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Diesel Locomotive

Posted on 1/20/2016 11:13:36 AM

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. 

Parts of a Diesel-Electric Locomotive

The following diagram shows the main parts of a US-built diesel-electric locomotive. 
Diesel Engine

This is the main power source for the locomotive.  It comprises a large cylinder block, with the cylinders arranged in a straight line or in a V .  The engine rotates the drive shaft at up to 1,000 rpm and this drives the various items needed to power the locomotive.  As the transmission is electric, the engine is used as the power source for the electricity generator or alternator, as it is called nowadays.
Diesel Engine Types

There are two types of diesel engine, the two-stroke engine and the four-stroke engine.  As the names suggest, they differ in the number of movements of the piston required to complete each cycle of operation.  The simplest is the two-stroke engine.  It has no valves.  The exhaust from the combustion and the air for the new stroke is drawn in through openings in the cylinder wall as the piston reaches the bottom of the downstroke.  Compression and combustion occurs on the upstroke.  As one might guess, there are twice as many revolutions for the two-stroke engine as for equivalent power in a four-stroke engine.

The four-stroke engine works as follows:  Downstroke 1 - air intake, upstroke 1 - compression, downstroke 2 - power, upstroke 2 - exhaust.  Valves are required for air intake and exhaust, usually two for each.  In this respect it is more similar to the modern petrol engine than the 2-stroke design.

In the UK, both types of diesel engine were used but the 4-stroke became the standard.  The UK Class 55 "Deltic" (not now in regular main line service) unusually had a two-stroke engine.  In the US, the General Electric (GE) built locomotives have 4-stroke engines whereas General Motors (GM) always used 2-stroke engines until the introduction of their SD90MAC 6000 hp "H series" engine, which is a 4-stroke design. 

The reason for using one type or the other is really a question of preference.  However, it can be said that the 2-stroke design is simpler than the 4-stroke but the 4-stroke engine is more fuel efficient.

Diesel-Electric Types

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 3-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 (3-phase) so that it can power the 3-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. 
In the US, traction alternators (AC) were introduced with the 3000 hp single diesel engine locomotives, the first being the Alco C630. The SD40, SD45 and GP40 also had traction alternators only. On the GP38, SD38, GP39, and SD39s, traction generators (DC) were standard, and traction alternators were optional, until the dash-2 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 C628 was the last locomotive to lead the horsepower race with a DC traction alternator.

Below is a diagram showing the main parts of a common US-built diesel-electric locomotive.  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. 

More Information

This page is just a brief description of the main points of interest concerning diesel locomotives.  There aren't too many technical sites around but the following links give some useful information:

US Diesel Loco Operating Manuals - Copies of some of the older US diesel locomotive manuals issued to staff.  Contains some very interesting details.
Diesel-Electric and Electric Locomotives - by Steve Sconfienza, PhD.D. - >Includes some technical background on the development of diesel and electric traction in the US, an illustration of the PRR catenary system and some electrical formulae related to different traction systems.
Diesel-Electric Locomotive Operation - A general list of US diesel locomotive types, designs and statistics with a summary of their development.  A useful introduction to the US diesel loco scene. 
China Balin Parts Plant - Reliable Manufacturers for diesel locomotive parts.It could be replacements for EMD,GE,ALCO,etc.

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