Diesel Locomotives



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Diesel Locomotives The Railway Technical Website PRC Rail Consulting Ltd


Diesel Locomotives
The Diesel Locomotive
The modern diesel locomotive is a self contained version of the electric locomotive. Like the electric locomotive, it has electric drive, in the form of traction motors driving the axles and controlled with electronic controls. It also has many of the same auxiliary systems for cooling,
lighting, heating, braking and hotel power (if required) for the train. It can operate over the same routes (usually) and can be operated by the same drivers. It differs principally in that it carries its own generating station around with it, instead of being connected to a remote generating station through overhead wires or a third rail. The generating station consists of a large diesel engine coupled to an alternator producing the necessary electricity. A fuel tank is also essential. It is interesting to note that the modern diesel locomotive produces about 35% of the power of a electric locomotive of similar weight.
Figure 1: A BNSF diesel electric locomotive a GE ES44C4 type, a
typical US heavy haul locomotive. Photo PorsHammer.
Diesel-Electric Types
Like an automobile, a diesel locomotive cannot start itself directly from a stand. It will not develop maximum power at idling speed, so it needs some form of transmission system to multiply torque when starting. It will also be necessary to vary the power applied according to the train weight or the line gradient. There are three methods of doing this mechanical, hydraulic or electric. Most diesel locomotives use electric transmission and are called "diesel-electric" locomotives. Mechanical and hydraulic transmissions are still used but are more common on multiple unit trains or lighter locomotives.
Diesel-electric locomotives come in three varieties, according to the period in which they were designed. These three are:
DC - DC (DC generator supplying DC traction motors AC - DC (AC alternator output rectified to supply DC motors) and AC - DC - AC (AC alternator output rectified to DC and then inverted to phase AC for the traction motors. The DC - DC type has a generator supplying the DC traction motors through a resistance control system, the AC - DC type has an alternator producing AC current which is rectified to DC and then supplied to the DC traction motors and, finally, the most modern has the AC alternator output being rectified to DC and then converted to AC (phase) so that it can power the phase AC traction motors. Although this last system might seem the most complex, the gains from using AC motors far outweigh the apparent complexity of the system. In reality, most of the equipment uses solid state power electronics with microprocessor-based controls. For more details on AC and DC traction, see Electric Traction
Power (../../../infrastructure/electric-traction-power.html) and Electric Locomotives (../electric-locomotives/) on this site.
In the US, traction alternators (AC) were introduced with the 3000 hp single diesel engine locomotives, the first being the Alco C. The
SD40, SD and GP also had traction alternators only. On the GP, SD, GP, and SD39s, traction generators (DC) were standard, and traction alternators were optional, until the dash era, when they became standard. It was a similar story at General Electric.
There is one traction alternator (or generator) per diesel engine in a locomotive (standard North American practice anyway. The Alco C was the last locomotive to lead the horsepower race with a DC traction alternator.
The diagram (Figure 2) shows the main parts of a US-built diesel-electric locomotive and these are described in the following paragraphs. I
have used the US example because of the large number of countries which use them. There are obviously many variations in layout and
European practice differs in many ways and we will note some of these in passing.

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