The construction of modern diesel locomotives

A general arrangement of the bodywork

Diesel locomotives fall into two main categories in this respect:

• those where the engine equipment is housed in one or two hoods, which do not occupy the entire width of the chassis, the machine being driven from a single driving cab;
• those with an approximately parallelepipedal body, whose central compartment, which houses the engine equipment, is framed by two driving cabs.

Locomotives with equipment housed in one or two hoods

The first arrangement, with hood and single cab, leads to an economical design, and is generally adopted for all units of moderate power, with which it is possible to give the hoods sufficiently small dimensions to provide good visibility to the driver in both directions of travel. The arrangement with two driver's cabs is necessary for reasons of visibility when power becomes important, or when, as multiple unit operation is frequent, it is advisable to allow personnel to move from one unit to the other.

To better protect the driving crew in the event of a collision, and also for aesthetic reasons, modern two-cab locomotives generally have a small "nose" at each end, topped by the slightly recessed driver's cab.

In the United States, single-cab locomotives are widespread, even for power ratings in excess of 1500 hp. In this case, two-man operation, which is the rule in the United States, is necessary because of the large dimensions of the hoods, which limit visibility.

When, as is frequently the case in the United States, the traction of trains requires significant power, several units operating as a multiple unit are used; most often, only the head unit, or the two extreme units, are equipped with a driver's cab.

In the case of single-cab and hooded locomotives, the chassis alone contributes to the overall strength; it must be particularly robust and, due to its low height, relatively heavy.

Locomotives with a parallelepiped body

In these machines of the 2^th type, it is generally possible to the sides of the boxes to participate in the resistance of the assembly, and thus save weight.

The heavy element of the locomotive consists of the generating set(s), which must be subtracted by appropriate links from the inevitable bending of the chassis, generally more sensitive than in machines with parallel axles.

An arrangement of the running gear, locomotives with and without bogies

Diesel locomotives have followed a similar development in this respect as electric locomotives . Whereas the first diesel locomotives had parallel driving axles, framed by bogie or bissel steering axles, today we mainly encounter two-bogie units, generally of the B-B or C-C type, and, more rarely, of the A1A-A1A type. The latter provision is adopted when the axle load has to be limited, without the required performance requiring full adhesion; it facilitates the construction of the bogie.

Modern motor bogies provide excellent track holding , without over-reacting on the rails; wheel wear is thus very reduced and, moreover, the ability to exchange bogies during maintenance operations is valuable on diesel equipment, whose recent engines require less frequent repairs than the running gear.

It should also be noted that the diesel locomotive, because of its lower power-to-weight ratio compared with electric locomotives, uses less powerful traction motors, which leads to lighter bogies; however, the body, which contains the generator(s), is relatively heavier. These factors are, on the whole, conducive to good dynamics of the machine.

3- or 4-axle locomotives

Three-axle and, more rarely, parallel four-axle locomotives are still in frequent use for low-speed service (in particular for shunting), because their construction is economical (coupling rods); moreover, in the case of non-electric transmissions, and for locomotives that regularly have to develop high forces, the control by connecting rods and dummy axles is still the only one that has been widely proven. Unfortunately, even with the use of devices to facilitate cornering and the lubrication of the wheel flanges, the wear of the axles on these locomotives is considerably more pronounced than on locomotives with bogies, and repairs bring the entire unit to a standstill; these difficulties increase, of course, with the load on the axles.

Diesel engines and transmissions

As these essential components are dealt with specifically in other communications, we will confine ourselves to highlighting their essential features.

Derivative and fast engines

There are now many motors specially adapted for rail traction. They belong to two still quite distinct classes:

• engines originally derived from marine engines, whose speed is generally between 600 and 1000 rpm, with 6 to 16 cylinders of 200 to 324 mm bore, whose unit power varies from 600 to 2000 HP, and whose weight per horsepower is of the order of 8 to 10 kg;
• fast engines, generally running at 1500 rpm, with 8 to 12 cylinders of 170 to 210 mm bore, developing 500 to 1000 HP, with a weight per horsepower varying from about 3 to 9 kg. Some 16-cylinder units, generally developing 1200 to 1500 HP, are beginning to be tested.

The maximum power currently achievable per locomotive is thus 1800/2000 HP, using one engine of the first category, or two of the second.

The different types of transmissions used

As far as transmissions are concerned, the electric transmission is still by far the most widespread. Technically well developed, very flexible, easy to manoeuvre, it adapts well to the various performances that may be required, from shunting to mixed line, passenger and freight services, even with heavy axle loads. However, it is relatively heavy and expensive, especially since the locomotive's power must be continuously developed over a wide range of speeds. Nevertheless, progress is still significant, and weight reductions can still be expected, thanks in particular to the use of new insulators.

