely, atmospheric. After this, while the piston is at the bottom of its stroke the inlet valve is closed by suitable mechanical means, the exhaust valve still remaining closed. We have thus arrived at a similar state to that begun with, and the air is ready for its compression stroke; the cycle of operations is now complete.
It will be seen that each consecutive working cycle of the Diesel engine is made up of four working strokes, these -
There is only one working, or power, stroke in the four strokes that make up this cycle, the other three being non-
To overcome the somewhat jerky or erratic nature of the working, it is usual to fit the engine crankshaft with a flywheel. If the engine has only one or two cylinders a larger flywheel will be necessary than for engines having four or more cylinders.
The object of the flywheel is to store some of the surplus energy from the power stroke and to give this energy out again for the three non-
There is another type of Diesel engine possessing advantages over the four-
The general method of accomplishing this is to arrange for the piston when moving downwards on its expansion stroke to uncover near the bottom of its travel -
Near the top of this stroke fuel is injected and combustion takes place, followed by expansion of the hot combustion products during the ensuing expansion stroke: the cycle of operations is thus completed.
The fresh air charge is usually forced through the inlet port by means of a separate air compressor, and in greater quantity than is needed completely to fill the cylinder. Part of this air is therefore used for clearing out the exhaust gases remaining in the cylinder, a process known as “scavenging”. This air also serves to keep the cylinder temperatures down to reasonable values, for it must be remembered that with the two-
We have seen that it is necessary to provide a clutch and transmission system -
The first system is similar in principle to the transmission of the ordinary motor car. This possesses the drawbacks of giving only three or four steps in the speed variations, and in requiring a certain skill on the part of the driver in making the gear-
The hydraulic transmission method is a decided advantage over the former one, since it gives a smooth continuously variable tractive effort, and does not require a clutch. Moreover, it requires very little skill on the part of the driver in making starts and speed changes.
The electrical transmission method has been used in several successful railcars. It consists in coupling the engine to a dynamo, then using the current obtained from the latter to drive electric motors placed on or near the axles of the driving wheels and, in the latter instance, connected by gearing, in order to obtain the required speed reduction. The electrical method is a particularly convenient one to employ, as it gives a continuously variable tractive effort, and affords the driver easy control.
Another advantage lies in the fact that the losses of the system are mainly heat losses, the friction losses being at a minimum, so that there is a minimum of wear and tear, while at the same time the engine is protected against rail shocks. The question of supplying energy for the auxiliary services, such as train lighting and heating, brakes and similar items, is also readily dealt with by the electrical system mentioned; no additional equipment is therefore necessary as with other transmission systems. And there is another advantage; the Diesel engine can readily be started by using the dynamo as a motor, the current being obtained from the storage battery forming part of the electrical system.
The dynamo used for Diesel-
FOR THE RAILWAY IN INDIA. This small Diesel locomotive weighs 4½ tons, has wheels of 16 in diameter and an overall length of 10 ft 2 in. It can haul a load of approximately 105 tons on the level.
Another interesting transmission system is that used in the Associated Equipment Company’s original railcar. The latter is fitted with a 130 horse power high-
The latter consists of an outer aluminium casing secured to the engine crankshaft, and an inner “paddle wheel” unit secured to the gear-
[From part 10, published 5 April 1935]
“The Flying Hamburger” on this website.