Evolution in the construction of diesel locomotives

Evolution in the construction of diesel locomotives

by Mr. ZENS, Director and Mr. HONNORE, Deputy Director of the Departmental Railway Company.

For many years, railway operators have been tempted to replace their steam locomotives, which have reached their wear limit, with diesel locomotives.

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The success of the Diesel

The financial success of these operations has been such that many operators are currently willing to their entire operations to diesel without waiting for their steam locomotive fleet to be fully amortised.

The advantages of thermal locomotives

The comparison of the two modes of traction confirmed that the advantages of locomotives powered by combustion engines were twofold:

  • In financial terms :
  • Significant reduction in labour costs and fuel savings.
  • Best use of equipment, always ready to go;
  • On the social plan :
  • These new machines avoid the handling of coal, the maintenance of a hearth that involves tedious and messy operations.

It is therefore superfluous to insist on the future of these new machines, which only find rivals in electrical machines when power can be supplied under very advantageous conditions.

Engine and transmission: a twofold problem

As far as the construction of diesel locomotives is concerned, two problems arose: the engine problem and the transmission problem.

The substitution of slow motors

Originally, diesel engines were "slow" and "heavy" because only the "slow" engines had the necessary resistance for railway operation ; but following numerous developments and studies from different parts of the world, the engine construction has been refined to the point where it is possible to reconcile the necessary robustness with increasingly higher power masses: the latest improvement is supercharging, which has increased these powers in significant proportions. As their power increased, their regularity of operation became more certain, as the protection of various wear parts became better and better, and their longevity increased day by day.

The technique is now sufficiently advanced to consider that an engine can remain in service until it has completed a specific task, which can be measured in particular by the number of litres of fuel consumed.

It should be noted that the distinction made between "slow" and "fast" engines was based solely on the notion of the number of crankshaft revolutions. However, there are currently "slow" engines whose piston speed is of the same order as that of the "fast" engines whose pistons are nevertheless lighter and less tiring to the shaft line.

Manufacturers who originally relied only on "slow" engines found it very difficult to transmit power to the locomotive axles.

The transmission for diesel locomotives

Any mechanical transmission system which, at first glance, would have seemed the simplest, proved to be almost unfeasible, due to the considerable torques to be transmitted.

As a result, they had to resort to a bias by using an electrical transmission. This consisted, as everyone knows, of using the power of the combustion engine to drive an electric generator, this generator feeding traction motors attacking the different axles of the locomotive.

This solution proved to be particularly interesting because it solved all three problems:

  • power distribution to the axles ;
  • starting flexibility ;
  • reversing.

It was sturdy but even heavier as the diesel engine was slow.

However, the cost of this manufacture was high, but the most serious disadvantage was and still is in the efficiency which, despite different control procedures, is hardly more than 80% overall.

The hydraulic torque converter

A problem converter as a solution?

As a result, other solutions were sought and the appearance of the hydraulic torque converter caught the attention of railway equipment manufacturers.

This converter, whose price and weight were more advantageous than those of the electric transmission, has unfortunately not been more efficient than the latter until now. The simplicity of its operation has however favoured its success.

The problem of the converter

However, this in turn posed a problem: transmitting the driving torque to the different axles, which it multiplied in a high ratio , which obliged the locomotive manufacturer to add a mechanical transmission subject to considerable torques, the work of which must be distributed equitably to the different driving axles.

These two modes of transmission had one thing in common: they are capable of continuously absorbing the power of the engines. As a result, to avoid fatigue, engine manufacturers have to set them at their continuous power about 10 % lower than the hourly single power.

The characteristics of diesel engines are such that, in their operating regime, they have a torque that varies relatively little and this operating range allows only relatively small variations in speed. In addition to the difficulty of distributing the driving force to the axles, they therefore pose two problems: that of starting and that of varying the vehicle's speed, which leads to the notion of gear changing.

These two problems have been solved for the electric transmission at the cost of an installation whose disadvantages we have just indicated above; and have also been solved for the hydraulic transmission at the cost of a number of artifices consisting in particular of using several torque transformers and using them alternately in order to maintain acceptable efficiency.

These hydraulic torque transformers are known to have efficiencies of up to 80% only in a small area of their use.

Therefore avoid using them for a long time outside this zone, and this can be achieved either by "stepping" them up or by adding a gearbox to them.