For locomotives of more than 500 HP, the purely mechanical transmission - and transmissions with a hydraulic coupler clutch, which are classified in this category - is not in common use, and only equips a few prototypes; these include some Renault B-B locomotives of 2 X 400 HP, and the Fell system locomotive of British Railways.

Hydraulic transmission, with torque converters, has made great progress in the last ten years, mainly in Germany; it is now for unit powers of up to about 1,000 hp . Locomotives exceeding this power have two separate engines and two separate transmissions.

The difficulties in the construction and maintenance of these transmissions lie much less in the hydraulic components themselves than in the mechanical components, which are always important and which necessarily accompany the preceding ones, particularly in transmitting power to the axles. The problem is practically solved on slow machines, with parallel axles with medium loads, by using a false axle and connecting rods; in the case of locomotives with bogies, it is necessary to use drive axles with cardan shafts, and very interesting realizations in this field are in service on the German railways (BB locomotives of 1000 and 2 X 1000 HP). However, there is still a lot of experience to be gained in heavy freight service, as the construction of drive axles for loaded axles (18 to 20 tons) working regularly at high torques poses delicate problems, and these must be fully resolved if hydraulic or mechanical transmissions are to compete with electric transmission on powerful "all service" diesel locomotives; this point is important because, because of the high cost of diesel locomotives and their frequent use on lines with medium-density traffic (heavily loaded lines are generally amenable to electrification), in order to use them intensively, it is generally important to have non-specialised units suitable for pulling trains of all kinds (fast passengers and heavy goods).

Ancillary services

Diesel locomotives have quite number of auxiliaries required by the diesel engine itself, by the transmission, and by the braking devices : cooling radiators, fuel supply devices, various filters, cooling radiators for hydraulic transmissions or cooling fans for electric machines, equipment for heating passenger carriages, air compressors, etc...

Originally, in locomotives with electric transmissions, the all the auxiliary services were operated by individual electric motors, direct current, powered by the auxiliary generator. This mode of operation, which facilitates the installation of the various components, is, however, cumbersome and expensive due to the high price of the DC motors and the significant increase in power required by the auxiliary generator. Many modern locomotives therefore use direct mechanical control for the most important auxiliaries, i.e. the air compressor, the radiator fans, and even the traction motor fans. However, some manufacturers have retained electrical control for the radiator fans, and some use alternating current produced by a diesel-driven alternator. This solution, which has the advantage of using robust and inexpensive engines, at the same time provides some control of the water temperature, as the speed of the fans is linked to that of the generator set. For the cooling of the lubricating oil, radiators through which the oil flows directly are increasingly being abandoned and water-oil exchangers are preferably used. Automatic regulation of the cooling water temperature, the need for which is now unanimously recognised, is still frequently provided by adjustable shutters in front of the radiators, or by a by-pass valve that bypasses the flow through the radiator; but the trend is towards regulation by varying the speed of the fans (progressive, or by "on-off"), and, to perfect the regulation, some recent units even have two completely separate cooling systems for water and oil. The use of high supercharging, which requires the air to be cooled before it enters the engine, means that a special water circuit, operating at a reduced temperature, must be provided for this purpose.

The air, fuel and lubricating oil filtering systems

Finally, we will mention the importance now given to air, fuel and lubricating oil filtration systems. In particular, for circulation in desert areas subject to sand winds, special precautions must be taken: adduction of pre-filtered air into the engine compartments, maintained at overpressure, double air filtration for the diesel engine , etc...

Heating of passenger trains

The heating of passenger trains towed by diesel locomotives is a difficult problem, the impact of which on the cost price is not negligible. The heat sources available on diesel locomotives cannot be used to heat the carriages, either because of their irregularity (exhaust fumes) or because their temperature is too low (engine cooling water). Therefore, a special source suitable for the equipment of the coaches must be used, i.e. electric heating or steam heating. Electric heating is very rarely used, because of its price, as it requires the installation of a special machine driven by the main diesel engine, or by an auxiliary engine, whose power can be up to about 20% of that required for traction. The train is therefore almost always heated by an automatic steam boiler, burning diesel oil. A water reserve of several m3 must be provided on board the locomotive, so that all the heating equipment results in a not insignificant increase in weight, which can reach about ten tonnes for a unit of 1800 hp, taking into account the lengthening of the body made necessary by the boiler.

source : Excerpt from the article L'EVOLUTION DE LA LOCOMOTIVE DIESEL by Ch. TOURNEUR, Chief Engineer at the S. N. C. F. appeared in the Revue Universelle de Mines July 55