Diesel locomotives for branch line operations

Diesel locomotives for branch line operations

By Mr. ZENS (Director) and Mr. HONNORÉ (Deputy Director) of the Compagnie des Chemins de fer Départementaux.

The operation of branch lines is generally considered to be responsible for part of the railways' deficit, although some authors have drawn attention to their role in the traffic carried by main lines.

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A traction unit solution that would reduce the deficit of the railways

The purpose of this note is to show how appropriate traction equipment should make it possible to reduce the cost price per tonne-kilometre on these branch lines.

The Chemins de Fer Départementeaux had this problem to solve; their research and experience led them to the following conclusions:

It is necessary that the purchase price of traction units for branch lines should be low enough to allow them to be amortised quickly, in about 10 years. Indeed, the evolution of technology is such that in 10 years the equipment is likely to become technically or economically obsolete, especially when compared with competing equipment used for road transport which, since it amortises quickly, is constantly being renewed and improved.

A success due to simplicity

To obtain this result, it is essential to use as much standard material as possible in the construction of these machines and to seek only simple solutions

What makes road transport successful is the simplicity of the means employed; road equipment manufacturers have made their fortunes and those of their customers thanks to this formula. For example, despite all the ingenious solutions that have been proposed to transmit engine power to the wheels, most heavy-duty trucks, which currently compete so fiercely with the railways on long routes, have gearboxes without synchronizers. The driver has to double-clutch when changing gears. And yet it is these simple, but well developed and practical solutions that are causing the railway, whose many technical achievements are incomparably better, to suffer the blows it is currently suffering.

This example shows that the problem is not to achieve technical performance, but to build with simple and honest means a fairly cheap equipment and a maintenance that is at the level of that which can be expected from a good garage.

The realisation of the 400hp Diesel tractor

It is to achieve this goal that the Chemins de Fer Départementeaux has, in order to make 400hp, 34-ton diesel tractors, been led to make them twin-engine (Fig. 1).

She used to equip them with one of the most powerful engines in France, built in "automobile" series: the 200hp Willème 8-cylinder Diesel engine.

The choice of a mechanical transmission

It has adopted the mechanical transmission and here it is necessary to justify this choice:

Mechanical transmission is the simplest, lightest, cheapest, and most easily repairable But to be suitable for track traction, it must be well adapted, well developed, and therefore not too powerful. For 200 to 300 hp it is now feasible and the chosen transmission has been tried and tested for a long time, since the first examples of this type were fitted 10 years ago on 200 hp single-engine tractors.

The mechanical transmission may not always be the one that provides the best performance but, as we have said, this is not the problem, on the other hand, it is the one that has the best efficiency. If it does not allow you to use all the power in all circumstances, it at least has the advantage of not consuming fuel and wearing out the engine only according to the power actually delivered. And in this respect it is worth recalling a notion that is all too often lost sight of: any engine is capable of delivering a number of kilogram-metres that is all the more important as it is not asked for them in too short a time, in other words, the more often it is asked for high power, the less work it will provide between 2 overhauls.

Fig 1. 400 hp, 34-tonne diesel locomotive for normal track use
Fig 1. 400 hp, 34-tonne diesel locomotive for normal track use

If you reduce the efficiency of a transmission by 10%, for example, you don't just increase fuel consumption by 10%: the extra horsepower that the transmission absorbs is the one that makes the engine the most tiring, and therefore by far the most expensive.

While the other types of electric or hydraulic transmission, which are less efficient, make it possible to use the full power of the engines at all times, this advantage is only obtained by accelerating their wear and tear. The horsepower gained in this way does not reach the rim completely: it is absorbed by the transmission to such an extent that, in order to cool it, it is necessary to provide installations capable of evacuating as many calories as the engine's radiators.

The real grievances made to the mechanical transmission are :

  • excessive dimensions for large powers ;
  • to the blows to the changes of speed.

In response, the engine flow was split over several mid-size powertrains, and automatic air control was used to alternate gear shifting between the different powertrains.

The pneumatic control that has been achieved, while remaining simple, gives good flexibility, continuous traction during gear changes and driving is as easy as that of an electric transmission.

This pneumatic control and the choice of engines and transmissions that do not exceed a unit power of more than 200 to 300 hp make it possible to build more powerful tractors with the same components of reasonable dimensions, as we will see later on.

source : Excerpt from the Revue Générale des Chemins de fer N° of September 1950

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