The mechanical transmission, which theoretically should have by far the best efficiency, was difficult to achieve for the big powers because it had to have, in the first place, a sliding device allowing starting. Although mechanical friction clutches have been perfected, the size and duration of the slip was such that wear on these components was prohibitive and required great dexterity on the part of the operator.

The manufacturers most favourable to the mechanical transmission have been led for starting operations to use the hydraulic coupler which transmits the engine torque completely and allows a rather prolonged slip without destruction of material.

The hydraulic coupling having, when "hooked up", a good efficiency (of the order of 98%), the sacrifice to be made on efficiency was quite acceptable.

Locomotive diagrams
Locomotive diagrams

The problem of speed variation and transmission of engine power to the various axles

The problem of the speed variation and the transmission of the driving force to the different axles had to be solved.

Speed variation

Speed variation was first attempted by using different gear trains, the engagement of which was facilitated by synchronization devices. These devices proved to be delicate for high power, but they presented a more serious disadvantage since the tractive effort was broken during the "gear change" operation. This variation in tractive effort produces ripples between cars that are more dangerous the heavier the trains and the longer the duration of the break in effort.

Some manufacturers have therefore imagined to shorten this duration by pre-selecting gear changes, for example, by "jumping" the power from one shaft to another by means of two clutches, one of which comes into action as soon as the other is released. This artifice reduces the breaking of the effort to a very short time, but by acting in this way, ? if the mass of the convoy keeps its speed, ? the instantaneous speed of the engine and part of the transmission vary brutally and these parts are subjected to overloads which have been the cause of the little success of this system.

The construction of mechanical transmissions is still faced with the difficult problem of continuity of effort, as the transmission of high power to the axles can always be solved at a reasonable price and weight.

As a result of the above considerations, it can now be said that the diesel locomotive problem is solved in a certain way regardless of whether the engine is "slow" or "fast" by using either electric or hydraulic transmission; the latter, however, presenting the expensive and difficult problem of transmission to the various axles with high torque.

The question of the day is no longer how to run diesel locomotives since this is common today, the question is how to improve them.

As the concept of long write-offs that had prevailed at the origins of the railway had disappeared, their purchase price had to be reduced so that they could be written off in a reduced number of years.

We must try to better adapt their power to the varying demands placed on them.

Their fuel consumption must be decreasing.

Maintenance costs must be reduced as well as the duration of immobilisation.

In order to reduce the purchase price of these machines, it is essential, on the one hand, to simplify their design and, on the other hand, to produce them using large series parts as much as possible.

In order for a machine to be simple, it seems necessary that it should be made up of parts of normal dimensions that can be produced in "general mechanics" workshops, not being subjected to exceptional work; it is therefore necessary to eliminate, in particular, all gears and transmission shafts subject to considerable torques. We consider, for example, that it is a simplification to attack the N axles with individual mechanisms that will be put into service according to the needs of the moment rather than with a single mechanism capable of transmitting the total power.

With regard to engine power, it is easy to see that engines with exceptional power are more expensive to buy per horsepower than standard engines, and that it is cheaper to buy more engines than to use a single high-powered engine.

The design of the axle or bogie power train is well suited to the fact that most operators require different locomotive horsepower requirements, and that the same power trains could equip different types.

If an operation needs machines with 250, 500 and 1000 HP, it seems obvious that it is in its interest that its "fleet" is made up of single, twin and four-engine engines.

With regard to adapting their power to the forces required, the design of a single-engine machine using its maximum power only under certain circumstances does not seem to lead to the best efficiency. A multi-motor machine where the individual drive elements can be put into operation almost instantaneously is less expensive in fuel and wear.

This idea was obviously inadmissible for steam engines, as it was inconceivable to equip them with several boilers and to have them come on demand.

By studying the efforts required from a locomotive during its use, we can see that these are of an almost constant value for quite long periods of time.

This is primarily due to the railway infrastructure that has been built to allow the use of steam engines. This is primarily due to the railway infrastructure, which was built to allow the use of steam engines. As far as possible, tracks were therefore laid out along the watercourses in order to avoid major differences in level, and when it was necessary to pass from one valley to another, major work was undertaken, establishing ramps that were as gentle as possible and therefore long and regular.

