Engine types of diesel locomotive

Diesel Multiple Units (DMUs)

The diesel engines used in DMUs work on exactly the same principles as those used in locomotives, except that the transmission is normally mechanical with some form of gear change system.  DMU engines are smaller and several are used on a train, depending on the configuration.  The diesel engine is often mounted under the car floor and on its side because of the restricted space available.  Vibration being transmitted into the passenger saloon has always been a problem but some of the newer designs are very good in this respect.

There are some diesel-electric DMUs around and these normally have a separate engine compartment containing the engine and the generator or alternator.

The Diesel Engine

The diesel engine was first patented by Dr Rudolf Diesel (1858-1913) in Germany in 1892 and he actually got a successful engine working by 1897.  By 1913, when he died, his engine was in use on locomotives and he had set up a facility with Sulzer in Switzerland to manufacture them.  His death was mysterious in that he simply disappeared from a ship taking him to London. 

The diesel engine is a compression-ignition engine, as opposed to the petrol (or gasoline) engine, which is a spark-ignition engine.  The spark ignition engine uses an electrical spark from a "spark plug" to ignite the fuel in the engine's cylinders, whereas the fuel in the diesel engine's cylinders is ignited by the heat caused by air being suddenly compressed in the cylinder.  At this stage, the air gets compressed into an area 1/25th of its original volume.  This would be expressed as a compression ratio of 25 to 1.  A compression ratio of 16 to 1 will give an air pressure of 500 lbs/in² (35.5 bar) and will increase the air temperature to over 800°F (427°C).

The advantage of the engine of diesel locomotives over the petrol engine is that it has a higher thermal capacity (it gets more work out of the fuel), the fuel is cheaper because it is less refined than petrol and it can do heavy work under extended periods of overload.  It can however, in a high speed form, be sensitive to maintenance and noisy, which is why it is still not popular for passenger automobiles.

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.