In the case of a line of valleys, a great deal of power is required at start-up and to maintain line speed, power can be significantly reduced.

For the other lines, a very high power is required for a fairly long time to climb the ramps, then in level flight it takes much less and to descend it takes no more power at all.

Therefore, it seems to be advantageous to distribute the maximum power among various drive elements that can be operated independently of each other.

The variable power of a locomotive is all the more appropriate since it is essentially required to tow variable loads.

We therefore believe it is advantageous to take advantage of the fact that diesel locomotives can easily be made to have variable power and grip, with the operating times at constant speed being long enough to start or stop part of the power train.

The following advantages result from the above:

  • fuel consumption can be significantly reduced ;
  • the wear of parts in service can be reduced ;
  • one can reduce the purchase price for any given available power, since it can be obtained by multiplying a single power unit and can benefit from "series" prices;

And you get:

  • a reduction in maintenance costs since the same components will have to be overhauled in all cases, whatever the power of the locomotive concerned, and these units are small and require only a small amount of tooling;
  • a significant reduction in the downtime of the entire locomotive, as the power units can be exchanged for overhaul on all types of machines in a very short time, and in particular on poly-engines by exchanging only the units with the highest fatigue.

In support of the above proposals, we show :

Schematic layouts of locomotives of different power ratings consisting of the same power units, i.e.: 250 HP diesel engine at 1500 rpm, hydraulic coupler, 6-speed synchronised gearbox, axle attack calculated for a grip of about 30 t.

The 250 HP, 30 t tractor has no special features. The coupler ensures starting and the power of this machine is not sufficient to ensure that breaks in the hook force during gear changes cause violent reactions on the couplers.

It should be noted, however, that with a mechanical transmission, it is essential that train reactions cannot influence engine speed; the transmission must therefore not be reversible and this is easily achieved by interposing a freewheel.

This freewheel between the gearbox and the axle drive also has the great advantage of considerably increasing the speed of gear shifting.

In the case of locomotives with several engines, the couplers provide the required flexibility during starting and, in order to ensure continuous traction during gear changes, all that is required is for the device that controls them to act at different times on the different gearboxes. All but one of the engines therefore remain under load at all times. The result is a continuous effort on the towing hook without any artifice or complication.

This device, thanks to the presence of the free wheels, does not present any danger to the engines, because at no time can we fear the overspeed of the engines.

These freewheels allow either to voluntarily isolate the engines whose power is not necessary for a certain time, or to allow safety devices to immediately stop an engine which would not be in good operating conditions. The safety devices in question are, moreover, very simple since they cut off the injection as soon as the oil pressure is no longer sufficient or the water temperature is too high; the engines can thus be left unattended with total safety.

To date, tests have been carried out on twin-engine engines built according to these principles, and the advantages we have outlined have been demonstrated.

The normal track test locomotives were of two types:

1°) Two-axle locomotives.

  • Operating weight: 34 t ;
  • Wheelbase: 4.92 meters;
  • Length out of buffer : 10m;
  • 2 "Willeme" engines, 8 cylinders, 200 HP, 1500 rpm ;
  • Vulcain Sinclair" couplers, 23?;
  • Dual-plate clutches;
  • Synchronized gearboxes, 6 speeds :
  • Rod Freewheels, calculated for a torque of 500 m/kg ;
  • Glaenzer" 1600 cardan shafts between clutches and gearboxes;
  • Glaenzer" 1800 gimbals between boxes and decks ;
  • Bridges wedged on each axle, are with triple reduction;
  • They have 3 bevel gears at the entrance forming a reverser;
  • This reverser is pneumatically controlled;
  • All the axles of the cranes are mounted on ball or roller bearings
  • 170 mm axles ;
  • The axleboxes are roller bearing and sealed.
Fig. 2. two-axle locomotive, 34 Tons, see normal
Fig. 2. two-axle locomotive, 34 Tons, see normal

Pneumatic control is by means of a rotary distributor and acts as follows:

The handwheel controls cams acting first on a clutch.

When disengaging the clutch, the engine is simultaneously switched to idling.

Once this operation has been completed, the distributor releases the cylinder controlling the speed that was engaged and this returns to neutral and then, through a second light, the compressed air acts on the cylinder corresponding to the speed to be obtained.

An interlock prevents the clutch from operating until the gear is shifted. Once this is done, the clutch engages again and the engine resumes the speed given by the position of the accelerator.

There is no disengagement pedal, because the throttle lever common to both engines has a stop in the idle position that operates a valve, putting the transmissions in neutral.

Driving is therefore very easy since to start the train, having set the steering wheel to 1re or 2nd gear, if the train is light, all you have to do is press the throttle lever to operate the stop, which thus "arms" the transmission by introducing air into the cylinders corresponding to the chosen speed.

Fig. 3: Two-axle locomotive, 34 tons, normal track
Fig. 3: Two-axle locomotive, 34 tons, normal track

So all you have to do is raise the throttle to increase the injection of the engines, and thus their power and speed.

Then, just maneuver the steering wheel to get the desired speed, without ever worrying about the clutches.

To stop, push the throttle lever all the way down.

The transmissions are in neutral and the engines idle.

The locomotive engineer only has to apply the brake to bring the vehicle to a stop.

Since the engine and gearbox are protected by the freewheels, no false manoeuvres can be made and at no time can a noisy gear shift be heard and it is permissible, regardless of the speed of the train, to engage any gearbox scale, thus "preparing" one's speed for a "pickup".

This control has been tested for four years on four locomotives on both standard and metric track and has shown excellent gearbox and clutch performance.

The compressed air system is maintenance free. The cylinders are automotive standard air brake cylinders. These are perfectly tuned since they are safety devices on the road.

The results of the tests, totalling several hundred thousand kilometres, have made it possible to build heavier and more powerful locomotives.

2°) Bogie locomotives.

To date, three 52 t bogie machines with 400 and 500 HP are in continuous operation.

They have the following characteristics:


  • total length over the buffers of the locomotive :11m 030;
  • truck centre distance: 5m 250
  • wheelbase of the bogies; 2 m 000;
  • overall width: 2 m 800;
  • diameter of the wheels at the bearing: 0 m 850;
  • height of the locomotive : 3 m 600 approx.

These locomotives fit into curves with a radius of 50 metres.

Gearbox ratios.

(2 variants)

1°) 4-speed gearbox :

  • 1re speed : 8.4 km
  • 2e » 11,8 »
  • 3e » 17,3 »
  • 4e » 25 »

2°) 6-speed gearbox :

  • 1re speed: 8,8 km
  • 2e » 13 »
  • 3e » 18,6 »
  • 4e » 26 »
  • 5e » 38 »
  • 6e » 54.5 »
  • (1500 rpm)


The locomotive frame consists mainly of 2 x 500 I-beams, braced, and has a sturdy box section and sheet metal box at each end, riveted together.

Bogies and suspension.

The trucks are of the "DIAMOND" type with a bolster supported by 8 coil springs.

Traction and braking forces are transmitted by connecting rods articulated on silent-blocks.

The "S.K.F." roller axle boxes are interior.

The axles are coupled by bronze bush-mounted connecting rods.

The end axle is driven by a triple reduction reversing axle.

POYAUD" engines.

  • ? type: 6 PDI 8 V PX1
  • ? number of cylinders: 6 8
  • ? bore in mm: 150
  • ? stroke in mm : 180
  • ? Displacement in litres:... 19 25.33
  • ? rpm : 1500 1500
  • ? power in C.V.: ... 200/220 250/300
  • ? consumptions per C. V. :
  • Comb. in Gr.: . 165 165
  • Oil in Gr. 1 to 2
  • ? bare weight in kg: 1615 2100
  • ? cast iron cylinder block;
  • ? hardened cast iron wet liners;
  • ? one cylinder head per cylinder;
  • ? crankshaft connecting rod bearings made of leaded bronze ;
  • ? injection by P.M. fuel pump;
  • ? P.M. injector;
  • ? starting under a voltage of 24 volts by means of a "BOSCH-LAVA- LETTE" 6 H.V. starter;
  • ? 24-volt, 300-watt "BOSCH-LAVALETTE" dynamos.
Fig 4. Truck Locomotive, 52 Tons, Normal Track
Fig 4. Truck Locomotive, 52 Tons, Normal Track
Fig 4bis. Bogie Locomotive, 52 Tons, Normal Track
Fig 4bis. Bogie Locomotive, 52 Tons, Normal Track


Each engine is equipped with a tubular water radiator comprising 2 elements and a nurse, as well as an oil radiator. The latter is powered by a gear pump which pumps the oil into the engine crankcase. The suction hose does not plunge to the bottom of the crankcase to avoid emptying it in the event of a radiator leak.

They are calculated to ensure the cooling of the 200 or 250 HP "Poyaud" engines in conditions of maximum temperature of 35° in the shade.

Each engine drives by V-belts the cooling turbine of the corresponding radiator.


For each of the locomotive's power units, the transmission includes :

  • 1 hydraulic coupler ;
  • 1 clutch;
  • 1 gearbox with freewheel;
  • 1 axle drive.

The gearbox has 4 or 6 gears all synchronized.

All gears in the gearbox are straight toothed and ground. The gears are always in mesh.

The gears are mounted on the gearbox shafts by means of ball bearings.

The hydraulic coupling is of the "FERODO" 23? type. It gives the tractor drive, especially during manoeuvring, a flexibility comparable to that of the electric transmission.

The clutch interposed between the hydraulic coupling and the gearbox is of the "COMETE-MECANO" 14 L type. F. bi-disc.

A "GLAENZER" cardan shaft n° 1600 connects the clutch to the gearbox.

A "GLAENZER" cardan shaft No. 1800 connects the gearbox to the reversing axle.

The reversing axle, which is mounted on the axle opposite the engine, comprises a reversing mechanism consisting of three bevel gears, a first reduction gear stage followed by a second reduction gear stage, to which the ring gear is also attached to the axle.


The locomotive is equipped with a large cab with two sliding doors on the side with detent locks.

The windows of the " SECURIT " type are descending.

Windshields have an adjustable incidence.

In the cab are all the on-board instruments: pressure gauges, speedometer, ammeters, etc...

Driving can be done on either side of the cab depending on the direction of travel. Any manoeuvre started at one station can be completed from the other.

For this purpose, there is only one brake valve driven by a joystick located at each operator's station.

The visibility is such that the driver can effortlessly see all 4 buffers of his machine. This convenience has a significant influence on manoeuvring speed and personnel safety.

All cables are under flexible sheathing and metal trim.

500 hp diesel locomotive. Power available at the rim
500 hp diesel locomotive. Power available at the rim
  • Voltage : 24 volts ;
  • 2 lead batteries 300 amp/hour.

The lighting complies with S standards. N. C. F. Each end has two white headlights, and two lanterns on each side of the cab can provide either white or red lights at will.


The locomotive service brake is of the direct and automatic "WESTINGHOUSE" type.

It acts on all the wheels by a shoe controlled directly by individual cylinders, thus eliminating the need for any linkage adjustment.

The two MPP 1000 compressors are driven by means of "TEXROPE" belts through the pulley located in front of the crankshaft of each engine.

A handbrake wheel is used to stop the locomotive.


The equipment includes 4 or 8 pneumatically operated gravity sanders, which sandblast each bogie.


An 800-litre fuel tank is located under the cab.

For each engine, a filter, whose filtering material consists of easily replaceable special fabric gaskets, ensures a very effective purification of the diesel oil.

These locomotives, specially built for hard maneuvering and traction of freight trains on secondary lines, are capable of the performances specified by the following curves :

(3) Locomotives under construction.

The performance obtained by the machines in service highlighted the good efficiency of the transmissions, the ease of operation and maintenance and the low manufacturing cost.

Under these conditions, Ateliers C. F. D. Workshops started the construction of a 800/1000 HP machine type.

The previous diagram shows us the arrangement of the main components.

As said before, these components are identical to those of the twin engine.

The four engines are 8-cylinder "POYAUD". This machine can reach 80 km/h, and with ballast can be loaded up to 75 tons.

The calculated performance is as follows:


Diesel multiple-engine locomotives with hydromechanical transmission with continuous traction, allow the necessary power to be adapted to the rhythm required by the operation, limiting consumption and wear of the driving part.

This process makes it possible to build:

  • the locomotives of different powers in a "Park", all with the same main components;
  • their purchase and their maintenance thus benefit from "series" prices;
  • the "continuous traction" is obtained without artifice or loss of efficiency.

They promise to be economical in construction, consumption and maintenance.

source : Extract from the International Association of Public Transport - Brussels Vol. III. NO. 3 - X. 1954

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